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Highly stretchable electroluminescent skin for optical signaling and tactile sensing

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Science  04 Mar 2016:
Vol. 351, Issue 6277, pp. 1071-1074
DOI: 10.1126/science.aac5082

Make it stretch, make it glow

The skins of some cephalopods, such as the octopus, are highly flexible and contain color-changing cells. These cells are loaded with pigments that enable rapid and detailed camouflaging abilities. Larson et al. developed a stretchable electroluminescent actuator. The material could be highly stretched, could emit light, and could also sense internal and external pressure. A soft robot demonstrated these combined capabilities by stretching and emitting light as it moved.

Science, this issue p. 1071

Abstract

Cephalopods such as octopuses have a combination of a stretchable skin and color-tuning organs to control both posture and color for visual communication and disguise. We present an electroluminescent material that is capable of large uniaxial stretching and surface area changes while actively emitting light. Layers of transparent hydrogel electrodes sandwich a ZnS phosphor-doped dielectric elastomer layer, creating thin rubber sheets that change illuminance and capacitance under deformation. Arrays of individually controllable pixels in thin rubber sheets were fabricated using replica molding and were subjected to stretching, folding, and rolling to demonstrate their use as stretchable displays. These sheets were then integrated into the skin of a soft robot, providing it with dynamic coloration and sensory feedback from external and internal stimuli.

biological systems employ a host of strategies for visual display and camouflage. Cephalopods, for example, can mimic their environment by changing skin color and texture, as well as posture (1). Recent developments in soft robotics (2, 3), bioinspired design (4, 5), and stretchable electronics (6) reveal strategies that enable us to engineer some of the functions of cephalopod skin synthetically. For example, microfluidic networks filled with liquid dyes have been used as active camouflage and displays for soft mobile robots, giving them the ability to change their appearance via color, texture, and luminescence (7). More recently, electro-mechano-chemically responsive films were exploited to render fluorescent patterns under the control of electric fields (8), and adaptive optoelectronic camouflage systems have been used to mimic the visual appearance of cephalopod skin (9). Another approach is the use of active display technologies, such as polymeric light-emitting devices (PLEDs) and organic light-emitting diodes (OLEDs), which use stretchable transparent electrodes based on indium tin oxide (ITO) films (10), graphene (11), single- or multi-walled carbon nanotubes (SWNTs or MWNTs) (12, 13), polyethylene-dioxythiophene:polystyrene-sulfonate (PEDOT:PSS) (14), or other percolated networks of conductive colloids or nanowires (15). Despite the broad applicability of LED-based systems for consumer displays, their electrical function is limited to ultimate strains, εult < 120% (16), well below the ultimate strain of elastomers (such as silicones; εult ~400 to 700%) that are used in soft robotics to mimic the movements of animals.

Biological skin also enables animals to sense their environments. A number of approaches have been used to create pressure-sensitive electronic skins, including arrays of organic field-effect transistors (FETs) deposited on flexible parylene-polyamide substrates (17, 18) and inside stretchable rubber (19), as well as thin Au films and liquid metal embedded in polydimethylsiloxane (PDMS) (20, 21). More recently, dielectric elastomer transducers (DETs), which are stretchable capacitors composed of highly extensible ionic hydrogels, have been used. These hydrogels are intrinsically soft, highly transparent in the visible spectrum (extinction coefficient μext ~ 10−6 μm−1) (22), can exhibit very high ultimate strain (εult ~2000%) and toughness (U ~ 9 kJ m−2) (23), and have relative changes in resistivity with strain that are orders of magnitude less than those of electrodes based on percolated networks of conductive particles (such as metal nanoparticles, carbon powder, or nanotubes) (24).

