Battery-free wearable sensors with ion-selective electrodes embedded in passive resonant antenna circuits for wireless sweat monitoring

Abstract

Compositional analysis of sweat as a liquid biopsy offers significant potential for advancing personal healthcare. Wireless and wearable technologies enable continuous, effortless monitoring of sweat during daily activities. However, conventional potentiometric methods face limitations due to complex wiring and bulky devices, making them unsuitable for wearable applications. Additionally, signals from traditional methods, such as potential changes or resonant antenna power spectra, are often unstable due to sensor placement and environmental variations. To address these challenges, we developed a battery-free wireless ion-sensing system combining ion-selective electrodes and varactor diodes within a resonant antenna circuit. This system converts interfacial potential changes, triggered by variations in sweat ion concentrations, into capacitance changes, which are detected as shifts in resonant frequency. The system demonstrated Nernstian linear responses for Cl⁻, Na⁺, and K⁺ ions across concentrations of 10⁻⁴ to 1 M, comparable to conventional potentiometric methods. Notably, resonant frequency-based detection maintained stable performance regardless of the distance or angle between the sensor and reader coil, thereby ensuring practicality for wearable applications. By integrating the resonant antenna circuit onto a flexible polydimethylsiloxane substrate, the system achieved selective detection of Cl⁻ and Na⁺ ions within physiological ranges in artificial sweat containing interfering ions. Combining stability, selectivity, and a battery-free design, this system provides a robust platform for real-time sweat ion monitoring, paving the way for practical, seamless health management in daily life.