Wearable sensors are rapidly advancing, becoming smaller yet more capable than ever of tracking physiological signals in real time. Recent studies have focused on developing skin patches that analyze sweat to monitor the concentration of important compounds, such as lactate and glucose. Sweat-based sensing is particularly attractive because it offers a noninvasive way to assess changes in metabolism. However, these devices require external batteries to function. Interestingly, to avoid relying on batteries, scientists have come up with self-powered alternatives. Enzymatic biofuel cells (EBFCs), which use enzymes as catalysts to convert chemicals in body fluids directly into electricity, are a prominent example.
Although EBFCs have shown promise in laboratory experiments, the main barrier to their widespread adoption boils down to limitations in manufacturing. Conventional EBFC fabrication requires multiple labor-intensive steps: printing a carbon electrode layer, separately drip-casting enzyme and mediator solutions onto the surface, and then drying. This process introduces significant variability between devices, making quality control difficult and mass production impractical.
Addressing this challenge, a research team led by Associate Professor Isao Shitanda from the Department of Pure and Applied Chemistry, Tokyo University of Science (TUS), Japan, has developed water-based 'enzyme inks' that simplify EBFC fabrication into a single printing step. Their findings, published online in the journal ACS Applied Engineering Materials on February 06, 2026, demonstrate a practical pathway toward the mass-production of wearable biosensors. Other members of the team included co-first author Ms. Mahiro Omori (first-year Master's student) from TUS and Mr. Mitsuru Hanasaki from RESONAC Co. Ltd, Japan.
"In order to avoid labor-intensive, inefficient, and expensive EBFC fabrication techniques, we need to bring an enzyme ink to the market that can be printed uniformly and is suitable for mass production," says Dr. Shitanda, as the motivation behind the study.
The concept of enzyme ink, which involves premixing enzymes, carbon materials, mediators, and binders into one printable formulation, had been proposed before. However, no inks have proven truly suitable for industrial screen-printing methods (a unique way of printing where the ink is pushed through a mesh screen onto the surface). Moreover, screen-printable inks for cathodes—the oxygen-utilizing electrodes in biofuel cells—are particularly difficult to realize.
To overcome these issues, the research team crafted an innovative ink formulation. They combined magnesium oxide-templated mesoporous carbon, which is a porous material with extremely high surface area, with chemical mediators that facilitate electron transfer and a novel water-based binder called POLYSOL. This polymer emulsion has the ability to bind strongly to carbon surfaces while maintaining a stable environment for enzymes within the porous structure. Carboxymethyl cellulose was added as a thickener to achieve the proper consistency for screen printing. They also included the specific target enzymes for the desired type of biosensor, such as lactate oxidase for detecting lactate, bilirubin oxidase for oxygen reduction, or glucose dehydrogenase for glucose. All components were dispersed in water-based solutions, avoiding organic solvents that could damage enzyme activity.
The researchers printed these inks directly onto lightweight paper substrates in a single manufacturing step. Through electrochemical tests, they showed that the printed electrodes significantly outperformed conventional drop-cast ones, producing higher catalytic currents and maintaining stable performance during long periods. Notably, drop-cast electrodes typically degraded to less than half their initial activity within minutes to hours, while the enzyme-ink electrodes exhibited minimal decay.
A complete lactate/oxygen biofuel cell assembled from these screen-printed electrodes achieved a maximum power output of 165 μW/cm2 with an operating voltage of 0.63 V, which is substantially higher than the 96 μW/cm2 reported in previous similar systems. This represents a major breakthrough, as it marks the first successful screen printing of the cathode side using enzyme ink. Moreover, the lactate detection range aligned well with physiological sweat concentrations observed during exercise, demonstrating the practical applicability of these biofuel cells for real-world monitoring.
The present system is specifically designed to measure lactate concentrations in sweat, accurately quantifying levels within the typical physiological range observed in healthy individuals (approximately 1–25 mM). This range is particularly relevant for monitoring exercise intensity and metabolic status. Based on their previous studies, the team also confirmed that the generated power is sufficient to support Bluetooth Low Energy wireless transmission. In fact, they have already demonstrated self-powered wireless monitoring of lactate concentration, confirming that the device can operate without an external power source.
To showcase the scalability of the proposed solution, the team conducted a practical roll-to-roll printing demonstration, achieving continuous printing on 400 m of substrate. Screen-printing offers a major advantage in manufacturing simplicity. If the entire device can be fabricated using full screen-printing processes, the number of fabrication steps can be significantly reduced compared with conventional methods. This streamlined approach could enable very low-cost production—potentially around 10 yen per device—making the technology highly attractive for disposable or large-scale wearable applications. "Together, our findings demonstrate that water-based enzyme ink formulations are a scalable, reproducible, and high-performance approach for fabricating EBFCs, offering practical advantages for integration into flexible, wearable, and self-powered biosensor platforms," remarks Dr. Shitanda.
Looking ahead, the researchers aim for practical implementation around 2030. This timeline reflects the need for further device optimization, long-term validation, and integration with wearable platforms. While specific commercial partners have not yet been identified, the team anticipates that printing companies and healthcare device manufacturers may be strong candidates for adopting this technology.
Overall, this work removes fundamental barriers that have kept self-powered wearable biosensors from reaching the market for years, with implications that extend across multiple domains. In sports, for example, real-time sweat lactate monitoring could provide immediate feedback on exercise intensity and muscle fatigue. For nursing and elderly care, continuous metabolic monitoring could allow for the early detection of health conditions. Similar biosensors may also contribute to heatstroke prevention systems by detecting early metabolic warning signs. "In these ways, this technology has the potential to contribute to the realization of a safer and healthier society by serving as the basis for sensors that monitor one's physical condition by simply wearing them," concludes Dr. Shitanda.