Presently, soft robots are primarily used because their low mechanical compliance enables safe human-robot interaction; however, their potential is limited by a lack of suitable electronics that can stretch continuously with their bodies. No soft robot can dynamically display information on its body, and there are relatively few examples that can sense external and internal stimuli. Here we present a hyperelastic light-emitting capacitor (HLEC) that enables both light emission and touch sensing in a thin rubber sheet that stretches to >480% strain (Fig. 1A). These HLECs are composed of ionic hydrogel electrodes and composites of doped ZnS phosphors embedded in a dielectric matrix of silicone elastomer. We used electroluminescent (EL) phosphor powders that emit light via excitations within intrinsic heterojunctions under an AC electric field; unlike current-driven LEDs, which require lithography to form p-n junctions, this material system can be processed using replica molding. Application of an AC electric field causes luminescence within the semiconducting phosphor at wavelength centers corresponding to the dopants in the ZnS lattice. Green and blue centers are typically produced using low [~0.01 weight % (wt %)] and high (~0.1 wt %) concentrations of Cu, whereas yellow is produced using Mn (~1 wt %) (25). White light can be achieved using combinations of these dopants.

Fig. 1 HLEC.

(A) Image of the HLEC conforming to the end of a pencil. (B) Exploded view of the HLEC showing its five-layer structure consisting of a ~1-mm-thick electroluminescent layer (ZnS-Ecoflex 00-30) that is sandwiched between two PAM-LiCl hydrogel electrodes and encapsulated in Ecoflex 00-30. (C) Stress-stretch curves of Ecoflex 00-30, the electroluminescent layer, and the composite device. The hydrogel data are shown in the inset because of its much lower elastic modulus.

The HLEC (Fig. 1B) is a five-layer structure consisting of an electroluminescent dielectric layer that is sandwiched between two electrodes and encapsulated in low elastic modulus (E ~ 30 kPa) (26) silicone (Ecoflex 00-30, Smooth-on Inc.). Our hydrogel electrodes are designed with a balance of high mechanical toughness, low volatility, and low electrical resistance under deformation (fig. S1 and data table S1). Aqueous lithium chloride (LiCl) is used as the ionic conductor because of its high conductivity (~10 S m−1), ionic strength, and hygroscopic nature, whereas polyacrylamide (PAM) is used as the elastomeric matrix because of its high toughness (27) and optical transparency. Electrodes are synthesized by first dissolving acrylamide monomer (AAm), polyacrylamide, and N,N′-methylenebisacrylamide crosslinker in aqueous LiCl and casting the solution onto an ultraviolet (UV)–ozone–treated silicone (Ecoflex 00-30) substrate. The aqueous PAM-AAm solution is then crosslinked under UV light (28), producing a highly stretchable and transparent electrode. The EL layer is formed by mixing commercially available phosphor powders (Global Tungsten & Powders) (25 μm, ~8% by volume) into silicone (Ecoflex 00-30) and then molding the dispersion into a 1-mm-thick sheet. Finally, we bond the EL layer between the two electrode-patterned silicone substrates and encapsulate the capacitor in an insulating layer of silicone.

The stress-strain curves of the HLEC and its silicone-containing layers (Ecoflex and Ecoflex-EL composite) are all coincident, whereas the elastic modulus of the hydrogel is two orders of magnitude lower, allowing the HLEC to stretch freely without delaminating. Mechanical testing data (Fig. 1C and data table S2) and images (Fig. 2A, data table S3, and movie S1) show the excellent adhesion between the layers. The HLEC achieved a mean strain of 487 ± 59% (SD), as measured at five locations across the width of the illuminated section, with portions exceeding 500% before the external copper leads lost contact with the hydrogel electrodes. For these tests, the HLECs were operated at 700 Hz under a nominal electric field of ~25 kV cm−1, with a power consumption of 0.2 W and a luminous efficacy of 43.2 millilumens per watt (mlm W−1) (28). We used this same replica molding technique to form an 8-by-8 array of 4-mm pixels (Fig. 3A). This HLEC display can undergo many deformation modes, including stretching, rolling, folding, and wrapping (Fig. 3, B to E, and movie S2). Dynamic control of the pixels is shown in Fig. 3, F to I.

