Fabrication of polyvinyl alcohol/gelatin nanofiber ...

Fabrication of polyvinyl alcohol/gelatin nanofiber composites and evaluation of their material properties Nguyen Thuy Ba Linh,1 Young Ki Min,2 Ho-Yeon Song,3 Byong-Taek Lee1 1

Department of Biomedical Engineering and Materials, School of Medicine, Soonchunhyang University, Cheonan, Chungnum 330-090, Korea 2 Department of Physiology, School of Medicine, Soonchunhyang University, Cheonan, Chungnum 330-090, Korea 3 Department of Microbiology, School of Medicine, Soonchunhyang University, Cheonan, Chungnum 330-090, Korea Received 22 October 2009; revised 31 May 2010; accepted 24 June 2010 Published online 24 August 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.31701 Abstract: Electrospinning of polyvinyl alcohol (PVA), gelatin (GE), and a PVA/GE blend was conducted with the aim of fabricating biodegradable scaffolds for tissue engineering. The process parameters including the concentration of GE in PVA/GE blends, electrical field, and tip-to-collector distance (TCD) were investigated. Electrospinning processes were conducted at three different GE concentrations (PVA/GE ¼ 2/8, 6/4, and 8/2), and the voltage and TCD were varied from 18 to 24 kV and 7 to 20 cm, respectively. The average diameter of the electrospun PVA, GE, and PVA/GE blend fibers ranged from 50 to 150 nm. The TCD had significant effects on the average diameter of the PVA/ GE nanofiber, while changes in the voltage did not significantly

affect the diameter of the PVA/GE nanofiber. The miscibility of the PVA/GE blend fibers was examined by differential scanning calorimetry, and X-ray diffraction was used to determine the crystallinity of the membrane. Tensile strength was measured to evaluate the physical properties of the membrane. Based on the combined results of this study, the PVA/GE membrane holds great promise for use in tissue engineering applications, C 2010 Wiley Periodiespecially in bone or drug delivery systems. V

INTRODUCTION

satile tool by which biodegradable plastics with a wide spectrum of properties can be obtained.9 In addition, gelatin (GE) is a natural biodegradable polymer derived from collagens that contains many functional groups (glycine, proline, glutamic acid, hydroxyproline, arginine, alanine, aspartic acid, and other amino acids). Owning to its excellent biocompatibility and biodegradability properties, GE has been widely used and studied in regards to many biomedical applications,10 including wound or burn dressings; surgical treatments; tissue engineering of bone, skin, and cartilage.11–13 GE has also been shown to be a good carrier for the controlled release of growth factor. Successful tissue regeneration and organ substitution by cell transplantation has been achieved by using GE hydrogels for growth factor release.14 For that reason, a number of studies on the preparation of GE in various forms for biomedical applications have been reported. However, only a few studies have focused on fabricating GE fibers through electrospinning.13,15–17 PVA/GE nanofibrous membranes with different compositions could have different clinical applications, especially in the controlled release of drugs and bone tissue engineering.18 Hence, attempts were made to develop a membrane by esterifying the hydroxyl group of PVA with the carboxyl group of GE.19

Electrospinning is a process that allows for the fabrication of continuous fibers with diameters ranging from submicrons to a few nanometers. This technique has received a lot of attention in recent years because of the relative simplicity with which a wide range of porous structures can be produced.1 Therefore, this method can be applied to natural polymers, synthetic polymers, and polymers loaded with nanoparticles as well as to metals and ceramics, to enhance cell migration and proliferation. In addition, this technique is especially suitable for biomedical applications, including tissue engineering of scaffolds, wound dressings, drug delivery, medical implants, and in fields as diverse as optoelectronics, sensor technology, catalysis, and filtration.2–7 Poly(vinyl alcohol) (PVA) is a semicrystalline, hydrophilic polymer with good chemical and thermal stability.8 The main advantageous property of PVA includes its biodegradability in physiological environments. PVA is a nonhazardous material, has no negative effects on animals, and does not cause any injuries to the skin upon contact. However, if PVA is injected under the skin or into the lungs it is not broken down by the tissue but remains in situ as a ‘‘foreign body’’. The blending of PVA with other biodegradable polymers, such as polysaccharides, biopolyesters, and biodegradable synthetic polyesters, may result in the production of a ver-

cals, Inc. J Biomed Mater Res Part B: Appl Biomater 95B: 184–191, 2010.

