Investigation of Reliability of a Flexible Pressure Sensor Based on a Polyethylene Carbon Composite - Velostat

Nanoparticle-based polymer composite materials are receiving attention for the development of sensors for various flexible and wearable electronic applications. However, it is important to test a material's reliability to validate the quality of the product and to ensure that the results obtained are accurate and repeatable. Flexible pressure sensors utilized in wearable and textile devices are subjected to continuous force and stress that may affect the sensor's response in the long run. Hence, reliability assessment is an essential factor to consider when designing any sensor. In this work, we have tested the reliability of a carbon-impregnated polymer composite named velostat, over repeated usage and bending cycles during the course of a year. It is an opaque conductive film of polyethylene impregnated with carbon particles, making it an electrically conducting composite material. It has been extensively explored because it is low-cost, thin, flexible, lightweight, and easy to fabricate. It works on the principle of piezoresistivity; the higher the pressure applied to the velostat, the higher the conductivity and the lower the resistance.<br/>We evaluated the reliability of velostat by implementing this material into the design of a flexible pressure sensor array. The reliability assessment was performed on the pressure sensor array for one year. The sensor array was designed and fabricated as a multi-layer matrix structure. The top and bottom layers were the protective layers made of PVC sheets on which the conductive layers were pasted. The protective layers were transparent, thin, soft, and inexpensive. The thickness of the protective layers was 135±1 µm each. The conductive layers were conductive adhesive copper tapes. The copper electrodes were pasted in a crossbar architecture format and had a rectangular cross-section. The ends of the copper tapes were soldered with wires connected to the microcontroller. The thickness of copper tape with an adhesive layer was measured to be 60±1 µm. The middle layer was the piezoresistive material, velostat. Four copper electrodes were pasted on each protective layer. The size of each sensor pixel was 10 mm x 10 mm, and they were separated by a distance of 50 mm. Hence, it was a 4x4 sensor matrix with 16 individual pressure sensors and a total thickness of 434 µm (as measured by a Mitutoyo micrometer).<br/>To analyze the reliability of the velostat-based pressure sensor, we conducted experiments for twelve months. In the first experiment, we tested the sensor's response every fortnight by applying a load of 1 kg to all sixteen sensors. We observed that with time, there is a change in the sensor response due to the non-homogeneous characteristics of the velostat. The sensor's response changed with an average standard deviation of 0.09 V. We also tested the sensor response to the application of higher loads by applying weights ranging from 1 kg to 12 kg every fortnight. We observed that there is less gradation in the resistance during the starting three months. As the molecules in the velostat start settling, the resistance shows more gradation on applying pressure and is closer to the ideal graph. We also evaluated the decay rate of the sensor. The decay rate is calculated by taking the sensor's response voltage ratio for all the weights. We observed that in the initial three months, there was a deviation in the decay rate by 0.11, which was reduced to 0.02 in the last six months' analysis.<br/>The results obtained from the experimental tests for reliability show a practical possibility of implementing velostat-based pressure sensors in wearable and healthcare devices. The sensor mat is highly scalable and can be used in various applications like foot pressure monitoring, gait analysis, sitting posture recognition and correction, body pressure measurement, detection of bed sores, and many more.