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

Abstract

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 H NMR,Si NMR, and 2D H-Si 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 (ρ = 0.11-0.2 g cm), high porosity (∼90%), high specific surface area (∼400-800 m g), 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 K. 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.

Silica-silk fibroin hybrid (bio)aerogels: two-step versus one-step hybridization

“…However, despite the fact given above, the density for ETT-20-SF-30 is rather low (ρ b = 0.18 g cm −3 ) compared to that of sample ETT-20-SF-15 (ρ b = 0.22 g cm −3 ), which can be explained by the steric hindrance of the functional groups of both phases due to the high concentration of surface functionalities and SF. Based on the previous study [11], with quantifying the overall organic moieties of the samples within the monolith by the thermogravimetric studies, a considerably higher organic contribution/ mass fraction of SF in case of ETT-20-SF-15 was also observed.…”

“…6), the compressibility, as well as the maximum compressive strength that is tolerated by the monoliths without structural failure, depends on the SF content in relation to the amount of silane, as well as the presence of coupling agents in the monolith. Compared to pure SF aerogels-AeroSF [11] (obtained by one-step acid catalyzed reaction), which is compressible up to 70% of strain, the aerogel hybrids obtained by the two-step sol-gel reaction show a lower compressibility (e.g., compressible up to 45% of strain). This can easily be explained by the higher inorganic content in these gels.…”

“…For the one-step solgel process, the reaction occurred entirely in one-step, although the gelation time has significantly increased from In both approaches, a coupling agent containing a carboxylic acid functionality, 5-(trimethoxysilyl) pentanoic acid (TMSPA), is co-condensed to give a homogeneous inorganic-organic hybrid silica network. This coupling agent is added to the sol-gel system for two reasons: on one hand, its long hydrocarbon chain increases the flexibility and reduces the fragility of the silica network and, on the other, the carboxylic acid moiety results in stronger interactions between both phases by various possible covalent and non-covalent interaction scenarios [11]. In addition, the acidic character of the coupling agent assists the concomitant hydrolysis and polycondensation of the alkoxysilanes with the physical gelation of the SF phase.…”

“…Silk fibroin (SF), a protein-based biopolymer extracted from B. mori silkworm cocoon has recently been utilized in the development of various functional materials, as this biopolymer has several assets in terms of biocompatibility, versatility in the processing into various shapes combined with a well-studied chemistry [10]. The concept of preparation of silk fibroin as aerogels and its utilization to mechanically support the delicate structure of silica and silsesquioxane aerogels is a new trend and has only recently been reported by our group [11,12]. In this regards, ultralight silk fibroin aerogels (so-called AeroSF) (ρ b = 0.02 g cm −3 ) have been synthesized.…”

“…Additive manufacturing approaches, such as 3D printing, have been successfully utilized to prepare silica-silk fibroin aerogels with a deliberately adjusted shape and morphology, thus opening new technical fields, e.g., as multi-pore biomaterials or customized anisotropic thermal insulators. For all hybrid systems, a carboxylic acid functionalized coupling agent, 5-(trimethoxysilyl) pentanoic acid (TMSPA), allowed a homogeneous chemical mixing of both phases at the molecular scale [11,12]. Hybridization of the silk fibroin with simple tetrafunctional silanes in the presence of an acid catalyst led to homogeneous mixing of both phases in a simple single step processing without any significant phase separation.…”