(4dc) Advanced Components and Methodology in Microfluidic Systems

Authors: 
Rhee, M., Sandia National Labs



For almost three decades, microfluidic systems have shown a wide variety of advantages and possibilities for biological, medical, and analytical studies, yet their use in the life sciences is still limited because of insufficient functionality of components, the required level of operational knowledge by non-experts, and the level of fabrication expertise required for construction. Here, I present novel microfluidic components with their simplest operational forms and most advanced functionalities as well as futuristic concept microfluidic systems. All of the systems listed here were based on entirely new concepts at the time of publications, and my inventions brought about very active follow-up research on similar systems around the world.

First, a novel assembly approach for microdevice construction using prefabricated microfluidic components is presented. Microfluidic assembly blocks are basic building units to assemble custom devices for diverse applications. Each Microfluidic Assembly Block (MAB) has a unique fluidic function such as inlet/ outlets, valves, straight/curved/bifurcated channels, and chambers. The MAB methodology allows for full flexibility in planar configuration. The MAB system provides a simple way for non-fluidic researchers to rapidly construct custom, complex microfluidic devices.

Second, I present microfluidic digital pneumatic circuits that operate upon serially encoded pneumatic signals. Digital pneumatic components include most of electronic counterparts such as flip-flops, logic gates, and shift registers. These circuit components are integrated to form advanced microprocessors capable of performing complex parallel operations with a single input source. The proposed digital pneumatic microprocessors can be a potent universal on-chip platform as a ‘self-thinking’ fluid chip to autonomously manipulate microfluids in a high throughput manner.

Third, a novel design for efficient three-dimensional hydrodynamic focusing is presented. 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 enable rapid diffusive mixing and controlled precipitation resulting in homogeneous nanoparticle synthesis, control of unpredictable 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 efficiently avoided by three dimensional hydrodynamic focusing where the polymer stream is constrained both horizontally and vertically. The system not only enables stable and efficient focusing but has also a very straightforward structure, minimizing complicated device preparation process.

Fourth, 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. 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.

Finally, I present a powerful microfluidic processor on a single chip, capable of performing (i) fluorescence-based flow cytometry and cell sorting, (ii) encapsulation of selected cells in picoliter droplets, and (iii) injection of amplification reagents into the droplets for single-cell genome amplification. Especially, to my best knowledge, this is the first report in the field that proposes on-demand droplet generation for single cell sorting. Encapsulation of bacteria in picoliter plugs in particular allows us to scale down conventional assays into much smaller reaction volumes better suited to the size of an individual microbe.  By dramatically reducing the reaction volume, the effective concentration of template is also increased, reducing amplification artifacts that often arise in single-cell reactions carried out at a conventional scale.