The research, led by Associate Professor Isao Shitanda of the Department of Pure and Applied Chemistry in the Faculty of Science and Technology at Tokyo University of Science, was published online in ACS Omega on September 21, 2026. Co-authored by second-year master’s student Takuma Fujisawa, the study describes a fully integrated system: screen-printed ion-selective electrodes for sodium and potassium, a printed liquid-junction reference electrode, a fabric-based sample transport layer, and custom digital electronics packaged in a wristwatch form factor that can process, store, and transmit data. The work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (B), Grant Number 24K02819.

To understand why this integration matters, it helps to look at how wearable ion sensors actually work. Most of them are potentiometric devices, which means they do not measure a concentration directly. Instead, they measure a voltage that changes predictably with the concentration of a specific ion in the sample touching the electrode. An ion-selective electrode is designed to respond preferentially to one ion species, such as sodium or potassium, generating a potential that tracks that ion’s activity in sweat. But a single electrode cannot give a meaningful reading on its own. Every potentiometric measurement is a comparison against a second, unchanging potential, and that is the job of the reference electrode.

The reference electrode is the quiet workhorse of electrochemistry, and it is also the weak link in many printed wearable sensors. Conventional printed reference electrodes can take considerable time to stabilize before they deliver a steady potential, and until they do, the readings from the ion-selective electrodes are unreliable. For a device meant to be strapped on and used immediately, whether on a factory floor or a running trail, a long pre-adjustment period is a serious practical obstacle. Continuous monitoring in dynamic, real-world contexts demands a reference electrode that is ready almost as soon as the sweat arrives.

The Tokyo University of Science team attacked this problem with a liquid-junction reference electrode built by screen printing. The key ingredient is silica gel embedded in the liquid junction, which facilitates water permeation into the electrolyte layer. That permeation allows the reference electrode to reach rapid initial stabilization, dramatically shortening the time between putting on the device and obtaining trustworthy measurements. Because the electrode is printed rather than assembled from bulky conventional components, it fits naturally into a flat, flexible, low-profile sensor that can sit against the skin on the wrist.

Getting sweat to the electrodes is its own engineering challenge, and the researchers addressed it with a dedicated sample transport layer. This layer consists of fabric and superabsorbent fiber placed over the electrodes. Its job is twofold: it helps wet the sensing area as sweat is produced, and it facilitates transport of the sample through the sensing region so that fresh sweat, carrying up-to-date ion information, continuously reaches the electrodes. Without such a layer, a wearable sensor can sit dry against the skin or read stale, pooled fluid, either of which undermines the goal of real-time monitoring.

The sensing elements alone do not make a wearable device, so the team integrated them with digital electronics in a wristwatch-type package. The custom device processes the signals from the sodium- and potassium-selective electrodes, transmits them wirelessly to a host computer, and simultaneously stores them locally on a micro-SD card. That combination of live transmission and onboard storage is important for field use, where wireless links can drop or where data may need to be reviewed after the fact. The wearer gets continuous monitoring without any interruption to their activity, and the researcher or safety officer gets a complete record.

The most striking demonstration came not in the laboratory but at an active construction site. One construction worker wore the wristwatch device through a normal workday, including breaks, hydration, and meals, while carrying out usual tasks around the site. Over approximately two hours and forty-five minutes, the device acquired sodium- and potassium-responsive signals in real time as the worker moved and worked. The measurements were successfully transmitted wirelessly to a host computer and simultaneously stored on the device, showing that the integrated system could acquire and relay ion-responsive signals under the messy, moving, sweaty conditions of genuine fieldwork rather than the controlled stillness of a lab bench.

Dr. Shitanda emphasized that the breakthrough is one of integration rather than novel materials. According to the researchers, the central achievement of the work lies not in proposing a new electrode material, but in combining a printed electrode, a sample transport mechanism, and a wireless measurement circuit, and demonstrating the feasibility of that combination in a real-world environment. This framing matters for anyone following the wearable-sensor field: elegant chemistry in a paper is common, but a printed, wireless, wrist-worn system that survives a full shift on a construction site is rare, and it is exactly the kind of systems-level proof that turns laboratory concepts into practical technology.

