Nature-inspired wood-like TPU/CB aerogels for high performance flexible strain sensors

Strain sensors based on porous conductive polymers (CPCs) have garnered growing research interest for their potential applications in motion detection, healthcare, human-computer interaction, and artificial intelligence. However, the complexity of CPC processing makes it difficult to achieve the controlled design of microscopic porous structures, leading to simple and random porous structures, thus limiting their further use in the field of pressure sensing. This paper presents a strain sensor with a high-performance, wood-like structure composed of flexible conductive carbon black/plastic polyurethane foam (BWCT) using a bidirectional freeze casting process. The results show that, compared with conventional random freezing and unidirectional freezing, the bidirectional freeze casting process can effectively realize multiscale control of the composite structure, which results in a good laminar porous structure of the prepared BWCT. This parallel laminar structure not only contributes to the layered transfer of stresses but also avoids the local concentration of stresses. At the same time, it significantly increases the directional electrical conduction ability, which results in high sensing stability performance. In particular, the BWCT sensors had a wide detection range (80%), a lower limit of detection (0.2%), rapid response and relaxation times (200 ms), as well as exceptional durability (>2000 cycles). Furthermore, the BWCT was integrated into a wearable sensor to monitor various human motions, including arm bending, squatting, and walking, demonstrating reliable detection performance. Altogether, the BWCT sensors are promising in expanding the application but also offer guidance for designing high-performance wearable strain sensors.