Researchers have harnessed beating heart cells and shimmering hydrogels to make a material that changes color as it moves. The biohybrid material could be used to make wearable electronics, soft robots that communicate using light, or for sensing materials and color-changing displays. It could also find uses in biomedical engineering. 

The hydrogel is an example of a structural color material, which are common in nature. The bright, iridescent hues of peacock feathers and some butterfly wings, for example, are a result of microscopic structures that interfere and interact with visible light. Researchers have been trying to mimic this process by making nanostructured surfaces that could be used for displays, anti-counterfeiting labels, and wearable electronics.

However, current artificial structural color materials all require external stimuli such as mechanical actuation or a chemical change to function. “This leads to complex systems and limits their further applications,” says Yuanjin Zhao, a professor of biological science and medical engineering at Southeast University in Nanjing, China.

Zhao and his colleagues wanted to make a self-regulating structural color material. They were in particular inspired by the structural color-shifting ability of chameleons. These lizards change color by stretching and shrinking skin cells, which tunes the spacing between light-reflecting guanine nanocrystals in the cells.

To make their color-changing material, Zhao and his colleagues first deposited spherical silica nanoparticles on a glass slide in a regular, closely packed hexagonal arrangement much like that found in precious opal. The particles self-assembled into a closely packed, ordered array with interconnected voids. The researchers soaked the array with a methacrylated gelatin solution, which permeated the voids, and then polymerized the solution to make a hydrogel. They etched away the silicon spheres, leaving behind a porous hydrogel film with an inverse opal-like structure that reflects certain wavelengths of light and displays vivid colors.

The team then cultured a layer of living rat heart cells on top of the film. It was observed that when the cells elongate and contract, the hydrogel’s nanostructure changes, shifting its photonic bandgap, and the film cycles through different iridescent colors. When patterned in a butterfly shape, the hydrogel film appears to flap its wings like a three-dimensional butterfly, the researchers demonstrate in a recent issue of Science Robotics.

As a practical demonstration, the researchers integrated the hydrogel into a microfluidic system to make a heart-on-a-chip device. Organ-on-a-chip systems are made to reproduce key features of tissues and organs in order to understand diseases and develop drugs. The hydrogel normally changed from red to green with the beating heart cells. But when the researchers added isoproterenol, a drug that increases heart rate, to the device, the heart cells beat more frequently per minute and the gel’s color changed to blue. The frequency went up with higher concentrations of the drug and the gel’s color shifted, becoming increasingly more blue. This could be a useful tool to screen new drugs.

“Biohybrid structural color hydrogels and their use in self-reporting heart-on-a-chip technology will play a profound role in the field of biomedical engineering,” Zhao says.

While materials that change color under stimuli and actuators based on muscle cells have been developed before, this work presents a very impressive integration of color-changing materials actuated by heart cells, says Xuanhe Zhao, a professor of mechanical engineering at Massachusetts Institute of Technology, who was not involved in the work. “This is a very interesting and nice work,’” he says. “The results are vividly beautiful.”

Read the article in Science Robotics.