FLEXIBLE ELECTRONICS

Highly stretchableelectroluminescent skin for opticalsignaling and tactile sensingC. Larson,1* B. Peele,1* S. Li,2* S. Robinson,2 M. Totaro,3 L. Beccai,3

B. Mazzolai,3 R. Shepherd1,2†

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 stretchingand surface area changes while actively emitting light. Layers of transparent hydrogelelectrodes sandwich a ZnS phosphor-doped dielectric elastomer layer, creating thinrubber sheets that change illuminance and capacitance under deformation. Arrays ofindividually controllable pixels in thin rubber sheets were fabricated using replicamolding and were subjected to stretching, folding, and rolling to demonstrate their useas stretchable displays. These sheets were then integrated into the skin of a softrobot, providing it with dynamic coloration and sensory feedback from external andinternal stimuli.

Biological systems employ a host of strat-egies for visual display and camouflage.Cephalopods, for example, can mimic theirenvironment by changing skin color andtexture, as well as posture (1). Recent devel-

opments in soft robotics (2, 3), bioinspired design(4, 5), and stretchable electronics (6) reveal strate-gies that enable us to engineer some of the func-tions of cephalopod skin synthetically. For example,microfluidic networks filled with liquid dyes havebeen used as active camouflage and displays forsoft mobile robots, giving them the ability tochange their appearance via color, texture, andluminescence (7). More recently, electro-mechano-chemically responsive films were exploited torender fluorescent patterns under the control ofelectric fields (8), and adaptive optoelectroniccamouflage systems have been used to mimic thevisual appearance of cephalopod skin (9). Anotherapproach is the use of active display technologies,such as polymeric light-emitting devices (PLEDs)and organic light-emitting diodes (OLEDs), whichuse stretchable transparent electrodes based onindium tin oxide (ITO) films (10), graphene (11),single- ormulti-walled carbonnanotubes (SWNTsor MWNTs) (12, 13), polyethylene-dioxythiophene:polystyrene-sulfonate (PEDOT:PSS) (14), or otherpercolated networks of conductive colloids ornanowires (15). Despite the broad applicability ofLED-based systems for consumer displays, theirelectrical function is limited to ultimate strains,eult < 120% (16), well below the ultimate strain ofelastomers (such as silicones; eult ~400 to 700%)

that are used in soft robotics to mimic the move-ments of animals.Biological skin also enables animals to sense

their environments. A number of approacheshave been used to create pressure-sensitive elec-tronic skins, including arrays of organic field-effect transistors (FETs) deposited on flexibleparylene-polyamide substrates (17, 18) and in-side stretchable rubber (19), as well as thin Aufilms and liquid metal embedded in polydi-methylsiloxane (PDMS) (20, 21). More recently,dielectric elastomer transducers (DETs), whichare stretchable capacitors composed of highlyextensible ionic hydrogels, have been used. Thesehydrogels are intrinsically soft, highly transpar-ent in the visible spectrum (extinction coefficientmext ~ 10−6 mm−1) (22), can exhibit very high ulti-mate strain (eult ~2000%) and toughness (U ~9 kJ m−2) (23), and have relative changes in resis-tivity with strain that are orders of magnitudeless than those of electrodes based on percolatednetworks of conductive particles (such as metalnanoparticles, carbon powder, or nanotubes) (24).Presently, soft robots are primarily used be-

cause their low mechanical compliance enablessafe human-robot interaction; however, their po-tential is limited by a lack of suitable electronicsthat can stretch continuously with their bodies.No soft robot can dynamically display informa-tion on its body, and there are relatively fewexamples that can sense external and internalstimuli. Here we present a hyperelastic light-emitting capacitor (HLEC) that enables bothlight emission and touch sensing in a thin rub-ber sheet that stretches to >480% strain (Fig. 1A).These HLECs are composed of ionic hydrogelelectrodes and composites of doped ZnS phos-phors embedded in a dielectric matrix of siliconeelastomer. We used electroluminescent (EL) phos-phor powders that emit light via excitations with-in intrinsic heterojunctions under an AC electric

