Preparation of PVP / PLLA Ultrafine Blend Fibers by Electrospinning

The Polyvinylpyrrolidone (PVP) / Poly (L-lactic acid) (PLLA) ultrafine blend fibers have been prepared for the first time by electrospinning. Trichloromethane was found to be the co-solvent for electrospinning. The PVP/PLLA blend solutions in various ratios were studied for electrospinning into ultrafine fibers. The morphology of the fibers was shown by scanning electron microscope (SEM). It was found that the morphology of the fibers became finer with the content of PLLA increasing. To show the molecular interactions, PVP/PLLA fibers were characterized by Fourier transform infrared spectroscopy (FTIR). The spun ultrafine fibers are expected to be used in the native extracellular matrix for tissue engineering.


Introduction
Electropinning is a simple and low-cost method for manufacturing polymer ultrafine fibers (Li et al., 2004).The ultrafine fibers produced by electrospinning method have showed amazing characteristics such as very large surface area-to-volume ratio and high porosity with very small pore size (Park et al., 2004), so they are used in biomedical area, including wound dressings (Kim et al., 2003), drug delivery (Zeng et al., 2003), tissue engineering scaffolds (Zong et al., 2003), and so on.For the tissue engineering scaffold, a highly porous microstructure with interconnected pores and large surface area is conducive to tissue ingrowth.The collected non-woven fibers mats just meet these requirements.The topology of these electrospun scaffolds closely mimics that of native extracellular matrix (ECM) (Li et al., 2005).Many materials, including natural macromolecule (Wnek et al., 2003;Chen et al., 2007), synthetic polymer (Yoshimoto et al., 2003;Yang et al., 2005) and their mixture (Nam et al., 2008), were manufactured into tissue engineering scaffold by electrospinning.Poly (L-lactic acid) (PLLA) was a biocompatible synthetic polymer which was approved by the Food and Drug Administration for specific human clinical applications, such as surgical sutures and some implantable devices, and had eximious mechanical property (Zhang et al., 2004).Its electrospinning product was increasingly used as the scaffold of tissue engineering.However, the further application of PLLA fiber in biomedical materials was hindered by its hydrophobicity.Polyvinylpyrrolidone (PVP) was also a biocompatible synthetic polymer, and was a hydrophilic polymer.So, PVP was a good material to prepare composite material with PLLA for improving the hydrophilicity of PLLA fiber.
In this study, PVP was introduced into PLLA fibers by electrospinning.The hydrophilicity of this blend fiber mat was tested.

Electrospinning
Different weights of PVP (1.0, 0.5 and 0.2 g) and different weights of PLLA (1.0, 1.5 and 1.8 g) were respectively added into 8 ml trichloromethane, the mixtures were stirred for 24 h at room temperature.Then, the spinning solution with different ratios by weight (PVP/PLLA: 1/1, 1/3, and 1/9) were prepared.At room temperature, the spinning solution was placed into a 2 ml glass syringe with the tip of inner diameter of 1 mm.A clamp connected with high voltage power supplier (0 -30 kV) was attached to the glass syringe.As grounded collector, a piece of aluminum foil was placed towards the tip at the distance of 27 cm.The polymer fibers generated from the tip by high voltage flied to the grounded collector and formed the ultrafine fiber mat.The apparatus for the electrospinning experiments was similar to previous report (Gupta et al., 2005).

Instruments
The morphology of the electrospun fibers were observed under a Scanning Electron Microscope (SHIMADZU SSX-550) at an accelerating voltage of 15 kV.FT-IR spectra were recorded on a Nicolet Instruments Research series 5PC Fourier Transform Infrared spectrometer.The contact angle of samples was determined by a contact angle determinator (Powereach JC2000C2).

Results and discussion
Figure 1 (A-C) showed SEM micrographs of the ultrafine fibers which were spun from the PVP/PLLA solutions with different ratios.There were some bead structures in the sample when the weight ratio of PVP and PLLA was 1:1 (Figure 1 (A)).With the increasing content of PLLA, the beads were decreasing and the morphology of fiber became finer (Figure 1 (B, C)).
Figure 2 (A) displayed the Fourier transform infrared spectroscopy (FTIR) spectra of the electrospun pure PVP and pure PLLA fibers.The FTIR spectrum of PLLA fibers depicted characteristic absorption band at 1745 cm −1 , which represent the backbone ester group of PLLA (Figure 2 A (a)).PVP fibers displayed characteristic absorption bands at 1650 cm −1 which represented the tert-amide characteristic absorption band (Figure 2 A (b)). Figure 2 (B) displayed the FTIR spectra of the electrospun PVP/PLLA blend fibers with different ratios.Obviously, with the increasing of PLLA, the relative strength of peak at 1745 cm −1 which belongs to carbonyl group in PLLA was increased, and the relative strength of peak at 1650 cm −1 which represent the tert-amide absorption band of PVP was decreased.The peaks position of these spectra almost did not changed.It could be explained that the molecular interaction between PVP and PLLA was weak.
Figure 3 was the relationship curve between the contact angles of PVP/PLLA blend fibers mat and the content of PLLA in the fiber.It could be seen that the contact angles of PVP/PLLA blend fibers mat was increased with the increasing of PLLA content (the decreasing of PVP content).It could be proved that the PVP content in the fiber was correlated positively with the hydrophilicity of the blend fibers mat.

Conclusion
In this study, we succeeded in preparing PVP/PLLA blended fibers by an electrospinning technique.We found that with the increasing of content of PLLA, the bead structure gradually disappeared and the morphology of fibers became finer.The hydrophilicity of fiber could be adjusted by controlling the content of PVP.It was assumed that the production would have a great potential application in the tissue engineering.