Engineers at the University of Glasgow, Scotland (UK), have developed a technology to power synthetic skin with energy from the sun. Using optical transparent graphene, the breakthrough approach could help to create advanced prosthetic limbs capable of returning the sense of touch to amputees.
The new skin requires 20 nanowatts of power per square centimeter. “The skin consists of single-layer graphene coplanar interdigitated capacitive (IDC) electrodes on a 125 µm-thick PVC substrate, connected to titanium/gold (10 nm/100 nm) contacts deposited on the edges of the electrodes,” says Dr Ravinder Dahiya, leader of the Bendable Electronics and Sensing Technologies (BEST) group at the University of Glasgow. The single-layer graphene, he explains, is first transferred to the PVC substrate through a hot lamination process, before etching out the copper foil on which the graphene is originally grown. Then metal contacts are deposited on the edges of the graphene layer, using electron beam evaporation and a shadow mask. Finally the graphene is patterned into interdigitated electrodes using a computer-controlled plotter blade. The sensors on the skin are completed by a 25 μm-thick layer of polymer (PDMS) spin-coated on top of the graphene channel. This ultimate layer not only serves as a deformable dielectric layer, it also encapsulates the device, according to its inventors.
Using amorphous silicon solar cells
“For a self-powered solution, we stacked the transparent skin on top of a commercially available amorphous silicon solar cell,” says Dahiya, adding that they intentionally picked the “poorest solar cell” in terms of efficiency and power conversion. “We demonstrated that while the 39.6 x 22.9 mm2 cell could produce 160 μW cm−2 of power, the skin consumed only 31 and 55 nW, before and during touch respectively. This means the large transparent skin stacked with flexible and stretchable photovoltaic cells could not only provide a self-powered prosthetics skin but could also contribute to improved system efficiency by storing the excess power or using it to drive the actuators in a robotic hand.”
Transparent tactile skin
The truly unique and innovative aspect of this solar-powered technology is the transparent tactile skin on the surface of the solar cell. “Without transparency, this would fail,” Dahiya stresses. “Existing tactile skins are non-transparent.” This is where the graphene plays its important rule. “Graphene provides the critical transparency,” the expert says. “But not only this, the excellent properties of graphene allowed us to develop skin that needs very little power for its operation.”
Promising for amputees
Being lightweight and very thin, the breakthrough technology could bring new hope for amputees. “Putting this skin on the artificial limbs will not add much weight,” the engineer confirms. “In fact, its energy generation capacity means reliance of external batteries will be minimal.”
Without batteries, the engineer notes, the weight of artificial limbs will be brought close to that of normal human limbs, thus improving the sense of ownership — i.e., amputees will start feeling the artificial hand as natural part of their body. “The sensitivity of skin to touch and pressure will significantly contribute to this as well,” the researcher adds. “In summary, this technology brings the prosthetics a step closer to a normal hand. As an analogy, normal human needs sunlight to convert fats under the skin to get Vitamin D. Amputees will do this to charge their (artificial) senses.”
Path and timeline to availability in the real world
The University of Glasgow scientists have already submitted a patent application. “Last year, we demonstrated low-cost manufacturing and transfer of high-quality graphene on flexible substrates," Dahiya notes. “With these works we have the demonstrated the capacity to scale up the research and make a quick transition to industry.”
The team is actively seeking collaborations with industry partners and aims to bring new solutions to market in next one to two years. As for skin, the researcher says the timeframe could be three to four years, as many more sensors will be required, compared with the current pressure and temperature.
Robots and other applications
The expert confirms that the pioneering technology could have “significant impact” on the field of robotics in the future. “With tactile feedback, robots will be able to interact safely with objects and humans. They will be able to exploit multiple contact from entire body (not just hands) to execute complex tasks. The skin will also find application in emerging applications such as driverless cars, smart cities and internet of things as energy autonomous sensory components will be key enablers for these applications.”
Dahiya and his colleagues are now working on further advances in skin technology. “We are adding the ultra-thin and flexible energy storage layer to the skin and would like to demonstrate the full operation of prosthetic limb without external power,” he says. “This demonstration will also indicate utility of our approach to new areas such as driverless cars, where both solar power and tactile skin will play important role in enhancing battery life and the functionalities, e.g., interiors based on touch or gestures.”
Written by Sandra Henderson, Research Editor, Solar Novus Today