Increasing the Efficiency of Thermoresponsive Actuation at the Microscale by Direct Laser Writing of Pnipam

Thermoresponsive hydrogels such as poly(N‐isopropylacrylamide) (pNIPAM) are highly interesting materials for generating soft actuator systems. Whereas the material has so far mostly been used in macroscopic systems, here it is demonstrated that pNIPAM is an excellent material for generating actuator systems at the micrometer scale. Two‐photon direct laser writing is used to precisely structure thermoresponsive pNIPAM hydrogels at the micrometer scale based on a photosensitive resist. This study systematically shows that the surface‐to‐volume ratio of the microactuators is decisive to their actuation efficiency. The phase transition of the pNIPAM is also demonstrated by nanoindentation experiments. It is observed that the mechanical properties of the material can easily be adjusted by the writing process. Finally, it is found that not only the total size and surface structure of the microactuator play an important role, but also the crosslinking of the polymer itself. The results demonstrate for the first time a systematic study of pNIPAM‐based microactuators, which can easily be extended to systems of microactuators that act cooperatively, e.g., in microvalves.