On July 23, 2024, Antarctic Bear learned that a research team at North Carolina State University used gold nanorods embedded in a hydrogel and processed through 3D printing technology to successfully create a structure that can shrink under the action of light. These structures automatically expand again after the light disappears. Due to their repeatable properties, these 3D printed structures can be used as remote control actuators.
Joe Tracy, a professor of materials science and engineering at North Carolina State University, said: "We know that you can use 3D printing to print hydrogels that shrink when heated. We also know that gold nanorods can be added to hydrogels to make them photostretchable, that is, they contract in a reversible way when exposed to light."
Preparation and application of photothermal responsive hydrogels
Hydrogels are polymer networks that contain water. From contact lenses to absorbent materials used in diapers, hydrogels are the direction of application. Technically, the researchers did not print the hydrogel directly with a 3D printer. Instead, they printed a solution that contained gold nanorods and all the ingredients needed to make a hydrogel.
Julian Thiele, head of the Department of Organic Chemistry at Otto von Gerrick University in Magdeburg, said: "When this printing solution is exposed to light, the polymers in the solution form cross-linked molecular structures. So the solution becomes a hydrogel, and the trapped gold nanorods are distributed throughout the material."
Since the viscosity of the pre-hydrogel solution from the 3D printer is very low, the solution cannot be printed onto the ordinary substrate, otherwise it will form a pool of water instead of a 3D structure.
To solve this problem, the researchers printed the solution into a translucent gelatin microparticle water paste. The printer nozzle can penetrate the gelatin paste and print the solution in the desired shape. Because the gelatin is translucent, light is able to penetrate the substrate and convert the solution into a solid hydrogel. Once finished, the entire model is placed in warm water to dissolve the gelatin, leaving a three-dimensional hydrogel structure.
When these hydrogel structures are exposed to light, built-in gold nanorods convert the light into heat. This causes the polymer in the hydrogel to contract, squeezing out water and shrinking the structure. However, once the light source is removed, the polymer cools and starts to absorb water again, returning the hydrogel structure to its original size.
The advantage of this technology is that it can use the photothermal effect of gold nanorods to achieve controlled deformation of hydrogel structures, providing a new way to make complex shapes and smart material printing that can respond to environmental changes.
Melanie Ghelardini, first author of the paper, said: "We have done a lot of work on hydrogels that shrink in response to heat. We have now shown that it is possible to do the same thing when the hydrogel is exposed to light, while also 3D printing the material. This means that applications that previously required direct heating can now be triggered remotely by lighting."
Thiele added: "3D printing of hydrogel structures replaces traditional casting and provides virtually unlimited freedom for design. And, it also allows for pre-programmed noticeable motion as our photostrictive material contracts and expands when triggered by light."
In general, this technology not only has a wide range of applications in the field of scientific research, but also may have a profound impact on everyday life in the fields of wearable devices, smart sensors and medical devices. Future research could further optimize the photothermal response properties of the material, expanding its potential for application in a wider range of fields.