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...

A schematic design of an epidermal touch panel is shown in Fig. 4A. The epidermal touch panel was built on a 1-mm-thick VHB film (3M, Maplewood, MN) so as to insulate the panel from the body. Because VHB film was originally developed as an adhesive, the panel could be attached to an arm without using extra glues (Fig. 4B). The epidermal touch panel was fully transparent so that it could convey visual content behind the touch panel. Moreover, the panel was mechanically soft and stretchable so that a user is comfortable with movement while wearing it. The currents measured before and after attachment are plotted in Fig. 4C. The base-line currents increased after the attachment owing to a leakage of charges through the VHB substrate. The thicker insulating layer generated a smaller baseline current. The effect of thickness of the insulating layers on the baseline currents is shown in fig. S8. The sensitivity to touch decreased after the attachment; however, the touching current was still sufficient to be detected. As shown in Fig. 4D, we subsequently touched from TP#1 to TP#4 on the epidermal touch panel, and the current was measured with the A1 current meter. The correlation between the measured currents and the touched position was not influenced by the attachment. The epidermal touch panel could successfully perceive various motions, such as tapping, holding, dragging, and swiping. Thus, various applications can be easily managed by integrating the panel. As shown in Fig. 4, E to G, writing words (Fig. 4E), playing music (Fig. 4F), and playing chess (Fig. 4G) were accomplished via adequate motions on the epidermal touch panel (movies S3 to S6). We have demonstrated a highly stretchable and transparent ionic touch panel. We used a PAAm hydrogel containing 2 M LiCl salts as an ionic conductor. We investigated the mechanism of position-sensing in an ionic touch panel with a 1D strip. The ionic touch strip showed precise and fast touch-sensing, even in a highly stretched state. We expanded the position-sensing mechanism to a 2D panel. We could draw a figure using the 2D ionic touch panel. The ionic touch panel could be operated under >1000% areal strain. An epidermal touch panel was developed based on the ionic touch panel. The epidermal touch panel could be applied onto arbitrarily curved human skin, and its use was demonstrated by writing words and playing the piano and games. Human performance is modulated by circadian rhythmicity and homeostatic sleep pressure. Whether and how this interaction is represented at the regional brain level has not been established. We quantified changes in brain responses to a sustained-attention task during 13 functional magnetic resonance imaging sessions scheduled across the circadian cycle, during 42 hours of wakefulness and after recovery sleep, in 33 healthy participants. Cortical responses showed significant circadian rhythmicity, the phase of which varied across brain regions. Cortical responses also significantly decreased with accrued sleep debt. Subcortical areas exhibite...

A highly elastic hybrid hydrogel of methacryloyl‐substituted recombinant human tropoelastin (MeTro) and graphene oxide (GO) nanoparticles are developed. The synergistic effect of these two materials significantly enhances both ultimate strain (250%), reversible rotation (9700°), and the fracture energy (38.8 ± 0.8 J m−2) in the hybrid network. Furthermore, improved electrical signal propagation and subsequent contraction of the muscles connected by hybrid hydrogels are observed in ex vivo tests.

