Ultra-conformable Tattoo electrodes for providing sensory feedback via transcutaneous electrical nerve stimulation

Electrical stimulation of the peripheral nervous system has been demonstrated to be effective in restoring somatotopic sensory information in subjects with amputations. It can be achieved using invasive electrodes, which require surgical implantation, or non-invasive electrodes, which are applied directly to the skin. Whereas less selective, non-invasive electrodes offer the advantage of avoiding surgical procedures. Additionally, commercially available wet Ag/AgCl electrodes, lack of adaptability to the irregular surface of a subject residual limb, leading to detachment during movement or use. This study aims to address some of the limitations of non-invasive electrodes by proposing and validating the use of ultra-conformable Parylene C-based tattoo electrodes to restore somatotopic sensory information through transcutaneous electrical stimulation of the peripheral nervous system. The skin-electrode impedance was characterized in a healthy subject at varying frequencies over a period of nine hours, showing a maximum impedance variation of 8% at the target stimulation frequency. Then, the performance of the proposed tattoo technology was compared to commercial Ag/AgCl electrodes through experimentation that involved 12 healthy participants. The study found no statistically significant differences in key stimulation parameters (rheobase, p>0.3 and chronaxie, p >0.15) or in the sensory perceptions elicited by the two compared electrodes. Additionally, tattoo electrodes exhibited a lower operational impedance compared to commercial electrodes, highlighting their potential advantages for practical applications.