Indian Journal of Fibre & Textile Research Vol. 39, June 2014, pp. 201-203
Effect of solvent concentration on morphology of electrospun Bombyx mori silk E Kamalha1,2,a, Y S Zheng1, Y C Zeng1 & J I Mwasiagi1,3 1
College of Textile Science and Engineering, Donghua University, 210620 Shanghai, China 2 Department of Textile Engineering, Busitema University, 236 Tororo, Uganda 3 iOTEX, Bahir Dar University, 600 Bahir Dar, Ethiopia Received 17 February 2013; revised received and accepted 23 July 2013 Effect of solvent (LiBr and CaCl2:methanol:water::1:2:8) concentration on the morphology of ectrospun Bombyx mori silk has been studied. Scanning electron microscopy is used to elucidate the appearance and diameter of the nanofibres. Concentrations of solvents in the range of 25-30% are found to yield fibres with fewer beads, and a better distribution of diameter. At lower concentrations, the fibres are found scattered and more discontinuous, although they appear finer as compared to the fibres produced using higher solvent concentration. The two solvents are found to yield similar results. Keywords: Bombyx mori, Electrospinning, Nanofibres, Silk fibroin
Bombyx mori fibres were first electrospun by Zarkoob et al.1,2 using aqueous hexafluoro-2-propanol (HFIP) as solvent. The fibre diameter was reported to be in the range of 6.5-200 nm. In later studies, Jin et al.3 electrospun silk nanomats of higher porosity. Solvents in current use for dissolving silk include; concentrated aqueous solutions of LiBr, CaCl2 and some organic solvents such as HFIP2,4,5. Choice for solvents has mostly been based on ease to dissolve, safety and need to minimize degradation of silk. For example, possibly due to more surface area, aqueous-derived silk fibroin scaffolds are said to degrade faster than HFIP-derived scaffolds. However, HFIP is highly toxic and hence must be used cautiously and in controlled amounts. It has also been found that uniform, finer and bead-free pure silk nanofibres are not easy to be produced by electrospinning6. Silk electrospinning solutions are associated with low viscosities and hence polyethylene oxide (PEO) is used along with silk and later removed after spinning7. However, residual PEO is said to affect the —————— a
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biocompatibility and mechanical properties of silk3,8,9. Higher concentrations of regenerated silk in formic acid, coupled with optimum processing parameters will improve the nanofibre yield10. Sukigara et al.11 noted that they could not spin fibres at silk concentrations lower than 5%, while concentrations below 10% yielded several beads and droplets during electrospinning. Concentrations in the range of 12-19.5% yielded continuous electrospun fibres, regardless of electric field and distance. The average diameter was larger in higher concentrations than at lower concentrations. At low concentrations, more beads are formed. At very high concentrations, formation of continuous fibres is inhibited due to restriction of flow of the polymer at the needle tip facilitating the formation of coarse fibres12. Although much work has been done on the electrospinning of silk, results on the effect of solvent concentration on morphology is inconsistent among researchers. There are insignificant comparative studies based on two or more solvents, to examine the effect of solution concentration on morphology, under the same experimental design. The present study is aimed at producing and comparing pure electrospun Bombyx mori nanofibres from two solvents, under the same experimental design using selected concentrations of electrospinning solvents. Experimental Materials and preparation
Bombyx mori fibres were purchased from Zhejiang Province, China. Shanghai Green Bird Science and Technology Co. Ltd, China supplied cellulose dialysis tubing (MWCO; nominal: 14000). Other reagents used were purchased from Sinorpham Chemical Co. Ltd, China, and were used in their original chemical analytical grades. Silk dissolution and preparation of SF films
Bombyx mori fibres were boiled in 0.5% (w/w) Na2CO3 at 100°C for 20 min. In one experiment, degummed silk fibres (DS) were dissolved in 10M LiBr to yield 15% (w/v), and then heated at 60°C for 6 h. The cooled silk solution was then dialyzed against distilled water using cellulose dialysis tubing to remove the lithium salts. The dialyzed solution was filtered and subjected to evaporation (at room
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temperature), to yield dry silk fibroin (SFLiBr). In another experiment, a solvent system consisting of CaCl2:methanol:water (1:2:8) was used to dissolve the degummed silk. This was also made to 15% (w/w) and followed through dialysis and evaporation to yield dry silk fibroin films (SFCaCl2).
Characterization of nanofibre morphology
The morphology of the electrospun fibres was observed with a Jeol scanning electron microscope (JSM-6360LVSEM, Japan). By use of photoshop software, about 300 fibre segments were analyzed randomly to get a mean diameter for each nonwoven web, and also check the distribution of the diameter.