Key Words: polyvinyl alcohol, gelatin, PVA/GE, electrospinning, nanofiber, tissue engineering

Correspondence to: B.-T. Lee; e-mail: [email protected] Contract grant sponsor: National Research Foundation of Korea (NRF); contract grant number: 2009—0092808

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ORIGINAL RESEARCH REPORT

The main purpose of this study was to develop a nanofiber system that could be used in biomedical applications, such as controlled drug delivery, tissue engineering, or orthopedic applications. In this study, PVA/GE biocomposite scaffolds prepared by electrospinning were evaluated. The morphology of these hybrid scaffolds were characterized by SEM. The miscibility of the PVA/GE blend fibers was examined by differential scanning calorimetry (DSC), and X-ray diffraction (XRD) was used to determine the crystallinity of the membrane. In addition, its mechanical properties were investigated. MATERIALS AND METHODS

Materials PVA with 99þ% hydrolyzed and number average Mw 115,000 g/mol was obtained from Aldrich Chemical Co (USA). Polymers of GE Type A (Approx. 300 Bloom, Sigma, St.Louis, MO) from porcine skin were obtained in powder form. Acetic acid (CH3COOH, glacial, 99.0%) was purchased from Duksan Pure Chemical Co., Korea. Preparation of polymer solutions Aqueous PVA solutions (12 wt %) were dissolved in deionized water at 80 C with constant stirring for at least 3 h. In a solvent consisting of deionized water:acetic acid ¼ 1:9, 12 w/v% GE solutions were prepared at room temperature under gentle stirring for 30 min.15 The GE solution was added into the PVA solution with specific volumes to obtain the PVA–GE solutions (PVA/GE ¼ 10/0, 8/2, 6/4, 2/8, 0/10, weight ratio). The mixture was the stirred for an additional 15 min before electrospinning. Electrospinning setting The electrospinning (eS-robot, Electrospinning/Spray system) solutions were placed into a 10-mL syringe fitted to a needle with a tip diameter of 25 gauges (inner diameter 0.25 mm), a syringe pump (lure-lock type, Korea) for controlled feed rates, and a grounded cylindrical stainless steel mandrel as collector of the mat. The electrospinning voltage was supplied directly by a high DC voltage power supply (NNC-30 kilovolts-2 mA portable type, Korea). Characterization Morphology analysis. The morphologies of the electrospun fibers and fiber diameter were examined by scanning electron microscopy (SEM, JSM-7401F). At least 10 different positions on the fiber mat were tested to measure the diameter of the electrospun fibers. A small section of the fiber mat was placed on the SEM sample holder and sputter coated with platinum (Cressington 108 Auto). Accelerating voltage of 15 kV was employed to take the SEM images. Differential scanning calorimetry DSC measurement (METTLER TOLEDO KOREA—DSC822e) was used with a sample weight of 3–5 mg under a nitrogen atmosphere with a scanning speed of 10 C/min. The samples were heated from 0 to 250 C at a rate of 10 C/min.

