Heterodimensional “nebula-cluster” hybrids from MOFs and BP nanosheets: towards multifunctional, flame retardant, and closed-loop recyclable flexible foam

The escalating demand for multispectral stealth and multifunctional integration in lightweight platforms, such as unmanned aerial vehicles (UAVs), presents a formidable challenge for conventional single-band materials. This work proposes a bioinspired “nebula-cluster” heterodimensional design to create a multifunctional sandwich composite for advanced skin applications. By coupling two-dimensional black phosphorus nanosheets (BPNS, acting as “nebulae”) with three-dimensional MOF-derived magnetic carbon clusters (MOF-C, as “star clusters”) and employing chitosan as a green binder, a uniform and robust CS-BPNS@MOF-C coating is integrated onto flexible polyurethane foam (FPUF) via an impregnation-freeze-drying process. The resulting composite achieves exceptional multifunctional synergy: (i) superior broadband electromagnetic wave absorption (effective absorption bandwidth of 4.43 GHz with a minimum reflection loss of −63.78 dB), (ii) rapid self-extinguishing flame retardancy with significant smoke suppression (peak smoke production rate reduced by 52.29%), (iii) high-sensitivity and stable piezoresistive strain sensing (14.6% relative resistance change at 66.7% strain), and (iv) enhanced acoustic-thermal stealth (perfect sound absorption at mid-frequencies and low thermal conductivity). Notably, the heterodimensional synergy ensures that the core radar-absorbing performance remains competitive with state-of-the-art single-purpose absorbers, effectively reconciling traditionally conflicting properties. Furthermore, a sustainable closed-loop recycling strategy based on the acid-responsive dissolution of chitosan enables the recovery of functional fillers and the regeneration of the foam substrate. This work provides a groundbreaking design paradigm and a viable material platform for next-generation intelligent stealth systems.