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...
Almost all aspects of cellular and multicellular activities depend on macromolecular interactions. [1] In this context, macromolecular displacement refers to the effect exerted by high in vivo concentrations of molecules in the cytoplasm. [2] Such crowding in living systems has major implications for all cellular activities and biochemical processes such as enzyme activity and protein stability. [1] Macromolecular solutions such as biopolymers, like polyethylene glycol (PEG) or proteins, have been used to replicate the crowded environment inside living cells. The advantage of well-defined molecules like PEG is that, unlike in vivo, they are easy to handle and chemically and physically well-defined to study protein interactions, enzymatic processes, [1] and cell membrane fusion. [3] PEG, for example, acts simply by volume exclusion, resulting in an osmotic force that fuses membranes in a dehydrated region; thus, the role of various lipids and fusion proteins has also been studied previously. [3] Recently, the mechanism of cellular uptake of PEG has been well described by Wang and his team. [3] The uptake of PEGylated nanoparticles occurs rapidly by endocytosis, while pure PEG does not form nanoparticles. [4] Also different mechanisms for cell entry have been described: Low molecular weight (MW) PEG up to 2000 Da (at various concentrations from 50 to 1000 μM) can enter the cell by passive diffusion; [4] however, with slower kinetics for higher MW PEG (2000 Da) than for lower MW PEG (750 Da). [4] On the contrary, the contribution of passive diffusion to the uptake of higher MW PEG is low and the uptake occurs mainly by endocytosis, suggesting aggregation of higher MW PEG. [4] Another study by Yang et al. [4] investigated the distribution, trafficking and exocytosis of PEG in vitro. After incubation of 3 Â 10 7 cells with 5 mM PEG of 5000 Da, a total amount of 62 μg PEG accumulated inside the cells. Specifically, internalized PEG is first transported to endosomes and from there to lysosomes. [5] After, it can either be transferred into the cytosol, endoplasmic reticulum, and mitochondria or leave the cells by exocytosis, [5] or is degraded by hydrolytic and proteolytic activities. [6] During this intracellular redistribution process, PEG has a very high residence time in cells, which can even exceed 96 h. [5] When the concentration of extracellular solutes increases by PEG addition, water is forced out of the cell by osmotic pressure which leads to a decreased cell volume with strong effects on the cells' physiological processes, cytotoxicity, and mechanics. [7,8] PEG is considered to be almost nontoxic in medical applications; [9] however, cytotoxicity of different MWs and concentrations of PEG has been reported in various crowding studies, [9][10][11][12] mainly due to hyperosmotic stress. [9] Metabolically activated low MW PEG (200-400 Da) has also been reported to have genotoxic effects on numerous cell lines. [13] While the mechanism and cytotoxicity of PEG internalization are well studied, and first studies hav...
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