X-ray diffraction The raw materials and the membrane were subjected to XRD (Rigaku, D/MAX—2500 V) using CuKa radiation generated at 40 kV and 200 mA. The diffraction angle was varied from 10.00 to 80.00 ; 2y. Mechanical characterization The mechanical properties of the electrospun fibrous membranes were determined using a universal testing machine (R&B UNITECH–T). All samples were prepared in the form of a rectangular shape with dimensions of width length ¼ 4 mm 20 mm from the electrospun fibrous membranes. The thicknesses of the samples were measured with a digital micrometer that had a precision of 1 lm. The frame was cut into rectangular pieces along the vertical lines. The gauge length (20 mm) of the electrospun fibers was determined by the gap between the parallel strips of the frame. The cardboard partitions were cut along the discontinuous lines (Figure 9) before the fiber was stretched. The measurements were taken with a 500 kgf load cell. Loaddeformation data were recorded at a deforming speed of 0.5 mm/s, and the stress–strain curve of the nanofibrous structure was constructed from the load-deformation curve. RESULTS

Morphology of electrospun membranes Figure 1 showed SEM images of PVA, GE, and PVA/GE nanofibers prepared from electrospinning at various PVA/GE ratios. The applied voltage and tip-to-collector distance (TCD) were fixed at 22 kV and 10 cm, respectively. The diameters of the resulting fibers ranged from 50 to 150 nm. The diameter of the electrospun fiber was found to increase as the ratio of PVA increased. The average diameter of the nanofibers is shown in Table I. The GE fibers had the smallest diameter and the highest tensile strength, whereas the PVA fiber had the lowest strength and largest diameter. The tensile and yield strength were shown to decrease with an increase in fiber diameter and the strain was found to increase with an increase in fiber diameter. Figure 2 shows the effects of the applied voltage on the morphological changes of electrospun PVA/GE ¼ 2/8 at a TCD of 10 cm. Based on these results, the optimum applied voltage during the electrospinning of PVA/GE blends was determined to be 22 kV. At the different applied voltages used in this study, the average fiber diameter was determined to be 90 6 20 nm. The dependence of the fiber diameter distribution for the PVA/GE blend as a function of TCD is shown in Figure 3. As is evident from Figure 3, the fiber diameter of the nanofiber increased from 80 to 350 nm with increasing TCD from 7 to 20 cm. Differential scanning calorimetry DSC investigations are widely used to examine the miscibility of polymer blends by measuring the thermal properties. Figure 4 shows the DSC thermograms of PVA, GE, and PVA/ GE blend nanofibers at various PVA/GE ratios prepared by electrospinning. One peak at 228 C was observed in the

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FIGURE 1. SEM images of PVA, GE, and their blend samples at various PVA/GE ratios of (a) 0/10, (b) 2/8, (c) 6/4, (d) 8/2, and (e) 10/0.

DSC thermogram of PVA nanofibers, which was attributed to the melting temperature of PVA. The endothermic peak of GE nanofibers appeared at 95.8 C. Two peaks were observed in the DSC scans of the blend membranes, where

one corresponded to PVA and the other GE. For the as-spun samples, the Tg and Tm values of the PVA/GE blends were not that much different from the values obtained for GE and PVA alone, irrespective of blend composition.

TABLE I. Properties of PVA/GE Electrospun Fibrous Membranes Ratio of PVA/GE

Average Diameter of Fibersa (nm)

0/10 2/8 6/4 8/2 10/0 a b

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50 90 120 130 150

6 6 6 6 6

10 10 20 10 20

Ultimate Strengtha (MPa) 3.70 3.55 3.10 1.20 0.85

6 6 6 6 6

0.50 0.50 0.80 0.40 0.60

Straina,b (%) 15.0 19.0 23.0 30.0 98.0

6 6 6 6 6

3.0 3.0 5.0 5.0 5.0

Yield Strengtha (MPa)

Tensile Toughness (kJ/m2)

6 6 6 6 6

39.0 52.4 54.4 59.2 64.0

3.20 3.00 2.50 0.90 0.50

0.05 0.08 0.02 0.05 0.06

All data are expressed as mean 6 standard deviation. At ultimate strength.

LINH ET AL.

