Photograph of the team's sensor mounted on a human eyelid show a conformal interface with the eyelid tissue when the eye is open.

Over the past few decades, electronics engineers have developed increasingly sophisticated sensors that can reliably measure a wide range of physiological signals, including heart rate, blood pressure, respiration rate and oxygen saturation. These sensors were used to create both biomedical and consumer-facing wearable devices, advancing research and the real-time monitoring of health-related metrics, such as sleep quality and physiological stress.

Fatigue, a mental state marked by a decline in performance due to stress, lack of sleep, excessive activity or other factors, has proved to be more difficult to reliably quantify. Most existing methods for measuring fatigue rely on surveys that ask people to report how tired they feel, a method to record the brain's electrical activity known as electroencephalography (EEG) or camera-based systems.

Most of these approaches are unreliable or only applicable in laboratory settings, as they rely on subjective evaluations, bulky equipment or controlled environments. These limitations prevent their large-scale deployment in everyday settings.

Researchers at University of California Los Angeles (UCLA) recently developed a new type of soft sensor that can reliably measure people's levels of fatigue based on their eyeball movements. The new device, presented in a paper published in Nature Electronics, can pick up how often a wearer blinks, by tracking changes in a material's magnetic properties prompted by mechanical stress.

"Our study started with a simple question: how can we monitor fatigue in the real world?" Jing Xu, Ph.D. candidate at UCLA, told Tech Xplore. "We've long known that fatigue is more than just feeling tired—it's a gradual breakdown in how well your body or mind can perform. It creeps in quietly, affecting attention, reaction time, and even physical safety. Yet, measuring fatigue outside of a lab and in a wearable manner has always been a challenge."

The main objective of this research team's study was to develop a new sensing device that could be used to reliably measure fatigue in real-time and outside of laboratory environments. When considering the physiological effects of fatigue, they soon realized that they could predict people's levels of fatigue based on their blinking patterns.

"There's something subtle and telling about how your eyes behave when you're fatigued," said Xu. "The blink rate changes, the speed slows down, and patterns begin to shift. But could we capture those changes continuously, comfortably, and in real-world conditions? We believed we could—and so we built something entirely new."

The soft sensor developed by the researchers can be gently worn against a human eyelid, adhering to it like a secondary skin. Notably, it is highly stretchable, does not rely on batteries for electrical power and responds swiftly each time a wearer blinks.

To fabricate the sensor, the team patterned a conductive gold coil onto a thin, thermoplastic elastomer. This elastomer was in turn placed over a magnetoelastic film filled with tiny magnets.

"This setup converts eyelid movement into high-fidelity electrical signals—essentially translating every blink into data," explained Xu. "What makes this special is not just the technology, but its potential impact. This is a fully wearable, self-powered system with onboard wireless transmission, designed for daily use—not just in clinics or research labs, but out in the world where fatigue matters: on the road, in classrooms, or in high-performance jobs."

Photograph of the team's sensor mounted on a human eyelid show a conformal interface with the eyelid tissue when the eye is closed.

Irrespective of whether they are wearable or implantable, bioelectronic devices should be able to reliably operate in highly humid environments, as they will unavoidably be exposed to sweat or internal bodily fluids. Yet most existing sensors for monitoring physiological signals are not intrinsically waterproof.

"Enhancing their water resistance typically requires additional encapsulation layers, which often increase device thickness and degrade performance, such as reducing sensitivity," said Dr. Jun Chen, Associated professor at UCLA who led and supervised the study.

"When I began my independent research at UCLA, I asked myself a fundamental question: Is it possible to develop intrinsically waterproof bioelectronic devices? To explore this, I considered various natural energy modalities—electricity, magnetism, heat, and light."

The operation of the sensor developed by the researchers relies on magnetic field variations, the invisible forces surrounding magnetic materials. As these forces can penetrate water and are not adversely impacted by humidity, Dr. Chen has long been exploring their potential for creating intrinsically waterproof devices.

"Historically, magnetoelasticity has been observed only in rigid metals and alloys since its discovery in 1865, requiring mechanical pressures as high as 10 MPa—conditions incompatible with soft, flexible electronics," explained Dr. Chen. "I hypothesized that it might be possible to extend the magnetoelastic effect to soft polymer systems."

In 2021, Dr. Chen's research team at UCLA discovered a giant magnetoelastic effect in soft polymer composites for the first time. Specifically, they found that when these materials were under mechanical pressures, the flux of magnetic fields through them was significantly altered.

"This groundbreaking study demonstrated that magnetoelasticity could be realized in soft materials, with pressure thresholds reduced to around 10 kPa—readily achievable through natural biomechanical activities such as heartbeat, respiration, and ocular motion," said Dr. Chen.

"Our team is now at the forefront of advancing this novel field of soft magnetoelastic bioelectronics, striving to apply it across a wide range of biomedical and health care technologies. The most pioneering contribution from my lab over the past five years has been the discovery of the giant magnetoelastic effect in soft materials, enabling new directions in bioelectronic applications."

The effect that the researchers observed in soft polymer composites, also known as the magnetoelastic effect, had already been observed in other materials in the past. The effect was discovered by Italian physicist Emilio Villari in 1865, but has so far primarily reported in rigid metals and metal alloys with an externally applied magnetic field.

"After joining UCLA, I led my research group in the discovery of the giant magnetoelastic effect in a soft polymer system, later in a liquid permanent fluidic magnet," said Dr. Chen. "The giant magnetoelastic effect was further coupled with magnetic induction to invent a soft magnetoelastic generator (MEG) as a fundamentally new platform technology for building up human-body-powered soft bioelectronics."

The inherently waterproof, soft magnetoelastic bioelectronics introduced by Dr. Chen and his research team could potentially be used to create a wide range of sensing devices. In addition to the measurement of fatigue, they could enable the prediction of other important health-related metrics, as well as environmental changes.

"This breakthrough opened alternative avenues for practical human-body-centered energy, sensing, and therapeutic applications," said Dr. Chen.

"With the continued effort of my UCLA group, the discovery of giant magnetoelastic effect in soft systems has been extensively introduced to various research areas as a fundamentally new working mechanism, including injectable and retrievable liquid bioelectronics, liquid acoustic sensing, pulse wave monitoring, speaking without vocal fold, haptic sensing, implantable cardiovascular monitoring, respiration monitoring, muscle physiotherapy, human-machine interface, personal thermoregulation, even wind, water wave, and biomechanical energy harvesting."

The new sensor for the measurement of fatigue developed by this team of researchers could soon be improved further and released on the market. Meanwhile, the researchers are working on other bioelectronic devices that leverage the giant magnetoelastic effects uncovered in their earlier works.

"In a broader view, the giant magnetoelastic effect in soft systems represents a transformative scientific discovery, yet its full theoretical and experimental potential remains to be unlocked," added Dr. Chen.

"Our group is deeply committed to pioneering a comprehensive understanding of this phenomenon and leveraging it as a foundational platform for a new class of intelligent, responsive technologies. By exploring its integration across a broad spectrum of applications—from bioelectronics to soft robotics—we strive to catalyze breakthroughs that redefine the interface between materials and life, ultimately driving profound societal advancement and future productivity."

More information: Jing Xu et al, A soft magnetoelastic sensor to decode levels of fatigue, Nature Electronics (2025). DOI: 10.1038/s41928-025-01418-x.

Journal information: Nature Electronics