ReGrid: A Highly Conformable and Ultrasound Transparent Patch for HD-sEMG Detection

This study describes a novel high-density EMG dry electrode grid (ReGrid) based on a thin-film patch highly conformable to the skin and fully ultrasound-transparent. ReGrid consists of a $15~\mu $ m-thick polyurethane membrane housing silver electrodes and traces inkjet-printed on the skin-facing side. A waterproof medical-grade adhesive layer protects the outer surface, while a flexible PCB connector ensures the connection with the acquisition system. Bench tests were conducted to assess mechanical conformability and US transparency. Results showed that ReGrid conformed to curved surfaces, other than allowing for B-mode US imaging without artifacts: both the support and the electrodes resulted transparent to ultrasound. In-vivo tests on the tibialis anterior muscle confirmed low and stable electrode-skin impedance and noise levels comparable to conventional gel-based electrodes. HD-sEMG signals were recorded during isometric contractions at two force levels (10% and 20% MVC), with and without a US probe placed directly over the electrodes. Conduction velocity estimates and HD-sEMG decomposition outcomes were not significantly affected by the US probe, nor was the level of power-line interference. Thanks to its conformability, ultrasound transparency, and high signal quality, ReGrid enables combined HD-sEMG and ultrasound acquisitions from the same muscle region, supporting novel applications such as 3-D and panoramic ultrasound imaging integrated with HD-sEMG.