Stimuli-Responsive Gels

Although the technological and scientific importance of functional polymers have been well established over the last few decades, the most recent focus that has attracted much attention concerns stimuli-responsive polymer gels. These materials are of particular interest due to their abilities to respond to internal and/or external chemo-physical stimuli; such responses are often large and macroscopic. Aside from the scientific challenges of designing stimuli-responsive polymer gels, the main technological interests concern numerous applications, ranging from catalysis in microsystem technology and chemo-mechanical actuators to sensors. Since the phase transition phenomenon of hydrogels is theoretically well understood, advanced materials based on predictions can be prepared. Since the volume phase transition of hydrogels is a diffusion-limited process, the size of the synthesized hydrogels is an important factor. Consistent downscaling of the gel size will result in fast smart gels with sufficient response times. To apply smart gels in microsystems and sensors, new preparation techniques for hydrogels have to be developed. For upcoming nanotechnology, nano-sized gels as actuating materials would be of great interest. Finally, new design concepts for tough polymer gels are of interest for overcoming the mechanical shortcomings of conventional gels. This Special Issue includes 17 papers covering a wide range of subjects including thermoand pH-responsive hydrogels, functionalized materials, supramolecular stimuli-responsive structures, composite hydrogels, sensors, and biomedical applications. Together, these contributions not only provide an excellent overview of the current state-of-the-art in the field, but also point out exciting challenges and opportunities for future work. In a number of reviews, the most recent findings on “Stimuli-Responsive Gels” are compiled in this book, also supplemented by original contributions. Thermo-responsive gelling materials constructed from natural and synthetic polymers can be used to provide triggered action and therefore are employed in customized products such as drug delivery and regenerative medicine products, as well as in other industries. Some materials enable Arrhenius-type viscosity changes based on coil to globule transitions. Others produce more counterintuitive responses to temperature change because of agglomeration induced by enthalpic or entropic drivers. The contribution of M. Joan Taylor et al. reviews recent developments in these thermo-responsive gels (doi:10.3390/gels3010004). Poly(vinyl caprolactam) (PNVCL) is one of the most important thermo-responsive polymers because it is similar to poly(N-isopropyl acrylamide) (PNIPAM). The use of PNVCL instead of PNIPAM is considered advantageous because of the assumed lower toxicity of PNVCL. The review by Chang-Sik Ha et al. focuses on the recent studies on PNVCL-based stimuli responsive three-dimensional hydrogels (macro, micro, and nano) for biomedical applications (doi:10.3390/gels2010006). In order to study the impact of a novel polyphenolic-based multiacrylate on the properties of N-isopropylacrylamide (NIPAM) hydrogels, J. Zach Hilt et al. synthesized a series of novel temperature-responsive hydrogels with varying contents of chrysin multiacrylate (ChryMA) (doi:10.3390/gels3040040). It was shown that the incorporation of ChryMA decreased the swelling ratios of the hydrogels and shifted their lower critical solution temperature (LCST) to a lower temperature, which could be attributed to the increased hydrophobicity being introduced by the ChryMA. Kuckling et al. synthesized new functional monomers based on