New method enables 3D printing of light-sensitive hydrogel structures

An international research team has embedded gold nanorods in hydrogels that can be 3D printed into structures that contract when exposed to light and expand again when the light is removed. Since this expansion and contraction can be performed repeatedly, the 3D-printed structures can serve as remote-controlled actuators.

Hydrogels are polymer networks that contain water and are found in products such as contact lenses and diapers. Technically, however, the researchers did not print a hydrogel directly, but a solution containing gold nanorods and all the necessary ingredients to produce a hydrogel.

“We knew that you could 3D print hydrogels that would contract when heated,” says Joe Tracy, co-corresponding author of a paper on the work and a 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.”

As the pre-hydrogel solution from the 3D printer has a very low viscosity, it cannot be printed on a normal substrate as it would otherwise run. To solve this problem, they printed the solution in a transparent mixture of gelatin microparticles in water. The printer nozzle can penetrate the gelatine mixture and form the solution into the desired shape. As the gelatine is translucent, light can penetrate the matrix and transform the solution into a solid hydrogel. The whole thing is then immersed in warm water, which melts the gelatine and releases the 3D hydrogel structure.

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

When these hydrogel structures are exposed to light, the embedded gold nanorods convert the light into heat. This causes the polymers in the hydrogel to contract, which pushes the water out of the hydrogel and causes the structure to shrink. When the light is removed, the polymers cool down and begin to absorb water again, causing the hydrogel to expand to its original dimensions.

“A lot of work has been done on hydrogels that contract when exposed to heat,” says Melanie Ghelardini, first author of the paper and a former Ph.D. 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 mold casting, 3D printing of hydrogel structures offers nearly unlimited freedom in design,” says Thiele. “And it allows for preprogramming distinct motion during light-triggered contraction and expansion of our photoresponsive material.”

The research results were published in the open access journal Polymers and co-authored by a team of scientists from the Leibniz Institutes of Polymer Research Dresden and the Technical University of Dresden.