Light Induced Training of 3D Printed Mechanical Metamaterials

Abstract Stimuli‐responsive materials often react to changes in environmental conditions by altering their shape. Here, it is shown that even changes in materials that are not directly observable, such as local stiffening, can be exploited to introduce the concept of trainable materials. A fully 3D‐printable filament based on thermoplastic polyurethane (TPU) functionalized with a bivalent crosslinker capable of undergoing a C,H insertion reaction under UV irradiation was developed. Specimens printed from this filament demonstrate a gradual increase in stiffness, reaching almost 300% of their initial stiffness after 50 hours of irradiation. To exploit this tunability, mechanical metamaterials incorporating the developed material are engineered. By utilizing an instability‐driven transformation under compression, it is demonstrated how local stiffening can be amplified through rational design. Moreover, by exploiting the unusual mechanical behavior of metamaterials arising from their internal architecture, a closed‐loop system is presented in which, under compressive load, the metamaterial closes an electric circuit that activates UV light, which in turn modifies the properties of the base material. Through this approach, two trainable systems are realized: one that progressively conforms its shape to mechanical compression, and another that gradually increases its resistance to an applied force, mimicking the physical training of biological tissues.