A photonic actuator with separate mechanisms for color change and motion could enable smarter soft robots and sensors.

Soft robots, artificial skins, and sensors increasingly rely on smart materials known as photonic actuators, which can change both color and shape in response to external stimuli such as humidity, light, or heat. While many structurally colored actuators rely on photonic crystals, researchers at the Chinese University of Hong Kong, Shenzhen, have developed a nature-inspired photonic actuator that separates color change from mechanical motion—two features that are typically closely linked in conventional designs.

“What makes our system different is the way it produces color,” says Yuhua Jin, an assistant professor at the university’s Guangdong Basic Research Center of Excellence for Aggregate Science.

Unlike most structurally colored actuators based on photonic crystals, the new device displays vivid structural color through a different mechanism. Structural colors—seen in the iridescent feathers of peacocks or the metallic sheen of beetles—arise from a material’s physical structure interacting with light, rather than from chemical pigments.

“Our system can switch between colorless and vivid structural colors depending on how the material is illuminated,” Jin explains. “This gives the device more dynamic and controllable optical behavior.”

Made from humidity-sensitive polyvinyl alcohol (PVA), the team’s butterfly-shaped device flaps its wings when touched by a moist finger. Contact on the underside causes the wings to bend upward, while touching the top surface induces downward bending.

As the wings move, their color also shifts—from purple to pink to green—as the angle of illumination from a white LED changes from 0 to 90 degrees.

A two-faced design

The material’s so-called “Janus” structure underpins both its optical properties and its humidity-driven actuation. Named after the two-faced Roman god Janus, such materials are engineered with two distinct sides that exhibit different physical or chemical characteristics.

One side of the actuator is flat, smooth, and translucent, appearing essentially colorless. The other side features ordered arrays of microscopic dome-like structures that generate structural color.

“The microdomes act like tiny, light-guiding structures,” explains Shuzhen Cui, a postdoctoral researcher in Jin’s group. “When white light enters them, it undergoes multiple internal reflections. As the light waves travel along different paths and recombine, they interfere with one another, producing visible structural color.”

Decoupling color from motion

The actuation mechanism, by contrast, arises from differences in how each side interacts with water. The smooth surface is more hydrophilic than the textured one, meaning it absorbs moisture more readily.

“When the two sides absorb different amounts of water, one side swells more than the other, causing the film to bend or flip,” Cui says. “This mismatch in swelling drives the motion.”

Because the structural features responsible for color rely on light interference and total internal reflection within the microdomes—and are not significantly affected by humidity—the material’s optical response remains independent of its mechanical deformation. This separation allows color and movement to be controlled separately, an uncommon feature in photonic actuators.

Jin says the new design could also simplify manufacturing. Traditional systems often rely on complex assemblies of nanoparticles or highly ordered nanostructures to produce color, making them difficult and costly to scale.

“We think this kind of material is promising because it can convert invisible environmental changes into signals that people can directly see, while also producing motion,” Jin says.

Such capabilities could be useful in lightweight devices designed to respond to moisture in confined or hard-to-reach environments, including environmental monitoring systems or small-scale soft robotic grippers.

“In sensing, it could be applied in wearable devices that visually indicate humidity-related information, such as breathing patterns, through direct color and shape changes, without requiring complex electronics,” Cui adds.

The team’s next goal is to develop materials that respond to multiple stimuli simultaneously, such as humidity combined with light or electric fields.

“At the same time, we want to improve the material platform itself by developing more robust humidity-responsive polymers with greater durability and long-term stability,” Jin says. Such advances could improve performance in complex real-world environments, facilitate integration into functional systems, and enable more precise control over actuation.

Reference: Shuzhen Cui, Fanxu Meng, and Yuhua Jin. Moisture-Driven Biomimetic Photonic Actuators Exhibiting Angle-Dependent Janus Structural Colors. Advanced Optical Materials (2026). DOI: 10.1002/adom.202503398

Featured Image: adapted from 10.1002/adom.202503398