The Short-Term Repeatability of Subdermal Electrical Stimulation for Sensory Feedback

Modern hand prostheses are used to restore the motor functions lost due to an amputation. However, the lack of sensory feedback remains a major challenge. Subdermal stimulation is a promising technique to restore tactile sensations when using prostheses, since it may overcome the disadvantages of surface electrodes without resorting to surgery that is required for a direct nerve interface. The present study evaluated the short-term repeatability of the perceptual properties of subdermal electrical stimulation over eight hours in healthy subjects and compared them to those of surface stimulation. Specifically, the detection threshold, pain threshold, dynamic range, just noticeable difference, resolution and quality of evoked sensations were tested and used for short-term repeatability evaluation. The results demonstrated that the detection threshold was more stable under subdermal stimulation, whereas the short-term repeatability of the pain threshold and just noticeable difference was better under surface stimulation. On the other hand, several psychometric parameters (dynamic range, resolution, sensation quality, intensity, and comfort) were equally stable and did not change significantly across sessions in either surface or subdermal stimulation. The subdermal stimulation was better localized and elicited fewer unwanted sensation modalities (p < 0.05), whereas surface stimulation was characterized by a higher resolution (p < 0.05). The results suggest that subdermal stimulation could be a viable alternative for the implementation of electro-tactile feedback as it generates sensations that are equally stable as in surface stimulation, and yet it has some important advantages for the practical applications (e.g., compact interface, permanent placement).