Fig. 2 The capacitive and luminescent behavior of the HLEC display under uniaxial stretching.

(A) A nominal electric field of ~25 kV cm−1 was applied to the HLEC at the start of the uniaxial test. Five lengths were measured using image analysis software to obtain λ1 across the width of the illuminated portion of the tensile bar. We report the mean and standard deviation of those measurements. At an engineering strain (grip to grip) of 395%, we measured the mean strain of the illuminated portion to be 487%, with a range of 420 to 549%. (B) The capacitance of the HLEC as a function of its uniaxial stretch (number of samples, n = 4). (C) The relative illuminance of the HLEC versus its uniaxial stretch (n = 4), plotted alongside predicted values (supplementary text).

Fig. 3 Multipixel electroluminescent displays fabricated via replica molding.

The device measures 5 mm thick, with each of the 64 pixels measuring 4 mm. We show the devices in various states of deformation and illumination: (A) undeformed, (B) stretched, (C) wrapped around a finger, (D) folded, (E) rolled, (F to H) with subsets of pixels activated, and (I and J) subsets of pixels activated while being deformed.

In addition to emitting light, the HLEC also serves as a dielectric elastomer sensor (DES), due to its construction as a parallel-plate capacitor. Changes in the electrode area (A) and separation distance (d) cause the capacitance (C) to change according to C/C0Ad–1, allowing the HLEC to sense deformations from pressure and stretching. The capacitance of the HLEC changes as it is stretched under uniaxial (Fig. 2B and data S4) and biaxial (fig. S2 and data S5) tension (28). We model the capacitance by expressing A and d in terms of the principal stretches, λ1, λ2, and λ3, which represent the axial, transverse, and out-of-plane orientations, respectively (supplementary text). For uniaxial boundary conditions, we observe that the relative capacitance increases linearly as the sample is stretched (eq. S11). For biaxial test conditions, we observe that the relative change in capacitance follows C/C0 = λ4 (eq. S12); however, at higher strains, the measured values are slightly lower, due to a decrease in the permittivity of the dielectric (24).

The illuminance of the HLEC also increases as the device is stretched. We attribute this change to two interrelated phenomena: (i) the increase in electric field (E) as d decreases and (ii) the decrease in areal number density of phosphor particles (η) as A increases. Starting with the Alfrey-Taylor equation (eq. S13, fig. S3, and data table S6) (29), we predict the scaling law in Eq. 1 by expressing E/E0 as a function of the principal stretches and by correcting for the change in η with stretching (η/η0A0/A) (supplementary text). The predicted trend is shown alongside luminescence measurements in Fig. 2C (data table S7)

Embedded Image(1)

To demonstrate the ability to monolithically integrate the HLEC into soft systems, we embedded three HLEC panels in a crawling soft robot by bonding six layers together. The top four layers make up the electroluminescent skin, whereas the bottom two are used for pneumatic actuation (Fig. 4A). Inspired by architectures developed for mobile soft robots (30), our pneumatic actuator uses a series of inflatable chambers embedded in silicone, with a bottom layer composed of an inextensible fiber-elastomer composite (28). The inextensible layer induces a net bending moment as the pneumatic chambers are inflated; the resulting curvature is exploited to create an undulating gait.

Fig. 4 HLEC skins endow soft robots with the ability to sense their actuated state and environment and communicate optically.

(A) Schematic of a three-chambered soft robot. A series of three independently actuated pneumatic chambers is embedded between the HLEC skin (top) and a strain-limiting layer (bottom). (B) Capacitance plotted versus the actuation amplitude, defined as the relative change in deflection between the uninflated and fully inflated states (number of samples, n = 5). (C) A firm finger press induces an ~25% increase in capacitance. (D) Change in capacitance versus applied pressure. We observed a negligible change in the capacitive response of the sensors over a period of 120 hours. (E) Array of three HLEC panels, each emitting a different wavelength through selective doping of the EL phosphor layer. Each HLEC panel is activated independently. (F) An undulating gait is produced by pressurizing the chambers in sequence along the length of the crawler. This sequence produces forward locomotion at a speed of ~4.8 m hour−1 (~32 body lengths hour−1). As each pneumatic chamber is pressurized, the outer electroluminescent skin is stretched, increasing the electric field across the EL layer and thus the luminescence.

