Highly stretchable electroluminescent skin for optical signaling and tactile sensing

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.

Toward Perceptive Soft Robots: Progress and Challenges

“…This direct fabrication of fully integrated perceptive soft actuators would open enormous possibilities in the development of soft self‐sensing robots. Alternatively, Larson et al57 developed a highly stretchable electroluminescent skin for a soft crawling robot that can either sense external pressures or the degree of deformation of the pneumatic chambers, during locomotion (Figure 3e), showing that skin deformation can also be communicated through luminescence. Here, soft hyperelastic capacitive sensors were built with ionic‐hydrogel electrodes and an elastomeric dielectric doped with Zn–phosphor powders.…”

“…e) A soft crawling robot with tactile or deformation sensing capabilities. Reproduced with permission 57. Copyright 2016, AAAS.…”

“…The key toward stretchable sensing is to develop highly stretchable, conductive materials as electrodes. So far, conductive fabric,85, 86 nanocomposites,87 conductive polymer/hydrogel,57 bulking thin film,88 and conductive liquid89 have been deployed as stretchable electrodes. Capacitive sensors have good linearity, high sensitivity, large dynamic range, and rapid response.…”

“…Unfortunately, most good conductors (metals and metal alloys) have a very poor stretchability (only 3–5%). Currently, there are two approaches to develop stretchable conductors ( Figure 7 ): material approach (e.g., nanocomposite,110 liquid conductor,111 conductive polymer/hydrogel,57, 112 etc.) and structural approach (e.g., bulking/wrinkling structure,113 kirigami patterning,114 textile,86 etc.).…”

Carbon Nanofiber versus Graphene‐Based Stretchable Capacitive Touch Sensors for Artificial Electronic Skin

“…Aside from being flexible, it is equally challenging to ensure that the capacitive devices function under elongation exceeding 100%,17 display negligible hysteresis while retaining mechanical robustness against touch and mild friction, are low cost, and can be easily installed on curvilinear topography 14, 16, 18, 19. Since fabrication of flexible and strain‐sensitive soft electronics has mainly relied on elastomeric matrices,20, 21, 22 the most utilized approach for the construction of “stretchable capacitors” is to print, deposit, or encapsulate carbon nanotubes (CnTs),23 metal nanowires,24 liquid metals such as GaInSn or hydrogels25 in silicone elastomers26, 27, 28 (polydimethylsiloxane, PDMS) as electrodes. Using elastomeric nanocomposites either as the dielectric layer or as the conducting coating is rather rare, even though they have significant potential for hyperelastic capacitors for soft robots 29.…”