Abstract
Stretchable capacitive devices are instrumental for new‐generation multifunctional haptic technologies particularly suited for soft robotics and electronic skin applications. A majority of elongating soft electronics still rely on silicone for building devices or sensors by multiple‐step replication. In this study, fabrication of a reliable elongating parallel‐plate capacitive touch sensor, using nitrile rubber gloves as templates, is demonstrated. Spray coating both sides of a rubber piece cut out of a glove with a conductive polymer suspension carrying dispersed carbon nanofibers (CnFs) or graphene nanoplatelets (GnPs) is sufficient for making electrodes with low sheet resistance values (≈10 Ω sq−1). The electrodes based on CnFs maintain their conductivity up to 100% elongation whereas the GnPs‐based ones form cracks before 60% elongation. However, both electrodes are reliable under elongation levels associated with human joints motility (≈20%). Strikingly, structural damages due to repeated elongation/recovery cycles could be healed through annealing. Haptic sensing characteristics of a stretchable capacitive device by wrapping it around the fingertip of a robotic hand (ICub) are demonstrated. Tactile forces as low as 0.03 N and as high as 5 N can be easily sensed by the device under elongation or over curvilinear surfaces.
Highlights
Flexible and stretchable capacitive devices are indispensable for rely on silicone for building devices or sensors by multiple-step replication
Analysis of the Raman spectra indicated that the graphene nanoplatelets (GnPs) are made up of ≈>9 layers[39] and that carbon nanofibers (CnFs) feature many defects/imperfections related to their graphitic structure compared to GnPs
The surface morphology of both CnF and GnP-based coatings resembles the morphology of rubber composites made with GnPs or CnFs or carbon nanotubes (CnTs)[51,52], since the coating matrix is blend of two rubbery polymers, namely, thermoplastic polyurethane (TPU) and high impact polystyrene (HIPS)
Summary
The surface morphology of the uncoated nitrile rubber has ribbon-like, wrinkled roughness features inherent to its fabrication. These features are known to help coating adhesion.[50] The surface morphology of both CnF and GnP-based coatings resembles the morphology of rubber composites made with GnPs or CnFs or CnTs[51,52] (see Figure S3, Supporting Information), since the coating matrix is blend of two rubbery polymers, namely, TPU and HIPS (see Figure S4, Supporting Information, for chemical details). Tape peel test results indicate that the adhesion strength (rubber to rubber) was around 1 N m−1 and this remained quite stable under elongation for CnF-based coatings (see Figure S5, Supporting Information). Note that adhesion to steel is a standard performance indicator for coatings or adhesives and adhesion strength of these coatings to metallic surfaces was around 100 N m−1
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