Antifreezing ionotronic skin based on flexible, transparent, and tunable ionic conductive nanocellulose hydrogels

Ionic hydrogels with excellent flexibility and good conductivity have great potential in diverse electric devices. However, it remains challenging to improve the biocompatibility of ionic hydrogels. Here, natural and environment-friendly cellulose nanofiber hydrogels were prepared without adding any organic active materials. The cellulose hydrogel networks with free metal ions (Li+/Ca2+/K+) were obtained by soaking in ion aqueous solution of different concentrations in order to endow tunable conductivity, thus a new kind of transparent ionic hydrogels with both excellent sensing performance obtained by a simple, low-cost and harmless process. The free metal ions locked in the negatively charged nanocellulose network through electrostatic interaction provided adjustable conductivity and sensing performance. Hydrogels doped with 4 mol L−1 lithium ions exhibited the best sensing performance with the conductivity of 4.36 × 10–4 S cm−1, and the current response value was as high as 127%. It was worth noting that the strain-sensitive performance of calcium ions was generally excellent even at low temperatures (−30 °C). The antifreezing of the hydrogel improved its service under extreme environment. This kind of hydrogel has great application prospects in artificial intelligence products, such as human healthy monitoring equipment and soft robotics at subzero temperature.