Soft and stretchable electronics are promising for a variety of applications such as wearable electronics, human-machine interfaces, and soft robotics. These devices, which are often encased in elastomeric materials, maintain or adjust their functionality during deformation, but can fail catastrophically if extended too far. Here, we report new functional composites in which stretchable electronic properties are coupled to molecular mechanochromic function, enabling at-a-glance visual cues that inform user control. These properties are realized by covalently incorporating a spiropyran mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that a strain threshold has been reached. The resulting colorimetric elastomers can be molded and patterned so that, for example, the word "STOP" appears when a critical strain is reached, indicating to the user that further strain risks device failure. We also show that the strain at color onset can be controlled by layering silicones with different moduli into a composite. As a demonstration, we show how color onset can be tailored to indicate a when a specified frequency of a stretchable liquid metal antenna has been reached. The multiscale combination of mechanochromism and soft electronics offers a new avenue to empower user control of strain-dependent properties for future stretchable devices.
Liquid metals adhere to most surfaces
despite their high surface
tension due to the presence of a native gallium oxide layer. The ability
to change the shape of functional fluids within a three-dimensional
(3D) printed part with respect to time is a type of four-dimensional
printing, yet surface adhesion limits the ability to pump liquid metals
in and out of cavities and channels without leaving residue. Rough
surfaces prevent adhesion, but most methods to roughen surfaces are
difficult or impossible to apply on the interior of parts. Here, we
show that silica particles suspended in an appropriate solvent can
be injected inside cavities to coat the walls. This technique creates
a transparent, nanoscopically rough (10–100 nm scale) coating
that prevents adhesion of liquid metals on various 3D printed plastics
and commercial polymers. Liquid metals roll and even bounce off treated
surfaces (the latter occurs even when dropped from heights as high
as 70 cm). Moreover, the coating can be removed locally by laser ablation
to create selective wetting regions for metal patterning on the exterior
of plastics. To demonstrate the utility of the coating, liquid metals
were dynamically actuated inside a 3D printed channel or chamber without
pinning the oxide, thereby demonstrating electrical circuits that
can be reconfigured repeatably.
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