Modern artificial limbs are stronger and lighter than ever, thanks to new materials and advanced plastics. They also provide improved control and capabilities through miniaturized electronics and greater computer power. Mechanical joints also adjust automatically, and pneumatic components and actuators generate realistic knee and ankle movements.

Improved manoeuvrability is, however, only half of the picture.  

A team of mostly Arab researchers from the University of Connecticut and the University of Toronto is working on prosthetics that will be able to seamlessly make sense of their environment and interface with the brain to help people whose skin has been burned, and those using artificial limbs, regain the sense of feeling through electronic sensors.

A key challenge for incorporating flexible, bio-compatible sensors into artificial skin or limbs is the need for power. A significant advance was the development of triboelectric nanogenerators (TENG). First demonstrated by Chinese scientists in 2012, TENGs generate an electrical current when two materials, one with positive polarity and the other with negative polarity, come into contact and then separate, or slide past each other. 

More usually used to convert mechanical energy into electrical energy, TENGs can also be used as sensors. Engineers at the University of Toronto, Canada, and chemists at the University of Connecticut in the US, created a flexible, waterproof sensor that can identify different forms of movement, pressure and various potential hazards. 

Restoring senses 

“We were initially trying to produce a skin sensor to restore a sense of feeling for those who lose limbs or those who have suffered burns,” said Islam Mosa, of the University of Connecticut. “Then we wanted to add extra functions that human skin does not have.” 

Mosa and his collaborators call their prototype the ferrofluid-based triboelectric nanogenerator (FO-TENG). Ferrofluids are liquids that contain nanoscale magnetic particles in suspension that become magnetic in the presence of a magnetic field. The FO-TENG consists of a liquid suspension of nanoscale iron oxide particles inside a silicon tube wrapped in a copper coil, all sealed into a silicone rubber gel. 

Stimuli like magnetic fields, acoustic waves and direct physical forces cause the silicone tube and ferroliquid to move with respect to each other, generating a positive charge in the tube and a negative charge in the ferroliquid. This creates an electric current which is picked up in the copper wire, activating the sensor. This electrical output varies based on the strength of the magnet and their proximity.

 This means it is able to produce different outputs for walking, running and jumping, with the signal strength varying according to movement intensity. Stretching the sensor to up three times its normal length, as well as repeated crumpling, bending and twisting, did not undermine its performance. 

Underwater simulations showed the sensor generated different electrical output patterns in response to different sizes and forms of wave. The researchers suggest the FO-TENG could even be used to track the performance of swimmers, and as an alarm in thre case of drowning. “A person who is drowning has a random style of motion that can be distinguished from the repetitive movements of a swimmer,” says Mosa. 

The researchers developed basic signal processing software to show it is possible to differentiate the outputs of the FO-TENG in different circumstances. They are investigating applications in prosthetics, robotics, artificial skin, remote healthcare monitoring and wearable hazard detection devices. 

Alex Chortos, a researcher who develops stretchable electronics at Stanford University, and who is not involved in the FO-TENG project, says the sensor’s stretchability makes it especially useful for certain applications. “This type of tubular structure is advantageous because it can easily be integrated with biological structures,” says Chortos. “That’s not easy to do with a flat, flexible device that will wrinkle and move around.” 

Advancing the field

Advances over recent years in flexible electronics are promising, however, components designed for advanced prosthetics and artificial skin must be able to interface smoothly with the human body. Chortos and co-workers in Professor Zhenan Bao’s group at Stanford University developed a stretchable circuit that can sense different degrees of pressure and transmit that information in signals the brain can recognize. 

“Because the information is encoded in the same way as the human body does it, we can directly input that signal into things like nerves,” says Chortos. 

In a 2018 paper, the group demonstrated the ability to recognize the direction and speed of an object moved across their pressure sensors. They were also able to identify braille characters pressed on an array of their sensors, and to control muscles in a disconnected cockroach leg via electrical signals generated by applying pressure to their sensors. 

Chortos was part of the team that last year published details of technology designed to provide sensitivity to pressure comparable to humans. Their ‘e-skin’ contains arrays of capacitors that can measure the intensity and direction of forces simultaneously and in real time. The researchers showed how it could be used to allow a robot to handle delicate objects like raspberries without damaging them. 

Another major challenge is finding new ways to manufacture artificial skins and limbs so that new flexible sensors, actuators, computer chips and other components are integrated effectively. Last year, a group led by researchers at Harvard University, in the US, demonstrated the ability to integrate multiple features and materials within 3D printed soft robots, using a conductive organic ink they developed. They produced and tested a three-fingered soft robotic gripper that can sense light and deep touch, curvature and temperature. 

The complex systems that allow that allow human bodies to understand what is going on evolved over millions of years. Developing the next generation of artificial limbs and skin that can mimic this ability and communicate with biological systems, including the body is a major challenge. Like other complex problems, it becomes less daunting when broken up into smaller, more manageable parts. Recent evidence suggests that scientists are close to turning the theoretical vision of allowing prosthetics and man-made skin to sense their environments into a technological reality. 

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