Effect of structural changes on the surface characteristics of regenerated and graft copolymerized silk fibroin films.

Abstract

Silk fibroin (SF) and polyacrylic acid (PAA)-grafted silk fibroin were dissolved in calcium chloride/ethanol/water mixture (1/2/8 in mole ratio) at 70°C for 4 hrs. The dissolved silk fibroin and SF-g-PAA polymer were regenerated by casting the dialyzed solutions into the films, respectively. The freshly prepared silk fibroin film was soluble in water and mainly consisted of random coil structure. By treating with aqueous solutions of organic solvents, such as formic acid, trichloroacetic acid, and methanol, the silk fibroin was denatured into insoluble β-pleated sheet structure. The contact angles of silk fibroin and SF-g-PAA films were measured after treating the films with aqueous methanol solution. The surface tensions of the films were calculated in three dimensions consisting of dispersion, polar, and hydrogen bond components from the contact angle data by using determinant method. As IR crystallinity indices of silk fibroin films treated with solvents increased, the total surface tension, polar, and especially, hydrogen bond components increased linearly. However, the dispersion component was nearly independent of the IR crystallinity index of the film. On the other hand, in SF-g-PAA film, with increasing graft yield the total surface tension, polar and hydrogen bond components increased and the dispersion component decreased.