Thermally triggered active metamaterials with shape memory polymers (SMPs) show greater potential for structural applications with reconfigurability than other programmable structures owing to their temporally stiff condition with shape locking. However, most SMP‐based active metamaterials have not shown complex transformation, such as multi‐modal and asymmetric deformations, because of the lack of an adaptable strategy with reasonable mechanics models. Moreover, conventional SMP has a critical drawback – irreversible transformability, limiting its reconfigurability for active metamaterials. Herein, a thermomechanical tool that allows a single material system to transform with reversible, multi‐modal, and asymmetric deformations is constructed and demonstrated. Using transformation aids (TAs), a localized pre‐stress and a temperature‐dependent reverse stiffness effect to exchange energy with a lattice is conceived. The deformation of a single SMP system whose energy is swapped from TAs by localized pre‐stress and reverse stiffness can transform into reversible, multi‐modal, and asymmetric deformations with shape‐locking. The methods can be used for reconfigurable structures, tuning symmetry, and chirality, especially for active acoustic metamaterials, deployable devices, and biomedical devices. The mechanics‐inspired design approach of local deformation of TA and the interaction with the temperature‐dependent stiffness drop of the lattice open an avenue to the robust design of thermally triggered active metamaterials.

Future active metamaterials for reconfigurable structural applications require fast, untethered, reversible, and reprogrammable (multimodal) transformability with shape locking. Magnetic control has a superior advantage for fast and remotely controlled deployment; however, a significant drawback is needed to maintain the magnetic force to hold the transformation, limiting its use in structural applications. The shape‐locking property of shape‐memory polymers (SMPs) can resolve this issue. However, the intrinsic irreversibility of SMPs may limit their reconfigurability as active metamaterials. Moreover, to date, reprogrammable methods have required high power with laser and arc welding proving to be energy‐inefficient control methods. In this work, a magneto‐thermomechanical tool is constructed and demonstrated, which enables a single material system to transform with untethered, reversible, low‐powered reprogrammable deformations, and shape locking via the application of magneto‐thermomechanically triggered prestress on the SMP and structural instability with asymmetric magnetic torque. The mutual assistance of two physics concepts—magnetic control combined with the thermomechanical behavior of SMPs is demonstrated, without requiring new materials synthesis and high‐power energy for reprogramming. This approach can open a new path of active metamaterials, flexible yet stiff soft robots, multimodal morphing structures, and mechanical computing devices where it can be designed in reversible and reprogrammable ways.

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Similar to Poisson's effect, mechanical coupling is a directional indirect response by a directional input loading. With the advance in manufacturing techniques of 3D complex geometry, architected materials with unit cells of finite volume rather than a point yield more degrees of freedom and foster exotic mechanical couplings such as axial–shear, axial–rotation, axial–bending, and axial–twisting. However, most structural materials have been built by the ad hoc design of mechanical couplings without theoretical support of elasticity, which does not provide general guidelines for mechanical couplings. Moreover, no comprehensive study of all the mechanical couplings of 3D lattices with symmetry operations has been undertaken. Therefore, we construct the decoupled micropolar elasticity tensor of 3D lattices to identify individual mechanical couplings correlated with the point groups. The decoupled micropolar elasticity tensors, classified with 32 point groups, provide 15 mechanical couplings for 3D lattices. Our findings help provide solid theoretical guidelines for the mechanical couplings of 3D structural materials with potential applications in various areas, including active metamaterials, sensors, actuators, elastic waveguides, and acoustics.