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Electrospinning versus fibre production methods: from specifics to technological convergence Downloaded by UNILEVER RESEARCH LABORATORY on 28 October 2012 Published on 22 May 2012 on http://pubs.rsc.org | doi:10.1039/C2CS35083A

C. J. Luo,*a Simeon D. Stoyanov,abc E. Stride,ad E. Pelanb and M. Edirisinghea Received 16th March 2012 DOI: 10.1039/c2cs35083a Academic and industrial research on nanofibres is an area of increasing global interest, as seen in the continuously multiplying number of research papers and patents and the broadening range of chemical, medical, electrical and environmental applications. This in turn expands the size of the market opportunity and is reflected in the significant rise of entrepreneurial activities and investments in the field. Electrospinning is probably the most researched top-down method to form nanofibres from a remarkable range of organic and inorganic materials. It is well known and discussed in many comprehensive studies, so why this review? As we read about yet another ‘‘novel’’ method producing multifunctional nanomaterials in grams or milligrams in the laboratory, there is hardly any research addressing how these methods can be safely, consistently and cost-effectively up-scaled. Despite two decades of governmental and private investment, the productivity of nanofibre forming methods is still struggling to meet the increasing demand. This hinders the further integration of nanofibres into practical large-scale applications and limits current uses to niche-markets. Looking into history, this large gap between supply and demand of synthetic fibres was seen and addressed in conventional textile production a century ago. The remarkable achievement was accomplished via extensive collaborative research between academia and industry, applying ingenious solutions and technological convergence from polymer chemistry, physical chemistry, materials science and engineering disciplines. Looking into the present, current advances in electrospinning and nanofibre production are showing similar interdisciplinary technological convergence, and knowledge of industrial textile processing is being combined with new developments in nanofibre forming methods. Moreover, many important parameters in electrospinning and nanofibre spinning methods overlap parameters extensively studied in industrial fibre processing. Thus, this review combines interdisciplinary knowledge from the academia and industry to facilitate technological convergence and offers insight for upscaling electrospinning and nanofibre production. It will examine advances in electrospinning within a framework of large-scale fibre production as well as alternative nanofibre forming methods, providing a comprehensive comparison of conventional and contemporary fibre forming technologies. This study intends to stimulate interest in addressing the issue of scale-up alongside novel developments and applications in nanofibre research.

1. Introduction Academic and industrial research on nanofibres is an area of intense global interest in terms of both fundamental and applied science. Electrospinning is an extensively studied and a

Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK. E-mail: [email protected]; Fax: +44 20 7388 0180; Tel: +44 20 7679 2193 b Unilever Research & Development Vlaardingen, Olivier van Noortlaan 120, 3133 AT, Vlaardingen, The Netherlands c Laboratory of Physical Chemistry and Colloid Science, Wageningen University, 6703 HB Wageningen, The Netherlands d Institute of Biomedical Engineering, Department of Engineering Science, Old Road Campus, University of Oxford, Oxford OX3 7DQ, UK

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widely applied method for nanofibre production from a remarkable range of organic and inorganic materials. The technique has enjoyed significant attention since the 1990s, on account of its versatility and economic competitiveness at the laboratory scale for producing nanofibres and composite nano-structures with tuneable properties for a broad range of applications.1–6 From photocatalytic self-cleaning car mirrors and building materials,7–11 stain-repellent, wrinkle-free, highly breathable detoxifying clothes,12–14 to multifunctional, stimuliresponsive bioengineered structures,15–28 and superfast miniature electronics,29–31 the applications of electrospun nanofibres and nanocomposites include not only domestic items such as clothing, batteries, optical devices, gravimetric and chemical sensors, and biomedical and healthcare products,5,32–35 but also large scale engineering on a worldwide level such as This journal is

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in sustainable construction, air and water filtration and purification, and energy generation using photovoltaic solar panels.6,36–46 The research and application areas of electrospun nanofibres and technical textiles are expanding rapidly and playing invaluable roles in a range of advances in nanoscience and technology, which continue to transform our standards of living. As a result, the demand for nanofibres is burgeoning. Notwithstanding the economic recession, the value of the global nanofibre market increased from $43.2 million in 2006 to $101 million in 2010, and is projected to reach 2.2 billion by 2020.47,48 Despite the wide-ranging interest and progress in electrospinning, after two decades of governmental and private investment, the modern capacity of semi-industrialised nanofibre production is struggling to meet the increasing market demand. The poor cost–yield efficiency of the current state-ofthe-art electrospinning and nanofibre forming technologies hinders the further integration of nanofibres into a wider range of practical large-scale applications and limits current

uses to high-end niche markets driven by cost–performance benefits. For example, while the U.S. Department of Defense purchases nano turbine-combustion air-filters for use on the M1 Abrams tanks,49 nanofiltration for affordable personal automobiles is still at a very early stage. This substantial gap between supply and demand was seen in conventional textile fibre production in the previous century and successfully addressed via extensive collaborative research and ingenious solutions. As the traditional textile fibre industry battled against failed attempts to reach a mere 100 kg per day target to stay profitable back in the 1890–1910s,50,51 the first 4000 trial-pairs of nylon stockings were sold out in a frenzy of 3 hours in 1939.52 The enthusiastic crowds were so large that they pushed back the counter, pinning the saleswomen to the wall.53 After decades of collaborative development and process optimisation, today commercial textile fibres can be prepared in tens of tonnes per hour.54 New advances in electrospinning and nanofibre production show a clear trend of interdisciplinary technological convergence.

Dr C. J. Luo holds a BSc in Biochemistry and PhD in Biomedical Engineering from University College London. After receiving her PhD in 2012, she continues to work at University College London in Professor Edirisinghe’s laboratory as a postdoctoral research associate. Her research interests centre on processing and forming of soft matter, particularly sustainable biopolymers for healthcare and environmental applications, C. J. Luo with an emphasis on technology transfer and translational research between biomedical/physical sciences and engineering disciplines.

Dr Simeon D. Stoyanov received his PhD from Essen University, Germany. He has worked in the Laboratory of Chemical Physics and Engineering in University of Sofia, Bulgaria, as a visiting scientist at the Ecole Normale Superieure, Paris, France, University of Erlangen and Henkel R & D in Dusseldorf, Germany. Currently he is a senior scientist in Colloids and Interfaces at Unilever R & D in Vlaardingen and a Simeon D. Stoyanov visiting professor in University of Wageningen, The Netherlands and University College London, UK. His research interests cover various topics of soft-condensed matter, materials science, self-assembly, surface science of liquid–liquid interfaces, foams and emulsions, physical-chemistry of digestion and encapsulation.

Dr Eleanor Stride holds a B.E. and PhD from University College London. Following the completion of her PhD in 2005 she was appointed to a lectureship and a Royal Academy of Engineering and EPSRC Research Fellowship and subsequently a Readership in 2010. She joined the Institute of Biomedical Engineering in October 2011 when she also became a Fellow of St. Catherine’s College, Oxford. Her main research interests are encapsulation, biomedical ultrasound and theranostics.

Dr Eddie Pelan is a Physics graduate from the Queens University of Belfast. He has more than 25 years of industrial applications in the formulation and processing of soft condensed matter primarily in the foods area with Unilever Research. His current role of a Platform Director in the area of Structured Materials & Processing Science is to generate internally, or find externally, novel insights in science and technology for improvement of the Unilever product portfolio, both in the Foods & Home and Personal Care parts of the Business.

E. Stride

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Recent efforts to improve the quality and performance of the nanofibres and increase productivity and reduce the cost of electrospinning and other nanofibre production methods involve merging key concepts of conventional textile fibre manufacture processes with nanotechnology. Examples include electroblowing, gas-jet/gas-assisted electrospinning and solution blowing, evolved from melt blowing,55–64 coupled electrocentrifugal processing,65 centrifugal nanospinning,66–68 nearfield electrospinning with dip-pen nanolithography,69–72 and XanoSheart combining shearing with wet spinning.73,74 A comprehensive study of conventional and contemporary fibre forming methods in parallel comparison to advances in electrospinning to improve the functionality, versatility, efficiency and yield was needed.41,75,76 Much has been written on the fundamental scientific principles of electrospinning, covering the physics, chemistry, materials science and applications of the technology. To stimulate further development in the scale-up of production, this review combines knowledge from the academic and industrial research communities and places fundamental research on electrospinning within a framework of commercialscale fibre production. It aims to provide a comprehensive comparison of conventional and contemporary industrial fibre forming technologies, with emphasis on bridging the key concepts and parameters among fibre-forming methods, in particular, spinning methods comparable to conventional and modified electrospinning methods. This work reflects on the past of conventional fibre technologies and illustrates that developments in scale-up of nanofibre productions can benefit from interdisciplinary technological convergence.

2. Conventional and contemporary fibre productions 2.1.

An historical overview

Natural fibrous materials are virtually ubiquitous and man has dreamed of mastering the art of fibre forming for centuries, Professor Mohan Edirisinghe is the Chair of Biomaterials at University College London. He gained his PhD and DSc from the University of Leeds and has published over 300 journal papers on advanced materials processing and forming. He leads a research team of 20 people developing novel methods for the preparation of bubbles, particles, capsules and fibres, covering therapeutics, healthcare-food M. Edirisinghe engineering, orthopaedic and tissue engineering. He has been awarded many research prizes, including the 2009 (Inaugural) EPSRC-Royal Society Interface Journal prize, the 2010 Materials Science Venture Prize, and the 2012 UK Biomaterials Society Presidents Prize for outstanding contributions to the Biomaterials field. 4710

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either by taking advantage of nature’s ingenuity such as to obtain silk from Bombyx mori, a secret highly guarded in ancient China, and the modern day bio-fabrication of cellulosic fibres from microbes; or by artificially mimicking nature such as in the synthesis of polymeric fibres from the molecular level to the finished yarn. The global demand for artificial fibres is high both in conventional textiles for clothing and furnishing for their outstanding hand-feel, softness, aesthetic beauty and comfort; and in the technical textile industry, which demands superior functionality over textural or aesthetic qualities. Technical textiles are produced for application areas such as electronics, optical technologies, military defences, automobiles, aerospace and marine engineering, protective clothing, reinforcement composites, and regenerative medicine.77,78 Fig. 1 maps the classification of natural and artificial fibres; the latter includes synthetic polymeric fibres and regenerated biopolymers. The notion of man-made fibre production was mooted in 1664, when Hooke first examined the structure of natural silkworm fibres at the microscopic level and envisaged the possibility of artificial silk. This was recorded in his book Micrographia and ordered for printing and publication by the Royal Society.79 However, it was more than 200 years later when man truly began to master the art of synthetic fibre production. The first factory-made nitrocellulose artificial silk was realised by Chardonnet, the ‘‘Father of Rayon’’, at Besançon in France in 1890 (Fig. 2). As the traditional textile fibre industry battled against failed attempts to reach a mere 100 kg per day target during the 1890s, Chardonnet’s company reached a production capacity of between 100 and 1000 kg per day at a price of 21.75–30.0 francs per kg (worth approximately 6.3–8.7 g of gold of the time)80 from 1894 to 1898, and made 20 200 francs (worth approximately 5.9 kg of gold of the time)80 profit and a 6.25% dividend in 1898.50,51 The first artificial synthetic textile Nylon 66, obtained purely from petrochemicals, was invented in 1935 and patented by DuPont.82–84 New stockings made from Nylon 66, as a replacement of silk stockings, were extremely well received. 36 million pairs had been sold in the first year of production by 194052,53 (Fig. 3). The gap between supply and demand seen in conventional textile fibre production over the last century was successfully addressed via extensive collaborative research and ingenious solutions and the productivity of conventional textile fibres has increased by a few orders of magnitude to reach the current tens of tons per hour. Today a large-scale integrated production line of synthetic fibres can reach a production capacity of 300 tonnes per day.54 For comparison, despite the increasing demand for nanofibres, the modern capacity of semi-industrialised nanofibre production techniques struggles to exceed few kg h1, a minimum initial target for artificial fibre production back in the 1890–1910s.50,51 Scale-up of nanofibre production may be accomplished in similar ways to that seen in conventional textiles via academic and industrial collaboration and interdisciplinary technological convergence, an emerging trend seen in the development of nanofibre forming methods. The following sections cover contemporary and conventional fibre production technologies, while paying special attention to electrospinning. This journal is

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Fig. 1 Classification of fibres.

Fig. 2 The first artificial fibre spinning machine developed by Chardonnet in 1889 for the production of nitrocellulose fibres. Spinnerets are positioned vertically upwards to eject fibres onto the reels at the top of the cabinet. Reproduced by permission of Oxford University Press from ref. 81, Plate 1 (figure inset between pp. 4 and 5) entitled ‘‘The Artificial Silk Spinning Machine’’ from Courtaulds: An Economic and Social History, Volume II by Donald Cuthbert Coleman (1969). Copyright 1969.

2.2.

Summary of fibre production methods

Current fibre-forming processes include artificial fibre spinning, thermally induced phase separation, molecular self-assembly and the bio-fabrication alternatives. Spinning is a term used to describe processing methods conventionally using spinneretextrusions to produce continuous polymeric fibres. Chain molecules are generally preferred in spinning, though recent work has demonstrated the spinnability of molecularly self-assembling materials (MSAs) using solution and melt electrospinning and melt blowing.85–87 Materials to be extruded must be in a liquid state. This is termed as ‘‘dope’’ or ‘‘spinning dope’’. Any polymer capable of fibre forming can be used for the process. The polymers are converted to a liquid This journal is

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Fig. 3 Photograph of crowds queuing to purchase the newly commercialised nylon stockings and a woman proudly putting on the coveted stockings in the street right after purchasing them – a glance of the demand, impact and importance of fibre production in history and society. Photograph reproduced with permission by courtesy of Hagley Museum and Library.

state by heating, dissolution, pressurisation, or combinations of these three procedures. Spinneret-extrusion spinning methods can be classified according to the nature of the spinning dope, namely melt spinning, solution spinning, and emulsion spinning. Melt spinning encompasses traditional melt spinning, microfibre melt blowing, multicomponent conjugate nanospinning via the island–sea method or the segmented pie method. Solution spinning includes dry spinning, conventional wet spinning, sheared wet nanospinning (XanoSheart), and gel spinning. Chem. Soc. Rev., 2012, 41, 4708–4735

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Fig. 4

A concise comparison of fibre spinning processes.

In addition, film splitting, drawing and thermal size-reduction process, centrifugal spinning are representative methods able to process both fibre-forming polymer melts and solutions. Highly flexible, electrospinning is suitable for processing solutions,1,88,89 gels and liquid crystals,90–92 melts93–96 as well as emulsions.22,97–99 The choice of the processing technique and the type of equipment used are important aspects to consider in the commercialisation of nanofibre production. The two factors are interrelated and a variety of nanofibre manufacturing routes were developed during the early stages of translation from laboratories to factories. Fig. 4 illustrates some of the key differences among the representative spinning methods mentioned here. In contrast to bottom-up nanofibre forming methods such as molecular self-assembly, which rely on more expensive and cumbersome processing to assemble matter at the molecular level (Table 1)100 into synthetic nanoscale discontinuous fibrils or fibrous objects, with limited control over the dimensions of the nanofibres produced,101 electrospinning employs a topdown engineering approach, reducing charged liquid materials in a spinning jet from macro-scale to nanoscale fibres. Electrospun nanofibres and the electrospinning technique have several unique advantages over nanofibres formed via other methods. Dispersion and alignment of nanofibres, nanowires and single-walled carbon nanotubes (SWNT) are major challenges in electronics and high strength composite reinforcements.16,102,103 Electrospinning has been shown to provide a solution to this problem, successfully inducing electrostatic alignment of nanofibres and nanotubes.104–109 In tissue engineering, biomimetic collagen scaffolds need to be assembled appropriately to exhibit the characteristic banding pattern typical of native collagen.24 It was shown almost a decade ago that electrospun collagen nanofibres could be successfully assembled to exhibit the 67 nm repeating banding patterns.29 Additionally, multicomponent co-electrospinning and emulsion electrospinning elaborated in further detail later Table 1

Nanoscale in perspective100

Matter

Diameter/nm

Hydrogen atom Width of 6 carbon atoms aligned Single wall carbon nanotube DNA Proteins White blood cell

0.1 1 0.4–1.8 2 5–50 10 000

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are important techniques for encapsulating drugs and biological agents.18,22,99,110,111 Melt co-electrospinning is particularly advantageous when materials of different melting points such as core–sheath nanofibres need to be incorporated for phase change applications.112,113 Further, most fibre spinning methods involve the use of spinnerets. This can limit the yield of nanofibres due to the delicate spinneret designs with relatively small orifice diameters as well as the need to have low feed rates, to obtain continuous homogeneous nanofibres. Electrospinning can be operated as a ‘‘spinneret-free’’ nanofibre forming method due to the use of electrostatic forces initiating self-assembled fibre spinning on charged free liquid surfaces.114,115

3. Electrospinning in an industrial framework This section provides a summary of the physical mechanism and key parameters in electrospinning and paves the way for the comparison of electrospinning with fibre production methods and the trend of interdisciplinary technological convergence in recent advances of nanofibre production. The race is on in the nanofibre industry to develop a principal production route capable of achieving high yields of high quality product at affordable prices. To achieve this, the governing process parameters in electrospinning are identified and evaluated together with comparable industrial spinning methods. 3.1.

