SCIENCE CHINA Physics, Mechanics & Astronomy • Article •
July 2012 Vol.55 No.7: 1189–1193
Special Topic: Nanotechnology for Bio/Energy Applications
doi: 10.1007/s11433-012-4786-6
Fabrication and characterization of electrospun biocompatible PU/PEGMA hybrid nanofibers by in-situ UV photopolymerization WANG HeYun1,2, FENG YaKai1,3*, YUAN WenJie1, ZHAO HaiYang1, FANG ZiChen1, KHAN Musammir1 & GUO JinTang1 1
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China; School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, China; Key Laboratory of Systems Bioengineering, Ministry of Education, Tianjin University, Tianjin 300072, China 2
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Received February 17, 2012; accepted May 16, 2012; published online May 28, 2012
Poly(ethylene glycol) methacrylate (PEGMA) was introduced into a polyurethane (PU) solution in order to prepare hemocompatible electrospun membranes for potential application as small diameter vascular scaffolds. Crosslinked PU/PEGMA hybrid nanofibers were fabricated by a reactive electrospinning process with N,N′-methylenebisacrylamide (MBAm) as crosslinker and benzophenone (BP) as photoinitiator. The photoinduced polymerization and crosslinking reaction took place simultaneously during the electrospinning process. No significant difference in the membrane morphology was found by SEM when PEGMA content was less than 20 wt%. The crosslinked fibrous membranes of PU/PEGMA exhibit higher hydrophilicity and mechanical strength than PU membrane. These nanofibrous membranes are potential substitutes for artificial vascular scaffolds. electrospinning, crosslink, photopolymerization, polyurethane, poly(ethylene glycol) methacrylate. PACS number(s): 81.05.Rm, 81.16.Rf, 83.80.Tc, 82.50.-m Citation:
Wang H Y, Feng Y K, Yuan W J, et al. Fabrication and characterization of electrospun biocompatible PU/PEGMA hybrid nanofibers by in-situ UV photopolymerization. Sci China-Phys Mech Astron, 2012, 55: 11891193, doi: 10.1007/s11433-012-4786-6
1 Introduction Polyurethanes (PUs), with excellent elasticity and mechanical properties, have been widely applied in many fields of materials, especially in the field of biomedical materials. PUs have better biocompatibility than other synthetic polymers because of the micro-phase separated structure [1,2]. However, the poor hydrophilicity and hemocompatibility of PUs become the bottleneck when they are applied in tissue engineering [3]. As we all know, the scaffolds should have appropriate hydrophilicity which could be in favor of cell attachment and proliferation. Poly(ethylene glycol) methacrylate (PEGMA) is one of the most usually used oligo*Corresponding author (email:
[email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2012
mers for preparing or modifying biomaterials [4–7]. However, because of its high solubility in water, it is necessary to attach it to polymer surface or crosslink to form a network. Until now, photo-crosslinked PEG-based materials have been fabricated by spin-coating of photosensitive macromer under UV exposure, but the surface area is limited. If the biocompatible PEGMA is used to modify nanofibers, these materials could be more efficient support for enzyme and cell immobilization because of the high specific surface [8,9]. Electrospinning technology is an inexpensive, effective and simple method to produce nanofibrous membranes from both synthetic and natural polymers. This technology can produce polymer fibers with diameters ranging from a few nanometers to many microns [10]. Electrospun membranes possess high porosity, large surface area and maximum inphys.scichina.com
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terconnectivity of pores with a non-woven nanofibrous structure [11–13]. In this paper, we aim to develop an in-situ method to prepare the crosslinked PU/PEGMA nanofibers by electrospinning technology. The photopolymerization of oligomer is carried out during this process. To the best of my knowledge, the crosslinked PU/PEGMA fibrous membrane by in-situ photopolymerization has not previously been reported. Combining the fiber fabrication process with photochemical reactions enables the polymerization and crosslinking of hydrophilic materials when they are made into the nanofibers [14]. The results indicate the feasibility of electrospinning of a photo-crosslinkable oligomer and provide a foundation for further optimization of engineered fibrous tissues.