The researchers are careful about what has and has not been proven. Quantitative sweat analysis will require controlled sweating experiments and further validation, so the device should be understood today as a demonstration of field feasibility for an integrated wearable electrolyte-sensing system rather than a finished clinical or occupational health product. Even so, the implications are broad. Because the electrodes can be fabricated using printing technology, the approach lends itself to low-cost manufacturing and large-scale production, which is essential if wearable electrolyte monitors are to move from research demonstrations to devices worn by thousands of workers and athletes. Dr. Shitanda noted that the 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, and that 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. As heat stress grows into an ever more pressing occupational and athletic concern, a printed wristwatch that quietly reads the body’s electrolyte story as it unfolds may prove to be one of the more consequential wearables on the horizon.

Subject of Research: Wearable potentiometric sweat sensors for real-time sodium and potassium monitoring

Article Title: Wristwatch-type wearable sensor enables real-time monitoring of sodium and potassium

Article References: Wristwatch-type wearable sensor enables real-time monitoring of sodium and potassium. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: wearable sensor, sweat analysis, sodium, potassium, ion-selective electrode, reference electrode, screen printing, potentiometry, real-time monitoring, hydration, occupational health, Tokyo University of Science

The central technical achievement documented in the review is the simultaneous optimization of two properties that have historically worked against each other: ionic conductivity and mechanical stretchability. In conventional solid polymer electrolytes based on polyethylene oxide, or PEO, ions migrate through amorphous regions of the polymer by coordinating with ether oxygen atoms and hopping between coordination sites, a process that depends on segmental mobility of the polymer chains. Crystalline regions impede this transport, which is why conductivity in neat PEO electrolytes has long been limited at room temperature. The review highlights how researchers now engineer these materials with nanofillers such as silicon dioxide nanoparticles, which disrupt chain packing and increase the amorphous fraction, pushing ionic conductivities above the benchmark of 10 to the power of minus 3 siemens per centimeter, a level once considered the practical threshold for device-grade performance.

Fluoropolymers occupy the other end of the synthetic polymer spectrum described in the review. Polyvinylidene fluoride copolymerized with hexafluoropropylene, known as PVDF-HFP, provides a chemically robust, hydrophobic matrix whose high dielectric constant helps dissociate salts into mobile charge carriers. When the crystalline PVDF phase supplies mechanical strength and the amorphous HFP-rich phase traps liquid electrolyte or ionic liquid, the resulting gel films deliver both durability and high ionic transport. The review traces this architecture back to foundational work on PVdF-based gel electrolytes and shows how modern formulations incorporate ionic liquids, salts such as lithium bis(trifluoromethanesulfonyl)imide, and graphene oxide fillers to tune viscosity, conductivity, and electrochemical stability windows for sensing applications rather than energy storage alone.

Perhaps the most striking trend the authors document is the rise of biopolymers as sensing electrolytes. Chitosan, a polysaccharide derived from crustacean shells, carries protonatable amine groups along its backbone, making it a natural proton-conducting membrane whose ionic conductivity responds directly to ambient humidity as absorbed water plasticizes the chains and liberates charge carriers. Cellulose and its nanofibrillar derivatives bring extraordinary mechanical toughness, biodegradability, and self-adhesive behavior to ionic hydrogels, as demonstrated in recent work on cellulose nanofibril-reinforced conductive hydrogels for strain and chemical sensing. Because these materials are abundant, renewable, and in many cases biocompatible, they align the sensor field with sustainability goals while offering surface chemistries, such as hydroxyl and amine groups, that can be readily functionalized with enzymes, ionophores, or recognition molecules.

The review devotes substantial attention to two-dimensional nanomaterials, and MXenes in particular, as conductive additives that transform the sensing capabilities of polymer electrolyte films. MXenes, a family of transition metal carbides and nitrides with metallic conductivity and hydrophilic surfaces, can be dispersed into hydrogel and gel matrices to create percolating conductive networks whose electrical properties shift in response to mechanical strain, ionic content, and molecular adsorption. Recent reviews of MXene-based electrochemical sensors document multiplexed detection of biomarkers in biofluids, and the Ionics review positions MXene-polymer hydrogels as a leading platform for next-generation wearable sensing, where the nanosheets provide both signal transduction pathways and reinforcement that prevents the soft electrolyte matrix from tearing under repeated deformation.

