Programmable mechanical metamaterials switching between soft and stiff on demand

Background

Materials are usually seen as either soft (like rubber) or hard (like steel). But what if materials could change their stiffness whenever needed—soft for cushioning, stiff for support? Mechanical metamaterials achieve special properties by using clever internal structures. These include designs like porous foams, micro-lattices, origami patterns, and linkage mechanisms. The key is how they behave under stress and strain, shown by their stress–strain curves.

Recently, scientists have focused on creating metamaterials whose stress–strain curves can be programmed to fit specific needs. For instance, zero stiffness helps reduce vibrations, while strain hardening protects against impacts. Yet most current materials only show simple hardening, lacking complex, programmable softening or full control over their mechanical response. Also, no existing design combines light weight, large stretchability, easy manufacturing (like 3D printing), and fully customizable nonlinear behavior.

Research Progress

Professor Yang Li team at Wuhan University compared multi-DOF and one-DOF systems. Multi-DOF systems act as “underactuated” setups, energy paths limit their deformation, and complex stress–strain responses fail. One-DOF systems control deformation paths precisely, enable arbitrary nonlinear behaviors. They designed mechanical metamaterial platform combining elastic components with one-DOF mechanisms, removing energy constraints. Each elastic component uses few design variables describing energy changes under compression, allowing full parametrization of stress–strain curves. Two-material 3D printing created samples matching four target curves, experimental results confirmed theory. Increasing elastic components improved curve fitting. Integration of shape memory alloys enabled dynamic stiffness tuning, rapid response switching, and reprogramming, achieving true adaptivity. Extension of design produced anisotropic metamaterials, responses vary with direction, unlocking new customized mechanical properties.

Future Prospects

This work shows that multi-DOF metamaterials are limited by their energy-driven deformation, restricting their mechanical behaviors. By using one-DOF mechanisms and inverse design, the team created lightweight, highly deformable, easy-to-make metamaterials with tunable, programmable, and anisotropic mechanical responses. This opens a new path toward smart, reconfigurable materials for advanced engineering and adaptive applications.

Sources: https://doi.org/10.34133/research.0715