Jetting mechanism and spin-line evolution

Electrospinning is in many ways similar to conventional spinning. Extensive attention has been paid to studies on the theoretical mechanism and modelling of the evolution of the spin-line and fibre jetting in conventional textile spinning116–120 as well as electrospinning.121–123 The fibre jet initiated during fibre spinning is generally subjected to tensile, rheological, gravitational, inertial and aerodynamic forces.117,121 The difference between electrospinning and conventional textile spinning lies in the origin of the tensile force initiating the fibre jet. In electrospinning, free charges carried by the liquid dope interact with the applied electric field and the tensile force inducing fibre jetting is due to the potential difference between the charged liquid in a spinneret and a grounded collector. On the other hand, conventional industrial fibre processing employs mechanical means using spindles and reels to generate the tensile force to initiate spinning.121 This journal is

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Fig. 5 Schematic diagrams showing the progressive understanding of the evolution of the spin-line and the physical mechanism of nanofibre formation in electrospinning. (a) ‘‘Splaying’’ of fibres previously believed to occur during electrospinning. Reprinted with permission from ref. 121. Copyright 1996, Institute of Physics Publishing Ltd. (b) Understanding of electrospinning of fibres due to bending instability, refuting the earlier theories on ‘‘splaying’’, reprinted with permission from ref. 122. Copyright 2000, American Institute of Physics.

The ejected thin viscoelastic liquid jet accelerates rapidly and demonstrates regular long wave-forms of instabilities during spinning.124 Fig. 5 shows the progressive understanding of the evolution of the spin-line and the physical mechanism of nanofibre formation in electrospinning. ‘‘Splaying’’ of fibres was believed to occur in the early investigations on electrospinning36,88 (Fig. 5a). During jet elongation, as the solvent evaporates and the diameter of the jet decreases, the surface charge density of the fibre increases which results in increased repulsive forces in the jet. This was believed to cause splitting of the jet, which splays again and repeats the cycle as the diameter of the jet further reduces.36,88 This theory was later refuted at the start of the new millennium by Reneker et al. (Fig. 5b)122 and the jetting motion was shown to undergo rapid bending instability (also known as whipping instability), which describes long wave-forms of perturbations of a liquid column driven by the lateral electric force and the aerodynamic interaction.122,123,125–128 Mathematical expositions and asymptotic analyses to model the instabilities on the jetting mechanism were discussed by Shin et al.125,126 and Hohman et al.127,128 after Reneker et al.122 and three instabilities in electrospinning were predicted: an axisymmetric instability dominated by surface tension and associated with the classical Rayleigh instability, which is suppressed at high electric fields when the applied electric field EN and the surface charge density s exceed a threshold given by: (e e)E2N + 4p2s2/e = 2pg/h

(1)

where g is the surface tension, h is the radius of the jet, e and e are, respectively, the dielectric constant inside and outside the jet, and e/e c1. At a high electric field, the classical Rayleigh instability becomes irrelevant and a second axisymmetric instability and a third non-axisymmetric instability (also known as bending/ whipping instability) become prevalent. The latter two are This journal is

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due to fluctuations in the dipolar component of the charge distribution, and termed ‘‘conducting modes’’ because they are electrically driven and essentially independent of the surface tension of the liquid. The electrically driven axisymmetric instability dominates at lower charge density and arises from the finite, non-zero conductivity of the liquid. As the jet travels further from the nozzle and elongates, aerodynamic interaction occurs together with increased surface charge density of the jet as the jet diameter decreases over the spinline. The non-axisymmetric bending/whipping instability becomes prevalent as a consequence of lateral perturbations in the centre axis of the jet due to aerodynamic interaction, bending torque produced from dipole charge distribution within the jet interacting with the external electric field, as well as bending caused by repulsion of surface charges.123,125 Bending/whipping instability is believed to be crucial in nanofibre formation via electrospinning.122,123,126 Later, Fridrikh et al.129 presented a model applicable to a charged liquid in an electric field with bending instability that predicted the diameter of a terminal jet: 1=3 Q2 2 ht ¼ ge 2 ð2Þ I pð2 ln w 3Þ The equation expresses terminal jet diameter ht as a function of flow rate Q, electric current I, surface tension g of the liquid, dielectric constant of the medium surrounding the jet e, and the dimensionless wavelength of the bending instability w B R/h, where R is the radius of the bending perturbation, h is the jet radius. The theoretical predictions were in agreement with experimental data obtained from electrospinning polycaprolactone (PCL) solutions.129 In addition to bending instability, buckling instability also occurs due to the longitudinal compressive force from jet impingement on a solid flat surface during collection.122,124,131,132 Bending and buckling instabilities are not limited to electrospinning,123,131 Chem. Soc. Rev., 2012, 41, 4708–4735

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Fig. 6 Comparison of the buckled patterns created by electrified nylon-6 jets collected on water (upper panel in each pair of comparison) to the buckling patterns130 resulting from uncharged gravity-driven syrup jets (lower panel in each pair of comparison, the scale is approximately 1000 higher than that of the upper panel). Adapted with permission from ref. 131. Copyright 2007. Elsevier Ltd.

and are also observed in uncharged liquid jets. Bending instability can be observed in uncharged jets moving in air at high speeds, such as in gas-assisted fibre processing methods including melt blowing and solution blowing, and other fibre spinning methods involving high-speed jetting in air.63,122,124,132,133 Buckling instability can be observed in fibres depositing on a hard flat surface from both uncharged and electrified jets and irrespective of whether the jets coming into contact with the collector are straight or bent (Fig. 6).130,131 3.2.

Material and process parameters

Good ‘‘electrospinnability’’ is defined as continuous fibre formation with uniform fibre diameter and minimal beadon-string defects from a viscoelastic liquid capable of jet initiation and stable spinning under an applied electrostatic field. Apart from jet initiation due to an applied voltage in electrospinning, the attributes of the spinning dope dominate the onset of fibre formation and the fibre morphology, in a similar way to how material properties influence conventional spinning methods.118,122,123,132,134 In this sense there is considerable overlap between conventional fibre processing methods and electrospinning, and the former provides useful insights for parameter optimisation in upscaling of the latter. To ensure continuity of the spinline and fineness of the fibres, common control parameters of spinneret-extrusion fibre forming methods in industry include: optimisation of the liquid properties of the fibre-forming material (i.e. the polymer should already possess fibre-forming properties, such as an appropriate degree of chain overlap), in the case of 4714

solution spinning – solvent and solution properties such as volatility, latent heat of vaporisation, dynamic surface tension and viscoelasticity, and processing parameters including spinneret orifice diameter, extrusion speed and collection distance, among others. In addition, the degree of molecular orientation in forming the fibres is influenced by a radial viscosity gradient during the solidification process. This is correlated to a temperature gradient in melt spinning, a concentration gradient in dry spinning and a heterogeneous diffusion process in wet spinning.116,117,135,136 Furthermore, important parameters unique for solution spinning are the critical solution concentration to ensure sufficient molecular chain entanglement for fibre formation and the solubility and volatility of the solvent for the fineness of the obtainable fibre.118,137 The material and processing parameters of electrospinning have been extensively studied in the laboratory environment. The outcome of an electrospinning process is influenced by a large number of interrelated variables, including operating parameters, such as applied voltage,138 flow rate, collection distance, nozzle design,125,139,140 and the physical nature and geometry of the collecting substrate;104,141,142 as well as material parameters including polymer molecular chain length,143,144 attributes of the solution such as concentration, presence of additives, and solvent and solution properties including viscosity, surface tension,125,138,145 conductivity,146–148 solvent boiling point, dielectric constant, molecular weight, solubility.149,150 Fibres can be spun from both positive and negative potentials.88 Shin et al.125 asserted the importance of nozzle design for stable jetting in electrospinning for determining the jet path and the shape and charge distribution on the jet in the vicinity of the nozzle. Increasing nozzle length was associated with a decrease in the applied voltage required to initiate jetting.126,138 Melt electrospinning studies suggested an optimal ratio of 4.5 with respect to the length of the nozzle to its diameter.139 This value was later confirmed by Mitchell and Sanders.140 A delicate balance between all the aforementioned variables governs the precise control of an electrospinning process.89,151

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3.3.

Scale-up

One of the most pressing issues in electrospinning and nanofibre production is the scale-up of the process. Various efforts have been made to improve the yield of electrospun fibres. The last decade saw the productivity of electrospinning increase from less than 0.5 g h1 in a typical single nozzle setup to a maximum reported value of B6.5 kg h1 today. It is worth mentioning that electrospun nanofibres found practical application and demonstrated significant advantages in filtration as early as the 1930s in the former USSR. More than half a century was dedicated to developing electrospun filters in Russia headed by Petryanov and co-workers at the Karpov Institute of Physical Chemistry in the Russian Federation.36,152 The electrospun ‘‘Petryanov’s filters’’ (PF) showed excellent ultrafiltration properties due to its high aerosol capture efficiency, low interference with breathing, long shelf-life of several years and a filtering effectiveness of >10 h.36 PF materials were used in gas masks for Russian soldiers during This journal is

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World War II and light-weight respirators to capture radioactive aerosols for nuclear and civil defence. The production capacity reached an equivalent of 6.5 kg h1 after an upgrade of the factory equipment during the 1950s–1960s.36 Petryanov and his co-workers were among the first who responded to the 1986 Chernobyl catastrophe to estimate the radiation situation and their ‘‘Lepestok’’ respirators helped to protect people at risk.152 The work by Filatov et al. and the invaluable scientific achievements contributed by I. V. Petryanov-Sokolov from the former USSR are information of great value to electrospinning and nanofibre production. However, this important series of work on the engineering basis of electrospinning was mainly published in Russian and translated texts by Filatov et al. currently have limited availability. More efforts to make these work accessible at a global scale will add valuable input for upscaling nanofibre production. Conventional single nozzle electrospinning has a yield of 0.11 g h1 for a typical production of polyethylene oxide nanofibres with an average fibre diameter of 292 29 nm.153 Spinning via multiple nozzles as seen in industrial fibre production has been investigated for upscaling electrospinning,34,154–162 and nanofibres of uniform diameters within the 100 nanometre range can be produced using nylon 6 at a yield of B6.5 kg h1 by Finetex Inc.161 However, issues such as potential nozzle clogging due to fine spinneret diameter in nanofibre spinning,163 neighbouring jet interference and inter-jet perturbations limiting the minimum distance between the extrusion holes of a spinneret157 are challenges faced in upscaling electrospinning via multiple spinneret extrusion. In addition, free surface nozzleless electrospinning is an attractive spinneretfree fibre forming technique yet to have its upscaling potential realised.114,115,164 For example, to produce polyvinyl alcohol nanofibres of 200 nm mean fibre diameter, the standard output of the semi-industrialised Nanospidert nozzleless spinning machine is o200 g h1.165,166 Multiple spinnerets and freesurface nozzleless spinning are discussed in further detail in Section 11. In addition, a critical aspect of any manufacturing process is its cost. At the laboratory scale, the low initial setup investment and the ease of maintenance brought conventional electrospinning widespread attention for making nanofibres. However, to upscale the technique within a commercial framework, current semi-industrialised electrospinning technologies do not have the aforementioned economic advantages of laboratorial electrospinning. For instance, despite offering a great improvement in productivity compared to conventional electrospinning, the capital investment to set up Elmarco’s Nanospidert nozzleless electrospinning machine is high and the maintenance of the equipment is demanding, with the company supplying cleaning devices tailored for the setup. To address the problem of upscaling, issues of processing need to be evaluated from an industrial perspective. Cost can be significantly reduced by reducing processing time and combining processing stages, as seen in textile fibre spunbond methods combining preparation, spinning and spin-finish in one production line. Recent studies in electrospinning have reported coupled spin-draw processing,72 which in principle is akin to industrial spin-draw processing. Considerations of how electrospinning can be incorporated into such a production line This journal is

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in similar ways to traditional large-scale textile spinning will be beneficial in realising upscaled profitable nanofibre mass-production. There are other limiting factors to be considered that affect the productivity, cost, consistency of fibre production, safety of the operations, the degree of fibre alignment, and the postspinning processing aspects. One common limitation on the yield of nanofibre production using spinneret-extrusion methods is the requirement for lower flow rates to achieve nano-scale fibre diameters. The mechanism of fibre drawing by electrostatic forces in electrospinning poses a constraint (although not a severe one) on the electrical conductivity, as well as the dielectric constant of the fibre-forming material, which precludes electrospinning of some important fluoropolymers. Another common phenomenon among aligned electrospun nanofibres is the need to control the inter-fibre distance due to like charge repulsion when the fibres are not collected at positions where the electric field is focused onto sharp points or edges.89,104,167 Furthermore, when the design of the setup requires charging of the fluid prior to spinneret extrusion, the spinneret nozzle and the spinning components in contact with the nozzle and all associated upstream equipment supplying the spinning dope need to be maintained at a high voltage in a conventional electrospinning setup. This is not practical at an industrial scale for multiple reasons, one of which is that the upstream equipment in a prototype spinning plant may be of sufficient size to ground the high voltage of the process unless they are physically separated. This is clearly undesirable in terms of the production time, cost and homogeneity of the process. Armantrout et al.61 investigated some of the aforementioned issues regarding industrialising charged liquid spinning methods and setups and patented some solutions to address these issues. This is described in further detail in Section 8.3 under the topic electroblowing. In addition, there are safety concerns relating to industrialising solution electrospinning due to the use of high voltages in the presence of flammable solvents. Similar commercial processes involving flammable materials and very high-applied voltages in the range of 50–150 kV, such as the well-established industrial process flocking and the use of electrostatic hand-held spraying equipment, have been surveyed.168 The safety standard for this process on a small scale has been established by the European industry since 1994 and has been consistently revised and updated four times from 1994 to 2011.169,170 Furthermore, the safety aspects of routine maintenance of charged equipment during a continuous manufacturing process need to be investigated. Table 2 lists some of the companies in nanofibre production. Given the potentially revolutionary impact of nanofibres, it is perhaps surprising to see that few multi-national business leaders are committed to commercialising nanofibres. From the perspective of serious investors, the reasons for this could be manifold: insufficient publications of marketing data on nanofibre production and application, poor collaborative business structure within the nanofibre market, concerns of the expensive price tag of ultrafine fibres, limited productivity in current nanofibre technology to enable cheap mass production at levels comparable to conventional textile manufacture, and the public anxiety towards the safety and environmental Chem. Soc. Rev., 2012, 41, 4708–4735

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Table 2

Company

Process

Dienes Apparatebau GmbH Donaldson DuPont Finetex

Nozzle centrifugal electrospinning Electrospinning Confidential Multi-nozzle electrospinning Nozzle centrifugal spinning Nozzle conjugate melt spinning ‘‘Nozzleless’’ sheared wet spinning Nozzle conjugate melt spinning Nozzleless electrospinning

FibeRio


Hills Inc Xanofi Teijin Elmarco

Minimum average fibre diameter published/nm

Typical yield and cost

Webpage

Ref.

80

—

www.dienes.net

171, 172

250 200 o100

— — 6.5 kg h1

http://www.donaldson.com/ http://www2.dupont.com/ http://www.finetextech.com/

49, 173 174, 175 161

45

http://www.fiberiotech.com

67, 176

www.hillsinc.net/

177

100

0.06 kg h1 per spinneret B5 kg h1 at $1–5/kg 6–12 kg h1

http://www.xanofi.com

73, 74, 178

700

—

http://www.teijinfiber.com

179, 180

50

0.171 kg h1

http://www.elmarco.com

165

Approx 50

and societal impact of integrating synthetic nanomaterials. Furthermore, the word ‘‘nano’’ has become a buzzword as an advertising strategy for many products and companies, some of which are not truly in the field of nanotechnology. Poor efforts to address the association of these broad societal issues with technological change discourage large-scale public use of nanofibres. Governmental institutions worldwide including the United States, the United Kingdom, the European Union and Japan are playing an increasing role in addressing challenges related to the social implications of nanotechnology.181,182 Understanding of the existing guidelines for similar industrialised procedures involving similar safety hazards and public apprehensions would be beneficial for the upscaling and integration of electrospinning and/or other nanofibre production methods in a future production line. As research on the electrospinning technique encompasses all three of the major fibre spinning categories, namely melt, solution and emulsion spinning, the advances in electrospinning in each of the three spinning categories are elaborated as subsections of the respective spinning methods in the following work. This places electrospinning within a framework of conventional and contemporary fibre production technologies, providing a comprehensive picture of the technique compared to other fibre forming methods.

4. Solution spinning methods Solution spinning involves the extrusion of a solution to form fibres. The solvent is removed from the fluid filament by vaporisation with hot gases in dry spinning, or by coagulation in a miscible non-solvent in wet spinning.118 4.1.

Wet spinning

Of all methods of fibre spinning, wet spinning has the longest history and was used to produce Rayon in the late 19th century.51 Wet spun deoxyribonucleic acids (DNAs) have been studied as early as the 1960s to produce oriented fibres of biopolymers, which were beneficial for physico-chemical investigations of the macromolecules using methods such as X-ray diffraction, birefringence, and dichroism.136,183 In wet spinning, 4716

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the spinneret is immersed in a coagulation bath, in which the polymer dissolved in a suitable solvent is extruded directly into a liquid which is miscible with the spinning solvent but a non-solvent of the polymer. This leads to solvent removal from the fibre jet and solidification of the fibre as precipitation occurs. Wet spinning involves mass transfer of the solvent and non-solvent for fibre solidification and is slower compared to both melt spinning which hardens the fibre by simple cooling, a heat transfer process; and dry solution spinning, in which fibre solidification occurs via evaporation, a mass transfer process more rapid than that in wet spinning, as solvent evaporation and diffusion into air is faster than solvent diffusion in a miscible liquid (Fig. 4).118,184 Examples of polymers commercially wet spun include viscose rayon, aromatic polyamides and polyvinyl alcohols.118 4.1.1. Dry-jet wet spinning and electrospinning. Wet spinning methods readily allow the manipulation of the cross-sectional shapes of the as-spun fibres by changing spinneret nozzle geometry and the rate of coagulation.118 Pomfret et al.185 compared conventional wet spinning to a modified dry-jet wet spinning method, in which the spinning jet comes in contact very briefly with the external gaseous environment before entering the coagulation bath. Many electrospinning and related electrohydrodynamic spraying processes in laboratories combine modified dry-jet wet spinning with electrospinning, in which the spinning jet comes in contact with ambient air very briefly and enters a liquid non-solvent medium placed a few millimetres to a few centimetres below the spinneret.186 This combination is advantageous particularly in the fields of drug delivery and tissue engineering, in which drugs may be encapsulated in carriers with specific shapes and morphologies to enhance efficiency or biocompatibility. 4.1.2. Sheared wet spinning: XanoSheart. Xanofi’s technology, XanoSheart, combines shearing with existing wet spinning.73,74 This method is able to control the aspect ratio of the nanofibres, with an average diameter of 500 nm. XanoSheart is superior in terms of yield, energy efficiency and simplicity of nanofibre spinning compared with nanofibre nozzle spinning methods which rely on spinneret designs, the number of This journal is

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spinnerets and low flow rates. The new technology, since its initial creation in 2008, leads to the spin-off company Xanofi, and the development of a pilot machine able to achieve continuous spinning of nanofibres with minimum production rates above 3.6 kg h1.73,74,178


4.2.

