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.