Microfluidics

We are using microfluidics to develop new techniques and applications integrating surface plasmon resonance (SPR), capillary electrophoresis (CE), sensors, and bead-based assays for use in the development of new diagnostic devices to analyze and detect biomarkers, novel methanol-based microfluidic fuel cells, fluorescence-based assays, separations. We are also using modeling to optimize experimental parameters in microfluidics and CE.

Bead-Based Assays

We are developing new microfluidic devices (MDs) fabricated from PDMS and other polymers for use in several applications. One area of research is in the development of microfluidic-based assays using magnetic beads. In recent work we have derivatized magnetic beads (2.7 micrometer in diameter) with antibiotics and proteins and have shown the feasibility of an assay on a microfluidic platform. For example, the magnetic beads were first derivatized with the glycopeptides antibiotic teicoplanin. The beads were then manipulated into a microfluidic channel and a solution of fluorescently-labeled ligand introduced into the channel. The solution was then manipulated to the place of the magnetic beads securely held with an external magnet. Flushing of the beads (or manipulation to another part on the chip) with buffer removed any ligand unbound to the receptor on the beads (vis-à-vis erased any possibility of non-specific binding) and the fluorescence recorded. A subsequent binding isotherm yielded the binding constant which was confirmed using fluorometric and flow cytometry experiments. Additionally, we have examined binding of the beads to Staphylococcus aureus further demonstrating the efficacy of the assay.

Fluorescent images of non-derivatized (A) and derivatized (B) magnetic beads after incubation with 100 µM FAM-(DA)3 for 15 minutes. Carboxylic terminated beads were derivatized with the antibiotic teicoplanin. Each sample was washed five times before the image acquisition.

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Chromatographic Separations

We are using MDs for chromatographic separations. We have developed and studied a disposable and inexpensive microfluidic chip, fabricated from PDMS incorporating conventional chromatographic reversed-phase silica particles (C18) without the use of frits, permanent physical barriers, tapers or restrictors. The packing of C18 modified silica particles into the microfluidic channels was made possible by the hydrophobic nature and excellent elasticity of PDMS. Keystone-, clamping- and anchor-effects providing the stability and the compactness of the packing and attenuated wall-effect were observed. As an extension to this work we have demonstrated electrochromatographic separations with these chips and have separated dyes and other small molecules.

Separation of two dyes in a serpentine channel.

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New Materials for Microfluidics

We are focused on the design and fabrication of chemically robust three-dimensional (3D) microfluidic valves using the fluorinated polymer Sifelin a collaboration with Dr. Axel Scherer (Caltech). These MDs are fabricated in a single monolithic layer that is resistant to most organic solvents with minimal swelling. A novel wax printing method allows for simple fabrication of topologically complex 3D microfluidic structures and offers many advantages over multilayer soft lithography (MSL) including ease of fabrication, rapid response time and high levels of integration.

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Valve Actuation

We are developing a simple, external in-line valve for use in MDs constructed of PDMS. The actuation of the valve is based on the principle that flexible polymer walls of a liquid channel can be pressed together by the aid of a permanent magnet and a small metal bar. In the presence of a small NdFeB magnet lying below the microfluidic channel of interest, a metal bar is pulled downward simultaneously pushing the thin layer of PDMS down thereby closing the channel stopping any flow of fluid. The operation of the valve is dependent on the thickness of the PDMS layer, the height of the channel, the gap between the chip and the magnet and the strength of the magnet. The microfluidic channels are completely closed to fluid flows commonly used in microfluidic applications.

"Thin" microchip with magnetic valves and electrodes for capillary electrophoresis separations.

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Affinity Capillary Electrophoresis on Microchips

We have developed a microfluidic/capillary electrophoresis (CE) technique employing partial filling affinity capillary electrophoresis (PFACE) to estimate binding constants of ligands to receptors using as model systems carbonic anhydrase B (CAB, EC 4.2.1.1) and vancomycin from Streptomyces orientalis. Using MSL, a MD consisting of fluid and control channels is fabricated and fitted with an external capillary column. Multiple flow channels allows for manipulation of a zone of ligand and sample containing receptor and non-interacting standards into the MD and subsequently into the capillary column. Upon electrophoresis the sample components migrate into the zone of ligand where equilibrium is established. Changes in migration time of the receptor are used in the analysis to obtain a value for the binding interaction. The manipulation of small volumes of solution on the MD minimizes the need of time-consuming pipetting steps.

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Surface Plasmon Resonance on Microfluidic Chips

We are developing CE-SPR on a microchip in a collaboration with Dr. Feimeng Zhou (CSULA). While much work has focused on using SPR for examining binding interactions, only one other group has coupled CE to SPR on-a-chip. The main advantage in SPR-CE lies in the inherent separation ability of CE whereby mixtures of many compounds can be assessed then examined using SPR. Preliminary work has demonstrated proof-of-concept. Further work is focused on examining a variety of receptor-ligand interactions using the CE-SPR microfluidic device.

Microchip for surface plasmon resonance.

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