Bioinspired mechanochromic devices (w/videos)

(Nanowerk Spotlight) Jellyfish can change from a transparent state to an opaque state when disturbed. Tactile stimulation can instantaneously evoke the contraction of radial muscles in the margin of a nectosac, resulting in a crumpled morphology with inward folds. Due to the wrinkles/folding that form, light will be scattered instead of travelling directly through the skin, resulting in a more opaque appearance.
Cephalopods are able to instantaneously change their skin color pattern for camouflage and communication. They are able to control this change through chromatophores – pigment sacs surrounded by radial muscles. When the radial muscle is relaxed, the chromatophore has a small exposure area that’s almost invisible to the naked eye. However, once the radial muscle contracts, the exposure area can be rapidly expanded into the millimeter scale and the color pattern can become visible.
In addition, the structural reflectors such as iridophores or leucophores beneath the chromatophore can reflect light from the pigment to enhance its visibility.
Inspired by marine life, a variety of these mechanochromic devices are created by researchers from University of Connecticut led by Prof. Luyi Sun, and his coworkers Songshan Zeng and Emily Huang as well as their collaborator Dr. Dianyun Zhang.
The team has reported their latest findings in Nature Communications ("Bio-inspired sensitive and reversible mechanochromisms via strain-dependent cracks and folds").
The series of devices designed by Sun's lab are able to undergo change from transparency to opaqueness, as well as a changes in color and pattern upon simply stretching and releasing the substrate.
In the first device, termed transparency change mechanochromism, a state from transparency to opaqueness is achieved, inspired by the change in appearance of jellyfish. It is composed of a thin, rigid transparent film bound to a soft, elastic substrate. Once the device is stretched, folds and cracks form in the rigid layer. When it’s released, the surface flattens out and returns to a smooth surface morphology.
The opacity of the stretched state can be attributed to strong trapping and scattering of light resulting from the strain-dependent cracks and folds.
Video 1: The strain-responsive behavior of transparency change mechanochromism.
The other three devices were inspired by the ability of cephalopods to undergo color change. These systems have similar structures to the transparency change devices – a bilayer structure composed of a rigid film atop an elastomer.
For the luminescent mechanochromism system, UV shielding materials are used for the rigid film and different fluorescent dyes were incorporated into the elastic material. When the device is released, the cracks of rigid UV shielding rigid film are closed, and thus the UV shielding layer can effectively block the UV radiation and the system is not luminescent.
When stretching the device, cracks would form in the rigid layer and this mechanism would allow the fluorescent dyes behind the cracks to be exposed to UV radiation and the fluorescence can be shown. These cracks act as ‘gates’ to adjust the exposure area of the fluorophore and the luminescence intensity.
The device also contains a reflector layer in order to enhance the luminescence of the system when stretched.
Video 2: The strain-responsive behavior of luminescent mechanochromism.
The color alteration mechanochromic device has a similar design to the luminescent device. However in this structure, the green fluorescent dye is incorporated into the rigid film layer, on the top of the rigid UV shielding layer, and the orange fluorescent dye is incorporated on the elastomer layer.
Thus, under the UV radiation, the device emit green fluorescence in released state and shows orange fluorescence when stretched due to the crack opening allowing the exposure of the orange dye.
This device is extremely sensitive to stretching, as it can achieve uniform color change at only 20% strain – a value much lower than in similar devices. After many cycles of stretching and releasing, the sample’s ability to undergo color change is not affected.
Video 3: The strain-responsive behavior of color alteration mechanochromism.
By adding a patterned reflector layer underneath the luminescent elastomer layer, the hidden pattern can be revealed when stretched. Similar to previous designs, when the device is stretched, the high density cracks that form allow the pattern to be exposed. Once released to its original length, the pattern is hidden by the UV shielding layer.
Video 4: The strain-responsive behavior of encryption mechanochromism.
All the devices have very good reversibility and the mechanical responsive optical properties can maintain upon thousands of stretching/releasing cycles. A variety of applications can be explored for these mechanochromism: the transparent change device can be used in smart windows – a small stretching strain allow the device to turn opaque. For the color changing devices, they can be used in fields such as encryption, strain sensing, or even toys.
Provided by Prof. Luyi Sun, University of Connecticut

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