doi:10.1016/j.bios.2006.12.017
Copyright © 2006 Elsevier B.V. All rights reserved.
Short communication
Target delivery in a microfluidic immunosensor
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Joel P. Goldena, 1, Tamara M. Floyd-Smithb, 1, David R. Mottc and Frances S. Liglera, 
, 
aCenter for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, DC 20375, United States
bChemical Engineering Department, Tuskegee University, Tuskegee, AL 36088, United States
cLaboratory for Computational Physics and Fluid Dynamics, Naval Research Laboratory, Washington, DC 20375, United States
Received 11 August 2006;
revised 1 December 2006;
accepted 7 December 2006.
Available online 16 January 2007.
Abstract
A study is presented that examines the effect of microfluidic mixing elements on direct and sandwich assays performed in microchannels. Patterned grooves were embossed in the top of microchannels made in PDMS using soft lithography. The grooves redirected the fluid flowing in the channel, enhancing delivery of the target from the bulk fluid to the surface and preventing the formation of a depletion layer at the surface. Comparing assays in grooved and plain channels demonstrated that the mixers improved assay results by 26–46%. A computational flow analysis showed that the grooves caused virtual particles in the bulk flow to come close to the surface (
11 μm) which is consistent with the signal increase seen experimentally. Direct assays for several concentrations of CY5-labeled biotin were performed in the microchannels. The mixers also improved signal intensity in sandwich assays for botulinum toxin which required mixing of the reagents as well as the direction of the target to the surface.
Keywords: Mixer; Grooves; Microchannels; Fluorescence; Immunoassay; Depletion
Fig. 1. (a) Flow recirculation patterns. Grooves in the top of the channel cause secondary flows in the fluid flowing down the channel, enhancing interaction of the bulk fluid with the surface. (b) Computational simulation of delivery of particles in bulk fluid to sensing surface. An array of 20 × 16 particles spaced 5 μm apart was tracked through channels with different numbers of groove feature sets. The closest approach each particle made with the sensing (bottom) surface was recorded. The average closest approach of all the particles (●) and the “worst-case” particle (○) as a function of the number of feature sets is shown.
Fig. 2. (A) Diagram of microchannels. The plain (top) and grooved (bottom) channels were formed in a single piece of PDMS using soft lithography and placed on a microscope slide that had the surface prepared for performing assays. The inlets and outlets, labeled (a)–(c), are used as described in the text. (B) Image of slide with PDMS channels removed after binding of CY5-labeled biotin to the avidin-coated surface. The top is the plain channel and the bottom is the channel with the groove features. Fluid flowed into the top of each channel.
Fig. 3. Line graphs along length of grooved and plain channels for direct binding assay showing integrated signal from four experiments at the same flow rate (1 μL/min), concentration (2.6 μg/mL) plug volume (4.5 μL), and exposure time (1.5 s). Shown is the average of four integrated line graphs through the channels for the grooved (black) and plain (gray) channels. Standard error is shown every 500 pixels along length of channel (n = 4). Flow is in the direction of increasing distance (left to right).
Fig. 4. Fluorescence intensity at the beginning (a) and end (b) of the grooved (black) and plain (gray) channels. Images were acquired during the course of one of the sandwich assays. An average of 100 pixels was obtained from inside the channel at the beginning (circles) and end (squares) of the plain and grooved channels in each of the images. Data was measured twice with an average coefficient of variation of 2.8%.
Corresponding author at: Center for Bio/Molecular Science and Engineering, Code 6900, Naval Research Laboratory, Washington, DC 20375, United States. Tel.: +1 202 404 6002; fax: +1 202 767 9594/404 8897.
1 These authors contributed equally to this work.
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