For years Ravinder Dahiya has been developing thin and flexible electronics, the kind that could be used for synthetic skin, and for years the material scientist has wrestled with various obstacles. These include making sensors that are small enough and electronics that can bend enough, not to mention how such things could be powered. His team is now reporting a breakthrough in which it has integrated solar cells into a graphene-based electronic skin, raising the possibility of prosthetic limbs that are both sensitive to touch and entirely self-powered.

Electrically conductive, just a single atom thick and stronger than steel, graphene has all sorts of advantages. But it hasn't always been so cheap. Back in 2015 Dahiya, who works at the University of Glasgow's School of Engineering, discovered a method of production that made graphene around 100 times cheaper than before. This was good news for any scientist working with graphene and its myriad applications, whether flexible displays for phones or medical patches for drug delivery. But for Dahiya, it made using the material as the basis for synthetic skin a whole lot more feasible.

For years, scientists have been trying to recreate the complex neural systems that enable human skin to sense touch by building pressure sensors into various materials. We have seen piezoelectric transistors incorporated into synthetic skins making them sensitive enough to read fingerprints, other approaches that use multipurpose sensors to detect temperature and humidity in addition to pressure, and others that use pressure-sensitive materials made from inorganic semiconductors to only use a small amounts of power. The issue with them all is that they need to be powered.

For their latest project, Dahiya and his team used single-layer graphene with a transparent polymeric protective layer on top, which is also pressure sensitive and enables the skin to detect minimum pressures of 0.11 kPa. Conveniently, graphene itself happens to be highly transparent and allows 98 percent of the light that hits its surface to pass through. Dahiya's team took advantage of this by placing a power-generating photovoltaic cell underneath, which provides it with the 20 nanowatts of power per square centimeter that the tactile skin needs to operate.

The next steps for the researchers involve exploring how some of that energy can be captured by batteries for later use, or perhaps even used to power an entire prostheses. One day, this could make for a self-powered prosthetic hand that can better handle soft materials. It could carry a cup of tea, for example, and even sense whether or not it is too hot to drink.

"We've already made some encouraging progress in this direction and we're looking forward to presenting those results soon," says Dahiya. "We are also exploring the possibility of building on these exciting results to develop wearable systems for affordable healthcare."

The team's latest research was published in the journal Advanced Functional Materials.