The crawling robot uses its HLEC skin to sense its physical state and environment (i.e., proprioception and exteroception). The capacitance of the HLEC changes with pneumatic actuation (Fig. 4B and data S8) and externally applied pressure (Fig. 4, C and D, and data table S9) (10). Actuation of the three underlying pneumatic chambers results in capacitance changes (ΔC) of up to 1000% when the chambers are fully inflated. Additionally, each HLEC panel is largely decoupled from the state of the surrounding pneumatic chambers (fig. S4 and data table S11) (28). The ability to identify the actuated state of the robot using the capacitive sensor readings enables proprioception. To demonstrate the tactile sensing capabilities of the electronic skin, we pressed each of the HLEC panels on the robot and measured the capacitive response (Fig. 4C). A firm finger press resulted in a ~25% increase in capacitance. The relative capacitance versus applied pressure, ranging from 0.9 to 30.9 kPa, remained nearly constant over a period of 120 hours (Fig. 4D). Arrays of these tactile sensors enable exteroception in soft robotic systems.

An array of three HLEC panels patterned into the three-chambered crawling robot enables eight distinct illuminated states (Fig. 4E). The embedded HLEC remains functional as the robot is actuated through its crawling sequence (Fig. 4F and movie S3). During actuation, the embedded HLEC undergoes stretches of λ1 = 2.63 and λ2 = 2.42 in the longitudinal (front to rear) and transverse (side to side) directions, respectively, to produce a ~635% increase in the skin’s surface area (fig. S5). Similar to the single-panel HLEC (movie S1), the luminescence of the embedded skin increases during actuation as its thickness is decreased.

Integrating these highly stretchable and compliant displays into soft actuators enables two new capabilities in soft electronics: (i) displays that actively change their shape and (ii) robots that actively change their color. Using replica molding, we fabricated a multipixel array of individually addressable HLECs, and we used the same process to monolithically integrate these displays into a soft robot capable of changing posture. The HLEC array imparts both dynamic coloration and the potential for feedback control, which would be useful in epidermal electronics (31) and robotics (32). Although the luminous efficacy of our HLEC (43.2 mlm W−1) is not as high as that of commercial AC powder electroluminescent devices (~4 lm W−1) (32), it can be greatly improved by tuning the materials system and device architecture (such as higher-transmissivity encapsulation layers, reduced thickness, and optimized particle size). For applications requiring higher display resolution, HLECs could be made compatible with photolithography and other microfabrication techniques by using photopolymerizable polymers. These techniques would also allow us to decrease the thickness of the electroluminescent layer, thereby reducing the voltage required to power the HLEC.

Supplementary Materials

www.sciencemag.org/content/351/6277/1071/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S7

Table S1

Reference (33)

Movies S1 to S3

Data Tables S1 to S11 (single Excel workbook)

REFERENCES AND NOTES

  1. Materials and methods are available as supplementary materials on Science Online.
  2. Acknowledgments: Data reported in the paper are included in the supplementary materials. This work was supported by the Army Research Office (grant no. W911NF-15-1-0464), the Air Force Office of Scientific Research (grant no. FA9550-15-1-0160), the NSF MRSEC program (DMR-1120296), and an NSF Graduate Research Fellowship (grant no. DGE-1144153). The hyperelastic electroluminescent capacitors presented in this work have been filed under a provisional patent application, no. 62/250,172 for Stretchable Electroluminescent Devices. The listed inventors are Chris Larson, Shuo Li, Bryan Peele, Sanlin Robinson, and Robert Shepherd.
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