Mechanical metamaterials have unprecedented properties. However, once the mechanical metamaterials are fabricated, the single configuration will limit their application, especially in areas requiring rich programmability and efficient shape reconfigurability, etc. To break through this limitation and explore more potential of mechanical metamaterials, a novel design strategy for 4D printed metamaterials with programmable stiffness was proposed. Deriving from the origami concept, the stiffness of the structure can be adjusted not only by changing the geometric parameters but also by changing the geometric configuration utilizing the shape memory property. With this strategy, the stiffness of the structure can be adjusted, and the mechanical programmability, shape reconfigurability and adaptability can be significantly improved. Utilizing excellent energy absorption performance, a kind of shoe sole was designed and fabricated. By integrating the sensor function of the Triboelectric Nanogenerator (TENG) into the sole, gait monitoring is successfully realized. The 4D-printed shape memory metamaterials show extraordinary potential in the field of programmable shock absorption and intelligent monitoring. 4D printing technology has unique advantages in structure fabrication and reprogramming after structure fabrication, which will bring more possibilities for the functional integration and application of TENG.
