An ultra-compressible piezoresistive strain and pressure sensor based on RGO-CNT-Melamine foam composite for biomedical sensing

• High gauge factor with low deformability is not optimum for sensing of body movements. • Conductive flake-based ultracompressible melamine foam can sense wide range of strain. • Tiny pulse waveform signal and large limb movements are faithfully measured. • Effect of temperature & humidity variations on sensor’s performance is insignificant. Many heitherto reported compressive piezoresistive foam sensors suffer from variable sensitivity over large compression range, limiting the applicability of the sensor for a narrow range of motion. This is because the area of contact between conductive foam struts is very small and varies linearly over only a small compression range. We resolved this problem by fabricating an ultracompressive foam sensor with large area conductive flakes using facile technique of soak-drying Melamine foam with the conducting aqueous RGO-CNT ink. Sensor exhibited a constant gauge factor (GF) 1.6 over a large and highly linear compression range (∼2% to 60 %) and detected both small and large body movements with superior detectability. Besides that all vital features of a tiny signal such as pulse waveform were delineated due to the excellent low pressure (<5 kPa) sensitivity 0.22 kPa −1 and the small relaxation time of 19 ms. The sensor showed a stable and repeatable performance over 11000 cycles. Its operation is also tested over a wide range of ambient temperature (-7 ℃ to 140 ℃ ) and humidity (25%–82%, RH) and even under water with no major deterioration in performance.