Effect of the Distribution of Fiber Orientation on the Mechanical Properties of Silk Fibroin/Polycaprolactone Nanofiber Mats

In this study, Silk Fibroin (SF)/Polycaprolactone (PCL) composite nanofiber mats were fabricated using an electrostatic spinning technology employing different drum rotation speeds. The morphology and tensile performance of the resulting nanofiber mats were characterized using thermal field emission scanning electron microscopy, multi-layer image fusion technology, pore size distribution analysis and uniaxial and biaxial tensile tests. The analytical results showed that the drum rotation speed had little effect on the diameter of the nanofibers, but it did effect the physical orientation of the nanofibers. When the drum rotating speed was lower than 2.38 m s, the nanofibers were randomly distributed, and there was no obvious mechanical anisotropy in the fiber mats. However, when the rotation speed was as high as 11.88 m s, the nanofibers were fully uniaxially oriented, which provided high mechanical anisotropy to the fiber mats. The distribution of the size of the aperture of the nanofiber mats was related to the distribution in the fiber orientation. If the degree of orientation of the fibrous layer was high, the variation in the individual fibers was low and the pore diameter of fibrous mats was smaller as a result of the centralized fiber distribution. In the case of the SF/PCL composite nanofiber mats fabricated with different drum rotation speeds, the variation in the mechanical performance of the resulting mat in biaxial tension was consistent with its performance in uniaxial tension; however, it was found that the fracture mechanism of fiber mats varied in biaxial tension and uniaxial tension. INTRODUCTION Nanofiber membranes are characterized by high porosity, and large specific surface area which afford specific properties. In recent years, researchers have reported the prospect of applying nanofiber membranes into such areas as reinforced composites, biomedical application, filtering separation and electronic devices [1-5]. The electrospinning technique is regarded as a simple and effective way to prepare nanofibers [6, 7]. In electrospinning, which is an electrostatic fiber fabricating technique, a charged jet flow is first produced by the polymers in high-voltage static electricity; the charged jet flow is then stretched thinner in the static electric field. The associated solvent is then evaporated and the jet flow solidifies, which finally results in the formation of a nanofiber network on the receiver. For common tablet receivers, the fibers in the nanofiber network are distributed in a disordered arrangement, and the irregular structure and the resulting low mechanical performance significantly limits the application of these nanofiber mats [8–10]. In order to prepare nanofiber mats with mechanical performance conforming to the application requirements, various receiving devices for the electrostatic spinning method have been designed by several researchers [11–13]. Compared to these receiving devices, high-speed rotating drums provide simpler and a more effective approach to produce nanofiber mats with the required orientation distribution and mechanical performance [14]. Exhibiting good mechanical strength and possessing abundant basic amino acids, SF has been widely used in tissue regeneration engineering as a bioactive dispersed phase in complex biological systems. Electrospun SF scaffolds provide large surface area, high porosity, and interconnection for cell adhesion and proliferation so they are promising candidates for tissue engineering applications. However, pure SF has a high elastic