Chameleons can rapidly change the colour of their skin between a “cryptic” (or camouflage) state and an excited state (that is mainly seen during courtship or combat). The animals do this by actively tuning a lattice of guanine nanocrystals within dermal iridophore cells. These cells are in fact tuneable photonic crystals.

Photonic crystals are nanostructured materials in which the periodic change of the refractive index on the length scale of visible light produces a photonic bandgap through which certain light wavelengths can pass through while light in other ranges is reflected. This means that the colour reflected by the crystals can be tuned by changing the bandgap. In chameleons, this gap is the distance between nonclose-packed guanine nanocrystals and it can be varied by deforming the surrounding elastic matrix. This allows the animal to change its colour over the entire visible spectrum, but it is usually from yellow to green.

Synchronous shifts in the photonic band gaps

“Inspired by the structural colour-shift mechanisms in these animals, we constructed flexible inverse-opal hydrogel films etched with nanocrystals patterns assembled with engineered rat cardiomyocyte tissue,” explains lead author of this study Fanfan Fu of the School of Biological Science and Medical Engineering, Southeast University in Nanjing, China. “As the heart cells beat, they contract and elongate and this causes the substrate hydrogel to do the same. This movement appears as synchronous shifts in the photonic band gaps, causing light of different wavelengths to be reflected from the gel nanostructure.”

The researchers, led by Yuanjin Zhao, integrated this material into a microfluidics system to make a heart-on-a-chip device in which the colour changing properties allowed them to measure the beat frequency of heart cells – for example, after administering isoproterenol (a drug that lowers heart rate). The colour shifts on the chip corresponded with measurements performed on live organisms that had also been given the drug.

Drug-testing applications

“This chip could be used to test different types of cardiomyocyte drugs and also provides an ideal platform for studying the growth and differentiation of heart cells,” Fu tells “We can use the chip to study how induced pluripotent stem cells develop, for example, or how other stem cells differentiate into cardiomyocytes.”

And that was not all: Fu and colleagues also organized the cardiomyocytes on the hydrogel film surfaces in an anisotropic way, so as to better mimic the conditions in a real heart. To do this, they used silicon wafers containing micro-groove patterns that allowed the hydrogel film to self-assemble into a particular structure – which has the same shape as a butterfly.

Intelligent robot actuators

When the heart cells beat, the hydrogel expands and contracts and the thrust from this expansion and contraction changes the bending angles of the film. “The ‘bionic butterfly’ thus has a specific structural fingerprint at each thrust, which makes it appear as though it is flapping its wings as the colour shifts from the outer edge of the structure and spreads to the inside of the wings,” explains Fu. “We believe that this type of structure could be useful for making intelligent robot actuators in the future.”

The team, reporting its work in Science Robotics DOI: 10.1126/scirobotics.aar8580, says that it would now like to make such devices from its material. “We also plan to further optimize the biohybrid structural hydrogel so that that we can detect single cardiomyocytes on our heart-on-a-chip platform,” adds Fu.