Design and validation of a wearable TENS stimulation system for restoring somatotopic sensory feedback in amputees

Abstract Upper limb and lower limb myoelectric prosthetic systems implementing open loop control have been shown to be hardly accepted by prosthesis users. The introduction of somatotopic sensory feedback (SSF) allows a closed-loop control strategy, potentially improving the functionality of the prosthetic device, leading the patient to its greater acceptance and control. Although less selective than invasive methods, transcutaneous electrical nerve stimulation (TENS) offers a reliable and non-invasive approach for eliciting somatotopic sensations, achieving promising results without the need for surgical intervention or the risk of long-term complications. Most studies addressing somatotopic TENS stimulation have relied on bulky, mains powered benchtop stimulators, thereby limiting the applicability of SSF systems in mobile or real-world scenarios. While several commercially available and research-grade wearable TENS stimulators exist, they are typically designed for pain relief applications. As a result, they often lack the electrical specifications, such as sufficient voltage compliance and waveform control capabilities, required for effective somatotopic stimulation. In parallel, although Ag/AgCl hydrogel electrodes remain the standard interface for TENS applications, their limitations become apparent in long-term use. Gel dehydration, increasing skin–electrode impedance, and limited conformability to non-planar anatomical regions all compromise stimulation reliability. To overcome these challenges, epidermal tattoo electrodes have emerged as a promising alternative. These ultra-thin interfaces adhere via van der Waals forces, ensuring stable contact and enhanced mechanical compliance. Their larger effective contact area contributes to lower interface impedance. While their effectiveness has been demonstrated in biopotential recording, their application as stimulation interfaces for somatotopic sensory feedback stimulation remains underexplored and warrants further investigation. The primary objective of this dissertation is to validate wearable TENS stimulators and a stimulation interface, designed to be integrated into a system capable of delivering somatotopic sensory feedback to improve upper and lower limb neuroprosthetic control. The work begins by defining the requirements for a TENS wearable stimulator, starting from the stimulation parameters. A comparative analysis of the main SSF mapping studies identified the stimulation parameters ranges necessary to evoke the full spectrum of somatotopic sensations in upper and lower limb scenario, thereby establishing the modulation capabilities the device must support. Therefore, the minimum voltage compliance needed to reliably deliver the required stimulation current across the skin was defined. An in-vivo study was conducted to characterize skin impedance under realistic SSF stimulation conditions. Measurements from ten able-bodied participants showed that the upper limb exhibits consistently higher impedance than the lower limb, and that electrode diameter significantly influences impedance levels. These findings informed the definition of minimum voltage compliance requirements, estimated at 30.14 ± 2.8 V for the upper limb and 60.4 ± 7.8 V for the lower limb. Guided by these specifications, a first generation, battery powered stimulator optimized for upper limb applications was developed, featuring a compact form factor (46 g, 62 × 49 × 20 mm) to ensure high wearability. Benchtop evaluations confirmed its capability to deliver charge balanced, biphasic pulses, while a novel healthy subjects protocol was implemented to assess the ability of the device to evoke somatotopic sensations. Specifically, the qualitative and quantitative characteristics of the elicited sensations were compared to those generated by a reference benchmark commonly used in the literature (i.e., the STG4008), demonstrating comparable performance. To provide a stimulator suitable for both upper and lower limb applications, and thereby ensure compatibility with the full range of somatotopic feedback use cases, a redesign was undertaken. Through a new power management and stimulation architecture, a second generation battery powered wearable device (52 g, 59 × 28 × 25 mm) was designed. The system features multiple independent stimulation channels, real time wireless programmability, and sufficient voltage compliance to accommodate the identified skin impedance ranges. Bench testing and human validation with twenty healthy participants confirmed reliable performance across skin-like resistive-capacitive loads and consistent elicitation of localized, somatotopic accurate sensations, demonstrating the suitability for future clinical studies. Finally, to address the limitations associated with conventional hydrogel electrodes, namely discomfort, instability, and impedance drift, the thesis investigates the use of ultra-conformable tattoo electrodes fabricated from biocompatible Parylene-C. Impedance measurements over a 9 hour period, coupled with psychophysical testing in twelve healthy participants, demonstrated that these dry electrodes maintain secure adhesion, exhibit reduced contact impedance, and evoke somatotopic sensations comparable in threshold and quality to those elicited by Ag/AgCl electrodes. These findings support their suitability for long-term, wearable SSF applications.