Dry spinning

As above, dry spinning differs from wet spinning in that the fibres solidify under a stream of hot air or inert gas and the solvent is removed by heating. Compared to wet spinning, dry spinning is faster as fibre solidification is via evaporation instead of precipitation. Dry spinning tends to result in simple round or collapsed oval, bean, and irregular cross-sectional fibre morphologies.187 Subsequent stretching and drawing along the fibre axis improve molecular chain orientation and the tensile strength of the product. The method is used in large-scale production of polymer fibres from polymers that are not thermoplastic and can be dissolved in industrially safety-approved solvents. Examples of polymers commercially dry spun include Lycrat, cellulose acetate and polyvinyl chloride. Recently a steady dry spinning process using a modified ‘‘spinneret based tuneable engineering parameter (STEP)’’ technique reported production of patterned nanofibrous mats with 50 nm fibre diameters (Fig. 7).137 4.2.1. Solution electrospinning. Conventional solution electrospinning, the most common form of electrospinning, produces ultrafine fibres from a polymer solution supplied via a single-nozzle spinneret under an applied electric field. Conventional electrospun fibres solidify by solvent evaporation as they do in dry spinning. Spinning temperature, humidity and pressure may be controlled in a conventional electrospinning setup to study and control the fibre morphology obtained under various external-spinning environments. As volatile solvents are often preferred in the process and the fibres obtained at the end of the spinning process are often in the sub-micrometre or nanometre range, the as-spun fibres are most commonly left to solidify via simple evaporation under ambient conditions. Similar to dry spinning, conventional electrospun nanofibres from solutions can produce simple fibre cross-sectional shapes such as round, ribbon and irregular morphologies.188 Buckling instability131 and bending instability122,134 during electrospinning limit the deposition precision. Improvements in the control of the deposition area and fibre alignment in electrospinning can be achieved by manipulating the electric field,104,134,189 such as in magnetic electrospinning,167 nearfield electrospinning69 and the case of ‘‘nanopottery’’190

illustrated in the following sub-sections. The speed of fabrication and potential clogging issues when using fine nozzles in many laboratory solution electrospinning processes including the modified techniques highlighted here may be difficult to upscale for mass-production. They are, however, significant in diversifying electrospinning technology to enable improved control and precision in patterning and fabrication of nanostructures, which like carefully handcrafted porcelain, may be slower and dearer, but are nevertheless important and in demand when high precision, functionalisation and sensitivity are called upon, such as in the field of electronics, optical technologies and biomedical engineering. 4.2.1.1. Magnetic electrospinning. Given the well known relationship between magnetism and electric potential, it is not surprising that a process combining the two, named magnetic electrospinning, has been explored.167,191 The setup is essentially the same as the conventional configuration except for the use of two magnets positioned at the sides of an aluminium foil collector.167 The process is able to align and pattern electrospun fibres. However, due to the like-charge repulsion, a 2 minute collection results in a fibre mat with a large inter-fibre distance of 30 mm. When the collection time was increased from 2 minutes to 5 minutes, the inter-fibre distance decreased from 30 mm to approximately 5 mm.167 Nevertheless, the inter-fibre distance in this case is large with respect to the nanofibre diameter. This phenomenon can be significantly reduced by focusing the electric field and collecting the nanofibres on sharp edges.104 4.2.1.2. Near-field electrospinning: bridging nanolithography and electrospinning. Near-field electrospinning,69 combining dip-pen nanolithography192,193 (DPN) with traditional electrospinning, is a good example of interdisciplinary technological convergence, which led to the creation of a nanofibre precisionprinting technique.69,194 ‘‘Dip-pen’’ refers to the ancient wellknown method to transport ink in a sharp object to a paper substrate via capillary action, the theory of which has been widely employed to transport molecules onto macroscale dimensions.192 Combining DPN with electrospinning, direct patterning from multiple nano-materials onto nano-scale structures can be achieved under ambient conditions using affordable laboratory equipment.194 Individual nanofibres are spun with the diameter gradually decreasing over the length of the fibre as the volume of the droplet reduces at the dip-pen nozzle. A nanofibre of less than 100 nm diameter is readily obtainable because of the fine

Fig. 7 Dry spun patterned nanofibres. Reproduced with permission from ref. 137. Copyright 2009, Wiley-VCH Publishers Inc.

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diameter of the nozzle (25 mm). However, this raises potential clogging issues. The nanofibre diameter distribution range in near-field electrospinning is narrower than that of a conventional process. The applied voltage required to achieve an electrical field strength to initiate spinning is much lower (B200 V) due to the significantly reduced distance between the nozzle and the substrate (between 500 mm and 3 mm) as well as the fine nozzle size.69,72 Although near-field electrospinning can produce nanofibre patterns with a good degree of precision, depending on the motion of the substrate (e.g. speed and direction of motion) and the collection distance of the process, buckling and bending instability can still be observed. The spinning process is discontinuous, though nanofibres obtained have high aspect ratios. The printing technique may be useful for localised deposition of functionalised nanofibres to achieve tailor-made structural modifications and increased sensitivity of the material for applications such as biosensors. 4.2.1.3. Nanopottery. Self-assembled structures such as in situ formation of tubular shapes can be obtained using electrospinning by taking advantage of the bending instability and manipulating the electric field to control the area of deposition. For instance, by focusing the electrostatic field on a grounded sharp tip (pin collector), long cylindrical coils of electrospun fibres in the form of a hollow tubular structure can be self-assembled in less than a second on a conductive collector, due to the electrically driven regular wave-forms of rapid bending perturbations in the spin-line.190 The tube has an approximate radius of 3 mm and a height of 40 mm (Fig. 8). The radius of the hollow tubular yarn R and the angular frequency O of the coiling were mathematically determined and empirically verified using a simple scaling law based on the electrical field strength E, the surface charge density qs, Young’s modulus Y of the nanofibre, the relative permittivity of free space e0, and the velocity U and radius r of the spinning jet. U and r have a range of measurements depending on the initial solution concentration, rate of solvent evaporation and the applied electrical field strength E. The radius of the tubular structure is approximated to: Rr

Y e0 E 2

1=3 ð3Þ

And following mass conservation, the angular frequency of coiling O is given by: O¼

4.3.

U U Y 1=3 R r e0 E 2

ð4Þ

Flash spinning and manipulating solvent quality

Flash spinning involves extruding a polymer solution under high pressure. The increased pressure enables dissolution of the polymer in a liquid that is a non-solvent of the polymer under atmospheric conditions.195,196 The selection of solvents is fundamental in solution spinning. The spinnability of a polymer solution is related to the viscoelasticity of the spinning dope, particularly the ability of the polymer solution to be spun without breaking.117,197 To achieve this, a polymer solution must have a critical minimum concentration ce to allow sufficient molecular chain entanglement.198,199 Insufficient chain entanglement leads to the formation of droplets and ‘bead-on-string’ fibre morphology. However, the as-spun fibre diameter increases with increasing polymer concentration in the solution. To achieve fibre diameters in the lower nanometre range, a lower ce is preferred. The value of ce and the degree of chain entanglement in a polymer solution are strongly influenced by the polymer molecular chain length, the degree of branching and the conformation of the polymer chain in solutions. This is closely associated with the intermolecular interactions between the polymer chain and the solvent molecules. The degree of this polymer–solvent intermolecular interaction defines the concept of solvent quality in general polymer solution rheology. Many reports indicated that a ‘‘good’’ solvent defined by general solution rheology for a polymer may not be a ‘‘good’’ solvent for producing the finest possible electrospun nanofibres of a polymer of interest.143,145,200,201 Poor and partial solvents have been discussed for their potential to allow electrospinning of nanofibres at lower critical solution concentrations than their good solvent counterparts of a polymer of interest.143,150 The addition of ethanol, a poorer solvent than water, in aqueous semi-dilute PEO solutions, showed an elongational thickening effect and promoted intermolecular interactions between PEO coils.145,150,202 Manipulations of solvent solubility not only influence the electrospinnability, viscoelasticity and the critical minimum concentration of

Fig. 8 A self-assembled electrospun hollow coiled structure built on the apex of a stainless steel conical tip. Reproduced with permission from ref. 190. Copyright 2010, American Chemical Society.

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the solution in electrospinning, but also the aspect ratio and morphology of the as-spun fibres.144,150 Changing solvent compositions of the polymer solution also showed correlation with the degree of molecular alignment of electrospun poly lactic acid (PLA) fibres. The tensile strengths of PLA fibres electrospun from binary solvent systems respectively mixing chloroform – tetrahydrofuran and chloroform – N,Ndimethylformamide demonstrated differences in crystallinity.65,203 Furthermore, preferential solvent–polymer interactions alone can be used to produce core–sheath fibres.98 Mixing poly(methyl methacrylate) (PMMA) and polyacrylonitrile (PAN) in dimethylformamide (DMF) resulted in a metastable solution with a preferential dissolution of PAN to PMMA in DMF. The resulting emulsion produced PMMA-core/PAN-sheath nanofibres when directly electrospun in a single nozzle.98 Other polymer emulsion blends able to produce core–sheath nanofibres directly from single nozzle electrospinning may be developed via differential solubility of a solvent system dissolving multiple polymers. Graphical solubility maps204 or the spinnability–solubility map recently introduced150 are useful for making such emulsions. Solvent mapping allows the overlapping of the solubility of different polymers onto one diagram and enable prediction of selective solvent solubility for a polymer blend or the mixing of different solvents at topographically calculated proportions to achieve a desirable selective solubility and electrospinnability for polymers of interest. 4.4.

Spinning high-tenacity high-modulus fibres

Fibres produced during the pre-industrial time, such as cotton, wool and silk, typically had tenacities in the range of 0.1–0.4 GPa and initial moduli of 2–5 GPa.119 Today, fibres with tenacity above 3 GPa and modulus above 50 GPa are considered as high-tenacity high-modulus fibres, which find technical uses in tyres, composites, aircrafts, ballistic applications, sports equipment and circuit boards.119 4.4.1. Gel spinning, liquid crystal spinning and electrospinning. Liquid crystals are viscoelastic materials possessing long-range orientational order.205 Fibres can be spun from two types of liquid crystal polymers: lyotropic liquid crystal polymers and thermotropic liquid crystal polymers. The former exhibits liquid crystallinity in solution form, such as poly(p-phenylene terephthalamide) (PPTA, also known as para-aramid or Kevlars). The liquid crystal formation is affected by the concentration, type of solvent and temperature. On the other hand, thermotropic liquid crystal polymer shows liquid crystalline properties when the material is heated to above its glass transition temperature, such as polyalirate (or Vectras). The Vectras variety with the apparent viscosity (3.358 Pa s at 340 1C and apparent shear rate = 1.918 104 s1) is the only known thermoplastic and thermotropic liquid crystal polymer commercially melt-spinnable into high-performance fibres with high strength and stiffness.206 In 1962, DuPont patented the production of dry-jet-wetspun high-performance para-aramid fibres, famously known today as ‘‘Kevlars’’ – a man-made fibre less dense yet tougher and more extensible than steel.207–209 This marked the beginning of commercial fibre production via liquid crystal spinning This journal is

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of high-tenacity high-modulus fibres.210 In the early work of electrospinning in the 1990s, Srinivasan and Reneker90 electrospun 2–3%w/w isotropic solutions of Kevlars 49 dissolved in 95–98% sulfuric acid. The fibre diameter ranged from 40 nm to sub-micrometre scale. The thermally resistant birefringent nanofibres showed good molecular orientation. Polymer gels are highly saturated polymer solutions with high solid contents. Examples include varieties of ultra-highmolecular-weight polyethylene (UHMWPE), such as Dyneemas and Spectras. Since the late 1970s, a new generation of gel spun high performance fibres with high modulus and high tenacity (HMHT fibres) has been created. HMHT fibres demonstrate a minimum tenacity of 3 to 6 GPa, and a modulus of between 50 and 600 GPa.119,211 With lower density but higher strength and stiffness of steel, HMHT fibres became the new material of choice in applications for military defence, automobiles, protective clothing, reinforcement composites, and aerospace and marine engineering. Both liquid crystal spinning (dry-jet wet spinning) and gel spinning (dry wet spinning) are still solution spinneretextrusion wet spinning processes, except that the state of the solutions extruded is between those of a conventional fluid solution and those of a solid. Polymers in gel spinning (e.g. UHMWPE) have long flexible molecular chains. The properties of gel spun fibres depend on the polymer molecular chain length and the degree of chain overlap, which in turn significantly increase the number of the weak intermolecular van der Waals forces between the overlapping chains, resulting in a strong cumulative inter-molecular strength.119 On the other hand, materials in liquid crystal spinning (e.g. paraaramid) exhibit rigid rod-like molecular structures; and liquid crystal spun fibres owe their high strength to stacking interactions between adjacent molecular strands and strong intermolecular interactions such as hydrogen bonding.119 Compared with conventional wet spinning, a fibre-forming material with such a high molecular orientational order produces fibres with an outstandingly high tenacity and modulus. The interference of molecular chain entanglements during drawing is minimised in liquid crystal and gel spinning and high drawing ratios and high molecular chain alignment are possible, producing fibres with high crytallinity and good tensile strength.212 Recently, composite fibres of thermotropic liquid crystals have been electrospun via coaxial electrospinning producing new functional fibres and providing new opportunities to study the impact of extreme confinement on liquid crystal phases.91,92 It should be noted that current industrial processing of high tenacity high modulus fibres often involves harsh processing conditions and environment. For example, the production of para-aramid fibres can release dust and fibre particulates, which can form explosives in the air and are irritants to the human eyes, nose, skin and respiratory system. Brief exposure to these air-born by-products causes coughing, sneezing, or mild irritation with redness or itching on the skin; and prolonged exposure at high concentrations can lead to lung damage.213 4.4.2. Spider dragline silk liquid crystal spinning. Many biological structures exhibit liquid crystalline behaviour. In fact, Chem. Soc. Rev., 2012, 41, 4708–4735

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studies on liquid crystals were initiated by physiologist Reinitzer during his studies on the physico-chemical properties of cholesterol-derivatives.214 Spider silk is an example of a lyotropic liquid crystalline protein. Spinning of animal silk from insects such as spiders and silkworms is nature’s way of fibre spinning – an important source of inspiration for sustainable biomaterials processing to produce multifunctional fibres with increased versatility and durability. Silkworm cocoon silk and spider silk are the two most studied types of silk. The former is a well-known deluxe textile material harvested from domesticated mulberry silkworm Bombyx mori since five thousand years ago in ancient China and the latter has been used for wound dressing in ancient Greece.209,215 Spider silk, in particular, dragline silk from Nephila clavipes, possesses marvellous properties such as low density, high tenacity and high modulus. Dragline silk is stronger than most man-made fibres, and comparable to the commercial state-of-the-art para-aramid fibres used in bullet-proofing or aircraft reinforcements.209,215,216 As mentioned in the previous section, artificial productions of high tenacity and high modulus functional fibres such as liquid crystal spinning of para-aramid involve harsh processing conditions and human and environmentally unfriendly procedures. In contrast, natural silk proteins can be stored in spiders, silkworms and other insects under physiological conditions and spun into mechanically robust and stable fibres at high concentrations in aqueous solutions or liquid crystal forms under close to ambient conditions.208,216,217 In terms of the stages of production from dope synthesis to optimisation of polymer rheology to fibre extrusion, artificial fibre spinning technologies are similar to that in spiders. However, nature combines biochemistry with mechanical engineering in spider silk spinning. In addition to external mechanical drawing upon spinneret extrusion and internal shearing and extensional flow, the biochemical and solvent environment prior to extrusion together with carefully designed molecular synthesis of the silk proteins allow precise folding and crystallisation of the material, and on-demand induction of liquid crystalline protein aggregation in aqueous solutions for mechanical drawing of highly oriented macromolecules.209,216,218 The highly conserved non-repetitive C-terminal domain of 100–140 amino acids in the molecular structure of spider silk proteins is a principal feature in spider silk fabrication. The non-repetitive domain consists of more hydrophobic amino acids compared with the repetitive backbone of the silk protein. Changes in the molecular behaviour of the non-repetitive domain in response to changes in the internal biochemical environment such as water percentage, pH and ionic strength, including concentrations of chaotropic ions (e.g. sodium and chloride) and kosmotropic ions (e.g. potassium and phosphate), regulate the solubility, thermo-responsiveness, pH sensitivity, as well as the supramolecular self-assembly of the macromolecule. This allows reversible tuning of amphiphilicity and molecular assembly, enabling a stimuli-response process of controlled properties such as crystallinity and strength.215,217–221 Hagn et al.217 analysed the amino acid sequence and molecular structure of the non-repetitive domain of spider dragline protein Araneus diadematus fibroin 3 (NR3) and established that the two salt bridges formed by the only two pairs of charged residues arginine 43, arginine 52, aspartic acid 93 4720

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Fig. 9 Natural spinning of spider dragline silk. Reproduced with permission from ref. 209. Copyright 2009, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

and glutamic acid 101, located in the most conserved region of NR3, are responsible for the structural integrity of the nonrepetitive domain. The salt bridges maintain the structure of the domain in a folded globular arrangement, exhibiting hydrophilic amino acids on the surface while concealing the hydrophobic residues for the storage of the proteins in the aqueous state. A decrease in the pH and the ionic strength of the aqueous environment disrupts the salt bridges and reveals the hydrophobic residues, which together with the intermolecular disulfide bridge acts as anchors for the correct alignment of the repetitive sequence, leading to protein aggregation and shear-induced formation of oriented b-sheets for spinneret extrusion (Fig. 9).209,217,219,220 Although much emphasis has been placed on silk proteins and the bio-inspired and biomimetic production of spider dragline silk, so far no study in the field has achieved fibres with tensile performance comparable to that of natural spider dragline silk.222 A better understanding of the synthesis of silk proteins and careful manipulation of the biomimetic extrusion-spinning process will enable further advance in biomimetic synthesis and processing of high quality fibres.

