Highly Conductive Ionohydrogels for Humidity Sensing

Polymeric hydrogel materials have excellent electrical conductivity and mechanical properties and will be potentially used in wearable electronic devices, soft robotics, and medical treatment. In this paper, a PAA-Fe³⁺-IL ionohydrogel (poly(acrylic acid)-Fe³⁺-ionic liquid ionohydrogel) with excellent mechanical and conductive properties is prepared by simple free radical polymerization. The presence of metal-ligand crosslinking within the ionohydrogel improves the mechanical properties of the hydrogel. When the IL content is 10 wt%, it has the maximum tensile strength and strain. When the ferric ion concentration is 0.3 mol%, the maximum tensile strength is 495.09 kPa. When the ferric ion concentration is 0.1 mol%, the maximum strain is 1151.35%. The tensile behavior of the ionohydrogels is quantitatively analyzed by the viscoelastic model. In addition, free metal ions and anions and cations in IL endowed the hydrogel with a conductivity of 1.48 S/m and a strain sensitivity of 8.04. Thus, the PAA-Fe³⁺-IL ionohydrogel can be successfully used as a humidity sensor due to the hydrophilic ionic liquid, which can increase the conductivity of the hydrogel by absorbing water. The physical crosslinking density inside the hydrogel is much higher than the chemical crosslinking density, which causes hydrogel dissolution in deionized water by swelling and is conducive to the recycling of the hydrogel. This is a promising material for use in intelligent wearable electronics and as a humidity sensor.