Stretchy, Soft, and Sticky: Advancing the Next Generation of Wearable and Implantable Sensors

Wearable and implantable biosensors have the potential to revolutionize health care by diagnosing, monitoring, and even treating a wide range of health conditions. Recent innovations in the lab of Wei Gao, professor of medical engineering at Caltech and a Heritage Medical Research Institute Investigator, are pushing the field forward through the creation of soft, stretchable, tissue-integrated bioelectronics for continuous sensing and adaptive therapy. Two new studies highlight complementary gains in materials and technology while addressing different challenges and applications.

Sensors That Stretch

To allow for medical sensing that can move flexibly with the body and even internal organs such as a beating heart, scientists in the Gao lab have developed a bioelectronic material that maintains conductivity and a strong connection with the skin or tissues as they deform. The super-stretchy biocompatible interface meets a key need for the next generation of wearable and implantable sensors.

The new material, aptly called a stretchable interface for resilient electrochemical sensing (SIRES), can stretch as much as 300 percent without losing its ability to transmit high-quality electrical signals. Gao and his colleagues describe SIRES in a paper that appeared in the May 28 issue of the journal Science.

When researchers attach a chemical sensor to an internal organ, they are trying to measure biomolecules that signal health status. But because of the materials used to create these sensors, often when the organ moves, the sensor either fails or becomes unreliable in its performance. To address this, Gao's team—led by Yadong Xu, a former postdoctoral scholar from Gao's lab, and Caltech graduate students Xiaotian Ma (MS '24) and Kexin Fan—developed a three-part material that makes use of polyurethane, an elastomer (a rubber-like solid) that is also biocompatible.

First, rather than using standard conducting wire within the device, the researchers turned to liquid metal to act as the conductor. Liquid metal can stretch yet maintain the same electrical resistance. Therefore, by mixing liquid metal with polyurethane, the scientists created a strain-resilient conductor.

Second, they created a stable flexible electrode for sensing. Normally, the electrodes in biosensors are made from a metal such as gold or from carbon nanotubes, but these can crack with even a tiny bit of stretching. The nanotubes are excellent at detecting electrical changes when a target molecule binds to their surface. Gao's team embedded carbon nanotubes in polyurethane, giving the nanotubes the ability to stretch while remaining interconnected. When the collection of nanotubes elongates, some of the connections between individual nanotubes break, causing electrical conductivity to decrease. But because the overall surface area of the electrode also increases with stretching, more molecules can be exchanged at the sensor, and this counteracts the decrease.

"If you tune the carbon nanotube level properly, the two effects balance out, and you get a stable response," says Gao, who is also a Ronald and JoAnne Willens Scholar. "So even in the case of an organ that deforms a lot, such as a beating heart, your sensor performance will not change."

The third component of SIRES is a stretchable functional coating of polyurethane that allows the researchers to embed any enzymes needed for chemical sensing inside.

"The conductor, the electrode, and the functional film are all made of polyurethane embedded with different things. That means the whole thing is very stretchable and also biocompatible," Gao says.

The team has tested SIRES with the group's sweat sensors, showing that even with vigorous exercise, the sensors maintain stable performance. They have also tested the material successfully in implantable sensors in animal models on organs such as the bladder, heart, stomach, and intestines—all of which deform significantly during normal functioning.

A Platform That Sticks

Another challenge in making reliable implantable sensors is they must stay attached to slick surfaces, hopefully for long periods of time. In a Nature Materials paper published June 10, Gao and members of his lab reported on a new device that not only sticks to organs and other internal structures but can also provide therapeutic interventions as needed.

"What's exciting about this work is that we developed a soft stretchable implantable platform that can firmly adhere to wet tissues while remaining stable even as the body moves," says Jiahong Li (PhD '26), a postdoctoral scholar in Gao's lab and first author of the paper. "The device can simultaneously monitor physical and chemical signals, and deliver electrical stimulation, which could help enable more seamless and long-lasting interfaces between electronics and the body."

Like the in SIRES study, the team used stretchable liquid-metal and elastomer composite strategies to make biophysical and biochemical sensors that maintain stable electrochemical performance under large deformation, such as might be caused by an expanding stomach. But for the platform to work inside the body, they had to develop a new adhesive to make it stick to wet tissues.

Gao and members of his lab created a molecular hydrogel, which is a 3D net-like structure made of water-rich materials to keep the adhesive soft and flexible. They incorporated a rubber-like elastomer to ensure the substance could stretch along with the biosensors. When the hydrogel encounters wet tissue, a chemical reaction called polymerization occurs that bonds the tissue and the device together.

"Not only does the device adhere to an organ, but we also show that the adhesion can be very, very strong," says Gao. "They can maintain stability for months so that you can have a stable interface."

By pairing a strong adhesive with stretchable chemical sensors and electrodes for physical sensing and electrical stimulation, the researchers were able to construct a small and implantable closed-loop platform called ElHyX (Elastic Hydrogel X, with X representing the platform's multifunctionality) that can monitor and treat disease. For example, Gao and Li show that the device could be used for in vivo electrocardiogram monitoring, glucose sensing, and nerve stimulation.

"In an animal model, we used the chemical sensor to monitor glucose levels and stimulated nerves that regulate insulin release when glucose got too high to keep them in a healthy range for diabetes management," Gao says. "Physical sensing of the heart can also track diabetes-related hypertension. This type of multifunctional closed-loop system is not something previously demonstrated."

The components of the ElHyX platform can be made using 3D-printing technology, meaning the devices can be made quickly and at low cost. Next, the researchers will work toward improving longer-term stability and reliability before testing the device in humans.

"Unlike a wearable sensor, we will need to do surgery to implant this platform," Gao says. "It's still quite a new direction for us, so one of the most important challenges is to ensure it can last many months and hopefully even years."

The team believes the ElHyX could be used to monitor and treat a wide range of health conditions, including pain, stress, and anxiety.

"In the future, we hope platforms like this could support more personalized monitoring and treatment for chronic diseases," Li says.

Additional authors of the Science paper, "Strain-resilient intrinsically stretchable electrochemical biointerfaces," are former Gao lab postdoctoral scholars Jihong Min (PhD '24), Wenzheng Heng (PhD '25), and Songsong Tang; current postdoctoral scholars Jin Qu, Jiahong Li (PhD '26), Gwangmook Kim, and Wenjian Li; and Caltech graduate students Ruixiao Liu (MS '25) and Shukun Yin (MS '24). Funding for the work was provided by grants from the National Institutes of Health, the National Science Foundation, the Army Research Office, the Office of Naval Research, the US Army Medical Research Acquisition Activity, and the Heritage Medical Research Institute. Critical support and infrastructure were provided by the Kavli Nanoscience Institute at Caltech.

Gao lab graduate students Kexin Fan, Xiaotian Ma (MS '24), and Yonglin Chen; former graduate students Canran Wang (PhD '26) and Jihong Min (PhD '24); and former postdoctoral scholars Yu Song and Yadong Xu were additional authors of the Nature Materials paper, "Strain-insensitive wet-tissue-adhesive biphasic bioelectronics for physicochemical monitoring and adaptive therapy." The work was supported by the National Institutes of Health, the Army Research Office, the National Science Foundation, the US Army Medical Research Acquisition Activity, and the Heritage Medical Research Institute.