Silk fibroin-based smart organohydrogels

Abstract

In the nature, living organisms develop certain vital strategies to increase their survival chance by increasing its adaptation to surround. One of the survival strategies of the living organisms, such as Rana Sylvatica, is the co-existence of hydrophilic and lyophilic components. Cell membrane and anti-freezing proteins are the examples of structures possessing hydrophilic and lyophilic structures together. Cell membrane acts as barrier between inside and outside of the cell and permits entering of the molecules selectively. All vital cellular processes, such as cell-cell recognition occur by the help of amphiphilic cellular membrane. It provides selectivity and mechanical strength to the cell. Antifreeze proteins provide advantage to living organism at which live in sub-zero temperatures. These proteins prevent freezing and provide continuum of cellular processes. By inspiring from the nature, organohydrogels (OHGs) are designed as a new class of polymeric gels. As the name of the OHGs implies it has organic and aqueous parts. Bio-inspired OHGs are the three-dimensional hydrophobic or hydrophilic network inholding aqueous or lyophilic solvent in their three-dimensional networks. The presence of lyophilic part provides mechanical strength and tolerance at extreme temperature or moisture conditions while the hydrophilic part provides biocompatibility. With the co-existence of antagonist parts, mechanically strong, tough, stable and biocompatible OHGs can be obtained. OHGs possess advantageous properties of traditional hydrogels and organogels, and their deficiencies are overcome by using them together. Generally, there are two methods used to fabricate OHGs. One of them is one-step solvent displacement method. First, hydrogel or organogel is synthesized, then their solvents are replaced with organic and aqueous solvents for hydrogel and organogel, respectively. The other method is two-step methods. At the first step hydrophilic or hydrophobic phase is dispersed in other phase by using surfactant to decrease surface tension between two phases. At the second step, in-situ polymerization of emulsion system occurs. Silk fibroin (SF) is a major structural protein of silk. As a biocompatible, mechanically strong, biodegradable and processable biopolymer, SF is widely used in biomedical applications in the form of polymeric gels, beads or nanoparticles. Biocompatibility, biodegradability and ease of processability of SF is highly foreseeable because it is naturally-derived protein. Mechanical strength of SF can be attributed to its structure. It has hydrophobic amino acids-dominated heavy chain and hydrophilic amino acids-dominated light chain. Heavy chain provides mechanical stability while light chain provides toughness to the SF. Hydrophilic parts are found as scattered between repeating heavy chain units, so SF is amphiphilic in nature and similar to the other proteins. Additionally, SF has primary, secondary and tertiary structures as an obligation for proteins. α-helix, β-sheet, β-turn and random coil conformations are secondary structures of SF. One of the most important secondary structures of SF is β-sheet structures. It forms by interaction of hydrophobic repeating domains via non-covalent and weak interactions. β-sheet structures cause formation of crystalline β-sheet domains and these structures act as physical crosslinking points and facilitates sol-gel transition in SF. β-sheet formation can be triggered with solvent, temperature or pH. In this thesis study, SF-based shape-memory OHGs that were mechanically strong, tough and biocompatible were created. Lyophilic and hydrophilic components of OHGs were prepared starting from n-octadecyl acrylate (C18A) monomer and aqueous SF solution, respectively.