Nanoparticles
are increasingly being used for biological applications,
such as drug delivery and gene transfection. Different biological
and bioinspired building blocks have been used for generating such
particles, including lipids and synthetic polymers. Proteins are an
attractive class of material for such applications due to their excellent
biocompatibility, low immunogenicity, and self-assembly characteristics.
Stable, controllable, and homogeneous formation of protein nanoparticles,
which is key to successfully delivering cargo intracellularly, has
been challenging to achieve using conventional methods. In order to
address this issue, we employed droplet microfluidics and utilized
the characteristic of rapid and continuous mixing within microdroplets
in order to produce highly monodisperse protein nanoparticles. We
exploit the naturally occurring vortex flows within microdroplets
to prevent nanoparticle aggregation following nucleation, resulting
in systematic control over the particle size and monodispersity. Through
combination of simulation and experiment, we find that the internal
vortex velocity within microdroplets determines the uniformity of
the protein nanoparticles, and by varying parameters such as protein
concentration and flow rates, we are able to finely tune nanoparticle
dimensional properties. Finally, we show that our nanoparticles are
highly biocompatible with HEK-293 cells, and through confocal microscopy,
we determine that the nanoparticles fully enter into the cell with
almost all cells containing them. Due to the high throughput of the
method of production and the level of control afforded, we believe
that the approach described in this study for generating monodisperse
protein-based nanoparticles has the potential for intracellular drug
delivery or for gene transfection in the future.