2 Experimental details Figure 1 shows the schematic electrospinning in-situ UV photopolymerization process. Firstly, PU (Mn=110000) was dissolved in the mixture solvent of N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) with 1:1 volume ratio to obtain a PU solution of 10 w/v%. A mixture of poly (ethylene glycol) methacrylate (PEGMA, Mn=400), N,N′methylenebisacrylamide (MBAm) as crosslinker and benzophenone (BP) as photoinitiator was prepared with the following weight ratio (wt%): PEGMA:MBAm:BP = 97.5:2.0:0.5. Then mixed solutions of PU/PEGMA with the different weight ratios (PU/PEGMA: 90/10, 80/20, 70/30, 60/40, 50/50) were prepared. The mixed polymer solutions were placed into a 10 mL plastic syringe with a 25 gauge (inner diameter 0.25 mm) capillary tip. The flow rate (0.6 mL/h) of PU/PEGMA solutions was controlled using a syringe pump. The positive lead from a high voltage supply was attached by an alligator clip to the external surface of the metal syringe needle. The electrospinning voltage (18 kV) was supplied. A grounded steel plate located 20 cm away from the tip of the syringe needle was used to collect the nanofibrous membranes. The UV light from a 200 W
Figure 1
Schematic illustration of the electrospinning equipment.
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Hg lamp was irradiated directly onto the jet traveling from the needle to the collector. The nanofiber morphologies were investigated with scanning electron microscopy (SEM). Based on the SEM images, the fiber average diameter and standard deviation were analyzed using an image analysis program (Adobe Photoshop 7.0). The surface chemistry of the membranes was characterized by Fourier transform infrared spectrometer (FTIR). The transmittance of each sample was recorded between 4000 cm1 and 500 cm1 with a resolution of 2 cm1. Mechanical properties of the electrospun membranes were tested with a tensile machine equipped with a 100 N load cell at a cross head speed of 10 mm/min in the ambient environment. The PU/PEGMA membranes were cut into a rectangular shape (10 mm×60 mm) with a gauge length of 30 mm. All the data reported for the tensile modulus, tensile strength, and break at elongation represented average results of six tests and were analyzed statistically by the method of the t test. To evaluate the hydrophilic/hydrophobic properties of the crosslinked PU/PEGMA membranes, we examined the water contact angle. The samples were cut into a rectangular shape (15 mm×8 mm). The contact angle was measured by the sessile drop method using a video contact angle instrument (Kruss EasyDrop goniometer, Germany) at room temperature. The contact angle was calculated with the AutoFAST algorithm of the image analysis software.
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Results and discussion
Because PEGMA is a low molecular weight oligomer, PEGMA content should influence the solution properties and the morphology of electrospun hybrid nanofibers. Figure 2 demonstrates the SEM micrographs of electrospun crosslinked PU/PEGMA fibers. With increasing PEGMA content, the viscosity of electrospinning solutions decreases because of the dilution of PEGMA [15]. The electrospun fibers change from fine fibers to bonded fibers as shown in Figure 2. When PU/PEGMA is 90/10 and 80/20, uniform fibrous scaffolds are formed with average fiber diameters of (622±110) nm and (547±77) nm (Figure 2(g)). However, when the PU/PEGMA weight ratio is 70/30, the electrospun fibers become non-uniform with distortion. Furthermore, many fibers are fused together and some thick fibers form a web-like fiber morphology. The negative effect of the PEGMA on the PU electrospinning behavior might be caused by low chain entanglement, when PU concentration is low in the spinning solution, which further affects fiber diameter. During the reactive electrospinning process, most of the solvent in the fibers is volatilized, but PEGMA mainly remains on the surface of the fibers. When PEGMA concentration was low, such as 10 wt%, UV photopolymerization happened instantly inside and on the surface of elec-
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Figure 2 SEM micrographs and fiber diameters of crosslinked electrospun PU/PEGMA fibrous scaffolds with different PU/PEGMA weight ratios: (a) 100/0, (b) 90/10, (c) 80/20, (d) 70/30, (e) 60/40, (f) 50/50, (g) average fiber diameters of crosslinked electrospun PU/PEGMA fibrous scaffolds.
trospun fibers. When PEGMA concentration was above 20 wt%, PEGMA took part in the formation of fibers during the electrospinning process, also adhered onto the fiber surface. Thus, the fibers distorted and even fused to form very thick fibers. Figure 3 demonstrates the FTIR spectra of PU, oligomer PEGMA and hybrid PU/PEGMA membranes. The peaks at 2952 and 2853 cm1 correspond to the anti-symmetric and symmetrical stretching vibration of -CH2- in PU membrane. The absorption peaks at 1731 and 1521 cm1 are responsible for the carbonyl and C—N bonds, respectively. The peak at 1468 cm1 is a symmetry bending vibration of -CH2-. The band at 1250 cm1 is assigned to the ester C—O—C stretching in the polycarbonate segment. The characteristic peaks (1108, 943, and 845 cm1) of PEGMA are clearly observed in the hybrid PU/PEGMA membrane. Particularly, characteristic peaks of the ether bond at 848 and 1108 cm1 strongly confirm that hybrid PU/PEGMA membrane is formed. Figure 3(b) shows the FTIR spectra of electrospun PU/PEGMA membranes with different weight ratios of PEGMA. The C==C bond at 1638 cm1 disappears completely, which indicates the acrylate double bonds have polymerized during the UV irradiation process. In the reactive electrospinning process, PEGMA polymerizes with crosslinker (MBAm) to generate a network
Figure 3 ATR-FTIR of macromonomer PEGMA, electrospun PU scaffold and crosslinked electrospun PU/PEGMA scaffold. (a) Wavenumber: 500–4000 cm1 and (b) wavenumber: 1600–1800 cm1.
