No abstract

Encapsulating cells within biocompatible materials is a widely pursued and promising element of tissue engineering and cell-based therapies. Recently, extensive interest in microfluidic-enabled cell encapsulation has emerged as a strategy to structure hydrogels and establish custom cellular microenvironments. In particular, it has been shown that the microfluidic-enabled photoencapsulation of cells within PEG diacrylate (PEGDA)-based microparticles can be performed cytocompatibly within gas-permeable, nitrogen-jacketed polydimethylsiloxane microfluidic devices, which mitigate the oxygen inhibition of radical chain growth photopolymerization. Compared to bulk polymerization, in which cells are suspended in a static hydrogel-forming solution during gelation, encapsulating cells via microfluidic processing exposes cells to a host of potentially deleterious stresses such as fluidic shear rate, transient oxygen depletion, elevated pressures, and UV exposure. In this work, we systematically examine the effects of these factors on the viability of cells that have been microfluidically photoencapsulated in PEGDA. It was found that the fluidic shear rate during microdroplet formation did not have a direct effect on cell viability, but the flow rate ratio of oil to aqueous solution would impart harmful effects to cells when a critical threshold was exceeded. The effects of UV exposure time and intensity on cells, however, are more complex, as they contribute unequally to the cumulative rate of peroxy radical generation, which is strongly correlated with cell viability. A reaction-diffusion model has been developed to calculate the cumulative peroxy radical concentration over a range of UV light intensity and radiation times, which was used to gain further quantitative understanding of experimental results. Conclusions drawn from this work provide a comprehensive guide to mitigate the physical and biochemical damage imparted to cells during microfluidic photoencapsulation and expands the potential for this technique.

No abstract

Hydrogels have been engineered for a variety of biomedical applications including biosensing, drug delivery, cell delivery, and tissue engineering. The fabrication of hydrogels into nanoscale and microscale particles has been a subject of intense activity and promises to extend their range of applicability. As hydrogels are reduced in size, their interfacial properties represent an increasingly significant contribution to their function and behavior. Hydrogel microparticle-based biosensing and drug delivery platforms, for instance, require delicate spatial control over the conjugation of biofunctional groups and network architecture, which impacts their mechanical properties and molecular permeability and diffusivity. Here, we demonstrate the ability to tune, with extraordinary precision, the interfacial properties of PEGDA particles generated in a droplet microfluidic device exploiting oxygen-inhibited photopolymerization. We demonstrate the broad utility of these engineered microgels by creating spherical particles with complex but predictable radial crosslinking density gradients. Immunoassays were conducted to examine the network properties of these particles, revealing a high degree of structural tunability, which, in turn, dictates macromolecule encapsulation and release profiles, as well as the presence of radial crosslinking gradients that impact the availability of functional groups.