Bio-composite design and 3D printing of soft multi-functional meta-structures with tuneable quasi-constant force

• QZS plateau 6 mm, 2.3–5.12 N; triple-unit triples force. • 86 % higher tensile strength with 3 % BC/1 % CNT. • Burning rate reduced by 35 % compared to pure TPU. • 88 % higher cyclic energy dissipation than pure TPU. • 98 % maximum-force retention after 1000 cycles; limited Mullins softening. This study presents a novel, 3D printable, multifunctional bio-composite material system for quasi-zero stiffness (QZS) mechanical metamaterials, transitioning from material development to structural implementation. Bio-based thermoplastic polyurethane (TPU) is reinforced with 3–5 wt.% bamboo charcoal (BC), 1 wt.% carbon nanotubes (CNT), and extruded for 3D printing via fused filament fabrication (FFF). The newly developed bio-composite shows up to 86 % strength enhancement and 35 % reduction in flammability. A surrogate-based optimisation method is implemented to calibrate a second-order Ogden hyper-elastic model using tensile data, enabling accurate prediction of nonlinear mechanical behaviours. Inspired by the human ribcage, QZS meta-structures were designed with dual-arched geometries and fabricated using the optimised TPU/BC/CNT composite. A finite element model is developed to digitally design the meta-structure and carry out a parametric study. Experimental and computational analyses demonstrate a materially tuneable constant-force plateau (e.g., 2.3–5.12 N) extending across a 6 mm displacement range, with excellent agreement between FEM and test results. Notably, the composite-based QZS structures show an 88 % increase in cyclic energy dissipation versus pure TPU. This response exhibits only limited early-cycle Mullins-type softening that stabilises by 10 cycles, retains 98 % of the maximum force at 1000 cycles, and remains durable under repeated loading-unloading. A modular triple-unit configuration further triples the force capacity without compromising QZS behaviour. This material-to-structure integration provides a scalable, sustainable pathway for engineering adaptive, load-bearing systems applicable to soft robotics, automotive interiors, and protective medical devices where force regulation, overload protection, safety, and comfort are desired.