Compressible, Thermally Insulating, and Fire Retardant Aerogels through Self-Assembling Silk Fibroin Biopolymers Inside a Silica StructureAn Approach towards 3D Printing of Aerogels

Thanks
to the exceptional materials properties of silica aerogels, this fascinating
highly porous material has found high-performance and real-life applications
in various modern industries. However, a requirement for a broadening
of these applications is based on the further improvement of the aerogel
properties, especially with regard to mechanical strength and postsynthesis
processability with minimum compromise to the other physical properties.
Here, we report an entirely novel, simple, and aqueous-based synthesis
approach to prepare mechanically robust aerogel hybrids by cogelation
of silk fibroin (SF) biopolymer extracted from silkworm cocoons.
The synthesis is based on sequential processes of acid
catalyzed (physical) cross-linking of the SF biopolymer and simultaneous
polycondensation of tetramethylorthosilicate (TMOS) in the presence
of 5-(trimethoxysilyl)­pentanoic acid (TMSPA) as a coupling agent and
subsequent solvent exchange and supercritical drying. Extensive characterization
by solid-state 1H NMR, 29Si NMR, and 2D 1H–29Si heteronuclear correlation (HETCOR)
MAS NMR spectroscopy as well as various microscopic techniques (SEM,
TEM) and mechanical assessment confirmed the molecular-level homogeneity
of the hybrid nanostructure. The developed silica–SF aerogel
hybrids contained an improved set of material properties, such as
low density (ρb,average = 0.11–0.2 g cm–3), high porosity (∼90%), high specific surface
area (∼400–800 m2 g–1),
and excellent flexibility in compression (up to 80% of strain) with
three orders of magnitude improvement in the Young’s modulus
over that of pristine silica aerogels. In addition, the silica–SF
hybrid aerogels are fire retardant and demonstrated excellent thermal
insulation performance with thermal conductivities (λ) of 0.033–0.039
W m–1 K–1. As a further advantage,
the formulated hybrid silica–SF aerogel showed an excellent
printability in the wet state using a microextrusion-based 3D printing
approach. The printed structures had comparable properties to their
monolith counterparts, improving postsynthesis processing or shaping
of the silica aerogels significantly. Finally, the hybrid silica–SF
aerogels reported here represent significant progress for a mechanically
customized and robust aerogel for multipurpose applications, namely,
as a customized thermal insulation material or as a dual porous open-cell
biomaterial used in regenerative medicine.