Inspired
by the octopus’s ability to change color and shape at will, researchers
have created a hydrogel ‘skin’ that can emit light and sense pressure
even when stretched [C. Larson et al., Science 351 (2016) 1071].
Soft robotic systems based on electromechanochemically
responsive films and organic or polymeric light-emitting devices with
stretchable indium tin oxide, graphene, carbon nanotube or metal
nanoparticle or nanowire electrodes are currently being explored. But
now researchers from Cornell University and the Istituto Italiano di
Tecnologia in Italy have come up with a different approach.
The team have developed a new highly luminescent
stretchable skin based on a hyperelastic light-emitting capacitor (HLEC)
that is twice as stretchy as previously demonstrated display materials
and can withstand stains of over 600%. The material is composed of
layers of highly elastic ionic hydrogel electrodes and dielectric
silicone embedded with electroluminescent ZnS phosphor powders. The ZnS
particles are doped with elements that emit light of different colors
under a high electric field. Low and high concentrations of Cu dopant
produce green and blue, respectively, while Mn produces yellow light.
Used in combination, a mixture of dopants can also generate white light.
The stretchable material can also sense pressure—since
changes in electrode area and separation distance affect the
capacitance. For example, as the material is stretched the capacitance
increases.
‘‘We have used this material system to make a grid of
pixels and applied it to the skin of a soft robot to enable feedback
control and visual communication,’’ explains researcher Bryan Peele of
Cornell.
The team created a small ‘crawling’ robot from the
material, which can wriggle along by inflating and deflating small
chambers, while emitting light from the top surface. All of the
materials making up the stretchy external skin can be processed in the
liquid state, with each layer cast in a 3D printed mold and then cured
either in an oven (for the silicone) or under UV illumination (for the
hydrogel).
‘‘We have only shown planar applications of our
system,’’ says Peele. ‘‘But the same materials and technique could be
used to cast stretchable displays into a wide variety of 3D structures
such spheres or more organic shapes that conform to the human body.’’
One of the most intriguing potential applications is
stretchable electronics such as a cell phone that fits into the pocket
but can be stretched to the size of a large tablet when desired.
‘‘The display material is not very sensitive to
stretching, which may make it suitable for displaying information
without being affected by strain,’’ comments Zhenan Bao of Stanford
University.
"The work is an impressive advance in materials
science and mechanics," adds John A. Rogers of the University of
Illinois at Urbana-Champaign. ‘‘This type of technology could be
important not only as soft skins for robots, but also as
indicatorlighting on thin, skin-like electronic systems that are rapidly
emerging as next generation wearables,’’ he says.
This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.03.001
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