Microfluidic devices can be used to visualize complex natural processes in real time and at the appropriate physical scales. We have developed a simple microfluidic device that mimics key features of natural porous media for studying growth and transport of bacteria in the subsurface.
I. Microfluidic Device Fabrication
II. Flow Quantification of Microfluidic Device by Gravimetric Analysis
III. Flow Visualization, Velocity Mapping, and Hydrophilic / Hydrophobic Interaction
IV. Microfluidic Device (EcoChip) for Modeling Bacterial Growth and Transport
V. Representative Results
The flow module provides simple means for regulating and distributing flows through multiple habitats. Gravimetric analysis proved to be an easy and straightforward way to determine the flow rates through the habitat structures, which depend on the hydraulic resistance of the habitats and the pressure differences between input and the output wells. For the bead flow experiments we have observed much larger bead accumulation at the device surfaces for the uncarboxylated beads (Figure 2). Additionally, larger diameter beads, 6 and 10 mm, were much more likely to become entrained in the smaller pore openings and begin to accumulate in the device. Faster flow rates reduced particle retention and entrainment.
During the bacteria growth experiments, the influence of flow conditions is clearly evident. Continuous shearing forces cause bacteria to aggregate together and form flocs, and not to be found as individual cells. Transport of large bacterial flocs is an important environmental process which is extremely difficult to study in a macroscopic system.
Figure 1. Schematic showing features and operation of flow module.
Figure 2. Flow rate through the structured habitat as determined by gravimetric analysis for different column heights. Column height ratio: 60 / 40 = 1.5, determined flow rate ratio: 1.04 / 0.71 = 1.46. Resulted flow rates for H = 40 mm is V = 0.71 μL/min and for H = 60 mm is V = 1.04 μ L/min. Estimated average linear velocities are 3.1 mm/s and 4.6 mm/s respectively.
Figure 3. Transport of 3um latex beads with and without carboxylation flowing through the structured habitats.
Figure 4. Change in bacterial growth and biofilm formation as a function of seed density in presence of a slow flow.
The EcoChip system is adaptable to the needs of an individual experiment. New masters can be created relatively easily, and once a master is fabricated, additional exactly replicated devices can be cast as needed. The flow module is simple to use, requires no special equipment or complex connections, and can be modeled as a simple falling head pressure-driven flow system. Additional extensions to this work are ongoing, and include creating humic acid coated channels, and systematically varying the aqueous chemistry of the flowing fluid. Using this approach, the micro-scale interactions of bacteria with surfaces and growth and transport phenomena in porous media can be observed directly and systematically investigated.
This study was supported by grant # 0649883 from the National Science Foundation, by the Vanderbilt Institute for Integrative Biosystems Research and Education (VIIBRE), and by the Searle Systems Biology and Bioengineering Undergraduate Research Experience (Searle SyBBURE).
Material Name | Type | Company | Catalogue Number | Comment |
---|---|---|---|---|
PDMS | Dow Corning | |||
SU8-2025 | MicroChem Corp. | |||
Fluorescent Beads | Polysciences, Inc. |