Design and Optimization of 3-D Geometry-Dependent Solid-State Sodium Ion Sensors for Wearable Sweat Sensing

The 3-D geometry of an ion-selective membrane (ISM) significantly impacts the practical performance of an ion-selective electrode (ISE), particularly regarding the ISEs conditioning time, potential drift, and electrode reproducibility. To investigate the ISM geometry-dependent characteristics, a two-layer structure ISE was proposed, with the carbon electrode serving both as the conductive substrate and the ion-to-electron transducer. Based on this, approximately 100 sodium ionizes (Na+-ISEs) were fabricated using a scalable printing method, with ISM thicknesses ranging from 30 to $60~\mu $ m and surface areas varying from $3\times 3$ mm2 to $5\times 5$ mm2. Potentiometric and other electrochemical analyses were applied to characterize the ISEs. The results indicate that increases in ISE thickness and surface area reduce potential drift, while increases in ISE area specifically decrease conditioning time. ISEs with $45~\mu $ m thickness and $5\times 5$ mm2 area exhibited optimal features, including highly linear sensitivity of $105~\pm ~5$ mV/decade, electrode reproducibility within 30 mV, insensitive to typical ions in sweat, long-term stability with a low drift rate of $90~\mu $ V/h over 72 h, and one-week repeatability. A sweat sensing patch with a single ISE is integrated with a wireless readout circuit, enabling real-time on-body monitoring of Na+ levels during various types of exercise, including rowing, running, badminton, and tennis. The results provide manufacturing guidance for adapting solid-state ISEs to be more versatile in various practical situations by altering the ISM geometry.