3D printed photoresponsive structures offer remotely controlled actuators

The international team’s findings are detailed in Polymers.

“We knew that you could 3D print hydrogels that would contract when heated,” said Joe Tracy, co-corresponding author and Professor of materials science and engineering at North Carolina State University. “And we knew that you could incorporate gold nanorods into hydrogels that would make them photoresponsive, meaning that they would contract in a reversible manner when exposed to light.

“We wanted to find a way to incorporate gold nanorods into hydrogels that would allow us to 3D print photoresponsive structures.”

Hydrogels - polymer networks that contain water – are present in everyday products including contact lenses and the absorbent material in nappies. Key to the team’s success was a printed solution containing gold nanorods and the items needed to create a hydrogel.

“And when this printed solution is exposed to light, the polymers in the solution form a cross-linked molecular structure,” said Julian Thiele, co-corresponding author of the paper and chair of organic chemistry at Otto von Guericke University Magdeburg, Germany. “This turns the solution into a hydrogel, with the trapped gold nanorods distributed throughout the material.”

The pre-hydrogel solution coming out of the 3D printer has a very low viscosity that cannot be printed onto a regular substrate.

To solve this problem, the researchers printed the solution into a translucent slurry of gelatin microparticles in water. The printer nozzle can penetrate the gelatin slurry and print the solution into the desired shape. Because the gelatin is translucent, light can penetrate the matrix, converting the solution into a solid hydrogel. Once this is done, the entire assembly is placed in warm water, melting away the gelatin and leaving behind the 3D hydrogel structure.

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When these hydrogel structures are exposed to light, the embedded gold nanorods convert that light into heat that causes the polymers in the hydrogel to contract, pushing water out of the hydrogel and shrinking the structure. When the light is removed, the polymers cool down and absorb water again, which expands the hydrogel structure to its original dimensions.

“A lot of work has been done on hydrogels that contract when exposed to heat,” said Melanie Ghelardini, first author of the paper and a former PhD student at NC State. “We’ve now demonstrated that you can do the same thing when the hydrogel is exposed to light, while also having the capability to 3D print this material. That means applications that previously required direct application of heat could now be triggered remotely with illumination.”

“Instead of applying conventional mould casting, 3D printing of hydrogel structures offers nearly unlimited freedom in design,” Thiele said in a statement. “And it allows for pre-programming distinct motion during light-triggered contraction and expansion of our photoresponsive material.”