During the lectures on sensors and sensing, we were shown a video of a ‘bionic arm powered by AI’. The video showed a man controlling a prosthetic hand using his mind, and got me thinking about the extent to which a prosthetic hand might be able to replicate the function of a biological hand, in particular if prosthetic hands could ever ‘feel’.

A touchy subject

So much of what we do and how we interact with the world relies on touch. Primarily, touch is important for perceiving pressure allowing you to interact with objects at just the right force. If you pick an egg up with too much force it will break, but not enough force and you’ll drop it. Either way you’ll break a few eggs, but won’t end up with an omelette. There are a range of receptors in the skin which give us our sense of touch. The mechanoreceptors act to convey tactile information from our fingertips to our nerves. Whereas the nociceptors are free nerve endings which conduct stimuli which we perceive as painful. In the past, prosthetic hands could replicate some of the hand functionality, but have not been able to sense tactile information. However, recent technological advances are paving the way for feeling with a prosthetic hand.

E-dermis: the prosthetic skin

John Hopkins have developed an engineered material skin, made with fabric and rubber, and implanted with sensors to act as pain and touch receptors. The sensors of the e-dermis can then stimulate the nerves in the residual limb or the amputee through the skin, to allow the perception of both painful and non painful tactile stimuli.

Brandon Preston’s story

After an industrial accident at his work in 2012, Brandon Prestwood lost his lower left arm and hand. After battling with depression after the accident, Prestwood volunteered for experimental research with Cleveland State Western University, leading to the insertion of electrical conductors to the remaining nerves in his residual left upper arm, with four wires then guided up through his residual left arm and out of his shoulder. As the nerves and their link to the brain remain in the residual limb, by incorporating sensors into the prosthetic hand, the signalling can be restored. The sensors in each prosthetic finger convert contact with a surface into an electrical signal, the signal is sent to a computer, then the computer stimulates the correct nerves through the implanted electrodes. By doing this, Prestwood could touch an object with a prosthetic finger and know which finger is touching it.

More than a feeling

You may wonder why the sense of touch is so important. Would it not be easier to shorten the loop, with the sensory receptors of the prosthetic feeding back to an internalised system to modulate the force used by the hand, and forget about transmitting signals to the brain all together? However, the need to perceive touch is for more than just to pick up an egg; touch is also a vital part of being human. From handshakes, to high-fives, to hugs, touching is integral to being human. It’s even engrained in our language, if someone buys you flowers you feel ‘touched’ by the kind gesture. Even Shakespeare alludes to the importance of hands and touch, ‘now join your hands, and with your hands your hearts’. For Brandon Prestwood it was as simple as being able to hold his wife’s hand again with his missing left hand:  “It’s the emotion that goes with any kind of touch. It is … it’s being complete.”

Reference links

Prosthesis with neuromorphic multilayered e-dermis perceives touch and pain | Science Robotics

From Research to Reward: Something Lost, Something Gained: High-Tech Prosthetics Build on New Understandings of the Human Body (nationalacademies.org)

New ‘E-Dermis’ Brings Sense of Touch, Pain to Prosthetic Hands – Johns Hopkins Biomedical Engineering (jhu.edu)

The audacious science pushing the boundaries of human touch | National Geographic

Advancements in prosthetics limb technology allow feeling, control | 60 Minutes – CBS News