Stretchable graphene and carbon nanofiber capacitive touch sensors for robotic skin applications

Stretchable capacitive tactile sensors are crucial for the current and future soft robotic technology, particularly for designing artificial electronic skin for robots. In this study, we fabricated stretchable electronic tactile sensors by spray coating of conductive graphene nanoplatelets (GNPs) or carbon nanofibers (CNFs) with liquid natural rubber (NR) binder over stretchable nitrile rubber (NBR) sheets. CNF-based coatings showed remarkably low sheet resistance of 30 Ω sq−1 and much better current transmission patterns under elongation, with ~50% current transmission reduction under 50% elongation, compared to GNP-based coatings with 600 Ω sq−1 resistance and ~99% current transmission reduction under 50% elongation. Structural damages/cracks within the coatings due to repeated stretch-release cycles could be healed by a rapid convective heat annealing process, restoring to initial current. Haptic sensing properties of the fabricated flexible capacitive device based on CNFs-coated nitrile rubber were evaluated by connecting it to a printed circuit board (PCB). The device could easily sense tactile forces from tens of mN to a few N, corresponding to (10−2–10) kPa pressure range over curvilinear surfaces or under elongation.