(139c) Reactive Microfluidic Production of Degradable Microgels for Drug Delivery

INTRODUCTION: Microgels, best defined as hydrogel particles, have great
potential to address many challenges in biomedicine. The facile injectability,
high surface area to volume ratio, and ready tunability of both the surface and
bulk of microgels present key advantages for a range of applications including
drug delivery or drug scavenging vehicles¹, cell encapsulation matrices², or biosensors³. Microgels prepared using ?smart?
materials that can dynamically change their physical properties (e.g. size,
charge, or colloidal stability) in response to external factors (e.g.
temperature, pH, or the concentration of a particular chemical in the microgel
environment) have particular utility in this regard, facilitating ?on-demand?
changes in microgel localization (i.e. aggregation versus circulation) or drug
diffusion in different environments⁴. Of particular note, microgel size
has a significant impact on the ultimate biological performance of the
microgel; for example, smaller microgels release drugs faster and circulate for
an extended period of time (ultimately localizing in the spleen or liver) while
larger microgels release drug slower and typically remain at their injection
site. However, conventional methods of fabricating environmentally-responsive
microgels are significantly limited in terms of their capacity to control
microgel size. While monodisperse thermoresponsive
microgels with sizes from 100-1000 nm are relatively easy to fabricate via a
precipitation-based technique⁴, inverse emulsion strategies are
typically required to synthesize non-thermoresponsive
microgels or any microgel with a diameter of >1 micron. Conventional methods
to generate inverse emulsions rely on sonication,
homogenization, or other types of bulk shear forces to generate the
water-in-oil droplets, resulting in polydisperse
microgels that give unpredictable drug release kinetics and biological
responses. However, microgels with >1 micron diameters are of significant
technological interest in that they are less likely to be endocytosed
by macrophages or quickly sequestered within the body upon injection. Of
particular utility for local drug delivery would be monodisperse populations of
large (>1 micron diameter) microgels crosslinked via degradable linkages;
such materials could be injected at a desired site of action, remain at the
site of injection to deliver drug locally over an extended period of time, and
then cleared from the body after use.

EXPERIMENTAL: Microfluidics chips were designed that facilitated on-chip
crosslinking between two reactive polymer precursors, pre-mixed prior to the
droplet formation step. Microgels were generated by reacting hydrazide-functionalized
polymers (prepared via EDC-mediated grafting of adipic
acid dihydrazide with a carboxylic
acid-functionalized polymer) with aldehyde-functionalized polymers (prepared
via carbohydrate oxidation using sodium periodate) to
spontaneously form a hydrazone-crosslinked microgel inside the microfluidics
chip. The chip (pictured) contacts the two aqueous reactive polymer streams,
passes the streams through a mixing channel, and then generates water-in-oil
droplets using paraffin oil as the oil phase and Span 80 as the surfactant.
Microgels were then extracted into an aqueous suspension via freeze-thawing and
subsequent solvent exchange. Particle size of the microgels was assessed via
light and fluorescence microscopy and surface charge was measured via
electrophoretic mobility measurements. Drug loading/release experiments were
performed by mixing purified microgels in a drug solution, centrifuging to
isolate the microgels, resuspending the microgels in
PBS inside a Float-a-Lyzer membrane bag (Spectrum
Labs, 50,000 MWCO), and assaying drug release as a function of time. Microgels
with the same composition were prepared using sonication
to compare with microgels synthesized using microfluidics.

RESULTS: Reactive microfluidics facilitates successfully production
of microgels with significantly improved monodispersity
and size control relative to traditional shear-based techniques. Microgels with
particle sizes ranging between 25-200 microns were generated with polydispersities <5% by varying both the relative flow
rates of the oil and (aqueous) polymer streams or the total flow rate of the
oil and polymer streams through the chip. Crosslinked microgels were generated
spontaneously upon droplet formation without the need for subsequent heating,
UV photopolymerization, or the use of additional
initiators, crosslinkers, or chain transfer agents, as required in previous
microfluidic microgel generation systems⁵. Continuous particle production was
possible over at least 72 hours, with increased microgel yield possible through
the use of parallel microfluidic reactors. Microgels were successfully
synthesized based on carbohydrates (carboxymethyl cellulose/dextran), mixtures
of synthetic oligomers and carbohydrates (poly(N-isopropylacrylamide)/
dextran) or synthetic oligomers only (poly(N-isopropylacrylamide)),
illustrating the adaptability of the device to generate multiple types of
degradable microgels with varying chemistries. Drug release for the model drug
bupivacaine (a local anesthetic) was a weak function
of microgel size and followed first-order, diffusion-based kinetics, with
sustained release over periods of up to two weeks.

CONCLUSIONS: Reactive microfluidics provides an effective route to the
preparation of monodisperse degradable microgels based on natural, synthetic,
or mixtures of natural and synthetic polymers. Such materials are ideal for
biomedical applications in that the crosslink density, composition, size, and
biocompatibility can be tuned appropriately for the particular desired end-use.

ACKNOWLEDGEMENTS:
This work was funded by the Natural
Sciences and Engineering Research Council of Canada and 20/20: NSERC Ophthalmic
Materials Research Network.

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