Real-Time Monitoring with a Wristwatch-Type Sodium- and Potassium-Ion Sensor Using a Screen-Printed Liquid-Junction Reference Electrode

Abstract Wearable ion sensors have gained increasing attention for the continuous noninvasive monitoring of ions in body fluids such as sweat. However, printed reference electrodes commonly used in potentiometric ion sensors require prolonged conditioning to achieve stable potentials, which limits practical wearable applications. Here, we integrated a screen-printed liquid-junction reference-electrode architecture with dual Na+- and K+-selective electrodes in a wearable wristwatch-type sensing platform. The liquid-junction reference becomes operable without the prolonged preconditioning required by poly(vinyl butyral) (PVB)-type printed references. The sensor exhibited sensitivities of 48.3 mV decade–1 for Na+ and 46.4 mV decade–1 for K+. The printed reference electrode drifted by approximately 6 mV over 3000 s against a commercial reference in 10 mM NaCl. A control reference electrode fabricated without the liquid-junction layer showed a substantially greater dependence on the external KCl concentration, supporting the functional role of the liquid-junction layer. A fabric layer and a superabsorbent-fiber (SAF) layer were incorporated as a physical barrier and a sample-handling structure. A wristwatch-type wireless device was designed and constructed for real-time data acquisition, wireless transmission, and local storage on an SD card. The complete system was deployed at an active construction site. During on-body testing with a volunteer construction worker, the system acquired Na+- and K+-responsive potentiometric signals in real time, which were converted to apparent ion concentrations using aqueous calibration curves, over a working period of approximately 2 h 45 min