field; unlike current-driven LEDs, which requirelithography to form p-n junctions, this materialsystem can be processed using replica molding.Application of an AC electric field causes lumi-nescence within the semiconducting phosphor atwavelength centers corresponding to the dopantsin the ZnS lattice. Green and blue centers aretypically 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 combina-tions of these dopants.The HLEC (Fig. 1B) is a five-layer structure con-

sisting of an electroluminescent dielectric layerthat is sandwiched between two electrodes andencapsulated in low elastic modulus (E ~ 30 kPa)(26) silicone (Ecoflex 00-30, Smooth-on Inc.). Ourhydrogel electrodes are designed with a balanceof high mechanical toughness, low volatility, andlow electrical resistance under deformation (fig.S1 and data table S1). Aqueous lithium chloride(LiCl) is used as the ionic conductor because ofits high conductivity (~10 S m−1), ionic strength,and hygroscopic nature, whereas polyacrylamide(PAM) is used as the elastomeric matrix becauseof its high toughness (27) and optical transpar-ency. Electrodes are synthesized by first dissolv-ing acrylamide monomer (AAm), polyacrylamide,and N,N′-methylenebisacrylamide crosslinker inaqueous LiCl and casting the solution onto anultraviolet (UV)–ozone–treated silicone (Ecoflex00-30) substrate. The aqueous PAM-AAm solu-tion is then crosslinked under UV light (28),producing a highly stretchable and transparentelectrode. The EL layer is formed by mixingcommercially available phosphor powders (GlobalTungsten & Powders) (25 mm, ~8% by volume)into silicone (Ecoflex 00-30) and then moldingthe dispersion into a 1-mm-thick sheet. Fi-nally, we bond the EL layer between the twoelectrode-patterned silicone substrates and en-capsulate the capacitor in an insulating layerof silicone.The stress-strain curves of the HLEC and its

silicone-containing layers (Ecoflex and Ecoflex-EL composite) are all coincident, whereas theelastic modulus of the hydrogel is two orders ofmagnitude lower, allowing the HLEC to stretchfreely without delaminating. Mechanical testingdata (Fig. 1C and data table S2) and images(Fig. 2A, data table S3, and movie S1) show theexcellent adhesion between the layers. The HLECachieved a mean strain of 487 ± 59% (SD), asmeasured at five locations across the width ofthe illuminated section, with portions exceed-ing 500% before the external copper leads lostcontact with the hydrogel electrodes. For thesetests, the HLECs were operated at 700 Hz undera nominal electric field of ~25 kV cm−1, with apower consumption of 0.2 W and a luminous effi-cacy of 43.2 millilumens per watt (mlm W−1) (28).We used this same replica molding technique toform an 8-by-8 array of 4-mm pixels (Fig. 3A).This HLEC display can undergo many deforma-tion modes, including stretching, rolling, folding,and wrapping (Fig. 3, B to E, and movie S2). Dyna-mic control of the pixels is shown in Fig. 3, F to I.

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1Department of Mechanical and Aerospace Engineering,Cornell University, Ithaca, NY 14853, USA. 2Department ofMaterials Science and Engineering, Cornell University, Ithaca,NY 14853, USA. 3Center for Micro-BioRobotics@SSSA,Istituto Italiano di Tecnologia, Viale Rinaldo Piaggio 34,I-56025 Pontedera, Italy.*These authors contributed equally to this work. †Correspondingauthor. E-mail: rfs247@cornell.edu

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In addition to emitting light, the HLEC alsoserves as a dielectric elastomer sensor (DES), dueto its construction as a parallel-plate capacitor.Changes in the electrode area (A) and separationdistance (d) cause the capacitance (C) to changeaccording to C/C0 º Ad–1, allowing the HLEC tosense deformations from pressure and stretch-ing. 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 din terms of the principal stretches, l1, l2, and l3,which represent the axial, transverse, and out-of-plane orientations, respectively (supplementarytext). For uniaxial boundary conditions, we ob-serve that the relative capacitance increases lin-

early as the sample is stretched (eq. S11). Forbiaxial test conditions, we observe that the rela-tive change in capacitance follows C/C0 = l4

(eq. S12); however, at higher strains, the mea-sured values are slightly lower, due to a decreasein the permittivity of the dielectric (24).The illuminance of the HLEC also increases as

the device is stretched. We attribute this change

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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 itsmuch lower elastic modulus.

