Sweat can serve as a real-time source of information about the body's current state. For example, wearable sensors designed to measure sodium and potassium in sweat could provide biochemical information relevant to physiological monitoring. Such devices could potentially contribute to future health-management applications for people working or exercising in hot environments by enabling measurements without interrupting their activities.
However, turning this idea into a fully functional device is not simple. Many wearable ion sensors are potentiometric, meaning they measure a voltage that changes with the concentration of a specific ion. To provide a meaningful reading, these ion-selective electrodes need a stable measurement potential from a reference electrode. Unfortunately, conventional printed reference electrodes can take considerable time to stabilize before stable measurements can begin, making them less suitable for continuous monitoring applications in dynamic contexts.
To tackle this problem, a research team led by Associate Professor Isao Shitanda from the Department of Pure and Applied Chemistry, Faculty of Science and Technology at Tokyo University of Science, Japan, developed a wristwatch-type sensor integrating a printed liquid-junction reference electrode that does not need lengthy pre-adjustment. Their study, published online in ACS Omega on September 21, 2026, was co-authored by second-year master's student Takuma Fujisawa from the same department.
The proposed design neatly integrates a screen-printed liquid-junction reference electrode with sodium- and potassium-selective electrodes, a sample transport layer, and digital electronics. The liquid junction contains silica gel, which facilitates water permeation into the electrolyte layer and enables rapid initial stabilization of the reference electrode. Meanwhile, the sample transport layer consists of fabric and superabsorbent fiber placed over the electrodes. It helps wet the sensing area and facilitates sample transport through the sensing region. The complete sensor was connected to a custom wristwatch-type device that can process the signals, transmit them wirelessly, and also store them locally on a micro-SD card.
To test their approach in the real world, the researchers took it to an active construction site, where one construction worker wore the wristwatch while carrying out their usual workday, including breaks, hydration, and meals. The device acquired sodium- and potassium-responsive signals in real time for approximately 2 hours and 45 minutes while the worker moved around the site. These measurements were successfully transmitted wirelessly to a host computer and simultaneously stored on the device, demonstrating that the integrated system could acquire and transmit ion-responsive signals during normal work in the field. "The central achievement of this work lies not in the proposal of a new electrode material, but in the integration of a printed electrode, sample transport mechanism, and wireless measurement circuit, demonstrating its feasibility in a real-world environment," says Dr. Shitanda.
Although quantitative sweat analysis will require controlled sweating experiments and further validation, this work demonstrates the field feasibility of an integrated wearable electrolyte-sensing system. "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," says Dr. Shitanda, "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."
Notably, because the sensor electrodes can be fabricated using printing technology, the approach could support the development of low-cost wearable systems suitable for large-scale production. The findings provide a foundation for future development of wearable systems for occupational, sports, and outdoor applications.
Reference
Title of original paper: Real-Time Monitoring with a Wristwatch-Type Sodium- and Potassium-Ion Sensor Using a Screen-Printed Liquid-Junction Reference Electrode
Journal: ACS Omega
DOI: https://doi.org/10.1021/acsomega.6c04240
About The Tokyo University of Science
Tokyo University of Science (TUS) is a well-known and respected university, and the largest science-specialized private research university in Japan, with four campuses in central Tokyo and its suburbs and in Hokkaido. Established in 1881, the university has continually contributed to Japan's development in science through inculcating the love for science in researchers, technicians, and educators.
With a mission of "Creating science and technology for the harmonious development of nature, human beings, and society," TUS has undertaken a wide range of research from basic to applied science. TUS has embraced a multidisciplinary approach to research and undertaken intensive study in some of today's most vital fields. TUS is a meritocracy where the best in science is recognized and nurtured. It is the only private university in Japan that has produced a Nobel Prize winner and the only private university in Asia to produce Nobel Prize winners within the natural sciences field.
Website: https://www.tus.ac.jp/en/mediarelations/
About Associate Professor Isao Shitanda from Tokyo University of Science
Dr. Isao Shitanda graduated from Tokyo University of Science (TUS) in 2001, and received a Ph.D. from The University of Tokyo, Japan, in 2006. Since 2020, he has served as an Associate Professor at the Department of Pure and Applied Chemistry at TUS, leading the Itagaki–Shitanda Laboratory. Dr. Shitanda specializes in electrochemical micro-/nanosystems, physical and analytical chemistry, bio-related chemistry, and environmental chemistry, among other fields. He has published over 200 papers and holds several patents.
Funding information
This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (B) (Grant Number: 24K02819).
