Stretchable hydrogel electronics

MIT engineers have designed a water-based ‘Band-Aid’ that can incorporate temperature sensors, LED lights and other electronics, as well as tiny, drug-delivering reservoirs and channels.

The key to the design is a hydrogel matrix designed by Xuanhe Zhao, the Robert N. Noyce Career Development Associate Professor in MIT’s Department of Mechanical Engineering.

The hydrogel is a rubbery material, mostly composed of water, designed to bond strongly to surfaces such as gold, titanium, aluminium, silicon, glass and ceramic.

Electronics coated in hydrogel may be used not just on the surface of the skin but also inside the body, for example, as implanted, biocompatible glucose sensors, or even soft, compliant neural probes, said Zhao.

“Electronics are usually hard and dry, but the human body is soft and wet. These two systems have drastically different properties,” Zhao said.

“If you want to put electronics in close contact with the human body for applications such as healthcare monitoring and drug delivery, it is highly desirable to make the electronic devices soft and stretchable to fit the environment of the human body. That’s the motivation for stretchable hydrogel electronics.”

A strong and stretchy bond

Synthetic hydrogels are brittle, barely stretchable and adhere weakly to other surfaces. “If you want to make an electronic device out of hydrogels, you need to think of long-term stability of the hydrogels and interfaces,” said Zhao.

To get around these challenges, his team came up with a design strategy for robust hydrogels, mixing water with a small amount of selected biopolymers to create soft, stretchy materials with a stiffness of 10 to 100 kilopascals — about the range of human soft tissues. The researchers also devised a method to strongly bond the hydrogel to various non-porous surfaces.

In the new study, the MIT researchers applied their techniques to demonstrate several uses for the hydrogel, including encapsulating a titanium wire to form a transparent, stretchable conductor. Zhao also created an array of LED lights embedded in a sheet of hydrogel. When attached to different regions of the body, the array continued working, even when stretched across highly deformable areas such as the knee and elbow.

A versatile matrix

Finally, the group embedded various electronic components within a sheet of hydrogel to create a ‘smart wound dressing’, comprising regularly spaced temperature sensors and tiny drug reservoirs.

The researchers also created pathways for drugs to flow through the hydrogel, by either inserting patterned tubes or drilling tiny holes through the matrix. They placed the dressing over various regions of the body and found that even when highly stretched the dressing continued to monitor skin temperature and release drugs according to the sensor readings.

The ‘smart wound dressing’ releases medicine in response to changes in skin temperature and can be designed to light up if, say, medicine is running low. When the dressing is applied to a highly flexible area, such as the elbow or knee, it stretches with the body, keeping the embedded electronics functional and intact.

An immediate application of the technology may be as a stretchable, on-demand treatment for burns or other skin conditions.

Delving deeper, Zhao envisions hydrogel to be an ideal, biocompatible vehicle for delivering electronics inside the body. He is currently exploring hydrogel’s potential as a carrier for glucose sensors as well as neural probes. “Currently, researchers are trying different soft materials to achieve long-term biocompatibility of neural devices. With collaborators, we are proposing to use robust hydrogel as an ideal material for neural devices, because the hydrogel can be designed to possess similar mechanical and physiological properties as the brain.”

Zhao’s co-authors on the paper are graduate students Shaoting Lin, Hyunwoo Yuk, German Alberto Parada, postdoc Teng Zhang, Hyunwoo Koo from Samsung Display, and Cunjiang Yu from the University of Houston. This research was funded, in part, by the Office of Naval Research, the MIT Institute for Soldier Nanotechnologies and the National Science Foundation.