5. Emulsion spinning and electrospinning Using an emulsion inter-polymerisation technique invented in the 1940s, emulsion spinning was first patented by DuPont in 1956 and further improved over the next decade to produce shaped, composite or matrix fibres composed of a mixture of two or more fibre forming components, with the major component being a desirable but intractable material originally poorly spinnable due to insolubility, or chemical or thermo-inertness.223–227 To improve the stability of a polymer emulsion, which is otherwise non-uniform and may precipitate on standing, producing non-uniform, discontinuous, poorly shaped and weak fibres of low practical and economical utility, such as pure polyvinyl chloride (PVC) emulsions, the insoluble polymer is dispersed in an aqueous solution of unsaturated polymerisable organic compound, such as polyvinyl alcohol (PVOH), in the presence of catalysts and emulsifiers. The emulsion is inter-polymerised to at least partly join the hydrophobic PVC polymer with the hydrophilic PVOH polymer.223,225 Homogeneous co-existence of hydrophobic and hydrophilic components in a fibre is a noteworthy feature of the This journal is

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emulsion spinning process.226 The resultant mixture is extruded into a coagulation bath consisting of an aqueous solution of an inorganic salt or an organic solvent in wet emulsion spinning; or simply into air and solidifies via water evaporation in dry emulsion spinning.227 The polymer component forming dispersion medium (the matrix) can either be removed by combustion (if the main dispersed polymer is flame resistant) or by dissolution; or be insolubilised by chemical treatments such as esterification or acetalisation in the case of PVC–PVOH matrix fibres.227 Post-processing such as heating coalesces the emulsion component and dimensionally stabilises the fibre. Additional mechanical drawing improves the strength of the product.226 The method has been mainly used to produce the flame retardant PVC and PVOH matrix fibres such as Cordelans and flame resistant fibre blends of the emulsion spun polyvinyl fibres with other fabrics such as cotton and polyesters.226,227 In addition to excellent flame redundancy, if forced to burn, toxic emissions from burning PVC/PVOH matrix fibres are comparable with cotton and significantly lower than natural textile fabrics such as wool and silk.226 It should be noted that the presence of low molecular weight emulsifying agents in the dope may encourage non-uniformity in the fibre structure and adversely affect the mechanical properties, transparency and lustre of the fibre products.228 Emulsion electrospinning has been studied for the encapsulation of drugs and/or biological agents either in a mixture of drug emulsified in a polymer solution or as an emulsion core co-electrospun with a polymer solution functioning as the sheath of the nanofibre.229 The as-spun core–sheath fibres can encapsulate particulates or fibrous cores that are stimuliresponsive, for example responsiveness to pH changes or temperature changes in the fibre surroundings.19,31 Emulsions of blends of polymers resulted from preferential solvent solubility have been used to produce core–sheath nanofibres from single-nozzle electrospinning.98 This was discussed in Section 4.3 earlier. Drug encapsulated nanofibres and/or core–sheath nanofibres in the case of emulsion co-electrospinning markedly ease the initial burst release of the drug.18,22,27,97,98,230,231 An interesting aspect to explore may be to integrate the concept of the emulsion inter-polymerisation technique employed in conventional emulsion spinning in electrospinning. This can produce alternating hydrophobic–hydrophilic core-supplementary components along the length of the fibre for enhanced functionality. Further, electrospinning of emulsions has been demonstrated to facilitate electrospinning and modulate fibre properties such as reducing the as-spun fibre diameters in comparison with conventional electrospinning.232 By mixing phases of different rheological properties, molecular interactions and other cooperative phenomena in the evolution of the spinning jet are different to conventional spinning systems, allowing electrospinning to occur at a much lower liquid viscosity range than that for a conventional solution system.232

6. Melt spinning methods Nanospinning of melts has been a challenge due to the high viscosity of melts impeding ultra-fine fibre formation. This journal is

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Many processes and different types of apparatus have been used for melt-spun nanofibre production. Some of the stateof-the-art melt spinning methods producing micrometre or submicrometre-scale fibres include melt blowing, classical centrifugal spinning and film-splitting centrifugal spinning methods (discussed in Section 9 under centrifugal spinning methods), island–sea multicomponent conjugate spinning and melt electrospinning. 6.1.

Classical melt spinning

Polymer melt processing is the preferred method for manufacturing polymeric materials. Melt spinning is a coupled transport process involving an interplay between fluid mechanics and heat transfer.120 Conventional melt spinning is a spinneret-extrusion process similar to dry spinning, except that a polymer melt or a system of polymer melts is used as the spinning dope. The continuous fibres extruded are mechanically drawn at the wind-up to reduce fibre diameters. The filament is quenched and solidified by cooling, which is the fastest fibre solidifying process involving one-way heat transfer.116,118 One fundamental flaw of solution spinning methods is the use of solvents, which not only pose environmental concerns, but also limit the productivity and energy efficiency of the process. Not only is concentration of the polymer an issue limiting spinnability and polymer output, the fibre solidifying process involves mass transfer, which is slower compared to the simple heat transfer during fibre cooling in melt spinning. Furthermore, residual solvent is an issue for nanofibre applications in healthcare. The exclusion of solvents in melt spinning eliminates the associated solution concentration, solvent residue and solvent recovery concerns. Hence, melt spinning has a lower manufacturing cost and 10–500 fold higher productivity than its solution spinning counterparts.233 In addition, to obtain composite fibres using multi-component systems, many combinations of components have few common solvents. With such clear advantages of environmental friendliness, no solvent use, solidification via simple cooling, high productivity, melt spinning has always been the preferred method in manufacturing. 6.2.

Melt electrospinning

Melt electrospinning was first reported in the 1980s93,139,234 but has not been studied as extensively as solution electrospinning due to the more expensive setup needed to maintain elevated temperature of the melt and the limitation of the low conductivity and high viscosity of polymer melts in conventional melt electrospinning setups to achieve significant fibre diameter reduction by electrostatic forces.235 In addition, surface tension instability and cohesive fracture of the spinline are factors adversely influencing the quality of melt electrospun fibres, and thus researchers struggled to achieve melt electrospun fibres with uniform diameters below the submicrometre range for more than two decades.93,94,236–238 As with all polymer fibre spinning methods, material parameters such as polymer molecular weight and molecular chain conformation are important. Polymers of higher molecular weights and atactic nature result in melt electrospun fibres of larger fibre diameter due to the stronger chain entanglement, Chem. Soc. Rev., 2012, 41, 4708–4735

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Fig. 10 Melt electrospun fibres. (a) PCL 3D scaffold demonstrating precision deposition of 3D architectures. Reproduced with permission from ref. 239. Copyright 2011, Wiley-VCH Verlag GMBH & Co. KGAA. (b–e) Melt electrospun nanofibres of PMMA sheath encapsulating TPC materials in the core. Adapted with permission from ref. 113. Copyright 2009, John Wiley & Sons, Inc. (b and c) Scanning electron micrographs, (d) transmission electron micrograph showing clear core–shell nanostructure and (e) fibres demonstrating fluorescence.

higher viscosity and poor crystallisation of the melt.94 Moreover, very low flow rates are desirable94 but the highly viscous nature of the melts makes nozzle blockage an issue at low flow rates. Nevertheless, melt electrospun PCL 3D scaffolds have been produced demonstrating excellent precision deposition of 3D architecture (Fig. 10a).239 Other recent advances in melt electrospinning have achieved fibres of nano-scale diameters with good consistency.96,112,113,206,239,240,241 Using concepts from melt blowing and non-woven technologies, addition of a viscosity-reducing agent (Irgatec CR 76 at 1.5%) allowed dramatic reduction of fibre diameters from 35.6 1.7 mm down to 0.84 0.19 mm.96 Coaxial melt electrospinning (or melt co-electrospinning) is particularly advantageous when materials with different melting points such as core–sheath nanofibres need to be incorporated for phase change applications. Nanofibres with a titanium dioxide–polyvinylpyrrolidone composite sheath encapsulating hexadecane and ocadecane cores with average diameters of 100 nm and 150 nm, respectively, have been demonstrated.112 Moreover, core–sheath nanofibres with thermochromic phase-change (TPC) materials (such as a mixture of crystal violet lactone–bisphenol A–1-tetradecanol, a typical TPC system comprising of a dye, a developer and a fatty alcohol113) encapsulated in an optically transparent PMMA shell via coaxial melt electrospinning have the advantage of fixing the fluid nature of the phase-change materials after melting, while allowing good body-temperature thermochromic fluorescent observations (Fig. 10b–e).113 6.3.

Spinning molecularly self-assembling materials

The high viscosity of polymer melts poses a constraint for melt spinning of nanofibres. Recently reported molecularly selfassembling materials (MSAs) offer a potential solution to this issue and have been successfully electrospun or meltblown into nanofibres.85–87 MSAs are oligomeric polymers, which when subjected to a triggering event (such as cooling, shearinduced crystallisation, or contact with a nucleating agent), 4722

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spontaneously form larger associated oligomers through intermolecular interactions of their chemical functional groups, such as hydrogen bonding, electrostatic interactions and van der Waals forces.85 MSAs within the range of 2000–50 000 g mol1 demonstrate desirable fibre-forming properties and mechanical strength similar to higher molecular weight polymers and a comparatively lower viscosity desirable for spinning finer fibres.85 Nanofibres in the approximate range of 30–1000 nm diameters were obtained using a melt electrospinning process.86 Claasen et al.85 obtained meltblown fibres with 95% of the fibre sample having a diameter of less than approximately 3 mm. Moreover, MSAs can have higher solution concentration while showing lower viscosity compared to high molecular weight polymers. Hence, more concentrated solutions of MSAs can be electrospun, allowing increased output of the electrospinning process.87

7. Multi-component conjugate spinning methods 7.1.

Industrial multi-component conjugate spinning

Multicomponent conjugate melt spinning conventionally spins a multicomponent system from a molten state to form a conjugate fibre, which is then converted into a bundle of fine filaments by mechanical splitting (via hydroentanglement, carding, twisting, drawing, etc.) as in the segmented pie method, or dissolving the sacrificial ‘‘sea’’ component as in the island–sea method.242 The latter employs a strategy more widely used in academic and commercial production of nanofibres and is discussed here in further detail.243 Current processing of the island–sea method can include post-spin operations such as pre-heating the filament and drawing at elevated temperatures to orientate and crystallise the composite fibres.179,244 The name island–sea describes the structure of the as-spun fibrous bundle in the lateral direction. Seckel patented the process to produce longitudinally separable conjugated multicomponent filaments as extruded thermoplastics in 1950.245 This journal is

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In island–sea conjugate spinning, two fibre-forming polymers in the molten state constitute the island and sea components, the latter of which is removed during postspinning processing, leaving the island polymers as a bundle of fine fibres. The polymer forming the island component should ideally have significantly lower solubility than the polymer forming the sea component (200 times lower or more), to ensure the surface island polymer in the fibre bundle staying intact while the sea polymer in the centre of the fibre bundle is being removed.163,242 Streams of molten polymeric island components and one stream of polymeric sea component having a higher melt viscosity are, respectively, extruded which then enter a spinneret setup that allows the alternative arrangement of the island and the sea streams in a side-by-side or core– sheath manner. The melt viscosity of the polymeric sea component is 1.1 to 2.0 times higher than that of the polymer forming the island component to ensure the individuality of the island fibrils, as well as the stability of the spinning process.163 The take up speed of the spinning filament is 400–6000 m min1.163 The number of island fibrils in each bundle should be at least 100 and less than 1000. When the number of spinneret holes is less than 100, productivity of the process is low and fibrils are not of desirable fineness. If the spinneret has more than 1000 holes, not only is the cost of making the spinneret high, but also the working precision of the spinneret is compromised. In addition, a diameter range of 100–700 nm for each extrusion hole of the fibril is preferred. Diameters less than 10 nm prevent formation of a stable fibre structure, and at more than 1 mm, the softness and hand feel typical of ultrafine fibres cannot be achieved.163 Island–sea conjugate spinning does not form non-homogeneous fibres.246 In addition, the removal and wastage of sea component is one disadvantage of the process. The current proportion of island to sea components is 1 : 1 or more, up to 90% of the island component is possible. However, it is hard to decrease the mass proportion of the sea component to incorporate more island fibres without increasing the size of the sea component or the total area of extrusion of the spinneret orifice to prevent the reversion of the island–sea relationship and the island components becoming too close together and forming a sea.247 It is a challenge to have the intervals between each island fibre less than 500 nm.163 A series of recent efforts have been made to increase the mass proportion of the island parts to the sea parts, and the intervals between each island fibre was reduced to less than 500 nm.163,248,249 7.2.

Multicomponent co-electrospinning

Coaxial or multicomponent electrospinning (co-electrospinning) refers to the simultaneous spinning of two or more liquids, extruded through separate dies arranged in either a side-by-side or concentric fashion. The liquids co-spun can be solutions, emulsions, melts or a combination of the three.99,229,250,251 Forming hollow tubular shapes via co-electrospinning employs a similar strategy to that of island–sea conjugate melt spinning, which sacrifices one of the components in the as-spun fibre bundle to produce core–sheath fibres and tubes. During this modified electrospinning process, the sacrificial component can be removed by heating or chemical vapor etching. This journal is

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Solution co-electrospinning of nanofibres allows non-fibreforming materials (for example, liquids with insufficient molecular chain entanglements) to be spun in fibre form as the core material within a core–sheath fibre structure. This is advantageous for encapsulating functional biological agents requiring a specific composition in the liquid medium which has low viscoelasticity and/or electrospinnability.110,111 In addition, melt co-electrospinning has been discussed in Section 6.2 on melt electrospinning; and emulsion co-electrospinning was discussed in Section 5.

8. Gas-assisted spinning methods Gas-assisted spinning technologies or ‘‘spin-blowing’’ methods have excellent productivity, and are especially useful for spinning nanofibres from poorly electrospinnable materials with low dielectric constant and/or electrical conductivity. Examples include the famous industrial melt blowing technology, as well as emerging methods such as electroblowing, gas-assisted electrospinning of solutions and melts, and solution blowing. These emerging methods are excellent examples of the potential to improve the quality and performance of nanofibres by combining principles of conventional textile technologies, such as melt blowing with contemporary techniques. 8.1.

Industrial melt blowing

Aiming to produce ultra-fine fibres back in the 1950s, Wendt et al.252 first developed melt blowing technology in the Naval Research Laboratory in Washington, D. C., USA. It is a onestep process to obtain micro and nanofibres as a non-woven mat by disrupting a low viscosity polymer melt extruding through a spinneret with a high velocity gas/air stream.253–255 Melt blowing technology allows the elongation of the fibres from macro size to sub-micrometre size in 5 1011 s.256 Fibres are attenuated because of the high velocity air/gas stream. Meltblown webs are instantly spunbonded due to the hot quenching air and the short collecting distance between the die and the substrate (Fig. 11). Melt blowing has the highest yield and is the most industrialised technology for ultra-fine microfibre production. An ingenious method, the principles of melt blowing have inspired a number of other fibre-forming methods to couple spinning with airflow. These include electroblowing, gas-jet electrospinning and solution blowing discussed in the following sub-sections. Though meltblown nanofibres have been achieved and the technique is one of the most well known methods of nanospinning,246 meltblown non-wovens often have a broad range of diameters from nano-scale to above 20 mm.253,256,257 The most consistent lower diameter range of commercialised meltblown fibres is 2 mm.243 Limitations include inconsistent fibre characteristics, limited number of usable polymers, and difficulties in achieving fibre of diameters less than 500–800 nm. The most cost-effective melt blowing process should produce consistent ultrafine fibres with a specific diameter and aspect ratio range, using the least amount of air while keeping amortisation, maintenance, operating temperatures, and other factors constant.253 This depends heavily on spinneret design. To investigate the optimum efficiency of melt blowing dies, Chem. Soc. Rev., 2012, 41, 4708–4735

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Fig. 11 Melt blowing technology: (a) photograph of Oerlikon Neumag’s melt blowing plant in production. Reproduced with permission. Copyright 2011. Courtesy of Oerlikon Neumag;258 (b) schematic of the melt blowing process; (c) schematic design of a melt blowing die. (b and c) Adapted with permission from ref. 253. Copyright 1988, American Chemical Society.

Shambaugh253 presented a comprehensive study of the macroscopic energy balance of the technique to understand the optimum spinneret designs for economical production of ultrafine fibres with varying diameter distribution profiles. 8.2.