FABRICATION AND MATERIAL PROPERTIES OF PVA/GE NANOFIBER COMPOSITES


FIGURE 2. SEM images of PVA/GE blend at a ratio of 2/8 with applied voltages of (a) 18 kV, (b) 20 kV, (c) 22 kV, and (d) 24 kV at a tip–target distance of 10 cm.

FIGURE 3. SEM images of PVA/GE blend at a ratio of 2/8 showing the variation in fiber diameter distribution with tip–target distance at (a) 7 cm, (b) 10 cm, (c) 15 cm, and (d) 20 cm at 22 kV.


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FIGURE 6. Stress–strain curves of electrospun PVA, GE, and PVA/GE at various ratios. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 4. DSC thermograms of electrospun PVA, GE, and their blends. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

X-ray diffraction The PVA and GE peaks in the XRD patterns of PVA, GE, and PVA/GE blend nanofibers (Figure 5) were around 19 2y and 22 2y, respectively and had intensities of 15,000 and 5000, respectively. PVA nanofibers of higher molecular weight had a superior crystalline property. The XRD pattern of the membranes revealed a prominent peak at around 22 2y that had an intensity of 7000, 7500, and 10,000 for PVA/GE ¼ 2/8, 6/4, and 8/2, respectively. As shown in Figure 5, the GE nanofibers gave a XRD pattern that was typical of a GE crystalline structure, which originates from the a-helix and triplehelical structure. Moreover, the XRD pattern showed that the crystalline structure of the PVA/GE nanofibers composite was not destroyed in acetic acid.

FIGURE 5. X-ray diffraction pattern of electrospun PVA, GE, and their blends. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Mechanical properties In this work, the mechanical properties of the scaffolds were evaluated using methods that have been commonly used to quantitatively measure the tensile strength of thin membranes. In order to use the as-fabricated fibers in the tensile tests, a method to handle and align the ultrafine fibers is required. The specimens were prepared as shown in Figure 9. At least five samples were stretched to failure for each type of electrospun fibrous membrane. The tensile strength and strain at the ultimate strength of the electrospun PVA/GE composites with various ratios are shown in Table I. The stress–strain curves of the electrospun fibrous membranes of PVA, GE, and the PVA/GE blends are shown in Figure 6. The tensile strength of the PVA/GE blends increased as the concentration of GE increased. The variation in tensile strength of PVA, GE, and their blends is given in Figure 7. The results of these experiments demonstrated that the incorporation of PVA and GE resulted in increased stress and, in general, the tensile strength decreased as the concentration of GE in PVA/GE blends was decreased. The interfacial adhesion in the biocomposites was apparent from

FIGURE 7. Variation of tensile strength of PVA, GE, and their blends.



FIGURE 8. SEM photomicrographs of the fracture tensile surfaces of (a) PVA, (b) GE, and (c) PVA/GE ¼ 2/8.

the SEM images of the tensile fracture surfaces of PVA, GE, and their blends, as shown in Figure 8a–c, respectively. The cross-sectional SEM image clearly shows the gradual break of PVA, which resulted from its elastic behavior, and an abrupt break of GE as well as the PVA/GE blend, which was because of their hardness and brittleness. DISCUSSION

The electrospinning conditions were optimized to produce a thinner and more uniform PVA/GE nanofiber. The optimized conditions were determined by examining the effect of different electrospinning parameters, including the PVA/GE ratio, applied voltage, and tip to collection distance (TCD) on the resulting PVA/GE nanofiber. Acetic acid was used as the solvent for electrospun GE. With natural polymers, including proteins, successful electrospinning cannot be obtained in a pure water system because of the limited solubility of proteins. Acidic solvents, mainly acetic acid in the case of GE, constitute another possible choice to dissolve proteins. Moreover, acetic acid is beneficial in that it reduces the surface tension of the solvent.15 Therefore, in this study, acetic acid was selected as the solvent for electrospun GE as well as the PVA/GE blends. The diameter of the electrospun fiber was found to increase as the ratio of PVA increased. The surface tension of aqueous PVA solutions exhibited a marked dependence on the degree of hydrolysis of the PVA. A 99þ% hydrolyzed PVA had a high surface tension, which resulted in an increase in the viscosity of the dope solution; thus, a higher

