Skin-like sensor can track body movement and electrical signals inside and out

A new skin-like sensor developed by an international team led by researchers at Penn State could help doctors monitor vital signs more accurately, track healing after surgery and even help patients with bladder control issues.

The sensor, which can be worn on the skin or implanted inside the body, can measure both physical movement and electrical signals. It is made from soft, stretchable materials that mimic human skin and is designed to work for long periods of time without losing performance.

“Usually, when you try to combine different materials to get the best of both worlds, you have to make a tradeoff,” said Huanyu “Larry” Cheng, James L. Henderson, Jr. Memorial Associate Professor of Engineering Science and Mechanics at Penn State and lead co-author of the study in Advanced Functional Materials. “With this design, we were able to avoid that. We achieved low contact resistance with the skin, high sensitivity, stretchability and long-term stability, all in one device.”

Most sensors rely only on electrical conduction, which moves electrons through metal or carbon materials. This dual-modal sensor combines two different types of conductivity: electrical and ionic conduction, which moves charged atoms through a liquid. That mix helps it interface more naturally with the body, especially for internal use where the environment is wet and ion-rich, according to Cheng.

“Ionic conductivity is more compatible with the body,” Cheng said. “It allows the sensor to stick better, especially inside the body, and gives us higher quality signals.”

The team built the sensor using a combination of flower-shaped metal-organic frameworks, carbon nanotubes and a soft rubber-like material filled with ionic liquid. This combination gives the sensor high performance and flexibility while helping reduce internal wear and tear over time, Cheng explained.

One key benefit of the design is that it can detect both large movements, like a bending wrist, and very small ones, such as subtle muscle vibrations. It can also record electrical activity like heart signals or brain waves.

That makes it useful for a wide range of medical and wellness applications, Cheng said, noting that one especially promising area is bladder function monitoring. The team tested the sensor in a rodent model, measuring both the stretching of the bladder and the electrical activity of the surrounding muscles. Such data, if applied to humans, could help patients with bladder control issues, according to Cheng.

“With this interface, we can extract information from the bladder surface and potentially provide electrical stimulation as well,” Cheng said. “That means we could monitor the problem and treat it at the same time.”

Because the sensor works in both dry and wet environments, it could be used on the skin or implanted without needing different materials or designs.

“You don’t need to choose one material for outside and another for inside the body,” said Songfang Zhao, professor of materials science and engineering at the University of Jinan and co-author of the study. “This sensor performs well in both conditions.”

Tests showed the sensor can withstand thousands of cycles of stretching without losing performance. It also accurately captured common movements such as wrist bends, knee flexes and even throat motion during speech or swallowing. When compared with commercial sensors, Cheng said that it performed equally well or better in measuring heart, muscle and eye activity.

Looking ahead, the researchers plan to take the sensor beyond monitoring. Cheng said they want to create a system that not only senses a problem but also delivers treatment automatically.

“If we can add the ability to deliver electrical stimulation, we could close the loop,” Cheng said, explaining how future work could allow for broader applications. “The sensor would detect a signal, decide what to do and trigger a response like nerve stimulation or pacing the heart. … With this one device, we could go from simply collecting health data to actually helping the body heal itself.”