Ultrasensitive multi-degree-of-freedom piezoionic sensor via synergistic hydrogel-ion interactions

Accurate body motion capture is of great significance for biomechanical assessment and immersive virtual/augmented reality (VR/AR) systems. Piezoionic flex-sensors, which mimic the ionic transport sensing mechanism of cutaneous mechanoreceptors, promises a self-powered approach for wearable body movements detection. However, their practical utility has been limited by low piezoionic flex-sensitivity (typically below 0.5 mV/degree) and unidirectional bending response, which restricts their ability to resolve complex multi-degree-of-freedom (multi-Dof) body motions. Here we report a soft, piezoionic and multi-Dof (SPIM) flex-sensor that overcomes these limitations. It ingeniously integrates zwitterionic dipole-ion interactions and size-induced steric hindrance effects to enable fast ion channels formation and amplified cation/anion transport imbalance, thus cooperatively contributing to a record-high piezoionic flex-sensitivity of 3.2 mV/degree. Theoretical simulations combining poroelastic mechanics and Poisson-Nernst-Planck model corroborated the synergistic enhancement. Furthermore, a unique architecture of square prism-shaped fiber with two pairs of symmetric nanomesh electrodes allows the SPIM flex-sensor to decouple multi-Dof joint flexion to distinguish complex body movements. We demonstrate a SPIM flex-sensor platform for metaverse applications, including a digital-twin of free-pose hand motions and complex yet intuitive VR control via single-joint dexterity equipped with a single compact sensor device.