Wearable Sweat Sodium Ion Biosensor for Personalized Hydration Strategy under Exercise-Induced Heat Stress

A flexible, screen-printed potentiometric biosensor was developed for the continuous, noninvasive monitoring of sodium concentration in human sweat.The sensor employed a poly(vinyl chloride) membrane doped with sodium ionophore X and sodium tetrakis [3,5bis(trifluoromethyl)phenyl]borate, with multiwalled carbon nanotubes (MWCNTs, 0-1 wt%) incorporated to enhance ion-to-electron transduction.Structural characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron Spectroscopy (XPS) confirmed homogeneous membrane formation and effective nanotube integration.Electrochemical analysis revealed a progressive improvement in performance with MWCNT loading: the sensitivity increased from 45.5 1.2 to 58.2 0.7 mV dec -1 , the detection limit decreased to 9.1 10 -5 M, and the potential drift was minimized to 0.1 mV h -1 .The optimized device responded within 10 s and maintained stable slopes after 5000 bending and 2000 stretching cycles, demonstrating strong mechanical durability.Selectivity coefficients (log K Na ,j pot -2.5 to -3.4) confirmed negligible interference from K + , NH 4 + , Ca 2+ , and Mg 2+ .During on-body trials involving 10 subjects performing 60 min of cycling at 27 , the wearable sensor accurately captured dynamic sweat Na + profiles ranging from 45 to 70 mM.Sensor readings correlated excellently with ion-chromatography reference values (R 2 = 0.985, mean bias -1.2 mM).These results validate the device as a reliable real-time platform for individualized fluid-electrolyte assessment.The integration of nanocomposite solid-contact membranes with flexible printed electronics enables the precise quantification of sweat electrolytes, supporting data-driven, personalized hydration management during exercise and heat exposure.