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 synthetic
microgels (mainly based on acrylamides); however, the general, fundamental features
of microgels are independent of the chemical nature of the building moieties. Microgels
allow combining features of chemical functionality, structural integrity, macromolecular
architecture, adaptivity, permeability, and deformability in a unique way to include
the "best" of the colloidal, polymeric, and surfactant worlds. This will open the
door for novel applications in very different fields such as, e.g., in sensors, catalysis,
and separation technology.