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 Gas
Permeation Microump
Students:
Mark Eddings
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Abstract
A PDMS diffusion-based membrane pump has been developed for on-chip fluid
handling within microfluidic devices. Vacuum and air pressure applied across a
thin PDMS membrane were used to bi-directionally manipulate fluid at flow rates
of 10-200 nl·min-1. Flow rates can be altered by changing diffusion
area, membrane thickness, and applied pressure or vacuum. The pump has been used
to rapidly fill and evacuate dead-end channels and chambers. Desired volume
plugs are easily isolated and manipulated. The pump can be integrated within
densely arrayed microfluidic channels and chambers for mixing, separation, and
analysis.
Experimental
In order to fabricate the diffusion membrane pump, previously reported
multi-layer soft lithography methods were utilized [1-2]. Devices consisted of a
microfluidic channel layer, a thin membrane layer, and the pressure/vacuum
control layer (See Figure 1a). Tests were run varying parameters such as
diffusion area, membrane thickness, and applied pressure/vacuum to determine the
change in flow rate. Separate devices were fabricated to demonstrate fluid
handling and dead-end channel filling (See Figure 1b). A description of the
permeation mechanism is shown in Figure 2.
Figure 1.
a) Drawing of a microfluidic device used for measuring flow rates.
The three-layer PDMS device consists of a fluid channel layer,
diffusion membrane, and vacuum source layer. b) Drawing of a
microfluidic device used for demonstrating dead-end chamber filling.
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 Results
and Discussion
The flow rate varied linearly with diffusion area and pressure/vacuum
(Figure 2). Flow rates from 10-200 nl·min-1 were achieved with the
fabricated device. Higher flow rates are theoretically possible assuming a 2-D
Steady-State diffusion model, neglecting flow into the bulk regions of the PDMS.
Membrane deflection was also modeled and was found to increase the diffusion
area by expanding the membrane and decrease the diffusion area by obstruction on
the channel ceiling.
Fluid was easily manipulated through turns in cross
intersections and to fill dead-end channels and chambers (Figure 3). Individual
chambers were filled with different colored fluids without mixing. Rapid fluid
plug dispensing and fluid mixing was possible by toggling chambers between
pressure and vacuum.
Figure 2.
A comparison of theoretical
and experimental flow rate results as for different pressures and
diffusion areas. The
deflection membrane had a thickness of 0.025 mm.
Data points represent experimental data while theoretical
results are denoted by lines.
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 Conclusion
The PDMS diffusion pump is a feasible option for complex lab-on-a-chip
applications. The micropump is easily fabricated using existing methods and can
be highly arrayed for multi-channel configurations. Rapid dead-end channel
filling and flow rates in the 200 nl·min-1 range have been demonstrated. It is
also possible to generate slow flow rates below 10 nl·min-1. Future
work will focus on prolonged, slow flow rates for drug delivery applications.
References
[1] D. Duffy, J. McDonald, O. Schueller, G. Whitesides, Rapid Prototyping of
Microfluidic Systems in Polydimethylsiloxane, Anal. Chem. 70, pp.
4974-4984 (1998)
[2] M. Unger, H. Chou, T. Thorsen, A. Scherer, S. Quake, Monolithic
Microfabricated Valves and Pumps by Multilayer Soft Lithography, Science,
Vol. 288, pp. 113-116 (2000)
[3] M. A. Eddings, B. K. Gale,
A PDMS-based
Gas Permeation Pump for On-chip Fluid Handling in Microfluidic Devices,
J. Micromech. Microeng. 16 (2006)
Figure 3.
Pictures
of three different fluids, red, green, and blue, filling dead-end
chambers (A-F). Pressure
differentials across the membrane below the chambers enabled the
flow to make turns at the intersection.
Each fluid was placed in the open well and pumped to each
individual chamber. The fluid
in each chamber can be individually returned to the well or can mix
with the other wells at the intersection by toggling between
pressure and vacuum below the membrane at each chamber.
This technique also enables the isolation of fluid plugs for
dispensing and metering similar to an electroosmotic flow setup.
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State of Utah Center of Excellence for Biomedical Microfluidics 
