Three-dimensional flexible porous dielectrics treated with a conductive surface are often used as force or pressure sensors, whereby any mechanical deformation of the pores results in detectable changes in the overall conductivity of the fabricated conductor. But under the repetitive mechanical deformation of their flexible substrate, the conductive surface can suffer from delamination and decay.
In a paper titled “3D-Printed Ultra-Robust Surface-Doped Porous Silicone Sensors for Wearable Biomonitoring“ published in the ACS Nano journal, the researchers present a simple fabrication process where graphene nanoplatelets are not directly deposited within the connected pores surface of the flexible shape, but instead are deposited on the surface of a sacrificial mold (dip coated with a solution of graphene nanoplatelets). When silicone is then poured into the sacrificial mold, the graphene nanoplatelets naturally end up being transferred and embedded onto the surface of the silicone material, effectively surface-doping the porous silicone pad.
Key in realizing this was the design of a sacrificial mold, using fused deposition modelling (FDM) 3D-printing to yield a mold internally shaped with ordered, interconnected, and tortuous internal geometries (with triply periodic minimal surfaces). Once dip coated with the graphene nanoplatelets and poured with silicone, the mold material is removed to leave only the flexible sensor.
With this fabrication method, the authors report a stable coating on various porous silicone samples, with long-term electrical resistance durability over a 12 months period and high resistance against harsh conditions, including the exposure to organic solvents. They also observed that the sensors retained their conductivity upon severe compressive deformations (over 75% compressive strain) with high strain-recoverability even across cyclic deformations, temperature, and humidity.