3D-printed multiscale hierarchical thermoplastic polyurethane/aramid nanofiber structures with enhanced energy absorption via in-Situ foaming technology

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Lightweight energy-absorbing materials with high mechanical strength hold significant promise for engineering applications. In this work, we engineered a multiscale hierarchical system through 3D printing in-situ foaming technology, integrating aramid nanofiber (ANF, nanoscale), thermoplastic polyurethane (TPU) microcellular foams (microscale), and honeycomb macrostructures (macroscale). The incorporated ANF enhance TPU's melt strength to stabilise closed-cell structures while serving as heterogeneous nucleation sites for microcellular architecture regulation. This structural hierarchy enables sequential collapse mechanisms where macroscale honeycomb buckling precedes microscale cell wall yield, achieving coordinated energy absorption–dissipation through progressive deformation modes. The optimised TPU/ANF (1.0 wt%) hierarchical structure demonstrated 16.6% and 53.7% enhancements in specific energy absorption (SEA) relative to TPU/ANF and pure TPU honeycombs, respectively. This approach not only enables precise control over cellular morphology through ANF-mediated foam stabilisation but also expands design possibilities for complex energy-absorbing geometries, positioning 3D-printed hierarchical structure as promising candidates for impact protection applications.