(4ee) Development of Novel Components for Next-Generation Microfluidic Systems

Although microfluidic systems are advantageous platforms for biological assays, their use in the life sciences is often limited by limited 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 a new concept system in microfluidics. First, a novel assembly approach for microdevice construction using prefabricated microfluidic components is presented (Fig. 1). Assembly blocks act as basic building units to form custom devices. Each Microfluidic Assembly Block (MAB) has a unique function such as inlet/ outlets, valves, straight/curved/bifurcated channels, and chambers. The MAB methodology allows for full flexibility in planar configuration. I have used MABs to prototype a variety of microfluidic devices including basic fluidic channels, a PDMS pneumatic valve to control fluid flow. The MAB system provides a simple way for non-fluidic researchers to rapidly construct custom, complex microfluidic devices. The proposed MAB methodology is thus suitable for constructing a variety of microfluidics devices. Second, I present here microfluidic digital pneumatic circuits that operate upon serially encoded pneumatic signals. Digital pneumatic components we constructed include flip-flops, logic gates, and shift registers and they are integrated to form advanced microprocessors capable of performing complex parallel operations with a single input source. Two distinct pressure levels are used for digital information: atmospheric pressure (value=0) and vacuum (value=1). The fundamental building unit is the asymmetric microfluidic inverter employing two-layered PDMS microvalves (normally closed but opens with vacuum input). With simple modifications of the symmetric inverter, NOR gates and NAND gates were constructed. Furthermore, with these cascadable universal gates along with the inverter, all sorts of logic gates were constructed including AND, OR, XOR, and XNOR. Such pneumatic logic gates comprise the building blocks for advanced circuits including bistable flip-flops, and micropneumatic shift registers (MSR). Serial-to-parallel converters are built by connecting multiple shift registers together. With n shift registers concatenated, n+1 bits of information are available at the parallel outputs. Fig. 2 shows that a 3-bit multiplexer controls individually accessible valves used for multiplexing eight different flow channels. The miniaturization and on-chip integration of control elements has the potential to significantly reduce the complexity of external controllers, interconnections and auxiliary equipment. Proposed digital pneumatic microprocessors can be a potent universal on-chip platform to autonomously manipulate microfluids in a high throughput manner. Third, I present a novel design for efficient 3D hydrodynamic focusing that has a simple and straightforward structure (Fig. 3). 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. Finally, 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.