Depositing gold onto a silicone rubber substrate transforms the ordinarily stiff metal into a stretchable material. Each line of metal is 1.6 cm long (top).
One researcher from the University of Cambridge (UK) is on a quest to make artificial skin as realistic as possible. Stéphanie Lacour, a research project manager, is designing stretchable electronic skin, made of thin gold strips and silicone rubber, which mimics human tactile senses via implants. Her ultimate goal is to create skin that's worn over prosthetic limbs.
To simulate real skin, electronics must be directly embedded into the material and connected to neural implants. For the first prototype, Lacour wants to achieve a glovelike skin that has embedded temperature and pressure sensors.
Work on the project began about five years ago, when Lacour joined a group at Princeton University (Princeton, NJ) that was interested in flexible electronics—electronics built onto plastic material that can roll, bend, and fold. There, the idea of making artificial skin to cover robots or airplane wings evolved into one of creating lifelike electronic skin for prosthetic limbs.
The active sensors need to be as compliant as human skin, so the researchers began exploring what materials could mimic skin's elasticity. “That's when we started thinking about using rubber as a substrate, because there's a whole range, particularly silicone rubber, that has mechanical properties very close to human skin,” says Lacour.
The flexible material could function as a special skin that covers prosthetic limbs.
Using electron-beam evaporation, gold metal lines were deposited onto the silicone substrate. Lacour was able to stretch the metallic lines by almost 100% without losing any electrical conduction. “That was the big, big thing that started the whole research,” says Lacour. “We were combining such soft substrates like silicone rubber with fairly stiff materials like a metal film, and then somehow the properties were transferred in a sense.” The metal, once bonded onto the silicone rubber, was behaving similarly to the rubber. If metal film alone is pulled, it usually fractures and fails electrically at 1–2% elongation.
When Lacour began working at the University of Cambridge, she applied the stretchable electrode concept to making implants that could help nerve regeneration in the brain.
“The idea was to combine the stretchable metal lines and make them like stretchable electrodes that we could use to record neuron activity while deforming brain slices in vitro,” says Lacour. By slicing a rat hippocampus, the small region in the back of the brain that is extremely sensitive to stretching, the researchers hope to observe what happens to neural activity upon and after stretching. From there it could be possible to create implants that promote nerve regeneration. Lacour emphasizes that this research is in its very early stages.
Lacour also continues to collaborate with the Princeton group working on the electronic skin concept. The next step is to examine how sensors will be integrated into the skin and determine the types of electrical signals needed to provide feedback to the nervous system. Much work needs to be done in this area, too. “To use the implants to control a prosthetic limb, you need as many electrical connections to sensors and actuators as possible, so that the person can feel as much as possible,” says Lacour.