Chemically Modified Silk Fibroins for Biomedical Applications. Vibrational Studies

There is considerable interest in exploiting natural polymers processing to create high-performing, environmentally friendly polymers for a range of biomedical applications. Silk fibroin is a natural polymer spun by silkworms and spiders, commonly used as a textile fibre. Recent investigations have proposed silk fibroin as a good basic component for new biomedical material development. In particular, silk fibres have been considered as starting material for the preparation of various kinds of medical devices, such as polymer-hydroxyapatite composites for bone regeneration, wire-ropes for the substitution of the anterior cruciate ligament, novel silk-based sutures and protective gauzes for the treatment of skin burns with improved blood compatibility. Silk films, which can be prepared by casting an aqueous silk fibroin solution, are highly attractive for their permeability to oxygen and water vapour. Likewise fibres, films exhibit the ability to support cell adhesion and growth, and their use as scaffolds for skin and bone regeneration has been proposed. Being a protein, fibroin is susceptible to biological degradation and can be chemically modified at side-chain groups thus improving its properties according to the function. Our studies on the degradation behaviour of silk fibres and films exposed to different proteolytic enzymes allow to elucidate the mechanism by which the material interacts with the biological environment, and to characterise the functional properties of the polymer. On the other hand, the possibility of modifying silk fibroin by using a non-hydrolitic enzyme (tyrosinase) and chemical agents (chlorosulphonic acid or metal cations with EDTA or tannic acid) to obtain functionalised biomaterials was studied. Tyrosinase allowed to obtain an intermediate able to graft chitosan onto Bombyx mori silk fibroin; chlorosulphonic acid was used to introduce covalently bound sulphate groups which confer to fibroin anticoagulant and antiviral activity. The metal binding (Ag+, Cu2+ and Co2+), which was enhanced by chemically modifying the fibres with chelating agents able to coordinate metal ions (EDTA or tannic acid), impart antimicrobial activity on fibroin. This review points out the contribution that Raman and IR spectroscopy can give to the knowledge of the structural modifications related to the obtainment of functionalised biomaterials such as the reaction mechanisms, the interactions between molecules and the intermediate which can form.