As the sun moves across the
sky, sunflowers continually orient themselves to soak up the most light (SN: 8/4/16). Now a type of human-made material can do that, too.
This is the first artificial material capable of phototropism, researchers report November 4 in Nature Nanotechnology. Stemlike
cylinders of the material, dubbed SunBOTs, can maneuver to capture about 90
percent of available sunlight, even when the sun comes in at an oblique angle, materials
scientist Ximin He of UCLA and her colleagues found. The technology could
someday be used to optimize solar panels, desalinate water or move robots, the
Other scientists have made artificial
substances that can bend toward light, but those materials stop arbitrarily.
SunBOTs can self-regulate, moving into the optimal position needed to absorb
the sun’s rays, then making small adjustments to stay there as the sun shifts.
That ability comes from a SunBOTs’
configuration: a stemlike polymer about 1 millimeter in diameter embedded with
a nanomaterial that responds to light. The nanomaterial absorbs light and
converts it into heat; the polymer shrinks in response to increased
When He and colleagues
trained a beam of light on one of these artificial stems, the illuminated side
heated up and contracted. That caused its top to bend toward the light. The
newly shaded underside of the stem then cooled, stopping the SunBOT’s movement
in a position best oriented to soak up the light. The process repeated as the angle
of the light beam changed.
To build their initial
SunBOTs, the researchers used gold nanoparticles and a hydrogel. But tests with
other materials — such
as carbon black nanoparticles and liquid crystalline polymers — revealed that the components could be mixed and
“If we have this big
repertoire of materials working with the same principle … scientists can use it
in different environments for different applications,” says Seung-Wuk Lee, a
bioengineer at the University of California, Berkeley who was not involved in
the study. For instance, hydrogel SunBOTs work in water, He’s team found.
SunBOTs can be lined up in
rows to cover an entire surface, creating a “mini sunflower forest,” she says. Coating
surfaces with this material could solve one of the biggest problems in solar
energy: As the angle of direct sunlight changes as the sun moves overhead,
conventional materials can’t keep up.
Materials that stay in one
position — like
solar cells on a solar panel —
capture about 24 percent of available energy from the sun. Simply by moving to
face the sun, He says, an array of SunBOTs can harvest about 90 percent of
energy from sunlight.
By creating a material that
can follow sunlight, the researchers were able to keep near-maximum solar
absorption as the sun moved overhead, Lee says. “That is a major thing that