A small piece of material sits nearly motionless in the dark. However, shine light on it and within seconds it begins to swell at an extraordinary rate. Aim the light differently, and the same material can curl itself into a left-handed helix, reverse into a right-handed one, twist into more complicated shapes, or mimic the vines of a climbing plant.
Then turn the lights off, and the instructions disappear.
Researchers at the University of Science and Technology of China have developed a new photoresponsive hydrogel whose 3D shape can effectively expand, change, or even be rewritten into an entirely different configuration depending on where light strikes it.
In experiments reported in Nature Communications, the material expanded more than 80,000% within 40 seconds and could be repeatedly programmed into helices, twists, and hybrid structures.
Researchers also used its light-controlled motion to lift objects and drive a miniature rotary system, pointing toward possible applications in smart materials and soft robotics.
“Reconfiguring handedness in synthetic systems with spatiotemporal control and reversible behavior remains a fundamental challenge in bioinspired robotics and materials science,” researchers write. “Here, we demonstrate homogeneous photoactive hydrogels capable of ultrafast, programmable handed shape-morphing, including helices and twists, through spatially controlled light illumination.”
The ability to program the material after it has already been formed makes the breakthrough especially unusual.
Many shape-changing materials are manufactured with their future behavior already built into them. Different layers, directional fibers, compositional gradients, and pre-patterned structures determine how they will eventually bend, curl, or twist. Once fabricated, those instructions can be difficult, if not impossible, to change.
The new hydrogel begins as a structurally uniform material, with the asymmetry required for complex motion instead created temporarily by the pattern of incoming light. The key to making that possible is an unusually powerful swelling response.
The researchers incorporated a light-sensitive molecule known as spiropyran into a polymer network. When illuminated, the molecule experiences a chemical change that alters the electrical interactions between portions of the polymer. Attractions that normally help keep the network compact weaken or become repulsive, allowing the chains to spread apart and absorb large amounts of water.
The resulting expansion is dramatic. The researchers measured more than 80,000% volume growth after only 40 seconds of irradiation, a combination of speed and magnitude they say exceeds previously reported photoresponsive hydrogels.
Yet, simply making the hydrogel swell was only part of the experiment.
By controlling where the light hits, the team could make some portions of the material expand faster than others. Illuminating one side of a narrow strip creates a swelling gradient through its thickness. The resulting strain mismatch causes the strip to bend first, then curl into a helix.
Illuminating both sides produces a different internal swelling distribution. Rather than curling in the same manner, the strip buckles and rotates along its length, creating a twisted structure. Those shapes are not locked in place. Once the light is removed, the hydrogel relaxes and can later be programmed again.
The direction of the illumination can even determine the material’s “handedness.” Light arriving from one angle can produce a left-handed helix, while lighting the same strip from the opposite direction produces a right-handed one after the material has relaxed in darkness. Researchers repeatedly switched samples back and forth and found that the material retained its reconfigurability after more than 20 consecutive cycles.
More complicated lighting designs produced even stranger results.
By illuminating separate regions of a longer strip independently, researchers created structures containing multiple forms at once. One section could form a helix while another became a twist. Other patterns produced neighboring helices with opposite handedness or helix-twist-helix combinations, creating shapes reminiscent of the spiraling tendrils plants use to wrap around supports.
That ability to rewrite a material’s geometry also translated into mechanical output.
In one demonstration, a 6 mg strip curled into a spring capable of lifting a 50 mg payload or more than eight times its own weight. Researchers also created a light-controlled underwater switch in which the hydrogel moved two conductors into contact, as well as a tendril-like actuator that encircled another object.
The most elaborate demonstration turned the hydrogel’s twisting motion into locomotion.
Researchers connected a floating propeller to a hydrogel strip that served as a rotating shaft, with a paddle attached below the water. When light caused the hydrogel to twist, the paddle rotated, and the propeller began both spinning and traveling around a circular path. Programming the hydrogel’s handedness allowed the system to produce clockwise or counterclockwise motion.
Potentially, the hydrogel breakthrough could lead to soft robots that change shape or movement depending on the task, light-controlled grippers that wrap around delicate objects, or adaptive components that reconfigure themselves without conventional motors and mechanical parts.
Similar hydrogel materials might eventually serve as tiny switches, valves, pumps, or rotating actuators in conditions where running wires or fitting traditional machinery is difficult. Because the same configuration can be reprogrammed rather than permanently locked into one behavior, it could also open the door to devices that perform several different mechanical functions at different times.
However, for now, the experiments remain laboratory demonstrations. Researchers say the material will need faster response times to compete with some other classes of soft actuators.
Further research might improve the efficiency of its molecular photoswitches and redesign the polymer network so water can move through it more quickly.
Still, the wider concept suggests a different way of thinking about programmable matter. Instead of manufacturing a material with one predetermined set of movements, its instructions could arrive afterward as a pattern of light, then disappear and be replaced whenever a different behavior is needed.
“The core innovation of this work lies in the achievement of precise, reversible regulation of structural handedness in homogeneous hydrogels purely via external optical modulation,” researchers write. “The design principle presented here—reversibly encoding spatiotemporally programmable handedness within a homogeneous network through simple light patterns—establishes a method for creating intelligent, adaptive materials with embedded logic and function.”
The study, “Reconfiguring handed shape-morphing and actuation in hydrogels via light-encoded rapid expansion,” appeared in Nature Communications.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com