Ionic skin

Abstract

Our skin is a stretchable, large-area sheet of distributed sensors. These properties of skin have inspired the development of mimics, with differing levels of sophistication, to enable wearable or implantable electronics for entertainment and healthcare. [1][2][3][4] "Electronic skin" is generally taken to be a stretchable sheet with area above 10 cm 2 carrying sensors for various stimuli, including deformation, pressure, light and temperature. The sensors report signals through stretchable electrical conductors [5] (e.g., carbon grease, [6] microcraked metal films, [1] serpentine metal lines, [2] graphene sheets, [7] carbon nanotubes, [8][9][10] silver nanowires, [11] gold nanomeshes, [12] and liquid metals [13,14] ). These conductors transmit signals using electrons.They meet the essential requirements of conductivity and stretchability, but struggle to meet additional requirements in specific applications, such as biocompatibility in biometric sensors, [15] and transparency in tunable optics. [16,17] By contrast, sensors in our skin report signals using ions. Here we explore the potential of ionic conductors in the development of a new type of sensory sheet, which we call "ionic skin".The sensory sheet is highly stretchable, transparent, and biocompatible. It readily monitors large deformation, such as that generated by the bending of a finger. It detects stimuli with wide dynamic range (strains from 1% to 500%). It measures pressure as low as 1 kPa, with small drift over many cycles. A sheet of distributed sensors covering a large area can report the location and pressure of touch. High transparency allows the sensory sheet to transmit electrical signals without impeding optical signals.Many ionic conductors, such as hydrogels and ionogels, are highly stretchable and transparent. [18][19][20] These gels are polymeric networks swollen with water or ionic liquids. They behave like elastic solids and eliminate the need for containers as required in the case of liquid metal conductors. Whereas familiar elastic gels, such as Jell-O, are brittle and easily rupture, the recent decade has seen the development of hydrogels and ionogels as tough as elastomers. [20][21][22] Many hydrogels are biocompatible. They can be made softer than tissues, achieving the "mechanical invisibility" required for biometric sensors, which monitor soft tissues without 3 constraining them. Although most hydrogels dry out in open air, hydrogels containing humectants retain water in environment of low humidity, and ionogels are nonvolatile in vacuum. [18][19][20] We have recently used ionic conductors-together with stretchable and transparent dielectrics-to make actuators, which deform in response to high voltages, on the order of kilovolts. [18] By contrast, the sensors described here deform in response to applied forces, giving signals that can be measured using voltages below 1 volt. To illustrate principles in our design of the ionic skin, consider a simple example-a dielectric sandwiched between two ionic conductors (Figure 1). In many...

Porous Ionic Membrane Based Flexible Humidity Sensor and its Multifunctional Applications

“…Especially, the rapid advances in the design of various flexible sensors have tremendously broadened the scope of flexible electronics to new classes of soft electronic systems 4, 5. For example, the flexible pressure and temperature sensors can be proposed for deriving various applications including the skin‐like electronics (E‐Skin), robotics human–machine interfaces, and biomedical applications 6, 7, 8, 9, 10. On the other hand, flexible humidity sensors were rarely reported, though humidity is one of most‐intensively measured variables in our daily lives, including comfortable living environment, medical facilities, and body health information 11.…”

Wearable and Washable Conductors for Active Textiles

“…Recent works have mimicked the myelinated axon using a hydrogel as the electrolyte and a hydrophobic elastomer as a dielectric [2][3][4] . Such a hydrogel-elastomer hybrid, called an artificial axon, or an ionic cable, has enabled many devices of unusual characteristics.…”

“…Examples include stretchable loudspeaker 3 , sensory skin [4][5] , electroluminescence [6][7] , and touchpad 8 . Whereas a metal conducts electricity with electrons, a hydrogel conducts electricity with ions.…”

3D Printing of Transparent and Conductive Heterogeneous Hydrogel–Elastomer Systems

“…Their high stretchability and transparency, when combined with recent improvements in toughness and stiffness, [19,20] have already enabled their use as stretchable electrical conductors, [21,22] capacitive strain sensors, [23][24][25] and chemical/pH sensors [26] . Gel-based ionic circuits thus represent a unique class of devices within stretchable electronics.…”

“…[24] Although recent studies have successfully fabricated stretchable electronics consisting entirely of soft materials, conductive hydrogels and dielectric elastomers, thus far these fabrication techniques have relied on casting or a combination of extrusion printing with other methods. [21][22][23][24][25] Here, we describe a simple approach to 3D extrusion printing of soft, stretchable electrical devices integrating a conductive hydrogel and a dielectric elastomer with sub-millimeter resolution. We show that both types of materials can be integrated into a single device using a single fabrication process.…”