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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 &#x2212;1.95 dB with a tuning range of 590 MHz, and at the second frequency (2.68 GHz) &#x2212;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&#x0027;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.

Wearable Flexible Radio Frequency Filtering System for Muscle Contraction Monitoring