A team of researchers at Karlsruhe Institute of Technology (KIT) and Heidelberg University have recently introduced functional 3-D hetero-microstructures based on Poly (N-isopropylacrylamide) (pNIPAM) a polymer that responds to changes in temperature close to its lower critical solution temperature.
Stimuli-responsive microstructures are of key importance for the creation of adaptable systems, which can have interesting applications in soft robotics and biosciences. For practical application, however, materials need to be compatible with aqueous environments while also enabling the manufacturing of 3-D structures, for instance, using 3-D printing.
“3-D printing by direct laser writing is a powerful technique enabling the manufacturing of almost all arbitrary stable structures in the micrometer range,” Marc Hippler, one of the researchers who carried out the study, told TechXplore. “However, for many applications, especially in the biomedical field, it is desirable to change the properties of the resulting microstructure on demand, as this enables the step from passive to active systems. We wanted to present a powerful and versatile technique to create such structures.”
In order to achieve complex actuation patterns, researchers need to use materials that react differently to external stimuli, such as temperature and light. Hippler and his colleagues thus developed new 3-D hetero-microstructures based on N-isopropylacrylamide, a temperature-responsive monomer that is commercially available.
“One important goal of our study was to achieve strong responses with a ‘mild’ stimulus,” Hippler said. “By increasing the temperature only slightly above room temperature we stay in a physiological range, which makes the system interesting for biological applications. One could, for example, think about single cells in 3-D scaffolds that are mechanically stimulated by their environment. We also demonstrated that this technique could be useful for other fields, such as microfluidics or soft robotics.”
Hippler and his colleagues demonstrated that by changing the local exposure dose in 3-D laser lithography, the material parameters could be altered on demand. They then explored this possibility further to create 3-D architectures with large amplitude and complex responses.
Using their method, the researchers successfully created active structures that exhibit a large-amplitude response to changes in temperature. In addition, they showed that the response of these structures can be activated both globally, by changing the water temperature, and locally, by illuminating the desired microstructure with a laser focus.
“We demonstrated a very versatile and powerful technique that can be employed and used by other people,” Hippler said. “I think three of the main aspects of our study are the creation of materials with largely different properties out of a single photoresist, the strong actuation due to a mild stimulus and the opportunity to use light to trigger the response. Due to this versatility, we didn’t focus on one particular application, but highlighted different possibilities.”
In the future, these findings could inform the development of materials with applications in a variety of fields, including microfluidics, soft robotics and biosciences. Hippler will now continue working on this system, specifically focusing on biological experiments.
“Additionally, we will be investigating other stimuli-responsive material systems with interesting properties that could be used for direct laser writing,” he said.
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