4D printing bio-inspired chiral metamaterials for flexible sensors

Chiral metamaterials were a typical metamaterial configuration, which had broad application prospects in vibration isolation, energy absorption and other fields. Although 4D-printed chiral metamaterials have been investigated by introducing stimulus-responsive materials to achieve programmable properties of metamaterials, chiral metamaterials are mostly fabricated by single materials, which limits the adjustable domains of the mechanical properties and the design freedom of metamaterials. In this work, inspired by the configuration of collagen fibers of biological tissues, the bio-inspired chiral metamaterials with wave ligaments were developed. The bi-phase (TPE@PLA-SMP) composite chiral metamaterials were fabricated by combining active phase (shape memory polylactic acid, PLA-SMP) with passive phase (thermoplastic elastomer, TPE). The influences of geometric parameters, ligament gradient forms, and TPE distribution modes on the mechanical properties of the developed metamaterials were characterized. When the TPE was distributed at 0 degrees and 45 degrees, the deformation of the mechanical metamaterials occurred mainly in the TPE cells, resulting in the J-shaped displacement-force curve. The programmable and reconfigurable properties of the metamaterials (including configuration, and energy absorption performance) under thermal stimulation were demonstrated. The specific energy absorption (SEA) of the metamaterial 2 theta = pi/2 was realized to transform between 0.92 kJ/kg and 0.38 kJ/kg by shape memory programming. Flexible sensors based on TPE@PLA-SMP composite metamaterials were developed by filling CNC-CNT@PVA composite hydrogels (minimum resistivity 8.1 Omega m), which enabled stable output of electrical signals under repeated loads. A motion state monitor was developed and realized to monitor the wearer's movement such as running, demonstrating its potential application prospects in fields such as flexible electronics.