All‐Elastomeric, Strain‐Responsive Thermochromic Color Indicators

Abstract

Recent advances in materials for stretchable electronics create new areas of application, particularly in systems that involve intimate integration with the human body. Examples include wearable electronics, [1][2][3][4][5][6] soft surgical instruments, [ 5,[7][8][9][10][11][12][13] and skin-integrated health/wellness monitors. [ 1,[14][15][16][17][18] Many envisioned systems require schemes for visual information display. Such functionality can be provided by organic or inorganic light emitting diodes (LEDs), either as arrays of non-stretchable devices joined by deformable interconnects, [ 7,19,20 ] or as elements in buckled geometries. [ 19,21 ] Intrinsically stretchable LEDs have also recently been achieved, using all polymer designs. [ 22 ] Here we present a simple, non-emissive option in display materials that likewise offer intrinsic stretchability. The approach uses elastomeric composites of thermochromic materials and metallic particles as the photonic and electronic components, respectively, in concepts that build on related materials [ 23 ] for fl exible displays. We show, additionally, that the types of stretchable displays introduced here can be used as colorimetric indicators of mechanical strain.The thermochromic elastomer uses a well-mixed dispersion of leuco dye (black 47C, LCR Hallcrest) contained in microcapsules and embedded in a silicone elastomer matrix (Smooth-on Inc.), in a construction that is similar to previous reports. [ 23 ] (See the Experimental Section). The dye uses a fl uoran chemistry, with the reversible ability to open (colored) and close (colorless) lactone rings in the presence of acids. [ 24,25 ] The microencapsules contain mixtures of leuco dye with a color developer and a low melting point solvent. At low temperature, the developer and leuco dye form a complex that favors the colored ring-open state. Upon heating, the solvent melts, and the developer and dye dissociate. The leuco dye then favors the colorless, ring-closed state. Such transformations occur in a reversible manner. [23][24][25][26] For the case explored here, at temperatures below ≈47 °C the dye is black; at higher temperatures, the mismatch between the index of refraction of the microencapsulated dye and the surrounding silicone matrix leads to strong optical scattering and a white appearance. The net effect, then, is a temperature activated, reversible switching between black and white. Figure 1 a,b presents confocal and scanning electron microscope images of this type of composite. At room temperature, the composite is black. As the temperature rises above 47 °C, the color changes to bright white, as illustrated by refl ectance measurements across the visible range (Figure 1 c). Mechanical stretching (e.g., deformation to strains of 30%, 60%, etc.) leads to minimal change in the color, as is also indicated in Figure 1 c.Patterns of stretchable conductors integrated directly with this type of thermochromic elastomer provide spatially controlled Joule heating for localized/selectable color switching. Here, we...

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