COMMUNICATION 1804080 (1 of 6) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Electroactive Soft Photonic Devices for the Synesthetic Perception of Color and Sound Do Yoon Kim, Sunglok Choi, Hyesung Cho,* and Jeong-Yun Sun* D. Y. Kim, Prof. J.-Y. Sun Department of Materials Science and Engineering Seoul National University Seoul 151-742, South Korea E-mail: jysun@snu.ac.kr S. Choi IT Convergence Technology Research Laboratory Electronics and Telecommunications Research Institute Daejeon 34129, South Korea Dr. H. Cho Department of Materials Science and Engineering University of Pennsylvania 3231 Walnut Street, Philadelphia, PA 19104, USA E-mail: hyesungc@seas.upenn.edu Prof. J.-Y. Sun Research Institute of Advanced Materials Seoul National University Seoul 1510744, South Korea The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201804080. DOI: 10.1002/adma.201804080 sensors, [8–10] and structural camouflage. [1,2] When paired with auditory counterpart, i.e., sound, the color facilitates the effec- tive transfer of information from devices to humans, therein rationalizing the stere- otypical designs of contemporary devices, which contain both sound and color mod- ules. With regard to color, nature has, via evolution, mastered the art of emit- ting and reflecting light, as evidenced by bioluminescence, [11] and structural color, [12] which in turn, can be juxtaposed with man-made emissive displays [13] and structural materials with photonic band gaps, [14] respectively. Indeed, while most artificial photonic devices intrinsically lack the ability to modulate their output color once their periodicity is fixed, [15] some creatures, such as cephalopods [4] and cha- meleons, [3] have overcome this constraint in an extrinsic manner by stretching their skin. Inspired by these naturally evolved systems, there has been an explosion of research [16] into deformable skins capable of color tunability to produce mechano- chromic systems, [17–19] though these sys- tems have their own inherent disadvantages, [8,16,20,21] including slow response rates, limited ranges of tunability, and high pro- pensity for electrical and mechanical failure. Transducing electrical energy in the form of excitation volt- ages in a dielectric elastomer actuator (DEA) [22–25] grants us access to large areal strains with high response rates by way of induced Maxwell stress across a dielectric medium. The intro- duction of a photonic layer between the two electrodes of the DEA, however, increases the requisite working threshold voltage to effect the required transduction and thereby the likelihood of dielectric breakdown. [26] This complication can be resolved by attaching the photonic layer to the exterior of the DEA. [27] In this regard, using hydrogels that are intrinsically soft, compa- rable to biological tissue (Young’s modulus, E < 1 MPa), stretch- able and able to withstand significant physical deformation in their swollen state [17,28,29] would appear to be an obvious choice. However, as their superior properties are so heavily reliant on their water contents, the ease of evaporation and high vapor pressure of their solvent (for water, ≈2.3 kPa) compared to other organic solvents (e.g., ethylene glycol, EG, ≈0.5 kPa) render them ill-suited for this particular application, which necessitates long-lasting softness. Here we demonstrate a photonic organogel [poly(2-hydroxyethyl methacrylate-co-acryla- mide) (pHEMA-co-AAm)] with EG as the solvent to sustain the Color, as perceived through the eye, transcends mere information in the visible range of electromagnetism and serves as an agent for communica- tion and entertainment. Mechanochromic systems have thus far only aimed at satisfying the sense of vision and have overlooked the possibility of generating acoustic vibrations in concert with their visual color responses that would enable the simultaneous satisfaction of the auditory system. Transcending the boundaries of the two senses (i.e., sound and color), herein a strategy for their concurrent and synesthetic fulfillment is elucidated by electrically actuating an organogel photonic device, controlled by a single input signal. This new class of devices that integrate a color module with a speaker is fabricated from a mechanochromic layer that comprises close-packed photonic lattice with an organogel matrix pervading the void fraction. Exploiting a dielectric elastomer actuator, the system’s mechanical response permits the simultaneous, yet independent, exploration of visible- light reflection alongside audible sound-wave generation. Large areal strains at low frequencies of actuation tune the photonic stop-band, whereas the layer remains incompressible and exhibits negligible strain when actuated at higher frequencies (e.g., tens of Hz), thereby making it amenable to modulate sound and color simultaneously yet independently. Synesthetic Devices With changes in shapes [1] and color, [2] human beings can visually perceive [3] or be deceived [4] by information or stimuli, as exploited in contemporary strain-governed applications [5] such as tactile displays, [6] and touch panel, [7] mechanochromic Adv. Mater. 2018, 1804080