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.
In parallel, electroencephalography (EEG) activity was recorded in the relevant somatosensory regions of the amputee’s brain to confirm phantom hand activation during stimulation.
Rich with all this experimental data, the researchers then developed a neuromorphic representation of the tactile signal, with the aim to transform the e-dermis (electronic skin) signal from a pressure signal into a biologically relevant signal, namely, a spiking response similar to what actual mechanoreceptors and nociceptors would provide. Fed into a transcutaneous electrical nerve stimulator, the neuromorphic signal transduction conveyed the changes in the tactile signal as changes in stimulation frequency and pulse width, matching the perceived levels of touch or pain recorded during all the previous sensory feedback experiments.
Ultimately, this neuromorphic signal transduction allowed the prosthesis wearer to differentiate between safe (innocuous) and painful (noxious) tactile sensations when grasping rounded or spiky objects between an e-dermis-covered thumb and index finger.
Pain is of no use if you don’t react to it. Hence to enforce prosthesis self-preservation, the scientists also modelled as a polysynaptic withdrawal reflex that would prevent damage and further pain. They note that although they implemented an autonomous pain reflex, such reflex component could be modulated by the user based on the perceived pain or even, for example using the amputee’s electromyography (EMG) signal as an additional input to the reflex model.
In the future, the researchers envisage an e-dermis with multiple types of sensors for a richer feedback (including the detection of temperature and possibly noxious chemicals). Such an electronic tactile skin could find applications in robotics too as a natural safeguard for sharp objects handling robots.