Nanocomposite Grafted Stretchable and Conductive Ionic Hydrogels for Use as Soft Electrode in a Wearable Electrocardiogram Monitoring Device

Hydrogels possessing stretchability, toughness, and conductivity together are promising candidates for soft electronics applications. In this report, ionic grafting of poly(acryloyl hydrazide) (PAHz)–silver (Ag) nanocomposites (NC) is used to improve the mechanical properties and conductivity of poly(acrylamidopropanesulfonic acid) (PAMPS) hydrogels. PAHz–Ag NCs possessing different sizes of Ag NPs (3–40 nm) are grafted in the PAMPS hydrogel matrix via −CONHNH3+---–O3S ionic linkage. The resulting PAHz–Ag NC grafted hydrogels at ∼62 wt % water content exhibited ultimate tensile strength (UTS) and fracture energy up to ∼1.14 MPa and ∼1600 J/m2, respectively. The UTS of hydrogels was dependent on the size of Ag NP in PAHz–Ag NCs, and the value increased from 0.70 to 1.14 MPa with the decrease in Ag NP size from ∼40 to 5 nm. The hydrogel samples exhibited adequate skin adhesiveness (tack adhesive strength ≈10.8 kPa), conductivity (50.5 mS/cm), and strain sensing ability (gauge factor ≈0.9), suggesting these samples are potentially useful for various soft electronics applications. As a proof of concept, the hydrogels were employed as soft electrode in an electrocardiogram (ECG) device, and the efficiency was monitored under real-time conditions. The data exhibited noise-free reproducible patterns of ECG with defined P, Q, R, S, and T peaks under different locomotion of the body, suggesting the viability of developed hydrogels for the ECG sensing applications.