voltage was needed to initiate the Taylor cone and the fiber jets.20 In addition, when the ratio of PVA was increased the conductivity of the dope solution decreased, the surface charge densities of the jet decreased, and the electrostatic repulsion forces of the charged jet was reduced. Thus, the diameter of the electrospun nanofibers tended to increase. The mechanical properties of samples are directly influenced by the fiber diameter. As the fiber diameter narrows, the mechanical strength increases. However, the fiber diameter of materials is dictated by their blend composition. Therefore, the mechanical strength of samples depends on the diameter of its fiber, which can be modified by the blend composition. Because of the excellent biocompatibility of GE, the PVA/ GE ¼ 2/8 biocomposite, which had the highest GE composition, was investigated further. Electrospinning conditions such as the voltage and TCD for PVA/GE ¼ 2/8 were evaluated to determine the optimum conditions for creating a good candidate system for applications in bone tissue engineering and drug delivery systems. Moreover, our experiments revealed that the applied voltages did not critically influence the electrospun fiber morphology of PVA/GE fiber blends. As shown in the SEM images in Figure 2, the fiber diameters did not differ when electrospinning was carried out at voltages ranging from 18 to 24 kV. Since the average diameter of the individual fibers that were collected in grounded cylindrical stainless steel mandrel were shown to be dependent on the distance between


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FIGURE 9. Image of the paper frame including electrospun mat (1), glue (2), double-sided tape (3), and the grip (4) used to prepare the tensile specimens from the electrospun mats.

the capillary tip and collector,5,21 we investigated the effects of the TCD on the diameter of the fiber. A series of experiments were carried out at a fixed applied voltage when the tip to target distance was varied. The formation of uniform fibers was obtained due to the high conductivity of acetic acid, which was used as the solvent for the PVA/GE blends. From these results, the optimum conditions for electrospinning of PVA/GE blends were determined. In this study, a PVA/GE nanofiber system was developed as a biodegradable scaffold for tissue engineering; thus, the biodegradability of the system had to be estimated in physiological environments. The degradation behavior of PVA is usually influenced by molecular weight, polydispersity, structure, and environmental conditions,22 whereas the main limitation of GE for the preparation of tissue substitutes is its rapid dissolution in aqueous environments, which leads to fast degradation of grafts at body temperature.23 Additionally, the extent of biodegradation seems to be higher in GE since it has lower crystalline phases when in GE than when in PVA, resulting in a higher susceptibility to degradation. Therefore, the degradation time for the PVA/GE may increase when the content of GE in the PVA/ GE composites was increased under the same degradation conditions. On the other hand, the rate of biodegradation increased with an increase in the GE content in the PVA/GE matrix. From Figure 4, the Tg and Tm values of the PVA/GE blends were not that much different from the values obtained for GE and PVA alone, irrespective of blend compo-

