FEATURE: The quest to make robots feel

Jennifer Johnson

The development of electronic skin could have important medical benefits and lead to industrial robots that can handle fragile materials (Credit: Shutterstock)
The development of electronic skin could have important medical benefits and lead to industrial robots that can handle fragile materials (Credit: Shutterstock)

Sentient robots have been the subject of numerous works of science fiction – some utopian, some dystopian and others downright terrifying.

If anything, these stories show us that creating artificial consciousness is fraught with ethical dilemmas (to say nothing of the technical challenges). It’s easy to see why building sentient robots isn’t a pressing research priority. However, there are advantages to having robots – especially those deployed in industry – experience the world as much like humans as possible.

The ability to sense pressure and temperature, for instance, would give industrial robots some clear operational skills. But first they’d need a sense of touch, and an organ like human skin to facilitate it. Around the world, there are a number of scientists working to create artificial skin, sometimes known as electronic skin, for use across the field of robotics and medicine. But formulating a material that is as durable and malleable as human skin is no easy task, especially when many electronic components are rigid by nature.

Finding flexibility

One of the first breakthroughs in this area came in 2003, when researchers at the University of Tokyo designed a flexible electronic mesh and wrapped it around the bones of a robotic hand. The material was made from a layer of rubbery polymer, which contained flakes of graphite to conduct electricity. When the sheet was squeezed, the electrical resistance was changed, and this was detected by a network of transistors under the rubber. The resulting skin was bendy, much like a sheet of paper, though not elastic as is human skin. 

In the next few years, the research team, led by Takao Someya, would create an electronic skin that was both flexible and stretchy. They made a sheet with embedded transistors and pressure sensors and used a mechanical punching process to slice away the material between the sensor nodes. This net-like material could be stretched by 25% – and there were still further improvements to come. By 2013, Someya and his team had set organic transistors and tactile sensors on an ultra-thin polymer sheet that measured just 1 micrometre thick. It could stretch up to 230% and accommodate repeated bending. 

The team would eventually wrap the ultra-thin skin over the heart of a rat during a three-hour surgery. By recording the electrical activity in the rodent’s heart, the skin was able to identify the position of a structural defect. Someya told CNN that this kind of technique could be deployed in human patients at some point, as the e-skin puts the heart under less stress than the electrodes traditionally used to monitor cardiac activity. Although some e-skins were designed to allow robots to “sense” their surroundings to some degree, it’s now clear that the materials can also facilitate new insights into the human body.

Sticky sensors

In 2019, engineers at Stanford University in California developed sensors that stick to human skin like plasters and which are able to detect physiological signals, such as pulse and respiration. The sticky sensors then send wireless readings to a receiver attached to the wearer’s clothes. According to Zhenan Bao, the chemical engineering professor behind the project, wearable e-skins could eventually be deployed to help monitor patients with sleep disorders or heart conditions. 

The stickers work in a similar way to the RFID tags that are widely used in identification badges. Inside is a small antenna that can harvest incoming RFID energy from the receiver on the wearer’s clothing. Readings are then taken directly from the skin and passed back to the receiver. In their first iterations, the stickers used tiny motion sensors to record respiration and pulse rates, although the researchers would subsequently look into integrating sweat and temperature sensors, too.

Soft robotics

Scientists at the University of Toronto in Canada have developed their own adhesive sensor to record the sensations of human skin, although this one notably doesn’t feature an antenna and receiver set-up. The material – dubbed artificial ionic skin – is made up of two oppositely charged sheets of hydrogels, hydrophilic polymeric networks cross-linked to make an elastic structure. In overlaying negative and positive ions the scientists create a “sensing junction” on the gel surface.

The artificial ionic skin could be used to make exo-suits for patients (Credit: Daria Perevezentsev/ University of Toronto Engineering)

The artificial ionic skin could be used to make exo-suits for patients (Credit: Daria Perevezentsev/ University of Toronto Engineering)

If the skin is subjected to strain, humidity or changes in temperature, it generates controlled ion movements across the junction, which can be measured as electrical signals. “When you press this gel, you create a separation of positive and negative charges,” explains Xinyu Liu, an associate professor at the university. “You could call it a power-generating structure because you can press and even measure a small voltage across the two layers.”

In future, the hydrogel structure could form the basis of skin-like wearables that measure athletic performance. The material could also help patients recovering from injuries. “Rehabilitation exo-suits are one of the major applications we’re working on,” Liu says. “If we create a hydrogel suit for patients, we can measure the realtime motion data when they do rehabilitation exercises.” He also envisions the highly pliable sensors being used in soft robotics, an emerging subfield in which robots are designed to move and adapt much like a living organism.

“This whole area was inspired by soft-body animals, like the octopus,” Liu says. “They can grasp things very gently, but also very effectively. If you attach some of these soft sensors onto each finger of a gripper, you can actually detect the motion and the bending angle of that gripper.” This means that soft robots would be uniquely equipped to handle brittle or fragile materials in a way that today’s rigid industrial materials are not. 

Solar sensing

Late last year, a paper published by researchers at the University of Glasgow described how a robotic hand wrapped in a flexible polymer embedded with solar cells could “interact” with objects without using traditional touch sensors. The solar cells generate enough energy to power microactuators – which control the hand’s movements – while also providing a sense of “touch” by measuring fluctuations in the output of the solar cells. 

“By measuring the change in the energy that is generated by the solar cells, we can detect whether there’s a contact or there’s a proximity,” says Professor Ravinder Dahiya of the University of Glasgow’s James Watt School of Engineering. “For example, when there’s a shadow, the intensity of light between two solar cells will decrease. As a result, energy decreases. And we can correlate that with the distance of an approaching object.”

If robotic arms in factories were equipped with this “skin that sees”, there could theoretically be a reduction in the number of accidents. And if the generated power was stored in devices, such as flexible supercapacitors, that work alongside the skin, it wouldn’t have to be constantly exposed to sunlight to function. In the medical field, the advent of this solar skin marks another step towards self-powered prosthetic devices. According to Dahiya, it might also be an important enabling component for autonomous humanoid robots capable of operating in extreme, remote environments. 

“We are thinking about robots in space,” he says. “Those robots will have to do some tasks but they need power also. Currently, if you look at humanoids, they cannot hold and manipulate a screwdriver because there’s no tactile feedback. These gaps could be bridged by e-skin – and, if that skin comes with energy, it solves other problems, too.”

While science fiction has taught us that there might be hazards to creating robots with feelings, there are compelling reasons to build robots that can more literally feel and respond to their surroundings. Although this level of autonomy is a long way off, improvements in e-skins are certain to help humans along the way.

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.


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