Bifunctional Metamaterials Incorporating Unusual Geminations of Poisson's Ratio and Coefficient of Thermal Expansion.

Natural materials overwhelmingly shrink laterally under stretching and expand upon heating. Through incorporating Poisson's ratio and coefficient of thermal expansion (PR and CTE) in unusual geminations, such as positive PR and negative CTE, negative PR and positive CTE, and even zero PR and zero CTE, bifunctional metamaterials would generate attractive shape control capacity. However, reported bifunctional metamaterials are only theoretically constructed by simple skeletal ribs, and the magnitudes of the bifunctions are still in quite narrow ranges. Here, we propose a methodology for generating novel bifunctional metamaterials consisting of engineering polymers. From concept to refinement, the topology and shape optimization are integrated for programmatically designing bifunctional metamaterials in various germinations of the PR and CTE. The underlying deformation mechanisms of the obtained bifunctions are distinctly revealed. All of the designs with complex architectures and material layouts are fabricated using the multimaterial additive manufacturing, and their effective properties are experimentally characterized. Good agreements of the design, simulation, and experiments are achieved. Especially, the accessible range of the bifunction, namely, PR and CTE, is remarkably enlarged nearly 4 times. These developed approaches open an avenue to explore the bifunctional metamaterials, which are the basis of myriad mechanical- and temperature-sensitive devices, e.g., morphing structures and high-precision components of the sensors/actuators in aerospace and electronical domains.