Wearable Flexible Body-Adaptable Frequency Reconfigurable Filter for Muscle Contraction Monitoring

This paper presents a wearable and flexible Radio Frequency (RF) filter that is designed to act as a sensor to monitor muscular contractions. The Coplanar Waveguide (CPW) fed filter incorporates logarithmically scaled spiral defected ground structures (DGSs) etched from the ground planes in addition to U -slot capacitors etched from the feeding line. The filter is fabricated on top of a Polyethylene Terephthalate (PET) substrate and is sandwiched between a Room Temperature Vulcanizing (RTV) silicon substrate. When the filter is at rest, it displays a dual bandstop operation at 2.5 GHz and 3.36 GHz and a dual bandpass operation at 1.62 GHz and 2.27 GHz. When stretched, these four frequencies change in an ascending order in a gradual fashion correlated with the intensity of the exerted muscular contraction on the stretchable filter. This flexible filter constitutes the front-end of the sensor, and it is accompanied by a flexible back-end circuit that extracts the reflection and transmission coefficients of the filter and derives their associated voltages. Each of these associated voltages that are extracted from the flexible back-end circuits indicates a specific level of muscle contraction.