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. Fivelengths were measured using image analysis software to obtain l1 across the width of the illuminatedportion of the tensile bar. We report the mean and standard deviation of those measurements. At anengineering strain (grip to grip) of 395%, we measured the mean strain of the illuminated portion to be487%, 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).

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to two interrelated phenomena: (i) the increasein electric field (E) as d decreases and (ii) thedecrease in areal number density of phosphorparticles (h) as A increases. Starting with the Alfrey-Taylor equation (eq. S13, fig. S3, and data tableS6) (29), we predict the scaling law in Eq. 1 byexpressing E/E0 as a function of the principalstretches and by correcting for the change in hwith stretching (h/h0 º A0/A) (supplementarytext). The predicted trend is shown alongside

luminescence measurements in Fig. 2C (datatable S7)

I

I0¼ ðl1l2Þ−1exp½5:68ð1 − l1=23 Þ� ð1Þ

To demonstrate the ability to monolithic-ally integrate the HLEC into soft systems, weembedded three HLEC panels in a crawlingsoft robot by bonding six layers together. Thetop four layers make up the electroluminescent

skin, whereas the bottom two are used for pneu-matic actuation (Fig. 4A). Inspired by architec-tures developed for mobile soft robots (30), ourpneumatic actuator uses a series of inflatablechambers embedded in silicone, with a bottomlayer composed of an inextensible fiber-elastomercomposite (28). The inextensible layer induces anet bending moment as the pneumatic chambersare inflated; the resulting curvature is exploitedto create an undulating gait.

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Fig. 3. Multipixel electroluminescent displays fabricated via replica molding.The device measures 5 mm thick, with each of the 64 pixels measuring4 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.

Fig. 4. HLEC skins endow soft robots with the ability to sense their actu-ated state and environment and communicate optically. (A) Schematic of athree-chambered soft robot. A series of three independently actuated pneumaticchambers is embedded between the HLEC skin (top) and a strain-limiting layer(bottom). (B) Capacitance plotted versus the actuation amplitude, defined as therelative change in deflection between the uninflated and fully inflated states(number of samples, n = 5). (C) A firm finger press induces an ~25% increasein capacitance. (D) Change in capacitance versus applied pressure.We observed

a negligible change in the capacitive response of the sensors over a period of120 hours. (E) Array of three HLEC panels, each emitting a different wavelengththrough selective doping of the EL phosphor layer. Each HLEC panel is activatedindependently. (F) An undulating gait is produced by pressurizing the chambers insequence along the length of the crawler. This sequence produces forwardlocomotion at a speed of ~4.8 m hour−1 (~32 body lengths hour−1). As eachpneumatic chamber is pressurized, the outer electroluminescent skin is stretched,increasing the electric field across the EL layer and thus the luminescence.

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The crawling robot uses its HLEC skin to senseits physical state and environment (i.e., proprio-ception and exteroception). The capacitance of theHLEC changes with pneumatic actuation (Fig. 4Band data S8) and externally applied pressure (Fig.4, C and D, and data table S9) (10). Actuation ofthe three underlying pneumatic chambers resultsin capacitance changes (DC) of up to 1000% whenthe chambers are fully inflated. Additionally, eachHLEC panel is largely decoupled from the state ofthe surrounding pneumatic chambers (fig. S4 anddata table S11) (28). The ability to identify theactuated state of the robot using the capacitivesensor readings enables proprioception. To dem-onstrate the tactile sensing capabilities of the elec-tronic skin, we pressed each of the HLEC panelson the robot and measured the capacitive response(Fig. 4C). A firm finger press resulted in a ~25%increase in capacitance. The relative capacitanceversus applied pressure, ranging from 0.9 to30.9 kPa, remained nearly constant over a periodof 120 hours (Fig. 4D). Arrays of these tactile sen-sors enable exteroception in soft robotic systems.An array of three HLEC panels patterned into

