Multifunctional Capabilities of Malonic Acid-Bridged Conductive Hydrogels for Wearable Electronic Devices

Flexible and strain-sensing smart materials have received significant attention from researchers due to their potential use in human motion detection, soft robotics, epidermis sensors, energy storage devices, etc. However, low mechanical strength, low range sensitivity, high time response, and low antifatigue resistance have hampered the application of previously fabricated materials. Herein, a malonic acid (MA)-reinforced hydrogel was prepared through one-pot-free radical polymerization, in which MA makes a bridge by connecting hydrophobically associated polyacrylamide (PAmm) and polydodecyl methacrylate (PDDMA) through physical cross-linking. Ethyl-hexadecyl dimethylammonium bromide (EHDDAB), a cationic surfactant, is used to ensure the formation of micelles. The micelles and polymer chains are bridged via interactions of electrostatic charge enhanced by the dicarboxylic groups present on MA molecules. Notable mechanical strength was observed for MA4 with a 2102% strain, a 2.36 MPa stress, and excellent cyclic stability. At 500% strain, the hydrogel suggests a significant sensitivity to tensile strain, as indicated by its gauge factor of 6.9 and with a fast response recovery time. Meanwhile, the ionic conductivity after the addition of LiCl was calculated as 0.20 S/m. Furthermore, practical applications were observed through the detection of different human motions like finger bending, wrist movement, and elbow and knee movements. Similarly, a small physiological larynx vibration was detected by speaking, coughing, and drinking water. The hydrogel as an electronic pen showed responses to multiple languages in both speaking and writing. The MA-regulated hydrogels show the possibility of use as flexible materials for many different applications, including flexible touch screens, biomedical monitoring, and soft robotic devices.