Much like sunflowers follow the sun’s movement across the sky, small pillars made of a light-sensitive polymer turn toward a light source, continually adjusting their position so that they directly face the source. This responsive materials system could enhance the efficiency of solar cells and solar thermal devices by helping them better harvest sunlight. It could also find novel uses in soft robots, smart windows, solar sails for spaceships, self-regulating optical devices, and guided surgery.

Materials that respond to triggers such as light, heat or chemicals are not new. These responses include shrinking or expanding, shape changes, and even motion. But making materials that respond by moving in specific directions has proven difficult. Such directional movement, called tropism, is found in many living things. Sunflowers, for instance, follow light, while plankton can move toward food or away from harmful chemicals.

Ximin He, a professor of materials science and engineering at the University of California, Los Angeles and her colleagues have developed phototropic materials that precisely follow the direction of incident light. “The key is to know where and when to stop,” she says. “How can we make our manmade material to have this intelligence?”

He and her colleagues started with a heat-responsive hydrogel such as poly(N-isopropylacrylamide) or poly(2-dimethylamino) ethyl methacrylate. These polymers shrink when heated, and expand when cooled. The researchers embedded the polymers with a light-absorbing material such as carbon black or gold nanoparticles. Then they made an array of pillars from the materials. The pillars were anywhere from micrometers to centimeters in width.

When light from a laser or a broader white-light source shines on a spot on one of these pillars from any arbitrary angle, the surface of the material gets locally heated because of the light absorbers, making the polymer contract, which causes the tip of the pillar to bend toward the light. The pillar continues bending toward the light, like a flexible straw tip, until it faces the light. If it bends too much, the bent tip blocks the light from shining on the original illuminated part, which cools, expands, and reverses the bending. “You don’t need any intervention, the pillar will find the right angle,” He says.

The pillars can continuously follow a light beam in a wide range of directions. To demonstrate their use, the research team used an array of these pillars on a solar vapor generation device that uses the heat from sunlight to vaporize water in order to purify or desalinate it. With light falling on the device at an angle, the sun-tracking pillars enhanced solar energy harvesting by up to 400%. 

Video showing a 1-mm diameter cylindrical pillar of poly(N-isopropylacrylamide) with homogeneously distributed gold nanoparticles that instantly bends toward an incident light (532 nm laser, 300 mW, 1 mm beam diameter), tracking the light at different angles and recovering when the light is removed. Credit: Ximin He   

By using other materials, the approach could be expanded for other uses. “We established a basic cyclic universal principle that people can use to make materials that track various energy sources. It doesn’t have to be light, it can be heat, chemicals, or acoustic signals,” says He.

“This work is significant since it established a universal principle for creating tropistic movements with nearly any reversibly stimuli-responsive soft material,” says Mingming Ma, a professor of chemistry at the University of Science and Technology of China. The major advance is the “built-in negative feedback loop” that uses the self-shading effect to control the movement of the pillars. “In the future, similar systems could be adopted to larger solar power harvesting systems,” Ma says.

Read the abstract in Nature Nanotechnology.