the three-chambered crawling robot enables eightdistinct illuminated states (Fig. 4E). The embeddedHLEC remains functional as the robot is actuatedthrough its crawling sequence (Fig. 4F and movieS3). During actuation, the embedded HLEC un-dergoes stretches of l1 = 2.63 and l2 = 2.42 in thelongitudinal (front to rear) and transverse (side toside) directions, respectively, to produce a ~635%increase in the skin’s surface area (fig. S5). Sim-ilar to the single-panel HLEC (movie S1), theluminescence of the embedded skin increasesduring actuation as its thickness is decreased.Integrating these highly stretchable and com-

pliant displays into soft actuators enables twonew capabilities in soft electronics: (i) displaysthat actively change their shape and (ii) robotsthat actively change their color. Using replicamolding, we fabricated a multipixel array of in-dividually addressable HLECs, and we used thesame process to monolithically integrate thesedisplays into a soft robot capable of changingposture. The HLEC array imparts both dynamiccoloration and the potential for feedback control,which would be useful in epidermal electronics(31) and robotics (32). Although the luminous ef-ficacy of our HLEC (43.2 mlmW−1) is not as highas that of commercial AC powder electrolumine-scent devices (~4 lm W−1) (32), it can be greatlyimproved by tuning the materials system anddevice architecture (such as higher-transmissivityencapsulation layers, reduced thickness, and opti-mized particle size). For applications requiringhigher display resolution, HLECs could be madecompatible with photolithography and othermicrofabrication techniques by using photo-polymerizable polymers. These techniques wouldalso allow us to decrease the thickness of theelectroluminescent layer, thereby reducing thevoltage required to power the HLEC.

REFERENCES AND NOTES

1. A. Barbosa, J. J. Allen, L. Mäthger, R. T. Hanlon, Proc. R. Soc.London Ser. B 279, 84–90 (2012).

2. F. Ilievski, A. D. Mazzeo, R. F. Shepherd, X. Chen,G. M. Whitesides, Angew. Chem. Int. Ed. Engl. 50, 1890–1895(2011).

3. D. Rus, M. T. Tolley, Nature 521, 467–475 (2015).4. M. J. Spenko et al., J. Field Robot. 25, 223–242 (2008).5. E. Kreit et al., J. R. Soc. Interface 10, 20120601 (2012).6. J. A. Rogers, T. Someya, Y. Huang, Science 327, 1603–1607

(2010).7. S. A. Morin et al., Science 337, 828–832 (2012).8. Q. Wang, G. R. Gossweiler, S. L. Craig, X. Zhao, Nat. Commun.

5, 4899 (2014).9. C. Yu et al., Proc. Natl. Acad. Sci. U.S.A. 111, 12998–13003

(2014).10. P. E. Burrows et al., Displays 22, 65–69 (2001).11. T. H. Han et al., Nat. Photonics 6, 105–110 (2012).12. T. Sekitani et al., Nat. Mater. 8, 494–499 (2009).13. M. K. Shin et al., Adv. Mater. 22, 2663–2667 (2010).14. M. S. White et al., Nat. Photon. 7, 811–816 (2013).15. L. Hu, H. S. Kim, J. Y. Lee, P. Peumans, Y. Cui, ACS Nano 4,

2955–2963 (2010).16. J. Liang, L. Li, X. Niu, Z. Yu, Q. Pei, Nat. Photon. 7, 817–824 (2013).17. T. Someya et al., Proc. Natl. Acad. Sci. U.S.A. 102, 12321–12325

(2005).18. K. Takei et al., Nat. Mater. 9, 821–826 (2010).19. T. Someya et al., Proc. Natl. Acad. Sci. U.S.A. 101, 9966–9970

