Direct laser writing is regarded as a particularly promising method: A computer-controlled and highly focused laser beam acts as a pen and creates the desired structure in a photoresist. In this way, any three-dimensional shape down to a size of a few micrometers can be created. "For many applications, especially in biology and biomedicine, however, it would be desirable not only to produce rigid structures but also active systems that are still movable after the printing process, i.e. that can change their shape by an external signal," explains Professor Martin Bastmeyer from the Institute of Functional Interfaces at KIT. Together with other colleagues from KIT as well as the Universities of Karlsruhe and Heidelberg, a printing process has now been developed for such mobile structures.
The scientists used stimuli-responsive polymers as printing inks, whose properties can be modified by external signals. The chemical compound poly(N-isopropyl acrylamide) changes its shape considerably when the temperature is raised only slightly above room temperature. The 3D structures produced in this way are functional in an aqueous environment and are therefore ideal for applications in biology and biomedicine.
"We have refined the method to such an extent that we can also produce complex structures in which the moving parts do not all react in the same way as a result of external stimulation, but show different but precisely defined reactions," explains researcher Marc Hippler. This is made possible by grayscale lithography: In this process, the photoresist is not exposed at all points to the same intensity, but in graduated steps. This allows the desired material properties - and thus the strength of the movement at a certain temperature change - to be set very exactly. With computer simulations, the resulting movements can be precisely predicted and therefore allow a rational design of complex 3D structures.