Highly Transparent and Flexible Iontronic Pressure Sensors Based on an Opaque to Transparent Transition

Abstract

Human–computer interfaces, smart glasses, touch screens, and some electronic skins require highly transparent and flexible pressure‐sensing elements. Flexible pressure sensors often apply a microstructured or porous active material to improve their sensitivity and response speed. However, the microstructures or small pores will result in high haze and low transparency of the device, and thus it is challenging to balance the sensitivity and transparency simultaneously in flexible pressure sensors or electronic skins. Here, for a capacitive‐type sensor that consists of a porous polyvinylidene fluoride (PVDF) film sandwiched between two transparent electrodes, the challenge is addressed by filling the pores with ionic liquid that has the same refractive index with PVDF, and the transmittance of the film dramatically boosts from 0 to 94.8% in the visible range. Apart from optical matching, the ionic liquid also significantly improves the signal intensity as well as the sensitivity due to the formation of an electric double layer at the dielectric‐electrode interfaces, and improves the toughness and stretchability of the active material benefiting from a plasticization effect. Such transparent and flexible sensors will be useful in smart windows, invisible bands, and so forth.

Recent Progress on Flexible Capacitive Pressure Sensors: From Design and Materials to Applications

“…The flexible capacitive pressure sensors with fluidic dielectrics are highly sensitive for small pressure ranges, very compatible with wearables, in‐vivo and in‐vitro measurements. [ 136 ] Moreover, using fluidic dielectrics in combination with a proper electrode material enable the design of transparent [ 114 ] flexible capacitive pressure sensors as opposed to the various structures and designs of dielectrics which have been reported and classified before. The major challenges which are being reported for fluidic dielectric based flexible pressure sensors are fluid leakage issues and integrating the iontronic devices with biocompatible integrated circuitries which could be lead to an excellent technological achievement in neuro‐prosthetics and artificial cardiac pacemakers.…”

“…[ 113 ] Ionic liquids offer multiple advantages such as improved flexibility, signal intensity, and sensitivity of sensors simply because ions provide electric double layer (EDL) capacitance. [ 114 ] N. Bai et al. presented a graded intra‐fillable architecture (GIA) based iontronic flexible pressure sensor [ Figure a].…”

“…designed a highly transparent (transmittance of 90.4%) flexible capacitive pressure sensor using ionic liquid as dielectric layer. [ 114 ] The ionic liquid is poured into a porous polyvinylidene fluoride (PVDF) film which has identical refractive index and increasing the transmittance of PVDF film from 0% to 94.8% in the visible spectrum. The ionic liquid filled PVDF film is sandwiched between two Ag nanowire (AgNW) electrodes which are transparent with 97.6% transmittance [Figure 8d].…”

“…(d,e)—Reproduced with permission. [ 114 ] Copyright 2020, Wiley. f) Schematic to show sensing principle of droplet (glycerol/electrolyte)‐based interfacial capacitive pressure sensor.…”

Controlled Assembly of MXene Nanosheets as an Electrode and Active Layer for High‐Performance Electronic Skin

“…Electronic skins with high sensitivity, portability and foldability have gained significant attention for their use in artificial intelligence, [ 1–3 ] human‐computer interaction, [ 4,5 ] health monitoring, [ 6,7 ] soft robotics, and disease detection. [ 8,9 ] A flexible pressure sensor is generally composed of a flexible substrate, an active layer, and a conductive electrode.…”