structure, where PU macromolecules penetrate to form a semi-interpenetrating network (SIPN). The mechanical properties of the fibrous SIPN membranes were studied by tensile tests [16]. The stress-strain tensile curves of the membranes with the different weight ratio of PU/PEGMA are shown in Figure 4 and the data are summarized in Table 1. Without PEGMA, the electrospun PU membrane has an elastic modulus of (2.83±0.11) MPa, a tensile strength of (2.34±0.17) MPa, and an elongation at break of 110±32%. The PU/PEGMA membranes demonstrate mechanical behavior like rubber. The stress-strain curves show linear elasticity as their intrinsic material property. Generally, the mechanical properties of the electrospun membranes depend on the inherent material character and geometric arrangement, which contributes to the tangle effect of fibers [17]. Hence, PEGMA and MBAm form the netpoints inner PU fibers through photopolymerization and enhance the fiber properties. When the weight ratio of PU/PEGMA is varied from 90/10 to 50/50, the tensile strength of crosslinked PU/PEGMA membrane and the elongation at break change from (6.34±0.72) MPa and 285±27% to 11.59±0.87 and
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Mechanical properties of the crosslinked PU/PEGMA hybrid membranes Sample ID
PEGMA content (wt%)
Tensile strength (MPa)
Elongation at break (%)
Young’s Modulus (MPa)
S-1
0
2.34±0.17
110±32
2.83±0.11
S-2
10
6.34±0.72
285±27
1.51±0.12
S-3
20
6.47±0.19
208±11
2.11±0.09
S-4
30
11.01±0.34
259±13
1.16±0.11
S-5
40
8.28±0.24
181±9
3.56±0.13
S-6
50
11.59±0.87
239±19
2.98±0.21
Figure 4 (Color online) Typical stress-strain tensile curves of the crosslinked electrospun PU/PEGMA membranes with the different weight ratios of PU/ PEGMA.
239±19%, respectively. Therefore, the in-situ photopolymerization of PEGMA oligomer can enhance the tensile properties significantly. Hydrophilicity is one of the most important factors that affect the compatibility of biomaterials [18]. The adhesion and growth of cells on a surface are considered to be strongly influenced by the balance of hydrophilicity/hydrophobicity, frequently described as wettability [18–20]. Many studies have demonstrated that cells adhere, spread and grow more easily on moderately hydrophilic substrates than on hydrophobic or very hydrophilic ones [19,20]. For investigating the effect of PEGMA content on the wettability of the crosslinked PU/PEGMA nanofibrous scaffolds, the water contact angle was performed and illustrated in Figure 5. The pure PU scaffold has a high contact angle value of 118.8°±7.8°. The contact angle of PU/ PEGMA nanofibrous scaffolds decreases with the increase of PEGMA weight ratio. When the weight ratio of PEGMA is 50%, the contact angle is only 20.2°±2.3°. The addition of PEGMA improves the hydrophilicity of hybrid PU/ PEGMA scaffolds because of the hydrophilicity of PEGMA.
Figure 5 Water contact angle of the crosslinked PU/PEGMA fibrous scaffolds with the different weight ratios of PU/PEGMA.
by the reactive electrospinning method. The hybrid PU/PEGMA (90/10, 80/20) uniform fibrous membranes are obtained with average fiber diameters of (622±110) nm and (547±77) nm. PEGMA can improve the mechanical properties and hydrophilicity of the electrospun PU membranes effectively. These nanofibrous membranes are potential substitutes for artificial vascular scaffolds. This work has been financially supported by the Program for New Century Excellent Talents in University “NCET”, Ministry of Education of China, and the International Cooperation from Ministry of Science and Technology of China (Grant No. 2008DFA51170). 1
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Hydrophilic crosslinked PU/PEGMA nanofibrous membranes are fabricated directly from PEGMA and PU solutions
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