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 inter-particle 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 model system, e.g. for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

We report on hollow shell-shell nanogels with two polymer shells that have different volume phase transition temperatures. By means of small angle neutron scattering (SANS) employing contrast variation and molecular dynamics (MD) simulations we show that hollow shell-shell nanocontainers are ideal systems for controlled drug delivery: The temperature responsive swelling of the inner shell controls the uptake and release, while the thermoresponsive swelling of the outer shell controls the size of the void and the colloidal stability. At temperatures between 32 °C < T < 42 °C, the hollow nanocontainers provide a significant void, which is even larger than the initial core size of the template, and they possess a high colloidal stability due to the steric stabilization of the swollen outer shell. Computer simulations showed, that temperature induced switching of the permeability of the inner shell allows for the encapsulation in and release of molecules from the cavity.

Microgels are nanometer-to-micrometer-sized cross-linked polymer networks that swell when dispersed in a good solvent. These soft colloids have emerged as versatile building blocks of smart materials, which are distinguished by their unique ability to adapt their behavior to changes in external stimuli. Using X-ray and neutron scattering and molecular simulation methods, we systematically measured and modeled the response to crowding of compressible, deformable microgels with varying cross-link densities and internal architectures. Our experiments and simulations demonstrate that incorporating a solvent-filled cavity during chemical synthesis provides an independent means of controlling microgel swelling that complements the influence of changing cross-link density. In other words, knowledge of the content of cross-links alone cannot be used to define microgel softness, but microgel architecture is another key property that affects softness. These results are potentially important for biomedical applications, such as drug delivery and biosensing.