The Biodegradation Behavior Of Silk Fibroin Materials

Silk fibroin has good biocompatibility. However, being used as tissue engineering scaffold, biodegradability is obviously important. Therefore, it is necessary for making a research on the factors which influence the degradation behavior of silk fibroin biomaterials and the ways of controlling over the rate of degradation. In this paper, we focused on the human body environment of dermal tissue, choosing collagenase IA which is typical enzyme in dermal tissue as proteolytic enzyme to degrade silk fibroin materials, and combining with in vivo experiment. We systematically investigated how the species of raw materials, molecular weight and degree of cross-linking affected the biodegradability of silk fibroin materials. The insights gained in this study can serve as a guide for adjusting the biodegradability of silk fibroin material used as skin regeneration scaffolds.Firstly, the silk fibroin fibers of Bombyx mori, Antheraea pernyi and Antheraea yamamai were dissolved under the same conditions. Three-dimensional porous silk fibroin scaffolds were prepared by freeze-drying method. Through analysis of secondary structures, the relative content of random coil in the B. mori silk fibroin porous scaffolds was highest, while in the A. pernyi silk fibroin porous scaffolds was least. The each group was degraded in vitro with collagenase IA (pH 7.4,37℃)and implanted into the skin defect in sprague-dawley (SD) rat. The results showed that the degradation rate of porous scaffolds ordered as B. mori silk fibroin > A. yamamai silk fibroin > A. pernyi silk fibroin. After degraded in vitro, the content of free amino acid and low molecular weight proteins in the degradation products of Bombyx mori was more than the other two. This indicated that the degradation of silk fibroin excreted from different species of silkworms exhibited significantly difference between each other.Secondly, the B. mori silk fibroin was dissolved with LiBr, Ca(NO3)2 and CaCl2, the content of low molecular weight proteins in the silk fibroin followed the order CaCl2 > Ca(NO3)2 > LiBr. The silk fibroin porous materials were degradaed in vitro with collagenase IA. The results showed that the degradation rate of silk fibroin scaffolds and the content of free amino acid increased with the increase of low molecular weight proteins content in silk fibroin. The molecular weight had obvious impact on the degradation of silk fibroin materials. The materials were implanted into the skin defect in sprague-dawley (SD) rat, and the results closely corresponded to that in vitro degradation. Therefore, the degradation behaviors of the silk fibroin scaffolds can be regulated by changing molecular weight.Additionally, Genipin (GP) was used as crosslinking reagent to prepare silk fibroin films with different degree of cross-linking. The content ofβ-sheet structure increased with increasing degree of cross-linking. The degradation in vitro indicated, as the degree of cross-linking increased, the degradation rate slowed down and the content of free amino acid decreased. The results demonstrated that the degree of cross-linking affected the degradation of silk fibroin films significantly. The in vivo degradation of the silk fibron films was carried out by subcutaneous implantation, after 28 days, the silk fibroin films of lower fixation index showed obvious degradation.The research in this paper revaled the relations between the species, molecular weight and crosslinking degree of silk fibroin materials and their biodegradation rate. All the results above could serve as a guide for the choice of raw materials and preparation of skin reconstruction scaffolds, which made the degradation rate of biomaterials match with the rate of tissue regeneration.