A Microfluidic Chip with On-Board Computer-Controlled Micropumps for Continuous Sampling and Hydrodynamic Flow Gate Injection

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A Microfluidic Chip with OnBoard Computer Controlled Micropumps for Continuous Sampling and Hydrodynamic Flow Gate injection

Hernan V. Fuentes1 and Robert T. Kennedy1,2

1Department of Chemistry and 2Department of Pharmacology

University of Michigan

Ann Arbor, MI 48109

Since first introduced in the early 1990’s, separation-based microfluidic devices are becoming increasingly appealing for many applications in chemistry and bioanalytical studies. Microfluidics provides many advantages in chemical analysis reducing costs and analysis time; in addition, many processes can be performed in parallel on a small footprint enhancing assay integration. Despite these unrivalled advantages, several challenges still remain. Indeed, connecting microfluidics to the real world and developing sample introduction methods for high efficiency separations are of particular interest.

We have developed a stand-alone microfluidic system for automated sampling, injection and high efficiency electrophoretic analysis of primary amine neurotransmitters. Our device uses computer-controlled peristaltic pumps driven by a set of pins from a Braille display.  Pumping action is created when a repeating pattern of pin states pushes a polydimethylsiloxane (PDMS) membrane upward against glass microchannels. An amino acid solution was sampled through a 20-cm long capillary and reacted on-line with naphthalene-2,3-dicarboxaldehyde (NDA)/CN. Labeled sample was then pumped continuously from the reaction channel to a flow gate interface in which a buffer cross flow prevented sample from reaching the separation channel. For injection, the cross flow was stopped and a small (~500 V) injection voltage was applied. After injection, and electric field (~700 V/cm) was applied across the separation channel resulting in fast CE separation of the amino acids in the sample.

Our results demonstrate that both, sample and reagents delivery can be performed with a single device without requiring external components. Sampling flow rate can be adjusted by varying the pin actuation frequency and/or microchannel dimensions. Flow rates of 35 – 250 nL/min have been measured with good reproducibility (RSD < 8.5%). The on-chip flow gate injection approach allows rapid, continuous and reproducible (RSD, 2.3 %, n=400) sample injection. Separation efficiency close to 200,000 theoretical plates has been obtained for glycine and the devices response time to a change in glutamate concentration was ~70 s.

This study demonstrates the feasibility of integrating micropumps with glass separation channels using Braille displays resulting in a highly automated separation-based biosensor for chemical analysis. Moreover, the ease of operation and control should facilitate the integration of these microfluidic systems with online sampling methods such as probes for in-vivo studies. We have already evaluated the ability of these devices to sample extra cellular fluid from the brain of a rat. Our current efforts are focus on using push-pull probes for in-vivo monitoring of neurotransmitters.