An ultra-compliant P(VDF-TrFE) fiber-based muscle patch sensor for a new wearable application: Monitoring muscle activities and fatigue through the peripheral length of a muscle’s physiological cross-sectional area

Current applications of physical sensors on wearable devices are primarily for monitoring gestures and body movement, which measure joint movements or action-induced compression pressure. Here, we present a new application of physical sensors: monitoring muscle activities and fatigue through the changes in the physiological cross-sectional area (PCSA) of muscles. The concept is that the contraction of excited muscle fibers results in muscle center bulges and PCSA enlargement. The peripheral length of a muscle perpendicular to the contracting direction is also increased. This increment in muscle peripheral length can be utilized to infer muscle activity and fatigue-induced tremors. To monitor muscle activities and fatigue through peripheral length change, we developed an ultra-compliant piezoelectric muscle patch sensor (MPS) that attaches to the skin above the muscle of interest. The MPS is constructed by a piezoelectric yarn composed of P(VDF-TrFE) fibers and is encapsulated inside silicon rubber to reach high repeatability, durability, and linearity. This MPS is highly compliant and can follow the bulge of the muscle contractions without introducing restrictions on PCSA change. We demonstrate that the MPS can faithfully monitor the changes in peripheral length during various isometric, concentric, and eccentric muscle contractions and fatigue. It is verified that the contribution of each muscle group to gestures and body movements can be directly monitored. The MPS can be broadly applied to sports, virtual reality, gaming, muscular disorders rehabilitation, and essential tremor disorder sensors. • Report a new wearable concept: measure the changes in muscle’s physiological cross-sectional area (PCSA). • Muscle contractions and fatigue can be quantitatively monitored through the PCSA changes. • Develop an ultra-compliant muscle patch sensor that can measure peripheral changes of a muscle’s PCSA during force contracting and fatigue. • The concept of using a highly compliant wearable sensor to measure muscle contractions and fatigue through PCSA can broadly apply to all physical sensors.