Solution blowing

In similar ways to melt blowing, driven by aerodynamic forces, solution blowing was recently studied to produce ultrafine fibres.63,64 Solution co-blown core–sheath PMMA–PAN nanofibres combined with post spinning carbonisation treatment resulted in mesoscopic carbon tubes with bores in the range of 50–150 nm and outer diameters of 400–600 nm.63 Furthermore, soy protein, an increasingly abundant by-product from SoyDiesel production, has been successfully blown in solution form into biodegradable nanofibres (Fig. 12).64 8.3.

Electroblowing/gas-assisted electrospinning

The idea to electrically charge meltblown webs was first developed by Moosmayer et al. in 1990.259 This concept was later incorporated in a technique termed ‘‘electroblowing’’, to process polymeric nanofibres from both solutions and melts by combining electrospinning and meltblowing.57,260 The process

Fig. 12 Solution blown nanofibres from a blend of nylon-6 and soy protein in formic acid: (a and b) a blend of nylon-6 and soy protein PRO-FAM 955; (c and d) a blend of nylon-6 and soy protein PRO-FAM 974. Reprinted with permission from ref. 64. Copyright 2011, American Chemical Society.

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extrudes electrically charged polymeric fluid through a spinneret, which is coupled to a gas stream forwarding in the same direction as the extruding spinline. Together, both high velocity air stream and the electrostatic forces act on the spinline and fine fibres are obtained.58,59 In particular, electroblowing extends the versatility of electrospinning and is capable of producing nanofibres with diameters below 100 nm from liquids with high viscosity and high surface tension such as aqueous hyaluronic acid, which cannot be satisfactorily electrospun conventionally.57 However, when the design of the setups requires charging the fluid prior to spinneret extrusion, many issues need to be addressed if electrostatic spinning methods are considered in an industrial environment. First, in upscaled production, the spinneret nozzle and the spinning components in contact with the nozzle and all associated upstream equipment supplying the spinning dope need to be maintained at high voltage in a conventional setup. This is not practical as the supply stream requires a motor and a gear box to drive the pump of the dope and a short circuit can occur if the motor is not electrically isolated from the pump, thus reducing the effective applied voltage potential to charge the spinning dope.60,61 In addition, the upstream equipment become large enough in a prototype spinning plant to ground the high voltage of the process unless they are physically separated, which is undesirable for the production time, cost and homogeneity of the process. Moreover, all charged equipment must be insulated and not have sharp edges, which will otherwise cause discharge.60,61 The high voltage applied to the spinning nozzle is about 1 to 300 kV.58 The routine maintenance of highly charged equipment during a continuous manufacturing process is a safety hazard to relevant personnel.61 Improvements addressing some of these issues of electroblowing related to high voltage processing of charged liquid spinning from the commercial perspective were published in a series of patents by DuPont.60–62 In this improved version, the electrically conductive polymeric fluid is uncharged as it exits the spinneret, which then becomes charged as it passes though an ion flow formed by corona discharge. As the liquid stream becomes charged, electrostatic forces stretch the spinning jet and fine fibres are obtained and deposited on a grounded moving belt. The moving belt is made of porous material which allows a vacuum to be drawn from beneath the collecting surface of the belt through a vacuum chamber connected to the inlet of a blower.60–62 This journal is

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Zhmayev et al.261,262 reduced the average diameter of melt spun fibres from 3.5 mm obtained in conventional melt electrospinning to 180 nm using gas-assisted melt electrospinning, a design combining melt blowing with electrospinning similar to the electroblowing process. The additional high velocity airflow resulted in an additional 10% thinning compared to conventional melt electrospinning. In addition, using an airflow of higher temperature led to a further twenty fold thinning of the jet.262 Further, spinneret-free laser melt electrospinning has been developed using modified laser setups to uniformly melt an end of a polymer rod206,263 or a length of a polymer sheet,264 where cone-jets self-assemble at the liquidised region. This is discussed in further detail in Section 11.2 (spinneret-free fibre production). Yao et al.55,56 coupled a nitrogen jet to solution electrospinning, and the average diameter of the as-spun polysulfone nanofibres reduced from 350 nm with conventional electrospinning to 200 nm in the gas-jet electrospinning process. In addition, by coupling the electrospinning jet with an inert-gas nitrogen jet, a significantly higher applied voltage (27–50 kV) could be achieved than would usually be possible in conventional laboratory setups under an ambient temperature and humidity (up to B30 kV) before spinning instability and corona discharge take place. More systematic and detailed investigation of the influence of the temperature and the composition of the gas-jet on the rate of evaporation of the solvent in the spinline, the control of fibre deposition, the resultant fibre diameter, morphology, porosity and crystallinity will be beneficial. For example, by increasing the temperature of air from 39 1C to 57 1C, Um et al.57 reported improved spinnability and reproducibility of the electroblowing process, producing relatively uniform hyaluronic acid nanofibres at 57 1C. Air temperature also influenced the average fibre diameter, which increased from 49 to 74 nm when temperature was increased by 10 1C from 47 1C to 57 1C.57

9. Centrifugal spinning methods Hooper patented the centrifugal spinneret in 1924.265 Classical centrifugal spinning involves supplying centripetal force using a rotary distribution disc with side nozzle holes. The rotation shears the spinning dope, classically thermoplastic materials, for instance, molten mineral or glass to form fibres such as mineral wool.233,266–268 Recent advances in centrifugal nanospinning

have demonstrated the potential of this technique for nanofibre production. The initial work done merging electrospinning with centrifugal concepts indicated a competitive edge compared with the technique alone in terms of its ability to produce homogeneous nanofibres of below 100 nm in size and a simultaneous one-step post spin-draw possibility to enhance molecular alignment and fibre crystallinity for improved tensile strength of the resultant fibre. 9.1.

Centrifugal electrospinning

Centrifugal electrospinning has been attempted and the setup typically comprises of rotary spinnerets similar to centrifugal spinning. In 2006, centrifugal electrospinning using a rotary spinneret was reported by Dosunmu et al.66 using a porous ceramic tube spinneret (Fig. 13) and by Andrady et al.269 using a rotatable spray head with four individual extrusion elements, which in turn can be made of bundles of multiple nozzles. Reiter Oberflachentechnik GmbH developed the HyperBell centrifugal electrospinning technology, which was subsequently acquired by Dienes Apparatebau GmbH.172 A centrifugalelectrospinning unit with three spin heads currently supplied by Dienes Apparatebau is able to increase the throughput of conventional nozzle electrospinning by a thousand fold with a minimum achievable nanofibre diameter of 80 nm.171 Coupling centripetal force with electrostatic force, highly aligned PLA electrospun fibres with improved modulus of 3.3 GPa (PLA in chloroform and tetrahydrofuran) were produced.65 9.2.

Centrifugal nanospinning

A spinning method using only centripetal force to produce nanofibres was recently developed.67,68,270 Forcespinningt by FibeRio Technology Corporation achieved a minimum as-spun fibre diameter of 45 nm. The first PEO nanofibres obtained by Forcespinningt demonstrated homogeneity with an average diameter of 300 105 nm (Fig. 14).67,68,270 Furthermore, aligned PLA nanofibrous scaffolds were prepared using a similar method by Badrossamay et al.68 under the name ‘‘Rotary-Jet Spinning’’ and were used to seed cardiomyocytes in mice. The result showed good tolerance of the nanofibres and the cell seedings successfully developed into pulsating multicellular tissues. The temperature, the rotational speed of the spinneret and the collection distance are parameters influencing the geometry and morphology of centrifugally spun nanofibres.

Fig. 13 Centrifugal spinning with porous ceramic tube spinneret. Reproduced with permission from ref. 66. Copyright 2006, Institute of Physics Publishing Ltd.

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Fig. 14 Centrifugal nanospinning. (a–k) Rotary jet-spinning process: (a and b) schematic illustrations, (c and d) 3D PLA nanofibrous structure, (e) PLA fibres produced with expedited solvent evaporation and >55% relative humidity, fibres spun from (f) aqueous PEO, (g and h) aqueous PAA, (i) gelatin in 20% (v/v) acetic acid, (j) encapsulated fluorescent polystyrene beads (0.2 mm diameter), (k) emulsion-spun gelatin in PLA. (a–k) reproduced with permission from ref. 68. Copyright 2010, American Chemical Society. (l–o) Forcespinningt melts and solutions: (l) PEO nanofibres from solution centrifugal spinning; (m) polystyrene melt spun mat produced by the Forcespinningt; (n) nanofibre web spinning in action and (o) as-spun free-standing nonwoven mat. (l–o) Adapted with permission from ref. 67. Copyright 2010, Elsevier Ltd.

Centrifugal nanospinning is as versatile if not more so than electrospinning with respect to the broad range of the materials that can be processed, including melts, solutions and emulsions (Fig. 14).67,68,270 The process offers higher productivity and simplicity in an equipment setup without the complication of high voltage in upscaled processing and the material constraint on electrical conductivity or relative 4726

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permittivity compared to electrospinning. Nevertheless, centrifugal nanospinning is an extrusion process limited by challenges relating to the material properties and the designs of the spinneret, which can lead to large differences in fibre quality and productivity. The fibre diameter in a single PLA sample can vary from 50 nm to 3.5 mm.68 The degree of complexity in the spinneret design is also proportional to the cost incurred. This journal is

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Although centrifugal nanospinning is a facile technique to generate 3D scaffolds with a moderately high degree of uniaxial alignment, formation of more complex 3D nanostructures with functionalised nano features or alignment of fibres in more than one direction may be difficult and has yet been demonstrated with this technique to date.


9.3.

Centrifugal film splitting and spin-coating

Lower viscosity melts of thermoplastic polymers such as polyethylene polymers and copolymers, polyesters such as thermotropic liquid crystal polymers and polyethylene terephthalate (PET) copolyesters can be spun into fibres via film splitting-based classical centrifugal spinning reported in a recent patent by DuPont.233 A thin melt film with a thickness in the low micrometre range is fully spread on the inner surface of a rotating distribution disc. Film splitting occurs at the forward discharge edge of the distribution disc to form nanofibres. A stationary or shear disc is placed downstream of the rotating distribution disc, and the polymer melt is issued through a gap between the rotating distribution disc and the shear disc, where the shear applied to the polymer melt causes shear thinning. The shear disc also acts as a melt distribution disc, helping to form a more uniform, fully spread, thin melt film. The desirable range of viscosity of the spinning melt is between 1000 and 50 000 mPa s, with a maximum acceptable limit of approximately 105 mPa s. Viscosity can be lowered by plasticising, hydrolysing or cracking.233 Concurrently, Weitz et al.271 reported solution nanospinning using a setup in principle similar to DuPont’s patent for nanofibre production by direct centrifugal film splitting of melts. Drops of polymeric solutions of high molecular weight PMMA were placed in the middle of the chuck of the spincoater and rotated at a minimum rotation speed of 3000 rpm. Nanofibres with diameters of 25 nm to 5 mm, and lengths of up to 0.5 mm formed at the edge of a spin-coater due to Rayleigh–Taylor instability at the air–liquid interface.271 Similar to conventional centrifugal solution spinning, the concept of critical minimum solution concentration and minimum rotation speed for fibre formation applies to the centrifugal spin-coating fibre forming process. The minimum rotation speed required for fibre spinning increased with decreasing viscosity of the polymer solution. It should be noted that defects such as beads were omnipresent under all solution-spinning

conditions reported on this spin-coating method. And contrary to the general knowledge that increasing solution viscosity decreases bead formation during spinning, the bead size increased with increasing solution viscosity (Fig. 15).271 As with any spin-coating and spinning methods, solution properties such as viscosity, solvent volatility, characteristics of the polymer and the geometry of the spin coater surface, as well as the balance between the centrifugal forces and the viscous forces are all factors influencing the occurrence of defects, and the morphology of the final product.272 Although the setup of the method is economical, depending on the rotational speed, it is possible to lose an excess of material during rotation.272 This could make the method unsuitable for processing expensive materials.

10.

Coupled or post-spinning processing

Post-spinning processing of the fibre such as drawing, stretching, and the simultaneous-coupled spin-draw processing have long been employed in the conventional textile industry to adjust the mechanical properties and the aesthetic appearance of the fibres.118,226 Among the extensive studies done on electrospinning, post-spinning processing of electrospun fibres via thermal, mechanical and chemical treatments is an important aspect. Core–sheath and multi-channel carbon tubes have been produced by post-spinning carbonisation of emulsionelectrospun core–sheath PMMA/PAN nanofibres.98,99 Electrospun micro/nanochannels can have coupled or post processing stages, and the sheath material can be carbonised, calcinated, or cooled rapidly.32,63,250 Combinations of these post-spinning processing techniques and coupling electrospinning to other technologies produce nanofibres with fine-tuned properties or novel structures. For example, coaxial electrospinning can be combined with sol–gel chemical processing to produce core– sheath nano-structures from inorganic metal oxides such as Ti2O nanofibres for photovoltaic and photocatalytic applications, among many other possibilities.273 High aspect ratio oxide nanofibres cannot be achieved via conventional solution spinning.273 Mechanical drawing has been demonstrated to produce nanofibres with dimensions comparable to single-wall carbon nanotubes. The process draws one nanofibre at a time and is capable of precisely positioning the nanofibre for manipulation

Fig. 15 Centrifugal film splitting and spin-coating: drops of polymeric solutions were placed in the middle of the chuck of the spin-coater and rotated at a minimum rotation speed of 3000 rpm. Resultant nanofibres formed at the edge of a spin-coater have diameters of 25 nm to 5 mm, and lengths of up to 0.5 mm. Reproduced with permission from ref. 271. Copyright 2008, American Chemical Society.

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with an AFM tip.274 Mechanical drawing was recently combined to electrospinning by Bisht et al.72 to induce electrospinning of ultrafine nanofibres of 20 nm diameters at a relatively low applied voltage of 200 V. It is well known in the spinning industry that molecular orientation is crucial for the final mechanical properties of the as-spun fibres. Improved mechanical strength of the as-spun fibres was often achieved via coupled spin-draw process in industrial spinning. Further, by coupling island–sea spinning to drawing, reduction in the fibre diameter and improvement in the uniformity of the island– sea melt spun fibres were achieved.179,244 The island–sea laserheated flow drawing and spinning process produced PET nanofibres of diameters below 50 nm and a modulus of 540 MPa – a strength comparable to microfibres obtained by conventional island–sea spinning.244 This demonstrates a noteworthy advantage of applying thermal drawing in postspinning processing to reduce fibre diameter and enhance fibre molecular alignment.

11. Multiple spinnerets and spinneret-free production 11.1.

Multiple spinnerets

Most spinning methods involve the use of spinnerets. The conventional concept to upscale fibre production, especially in the traditional textile industry, is by increasing the number of spinnerets in a spinning process, i.e. using multiple spinnerets. This has been investigated for upscaling electrospinning.34,157–160,162 However, because of the great reliance on spinnerets in extrusion spinning methods,253 there are a number of crucial disadvantages hindering large-scale production of nanofibres using multiple spinnerets. Very fine spinneret extrusion diameters often in the micrometre or even nanometre range are desirable to produce ultrafine fibres.235 Further, due to inter-jet perturbations, the distance between the extrusion holes of a spinneret needs careful calculation and optimisation.157,159,160 The distance between the extrusion holes affects the productivity and quality of the yield. The low feed rates from the spinnerets required to obtain continuous nano-scale fibre diameters also limit the throughput of spinneret-extrusion processes.

A recent experiment conducted by Varabhas et al.158 showed that a 130 mm long tube with 20 holes could only produce 0.3–0.5 g h1 of nanofibres with average diameters between 300 and 600 nm. Moreover, spinnerets capable of nanospinning can involve complicated designs and expensive manufacture, such as the spinnerets in conjugate island–sea spinning.247 Nevertheless, Finetex Inc. patented a multi-nozzle spinneret design in blocks which can produce electrospun nylon 6 nanofibres of uniform diameters within the 100 nanometre range at 6.5 kg h1.161,162 11.2.

Spinneret-free fibre production

It is possible to produce nanofibres without involving spinneretextrusion (Fig. 16).114 Electrospinning has a unique advantage in that it is conducive to jet initiation. Unlike most spinning methods which utilise some form of mechanical drawing to induce spinning at the spinnerets, the charges carried by the spinning material allows spontaneous jet initiation on a free liquid surface (Fig. 16a). This is known as needleless electrospinning (also known as nozzleless or spinneret-free electrospinning). As the spinning jets are self-assembled and the distance between each spinline naturally self-stabilises, the needleless electrospinning process affords hassle-free natural multiple-spinnerets spontaneously operating under self-regulated and self-assembled cone-jet conditions. Though the improvement on productivity using nozzleless spinning methods is still below 10 kg per day, these methods have attracted attention for their potential to by-pass cost and productivity issues related to spinneret nanofibre spinning. Miloh et al.115 presented a comprehensive study predicting the electrically driven instability causing the formation of protrusions when a viscous leaky dielectric layer was poured on a spherical metal surface and subjected to an initially radial electric field. Thoppey et al.153,275 explored ‘‘edge electrospinning’’ producing fibres at the circumference of a bow, and achieved a ten-fold increase in productivity compared to conventional spinneret electrospinning. Moreover, nanofibres have been prepared via spinneret-free laser melt electrospinning, which applies more even heating to the material compared to many melt nanospinning methods such as the current centrifugal techniques. Laser melt electrospinning feeds a polymer rod or sheet individually into a

Fig. 16 Spinneret-free electrospinning. (a) Schematic drawing of a spinneret-free free-surface solution electrospinning setup. Adapted with permission from ref. 114. Copyright 2004, Elsevier Ltd. (b–d) Laser melt electrospinning. Adapted with permission from ref. 264. Copyright 2010, John Wiley & Sons, Inc. (b) Self-assembled cone-jets along the molten line of the polymer sheet of 0.75 mm in thickness, fed to laser irradiation at a speed of 1.0 mm min1. Fibres were collected at a collection distance of 114 mm. The distance between cone-jets is dependent on the electrostatic repulsion between the cones, at a range below 10 mm; (c) line laser electrospinning; (d) spot laser electrospinning.

