Nanogels and microgels are soft, deformable, and penetrable objects with an internal gel-like structure that is swollen by the dispersing solvent. Their softness and the potential to respond to external stimuli like temperature, pressure, pH, ionic strength, and different analytes make them interesting as soft model systems in fundamental research as well as for a broad range of applications, in particular in the field of biological applications. Recent tremendous developments in their synthesis open access to systems with complex architectures and compositions allowing for tailoring microgels with specific properties. At the same time state-of-the-art theoretical and simulation approaches offer deeper understanding of the behavior and structure of nano- and microgels under external influences and confinement at interfaces or at high volume fractions. Developments in the experimental analysis of nano- and microgels have become particularly important for structural investigations covering a broad range of length scales relevant to the internal structure, the overall size and shape, and interparticle interactions in concentrated samples. Here we provide an overview of the state-of-the-art, recent developments as well as emerging trends in the field of nano- and microgels. The following aspects build the focus of our discussion: tailoring (multi)­functionality through synthesis; the role in biological and biomedical applications; the structure and properties as a model system, e.g., for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

This chapter reviews recent work on the synthesis of aqueous microgel particles by precipitation polymerization. Precipitation polymerization allows flexible control over important physicochemical properties of aqueous microgels, such as size distribution, surface charge, chemical composition, and microstructure. The microgel systems discussed in this review are mainly based on poly(N-isopropyl acrylamide) and poly(N-vinylcaprolactam) due to their ability to react to external stimuli such as the pH or temperature of the surrounding medium. We discuss synthetic routes to obtain microgels based on homo-or copolymers as well as colloids with complex core-shell morphology. The functionalization of microgels is of crucial importance from the application point of view. Different routes for incorporation of functional groups, synthetic polymers, proteins, or nanoparticles in microgel structures are discussed.

such as aerospace, transportation, and biological engineering. [2,3] In general, the combination of great toughness, large ductility, and high strength in polymers is essential for enabling their real-world applications. However, existing strengthening and/or toughening strategies fail to realize the desirable mechanical combination in polymers owing to mutually exclusive governing mechanisms between strength and modulus. [4] In addition, the ability to self-heal is another desirable yet key factor for extending their lifespan after damage, [5][6][7] whereas a good biocompatibility is a prerequisite for their practical applications as artificial tissues. For example, in addition to the lack of a selfhealing ability, existing artificial ligament materials usually suffer from lower ductility and/or toughness relative to natural counterparts. [8,9] Therefore, it has been highly attractive but remained a grand challenge to create strong, tough, and ductile polymeric materials that are also healable and biocompatible so far.Many natural materials such as nacre, [10,11] bones, [12] and spider silk fibers (SSF) [13] are valid examples of how the evolutionary forces address the issue of the trade-off between mechanical strength and stretchability. In particular, SSF exhibits an outstanding fracture toughness over 150 J g −1 and a large breaking strain (>50%) as well as high tensile strength (>1 GPa). [14] The unique mechanical combination has been revealed to originate from Lightweight polymeric materials are highly attractive platforms for many potential industrial applications in aerospace, soft robots, and biological engineering fields. For these real-world applications, it is vital for them to exhibit a desirable combination of great toughness, large ductility, and high strength together with desired healability and biocompatibility. However, existing material design strategies usually fail to achieve such a performance portfolio owing to their different and even mutually exclusive governing mechanisms. To overcome these hurdles, herein, for the first time a dynamic hydrogen-bonded nanoconfinement concept is proposed, and the design of highly stretchable and supratough biocompatible poly(vinyl alcohol) (PVA) with well-dispersed dynamic nanoconfinement phases induced by hydrogenbond (H-bond) crosslinking is demonstrated. Because of H-bond crosslinking and dynamic nanoconfinement, the as-prepared PVA nanocomposite film exhibits a world-record toughness of 425 ± 31 MJ m −3 in combination with a tensile strength of 98 MPa and a large break strain of 550%, representing the best of its kind and even outperforming most natural and artificial materials. In addition, the final polymer exhibits a good self-healing ability and biocompatibility. This work affords new opportunities for creating mechanically robust, healable, and biocompatible polymeric materials, which hold great promise for applications, such as soft robots and artificial ligaments.