Metamaterial Sensing and Memory via Multistability and Viscoelastic Effects

The concept of intelligence in materials is gaining increasing attention as a means to enhance the inherent responsiveness and adaptability of engineered systems. Basic changes in material properties, such as stiffness or density, or even larger scale reactions, such as shape memory, are well-established material responses to environmental stimuli, including shifts in temperature, pH, or humidity. Engineered metamaterials may incorporate these fundamental material responses in combination with multiscale structural architectures and carefully designed unit cells to achieve advanced functionalities, including programmable morphing, sensing, and memory.<br/><br/>In this research, we examine a polymer metamaterial composed of dome-shaped unit cells connected by a base membrane. The spatially distributed domes have varying geometries and corresponding stability properties. These metamaterials are cast or 3D printed using viscoelastic, polymeric materials, with embedded piezoelectric sensing strips composed of a polymer-based filament with conductive additives. The interaction of the material viscoelasticity with the structural mono, meta, or bistability of the unit cells allows for the metamaterial to have unique, time-dependent behaviors which may be used for distributed mechanical sensing and memory. These include the critical mechanosensing capabilities of nonlinear signal amplification and filtering and thresholding. Inspired by mechanosensors found in nature, the metamaterial amplifies mechanical signals above a predetermined threshold, corresponding to the minimum snap-through force of the domes, while filtering out those below this threshold. This amplification is due to the highly nonlinear strains experienced by the unit cell during snap-through. Furthermore, the viscoelastic behaviors of stress relaxation, creep, and creep recovery may be used to sense the time elapsed in a local stable state or since a snap-through event triggered by an environmental stimulus, such as a mechanical load or change in temperature. The phenomenon of pseudo-bistability, in which a unit cell structure exhibits bistability for only a limited amount of time due to viscoelastic material relaxation, is particularly interesting as it allows the metamaterial to sense, react, and self-reset after a programmed delay. The pseudo-bistability may be further manipulated by exploiting thermal properties of the polymer material. These combined material and structural capabilities ultimately enable the metamaterial to sense and remember spatiotemporal patterns that convey information about the metamaterial’s environmental history. This class of intelligent metamaterials that can sense and respond to distributed external stimuli over large areas have potential applications in a wide range of industries, including soft robotics, advanced packaging, and smart buildings.