Extraordinary sensitive mechanochromic hydrogels for visually detecting ultrasmall pressure

Similar to chameleon skins, colloidal mechanochromic photonic crystals (MPCs) have been receiving increasing attention due to their capability of adjusting colors by force. However, their applications are hindered by poor sensitivity (<1 nm/kPa or nm/%), responsiveness (<1 nm/ms), and tuning range of wavelength (Δλ < 180 nm), mainly due to the small surface-to-surface distance (Ds-s) between neighboring particles. Here, we have fabricated supersensitive SiO2/water-hydroxyethyl acrylate MPC hydrogels with large Ds-s and thus extreme sensitivity (106.9 nm/kPa and 3.7 nm/%), fast responsiveness (11.3 nm/ms), broad Δλ (340 nm), and good reversibility (>100 times) by swelling non-closely packed structures combined with a unique interface etching-induced swelling mechanism. The unique compositions, structures, and swelling mechanisms are the keys to precisely regulating the Ds-s and mechanochromic properties. These MPC hydrogels can monitor the small jump pressure of frogs dynamically, visually, and instantly, showing their potential applications in sensing and wearable devices.