Achieving significant mechanical improvement of chitosan aerogel with embedding or bridging structures mediated by size-dependent silk microfibers

Building high-performance aerogels with biomass-derived rather than fossil-derived polymers is an eco-friendlier option given the increasingly serious sustainability issues. Chitosan (CS) aerogels with oriented pore structures exhibit broad application prospects owing to light weight, high porosity, and favorable bioactivity, but the dominating drawback in low mechanical strength greatly hinders their functional advantages. In this study, two types of silk microfibers with similar diameter yet different aspect ratios (1–3 (denoting as SmSF) and 50–100 (denoting as LmSF)) were used as fillers to reinforce CS aerogels prepared by directional freeze casting. The distinction of SmSF and LmSF in size led to their notable variations in distribution pattern, as SmSF embedded within the individual CS lamellae while LmSF traversed throughout the adjacent CS lamellae, which in consequence significantly influence their mechanical reinforcing efficiency. The compressive strength values could be improved from 61.67 kPa (pure CS aerogel) to 82.13 kPa (SmSF/CS aerogel) and 165.03 kPa (LmSF/CS aerogel), respectively, attributing to the transition in deformation mechanisms from a bending- to crumpling-dominated mode. In addition, the embedding or bridging structure could also change the liquid transportation property of CS aerogels. The results of this study demonstrated the feasibility of applying filler-size-mediated strategy for material structural optimization.