Silica-Silk Fibroin Aerogels

The project Silica-Silk Fibroin Aerogels aims at synthesizing a novel class of light-weight materials with a unique set of properties, such as high mechanical strength, meso- and macroporosity and biocompatibility. The key idea is to improve the compressive strength as well as the porous structure of organofunctional silica aerogels by forming a homogeneous, covalently bonded, hybrid materials with silk fibroin proteins (from Bombyx Mori). Silk is a multiprocessable polymer with biocompatible and biodegradable properties that is already in use as biomaterial. In this project it serves as an organic scaffold supporting the mechanical properties of the inorganic gel network, while silica can impart bioactivity as an additional functionality to the composite. As a starting point in this project fundamental investigations regarding the functionalization and optimization of the porous nature of the inorganic network are necessary to continue in the second part with detailed investigations on the formation of the silica/silk fibroin hybrid network. The ultimate objective is to combine the excellent material properties of nanostructured silica aerogels with those of silk fibroins for fabricating a novel class of composite scaffolds for bone tissue engineering applications.

Thanks to the exceptional materials properties of silica and polymethylsilsesquioxanexane (PMSQ) aerogels, these fascinating highly porous materials have found advanced applications in various modern industries and life science applications. However, a requirement for a broadening of these applications is based on the further improvement of their properties especially with regard to mechanical strength, surface chemistry, and post- synthesis processability with minimum compromise to the other physical properties. Here, in her Lise Meitner project, Dr. Maleki has reported entirely novel, simple and aqueous based synthesis approaches to prepare mechanically robust aerogel hybrids by co-gelation of silk fibroin (SF) biopolymer, extracted from silkworm cocoons with various organo-silanes. The synthesis is based on the novel one-step sequential processes of acid catalysis (physical) crosslinking of the SF biopolymer and simultaneous polycondensation of tetra, and tri- functional silanes in the presence of an appropriate silane coupling agent and subsequent solvent exchange and supercritical drying. Extensive characterizations with various analytical techniques mainly solid-state MAS NMR spectroscopies as well as various microscopic techniques (SEM, TEM) and mechanical assessment, confirmed the molecular-level homogeneity of the hybrid nanostructure. The developed PMSQ-, silica-SF aerogel hybrids contained an improved set of material properties, such as low density (b, average = 0.08 - 0.2 g cm-3), high porosity (~90-99%), high specific surface area (~ 400-1000 m2 g-1), and excellent flexibility in compression (up to 80% of strain) with more than three-order of magnitude improvement in the Youngs modulus over that of pristine silica and PMSQ aerogels. In addition, most of the hybrid aerogels are fire retardant and demonstrated excellent thermal insulation performance with thermal conductivities () of 0.028-0.039 Wm-1 K-1. Due to the superhydrophobicity and superoleophilicity, the hybrid PMSQ-SF aerogels exhibited excellent potential for continues separation of oil from water. Also, Dr. Maleki, showed an excellent printability in the wet state for hybrid silica- SF aerogel using a micro-extrusion- based 3D printing approach, improving the current challenge of low post-synthesis processing or shaping of the silica aerogels. In addition, with the aim of preparation of an ideal scaffold for bone tissue engineering, Dr. Maleki has improved the set of properties in the silica-SF aerogel with analogizing the sol-gel process with uni-directional freeze casting method. The prepared aerogel scaffolds showed excellent anisotropy in the microstructure with multi-scaled porosity with pore sizes up to 20 microns without any compromising in the mechanical properties. The developed aerogel scaffolds were biocompatible with promising cell attachment features opening the opportunities for using the aerogels as scaffold for regenerative medicine.