The bi vitro biodegradation of Bombyx mori silk fibroin was studied by incubating fibers and films with proteolytic enzymes (collagenase type F, a-chymotrypsin type I-S, protease type XXI), for times ranging from 1 to 17 days. The changes in sample weight and degree of polymerization of silk fibers exposed to proteolytic attack were negligible. However, tensile properties were significantly affected, as shown by the drop of strength and elongation as a function of the degradation time. Upon incubation with proteolytic enzymes, silk films exhibited a noticeable decrease of sample weight and degree of polymerization, the extent of which depended on the type of enzyme, on the enzyme-to-substrate ratio, and on the degradation time. Protease was more aggressive than a-chymotrypsin or collagenase. Film fragments resistant to enzymatic degradation were enriched in glycine and alanine. FT-IR measurements showed that the degree of crystallinity of biodegraded films increased. Soluble degradation products of silk films consisted of a range of peptides widely differing in size, deriving from the amorphous sequences of the silk fibroin chains. Biodegraded fibers showed an increase of surface roughness, while films displayed surface cracks and cavities with internal voids separated by fiber-like elements. (C) 2003 Wiley Periodicals, Inc.