Nanogels: Synthesis, properties, and recent biomedical applications

Nanogels, also termed hydrogel nanoparticles, have the properties and functions of both nanomaterials and hydrogels. Till date, a considerable number of nanogels have been developed to satisfy increasing requirements from different fields. Therefore, it is crucial to summarize the existing preparation methods, physicochemical and biological properties, and recent applications of nanogels, which may be beneficial for seeking new developmental directions of nanogels. In this review, we first introduce the history, ingredients, and classification of nanogels, and the differences of nanogels from macro-sized hydrogels and other kinds of nanoparticles. Second, we focus on the synthetic methods of nanogels from the aspects of physical crosslinking approaches and chemical crosslinking reactions, and discuss the pros and cons of these methods. Third, the common properties of nanogels are introduced, including the reasons why nanogels possess these properties and some representative examples illustrating the merits brought about by these properties. Fourth, we elaborate on the current applications of nanogels in drug delivery, imaging, detection, and regenerative medicine. Fifth, we also summarize the clinical trials and clinical applications in the nanogel field. Finally, the current problems and challenges in the nanogel field are raised. This review is expected to inspire researchers to develop more functional nanogels and promote their practical applications in a variety of fields.