NC State 3D prints light-activated hydrogel actuators
Creating structures that contract when exposed to light and expand again when the light is removed
According to NC State, an international team of researchers has embedded gold nanorods in hydrogels that can be processed through 3D printing to create structures that contract when exposed to light and expand again when the light is removed. This reversible expansion and contraction can be repeated multiple times, allowing the 3D printed structures to serve as remotely controlled actuators.
“We knew that you could 3D print hydrogels that would contract when heated,” said Joe Tracy, co-corresponding author of a paper on the work and a professor of materials science and engineering at NC State. “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 are polymer networks that contain water, commonly found in items like contact lenses and the absorbent material in diapers. The researchers did not technically 3D print a hydrogel but rather printed a solution containing gold nanorods and all the necessary ingredients 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. “This turns the solution into a hydrogel, with the trapped gold nanorods distributed throughout the material.”
The 3D printed pre-hydrogel solution has very low viscosity and cannot be printed onto a regular substrate, as it would result in a puddle instead of a 3D structure. To address this issue, the researchers printed the solution into a translucent slurry of gelatin microparticles in water. The printer nozzle penetrates the gelatin slurry to print the solution into the desired shape. Since the gelatin is translucent, light can penetrate the matrix, converting the solution into a solid hydrogel. The entire structure is then placed in warm water, melting away the gelatin and leaving behind the 3D hydrogel structure.
When these hydrogel structures are exposed to light, the embedded gold nanorods convert the light into heat, causing the polymers in the hydrogel to contract, pushing water out and shrinking the structure. When the light is removed, the polymers cool down and reabsorb water, expanding the hydrogel structure back 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 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,” said Thiele. “And it allows for preprogramming distinct motion during light-triggered contraction and expansion of our photoresponsive material.”
The paper, titled ‘3D-Printed Hydrogels as Photothermal Actuators‘, is published in the open-access journal Polymers. The paper was co-authored by Jameson Hankwitz, a graduate student at NC State; Martin Geisler, Niclas Weigel, Nicolas Hauck, and Jonas Schubert of the Leibniz Institute of Polymer Research Dresden; and Andreas Fery of the Leibniz Institute of Polymer Research Dresden and Technische Universität Dresden.
This research was done with support from the National Science Foundation, under grant 1803785; the German Research Foundation (DFG) Research Training Schools 1865: Hydrogel-based microsystems and 2767, under project number 451785257; the Alexander von Humboldt Foundation; the Dresden Center for Intelligent Materials; and the European Union’s Horizon 2020 research and innovation program, under grant 852065.
