(3dn) Merging Microfluidics Into Nanoparticle Drug Delivery

Minsoung Rhee^1,2

¹Massachusetts Institute of Technology, USA.

²Brigham and Women’s Hospital, Harvard Medical School, USA.

First, I present a novel design for efficient 3D hydrodynamic focusing that has a simple and straightforward structure. Hydrodynamic focusing in a microfluidic channel provides homogeneous reaction conditions that allow for various synthesis applications including polymeric nanoparticle synthesis by nanoprecipitation. Although microfluidic platforms have advantages of rapid mixing and controlled precipitation resulting in homogeneous particles, control of aggregation is a nontrivial issue since polymer particles tend to stick to channel walls and rapidly agglomerate inside, blocking the entire channel. Polymer aggregation can be avoided by 3D hydrodynamic focusing where the polymer stream is constrained both horizontally and vertically. Our system is constructed on a single PDMS layer consisting of three sequential inlets to a conventional 2D hydrodynamic focusing system. The sample flow is first vertically focused by two vertical sheath flows and then horizontally squeezed into a narrow stream flowing in the outlet channel. Computer simulations showed that the size and position of inlet holes play a critical role in determining the shape of hydrodynamic focusing in the channel. When the inlet hole size is comparable to the channel width, the sample stream is successfully constrained in a horizontal band. In case of such complete vertical lamination, two-dimensional analytical models become sufficiently accurate. One can also simply increase the channel height to achieve similar focusing results. Molecules in the vertically focused stream gradually diffuse into both sheath streams, eventually reaching the top and bottom walls. Simulated bottom wall concentrations perfectly matched with the predicted values from two-dimensional mathematical modeling. The performance of 3D focusing was also confirmed experimentally using Poly(lactic-co-glycolic acid) (PLGA) particles known to agglomerate rapidly. Our 3D hydrodynamic focusing can prevent the precipitation at the wall. In addition, confocal micrographs with fluorescent dyes show cross-sectional views with vertical focusing. We have synthesized various polymeric nanoparticles that could neither be synthesized by 2D hydrodynamic focusing nor by bulk mixing in a conventional way. The sample stream containing polymer precursors dissolved in acetonitrile (ACN) becomes vertically squeezed by two vertical ACN sheath streams and then horizontally squeezed by water sheath streams, producing nanoparticles by nanoprecipiation.

Second, a novel single-channel multiple 3D hydrodynamic focusing device for high-throughput systems is presented here.  The device consists of two PDMS layers; the bottom layer contains a single channel for multiple focusing and the top layer governs fluidic supply and arrangement, respectively. Our prototypic device can produce 10 multiple focused streams in a parallel manner in a single channel. Each of focused streams can be adjusted to a different flow rate, resulting in a differently squeezed stream. Such multiple parallel focusing is ideal for high-throughput synthesis system where particles are synthesized by diffusive mixing that relies on hydrodynamic focusing. sson one layer and ed  PDMS layer consisting of three sequential inlets to a conventional 2D hydrodynamic focusing system. In conclusion, I present here an innovative microfluidic fabrication method, a novel air-operable microfluidic circuit, and new designs for single and multiple 3D hydrodynamic flow focusing systems that have a variety of applications in biological sciences including flow cytometry, drug delivery, drug synthesis, and screening.  

Finally, I have designed a microfluidic technology that performs counter-current flow dilution where prepared NPs solution with impurities and washing deionized water are oppositely directed in the microchannel network. This counter-current flow maintains maximum concentration gradients of impurities that in turn facilitate diffusive dilution by washing flow. Since the organic solvent and non-encapsulated drug molecules have higher diffusivities than that of polymeric NPs, the impurities are washed out and NPs can be purified and concentrated with the specially designed microfluidic system where convection between two opposite flows is minimized by imposing high hydrodynamic resistance in connecting bridges. The device operates with input polymer precursors along with drug molecules at programmed flow rates by syringe pumps, and drug-encapsulated NPs assembled in the channel will flow into the purification microchannel network where organic solvent and non-encapsulated drug will be rapidly removed. Such integration of purification functionality eliminates the need for manual collection, delivery, and filtration and thus accelerates the NP preparation and screening process by orders of magnitude. Reduction in processing time also results in faster stabilization of NPs, minimizing the possibility of premature release of encapsulated drug molecules or NP aggregation.