Preparation of the spinning dope and electrospinning
For electrospinning, SF films were dissolved in 98% formic acid for 6 h, yielding viscous polymer solutions of 5-30% (w/v) concentration, which were then filtered. This was done for both SFLiBr and SFCaCl2, separately. Electrospinning was done using similar selected parameters for both kinds of SF. A 10 mL syringe, with a stainless steel needle was used to contain the viscous dope. The inner diameter of the needle was 0.7 mm. A DC high voltage generator was then connected to the tip of the needle. The syringe was mounted and clamped onto a pump (Cole-Parmer Instrument Co., Vernon Hills IL, USA) to point in the horizontal direction. Aluminum foil sheet was clipped onto a grounded thin metal board and placed 20cm from the tip of the needle, to act as the fibre collector. All electrospinning experiments were performed at room temperature (21-25°C) and 60-75% humidity. The polymer flow rate and applied voltage during electrospinning were set at 0.1 mL/h and 15 kV respectively, for the two fibroin films.
Results and Discussion SEM micrographs of electrospun silk at recorded concentrations are given in Figs 1 and 2. The distribution of diameter shows that ESLiBr nanofibres are little finer compared to ESCaCl2 nanofibres. At concentrations below 10%, there are hardly any fibres electrospun; instead, beads are formed. With lower concentrations, the fibres are found more entangled and open-meshlike. The low concentrations also yield more discontinuous fibres. At high concentration, the fibres are more uniform compared to those at lower concentrations. This is supported by the evidence of different standard deviations. The presence of beads is found more pronounced at lower concentrations than in higher concentrations. The concentration of the polymer solution influences the viscosity. Hence, at low concentrations, the polymer in acid has fewer molecules. This reduces the cohesion, and the solution is easily blown by
Fig. 1—SEM micrographs (× 10000) and diameter distribution for ESCaCl2 Bombyx mori fibres
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Fig. 2—SEM micrographs (× 10000) and diameter distribution for ESLiBr Bombyx mori fibres
electrostatic forces producing an unstable electrospinning jet that leads to poor fibre deposition and more broken fibres. Besides, the polymer solution reaches the collecting plate before all the acid evaporates off. This promotes formation of beads. Other factors which may influence formation of beads include collecting distances, flow rates and electric field among others. However, with these factors fixed, these findings show that higher concentration favors better fibre lay and reduces occurrence of beads in the nanofibre web. Generally, higher concentrations exhibit closer fibre packing, fewer beads and more uniform webs than lower concentrations. The minimum fibre diameter measured is 19 nm, while the maximum fibre diameter is 394 nm. Considering errors in experiment, testing and analysis, the mean diameter generally increases with concentration. The minimum fibre diameter also increases as the concentration is increased. At higher concentration, the time for the electrospinning jet thinning reduces. Hence, the diameter remains higher than observed in lower concentrations. At low concentration, polymer molecules are few in the spinning solution and hence there is less interaction of molecules and aggregate formation. This lowers fibre formation and increases the formation of beads. The study of the effect of solvent concentration on the morphology of electrospun fibre shows that higher
concentrations favor production of better quality electrospun Bombyx mori fibres. The fibre diameter of electrospun Bombyx mori fibres increases with concentration. Nanofibres produced using LiBr as the solvent slightly exhibit finer diameters and better distribution, especially at lower concentrations than those made using CaCl2. References 1
Zarkoob S, Structure and morphology of regenerated silk nano-fibres produced by electrospinning, PhD thesis, University of Akron, 1998. 2 Zarkoob S, Reneker D H, Ertley D, Eby R K & Hudson S D, US Pat 6,110,590, 2000. 3 Jin H J, Fridrikh S V, Rutledge G C & Kaplan D, Biomacromolecules, 3 (2002) 1233. 4 Zhang Q, Yan S & Li M, Materials, 2 (2009) 2276. 5 Ayutsede J, Regeneration of Bombyx mori silk nanofibres and nanocomposite fibrils by the electrospinning process, PhD thesis, Drexel University, 2003. 6 Kim U, Park J, Kim J H, Wada M & Kaplan D L, Biomaterials, 26(15) (2005) 2775. 7 Meinel A J, Cell instructive silk fibroin scaffolds for tissue engineering. PhD thesis, University of Wurzburg, 2010. 8 Cappello J & McGrath K P, in Silk Polymers, Materials Science and Biotechnology, Vol. 544, edited by D Kaplan, W W Adams, B Farmer & C Viney (American Chemical Society), 1994. 9 Zhang Q, Yan S & Li M, Materials, 2 (2009) 2276. 10 Fong H, Chun I & Reneker D H, Polymer, 40 (1999) 4585. 11 Sukigara S, Gandhi M, Ayutsede J, Micklus M & Ko F, Polymer, 44 (2003) 5721. 12 Deitzel J M, Kleinmeyer J, Harris D & Tan B N C. Polymer, 42 (2001) 261.
