Subtleties of UV-crosslinking in microfluidic particle fabrication: UV dosage and intensity matter

Curable hydrogels have tunable properties that make them well-suited for applications in drug delivery, cell therapies, and 3D bioprinting. Advances in microfluidic droplet generation enable rapid fabrication of polymer-filled droplets. UV-curable polymers offer a clear path toward using fluidic generation to produce monodisperse microgels with uniform properties. In flow, polymer concentration and UV exposure both control the degree of crosslinking. High UV intensity is often used to ensure complete gelation and avoid complications that may arise from partial curing. Optical microscopy can assess droplet and particle sizes in flow. However, optimizing formulations for mechanical properties usually requires removal of generated material and external measurement outside of flow. In this study, we couple droplet generation, microgel fabrication, and mechanics assessment within a single fluidic device. We make and measure soft polyethylene glycol diacrylate (PEGDA) microgels by curing polymer-filled water drops in mineral oil. Crosslinking is tuned by varying UV dosage, allowing us to study how gelation degree influences microgel properties. Within the device, we use shape deformation in flow to measure the restoring stress of both droplets and particles. Our results suggest that PEGDA droplets gel from the inside out. If gelation is incomplete, a particle resides within a fluid drop. Independent measurements outside of flow corroborate this observation. Crosslinking PEGDA-filled droplets in a pendant drop geometry, with dye, suggests the persistence of an aqueous shell around the gel. Similarly, microparticles in PEGDA-filled drops undergoing gelation exhibit diffusive arrest near the drop center, while maintaining mobility in an outer region. Together, these results suggest the importance of considering the extent of gelation when fabricating microgels using fluidics.