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Research Papers

# A Microfluidic Device to Establish Concentration Gradients Using Reagent Density Differences

[+] Author and Article Information
Qingjun Kong, Richard A. Able, Veronica Dudu

Department of Biomedical Engineering, The City College of The City University of New York (CCNY), Room 403D, Steinman Hall, 160 Convent Avenue, New York, NY 10031

Maribel Vazquez1

Department of Biomedical Engineering, The City College of The City University of New York (CCNY), Room 403D, Steinman Hall, 160 Convent Avenue, New York, NY 10031vazquez@ccny.cuny.edu

1

Corresponding author.

J Biomech Eng 132(12), 121012 (Nov 16, 2010) (9 pages) doi:10.1115/1.4002797 History: Received March 25, 2010; Revised September 17, 2010; Posted October 15, 2010; Published November 16, 2010; Online November 16, 2010

## Abstract

Microfabrication has become widely utilized to generate controlled microenvironments that establish chemical concentration gradients for a variety of engineering and life science applications. To establish microfluidic flow, the majority of existing devices rely upon additional facilities, equipment, and excessive reagent supplies, which together limit device portability as well as constrain device usage to individuals trained in technological disciplines. The current work presents our laboratory-developed bridged $μLane$ system, which is a stand-alone device that runs via conventional pipette loading and can operate for several days without need of external machinery or additional reagent volumes. The bridged $μLane$ is a two-layer polydimethylsiloxane microfluidic device that is able to establish controlled chemical concentration gradients over time by relying solely upon differences in reagent densities. Fluorescently labeled Dextran was used to validate the design and operation of the bridged $μLane$ by evaluating experimentally measured transport properties within the microsystem in conjunction with numerical simulations and established mathematical transport models. Results demonstrate how the bridged $μLane$ system was used to generate spatial concentration gradients that resulted in an experimentally measured Dextran diffusivity of $(0.82±0.01)×10−6 cm2/s$.

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## Figures

Figure 1

Images and schematic of the bridged μLane system. After photolithography and elastomeric molding, a microchannel with two reservoirs was fabricated in polymerized PDMS, defined as the first layer PDMS of the bridged μLane system. (a) This PDMS layer was then bonded onto an ultrasonically cleaned microscope glass slide. (b) A second layer of PDMS, consisting of a bridge channel and two chambers, was molded on top of the first layer PDMS. This layer of PDMS was defined as the second layer PDMS, or user interface layer, of the system. (c) The chambers in the second layer PDMS were fluidically connected with the reservoirs in the first layer PDMS, as shown in the bridged μLane system schematic (not to scale). The microchannel approximately measures 13 mm in length, 90 μm in depth, and 100 μm in width (averaged with the upper side of 95 μm and the lower side of 105 μm), as its semihemispherical cross section shown in inset.

Figure 2

Simulation of Dextran transport within the bridged μLane system. (a) The SRC, SRR, microchannel, SKR, SKC, and bridge channel were modeled using finite element software. The Dextran concentration in the SRC was set to a maximum of CH=40 ng/ml, while the Dextran concentration in the SKC was set to a minimum of CL=0 ng/ml. The bulk velocity within the microchannel was measured using fluorescent beads and inputted into the Dextran transport model for numerical simulation. All boundaries of the bridged μLane system were modeled as insulated from mass transfer. Dextran transport at different positions within the microchannel, x=5 mm, x=8 mm, and x=11 mm, was examined over time. One-dimensional convective diffusion equation was used to model the transport of Dextran within the microchannel, where C represents concentration, V is bulk velocity, and D is the diffusion coefficient. (b) The simulation illustrates that the Dextran concentration at a representative microchannel position of x=5 mm increases over time and reaches steady-state in 11 h.

Figure 3

(a) Images of Dextran transport within the microchannel at three representative positions over time. Fluorescence was initially observed throughout the microchannel in approximately 4 h with increasingly fluorescence intensity measured for up to 20 h. (b) Average intensity values measured every hour across three different channel sections are plotted to illustrate the Dextran concentration profile obtained experimental at steady-state. Scale: 100 μm.

Figure 4

Images of the bulk flow of beads within a μLane device (a) in the absence of the bridge channel and (b) within the bridged μLane system. Images illustrate the motion of individual beads across a fixed, representative channel section. Scale: 100 μm.

Figure 5

Graph of normalized experimental data obtained from Dextran transport experiments at detection positions within the bridged μLane system, compared against normalized simulation data generated from the 1D convective diffusion equation. These normalized data were plotted as a function of time. A range of Dextran diffusion coefficients between 0.7×10−6 cm2/s and 1.0×10−6 cm2/s and the experimentally measured mean bulk velocity for Dextran of V=0.37 μm/s were used in the numerical simulation to generate the Dextran concentration profiles shown at different detection positions over time.

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