Microgel-Based Smart Materials: How Do You Design a Microgel?

Microgels, which are submicrometer- to micrometer-sized hydrogels, have been investigated for more than four decades and are now widely applied in modern advanced smart materials. The "smartness" of microgel-based materials is attributed to their material composition, cross-linking strategy, and responsiveness to stimuli. These characteristics are inherently influenced and constrained by the fabrication method, which, in turn, affects the properties of the resulting microgel particles. While numerous studies have reported on the applications of microgels, the translation of fundamental research findings into practical applications remains limited. For example, while recent research in biomedical applications has focused on controlled and smart drug release based on novel environmentally responsive mechanisms, this Review highlights that the responsiveness still requires further refinement in terms of selectivity and precision. Moreover, the variety of drugs that can be used remains limited, and as this Review clarifies, microgel-based materials frequently do not possess adequate biocompatibility for biomedical applications. This Review initially summarizes the relationship between microgel synthesis techniques and their resulting properties. Furthermore, we observe that recent reports on the applications of microgels fall primarily into the categories of sensing, separation, biomedical applications, and additive manufacturing. These reports highlight recent advances in microgel applications; however, several challenges specific to each application area still need to be addressed. For instance, improving sensitivity and selectivity is a key concern in the sensing field. This Review identifies these challenges and proposes future directions for the advancement of microgel-based smart materials.