Their idea is to enable amputees to receive sensory feedback from their prosthetic limb not only for grasping objects with relevant force, but also to enable them to feel more accurately the objects' shapes and eventually experience their sharpness or pointiness through different pain thresholds.
Although some could argue that pain sensations are not a must have, coming from prosthesis hardware, the researchers claim that adding pain perception to touch helps amputees learn about their environment while also preventing damage to the sensor-laden prosthetic fingertips.
Detailed in a paper titled "Prosthesis with neuromorphic multilayered e-dermis perceives touch and pain" published in Science Robotics, the e-skin combines piezoresistive-based taxels (tactile pixels) distributed across two different layers (with differentiated response levels) with a neuromorphic interface so the taxels' outputs can be appropriately mapped as spiking input stimulus relayed via transcutaneous electrical nerve stimulation (TENS) at the periphery of the amputated limb.
If the taxels themselves were fairly simple to implement, each capped under a 1mm-thick rubbery layer and calibrated for a range of 0 to 300kPa, much work was done to properly quantify the stimulation parameters required to deliver appropriate innocuous (non-painful) and noxious (painful) tactile perceptions in the amputee's phantom hand.
First, the researchers used targeted TENS to extensively map and understand the perception of a transhumeral amputee’s phantom limb during sensory feedback. To gather as much data, the participant received more than 25 hours of sensory mapping as well as participating to over 150 trials of sensory stimulation experiments, to quantify the perceptual qualities of the stimulation.
This extensive mapping of the limb's residual innervation allowed the researchers to identify a localized correspondence in activation of the amputee’s phantom hand. The authors also report that through repeated psychophysical experiments, they were able to correlate the amputee's perception of painful tactile sensations in his phantom hand to changes in both stimulation frequency and pulse width.
Uncomfortable but tolerable pain was perceived at relatively low frequencies between 10 and 20Hz, while higher frequency stimulation gave off a more pleasant tactile sensation, the paper reads.