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Naturally derived cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) are emerging nanomaterials that display high strength, high surface area, and tunable surface chemistry, allowing for controlled interactions with polymers, nanoparticles, small molecules, and biological materials. Industrial production of nanocelluloses is increasing rapidly with several companies already producing on the tons-per-day scale, intensifying the quest for viable products across many sectors. While the hydrophilicity of the nanocellulose interface has posed a challenge to the use of CNCs and CNFs as reinforcing agents in conventional plastics, it is a significant benefit for creating reinforced or structured hydrogel composites (or, when dried, aerogels) exhibiting both mechanical reinforcement and a host of other desirable properties. In this context, this Review describes the quickly growing field of hydrogels and aerogels incorporating nanocelluloses; over 200 references are summarized in comprehensive tables covering the chemistry, preparation, properties, and applications of “nanocellulose-only” and “nanocellulose-containing” gels. Physical and chemical cross-linking strategies, postmodification steps, and routes to control gel structure are discussed, along with key developments and ongoing challenges in the field. Nanocellulose hydrogels and aerogels show great promise in a wide range of biomedical, energy storage, construction, separations, cosmetic, and food applications.

Degradable microparticles have broad utility as vehicles for drug delivery and form the basis of several FDA-approved therapies. Conventional emulsion-based methods of manufacturing produce particles with a wide range of diameters (and thus kinetics of release) in each batch. This paper describes the fabrication of monodisperse, drug-loaded microparticles from biodegradable polymers using the microfluidic flow-focusing (FF) devices and the drug delivery properties of those particles. Particles were engineered with defined sizes, ranging from 10 μm to 50 μm. These particles were nearly monodisperse (polydispersity index = 3.9 %). We incorporated a model amphiphilic drug (bupivacaine) within the biodegradable matrix of the particles. Kinetic analysis showed that the release of drug from these monodisperse particles was slower than that from conventional methods of the same average size but a broader distribution of sizes and, most importantly, exhibited a significantly lower initial burst than that observed with conventional particles. The difference in the initial kinetics of drug release was attributed to the uniform distribution of drug inside the particles generated using the microfluidic methods. These results demonstrated the utility of microfluidic FF for the generation of homogenous systems of particles for the delivery of drugs.

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.

Temperature-responsive microgels based on poly(N-isopropylacrylamide) (PNIPAM) and functionalized with vinylacetic acid (VAA) are observed to exhibit a host of novel swelling responses compared with equally functionalized microgels prepared using the conventional acrylic acid (AA) and methacrylic acid (MAA) comonomers. VAA−NIPAM microgels are ionized over a narrow pH range and show functional group pK a values which are independent of the degree of ionization. Ionization induces a much larger swelling response in VAA−NIPAM microgels than in the conventional microgels; upon ionization at physiological temperature, VAA−NIPAM swells 3 times more than either AA−NIPAM or MAA−NIPAM. VAA−NIPAM microgels also display sharp, PNIPAM-like thermal deswelling profiles when protonated but, upon ionization, undergo no volume phase transition up to at least 70 °C. The highly responsive and tunable ionization and swelling profiles observed for VAA−NIPAM are consistent with the tendency of VAA to behave as a chain transfer agent, resulting in the incorporation of a large number of well-separated VAA units on highly mobile chain ends at or near the microgel surface. VAA−NIPAM microgels may thus be ideal for use in biomolecule separation, medical diagnostics, and biodelivery applications in which sharp responses to multiple environmental stimuli are required.