Multifunctional Janus structured flexible sensors based on PU-TA@HCB: Enhanced mechanical, self-healing, and underwater sensing

Conventional flexible sensors often suffer from limited mechanical durability and restricted functionality, which constrain their applicability in practical scenarios. Drawing inspiration from the asymmetric structure of human skin, this study introduces a Janus-structured film composed of polyurethane (PU) and tannic acid (TA)-grafted hydroxylated carbon black (HCB). The surface modification of HCB with TA introduces a high density of polar functional groups, enabling strong intermolecular hydrogen bonding with PU chains and thereby enhancing stress transfer across the composite matrix. Under gravitational influence, the gradient distribution of TA@HCB within the PU matrix facilitates the formation of an intrinsic double-sided structure. The resulting PU–TA@HCB composite exhibits superior mechanical performance, including a tensile elongation at break of 1854.65 %, tensile strength of 35.37 MPa, and a toughness of 293.52 MJ/m³, significantly outperforming the metrics of previously reported PU-based sensors (Table S1). In addition, the presence of a dynamic hydrogen-bonding network contributes to the material's exceptional self-healing and recyclability, supporting its sustainable use. Moreover, the asymmetric conductive structure enables reliable differentiation between directional bending and stretching motions, while maintaining high sensing stability in underwater environments. Owing to this synergistic design, the sensor demonstrates versatility in applications such as motion detection, handwriting recognition, and Morse code communication. This work presents a rational design strategy that integrates ultra-high toughness, multifunctionality, and environmental adaptability, offering valuable insights for the advancement of next-generation wearable electronics and human–machine interface technologies.