Wearable Flexible Radio Frequency Filtering System for Muscle Contraction Monitoring

This paper introduces a dual band-pass and dual band-stop filter that is designed along its flexible back-end circuitry to sense and monitor muscle contractions. The filter and its back-end circuit are proposed to be wearable, flexible, and stretchable. The presented design is composed of several logarithmically scaled spiral-shaped defected ground structures (DGS) located along the ground plane of a co-planar waveguide transmission line. In addition, U-shaped slots are integrated within the transmission line to maintain the sensing operation of the filter when its structure is stretched. The entire structure is fabricated on a multi-part flexible Polyethylene Terephthalate (PET) substrate and its stretchable configuration is enabled through the integration of a Room-Temperature-Vulcanizing (RTV) silicon substrate. Such stretchable ability is obtained through the movement of the multiple parts that compose the filter and is exhibited by the tuning of its band-pass and band-stop frequencies of operation between 1 GHz and 4 GHz. Correspondingly, the stretchable ability of the filter is also indicated by the change in magnitudes of its reflection and transmission coefficients. As a result, for the band-pass operation, the insertion loss of the flexible wearable filter, placed above the human arm, at the first frequency (1.39 GHz) is -1.95 dB with a tuning range of 590 MHz, and at the second frequency (2.68 GHz) -1.94 dB with a tuning range of 330 MHz. The change in the response of the presented system is proportional to the intensity of the muscle contraction. To capture this change, a custom-designed integrated flexible back-end circuit interrogates the sensor, collects the magnitudes of the reflection and transmission coefficients, and outputs corresponding voltages. As a result, monitoring the output voltage of the back-end circuit indicates the muscle contraction level, which is sensed from the stretching movement of the filter's structure. The back-end circuit and the sensor are fabricated and tested over multiple measurement cycles where the ability of the sensor to track muscle contraction is demonstrated.