Inspired by chameleon skin, researchers at Emory University have developed a photonic crystal material that changes color in response to thermal stimuli and sunlight. Unlike conventional designs for “smart skins” that change color on demand but require large deformations, the new material maintains its size throughout the process. As reported in ACS Nano, the key is a strain-absorbing design composed of two different hydrogels.

Color-changing photonic crystal materials have intriguing potential in chemical sensing, camouflage, and anti-counterfeiting applications. They are made by embedding periodic arrays of photonic crystals in a hydrogel or flexible polymer matrix. The photonic crystals generate color through optical interference effects. The wavelength is a function of crystal lattice spacing, so as the skin expands or contracts in response to thermal and optical input, the color shifts. But the size change required to generate an observable chromatic shift is problematic—a spectral change of 100 nm requires a linear expansion of at least 20%. Such a large change can cause structural instabilities or buckling.

To address this challenge, a research team led by Khalid Salaita and Yixiao Dong, a graduate student working with Salaita, turned to nature. “We studied [a] video of chameleon skin cells and found that the colored photonic crystal cells were surrounded by non-colored cells,” Salaita says. “These non-colored cells acted like a spring; as the photonic crystals swelled or contracted, these non-colored cells filled in the void and helped maintain a constant size of the skin.”

Guided by this insight as well as computational models, the team devised a strategy for creating a color-changing, strain-absorbing smart skin (SASS): patterning a temperature-responsive photonic crystal hydrogel onto a nonresponsive base layer that absorbs the expansions and contractions without deforming.

The researchers then fabricated the responsive photonic crystal hydrogel by dispersing silica-coated iron oxide nanoparticles in the thermally-sensitive hydrogel poly(N-isopropylacrylamide) (pNIPAM). The particles, 180 nm in diameter, were aligned into periodic arrays by an external magnetic field and locked in place with crosslinking. The base layer was a film of tetra-polyethylene glycol (PEG), a nonresponsive hydrogel with properties similar to soft tissue, that was doped with a nanoclay to make it more mechanically robust.

To create SASS, the team patterned 4 × 4 mm² segments of the responsive photonic crystal hydrogel into arrays on the base layer. Measurements of the reflection spectra as a function of temperature agreed well with those of conventional responsive hydrogels. Mechanical characterizations revealed SASS to be stretchy and significantly stronger than conventional pNIPAM hydrogels. Under tensile testing at a strain of 1.5, SASS ruptured at a force more than an order of magnitude larger than conventional hydrogels.

To examine how SASS deformed in response to thermal input, the researchers restricted samples to expand or contract in only one plane. When they introduced a thermal change from 20°C to 40°C, the SASS samples stayed the same size. In comparison, conventional samples contracted by around 23% in length.

High-resolution in situ optical microcopy of SASS samples as they underwent heating and cooling cycles showed that the photonic crystal chains were displaced in response to temperature changes and reverted to their initial positions on completion of a cycle.

The team then explored the chromatic shifts in more detail. The new material responded to optical input in addition to thermal input, as illumination by a white LED light source stimulated a rapid, reversible color shift similar to that caused by thermal heating. When a 532 nm laser was focused onto a sample so that it irradiated a spot 2 mm in diameter, the spot noticeably changed color while the rest of the sample was unchanged. According to the researchers, this demonstrates that chromatic shifts in SASS can be spatially controlled and that dynamic color patterning may be possible.

The team also demonstrated that that the material can be spectrally tuned by embedding nanoparticles of different sizes, and that SASS responds to sunlight. SASS samples placed in sunlight underwent an observable, reversible chromatic shift in about 10 minutes. In one case, the researchers created a yellow, leaf-shaped SASS sample. Then they tailored its response so that when stimulated by sunlight, the leaf appeared green and matched the real leaves in its surrounding.

The most challenging aspect of this work was matching the right polymer properties with the necessary mechanical compliance, according to Salaita. “[I]t was an evolutionary screening process through different polymers, initiators, and crosslinker concentrations,” he says. Though challenging, the end result suggests that this research offers a useful framework for smart skin development.

“[This work] solves the long-standing problem of mechanical buckling and instability in conventional responsive photonic crystal hydrogels,” according to Ximin He, an expert on responsive polymers and micro/nano-structure fabrication at the University of California, Los Angeles. “SASS demonstrates a generalizable concept of how to create responsive photonic crystals that maintain constant size during chromatic shifting,” she says, also noting that the material “has broad potential applications ranging from camouflage and communications to anti-counterfeiting and biosensing.”  

Salaita and his team are now exploring the creation of SASSs that change color permanently. “While having a fully reversible color change is advantageous since this provides real-time information, we are [also] interested in building SASS films that produce a non-reversible color change. This is advantageous in sensing applications where you need to know if the input was present at some point,” Salaita says. The research team is also studying the minimum segment size necessary to preserve photonic crystal coloration and ways to tune the mechanical properties of the films to achieve desired levels of rigidity.

Read the abstract in ACS Nano.