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

Abstract

based on a reversible phase transition at the lower critical solution temperature (LCST) due to a globule-to-coil transition. [8] Below the LCST the polymer is in a hydrophilic, hydrated state, while above the LCST it is in a hydrophobic state. In this state, the interactions of the polymer with itself lead to a higher gain in free energy than would be achieved by solvation energy. The volume of pNIPAM materials in the hydrated, swollen state is much higher than in the hydrophobic state, because water is integrated into the material in the hydrated state. [9] As the actuation of pNIPAM hydrogel is based on its water-dependent shrinkage and swelling, the actuation efficiency and the forces generated by pNIPAM-based actuators critically depend on the in-and outflow of water through the pores of the material. Therefore, the actuation effect, including actuation dynamics as well as stroke forces, can be improved by introducing pores into the material. [10,11] Whereas in previous studies template-assisted methods have proven highly suitable for introducing pores into hydrogels, they lack local microstructural control, while being excellent for structuring macroscopic samples with an interconnected structure. [10] For generating highly efficient pNIPAM-based microactuators, a precisely defined design is required and thus methods are needed that allow for their highly precise 3D structuring at the micrometer scale.The technology of additive nano-and micromanufacturing opens doors to novel and remarkable microengineering possibilities and even enables printing of simple 3D volumetric structures at small scales. [12] Direct laser writing (DLW) involving two-photon polymerization of photoresists is one of the additive manufacturing techniques that can be used to fabricate intricate and miniaturized 3D structures, as they are required for the fabrication of microactuators. By utilizing twophoton absorption and a femtosecond pulsed laser, this fabrication method allows to print in sub-micrometer resolution, even with several materials and on different types of substrates. [13,14] Direct laser writing has also proven highly suitable for fabricating 4D materials, i.e., responsive materials that change their properties due to an external stimulus. [15] For example, the shape of such materials could be controlled at the micrometer scale by temperature and light. [16] 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...

Femtosecond laser direct writing of functional stimulus-responsive structures and applications

“…The response processes of most temperature-sensitive materials, including hydrogels, shape-memory materials, and LC polymers, are caused by changes of molecular structure, anisotropic properties, or different swelling behaviors. For thermo-responsive hydrogels, their response principles mainly rely on the change in wettability and solubility with temperature [204,205]. For example, Hippler et al manufactured functional hetero-microstructures by utilizing poly(N-isopropylacrylamide) (PNIPAM)-based hydrogels and two-photon polymerization strategy.…”

Two-photon polymerization-based 4D printing and its applications

“…Light-responsive hydrogels [112,113], thermo-responsive hydrogels [113][114][115][116], and solvent-responsive hydrogels [117][118][119] are also pivotal components in two-photon 4D printing. Light-responsive hydrogels respond to light, enabling remote and precise spatial and temporal control.…”

“…Thermo-responsive hydrogels offer ease of operation and tunable temperature range responses. An innovative approach by Spratte et al employed pNIPAM-based microactuator systems designed with precision via DLW technology [114]. Their study comprehensively explored the shrinkage and swelling properties of these systems, placing emphasis on actuator size, design variations, and DLW parameter influence on material actuation.…”

Recent Advances in Multi‐Photon 3D Laser Printing: Active Materials and Applications

“…Using the same system, Spratte et al were able to show an efficiency increase of the thermoresponsive actuation behavior in dependence on geometry as well as the adjustability of mechanical properties by fabrication parameters. [49] This approach, termed grey tone lithography, gave access to a range of different materials featuring distinct mechanical, thermal, and dynamic properties -utilizing only one ink system. In another example, Decroly et al fabricated structural elements for bending compression and twisting deformation, demonstrating complex geometries based on hydrogels featuring advanced kinematics.…”

“…were able to show an efficiency increase of the thermoresponsive actuation behavior in dependence on geometry as well as the adjustability of mechanical properties by fabrication parameters. [ 49 ] This approach, termed grey tone lithography, gave access to a range of different materials featuring distinct mechanical, thermal, and dynamic properties – utilizing only one ink system. In another example, Decroly et al.…”