Regenerated Silk Fibroin Materials: Preparation, Structure And Properties

In recent years, exhaustion of petroleum sources and high price of crude oil have stimulated the fast growth of natural polymer industries. Our country is not only the home of Bombyx mori silkworm but also the world's largest silk producer with annual production of 100,000 tons of cocoon silk. So the exploration of silk and its protein (fibroin) for non-textile applications has attracted considerable interests from material researchers.Silk fibroin, as a structural protein without biological activity, shows unique advantages including low cost, renewability, biocompatibility, biodegradability and acceptable specific strength. A standardized preparation process of regenerated silk fibroin (RSF) from silkworm cocoon silk has been well developed in the past decades. Therefore, RSF was studied as a biomaterial rather than a protein in the present works. We investigated the various structures of RSF especially the relationship between its fine structures and dynamic mechanical behaviors. Then we used the knowledge obtained on RSF's structures to guide our creative preparations of new-typed RSF materials, i.e. all-fibroin composite and RSF nanosphere.At first, for the insight of the complex structural transition of silk fibroin after several processes including ethanol treatment, heating and post stretching, the well studied air-cast RSF film was used here as a model sample. Dynamic mechanical analysis (DMA) technique was applied to investigate the correlation between dynamic mechanical behaviors of RSF film and its structural transition which was characterized by near-infrared (NIR) spectroscopy. The tanδpeak split of DMA demonstrated that the apparent glass transition of air-cast RSF film (at around 177℃) was the contribution of both uncrystallizable and crystallizable segments, which were gradually separated from each other and produced both disordered andβ-sheet domains during ethanol treatment. Furthermore, DMA and NIR were also employed to investigate the effects of heating and post stretching on RSF film. The results showed that glass transition temperature of permanently disordered domains was increased from 155℃to 190℃and 205℃, respectively. It suggested the forming of more ordered structure in RSF after either process, which was further examined by applying other RSF material, i.e. RSF scaffold, for DMA test under compression mode. Bombyx mori silk possesses remarkable mechanical properties, while RSF film shows a brittle mechanical behavior. Base on the work in the first part, we believed that the fine structures between RSF materials and silk were fairly different, so directly composing silk fiber with RSF matrix could not help the improvement of mechanical properties of RSF matrix itself. Therefore, a proper surface modification of silk fiber is necessary. We pretreated uniaxially-loaded silk fibers with LiBr aqueous solution, then impregnated them in a high concentration of RSF aqueous solution, and finally obtained a high performance all-fibroin composite via a solution casting process. Both effects of LiBr pretreatment and methanol post treatment on interface adhesion between silk fiber and RSF matrix were further investigated. The results demonstrated that, under optimized 6 mol L-1 LiBr pretreatment on aligned silk fibers for 10 min, the produced silk fiber/fibroin composite with 25% fibers showed a breaking stress of 151 MPa, a breaking elongation of 27.1% and a breaking energy of 23.2 kJ kg-1 in the direction parallel to fiber array, and a compression modulus of 1.1 GPa in the perpendicular direction, much better than those of pure RSF matrix (air-cast film). After methanol post treatment, the interface adhesion of such silk fiber/fibroin composites was further enhanced as compared to untreated ones, the transverse breaking stress and breaking elongation were 46 MPa and 2.4%(vs.20 MPa and 1.3%), respectively. So this work may help the exploration of animal silk-based composites for various applications.RSF is an ideal material for drug delivery especially in form of nanosphere, due to its distinctive properties like biocompatibility, biodegradability and nontoxicity. We applied electrospraying technique to well disperse RSF molecules in alcohol solvent (i.e. methanol, ethanol or n-propanol), and at meanwhile used the alcohol to promote the conformation transition of RSF molecules into a water-insolubleβ-sheet structure (SilkⅡ). A kind of RSF nanosphere which could be easily redispersed in water with good stability was successfully produced. By adjusting the processing parameters including RSF concentration, alcohols, flow rate, electrical voltage and receipt distance, we produced such RSF nanospheres with a controllable size of 100-400 nm and a polydispersity index (PDI) below 0.2. Based on that, we further applied the electrosprayed RSF nanosphere to encapsulate model drug chlorpheniramine for in vitro drug release. The results showed that the chlorpheniramine-loaded RSF nanospheres produced in either methanol or ethanol solvent achieved a high drug encapsulation efficiency of 80% and 64%, and a total drug release of 69% and 90% within a week, respectively. Moreover, we attempted electrospraying RSF/Fe3O4 nanosphere for a future application such as magnetic targeting.