Much effort is underway to bridge the technological gap between real and artificial skin, and researchers have already made flexible and stretchable tactile sensors based on various micro- and nanomaterials. Pressure-sensitive rubbers used as resistive elements that respond to tensile strains and that are integrated with flexible organic electronics and nanomaterial-based transistors are some examples. Such devices rely on the electrical properties of material and composites to measure a signal.

Now, a team led by Robert Shepherd has taken a new approach by using stretchable optical waveguides for sensing strain in a prosthetic hand. The sensors are easy to fabricate, are made from non-toxic components, and output high-quality data, explains team member Huichan Zhao, and the hand in which they are integrated can measure the softness and the roughness of the objects it touches.

Waveguide sensors are directly integrated into each finger

The researchers made their optical waveguides from rubber filled with a light-emitting diode and photodetectors. They made the core of the waveguides by moulding an optically transparent rubber using soft lithography techniques. They then encapsulated this rubber with a lower index refraction rubber. The waveguides can measure elongation, curvature and pressure, says Zhao, and they are very precise – even when greatly stretched.

“We pre-assembled three waveguides into the mould we used for each finger of the hand and then poured the silicone used to make the fingers over them, she adds. “The waveguide sensors are thus directly integrated into each finger and move with it.”

“Sophisticated grasping and sensory feedback”

By then measuring the ways in which light from the waveguides passed through the fingers when the waveguides were stretched, bent or pressed while they touched and grasped objects, the researchers say they were able to record tactile information, such as shape, texture and softness for various objects like sponges, silicon rubber and tomatoes. The hand, which itself was attached to the end of a robotic arm, was even able to select the ripest of three tomatoes in several trial experiments. “It could feel roughness on the scale of several hundreds of microns, though not close to the nanometre-scale recognition ability of human hands,” Huichan tells

“Most robots today have sensors on the outside of the body that detect things from the surface,” she adds. “Our sensors are integrated within the body, so they can actually detect forces being transmitted through the thickness of the robot, a lot like we and all organisms do when we feel pain, for example. Our experiments also demonstrate the high resolution of our sensors as well as their compatibility with soft robotic systems, and the application we explored in this work could potentially provide sophisticated grasping and sensory feedback for advanced, custom prosthetics at low cost."

The team, reporting its work in Science Robotics DOI: 10.1126/scirobotics.aai7529, says that it now plans to increase the density of the waveguides “innervated” into a prosthetic hand and then use machine learning tools to better interpret the signals produced by the devices. The project is funded by the AFOSR (Air Force Office of Scientific Research).