(2004).20. Y. L. Park, B. R. Chen, R. J. Wood, IEEE Sens. J. 12, 2711–2718

(2012).21. D. P. J. Cotton, I. M. Graz, S. P. Lacour, IEEE Sens. J. 9,

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26, 7608–7614 (2014).25. A. Kitai, Luminescent Materials and Applications (Wiley, West

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ACKNOWLEDGMENTS

Data reported in the paper are included in the supplementarymaterials. This work was supported by the Army Research Office(grant no. W911NF-15-1-0464), the Air Force Office of ScientificResearch (grant no. FA9550-15-1-0160), the NSF MRSEC program(DMR-1120296), and an NSF Graduate Research Fellowship(grant no. DGE-1144153). The hyperelastic electroluminescentcapacitors presented in this work have been filed under aprovisional patent application, no. 62/250,172 for StretchableElectroluminescent Devices. The listed inventors are Chris Larson,Shuo Li, Bryan Peele, Sanlin Robinson, and Robert Shepherd.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/351/6277/1071/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S7Table S1Reference (33)Movies S1 to S3Data Tables S1 to S11 (single Excel workbook)

5 May 2015; accepted 4 February 201610.1126/science.aac5082

HUMAN ALTRUISM

The brain’s functional networkarchitecture reveals human motivesGrit Hein,1* Yosuke Morishima,1,2,3 Susanne Leiberg,1 Sunhae Sul,4 Ernst Fehr1

Goal-directed human behaviors are driven by motives. Motives are, however, purely mentalconstructs that are not directly observable. Here, we show that the brain’s functionalnetwork architecture captures information that predicts different motives behind the samealtruistic act with high accuracy. In contrast, mere activity in these regions contains noinformation about motives. Empathy-based altruism is primarily characterized by apositive connectivity from the anterior cingulate cortex (ACC) to the anterior insula (AI),whereas reciprocity-based altruism additionally invokes strong positive connectivity fromthe AI to the ACC and even stronger positive connectivity from the AI to the ventralstriatum. Moreover, predominantly selfish individuals show distinct functionalarchitectures compared to altruists, and they only increase altruistic behavior in responseto empathy inductions, but not reciprocity inductions.

The theory of revealed preference (1) pro-vides the choice-theoretic foundations formodern economics. In this view, prefer-ences cannot be identified independentlyof behavior, andmotives play no causal role

in economists’ explanatory toolbox—a view thatis in direct contradiction to the neuroeconomicapproach (2–4). In psychology, motives are alsoconsidered to be independent drivers of goal-directedhumanbehavior (5).Motives are, however,mental constructs that are not directly observableand frequently not even accessible introspectively,meaning that asking people does not provide rel-

evant information about motives (6, 7). There-fore, humanmotives have been typically inferredfrom individuals’ behavior by assuming that dif-ferent motives lead to different behaviors.

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1Laboratory for Social and Neural Systems Research,Department of Economics, University of Zurich, Switzerland.2Division of Systems Neuroscience of Psychopathology,Translational Research Center, University Hospital ofPsychiatry, University of Bern, Switzerland. 3JapaneseScience and Technology Agency, PRESTO, Japan.4Department of Psychology, Pusan National University,Pusan, South Korea.*Corresponding author. E-mail: grit.hein@econ.uzh.ch (G.H.);ernst.fehr@econ.uzh.ch (E.F.)

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Highly stretchable electroluminescent skin for optical signaling and tactile sensingC. Larson, B. Peele, S. Li, S. Robinson, M. Totaro, L. Beccai, B. Mazzolai and R. Shepherd

DOI: 10.1126/science.aac5082 (6277), 1071-1074.351Science 

, this issue p. 1071Sciencemoved.internal and external pressure. A soft robot demonstrated these combined capabilities by stretching and emitting light as itstretchable electroluminescent actuator. The material could be highly stretched, could emit light, and could also sense

developed aet al.cells are loaded with pigments that enable rapid and detailed camouflaging abilities. Larson The skins of some cephalopods, such as the octopus, are highly flexible and contain color-changing cells. These

Make it stretch, make it glow

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