Conductive Adhesive Is Over 90 Percent Water

Hydrogel inspired by nature is electrically conductive and biocompatible.

Hydrogel being peeled away from a glass surface. (Image courtesy of MIT)

Hydrogel being peeled away from a glass surface. (Image courtesy of MIT)

Researchers at MIT have developed a new hydrogel adhesive with a toughness  comparable to the bonds between tendon, cartilage and bone.  This new material is also electrically conductive and over 90 percent water.

The hydrogel can currently bond to glass, silicon, ceramics, aluminium and titanium. It is also biocompatible, which makes it suitable as a coating for medical applications like catheters or in vivo sensors.

Researchers demonstrated the robustness of the hydrogel by bonding it to a silicon wafer. The composite material was then smashed with a hammer. Though the silicon shattered, the adhesive held it firmly in place.

A silicon wafer is bonded to hydrogel then smashed. (Video courtesy of MIT)

Chemical Anchorage & Bulk Dissipation

Hydrogels create a tough bond by dissipating the energy used to stretch the gel while remaining chemically anchored to a surface.

“Chemical anchorage plus bulk dissipation leads to tough bonding,” explained Xaunhe Zhao, who led the research team. “Tendons and cartilage harness [this concept], so we’re really learning this principle from nature.”

The adhesive hydrogel was created by mixing water with an undisclosed dissipative ingredient to create a stretchy, rubbery material. It was then chemically bonded to various test surfaces using silanes. Similar to alkanes, silanes are long chained silicon-based compounds saturated with hydrogen atoms.

Researchers used a standard peeling test to assess the force needed to separate the adhesive from the surface. On average, the adhesive hydrogel required one kilojoule per square meter to separate from the surface (0.088 BTU/ft2).

The researchers explain that the new hydrogel has a stronger bond than any previous hydrogel, elastomer, tissue adhesive or nanoparticle gel.

Hydrogel Actuators

MIT is currently investigating whether the hydrogel could serve as synthetic tendons or cartilage in soft robotics.

“Hydrogels can act as actuators,” said Zhao. “Instead of using conventional hinges, you can use this soft material with strong bonding to rigid materials… It can give a robot many more degrees of freedom.”

The hydrogel can also be used as an electrical conductor. Researchers added salt to a hydrogel sample which was then used to connect two metal plates wired to an LED.

When the circuit was closed, they found that the hydrogel enabled the flow of salt ions within the electrical loop, lighting up the LED even when the gel was stretched. As a result, the material could also have a future in flexible solid batteries.

For more information, visit the MIT Soft Active Materials Lab website.