Optimising polyurethane/CNTs piezoresistive pressure sensors by varying the modulus of the polymer matrix

Varying the elastic modulus of the thermoplastic polyurethane (TPU) matrix on the sensing behaviour of pressure sensors made from TPU/carbon nanotubes (CNTs) elastomeric composites was studied. Piezoresistive sensors have a wide range of applications in wearable devices such as smart shoe insoles. Compounds were prepared using different grades of TPU with various levels of hardness, ranging from 37 to 98 Shore A, using a melt extrusion technique. The percolation threshold of CNTs was about 2 wt%, with the best sensing performance being at a weight fraction above this, at 2.5 wt% CNTs. The elastomeric nanocomposites were characterised for their pressure sensing behaviour. Sensors were constructed by assembling the composite films and an interdigitated silver electrode, and their sensing properties were characterised using a voltage divider circuit under cyclical loading. This work evaluates the reference resistor-dependency, test frequency-dependency and the long-term stability under cyclic loading. The results indicate that the polymer modulus has a significant impact on the sensing performance and piezoresistive hysteresis value for each sensor. As the modulus increases, the hysteresis value between loading-unloading is reduced and the sensor displays a much more stable response during longer cyclical loading tests.