Elastic Conductive Copolymer Hydrogel Reinforced with Biobased Carbon Dots and Ti3C2Tx MXene for Wearable Strain Sensing and Morse-Code Signal
Abstract
Conductive hydrogels are attractive substrates for wearable strain sensors because their softness, high water content, and tissue-like compliance enable conformal contact with moving skin. However, simultaneously achieving high deformability, stable electrical transport, and repeatable signal transduction remains challenging. Here, an acrylamide/maleic acid/butyl acrylate copolymer hydrogel containing stevia-derived carbon dots (CDs) and Ti3C2Tx MXene was investigated as a flexible piezoresistive platform. The 10 wt % 2CD:1MXene formulation exhibited the highest measured conductivity, (2.11 ± 0.11) × 10–4 S cm–1, and the lowest Tauc-derived apparent optical transition energy, 2.026 ± 0.013 eV, within the tested formulation range. The optimized hybrid formulation combined enhanced stretchability with a tensile toughness of approximately 41.4 kJ m–3 and maintained an elastic-dominant viscoelastic response, with tanδ values of 0.286 ± 0.008 in the strain sweep and 0.191 ± 0.005 in the frequency sweep at the common comparison condition. The sensor showed gauge factors of 3.088 and recovery times of 8.50 ± 0.48 and 8.63 ± 0.51 s, respectively. After 1000 loading–unloading cycles, 94.57% of the initial response amplitude was retained. On-body measurements demonstrated qualitative detection of joint and physiological motions, while controlled short and long inputs produced distinguishable Morse-code-like temporal patterns. As a wearable strain sensor, the material showed stable and repeatable resistance responses toward finger bending, wrist motion, facial micro-movements, chewing, blinking, and breathing. Importantly, the sensor could translate controlled mechanical inputs into distinguishable Morse-code patterns, enabling the encoding of messages such as “SOS”, “HELP”, and “MXENE”. The resulting hydrogel is, therefore, a promising platform for wearable motion monitoring, soft human-machine interfaces, and Morse-code-assisted communication.