Tough and adhesive conductive hydrogels with fast gelation from a polyphenol–aluminium ion dual self-catalysis system for wearable strain sensors and triboelectric nanogenerators†

Abstract

Hydrogels have been widely used as flexible electrodes for the construction of strain sensors and triboelectric nanogenerators (TENGs) with high performance owing to their attractive flexibility and conductivity. However, traditional fabrication methods of hydrogels involve time-consuming synthesis and/or use of external stimuli (i.e., heat and light). Herein, a tough and adhesive conductive double network hydrogel (PVA/PHEAA–TA–Al³⁺ gel) was prepared via rapid in situ room temperature gelation processes (25 °C, 215 s) in a tannic acid–aluminium ion (TA–Al³⁺) dual self-catalysis system. This involved the collaborative use of TA–Al³⁺ to induce the decomposition of ammonium persulfate (APS), which generated abundant free radicals to trigger the polymerization of the HEAA monomer within a polyvinyl alcohol/N-(2-hydroxyethyl)acrylamide/tannic acid (PVA/HEAA/TA) aqueous solution. The obtained hydrogel showed excellent mechanical properties (tensile stress/strain of 240 kPa/920%), adhesion, and self-healing ability. Benefitting from the ultra-wide sensing range (1–600%), high sensing sensitivity (GF = 2.7) and long-term stability (500 cycles), the PVA/PHEAA–TA–Al³⁺ gel was used to construct a strain sensor, which can accurately identify and distinguish the changes in human expression and joint movement. Furthermore, the PVA/PHEAA–TA–Al³⁺ gel was used to fabricate TENGs (named PT-TENGs). PT-TENGs with an area of 2 × 2 cm² exhibited attractive electrical output properties (V_OC = 109 V, I_SC = 1.3 μA, and Q_SC = 35 nC at a fixed frequency of 2.0 Hz), which can power 22 LED arrays. This TA–Al³⁺ dual self-catalysis system is expected to provide a new way for the fabrication of tough and adhesive conductive hydrogels toward health monitoring sensors and energy supply.