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Free-standing carbon nanomaterial hybrid sheets, consisting of multi-walled carbon nanotubes (MWCNTs), exfoliated graphite nanoplatelets (xGnPs) and nanographene platelets (NGPs) of different lengths and lateral dimensions, have been prepared using various material combinations and compositions. When subjected to tensile strains, the carbon nanomaterial sheets showed piezoresistive behavior, characterized by a change in electrical resistance with applied strain. Simultaneous measurement of resistance changes among multiple electrodes placed on the periphery of the hybrid sheets showed the dependence of resistance changes on strain direction, which potentially allows multi-directional strain sensing. Various combinations of MWCNT length, xGnP size and MWCNT-to-xGnP/NGP ratio result in different specific surface areas and nanoparticle interactions, which serve as critical factors for controlling the sensitivity of hybrid sheets. The smaller the nanoplatelet size and the higher the content as compared to MWCNT, the higher the sensitivity. Buckypapers, which are free-standing sheets composed of CNTs, are used as the control materials, and the unique characteristics of hybrid sheets are discussed.

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The development of a flexible electronic skin (e-skin) highly sensitive to multimodal vibrations and a specialized sensing ability is of great interest for a plethora of applications, such as tactile sensors for robots, seismology, healthcare, and wearable electronics. Here, we present an e-skin design characterized by a bioinspired, microhexagonal structure coated with single-walled carbon nanotubes (SWCNTs) using an ultrasonic spray method. We have demonstrated the outstanding performances of the device in terms of the capability to detect both static and dynamic mechanical stimuli including pressure, shear displacement, and bending using the principles of piezoresistivity. Because of the hexagonal microcolumnar array, whose contact area changes according to the mechanical stimuli applied, the interlock-optimized geometry shows an enhanced sensitivity. This produces an improved ability to discriminate the different mechanical stimuli that might be applied. Moreover, we show that our e-skins can detect, discriminate, and monitor various intensities of different external and internal vibrations, which is a useful asset for various applications, such as seismology, smart phones, wearable human skins (voice monitoring), etc.

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