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modified laser setup, which controls the intensity of heating focused on an intended location of the material, and uniformly heats and melts the tip of the polymer rod in situ (spot laser melt electrospinning, Fig. 16d) or along the line of the polymer sheet (line laser melt electrospinning, Fig. 16c) and generates selfassembled cone-jets at the molten location (Fig. 16b–d).206,263,264 Other spinneret-free nanofibre spinning methods illustrated elsewhere in this review include modified film-splitting based centrifugal spinning271 and Xanosheart sheared wet-spinning.73,74,178


12.

Bio-fabrication and nanofibre forming alternatives

In addition to electrospinning and fibre-forming technologies already in industrial production lines, it is worth noting alternative nanofibre forming methods. Understanding the alternatives

will facilitate the development of new nanofibre fabrication technologies, potentially integrating the unique advantages of different methods. The alternatives include thermally induced phase separation, peptide nanofibre self-assembly and the natural bio-fabrication of cellulose nanofibres using microbes such as bacteria (Fig. 17). Thermally induced phase separation has the advantage of producing 3D nanofibrous scaffolds with good control over pore size and 3D shape when combined with other techniques such as porogen leaching and solid freeform fabrication, though a slower process with a lower degree of control over the alignment and aspect ratios of the fibres compared to the aforementioned nanospinning techniques.12,276 Biopolymers originally designed by nature such as polypeptides, polynucleic acids and polysaccharides (Fig. 1) have long been

Fig. 17 Nanofibres formed via representative techniques. (a–c) Bio-fabrication of bacterial nanocellulose. Adapted with permission from ref. 290. Copyright 2010, Springer New York LLC. (a) Random unaligned fibrous mat; (b and c) electrically induced alignment of nanofibrous mats under an electric field of 0.045 V mm1. (d–f) Self-assembled peptide-amphiphile nanofibres with varying lengths. Adapted with permission from ref. 284. Copyright 2002, National Academy of Sciences, USA. (g–i) Nanofibrous scaffolds and hollow microspheres produced via thermally induced phase separation. (g) Adapted with permission from ref. 291. Copyright 2000, John Wiley & Sons; (h and i) adapted with permission from ref. 292. Copyright 2011, Nature Publishing Group. (j–l) Electrospun nanofibres. (j) Adapted with permission from ref. 6. Copyright 2010, Springer New York LLC; (k) adapted with permission from ref. 26. Copyright 2004, American Chemical Society. (l) Adapted with permission from ref. 26. Copyright 2011, John Wiley & Sons, Inc.

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explored to produce fibres for their excellent functionality.51 Artificial fibres from biopolymers can be spun using spinning methods previously described; self-assembled from genetically modified biomaterials with specifically designed coding sequences;51,136,277 or harvested from microbial bioreactors.278–282 The bottom-up self-assembly method readily forms peptide amphiphile nanofibres with diameters below 10 nm, with considerable variation in the length and stiffness of the fibres.283,284 The method relies on the design and synthesis of peptides and proteins with sequences to allow self-assembled structures such as monolayers, tapes, belts, fibrils, tubes, vesicles and membranes.2,217,285 Bio-fabrication such as regenerated cellulose harvested from microbes is an attractive natural alternative to obtain polysaccharide nanofibres and large hydrogel 3D structures with competitive and stable mechanical properties. Over the last century, the choice of materials for technical textiles moved from natural fibres to regenerated cellulose to synthetic polymeric fibres made purely from petrochemicals today. For instance, the material used for automobile tyre reinforcements changed from cotton cords in the 1900s, to regenerated cellulosic rayon in the 1930s–1950s, to Nylon, polyester and metallic materials.119 Despite the broad range of tailorable properties, fibres made from petrochemicals are not environmentally sustainable. Cellulose is the most abundant biopolymer and its superior credentials as a sustainable raw material is undeniable.51 Cellulosic fibres were among the first synthetic fibres to be made. However, conventional cellulose processing methods have difficulty in obtaining nanofibres without using harsh solvents.286 Melts of cellulose and cellulose derivatives are prone to oxidation if spinning is carried out in air. Microbial cellulose nanofibres with diameters within the range of 20–100 nm can be easily isolated in high yield from extracellular cellulose generated by Gluconacetobacter strains such as Gluconacetobacter xylinus.278–280 By changing the Gluconacetobacter strain, the constituents, chemical reagents and additives of the culture medium,282 and the composition and shape of the substrate or the bioreactor, this natural alternative has the advantage of in situ direct regulation of the biosynthesis and control of the formation and mechanical properties of the cellulose and cellulose composite nanofibres.280,282 For example, composite cellulose nanofibres incorporating organic and inorganic materials such as bioactive agents, metals and metal oxides can be achieved by simple addition of the substance to the culture medium;280,287 bacterial nanocellulose with exceptional mechanical properties can be oriented to form tubular structures with uniaxially aligned fibrils by manipulating the cultivation template.288,289 Furthermore, Sano et al.290 showed the possibility to induce alignment of the bacterial nanocellulose by applying an electric field (Fig. 17a–c). Another desirable property of the bacterial cellulose is the versatility to manipulate its water content. Cellulose is traditionally regenerated from plant cellulose, which is associated with other substances such as lignin, pectin and hemicelluloses, and a water content of approximately 6%.281 Biofabricated bacterial cellulose nanofibres have similar molecular structure to plant cellulose, but a markedly higher purity, equal or more 4730

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than 90% water content, and a high degree of polymerisation (DP) and crystallinity with DP values of 2000–8000 and 60–90% crystallinity. The naturally high water content makes bacterial cellulose hydrogel a good candidate for soft tissue repair. In addition, it is possible to re-introduce up to 70% water content after gentle freeze-drying of the cellulose. And other solvents such as methanol, acetone and n-hexane can be exchanged with the water content of the cellulose without damaging the porosity and integrity of the structure. In this way, largely re-swellable aero-gels with dimensions comparable to the starting cellulose material can be obtained.280,281 Dried bacterial cellulose structures are mechanically stable nanofibre networks with reduced water uptake, which are desirable properties for membrane barriers and films in the technical textile industry.280 In addition, post-processing allows the potential of further development of bacterial cellulose nanocomposites where specific functionalisations are needed. For instance, titanium oxide nanotubes can be produced using the cellulose nanofibre structure as a precursor and template.280 Future efforts to integrate concepts of different nanofibre forming methods into novel processing techniques will be advantageous. For example, the characteristics of bacterial cellulose nanofibres may be enhanced by post-process functionalisation using near-field electrospinning to produce high quality biocompatible 3D nanofibrous scaffolds with controlled deposition of surface nanofibre composite patterns and localised functionality within the 3D structure. Furhtermore, the superamolecular amphiphilicity of spider silk proteins may be incorporated to molecularly engineered smart polymers or genetically modified biopolymers, which are capable of reversible tuning of amphiphilicity and molecular assembly, allowing stimuli-responsive processing and formation of nanofibres with adjustable properties such as crystallinity and strength.

13.

Conclusions

Improvements in the productivity of electrospinning and other nanofibre production methods are needed to further integrate and realise the considerable potential of nanofibres in practical applications. New advances in electrospinning and nanofibre production methods show a clear trend of interdisciplinary technological convergence, and principles of industrial fibre processing are being combined with nanofibre forming technologies. Many important parameters of electrospinning and nanofibre spinning methods overlap parameters extensively studied in industrial fibre processing. A comprehensive understanding of nanofibre forming methods in both academic and industrial environments is essential in addressing the issue of scale-up alongside novel developments and applications in nanofibre research. Future efforts to combine interdisciplinary concepts of different nanofibre forming methods into novel processing techniques will be advantageous. The governing parameters of a given nanofibre forming technique that affect the yield, cost, robustness of the process, safety of the operations, the degree of fibre alignment, and the post-spinning processing aspects should be further evaluated and recognised within a framework of an industrial setup. This journal is

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Acknowledgements CJ Luo et al. thank EPSRC and a UCL Knowledge Transfer & Enterprise Award for supporting their research.


References

35 36

1 D. H. Reneker and A. L. Yarin, Polymer, 2008, 49, 2387–2425. 2 J. G. Hardy, L. M. Romer and T. R. Scheibel, Polymer, 2008, 49, 4309–4327. 3 B. Dong, O. Arnoult, M. E. Smith and G. E. Wnek, Macromol. Rapid Commun., 2009, 30, 539–542. 4 K. Ohkawa, D. I. Cha, H. Kim, A. Nishida and H. Yamamoto, Macromol. Rapid Commun., 2004, 25, 1600–1605. 5 Z. M. Huang, Y. Z. Zhang, M. Kotaki and S. Ramakrishna, Compos. Sci. Technol., 2003, 63, 2223–2253. 6 S. Ramakrishna, R. Jose, P. S. Archana, A. S. Nair, R. Balamurugan, J. Venugopal and W. E. Teo, J. Mater. Sci., 2010, 45, 6283–6312. 7 M. Lombardi, P. Palmero, M. Sangermano and A. Varesano, Polym. Int., 2011, 60, 234–239. 8 S. Chuangchote, J. Jitputti, T. Sagawa and S. Yoshikawa, ACS Appl. Mater. Interfaces, 2009, 1, 1140–1143. 9 R. Benedix, F. Dehn, J. Quaas and M. Orgass, Lacer, 2000, 5, 157–168. 10 T. He, Z. Zhou, W. Xu, F. Ren, H. Ma and J. Wang, Polymer, 2009, 50, 3031–3036. 11 S. Madhugiri, B. Sun, P. G. Smirniotis, J. P. Ferraris and K. J. Balkus, Microporous Mesoporous Mater., 2004, 69, 77–83. 12 V. J. Chen, L. A. Smith and P. X. Ma, Biomaterials, 2006, 27, 3973–3979. 13 M. Ma, R. M. Hill and G. C. Rutledge, J. Adhes. Sci. Technol., 2008, 22, 1799–1817. 14 J.-M. Lim, G.-R. Yi, J. H. Moon, C.-J. Heo and S.-M. Yang, Langmuir, 2007, 23, 7981–7989. 15 M. Gandhi, R. Srikar, A. L. Yarin, C. M. Megaridis and R. A. Gemeinhart, Mol. Pharmacol., 2009, 6, 641–647. 16 Y. Zhang and A. L. Yarin, J. Mater. Chem., 2009, 19, 4732–4739. 17 T. J. Sill and H. A. von Recum, Biomaterials, 2008, 29, 1989–2006. 18 Y. Q. Wu and R. L. Clark, J. Biomater. Sci., Polym. Ed., 2008, 19, 573–601. 19 R. Srikar, A. L. Yarin, C. M. Megaridis, A. V. Bazilevsky and E. Kelley, Langmuir, 2008, 24, 965–974. 20 G. C. Rutledge and S. V. Fridrikh, Adv. Drug Delivery Rev., 2007, 59, 1384–1391. 21 M. J. Olszta, X. Cheng, S. S. Jee, R. Kumar, Y.-Y. Kim, M. J. Kaufman, E. P. Douglas and L. B. Gower, Mater. Sci. Eng., R, 2007, 58, 77–116. 22 Q. P. Pham, U. Sharma and A. G. Mikos, Tissue Eng., 2006, 12, 1197–1211. 23 M. Yousefzadeh, M. Latifi, W. E. Teo, M. Amani-Tehran and S. Ramakrishna, Polym. Eng. Sci., 2011, 51, 323–329. 24 S. Y. Chew, Y. Wen, Y. Dzenis and K. W. Leong, Curr. Pharm. Des., 2006, 12, 4751–4770. 25 M. S. Khil, D. I. Cha, H. Y. Kim, I. S. Kim and N. Bhattarai, J. Biomed. Mater. Res., Part B, 2003, 67, 675–679. 26 J. A. Matthews, G. E. Wnek, D. G. Simpson and G. L. Bowlin, Biomacromolecules, 2002, 3, 232–238. 27 S. Sinha-Ray, Y. Zhang, D. Placke, C. M. Megaridis and A. L. Yarin, Langmuir, 2010, 26, 10243–10249. 28 Y. Zhang, S. Sinha-Ray and A. L. Yarin, J. Mater. Chem., 2011, 21, 8269–8281. 29 R. Sen, B. Zhao, D. Perea, M. E. Itkis, H. Hu, J. Love, E. Bekyarova and R. C. Haddon, Nano Lett., 2004, 4, 459–464. 30 X. L. Xie, Y. W. Mai and X. P. Zhou, Mater. Sci. Eng., R, 2005, 49, 89–112. 31 H. Wu, L. Hu, M. W. Rowell, D. Kong, J. J. Cha, J. R. McDonough, J. Zhu, Y. Yang, M. D. McGehee and Y. Cui, Nano Lett., 2010, 10, 4242–4248. 32 S. Sinha-Ray, Y. Zhang and A. L. Yarin, Langmuir, 2011, 27, 215–226. 33 S. Chen, H. Hou, F. Harnisch, S. A. Patil, A. A. CarmonaMartinez, S. Agarwal, Y. Zhang, S. Sinha-Ray, A. L. Yarin,

This journal is

34

c


37 38 39 40 41 42 43 44 45

46 47 48 49

50 51 52 53 54

55 56 57 58

59 60 61 62 63

A. Greiner and U. Schroeder, Energy Environ. Sci., 2011, 4, 1417–1421. B. Ding, J. H. Kim, Y. Miyazaki and S. M. Shiratori, Sens. Actuators, B, 2004, 101, 373–380. X. Y. Wang, C. Drew, S. H. Lee, K. J. Senecal, J. Kumar and L. A. Samuelson, J. Macromol. Sci., Part A: Pure Appl. Chem., 2002, 39, 1251–1258. Y. Filatov, A. Budyka and V. Kirichenko, Electrospinning of micro- and nanofibers: fundamentals and applications in separation and filtration processes, Begell House, Inc, New York, 2007. M. Li, J. Zhang, H. Zhang, Y. Liu, C. Wang, X. Xu, Y. Tang and B. Yang, Adv. Funct. Mater., 2007, 17, 3650–3656. S. Pagliara, A. Camposeo, E. Mele, L. Persano, R. Cingolani and D. Pisignano, Nanotechnology, 2010, 21, 215304. L. M. Bellan and H. G. Craighead, Polym. Adv. Technol., 2011, 22, 304–309. I. S. Chronakis, J. Mater. Process. Technol., 2005, 167, 283–293. A. Greiner and J. H. Wendorff, Angew. Chem., Int. Ed., 2007, 46, 5670–5703. D. Li and Y. N. Xia, Adv. Mater., 2004, 16, 1151–1170. K. Onozuka, B. Ding, Y. Tsuge, T. Naka, M. Yamazaki, S. Sugi, S. Ohno, M. Yoshikawa and S. Shiratori, Nanotechnology, 2006, 17, 1026–1031. Y. Zhu, J. C. Zhang, J. Zhai, Y. M. Zheng, L. Feng and L. Jiang, ChemPhysChem, 2006, 7, 336–341. V. Tomer, R. Teye-Mensah, J. C. Tokash, N. Stojilovic, W. Kataphinan, E. A. Evans, G. G. Chase, R. D. Ramsier, D. J. Smith and D. H. Reneker, Sol. Energy Mater. Sol. Cells, 2005, 85, 477–488. C. Drew, X. Y. Wang, K. Senecal, H. Schreuder-Gibson, J. N. He, J. Kumar and L. A. Samuelson, J. Macromol. Sci., Part A: Pure Appl. Chem., 2002, 39, 1085–1094. BCC Research, Nanofibers: Technologies and Developing Markets. Report Code NAN043A, 2007. BCC Research, Nanofibers: Technologies and Developing Markets. Report Code NAN043B, 2010. T. H. Grafe, M. Gogins, M. Barris, J. Schaefer and R. Canepa, Nanofibers in filtration applications in transportation, Donaldson Company Inc., Filtration 2001 International Conference and Exposition of the INDA (Association of the Nowovens Fabric Industry), Chicago, Illinois, USA, 2001. Time-life Books, Inventive Genius (Library of Curious and Unusual Facts), Time Life UK, 1992, p. 52. K. Woodings, Regenerated Cellulose Fibers, CRC Press LLC, Woodhead Publishing Ltd, Cambridge, UK, 2001. German Hosiery Museum, Nylon, Last accessed 11 Nov 2011, http://www.german-hosiery-museum.de/technik/garne/nylon/ nylon.htm, 2011. P. C. Painter and M. M. Coleman, Essentials of polymer science and engineering, DEStech Publications Inc., USA, 2009, p. 56. Oerlikon Neumag, Oerlikon Neumag. Oerlikon Neumag sold 300 tons per day line to Taiwan, Last accessed 2 October 2011, http://www.neumag.oerlikontextile.com/desktopdefault.aspx/ tabid-281/133_read-6706/, 2010. Y. Y. Yao, P. X. Zhu, H. Ye, A. J. Niu, X. S. Gao and D. C. Wu, Acta Polym. Sin., 2005, 687–692. Y. Y. Yao, P. X. Zhu, H. Ye, A. J. Niu, X. S. Gao and D. C. Wu, Front. Chem. Chin., 2006, 3, 334–339. I. C. Um, D. F. Fang, B. S. Hsiao, A. Okamoto and B. Chu, Biomacromolecules, 2004, 5, 1428–1436. Y. M. Kim, K. R. Ahn, Y. B. Sung and R. S. Jang, Manufacturing device and the method of preparing for the nanofibers via electro-blown spinning process, United States Patent Application 20050067732, 2005. J. B. Hovanec, Blowing gases in electroblowing process, United States Patent 7846374, E. I. du Pont de Nemours and Company, 2010. J. E. Armantrout, B. S. Johnson, C. A. Rude and M. A. Bryner, Fiber charging apparatus, United States Patent 7465159, 2008. J. E. Armantrout, M. A. Bryner and C. B. Spiers, Electroblowing fiber spinning process, United States Patent 7582247, E. I. du Pont de Nemours and Company, 2009. M. A. Bryner, J. E. Armantrout and B. S. Johnson, Electroblowing web formation, United States Patent 7931456, 2011. S. Sinha-Ray, A. L. Yarin and B. Pourdeyhimi, Carbon, 2010, 48, 3575–3578.

