Schematic of the simplified programming process of the smart metamaterial (left); stimuli-sensitive response (center); and photos of topological lattices (right).
Schematic of the simplified programming process of the smart metamaterial (left); stimuli-sensitive response (center); and photos of topological lattices (right).

The mechanical properties and functions of most materials are unchanging. Shape memory materials are an exception. These materials can take up a temporary shape, reverting to their original configuration when exposed to an external stimulus such as heat, light, moisture, or pH. While smart materials are very useful in biomedical devices, flexible electronics, deployable aerospace devices, and soft robotics, their response time can be slow and their thermal programming time-consuming.

Now researchers at McGill University in Canada and Harbin Institute of Technology in China have combined two 3D-printable polymers to create mechanical metamaterials with a shape memory response that overcome these limitations [Yang et al., Materials Today (2023),].

“Our concept is an alternative to current SMPs, which exhibit a number of drawbacks [that] limit applications and prevent their use in a broad spectrum of industrial sectors,” explains Damiano Pasini of McGill, who led the study with Li Ma. “We have realized a simplified shape memory effect [using] a set of mechanical metamaterials.”

Traditional shape memory materials maintain a temporary shape until a stimulus prompts them to return to their original configuration. The programming procedure for thermally activated shape memory materials is laborious and requires high temperatures. Response time can also be slow and inaccurate, with differences between the temporary and original shapes. Instead, Pasini and Ma’s metamaterial comprises two different polymers that together produce a simplified shape memory effect.

The metamaterial consists of a lattice of curved beams of two 3D-printable polymeric materials, which differ in the response of their elastic moduli to temperature. The elastic modulus of one hardly changes with increasing temperature while the other changes radically. At low temperatures the metamaterial exists in either an undeformed or deformed state if loaded. When the material is heated, the metamaterial snaps back into its original configuration. Since the shape reconfiguration and recovery is governed by temperature and loading alone, the response is faster than conventional shape memory materials and can be programmed to perform complex functions such as releasing a cargo or picking up an object in response to heat.

“By changing the relative modulus of the two constitutive materials through an external stimulus, we can design and realize stimuli-responsive metamaterials with shape reconfiguration and rapid recovery,” points out Pasini.

The researchers believe this approach could be extended to other 2D and 3D architectures and other polymeric materials that respond to light, moisture, or electromagnetic fields.