  Droplet microfluidics are a powerful approach for hydrogel cell
encapsulations. Much of the field has focused on single-cell encapsulations
with pico-nanoliter droplet volumes necessary for single-cell sequencing or
high-throughput screening. These small volumes, however, limit the use of
hydrogel droplets for tissue engineering or cell therapies. We describe simple
droplet microfluidics to generate millimeter-sized alginate droplets and
demonstrate their use for cell encapsulations. This effort builds on our recent
efforts, specifically by replacing the glass slide forming the bottom layer of
the chamber with a more hydrophobic acrylic (PMMA) layer to improve the
alginate-in-oil droplet formation. Using glass layer and PMMA layer devices, we
characterized the tunable production of water-in-oil droplets (average droplet
lengths ranged from 0.8 to 3.7 mm). Next, PMMA layer devices were used to
demonstrate the tunable generation of alginate-in-oil droplets (average droplet
lengths ranged from 3-6 mm). Increasing the flow ratio (Q.ratio =
Q.oil/Q.alginate) led to more uniform droplets as measured by the coefficient
of variance, which was approximately 5%. Finally, a proof-of-use experiment
used HUVEC-encapsulated alginate droplets as part of a scratch-healing assay.
Specifically, HUVEC-encapsulated droplets (AH droplets) led to the recovery of
3T3 fibroblast monolayers compared to no droplets or cell-free droplets (A
droplets). Our results extended the use of simple microfluidics to generate and
retrieve millimeter-sized alginate droplets for effective cell encapsulations.
