Battery-free wearable sensor continuously monitors health through sweat

University of California, Irvine researchers have invented a wearable, wireless, battery-free, bioelectronic sensor to monitor users' health by analyzing molecular biomarkers in human sweat.

The device is called the In-Situ Regeneratable, Environmentally Stable, Multimodal, Wireless, Wearable Molecular Sweat Sensing System, or IREM-W2MS3, and is described in a study published today in Nature Biomedical Engineering.

According to Rahim Esfandyar-pour, senior author of the study and assistant professor of electrical engineering and computer science, the standout features of the invention include its ability to regenerate sweat-sensing surfaces, induce perspiration in wearers when needed and operate continuously over long timespans.

The regenerative capability of the IREM-W2MS3 addresses one of the biggest obstacles in long-term wearable biosensing, which is sensor surfaces that lose performance after repeated measurements because molecules remain bound to the sensing layer. By being able to refresh itself, generate sweat and be worn for long durations outside of laboratory or clinical settings, the device offers users a health monitoring platform that is robust and highly practical."

Rahim Esfandyar-pour, senior author of the study 

An improved approach for sensing biomarkers

Worn as a flexible skin patch and paired with a standard Android smartphone or a custom wrist-watch-like reader, the system simultaneously tracks cortisol, glucose, lactate and urea in sweat. Those biomarkers are associated with stress response, metabolic activity, physical exertion and kidney function.

The researchers say the platform could support future applications in chronic disease management, stress and mental health monitoring, sports performance, preventive medicine, early disease detection research, and remote community health monitoring.

"Chronic illnesses and stress-related conditions affect hundreds of millions of people worldwide, making early diagnosis and consistent health monitoring essential in reducing disease burden and improving patients' quality of life," said Esfandyar-pour. "Our IREM-W2MS3 wearable accomplishes the goal of stable, ongoing and long-term sweat monitoring."

Allowing for continuous tracking

Wearable sensors have emerged as promising tools for continuous health tracking because they can be cost-effective and easy to use. Sweat is an especially attractive target because it can be collected noninvasively and contains ions and metabolites that reflect biological changes occurring in the body.

But reliable long-term sweat sensing has been difficult to achieve, according to Esfandyar-pour, who said many molecular sensors lose accuracy as target molecules accumulate on the sensing surface.

Other sensors rely on collecting enzymes, antibodies or aptamers that can degrade under changing temperature, humidity and pH conditions. Existing systems also often struggle to measure multiple biomarkers simultaneously with high precision.

"Despite rapid progress in the field, existing wearable devices have consistently fallen short in several critical areas," Esfandyar-pour said. "They lack environmental stability for real-world, outside-the-lab use; they cannot regenerate their sensing surfaces for repeated, long-term use; and they are largely incapable of simultaneously detecting multiple molecular biomarkers with high precision."

According to Esfandyar-pour, the defining advance of the IREM-W2MS3 is its ability to regenerate its sweat-sensing surfaces. As it's being operated, the device automatically applies a low voltage to the sensing surface, which releases signals to restore the sensor's sensitivity and selectivity without manual cleaning, replacement or intervention. In testing, the regeneration process achieved a nearly full recovery rate across multiple cycles, according to the researchers.

Solving a key challenge

The system also solves another key challenge in sweat monitoring: producing enough fresh sweat for analysis without requiring exercise.

The IREM-W2MS3 patch does not need an internal battery. Instead, it draws power wirelessly from a near field communication-enabled smartphone or a custom wrist-watch-like reader. When a user brings the phone or reader near the patch, an induced electromagnetic field supplies a small current to activate a biocompatible hydrogel embedded in the device. This process allows the system to generate and collect sweat samples without requiring strenuous physical activity.

The research team tested IREM-W2MS3 under varying pH and temperature conditions over a continuous 21-day period. The device demonstrated consistent sensing performance with no measurable signal degradation. The researchers said this stability is important for moving wearable molecular sensing from controlled laboratory environments into real-world settings.

Sensing four clinically relevant biomarkers

The sensor simultaneously detects four clinically relevant sweat biomarkers. Cortisol is a stress-responsive hormone that provides insight into anxiety, depression and hypothalamic-pituitary-adrenal axis dysregulation. Glucose is relevant to prediabetes and diabetes monitoring. Lactate can reflect physical exertion or metabolic dysfunction. Urea is associated with kidney health and function.

By tracking these molecules together, continuously and over time, the IREM-W2MS3 can provide a broader picture than devices that monitor only one biomarker.

"Applications for the IREM-W2MS3 are numerous and varied," Esfandyar-pour said. "Potential uses include chronic disease management, stress and mental health monitoring, sports science and performance optimization, preventive medicine and early disease detection and remote community health monitoring. We designed this wearable to be durable, easy to use and highly reliable."

The research team submitted a patent application through UC Irvine's Beall Applied Innovation, and the technology is currently under further development. The team is exploring pathways toward translation and manufacturing.

Joining Esfandyar-pour on this project were Jerome Rajendran, postdoctoral scholar, Xiaochang Pei, Ph.D. student; Anita Ghandehari, PhD student; Shingirirai Chakoma, Ph.D. student; Jorge Tavares-Negret, Ph.D. student; and Sahar NajafiKhoshnoo, Ph.D. student in the UC Irvine Department of Electrical Engineering and Computer Science. Funding was provided UC Irvine's Samueli School of Engineering.