Microgels

Microgels are crosslinked polymer particles that swell in a solvent to form soft, hydrogel-like colloids. These particles are typically submicrometer to a few micrometers in diameter and consist of networks of polymer chains held together by crosslinks. The extent of swelling, mechanical stiffness, and surface properties are governed by the crosslink density, particle size, and chemical composition.

Synthesis and composition: Microgels are commonly prepared by polymerization in dispersed media, such as precipitation polymerization, emulsion polymerization, or microfluidic methods, producing discrete particles without the need for post-synthesis drying. The most widely studied microgels are based on poly(N-isopropylacrylamide) PNIPAM, polyacrylates, polyacrylamides, and their copolymers. Crosslinkers such as N,N'-methylenebisacrylamide (BIS) or divinylbenzene control network density. Functional groups can be introduced for bioconjugation, charge, or responsiveness to stimulus.

Stimuli-responsive behavior: Many microgels are stimuli-responsive, changing volume or porosity in response to temperature, pH, ionic strength, or light. PNIPAM microgels exhibit a volume phase transition at around 32 C in water. By tuning composition and crosslink density, microgels can be engineered to release encapsulated cargo or alter permeability on demand.

Applications and characterization: Microgels find use in drug delivery, imaging, diagnostics, biosensing, catalysis, and wastewater treatment. They are characterized by dynamic light scattering for size distribution, electron or atomic force microscopy for morphology, zeta potential for surface charge, and techniques such as small-angle X-ray scattering for internal structure. Swelling ratio and loading/release behavior are key performance metrics.