New methodologies of Silk Proteins processing for advanced applications

Abstract

Silk fibroin is a widely studied material in the context of tissue engineering. Thanks to its versatility and impressive properties, the fields where silk fibroin is used have grown. In particular, silk fibroin has proved to be useful in all the cases when an interface with living tissues is needed (e.g. biophotonics, bioelectronics). As a consequence of this increasing interest, a wide range of protocols have been developed to prepare different materials starting from cocoons. The aim of this thesis is to investigate new strategies to fabricate silk fibroin-based materials, either improving previously developed protocols or proposing new methodologies both with the purpose to overcome certain limitations of current approaches and to propose new areas of application. We choose to work on three topics: the production of patterns using photolithography on a fibroin photoresist films (fibroin photocrosslinkable photoresist, FPP), the production of sponges made from a chemically modified version of the native protein (Methacrylated fibroin, Sil-MA), and the production of a solid bulk resin made starting from the regenerated protein.  In the case of the FPP (and its counterpart made of sericine, SPP) the fabrication of films and pattern was restricted to the use of harsh chemicals. In addition, the resulting material had a roughness that limits its use in optical applications, making the determination of the refractive index (RI) not possible. The novelty of our work consisted in the modification of the original protocol to make it environmentally sustainable and to decrease the roughness in order to use ellipsometry to determine the RI dispersion. The broadly used silk-based sponges can be prepared by several protocols but they all suffer of the same limitations: the sponges are stabilized only by physical crosslinking (the change from the random to the crystalline secondary structure), and there are no clear models that correlate the sponge properties to their composition. We produced a new sponge, chemically crosslinked, whose stability was ensured by the creation, of chemical bonds between the protein chains during an UV curing. This task was accomplished using a simple protocol and a statistical method to model the composition properties relations. The possibility to obtain a bulk, non-porous solid monolith from fibroin (solid-fibroin) has been received attention only in the last few years. This material is produced by a transition from solution to solid through solvent evaporation, a very slow process that takes weeks to be completed. The advantage of this transition is that it occurs at room temperature, allowing the addition of thermally degradable molecules (e.g. enzymes). We were able to optimize a procedure to produce the same material by compression of a silk sponge at high pressure and low temperature. The advantage of this method is the lower amount of time required to produce the material, minutes instead of days.