Preparation Of Silk Fibroin-based Electrospun Mat And Deposition Of Inorganic Mineral On Silk Fibroin Matrix

Silk protein (silk fibroin, SF) is one of the most extensively studied materials among the natural polymers, and has the broad applications (e.g. medical suture, tissue engineering scaffold, etc.) due to its distinctive properties such as biocompatibility and degradability. The electrospinning technique is a simple and effective method to produce ultra-thin fibers with diameters ranging from several nanometers to submicrons. Electrospinning has been widely explored recently because electrospun fibers are ideal candidates for a wide variety of applications, including high performance filters, delivery carriers, etc. These applications benefit from the large specific surface area and high porosity of the electrospun mat.In order to avoid the use of organic solvents in the electrospinning process and obtain SF electrospun mat with good mechanicl property, the procedures invented by our laboratory, which can prevent the fibroin molecular chain from severe degradation, are applied in this thesis. By adjusting the processing parameters (concentration and electrical conductivity of the fibroin solution, electrical field applied etc.), SF electrospun mats with controllable morphology and satisfied mechanical properties (the apparent stress and strain at break are 11.1±0.7 MPa and 10.2±1.6 %, respectively) are successfully prepared from the SF aqueous solution, which is important in the further application such as matrix of biomineralization.In order to enlarge the application of silk fibroin, in the second part of the thesis, silk-based co-electrospun nanofibers are successfully prepared. PVA/SF, Gelatin/SF and Hydroxyapatite (HAP)/SF double-layered nanofibers (mats) are prepared by co-electrospinning technique based on the achievement of electrospinning SF nanofibers. PVA is a water-soluble polymer, while silk fibroin is insoluble in water after ethanol treatment. When the PVA(core)/SF(shell) double-layered fiber is treated with ethanol and washed with water, PVA(core) can be removed while SF is reserved. Gelatin has good biocompatibility and degradability. However, gelatin aqueous solution can hardly flow at room temperature. And Gelatin is often dissolved in formic acid for electrospinning. If gelatin-formic acid solution and SF aqueous solution are simply mixed for electrospinning, the existance of formic acid may result in the degradation of SF. So co-electrospinning technique is more proper to prepare Gelatin/SF composite. During the co-electrospinning process, Gelatin-formic acid solution and SF aqueous solution are placed in different syringes in order to minimize the osculant time. Gelatin/SF double-layered electrospun mat, which has the good application in the tissue engineering scaffold such as artifical skin and cartilage, is successfully prepared by co-electrospining technique. Hydroxyapatite is the main inorganic component of bone and nano-scaled hydroxyapatite is found to disperse within the collagen matrix. Meanwhile, SF has the similar ability to improve cell attachment and growth as collagen. So HAP/SF composite has a possible further application in bone repairing.The third part of the thesis mainly focuses on inorganic deposition and mineralization on the SF electrospun mat. Biomineralization is the process in which organisms transform organic matter into mineral matter. During biomineralization process, inorganic matter nucleates and grows slowly at the organic/inorganic interface through the modulation of specific organic template, which controls the morphology and ultimate property of the organic/inorganic composite. To some extent, carboxyl groups in SF molecule can influence the deposition of inorganics because there are strong chelation interactions between Ca²⁺ and carboxyl groups. In this thesis, by the alternating preparation of organic and inorganic layers, we successfully fabricate multi-layered SF-matrixed composite, which is similar to the structure of natural nacre, and has the corresponding importance in exploring the natural mineralization process. Furthermore, the existance of SF can alleviate the brittleness of hydroxyapatite by adjusting the proportion of SF and hydroxyapatite during the mineralization process. This advantage has the pratical significance in the field of bone repair. Besides, in order to clarify whether the existance and content of hydroxyapatite in the electrospun mat will influence the biomineralization process, we have researched on the mineralization of hydroxyapatite using electrospun mats of SF, SF/HAP composite and HAP (core)/SF(shell) mat as organic templates, respectively. The results indicate that HAP (core)/SF(shell) mat is more suitable for mineralization process. Finally, we have a preliminary study on the mineralizaiton of calcium carbonate with the organic template of SF electrospun mat. Two systems for mineralization are used in this paper. One system is that ethanol-treated SF electrospun mat is immersed in SF/NaHCO₃/CaCl₂ solution to mineralize, and the result shows that pH value of the solution has some influence on the polymorph of CaCO₃ crystal. The other system for mineralization is to prepare organic/inorganic composite via CO₂ diffusion method. The polymorphs of CaCO₃ crystal are affected by pH value of CaCl₂ solution and the additives in the solution as well.