A multifunctional flexible sensor with dual-conductive networks for monitoring human motion signals and sweat pH/Lactic acid

Flexible wearable sensors have garnered significant attention for monitoring human motion signals and detecting sweat composition due to their high flexibility, consistency, and low cost. However, existing problems such as poor conductivity, inadequate flexibility, low sensitivity and unstable sensing performance present bottlenecks to their further development. This paper presents a novel highly-conductive, ultra-soft, and freeze-resistant poly(acrylamide-acrylic acid)/polyaniline/lithium bromide (PAMAAni/LiBr) hydrogel for monitoring human motion signals and detecting pH and lactic acid in sweat. The hydrogel establishes a dual conductive network by integrating the conjugated structure of polyaniline with the ionization principle of LiBr, which endows the hydrogel with excellent conductivity (4.6 S/m). Interactions between hydrogel networks, LiBr, and water molecules contribute to the hydrogel's low modulus (3.47 kPa) and remarkable freeze-resistant ability. Flexible sensors assembled from this hydrogel demonstrate a wide detection range (0.5%–200%), high sensitivity (strain range from 50% to 150%, gauge factor=3.24), and excellent fatigue resistance (exceeding 2000 cycles). This flexible sensor demonstrates high sensitivity in monitoring human activities in multiple scenarios, including joint movements, handwriting, and precise robotic hand control. Additionally, it exhibits excellent electrochemical performance for pH detection (pH 1–12) and lactic acid measurement with a wide detection range (0.25–50 mM) and a low detection limit (1.98 μM), providing valuable insights for the development of innovative flexible wearable sensing devices.