On the fabrication side, the review identifies three production routes that together make large-area, low-cost sensor films realistic. Solution casting, the simplest approach, involves dissolving polymer, salt, and filler in a common solvent and evaporating the solvent to leave a free-standing film, a method compatible with laboratory screening and small-batch production. Electrospinning draws polymer solutions into nanofibrous nonwoven membranes under high voltage, producing mats with enormous surface area and interconnected porosity that shorten ion diffusion paths and accelerate sensor response. Roll-to-roll printing and coating, adapted from the organic photovoltaic industry, brings these materials to manufacturing scale, and the review cites production costs below one dollar per square meter as an achievable target. Inkjet printing adds a digital, maskless route for patterning electrodes and electrolyte layers directly onto flexible substrates, textiles, and paper.

The sensing mechanisms themselves fall into three broad categories that the review dissects in detail. Potentiometric electrochemical sensors rely on redox coupling and ion-selective membranes to generate a voltage proportional to the logarithm of ion activity, a mature principle now miniaturized onto flexible substrates for wearable sodium, potassium, and pH monitoring. Amperometric sensors measure current from electrochemical reactions at a working electrode, the basis of enzymatic glucose detection in sweat and interstitial fluid. Humidity sensing, by contrast, exploits proton conduction and dielectric modulation: as water molecules adsorb onto hydrophilic polymer chains, dissociated protons hop through the hydrated network while the film’s permittivity rises, producing resistive or capacitive signals that track relative humidity across wide dynamic ranges. The review reports response times below one second and minimal hysteresis in the best-performing polymer composite humidity sensors, including crosslinked polyethyleneimine systems and PEO-PVA blends engineered for temperature-independent operation.

What elevates these laboratory metrics into headline territory is the demonstrated clinical-grade performance of wearable devices built on flexible polymer electrolytes. The review surveys sweat-analysis patches that use microfluidic harvesting and ion-selective electrodes to track electrolyte loss in real time, building on landmark demonstrations of fully integrated wearable arrays for multiplexed perspiration analysis. Respiratory monitoring benefits from fast, flexible humidity sensors that detect the moisture signature of each breath, enabling respiratory rate tracking and noncontact interfaces. Metabolite and nutrient biosensors worn on the skin have reached the point of continuous monitoring in naturalistic settings, and the review notes that recent wearable electrochemical platforms achieve accuracy comparable to clinical benchtop instruments, a threshold that regulatory adoption and telemedicine applications demand.

The authors are candid about the obstacles that remain. The conductivity-durability trade-off is fundamental: soft, highly swollen hydrogels conduct ions well but dehydrate, fatigue, and lose adhesion over days of wear, while tougher matrices often sacrifice transport. Environmental stability, particularly tolerance to temperature swings, drying, and repeated mechanical cycling, remains inconsistent across material systems. Biofouling, the accumulation of proteins and cells on implanted or skin-contacted surfaces, degrades sensitivity over time and motivates antifouling surface chemistries borrowed from biomaterials research. Self-healing electrolytes, including zwitterionic hydrogels that recover their structure and conductivity after damage, offer one route to longer device lifetimes, and dynamic covalent crosslinking strategies originally developed for battery electrolytes are migrating into the sensing domain.

Looking forward, the review sketches a research agenda built on multifunctional nanofiller hybrids, self-healing systems, and artificial intelligence-driven material design. Machine learning models trained on composition-property datasets are already accelerating the discovery of polymer electrolyte formulations, replacing iterative trial-and-error synthesis with predictive screening, and deep learning applied to multimodal sweat data promises to convert raw sensor signals into actionable health insights. The authors envision autonomous, sustainable sensing platforms in which biodegradable polymer electrolytes, printed manufacturing, and AI analytics converge into devices that monitor health, air quality, and industrial processes continuously and invisibly. If the trajectory documented in this review holds, the humble ion-conducting film, a material with no moving parts and no electronics of its own, may become one of the most consequential components of the ambient sensing infrastructure now taking shape across healthcare and environmental monitoring.

Subject of Research: Flexible polymer electrolyte films for electrochemical and humidity sensing applications

Article Title: Recent progress in flexible polymer electrolyte films for electrochemical and humidity sensing applications: a short review

Article References: Recent progress in flexible polymer electrolyte films for electrochemical and humidity sensing applications: a short review. (n.d.). https://doi.org/10.1007/s11581-026-07542-8

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07542-8

Keywords: flexible polymer electrolytes, electrochemical sensors, humidity sensing, wearable devices, ionic hydrogels, nanofiller composites, MXenes, chitosan, cellulose, sweat analysis, roll-to-roll printing, ion transport