Microgels are macromolecular networks swollen by the solvent in which they are dissolved. They are unique systems that are distinctly different from common colloids, such as, e.g., rigid nanoparticles, flexible macromolecules, micelles, or vesicles. The size of the microgel networks is in the range of several micrometers down to nanometers (then sometimes called "nanogels"). In a collapsed state, they might resemble hard colloids but they can still contain significant amounts of solvent. When swollen, they are soft and have a fuzzy surface with dangling chains. The presence of cross-links provides structural integrity, in contrast to linear and (hyper)branched polymers. Obviously, the cross-linker content will allow control of whether microgels behave more "colloidal" or "macromolecular". The combination of being soft and porous while still having a stable structure through the cross-linked network allows for designing microgels that have the same total chemical composition, but different properties due to a different architecture. Microgels based, e.g., on two monomers but have either statistical spatial distribution, or a core-shell or hollow-two-shell morphology will display very different properties. Microgels provide the possibility to introduce chemical functionality at different positions. Combining architectural diversity and compartmentalization of reactive groups enables thus short-range coexistence of otherwise instable combinations of chemical reactivity. The open microgel structure is beneficial for uptake-release purposes of active substances. In addition, the openness allows site-selective integration of active functionalities like reactive groups, charges, or markers by postmodification processes. The unique ability of microgels to retain their colloidal stability and swelling degree both in water and in many organic solvents allows use of different chemistries for the modification of microgel structure. The capability of microgels to adjust both their shape and volume in response to external stimuli (e.g., temperature, ionic strength and composition, pH, electrochemical stimulus, pressure, light) provides the opportunity to reversibly tune their physicochemical properties. From a physics point of view, microgels are particularly intriguing and challenging, since their intraparticle properties are intimately linked to their interparticle behavior. Microgels, which reveal interface activity without necessarily being amphiphilic, develop even more complex behavior when located at fluid or solid interfaces: the sensitivity of microgels to various stimuli allows, e.g., the modulation of emulsion stability, adhesion, sensing, and filtration. Hence, we envision an ever-increasing relevance of microgels in these fields including biomedicine and process technology. In sum, microgels unite properties of very different classes of materials. Microgels can be based on very different (bio)macromolecules such as, e.g., polysaccharides, peptides, or DNA, as well as on synthetic polymers. This Account focuses on ...

We investigated the thermoresponsive behavior of aqueous solutions of star-shaped and linear poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). The observed cloud points strongly decrease with increasing pH of the solution. This is explained by a weak charging of the star polymers with decreasing pH. A significant decrease of the cloud points with increasing molecular weight for high pH, i.e., for the almost uncharged state, was found to be virtually independent of the arm number and arm length. These findings are explained by classical Flory-Huggins theory. The increase of cloud points upon charging is captured by introduction of an effective degree of polymerization. Polymers with shorter arms show slightly higher cloud points at low pH than polymers with longer arms. The intramolecular segment density also influences the observed apparent pK b values, leading to higher values for stars with higher arm numbers.

The presence of multivalent counterions induces an upper critical solution temperature (UCST) in addition to the known lower critical solution temperature (LCST) of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). The LCST-type cloud points can be adjusted by pH of the buffer, whereas the UCST-type cloud points can be adjusted by the concentration of trivalent counterions. High pH favors the LCST transition, whereas lower pH extends the UCST-type miscibility gap at constant concentration of trivalent counterions. By use of hexacyanocobaltate(III) as a trivalent counterion, we can even switch off again the UCST-behavior by UV-illumination (photoinduced dissolution).

Summary: We report the synthesis of star‐shaped poly(acrylic acid) (PAA), with 5, 8, and 21 arms, by atom transfer radical polymerization of tert‐butyl acrylate. We employ the core‐first approach using glucose‐, saccharose‐ and cyclodextrin‐based initiators. Subsequent acidic treatment of poly(tert‐butyl acrylate) (PtBA) leads to star‐shaped poly(acrylic acid) (PAA). Alkaline cleavage of the arms enabled us to determine the initiation site efficiency. The PAA stars and arms were esterified to poly(methyl acrylate) (PMA). Molecular weight determination by means of GPC/viscosity, MALDI‐TOF MS and NMR end‐group determination showed that the initiation site efficiency is close to unity. Results from potentiometric titration of PAA arms and stars show that the apparent pKa values increase with increasing arm number, which is a direct result of increasing segment density. Osmometry measurements of aqueous solutions of the PAA stars result in osmotic coefficients between 0.05 and 0.38, indicating that most of the counterions are confined within the star. The confinement increases with arm number.

This work examines the fabrication regime and the properties of microgel and microgel/enzyme thin films adsorbed onto conductive substrates (graphite or gold). The films were formed via two sequential steps: the adsorption of a temperature- and pH-sensitive microgel synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAM) and 3-(N,N-dimethylamino)propylmethacrylamide (DMAPMA) (poly(NIPAM-co-DMAPMA) at the pH-condition corresponding to its noncharged state (first step of adsorption), followed by the enzyme, tyrosinase, adsorption at the pH-condition when the microgel and the enzyme are oppositely charged (second step of adsorption). The stimuli-sensitive properties of poly(NIPAM-co-DMAPMA) microgel were characterized by potentiometric titration and dynamic light scattering (DLS) in solution as well as by atomic force microscopy (AFM) and quartz crystal microbalance with dissipation monitoring (QCM-D) at solid interface. Enhanced deposition of poly(NIPAM-co-DMAPMA) microgel particles was shown at elevated temperatures exceeding the volume phase transition temperature (VPTT). The subsequent electrostatic interaction of the poly(NIPAM-co-DMAPMA) microgel matrix with tyrosinase was examined at different adsorption regimes. A considerable increase in the amount of the adsorbed enzyme was detected when the microgel film is first brought into a collapsed state but then was allowed to interact with the enzyme at T < VPTT. Spongelike approach to enzyme adsorption was applied for modification of screen-printed graphite electrodes by poly(NIPAM-co-DMAPMA)/tyrosinase films and the resultant biosensors for phenol were tested amperometrically. By temperature-induced stimulating both (i) poly(NIPAM-co-DMAPMA) microgel adsorption at T > VPTT and (ii) following spongelike tyrosinase loading at T < VPTT, we can achieve more than 3.5-fold increase in biosensor sensitivity for phenol assay. Thus, a very simple, novel, and fast strategy for physical entrapment of biomolecules by the polymeric matrix was proposed and tested. Being based on this unique stimuli-sensitive behavior of the microgel, this stimulated spongelike adsorption provides polymer films comprising concentrated biomaterial.