Abstract
This article provides a systematic review of the crosslinking strategies used to produce microgel particles in microfluidic chips. Various ionic crosslinking methods for the gelation of charged polymers are discussed, including external gelation via crosslinkers dissolved or dispersed in the oil phase; internal gelation methods using crosslinkers added to the dispersed phase in their non-active forms, such as chelating agents, photo-acid generators, sparingly soluble or slowly hydrolyzing compounds, and methods involving competitive ligand exchange; rapid mixing of polymer and crosslinking streams; and merging polymer and crosslinker droplets. Covalent crosslinking methods using enzymatic oxidation of modified biopolymers, photo-polymerization of crosslinkable monomers or polymers, and thiol-ene “click” reactions are also discussed, as well as methods based on the sol−gel transitions of stimuli responsive polymers triggered by pH or temperature change. In addition to homogeneous microgel particles, the production of structurally heterogeneous particles such as composite hydrogel particles entrapping droplet interface bilayers, core−shell particles, organoids, and Janus particles are also discussed. Microfluidics offers the ability to precisely tune the chemical composition, size, shape, surface morphology, and internal structure of microgels by bringing multiple fluid streams in contact in a highly controlled fashion using versatile channel geometries and flow configurations, and allowing for controlled crosslinking.
Highlights
Hydrogels are three-dimensional networks of hydrophilic crosslinked polymers that can hold large amounts of water in their intermolecular space, but they are not soluble in water in their crosslinked form
The objective of this paper is to review recent developments in crosslinking polymers or monomers in microfluidic chips for the purpose of producing both matrix-type and core−shell microgels
Microfluidic encapsulation of single cells cells within Dex-Tyr microgels achieved by mixing the polymer, cells, and crosslinkers in within Dex-Tyr microgels achieved by mixing the polymer, cells, and crosslinkers in situ situ just before droplet pinch-off is shown in Figure 19a [75,76]
Summary
Hydrogels are three-dimensional networks of hydrophilic crosslinked polymers that can hold large amounts of water in their intermolecular space, but they are not soluble in water in their crosslinked form. Hydrogels are widely used as excipients for drug delivery systems [1,2]; scaffolds in tissue engineering [3,4]; wound dressings [5]; absorbents for hygiene products (diapers, napkins, hospital bed sheets, and sanitary towels) [6]; gelling agents, thickeners, and packaging materials in food products [7]; and irritationfree, transparent materials for contact lenses [8]. Their high-water retention capacity and soft, porous structure mimic the in vivo extracellular matrix (ECM) microenvironment. The mechanisms of droplet formation in microfluidic devices have been described in many excellent review papers [21,22,23,24,25] and will not be discussed here
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