Carbon-laden foam makes super-sensitive pressure sensor
covered conductive pores.
The capacitive sensors detailed in a paper titled “Polymeric foams for flexible and highly sensitive low-pressure capacitive sensors” published in Nature’s npj Flexible Electronics journal consist of a composite foam whose closed pores are covered with conductive carbon black particles. The authors designed the composite material using a water-in-oil emulsion method, whereby a water-based emulsion of carbon black (CB) droplets is dispersed in a matrix of PDMS before it is cured. Evaporating the water after curing the polymer leaves an elastomeric structure with spherical pores covered with carbon black particles. The composite foam is then sandwiched between two strips of conductive flexible carbon tape acting as top and bottom electrodes, forming the complete capacitive sensor.
Experimenting with various concentrations of carbon black, dispersing concentrations and material thickness, the researchers found that a 10 wt% of CB in a 1.2mm thin foam pad led to excellent sensitivity. The paper reports a pressure sensitivity of 35.1 kPa−1 in the 0 to 0.2 kPa pressure range and 6.6 kPa−1 in the 0.2 to 1.5 kPa range.
What’s more, the high capacitance (over 100pF) can easily be measured with low-cost electronics, without any signal amplification. It only takes a 1V bias to record the device’s capacitive responses, and because the material’s resistivity is high, the sensor draws only about 100nW at 1 kPa of pressure sensing (then showing a resistance of 10MΩ). This is orders of magnitude more efficient than conventional silicon piezo-resistive pressure transducers, claim the researchers.
The authors anticipate that such ultrasensitive sensors could be designed as a simple patch to monitor blood-pressure by recording arterial pulse waves (as capacitive signals) and converting them into a pressure value using AI-based algorithms.
But such composite materials could also be used to design pressure sensitive artificial skins for robotics. One particular medical application mentioned in the paper is the use of robotics for the palpation of soft tissue organs, for the detection of embedded nodules in soft tissues. For millimetre-sized nodules, the variation of stiffness is below 0.1 kPa, the authors note, requiring highly sensitive sensors like the one they demonstrated.
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