A Flexible and Wearable Sparse Ultrasound Array Integrated with a Functional Electrical Stimulation Array for Spatially Distributed Sequential Stimulation

Functional electrical stimulation (FES) is commonly utilized in rehabilitation to induce muscle contractions, yet its effectiveness is limited by the rapid onset of muscle fatigue. Spatially distributed sequential stimulation (SDSS) has recently emerged as a promising solution by temporally shifting FES pulses across multiple electrodes. This protocol activates motor units at a lower frequency than conventional FES, which minimizes FES-induced muscle fatigue while maintaining or even increasing the overall muscle power output. To optimize SDSS parameters and electrode placement, it is essential to assess muscle contractility in real time. Ultrasound (US) technology plays a pivotal role, offering deep wave penetration non-invasively, furnishing functional insights into musculoskeletal activities. Conventional US transducers suffer from a lack of flexibility and wearability, thereby constraining mobility. To overcome these limitations, we integrated a flexible/wearable sparse US (fWSUS) 2D array (4 by 4) with an SDSS electrode array. A center frequency of 6 MHz was chosen to achieve the sufficient penetration depth of US wave for monitoring the lower limb muscle activity. The SDSS array, fabricated using silver nanowire/polydimethylsiloxane (AgNW/PDMS) composite, was tested against the commercial FES electrodes. US-compatibility was investigated by collecting A-mode US signals with the fWSUS array placed vertically on the SDSS array. The results suggest that the proposed approach is promising to reduce FES-induced muscle fatigue while enabling real-time monitoring of targeted muscle activity, offering significant potential to advance rehabilitation technology.