Self-reporting of damage in underwater hierarchical ionic skins via cascade reaction-regulated chemiluminescence

Self-reporting of damage in underwater materials allows on-demand maintenance and, therefore, improves the reliability of materials used in aquatic environments. Here, we report a chemiluminescence-based strategy to self-report the mechanical damage (e.g., fracture or puncture) in underwater hierarchical ionic skins (HI-skins). The chemiluminescence-based self-reporting is regulated by a cascade reaction, which first occurs at the interface between water and the damage location and then spreads through the whole material. When the HI-skins were mechanically damaged underwater, the pre-embedded calcium peroxide became exposed to and reacted with water to generate hydrogen peroxide that further activated the peroxyoxalate chemiluminescence reaction for reporting the damage. The luminescence wavelength could be tuned (439, 508, or 603 nm) and the damage-induced luminescence lasted for up to 12 h. The self-reporting HI-skins also displayed high mechanical and electronic restorability (93% healing efficiency), excellent stretchability (1600%), impressive room-temperature ionic conductivity (1.7 × 10^-4 S cm^-1), and durable strain sensing performance (highly reproducible electrical response over 1000 uninterrupted strain cycles), making them suitable and reliable candidates for underwater soft ionotronics.