From coral to control: bio-inspired, 3D-printable metamaterials with tuneable quasi-zero stiffness and multi-functional bio-composites

This study introduces a new class of quasi-zero stiffness (QZS) mechanical metamaterials featuring bio-inspired, variable-thickness curvilinear architectures and sustainable, 3D-printable reinforced bio-composites. Drawing inspiration from coral geometries, the proposed metamaterials integrate non-uniform curved beams with programmable plateau regions and controllable constant-force levels. A hybrid framework combining visco-hyper-elastic finite element modelling (FEM) with multi-objective optimisation enables geometry tailoring for mechanical precision. FEM is established based on the Mooney Rivlin model in conjunction with Prony series. Reinforcement of thermoplastic polyurethane with 1 wt% Chitosan with antibacterial properties produces a 33 % increase in strength, 152 % enhancement in stress impulse, and over 110 % gain in cyclic energy dissipation, along with 32.5 % burning rate reduction, and better moisture retention. Experimental/numerical analyses confirm stable force–displacement responses, durable hysteresis loops, and negligible Mullins effect over repeated loading–unloading-reloading cycles. The optimised metamaterial achieves a 5 mm plateau-region and tuneable constant-force output ranging from 0.5 to 1.15 N. Additionally, a dual-unit configuration yields double the force capacity while preserving QZS characteristics. This multifunctional platform addresses key limitations in force regulation, overload protection, safety, comfort, hygiene, and sustainability. It demonstrates strong potential for integration into healthcare, sports, and mobility sectors, including orthopaedic devices, rehabilitation grips, and protective sports gears.