Bioprocess-inspired synthesis of printable, self-healing mineral hydrogels for rapidly responsive, wearable ionic skin

Oyster reefs, which can maintain coastal ecosystems by absorbing storm surge energy, are composed of aggregated oyster shells bonded by bioadhesive secretions containing inorganic minerals. Bioadhesive secretions are formed by cross-linking extracellular liquid, which is rich in organic and inorganic ions, with sand, bacteria, or diatoms. Inspired by such bioprocesses, a mineral hydrogel with excellent 3D printability, rapid self-healing ability (85% recovery within 1 min), high stretchability (>1500% tensile elongation) and ionic conductivity was synthesized by the chelation of polyacrylic acid (PAA) with calcium ions (Ca2+) and subsequent physical cross-linking of PAA with modified amorphous calcium phosphate (ACP) nanoparticles. With such mineral hydrogels, ionic skin was successfully prepared, which could recognize the bending degrees of the index finger and sensitively sense the tensile strain (approximately 33 ms). Moreover, the ionic skin can distinguish external temperature stimuli and work stably at a temperature of 75 °C. These performances exhibit the great mechano-sensing and thermosensing abilities of ionic skin, showing promising applications in various fields, such as wearable sensors, for withstanding high ambient temperatures and the next generation of soft intelligent robots.