A stiff skin forms on surface areas of a flat polydimethylsiloxane (PDMS) upon exposure to focused ion beam (FIB) leading to ordered surface wrinkles. By controlling the FIB fluence and area of exposure of the PDMS, one can create a variety of patterns in the wavelengths in the micrometer to submicrometer range, from simple one-dimensional wrinkles to peculiar and complex hierarchical nested wrinkles. Examination of the chemical composition of the exposed PDMS reveals that the stiff skin resembles amorphous silica. Moreover, upon formation, the stiff skin tends to expand in the direction perpendicular to the direction of ion beam irradiation. The consequent mismatch strain between the stiff skin and the PDMS substrate buckles the skin, forming the wrinkle patterns. The induced strains in the stiff skin are estimated by measuring the surface length in the buckled state. Estimates of the thickness and stiffness of the stiffened surface layer are estimated by using the theory for buckled films on compliant substrates. The method provides an effective and inexpensive technique to create wrinkled hard skin patterns on surfaces of polymers for various applications.focused ion beam surface modification ͉ polydimethylsiloxane ͉ surface wrinkles W rinkle patterns shown in Fig. 1 are formed by exposing the surface area of a flat polydimethylsiloxane (PDMS) sheet (thickness Ϸ3 mm, Young modulus Ϸ2 MPa) (1) to a focused ion beam (FIB) of Ga ϩ ions as shown schematically in Fig. 1 A. This method can create wrinkle patterns of various widths and complexity by controlling the relative motion of the polymeric substrate and the FIB to scan selected areas as shown in Fig. 1 B-E. The wrinkles appear only on the areas of the PDMS exposed to FIB (see Fig. 1 B and C), due to buckling of the stiff skin formed on the areas of the PDMS exposed to FIB. FIB exposure creates a tendency for the skin to expand in the direction perpendicular to the direction of FIB irradiation if it was not constrained by the PDMS substrate, similar to the effect observed in exposing metallic surfaces to ion beam irradiation (2-4). The mismatch strain between the stiff skin and its substrate give rise to skin buckling and the formation of the wrinkle patterns (5-9). FIB exposure differs from UV/ozone treatment of PDMS in that the latter produces a stiff skin by increasing cross-links with relatively little strain mismatch (10, 11). The morphology of the wrinkle patterns on the surface areas of PDMS is mainly a function of ion fluence as shown in Figs. 1C and 2. Fig. 1D shows that the path of the wrinkle patterns can be selected by controlling the relative motion of the substrate and ion beam. In addition, one can create islands of buckled stiff skins on the PDMS by controlling the ion beam spot diameter and spacing (see Fig. 1E).Various morphologies shown in Fig. 2 A are created by a single mode FIB scanning with the beam current of 1 nA and the fluences indicated. When the PDMS substrate is exposed with a fluence on the order of 1 ϫ 10 13 ions per cm 2 , the...

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PSS are well preserved during the mechanical deformation.

We present the mechanics of folding surface-layer wrinkles on a soft substrate, i.e. inter-touching of neighbouring wrinkle surfaces without forming a cusp. Upon laterally compressing a stiff layer attached on a finite-elastic substrate, certain material nonlinearities trigger a number of bifurcation processes to form multi-mode wrinkle clusters. Some of these clusters eventually develop into folded wrinkles. The first bifurcation of the multi-mode wrinkles is investigated by a perturbation analysis of the surface-layer buckling on a pre-stretched neo-Hookean substrate. The postbuckling equilibrium configurations of the wrinkles are then trailed experimentally and computationally until the wrinkles are folded. The folding process is observed at various stages of wrinkling, by sectioning 20-80 nm thick gold films deposited on a polydimethylsiloxane substrate at a stretch ratio of 2.1. Comparison between the experimental observation and the finite-element analysis shows that the Ogden model deformation of the substrate coupled with asymmetric bending of the film predicts the folding process closely. In contrast, if the bending stiffness of the film is symmetric or the substrate follows the neo-Hookean behaviour, then the wrinkles are hardly folded. The wrinkle folding is applicable to construction of long parallel nano/micro-channels and control of exposing functional surface areas.

Human–machine interfaces have benefited from the advent of wireless sensor networks and the internet of things, but rely on wearable/attachable electronics exhibiting stretchability, biocompatibility, and transmittance. Limited by weight and volume, wearable devices should be energy efficient and even self-powered. Here, we report practical approaches for obtaining a stably self-cleanable, transparent and attachable ionic communicator based on triboelectric nanogenerators. The communicator can be easily applied on human skin due to softness and chemically anchored robust layers. It functions as a means of real-time communication between humans and machines. Surface functionalization on the communicator by (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane improves sensitivity and makes the communicator electrically and optically stable due to the self-cleaning effect without sacrificing transmittance. This research may benefit the potential development of attachable ionics, self-powered sensor networks, and monitoring systems for biomechanical motion.