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-by-layer polymer deposition with the capability for dramatic and reversible changes in volumes upon environmental triggers, a characteristic of chemically cross-linked 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.

中文翻译：

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通过逐层组装的二维和三维超薄多层水凝胶

刺激响应的多层水凝胶为设计具有在纳米尺度上控制特性的分层组织网络开辟了新的机会。这些多层材料集成了由交替的逐层聚合物沉积提供的结构、形态和组成的多功能性，以及在环境触发下体积发生显着和可逆变化的能力，这是化学交联响应网络的特征。尽管它们具有有趣的潜力，但对多层水凝胶的结构-性能关系的了解有限，部分原因是在调节网络结构组织方面存在挑战，以及在纳米空间分辨率下解析结构和性能的仪器池有限。这篇专题文章重点介绍了我们最近关于推进多层水凝胶组装技术、基础和应用的研究。讨论了合成策略、化学成分和水凝胶结构之间的基本关系，并介绍了它们对刺激引起的体积变化、形态和机械响应的影响。我们概述了我们对薄多层水凝胶涂层的研究，重点是控制和量化层混合的程度，这是设计具有可预测性能的水凝胶的关键问题。我们还揭示了分层“多室”水凝胶响应 pH 值和温度变化的行为。我们总结了独立式多层水凝胶的机械响应，包括具有封闭几何形状的平面薄涂层和薄膜，例如具有球形和非球形形状的中空微胶囊和非中空水凝胶微粒。最后，我们将展示对 pH 和温度敏感的多层水凝胶在传感和药物输送中的潜在应用。关于多层水凝胶的知识可以促进具有可预测和良好可调特性的聚合物网络的合理设计，有助于现代聚合物科学和拓宽水凝胶的应用。