Energy-Autonomous, Flexible, and Transparent Tactile Skin

© 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1606287 (1 of 12) such as diabetes. To satisfy the requirements of such a system, active materials with intrinsic properties, including good mechanical, electrical, optical, and structural properties, are in high demand.[1] The development of suitable flexible pressure sensors for e-skin applications has been a challenge, due to inadequate flexibility, conductivity, large-area manufacturability, and reliable and repeatable performance of the structure, to be applicable in practical robots.[7] In this regard, only very few approaches have been successfully employed in actual robots.[7,8] Further, making the e-skin transparent adds an extra dimension in the functional design space of e-skin, as it enables incorporating photovoltaic (PV)-energy harvesting, electro/thermochromicity, chameleon effect, etc. Along with a new generation of flexible and stretchable solar cells,[9] this will allow fabrication of energy-autonomous, stretchable e-skins. Accordingly, a novel approach is explored in this work, of a vertical-layered-stack structure consisting of a photovoltaic cell attached to the back plane of a transparent tactile skin; where skin transparency is a crucial feature that allows light to pass through, making the building block unique and opening a new, promising line of energy-autonomous devices for flexible electronics. In this regard, graphene is a promising material as it offers key parameters to develop nonplanar, transparent electronic or tactile skin. It has been shown that graphene has a good combination of stiffness (≈1000 GPa) and tensile strength (≈100 GPa).[10] Together with its sunlight blindness[11] and good electrical conductivity,[12] graphene has also emerged as a viable candidate for various flexible, transparent electronic and optoelectronic devices.[13–16] Moreover, in our recent work, we demonstrated that high-quality graphene can be synthesized and transferred on large area, flexible substrates (400 cm2) with a very low-cost and easy fabrication process.[15] Owing to the intrinsic properties and advances in the synthesis and fabrication of devices, graphene is also a promising candidate for the development of high-performance e-skin, requiring large area device fabrication on nonplanar surfaces. A few flexible pressure sensors reported in literature, based on capacitive,[17–20] piezoelectric,[21] and piezoresistive sensing mechanisms,[2,22–28] use graphene as an active material. Piezoresistive sensors transduce the pressure imposed on the sensor’s active area in terms of resistance change, and offer an attractive solution for pressure sensing due to advantages such as low cost and easy signal collection. Graphene-based piezoresistive pressure sensors have been reported in various configurations. For example, Yao et al. demonstrated the fabrication of flexible pressure sensors based on a graphene nanosheet on Energy-Autonomous, Flexible, and Transparent Tactile Skin