We demonstrate a novel low power inductive wearable plethysmography system. This consists of ultrasensitive 3D knitted helical coils integrated in a garment and an oscillator circuit with high quality factor. The low power oscillator is built using two cross coupled FET pairs with low capacitance drawing only 95 A during operation and with a response time smaller than 10 s. The sensor system is linear, with negligible hysteresis. The best compromise in sensitivity and power consumption is obtained with a 3D knitted helical coil using jersey knit with elastic yarn, a lower knitting needle size than recommended for the yarn and minimizing both the number of stitches per winding as well as the stitches containing metal. A sensitivity of 2.7 kHz per mm change in circumference with a power consumption of 6.85 mW per 30 ms measurement time is reported. This system can be used for long term breathing monitoring using a garment indistinguishable from everyday clothing.

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