Sweat contains valuable clues about what is happening inside the body. By measuring electrolytes such as sodium and potassium, wearable sensors could help researchers better understand the body's physiological responses during exercise or work in hot conditions. Because these measurements can be taken while people go about their normal activities, the technology may eventually support health monitoring without disrupting daily routines.
Developing a reliable wearable sensor, however, comes with technical challenges. Many existing devices use potentiometric sensors, which detect changes in electrical voltage associated with the concentration of a particular ion. These sensors depend on a reference electrode to maintain a stable electrical potential. Conventional printed reference electrodes can take a long time to settle into a stable state, limiting their usefulness for continuous monitoring in changing environments, reports Medical Lifesciences.
A research team at Tokyo University of Science in Japan has developed a wristwatch-style sensor designed to address this limitation. Led by Associate Professor Isao Shitanda of the university's Department of Pure and Applied Chemistry, the team incorporated a printed liquid-junction reference electrode that requires little initial adjustment. The findings were published online in ACS Omega on September 21, 2026. Takuma Fujisawa, a second-year master's student in the same department, also contributed to the study.
The compact device combines sodium- and potassium-selective electrodes with a liquid-junction reference electrode, a sweat-transport layer, and electronic components. A key feature is the use of silica gel within the liquid junction. The material allows water to pass into the electrolyte layer, helping the reference electrode reach a stable operating condition more quickly.
The researchers also developed a sample-transport layer made from fabric and superabsorbent fibers. Positioned over the sensing electrodes, this layer helps moisture reach the sensing area and move across it. Together, these components allow the sensor to collect ion-responsive signals while connected to a custom wristwatch unit. The electronics process the measurements, send data wirelessly, and save a local copy to a microSD card.
To assess the system outside the laboratory, the team tested it at an active construction site. One construction worker wore the device during a regular workday that included physical activity, rest breaks, meals, and drinking water. Over approximately two hours and 45 minutes, the sensor recorded signals responsive to sodium and potassium as the worker moved around the site.
The data were transmitted wirelessly to a host computer while also being stored on the watch itself. This demonstrated that the integrated system could collect and transfer sensor readings under everyday working conditions, rather than only in a controlled laboratory setting.
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Dr. Shitanda explained that the main contribution was not the invention of a new electrode material. Instead, the achievement was bringing together a printed electrode, a sample-transport mechanism, and a wireless measurement circuit in a wearable system and demonstrating that it could operate in a real-world environment.
“This technology could serve as a foundation for acquiring information about ions contained in sweat without interrupting work or exercise in construction sites, manufacturing plants, sports, and outdoor activities. In the future, by combining this information with data, such as sweat volume and water intake, it could be applied to health management and hydration support in hot environments,” says Dr. Shitanda.
There are still important steps to take before the technology can provide reliable quantitative measurements of sweat electrolytes. Further research, including controlled sweating experiments and additional validation, will be needed to establish how accurately the device measures sodium and potassium concentrations under different conditions.
Nevertheless, the study offers a promising starting point for practical wearable electrolyte monitoring. Since the electrodes can be manufactured using printing techniques, the approach may also help make production more affordable and scalable.
In the future, similar devices could find applications in construction, sports, outdoor work, and other settings where monitoring the body's response to heat and physical exertion is important. While the technology is not yet a substitute for established medical tests, it represents a step toward wearable sensors that can gather useful physiological information as people go about their daily lives.
