Squid-inspired probe changes color in response to force

Taking a clue from squids that change color to blend into their surroundings, NCCR Bio-Inspired Materials researchers at the Adolphe Merkle Institute have designed pigments that make a material change its appearance in response to mechanically induced deformations.

The pigments could act as stress detectors in materials used for packaging as well as for structural and biomedical applications, thus helping to prevent damages and failures. Scientists could also use the pigments to study how different materials respond to stress, says NCCR principal investigator Jessica Clough, assistant professor and head of the Mechanoresponsive Materials research group at the Adolphe Merkle Institute, who led the study. The pigments, she adds, “could give us more insights into the mechanisms by which materials fail, so we can design better ones.”

A long-standing challenge in materials science has been making stress sensors that are easy to incorporate into different materials and that can rapidly change their color when a force is applied. Most current sensors turn on just as the material is about to break, so one of the goals of Clough’s team was to make sensors that are responsive to smaller forces.

To do so, the researchers took inspiration from squids — marine animals that can change their skin tone to match their surroundings. The squid’s skin is packed with color-changing cells, each of which contains a sac full of pigments. When the animal contracts its muscles, the sac expands, making the color more visible. Underneath these cells lies a layer of mirror-like cells that are capable of reflecting all the light that hits them. By scattering this light, they create iridescent colors.

Clough and her colleagues set out to develop probes that combine the features that squids use to change color. The team designed tiny spheres made of silica nanoparticles that assemble into mirror-like photonic structures. The spheres also contain force-sensitive molecules called mechanophores, which emit fluorescence when a mechanical force brings about a structural rearrangement.

The researchers embedded these spheres, dubbed “mechano-pigments”, into several polymers, including polyethylene, the most widespread plastic in the world, and polydimethylsiloxane, a rubbery material with a wide variety of uses — ranging from electronics to biomedicine. Just as color pigments give color to a paint, so the mechano-pigments make polymers change their colors in response to forces when embedded into them, Clough says. “You don’t have to do complicated chemistry, you can just mix the mechano-pigments in.”

Compressing the materials containing the mechano-pigments changed their appearance markedly. Mechanical strains as small as 1% altered the arrangement of the photonic structures within the mechano-pigments, scattering light in ways that changed the materials’ color from bright green to pale blue. Higher strains, ranging from 30 to 70%, caused the mechanophores to emit fluorescence.

The study, published in the journal Advanced Science, is the first of its kind to develop mechano-pigments that combine photonic structures with mechanophores. The work also pioneered the application of these mechano-pigments in a range of polymers — from relatively stiff ones such as polyethylene to the rubbery polydimethylsiloxane. If applied to softer materials, such as hydrogels, the mechano-pigments may help scientists to better understand and predict failure in all kinds of materials, from 3D printed plastics to artificial tissues.

Clough, who had moved to Switzerland to take up a Women in Science postdoctoral fellowship from the NCCR Bio-Inspired Materials, was awarded a PRIMA grant from the Swiss National Science Foundation in 2022. The grant allowed her to start her own research group at the Adolphe Merkle Institute, where she’s working towards developing new probes of mechanical deformation in polymers. “Many macroscopic phenomena, like a crack formation, start off with a little defect that gets bigger and bigger,” she says. “We’re interested in developing a way to study this process from its nanoscopic origins.”

Reference: Clough, J. M.; Kirchner, C.; Wilts, B. D.; Weder, C. Hierarchically structured deformation-sensing mechanochromic pigments, Adv. Sci., 2023, 10, 2206416. https://doi.org/10.1002/advs.202206416 (open access)