Anti-swelling and adhesive hydrogels enable amphibious sensors with long-lasting conductivity, adhesion and robustness

Abstract

Conductive hydrogels (CHs) have been widely applied in actuating, sensing, health monitoring, etc. However, their applications in aquatic environments face challenges, due to excessive swelling and poor underwater adhesion. To address this issue, it was necessary to combine anti-swelling, adhesion, and long-term usability into one strain sensor. Hence, bilayer-structured CHs with stable sensing properties both in water and on land were developed via radical copolymerization of 2-hydroxyethyl methacrylate (HEMA) and L-serinyl acrylate–copper complex (LSA) to form an anti-swelling yet conductive matrix, followed by painting the adhesive layer of gelatin–tannin acid at the bottom. Because of the Cu²⁺ ions, it achieved ion conductivity, while the strong interaction between Cu²⁺ and carboxylic acid (–COOH)/amine (–NH₂) groups stabilized Cu²⁺ ions and reduced leakage of conductive components. Due to the delicate balance of hydrophilic hydroxyl groups and hydrophobic backbones, the hydrogel displayed excellent swelling-resistance (with a swelling ratio of approximately 20%, tensile stress of 215 kPa, tensile strain of 180%, and conductivity of 17.9 mS m⁻¹) even after immersing in water for a month. Meanwhile, the hydrogel maintained robust adhesion (15 kPa) in aquatic environments for a month. Strain sensors based on the bilayered hydrogel exhibited linear sensitivity in monitoring human motions both on land and underwater, and enabled information transmission through the Morse code. Consequently, this strain sensor has significant potential in underwater wearable devices.