Chem. Soc. Rev., 2012, 41, 4708–4735

4731


View Online 64 S. Sinha-Ray, Y. Zhang, A. L. Yarin, S. C. Davis and B. Pourdeyhimi, Biomacromolecules, 2011, 12, 2357–2363. 65 C.-C. Liao, C.-C. Wang and C.-Y. Chen, Polymer, 2011, 52, 4303–4318. 66 O. O. Dosunmu, G. G. Chase, W. Kataphinan and D. H. Reneker, Nanotechnology, 2006, 17, 1123–1127. 67 K. Sarkar, C. Gomez, S. Zambrano, M. Ramirez, E. de Hoyos, H. Vasquez and K. Lozano, Mater. Today (Oxford, UK), 2010, 13, 12–14. 68 M. R. Badrossamay, H. A. McIlwee, J. A. Goss and K. K. Parker, Nano Lett., 2010, 10, 2257–2261. 69 D. H. Sun, C. Chang, S. Li and L. W. Lin, Nano Lett., 2006, 6, 839–842. 70 C. Chang, K. Limkrailassiri and L. Lin, Appl. Phys. Lett., 2008, 93, 123111. 71 C. Chang, V. H. Tran, J. Wang, Y.-K. Fuh and L. Lin, Nano Lett., 2010, 10, 726–731. 72 G. S. Bisht, G. Canton, A. Mirsepassi, L. Kuinsky, S. Oh, D. Dunn-Rankin and M. J. Madou, Nano Lett., 2011, 11, 1831–1837. 73 O. D. Velev and R. G. Alargova, Process for preparing microrods using liquid-liquid dispersion, United States Patent 7323540, 2008. 74 O. D. Velev, S. Smoukov and M. Marquez, Nanospinning of polymer fibers from sheared solutions, United States Patent Application 20100247908, 2010. 75 A. Greiner, Nanofiber based nonwovens by electrospinning – from fundamental research to real world applications, Swiss Nano Convention 2011, Culture & Congress Centre TRAFO, Baden, Last accessed 2 October 2011, http://www.swiss-nanoconvention.ch/ Speaker/abstract-Greiner.html, 2011. 76 T. Subbiah, G. S. Bhat, R. W. Tock, S. Pararneswaran and S. S. Ramkumar, J. Appl. Polym. Sci., 2005, 96, 557–569. 77 M. Milwich, T. Speck, O. Speck, T. Stegmaier and H. Planck, Am. J. Bot., 2006, 93, 1455–1465. 78 A. R. Horrocks and S. C. Anand, Handbook of technical textiles, Woodhead Publishing Limited, 2000. 79 R. Hooke, Micrographia: or some physiological descriptions of minute bodies made by magnifying glasses: with observations and inquiries thereupon, Printed for John Martyn, printer to the Royal Society, London, 1667, pp. 6–10. 80 F. W. Taussig, Principles of economics, The Macmillan Company, USA, 3rd edn, 1921, vol. 1, p. 229. 81 D. C. Coleman, Courtaulds: An Economic and Social History, Oxford University Press, London, UK, 1969, vol. II, p. 5. 82 W. H. Carothers, Alkylene carbonate and process of making it, United States Patent 1995291, DuPont, 1935. 83 W. H. Carothers, Linear Condensation Polymers, United States Patent 2071250, 1937. 84 P. J. T. Morris, Polymer pioneers: a popular history of the science and technology of large molecules (Center for History of Chemistry, No 5), Chemical Heritage Foundation, Philadelphia, PA, USA, 2005, p. 56. 85 G. Claasen, G. J. Brands, L. C. Lopez, R. Broos, T. Allgeuer, W. Chen and J. F. Sturnfield, Process for producing micron and submicron fibers and nonwoven webs by melt blowing, United States Patent Application 20100037576, 2010. 86 G. J. Brands, R. Broos, R. J. Koopmans, J. W. Storer, L. C. Lopez and J. F. Sturnfield, Polymer or oligomer fibers by solvent-free electrospinning, United States Patent Application 20100064647, 2010. 87 J. W. Storer, J. F. Sturnfield, L. C. Lope, R. J. Koopmans, R. Broos, W. Chen and G. J. Brands, High-output solvent-based electrospinning, United States Patent Application 20100200494, 2010. 88 J. Doshi and D. H. Reneker, J. Electrost., 1995, 35, 151–160. 89 D. H. Reneker, A. L. Yarin, E. Zussman and H. Xu, Adv. Appl. Mech., 2007, 41, 43–195. 90 G. Srinivasan and D. H. Reneker, Polym. Int., 1995, 36, 195–201. 91 J. P. F. Lagerwall, J. T. McCann, E. Formo, G. Scalia and Y. Xia, Chem. Commun., 2008, 5420–5422. 92 E. Enz, U. Baumeister and J. Lagerwall, Beilstein J. Org. Chem., 2009, 5, 1–8. 93 L. Larrondo and R. S. J. Manley, J. Polym. Sci., Part B: Polym. Phys., 1981, 19, 909–920. 94 J. Lyons, C. Li and F. Ko, Polymer, 2004, 45, 7597–7603.

4732

Chem. Soc. Rev., 2012, 41, 4708–4735

95 H. Zhou, T. B. Green and Y. L. Joo, Polymer, 2006, 47, 7497–7505. 96 P. D. Dalton, D. Grafahrend, K. Klinkhammer, D. Klee and M. Moller, Polymer, 2007, 48, 6823–6833. 97 E. R. Kenawy, G. L. Bowlin, K. Mansfield, J. Layman, D. G. Simpson, E. H. Sanders and G. E. Wnek, J. Controlled Release, 2002, 81, 57–64. 98 A. V. Bazilevsky, A. L. Yarin and C. M. Megaridis, Langmuir, 2007, 23, 2311–2314. 99 A. L. Yarin, E. Zussman, J. H. Wendorff and A. Greiner, J. Mater. Chem., 2007, 17, 2585–2599. 100 S. A. Edwards, The nanotech pioneers: where are they taking us? Wiley-VCH Verlag Gmbh & Co., KgaA, Weinheim, Germany, 2006. 101 Y. Dzenis, Science, 2004, 304, 1917–1919. 102 P. Calvert, Nature, 1999, 399, 210–211. 103 M. C. P. Wang and B. D. Gates, Mater. Today (Oxford, UK), 2009, 12, 34–43. 104 A. Theron, E. Zussman and A. L. Yarin, Nanotechnology, 2001, 12, 384–390. 105 Y. Dror, W. Salalha, R. L. Khalfin, Y. Cohen, A. L. Yarin and E. Zussman, Langmuir, 2003, 19, 7012–7020. 106 E. Zussman, A. Theron and A. L. Yarin, Appl. Phys. Lett., 2003, 82, 973–975. 107 F. Ko, Y. Gogotsi, A. Ali, N. Naguib, H. H. Ye, G. L. Yang, C. Li and P. Willis, Adv. Mater., 2003, 15, 1161–1165. 108 R. Sen, B. Zhao, D. Perea, M. E. Itkis, H. Hu, J. Love, E. Bekyarova and R. C. Haddon, Nano Lett., 2004, 4, 459–464. 109 J. B. Gao, A. P. Yu, M. E. Itkis, E. Bekyarova, B. Zhao, S. Niyogi and R. C. Haddon, J. Am. Chem. Soc., 2004, 126, 16698–16699. 110 Z. C. Sun, E. Zussman, A. L. Yarin, J. H. Wendorff and A. Greiner, Adv. Mater., 2003, 15, 1929–1932. 111 A. Greiner, J. H. Wendorff, A. L. Yarin and E. Zussman, Appl. Microbiol. Biotechnol., 2006, 71, 387–393. 112 J. T. McCann, M. Marquez and Y. Xia, Nano Lett., 2006, 6, 2868–2872. 113 F. Y. Li, Y. Zhao, S. Wang, D. Han, L. Jiang and Y. L. Song, J. Appl. Polym. Sci., 2009, 112, 269–274. 114 A. L. Yarin and E. Zussman, Polymer, 2004, 45, 2977–2980. 115 T. Miloh, B. Spivak and A. L. Yarin, J. Appl. Phys., 2009, 106, 114910. 116 A. Ziabicki, in Man-made Fibres: Science & Technology, Interscience Publishers, New York, 1967, vol. 1, p. 169. 117 A. Ziabicki, Fundamentals of fibre formation: the science of fibre spinning and drawing, London and New York, 1976. 118 V. B. Gupta and V. K. Kothari, Manufactured fibre technology, Chapman & Hall, London, 1997. 119 J. W. S. Hearle, High-performance fibres, Woodhead Publish Ltd, Cambridge, UK, 2001. 120 M. M. Denn, Polymer Melt Processing: Foundations in Fluid Mechanics and Heat Transfer, Cambridge University Press, USA, 2008. 121 D. H. Reneker and I. Chun, Nanotechnology, 1996, 7, 216–223. 122 D. H. Reneker, A. L. Yarin, H. Fong and S. Koombhongse, J. Appl. Phys., 2000, 87, 4531–4547. 123 A. L. Yarin, S. Koombhongse and D. H. Reneker, J. Appl. Phys., 2001, 89, 3018–3026. 124 A. L. Yarin, in Handbook of Atomization and Sprays: Theory and Applications, ed. N. Ashgriz, Springer Science & Business Media, LLC, 2011. 125 Y. M. Shin, M. M. Hohman, M. P. Brenner and G. C. Rutledge, Appl. Phys. Lett., 2001, 78, 1149–1151. 126 Y. M. Shin, M. M. Hohman, M. P. Brenner and G. C. Rutledge, Polymer, 2001, 42, 9955–9967. 127 M. M. Hohman, M. Shin, G. Rutledge and M. P. Brenner, Phys. Fluids, 2001, 13, 2201–2220. 128 M. M. Hohman, M. Shin, G. Rutledge and M. P. Brenner, Phys. Fluids, 2001, 13, 2221–2236. 129 S. V. Fridrikh, J. H. Yu, M. P. Brenner and G. C. Rutledge, Phys. Rev. Lett., 2003, 90, 144502. 130 S. Chiu-Webster and J. R. Lister, J. Fluid Mech., 2006, 569, 89–111. 131 T. Han, D. H. Reneker and A. L. Yarin, Polymer, 2007, 48, 6064–6076. 132 A. L. Yarin, Free liquid jets and films: hydrodynamics and rheology, Wiley, New York, 1993.

This journal is

c



View Online 133 A. L. Yarin, S. Sinha-Ray and B. Pourdeyhimi, Polymer, 2011, 52, 2929–2938. 134 J. M. Deitzel, J. Kleinmeyer, D. Harris and N. C. B. Tan, Polymer, 2001, 42, 261–272. 135 A. Ziabicki and H. Kawai, High-speed Fiber Spinning: Science and Engineering Aspects, Wiley, New York, 1985. 136 A. Rupprecht, Acta Chem. Scand., 1966, 20, 494–504. 137 A. S. Nain, M. Sitti, A. Jacobson, T. Kowalewski and C. Amon, Macromol. Rapid Commun., 2009, 30, 1406–1412. 138 G. Taylor, Proc. R. Soc. London, Ser A, 1969, 313, 453–475. 139 L. Larrondo and R. S. J. Manley, J. Polym. Sci., Part B: Polym. Phys., 1981, 19, 921–932. 140 S. B. Mitchell and J. E. Sanders, J. Biomed. Mater. Res., Part A, 2006, 78, 110–120. 141 D. Li, Y. L. Wang and Y. N. Xia, Adv. Mater., 2004, 16, 361–366. 142 W. E. Teo and S. Ramakrishna, Nanotechnology, 2006, 17, R89–R106. 143 S. L. Shenoy, W. D. Bates, H. L. Frisch and G. E. Wnek, Polymer, 2005, 46, 3372–3384. 144 C. J. Luo, E. Stride, S. Stoyanov, E. Pelan and M. Edirisinghe, J. Polym. Res., 2011, 18, 2515–2522. 145 H. Fong, I. Chun and D. H. Reneker, Polymer, 1999, 40, 4585–4592. 146 A. Jaworek and A. Krupa, J. Aerosol Sci., 1999, 30, 873–893. 147 B. Kim, H. Park, S. H. Lee and W. M. Sigmund, Mater. Lett., 2005, 59, 829–832. 148 K. Morota, H. Matsumoto, T. Mizukoshi, Y. Konosu, M. Minagawa, A. Tanioka, Y. Yamagata and K. Inoue, J. Colloid Interface Sci., 2004, 279, 484–492. 149 C. J. Luo, E. Stride and M. Edirisinghe, Macromolecules, 2012, DOI: 10.1021/ma300656u. 150 C. J. Luo, M. Nangrejo and M. Edirisinghe, Polymer, 2010, 51, 1654–1662. 151 A. L. Andrady, Science and technology of polymer nanofibers, John Wiley & Sons, Inc., USA, 2008. 152 Editorial Board, Russ. J. Phys. Chem. A, 2007, 81, 1355–1357. 153 N. M. Thoppey, J. R. Bochinski, L. I. Clarke and R. E. Gorga, Polymer, 2010, 51, 4928–4936. 154 A. Formhals, Process and apparatus for preparing artificial threads, United States Patent 1975504, 1934. 155 H. L. Simons, Process and apparatus for producing patterned nonwoven fabrics, United States Patent 3280229, 1966. 156 P. Gupta and G. L. Wilkes, Polymer, 2003, 44, 6353–6359. 157 S. A. Theron, A. L. Yarin, E. Zussman and E. Kroll, Polymer, 2005, 46, 2889–2899. 158 J. S. Varabhas, G. G. Chase and D. H. Reneker, Polymer, 2008, 49, 4226–4229. 159 A. Varesano, R. A. Carletto and G. Mazzuchetti, J. Mater. Process. Technol., 2009, 209, 5178–5185. 160 A. Varesano, F. Rombaldoni, G. Mazzuchetti, C. Tonin and R. Comotto, Polym. Int., 2010, 59, 1606–1615. 161 H.-y. Kim, Electronic spinning apparatus and a process of preparing nonwoven fabric using the same, United States Patent 7332050, 2008. 162 J. Park, Electric spinning apparatus for mass-production of nano-fiber, United States Patent 7980838, Finetex Ene, Inc., Seoul, KR, 2011. 163 M. Kamiyama and M. Numata, Island-in-sea type composite finer and process for producing same, United States Patent Application 20100029158, 2010. 164 J. C. Almekinders and C. Jones, J. Aerosol Sci., 1999, 30, 969–971. 165 Elmarco, Nanofibers metric for Nanospider., Last accessed 2 October 2011. http://www.elmarco.cz/upload/soubory/obsah/ 63-2-nanofibers-metric-for-nanospidertm.pdf, 2011. 166 D. Petras, M. Maly, J. Pozner, J. Trdlicka and M. Kovac, Rotary spinning electrode, United States Patent Application 20100034914, 2010. 167 D. Yang, B. Lu, Y. Zhao and X. Jiang, Adv. Mater., 2007, 19, 3702–3706. 168 H. Kramer, J. Electrost., 1993, 30, 77–92. 169 European Standards, EN 50223, Reference EN 50223:2010, 2010. 170 European Standards, EN 50050, Reference prEN 50050-3:2011, 2011. 171 Technical Textiles International, ITMA, Barcelona preview, Last accessed 7 Oct 2011. http://www.intnews.com/ITMA2011%20 preview.pdf, 2011.

