Hydrogels are lightweight, stretchable, and biocompatible, making them suitable materials for contact lenses and tissue engineering scaffolding. They are, however, poor at conducting electricity, which is needed for digital circuits and bioelectronics applications.
The team at Soft Machine Labs has suspended micrometer-sized silver flakes in a polyacrylamide-alginate hydrogel matrix. After going through a partial dehydration process, the flakes formed percolating networks that were electrically conductive and robust to mechanical deformations. By manipulating this dehydration and hydration process, the flakes could be made to stick together or break apart, forming reversible electrical connections.
"With its high electrical conductivity and high compliance or 'squishiness,' this new composite can have many applications in bioelectronics and beyond," explained Carmel Majidi, professor of mechanical engineering. "Examples include a sticker for the brain that has sensors for signal processing, a wearable energy generation device to power electronics, and stretchable displays."
Previous attempts to combine metals and hydrogels have tended to have resulted in a trade-off between improved electrical conductivity and lowered compliance and deformability. Majidi and his team have attempted to tackle this challenge, building on their expertise in developing stretchable, conductive elastomers with liquid metal.
The silver-hydrogel composite can be printed by standard methods like stencil lithography, similar to screen printing. The researchers used this technique to develop skin-mounted electrodes for neuromuscular electrical stimulation. According to Majidi, the composite could cover a large area of the human body, "like a second layer of nervous tissue over your skin."
Future applications could include treating muscular disorders and motor disabilities, such as assisting someone with tremors from Parkinson's disease or difficulty grasping something with their fingers after a stroke.
The research findings have been published in Nature Electronics.