A green MXene-based organohydrogel with tunable mechanics and freezing tolerance for wearable strain sensors

Conductive hydrogels have attracted considerable attention owing to their potential for use as electronic skin and sensors. However, the loss of the inherent elasticity or conductivity in cold environments severely limits their working conditions. Generally, organic solvents or inorganic salts can be incorporated into hydrogels as cryoprotectants. However, their toxicity and/or corrosive nature as well as the significant water loss during the solvent exchange present serious difficulties. Herein, a liquid-like yet non-toxic polymer-polyethylene glycol (PEG) was attempted as one of the components of solvent for hydrogels. In the premixed PEG-water hybrid solvent, polyacrylamide (PAAm) was in situ polymerized, overcoming the inevitable water loss induced by the high osmotic pressure of the PEG solution and achieving tailored water capacity. Interestingly, the mechanical strength (“soft-to-rigid” transition) and anti-freezing properties of organohydrogels can be simultaneously tuned over a very wide range through adjusting PEG content. This was due to that with increasing PEG in solvent, the PAAm chains transformed from stretching to curling conformation, while PEG bonded with water molecules via hydrogen bonds, weakening the crystallization of water at subzero temperature. Additionally, a highly conductive Ti3C2Tx-MXene was further introduced into the organohydrogels, achieving a uniform distribution triggered by the attractive interaction between the rich functional groups of the nanofillers and the polymer chains. The nanocomposite hydrogels demonstrate high electrical conductivity and strain sensitivity, along with a wide working temperature window. Such a material can be used for monitoring human joint movement even at low temperature and has potential applications in wearable strain sensors.