Researchers Hack Electronics’ Surface Chemistry To Bond With Hydrogels

Hydrogels are a lot like people: soft, squishy, and made mostly of water.

Because their materials properties match those of biological tissue so well, hydrogels provide attractive foundations for emerging biomedical devices, such as implantable sensors and diagnostic wound dressings.

But scientists have had trouble interfacing hydrogels that behave like soft tissue with sensors and other electronics that are rigid and work only when dry, says Xuanhe Zhao of Massachusetts Institute of Technology.

Zhao and his team have now developed hydrogel devices that cling to electronic components without compromising their ability to flex and stretch like tissue (Adv. Mater. 2015, DOI: 10.1002/adma.201504152).

To improve the bond between the mismatched materials, the team tailored the surface chemistry of a device’s onboard electronics to better mesh with the hydrogels. The first step in making a device was encasing an active electronic component, such as a temperature sensor, in a silicone elastomer to protect it from the hydrogel’s moisture. The researchers then bound the silicone to a thin piece of glass.

They then functionalized the surface of the glass by marinating it in a solution of 3-(trimethoxysilyl)propyl methacrylate. When introduced to a hydrogel matrix, this surface could then bond with polyacrylamide in the gel, the team reports. A similar marinating solution is also capable of functionalizing the surface of flexible titanium wiring used to connect electronics within a hydrogel.

These covalent bonds keep the electronics anchored to the hydrogel, even as it bends and stretches, Zhao says.

“That’s very clever,” says A. Toby A. Jenkins of the University of Bath, who was not involved with the study. Earlier this year, Jenkins and his team created hydrogel wound dressings that detect bacterial toxins (ACS Appl. Mater. Interfaces 2015, DOI: 10.1021/acsami.5b07372). The sensing elements in these devices relied on fluorescent dyes housed within soft vesicles that are stable in the wet matrix of a hydrogel.

Because hydrogels can now accommodate rigid electronics, it’s easier to envision a future where dressings or implants contain radio-frequency ID chips that could send diagnostic information to cell phones, Jenkins adds.

Although the MIT team demonstrated its new approach using select materials—titanium wires and a polyacrylamide hydrogel—the method could easily be adapted for other common wire and hydrogel compositions, Zhao tells C&EN.

The team also developed hydrogel-electronic devices to demonstrate the potential of their method, including wearable patches with light-emitting diodes and “smart” wound dressings with temperature sensors and hollow channels to deliver drugs to damaged tissue.

But Zhao is most excited about the smart implants this approach could enable, such as glucose sensors or neurological theranostics—devices that are both therapeutic and diagnostic.

“The body will only see the hydrogel matrix,” Zhao says. “We’ve developed a seamless interface between the body and external electronics.”