Tough and Elastic\nCellulose Composite Hydrogels/Films\nfor Flexible Wearable Sensors

Abstract

Cellulose and its composites, despite being abundant\nand sustainable,\nare typically brittle with very low flexibility/stretchability. This\nstudy reports a solution processing method to prepare porous, amorphous,\nand elastic cellulose hydrogels and films. Native cellulose dissolved\nin a water–ZnCl₂ mixture can form ionic gels through\nin situ polymerization of acrylic acid (AA) to poly(acrylic acid)\n(PAA). The addition of up to 30 vol % AA does not change the solubility\nof cellulose in the water–ZnCl₂ mixture. After polymerization,\nthe formation of interpenetrated networks, resulting from the chemical\ncross-linking of PAA and the ionic/coordination binding among cellulose/PAA\nand ZnCl₂, gives rise to strong, transparent, and ionically\nconductive hydrogels. These hydrogels can be used for wearable sensors\nto detect mechanical deformation under stretching, compression, and\nbending. Upon removal of ZnCl₂ and drying the gels, semitransparent\namorphous cellulose composite films can be obtained with a Young’s\nmodulus of up to 4 GPa. The rehydration of these films leads to the\nformation of tough, highly elastic composites. With a water content\nof 3–10.5%, cellulose-containing films as strong as paper also\nshow typical characteristics of elastomers with an elongation of up\nto 1300%. Such composite films provide an alternative solution to\nresolving the material sustainability of natural polymers without\ncompromising their mechanical properties.