Equipping chitosan cryogels with a silicon armor for integrated elasticity, hydrophobicity, and flame retardancy.

Chitosan cryogels, characterized by highly porous and lightweight architectures, offer the advantages of renewability and excellent biocompatibility, but their inherent brittleness, poor moisture resistance, and limited thermal stability severely restrict practical applications. Herein, a "silicon armor" strategy was developed by employing hyperbranched epoxy silicone resin (HESR) as a chemical bridge to construct a covalently bonded silicon-containing hybrid shell on the chitosan framework through epoxy-amine ring-opening reactions. Benefiting from the abundant reactive epoxy groups, the hybrid shell further enabled the multiscale integration of functional components. An inner-outer dual‑phosphorus flame-retardant architecture was established by incorporating melamine polyphosphate (MPP) and diethyl phosphoramidate (DEPA), while the Si-O-Si network simultaneously contributed to structural reinforcement and thermal stability. In addition, polyetheramine flexible segments introduced entropy elasticity into the hybrid network, effectively alleviating the intrinsic brittleness of the rigid silicon framework and improving its deformation recovery capability. Subsequent hydrophobic modification further reduced the surface energy and significantly enhanced the moisture resistance and environmental stability of the cryogel. As a result, the obtained CS-Si cryogel integrates efficient flame retardancy, excellent elasticity, mechanical robustness, thermal insulation, and hydrophobic protection into a single biomass-derived system, providing a simple, efficient, and versatile strategy for the design of multifunctional sustainable cryogel materials.