In a study published in June 2018 in Science Robotics, a team of researchers led by Johns Hopkins Biomedical Engineer Luke Osborn report altering a prosthetic arm so that it can feel a version of pain. This effort is meant to increase the realistic nature of an amputee’s experience and to help them avoid damaging the prosthetic limb. The pain is not equivalent to what human skin typically experiences.
“It's like a steady growth of discomfort leading up to this localized sharp pressure, but there's also some tingling aspect as well,” Osborn says.
“This work demonstrates possibilities for creating a more natural sensation spanning a range of tactile stimuli for prosthetic hands,” the abstract states.
To create this sensation, the team transformed a Bebionic hand, an advanced, commercially available prosthetic. They modified the hand to give it sensing capabilities in a multilayered electronic dermis, or e-dermis. Human skin possesses various receptors; mechanoreceptors, which specialize in feeling blunt objects, and nociceptors, which are better at receiving painful input. The novel prosthesis was designed to emulate these natural sensing abilities, and to then translate the pressure received into electrical signals that can reach a wearer’s brain.
When in contact with a curved or blunt object, a wide range of the sensors activates at multiple layers. When touching a sharp object, the pressure sensation is markedly different, just as it would be with an organic hand.
“With a pointy object, it's a highly localized pressure when the fingers grab it, and so from the prosthesis’ point of view, that's something that's more uncomfortable,” Osborn says. Electrodes are used to send electrical information about the pressure to nerves where the device meets the patient's arm. These nerves run to the spinal cord and then the brain, and create a “phantom” experience of either a non-threatening or painful sensory encounter. In “Prosthesis with neuromorphic multilayered e-dermis perceives touch and pain,” the authors write:
Through transcutaneous electrical nerve stimulation (TENS) with an amputee, we discovered and quantified stimulation parameters to elicit innocuous (nonpainful) and noxious (painful) tactile perceptions in the phantom hand.
While this project does improve amputees' ability to feel a variety of sensations, Matt Simon of Wired points out that there are a few potential issues: first, doctors pledge to “do no harm.” Second, pain is highly subjective. Also, many prosthetics are already somewhat uncomfortable, if not painful, to wear -- could amputees be alerted of sensory danger via another kind of signal?
Delivering the right amount of unpleasant experience so that users can realize they may be damaging a precious and expensive artificial limb without causing them unnecessary discomfort may be challenging. Each person may also have a different idea of what constitutes an acceptable amount of pain.
During tests of the new hand, volunteers were empowered to end the experiment whenever they chose, and a preset sensation limit was in place.