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sition. These results clearly show that the blend fibers of PVA and GE were immiscible in the as-spun state because of the limited solubility of one in the other. As can be seen from these experiments, it is difficult to form miscible polymer blends based on 99þ% hydrolyzed PVA because of the high surface tension and high viscosity. Because of the immiscibility of PVA and GE, the GE component of the PVA/ GE scaffold was gradually dissolved during cell culture, which created more space for cell migration. To examine the effect of the crystallinity of PVA, GE, and PVA/GE blend nanofibers, XRD patterns were investigated. From these results in Figure 5 we can infer that the crystallinity of the membrane was mainly because of GE rather than PVA. The 99þ% hydrolyzed PVA had a superior crystalline property, and the tensile strength of the electrospun PVA membranes may not have changed significantly during in vitro degradation. Accordingly, the XRD of PVA did not significantly change prior to failure. The degradation of GE occurred through dissolution in physiological environments so the mechanical property of GE may have decreased remarkably, although Figure 5 showed the XRD pattern of a GE crystalline structure. The effect of GE concentration on the mechanical properties of PVA/GE composites was investigated. The stress– strain curves of the electrospun fibrous membranes of PVA, GE, and the PVA/GE blends are shown in Figure 6. This result implies that the toughness of PVA may compensate for the brittle characteristics of GE. When the GE concentration was increased, the stresses accelerated whereas the strains slowly increased. The cross-sectional SEM image clearly shows the gradual break of PVA, which resulted from its elastic behavior, and an abrupt break of GE as well as the PVA/GE blend, which was because of their hardness and brittleness. As shown in Table I, the GE fibers had high strength but limited strain values, therefore, the GE nanofibers were not tough. In addition, the PVA fibers had a high toughness value, which is typical of plastic material. The electrospun PVA membrane might have maintained its elongation-at-break, while the elongation-at-break of the GE and PVA/GE composite membranes may have decreased during the first stage of degradation. PVA has been shown to degrade predominantly via chemical hydrolysis and molecular weight. Change of mechanical properties of electrospun scaffolds during degradation has rarely been reported. The ideal tissue-engineering scaffolds must have the required mechanical integrity to maintain the predesigned tissue structure and not be crushed when implanted. Therefore, understanding the mechanical properties and how they change during degradation is very important. The addition of PVA can compensate for the rigid association of GE molecules because of the interaction between the hydroxyl group in PVA and primary amino group in GE through hydrogen bonding. The rigidity of different fractions of GE is independent of the specific structural nature and possibly the molecular weight, above a limiting value. The ability to form extensively hydrogen bonded systems may control the interchain stability of GE. However, the tensile strength of



electrospun GE membranes decreased dramatically because of dissolution in physiological environments. In contrast, the tensile strength of electrospun PVA membranes may not change significantly during in vitro degradation. The water resistance of PVA increased with an increase in the degree of hydrolysis (PVA with 99þ% hydrolyzed in this case). The sudden decrease in tensile strength of composite membranes can occur at the beginning of degradation because of the hydrophilicity of both PVA and GE and may remain constant throughout the degradation period. CONCLUSION

PVA, GE, and PVA/GE blends were fabricated by electrospinning. Optimum electrospinning conditions for PVA/GE were determined through morphological investigation. The effect of processing parameters such as the concentration of GE in the PVA/GE composite, voltage, and tip-target distance was evaluated. The average diameter of the PVA/GE nanofiber slightly increased with an increase in PVA content. By increasing TCD, the PVA/GE nanofiber diameter was increased. However, the applied voltage did not affect the diameter of the PVA/GE nanofiber. The mechanical properties of PVA/GE biocomposites were characterized and optimized. PVA showed improved crystalline structure and thermal stability wherteas GE showed improved tensile strength. These PVA/GE nanofiber composites hold promise for use as scaffolds in tissue engineering applications, especially in bone and drug delivery systems. REFERENCES 1. Andreas G, Joachim HW. Electrospinning: A fascinating method for the preparation of ultrathin fibers. Angew Chem Int Ed 2007; 46:5670–5703. 2. Rujitanaroj P, Pimpha N, Supaphol P. Wound-dressing materials with antibacterial activity from electrospun gelatin fiber mats containing silver nanoparticles. Polymer 2008;49:4723–4732. 3. Bader R, Herzog K, Kao W. A study of diffusion in poly(ethyleneglycol)-gelatin based semi-interpenetrating networks for use in wound healing. Polym Bull 2009;62:381–389. 4. Barhate RS, Ramakrishna S. Nanofibrous filtering media: Filtration problems and solutions from tiny materials. J Membr Sci 2007;296:1–8. 5. Sill TJ, von Recum HA. Electrospinning: Applications in drug delivery and tissue engineering. Biomaterials 2008;29:1989–2006.

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