Two-Dimensional and Three-Dimensional Ultrathin Multilayer Hydrogels through Layer-by-Layer Assembly
Stimuli-responsive multilayer hydrogels have opened new
opportunities to design hierarchically organized networks with properties
controlled at the nanoscale. These multilayer materials integrate structural,
morphological, and compositional versatility provided by alternating layer-bylayer
polymer deposition with the capability for dramatic and reversible changes
in volumes upon environmental triggers, a characteristic of chemically crosslinked
responsive networks. Despite their intriguing potential, there has been
limited knowledge about the structure−property relationships of multilayer
hydrogels, partly because of the challenges in regulating network structural
organization and the limited set of the instrumental pool to resolve structure and
properties at nanometer spatial resolution. This Feature Article highlights our
recent studies on advancing assembly technologies, fundamentals, and
applications of multilayer hydrogels. The fundamental relationships among
synthetic strategies, chemical compositions, and hydrogel architectures are
discussed, and their impacts on stimuli-induced volume changes, morphology,
and mechanical responses are presented. We present an overview of our studies
on thin multilayer hydrogel coatings, focusing on controlling and quantifying the
degree of layer intermixing, which are crucial issues in the design of hydrogels with predictable properties. We also uncover the
behavior of stratified “multicompartment” hydrogels in response to changes in pH and temperature. We summarize the mechanical
responses of free-standing multilayer hydrogels, including planar thin coatings and films with closed geometries such as hollow
microcapsules and nonhollow hydrogel microparticles with spherical and nonspherical shapes. Finally, we will showcase potential
applications of pH- and temperature-sensitive multilayer hydrogels in sensing and drug delivery. The knowledge about multilayer
hydrogels can advance the rational design of polymer networks with predictable and well-tunable properties, contributing to modern
polymer science and broadening hydrogel applications.
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