Example of a skin patch health monitor. (Image Credit: Bing Image Creator)
Wearable biosensors are quickly evolving, with smaller devices like flexible skin patches continuously monitoring physiological signals. These analyze sweat to measure metabolites like lactate and glucose but require external batteries, adding bulk and limiting long-term usability. Scientists explored enzymatic biofuel cells (EBFCs) to generate electricity by converting chemicals in body fluids into electrical energy. EBFCs involve complex manufacturing steps, such as printing carbon electrodes, applying enzyme and mediator solutions, and drying the layers. Researchers at the Tokyo University of Science developed water-based enzyme inks that simplify this process by printing the components in one step.
Previously, scientists suggested using enzyme inks, which blend carbon materials, enzymes, and mediators into a printable mixture for biofuel cells. However, the formulations were incompatible with industrial screen printing, especially for cathodes. The team solved that issue by creating a water-based ink using high-surface-area mesoporous carbon, electron-transfer mediators, and a polymer binder called POLYSOL that stabilizes the enzyme while sticking to the carbon.
Their formulation has a thickener for printing consistency and target enzymes like lactate oxidase, bilirubin oxidase, or glucose dehydrogenase. Using water-based solutions rather than organic solvents allows the ink to preserve enzyme activity while enabling large-scale manufacturing.
The team used the enzyme inks to screen print electrodes on lightweight paper substrates in one fabrication step. Electromechanical tests demonstrated that the printed electrodes distributed stronger catalytic currents and improved stability compared to traditional drop-cast electrodes, which lose over half of their activity within minutes to hours. The enzyme-ink electrodes performed consistently over extended use.
Assembling it onto a lactate-oxygen biofuel cell enabled the printed electrodes to generate a maximum power density of 165 uW/cm2 at 0.63 V, higher than the 96 uW/cm2 for comparable systems. This is a notable achievement as it represents the first-ever screen printing of the cathode using enzyme ink. The device detects lactate levels in sweat within the physiological range of 1-25 mW, making it ideal for monitoring exercise intensity and metabolic activity.
The researchers also verified that the generated power is sufficient for supporting Bluetooth Low Energy communication. They demonstrated self-powered wireless monitoring of lactate levels without relying on an external battery.
In addition, the team performed a roll-to-roll printing test and produced continuous patterns across 400 meters of substrate. Since the device is fabricated via screen printing, this technique simplifies manufacturing compared to traditional EBFC production. The new streamlined process cuts costs by ten yen per unit, making the technology ideal for disposable or large-scale wearable sensors. Water-based enzyme inks are a scalable and reliable method for producing high-performance EBFCs, leading to flexible, self-powered biosensors.
By 2030, the team hopes to implement these biosensors. This gives them enough time to perform device optimizations, ensure long-term validation, and integrate it with wearable platforms. They believe printing companies and healthcare device manufacturers could adopt this technology.
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