This journal is

c


172 A. Desch and F. Reiter, Nanofiber spinning installations by Dienes Apparatebau, with the technology of Reiter OFT, Last accessed 8 November 2011. http://www.dienes.net/docs/Dienes%20Article% 20%20Nanofiber%20Spinning%202010-10%20en.pdf, 2010. 173 T. H. Grafe and K. M. Graham, Nanofiber webs from electrospinning, Donaldson Company Inc., Nonwovens in Filtration – Fifth International Conference, Stuttgart, Germany, 11–12 March, 2003. 174 DuPont, Energaint separators bring batteries more power, longer life, Last accessed 8 November 2011. http://www2.dupont.com/ Energy_Storage/en_US/tech_info/technical_info.html, 2011. 175 DuPont, DuPont, in: Hybrid membrane technology (HMT), Last accessed 8 November 2011. http://www2.dupont.com/Separa tion_Solutions/en_US/tech_info/hmt/hmt.html, 2011. 176 FibeRio, Last accessed 7 November 2011. http://fiberiotech.com/ technology/how-it-works/, 2011. 177 Hills Inc., Last accessed 12 Mar 2012. www.hillsinc.net/, 2012. 178 Xanofi, Xanoshear, Last accessed 7 Oct 2011. http://www.xanofi.com/ tech.html, 2011. 179 H. Goda, M. Numata, M. Kamiyama, M. Yamamoto and T. Yamamoto, Method of producing island-in-sea type composite spun fiber, United States Patent Application 20090042031, Teijin Fibers Limited, 2009. 180 Teijin Fibers Ltd., Nanofrontt 700 nanometer ultra fine polyester nanofiber, Last accessed 8 Nov 2011. http://www.teijinfiber.com/ english/products/specifics/nanofront.html, 2008. 181 Royal Society and Royal Academy of Engineering, Nanoscience and nanotechnologies: Opportunities and uncertainties, The Royal Society, London, UK, 2004. 182 S. Ishizu, M. Sekiya, K. Ishibashi, Y. Negami and M. Ata, J. Nanopart. Res., 2008, 10, 229–254. 183 J. D. Watson and F. H. C. Crick, Nature, 1953, 171, 737–738. 184 B. Gupta, N. Revagade and J. Hilborn, Prog. Polym. Sci., 2007, 32, 455–482. 185 S. J. Pomfret, P. N. Adams, N. P. Comfort and A. P. Monkman, Polymer, 2000, 41, 2265–2269. 186 M. Enayati, Z. Ahmad, E. Stride and M. Edirisinghe, J. R. Soc., Interface, 2010, 7, S393–S402. 187 A. J. Guenthner, S. Khombhongse, W. X. Liu, P. Dayal, D. H. Reneker and T. Kyu, Macromol. Theory Simul., 2006, 15, 87–93. 188 S. Koombhongse, W. X. Liu and D. H. Reneker, J. Polym. Sci., Part B: Polym. Phys., 2001, 39, 2598–2606. 189 D. Li, Y. L. Wang and Y. N. Xia, Nano Lett., 2003, 3, 1167–1171. 190 H.-Y. Kim, M. Lee, K. J. Park, S. Kim and L. Mahadevan, Nano Lett., 2010, 10, 2138–2140. 191 Y. Liu, X. Zhang, Y. Xia and H. Yang, Adv. Mater., 2010, 22, 2454–2457. 192 R. D. Piner, J. Zhu, F. Xu, S. H. Hong and C. A. Mirkin, Science, 1999, 283, 661–663. 193 F. Huo, Z. Zheng, G. Zheng, L. R. Giam, H. Zhang and C. A. Mirkin, Science, 2008, 321, 1658–1660. 194 G. A. Ozin, A. C. Arsenault and L. Cademartiri, Nanochemistry: a chemical approach to nanomaterials, Royal Society of Chemistry, Cambridge, 2009. 195 H. Blades, J. R. White, W. R. White and C. Ford, Fibrillated strand, United States Patent 3081519, 1962. 196 H. Shin and S. L. Samuels, Flash-spinning polymeric plexifilaments, United States Patent 5147586, 1992. 197 Z. Tadmor and C. G. Gogos, Principles of polymer processing, John Wiley & Sons, Inc., New Jersey, 2006. 198 R. H. Colby, L. J. Fetters, W. G. Funk and W. W. Graessley, Macromolecules, 1991, 24, 3873–3882. 199 M. G. McKee, G. L. Wilkes, R. H. Colby and T. E. Long, Macromolecules, 2004, 37, 1760–1767. 200 Z. Chen, X. Mo and F. Qing, Mater. Lett., 2007, 61, 3490–3494. 201 T. Jarusuwannapoom, W. Hongrojjanawiwat, S. Jitjaicham, L. Wannatong, M. Nithitanakul, C. Pattamaprom, P. Koombhongse, R. Rangkupan and P. Supaphol, Eur. Polym. J., 2005, 41, 409–421. 202 M. F. Torres, A. J. Muller and A. E. Saez, Polym. Bull., 2002, 47, 475–483. 203 M. C. Branciforti, T. A. Custodio, L. M. Guerrini, L. Averous and R. E. Suman Bretas, J. Macromol. Sci., Part B: Phys., 2009, 48, 1222–1240.

Chem. Soc. Rev., 2012, 41, 4708–4735

4733


View Online 204 J. Burke, Solubility parameters: theory and application, in: AIC Book and Paper Group Annual, 1984, vol. 3, pp. 13–58. 205 A. D. Rey, Soft Matter, 2007, 3, 1349–1368. 206 N. Ogata, S. Yamaguchi, N. Shimada, G. Lu, T. Iwata, K. Nakane and T. Ogihara, J. Appl. Polym. Sci., 2007, 104, 1640–1645. 207 DuPont, Technical guide Kevlars aramid fiber, Last accessed 8 November 2011. http://www2.dupont.com/Kevlar/en_US/assets/ downloads/KEVLAR_Technical_Guide.pdf, 2011. 208 J. M. Gosline, P. A. Guerette, C. S. Ortlepp and K. N. Savage, J. Exp. Biol., 1999, 202, 3295–3303. 209 M. Heim, D. Keerl and T. Scheibel, Angew. Chem., Int. Ed., 2009, 48, 3584–3596. 210 S. L. Kwolek, P. W. Morgen and W. R. Sorenson, Process of making wholly aromatic polyamides, United States Patent 3063966, DuPont, 1962. 211 N. Erdem and U. H. Erdogan, Recent advances in the spinning processes used for high performance fibers, Last accessed 2 October 2011, http://fs.tx.ncsu.edu/Past_Meetings/Spring_2003_ Loughborough/papers/143-Erdem.pdf, 2003. 212 S. Michielsen, M. L. Realff and J. Leisen, Fundamentals of High Modulus, High Tenacity Melt Spun Fibers, National Textile Center Annual Report: M01-G01, Last accessed 2 Octerber 2011. URL: http://www.tfe.gatech.edu/faculty/michielsen/michelsen-highmod.htm, 2001. 213 DuPont, KEVLARs Para-aramid Material Safety Data Sheet, Version 2.2, Ref. 150000002634, 2008. 214 F. Reinitzer, Monatsh. Chem., 1888, 9, 421–441. 215 L. Eisoldt, A. Smith and T. Scheibel, Mater. Today (Oxford, UK), 2011, 14, 80–86. 216 F. Vollrath and D. P. Knight, Nature, 2001, 410, 541–548. 217 F. Hagn, L. Eisoldt, J. G. Hardy, C. Vendrely, M. Coles, T. Scheibel and H. Kessler, Nature, 2010, 465, 239–242. 218 Z. Lin, W. Huang, J. Zhang, J.-S. Fan and D. Yang, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 8906–8911. 219 H. J. Jin and D. L. Kaplan, Nature, 2003, 424, 1057–1061. 220 G. Askarieh, M. Hedhammar, K. Nordling, A. Saenz, C. Casals, A. Rising, J. Johansson and S. D. Knight, Nature, 2010, 465, 236–238. 221 X. Zhang and C. Wang, Chem. Soc. Rev., 2011, 40, 94–101. 222 F. Vollrath, D. Porter and C. Holland, Soft Matter, 2011, 7, 9595–9600. 223 W. Heuer, Interpolymers of vinyl sulphonic acid with another vinyl compound and aqueous emulsions thereof, United States Patent 2300920, Gen Aniline & Film Corp, 1942. 224 B. L. Arthur and J. W. Edwin, Composition comprising a polyhalogenated ethylene polymer and viscose and process of shaping the same, United States Patent 2772444, DuPont, 1956. 225 O. Seizo, M. Tadashi, A. Hiroshi and C. Ippei, Process for the production of fibers having polyvinyl chloride as the principal constituent and also containing polyvinyl alcohol, United States Patent 3111370, Toyo Kagaku Co Ltd, 1963. 226 T. Koshro, Appl. Macromol. Chem. Phys., 1974, 40, 277–290. 227 T. Tsuji and M. Korematsu, Highly flame-retardant shaped articles and method for preparing the same, United States Patent 3907958, Kabushiki Kaisha Kohjin, 1975. 228 K. Matsuo, M. Araki, T. Matsugu and T. Mikami, Emulsions useful in the preparation of heat resistant fibers and films, United States Patent 3925290, Kohjin Co. Ltd., 1975. 229 A. L. Yarin, Polym. Adv. Technol., 2011, 22, 310–317. 230 X. L. Xu, L. X. Yang, X. Y. Xu, X. Wang, X. S. Chen, Q. Z. Liang, J. Zeng and X. B. Jing, J. Controlled Release, 2005, 108, 33–42. 231 Y. Yang, X. Li, M. Qi, S. Zhou and J. Weng, Eur. J. Pharm. Biopharm., 2008, 69, 106–116. 232 J. C. Sy, A. S. Klemm and V. P. Shastri, Adv. Mater., 2009, 21, 1814–1819. 233 T. Huang, L. R. Marshall, J. E. Armantrout, S. Yembrick, W. H. Dunn, J. M. Oconnor, T. A. Mueller and M. D. Wetzel, Production of nanofibers by melt spinning, United States Patent Application 20080242171, 2008. 234 L. Larrondo and R. S. J. Manley, J. Polym. Sci., Part B: Polym. Phys., 1981, 19, 933–940. 235 T. Kamisasa and N. Ogata, Ultrafine composite fiber, ultrafine fiber, method for manufacturing same and fiber structure,

4734

Chem. Soc. Rev., 2012, 41, 4708–4735

236 237 238 239 240 241 242 243

244 245 246 247

248 249 250 251 252

253 254 255 256 257 258

259 260

261 262 263 264

United States Patent Application 20100297443, Daiwanbo Holdings Co. Ltd., Daiwanbo Polytech Co. Ltd., 2010. S. Lee and S. K. Obendorf, J. Appl. Polym. Sci., 2006, 102, 3430–3437. P. D. Dalton, K. Klinkhammer, J. Salber, D. Klee and M. Moller, Biomacromolecules, 2006, 7, 686–690. P. D. Dalton, J. L. Calvet, A. Mourran, D. Klee and M. Moeller, Biotechnol. J., 2006, 1, 998–1006. D. W. Hutmacher and P. D. Dalton, Chem.–Asian J., 2011, 6, 44–56. P. D. Dalton, N. T. Joergensen, J. Groll and M. Moeller, Biomed. Mater., 2008, 3, 034109. K. Garg and G. L. Bowlin, Biomicrofluidics, 2011, 5, 013403. A. L. Breen and W. E. Jordan, Two component convoluted filaments, United States Patent 3017686, 1962. M. Satcher, R. E. Gorga, C. Rinaldi, S. Michielsen, C. Barrera, N. Fedorova, J. P. Hinestroza, S. Chhaparwal, W. Dondero, T. Ghosh and B. Pourdeyhimi, Textile Nanotechnologies, in Chapter 21, Handbook of Nanoscience, Engineering, and Technology, CRC Press, Taylor & Francis Group, LLC, Boca Raton, Florida, 2007. M. S. Ndaro, X. Jin, T. Chen and C. Yu, J. Eng. Fibers Fabr., 2007, 2, 1–9. P. H. Seckel, Longitudinally separable extruded thermoplastic strip and process of producing same, United States Patent 2531234, 1950. T. Nakajima, K. Kajiwara and J. E. McIntyre, Advanced fiber spinning technology, Woodhead Publishing, Cambridge, UK, 1994. Y. S. Kim, D. J. Jo, J. S. Kim, D. H. Kim and I. Y. Yang, Spinneret for preparing island-in-the-sea yarns, United States Patent Application 20100205926, Woongjin Chemical Co. Ltd., 2010. M. Kamiyama and M. Numata, Island-in-sea type composite fiber and process for producing the same, United States Patent Application 20070196649, 2007. M. Kamiyama and M. Numata, Islands-in-sea type composite fiber and process for producing the same, United States Patent 7622188, 2009. E. Zussman, A. L. Yarin, A. V. Bazilevsky, R. Avrahami and M. Feldman, Adv. Mater., 2006, 18, 348–353. Y. Dror, W. Salalha, R. Avrahami, E. Zussman, A. L. Yarin, R. Dersch, A. Greiner and J. H. Wendorff, Small, 2007, 3, 1064–1073. V. A. Wendt, E. L. Boon and C. D. Fluharty, Manufacture of Super-Fine Organic Fibers (NRL report 4364), U. S. Department of Commerce. Office of Technical Services, Naval Research Laboratory, 1954. R. L. Shambaugh, Ind. Eng. Chem. Res., 1988, 27, 2363–2372. M. A. J. Uyttendaele and R. L. Shambaugh, AIChE J., 1990, 36, 175–186. B. Majumdar and R. L. Shambaugh, J. Rheol., 1990, 34, 591–601. R. S. Rao and R. L. Shambaugh, Ind. Eng. Chem. Res., 1993, 32, 3100–3111. G. Eian and P. G. Cheney, Durable melt-blown fibrous sheet material, United States Patent 4681801, Minnesota Mining and Manufacturing Company (St. Paul, MN), 1987. Oerlikon Neumag, Oerlikon textile, non-woven technology, meltblown, J & M meltblown technology, Last acessed 2 October 2011. http://www.neumag.oerlikontextile.com/en/desktopdefault.aspx/ tabid-541/, 2011. P. Moosmayer, J. Budliger, E. Zurcher and L. C. Wadsworth, Apparatus for electrically charging meltblown webs (B-001), United States Patent 4904174, 1990. B. Chu, B. S. Hsiao, D. Fang and A. Okamoto, Electro-blowing technology for fabrication of fibrous articles and its applications of hyaluronan, United States Patent 7662332, The Research Foundation of State University of New York, 2010. E. Zhmayev, D. Cho and Y. L. Joo, Polymer, 2010, 51, 274–290. E. Zhmayev, D. Cho and Y. L. Joo, Polymer, 2010, 51, 4140–4144. S. Tian, N. Ogata, N. Shimada, K. Nakane, T. Ogihara and M. Yu, J. Appl. Polym. Sci., 2009, 113, 1282–1288. N. Shimada, H. Tsutsumi, K. Nakane, T. Ogihara and N. Ogata, J. Appl. Polym. Sci., 2010, 116, 2998–3004.

This journal is

c



View Online 265 J. P. Hooper, Centrifugal spinneret, United States Patent 1500931, 1924. 266 R. M. Downey, Apparatus and process for making mineral wool, United States Patent 2587710, 1952. 267 W. Hans, Method and means for the manufacture of fibres of thermoplastic material, United States Patent 3097085, 1963. 268 W. Wagner, P. R. Nyssen, D. Berkenhaus and H. Van Pey, Production of very fine polymer fibres, United States Patent 4937020, Bayer Aktiengesellschaft, 1990. 269 A. L. Andrady, D. S. Ensor and R. J. Newsome, Electrospinning of fibers using a rotatable spray head, United States Patent 7134857, Research Triangle Institute (Research Triangle Park, NC, US), 2006. 270 K. Lozano and K. Sarkar, Methods and apparatuses for making superfine fibers, United States Patent Application 20090280325, 2009. 271 R. T. Weitz, L. Harnau, S. Rauschenbach, M. Burghard and K. Kern, Nano Lett., 2008, 8, 1187–1191. 272 C. Sanchez, P. Belleville, M. Popall and L. Nicole, Chem. Soc. Rev., 2011, 40, 696–753. 273 G. Larsen, R. Velarde-Ortiz, K. Minchow, A. Barrero and I. G. Loscertales, J. Am. Chem. Soc., 2003, 125, 1154–1155. 274 T. Ondarçuhu and C. Joachim, Europhys. Lett., 1998, 42, 215–220. 275 N. M. Thoppey, J. R. Bochinski, L. I. Clarke and R. E. Gorga, Nanotechnology, 2011, 22, 345301. 276 J. M. Holzwarth and P. X. Ma, J. Mater. Chem., 2011, 21, 10243–10251. 277 M. Zelzer and R. V. Ulijn, Chem. Soc. Rev., 2010, 39, 3351–3357.

This journal is

c


278 R. M. Brown, J. H. M. Willison and C. L. Richardson, Proc. Natl. Acad. Sci. U. S. A., 1976, 73, 4565–4569. 279 A. N. Nakagaito, S. Iwamoto and H. Yano, Appl. Phys. A: Mater. Sci. Process., 2005, 80, 93–97. 280 D. Klemm, F. Kramer, S. Moritz, T. Lindstrom, M. Ankerfors, D. Gray and A. Dorris, Angew. Chem., Int. Ed., 2011, 50, 5438–5466. 281 D. Klemm, B. Heublein, H. P. Fink and A. Bohn, Angew. Chem., Int. Ed., 2005, 44, 3358–3393. 282 S. Yamanaka, M. Ishihara and J. Sugiyama, Cellulose, 2000, 7, 213–225. 283 G. A. Silva, C. Czeisler, K. L. Niece, E. Beniash, D. A. Harrington, J. A. Kessler and S. I. Stupp, Science, 2004, 303, 1352–1355. 284 J. D. Hartgerink, E. Beniash and S. I. Stupp, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 5133–5138. 285 K. H. Smith, E. Tejeda-Montes, M. Poch and A. Mata, Chem. Soc. Rev., 2011, 40, 4563–4577. 286 M. W. Frey, Polym. Rev., 2008, 48, 378–391. 287 H. Yano, J. Sugiyama, A. N. Nakagaito, M. Nogi, T. Matsuura, M. Hikita and K. Handa, Adv. Mater., 2005, 17, 153–155. 288 A. Putra, A. Kakugo, H. Furukawa, J. P. Gong and Y. Osada, Polymer, 2008, 49, 1885–1891. 289 A. Putra, A. Kakugo, H. Furukawa, J. P. Gong, Y. Osada, T. Uemura and M. Yamamoto, Polym. J., 2008, 40, 137–142. 290 M. B. Sano, A. D. Rojas, P. Gatenholm and R. V. Davalos, Ann. Biomed. Eng., 2010, 38, 2475–2484. 291 R. Y. Zhang and P. X. Ma, J. Biomed. Mater. Res., 2000, 52, 430–438. 292 X. Liu, X. Jin and P. X. Ma, Nat. Mater., 2011, 10, 398–406.

Chem. Soc. Rev., 2012, 41, 4708–4735

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