We demonstrate protocols for manufacturing and automating elastomeric polydimethylsiloxane (PDMS)-based microvalve arrays that need no extra energy to close and feature photolithographically defined precise volumes. A parallel subnanoliter-volume mixer and an integrated microfluidic perfusion system are presented.
Miniaturized microfluidic systems provide simple and effective solutions for low-cost point-of-care diagnostics and high-throughput biomedical assays. Robust flow control and precise fluidic volumes are two critical requirements for these applications. We have developed microfluidic chips featuring elastomeric polydimethylsiloxane (PDMS) microvalve arrays that: 1) need no extra energy source to close the fluidic path, hence the loaded device is highly portable; and 2) allow for microfabricating deep (up to 1 mm) channels with vertical sidewalls and resulting in very precise features.
The PDMS microvalves-based devices consist of three layers: a fluidic layer containing fluidic paths and microchambers of various sizes, a control layer containing the microchannels necessary to actuate the fluidic path with microvalves, and a middle thin PDMS membrane that is bound to the control layer. Fluidic layer and control layers are made by replica molding of PDMS from SU-8 photoresist masters, and the thin PDMS membrane is made by spinning PDMS at specified heights. The control layer is bonded to the thin PDMS membrane after oxygen activation of both, and then assembled with the fluidic layer. The microvalves are closed at rest and can be opened by applying negative pressure (e.g., house vacuum). Microvalve closure and opening are automated via solenoid valves controlled by computer software.
Here, we demonstrate two microvalve-based microfluidic chips for two different applications. The first chip allows for storing and mixing precise sub-nanoliter volumes of aqueous solutions at various mixing ratios. The second chip allows for computer-controlled perfusion of microfluidic cell cultures.
The devices are easy to fabricate and simple to control. Due to the biocompatibility of PDMS, these microchips could have broad applications in miniaturized diagnostic assays as well as basic cell biology studies.
Microfluidic device design using CorelDraw or AutoCAD software
Principle of PDMS microvalves-based devices: The devices consist of three layers: a fluidic layer containing microchambers of various sizes, a “control layer” containing the microchannels necessary to actuate the fluidic path with microvalves, and a middle thin PDMS membrane that is bound to the control layer. At rest, due to the compliance and hydrophobicity of PDMS, the membrane seals (reversibly) against its seat, therefore the chambers remain isolated from each other without energy input. Valves can be opened by applying negative pressure (e.g., house vacuum), so the PDMS membrane deflects down and separates from the surface that supports the wall between two fluidic chambers, thus connecting the fluidic path. Valve closure can be achieved by switching the pressure setting from vacuum to atmospheric pressure.
Fluidic layer and control layer patterns were designed using CorelDraw or AutoCAD software. Masks containing these designs were printed at high resolution (8,000 to 20,000 dpi) on transparency films through commercial services (CAD/Art services, Bandon, OR) (masks not shown).
Fabrication of silicon masters using standard SU-8 photolithography
Replica molding of PDMS from the masters
Thin PDMS Membrane Manufacturing
Multilayer PDMS device bonding and assembly
Computer-controlled opening and closing of PDMS microvalves by vacuum or pressure
Parallel mixing of two different color dyes in different defined nanoliter volumes
We demonstrate the operation of a parallel mixer that allows for storing and mixing precise sub-nanoliter volumes of aqueous solutions at various mixing ratios:
An integrated microfluidic system for computer-controlled perfusion of microfluidic cell cultures
We demonstrate a microfluidic system that is capable of the automated perfusion of multiple solutions to a single cell culture chamber. The inlets are controlled by microvalves, which can be activated in any sequence of single inlets, various combinations, or all at once. The device is capable of producing gradients or mixtures of the various solutions.
This device also consists of three layers: a fluidic layer, a control layer, and a middle thin PDMS membrane.
Alternative fabrication steps for this device:
The features of our integrated microfluidic system: The device is capable of automated perfusion of 16 different solutions to a cell culture chamber using a multiplexed valving scheme. The channel design ensures that the resistance of all the inlets is balanced. Our microvalve design isolates solutions and controls rinsing through integrated channels for the rapid removal of fluid, which limits cross-contamination. An integrated herringbone mixer can be activated to produce mixtures of different inlets. Additionally, there are four varying resistance channels that can be activated to alter the flow rate.
Main advantages of our microvalve design:
Advantages of the parallel mixer:
Advantages of the integrated microfluidic perfusion chamber:
Main cautions for the fabrication processes:
This work was supported by the National Institute of Biomedical Imaging and Bioengineering grant #EB003307 and by the National Science Foundation Career Award to A.F.
Material Name | Type | Company | Catalogue Number | Comment |
---|---|---|---|---|
Clean silicon wafers | Supplies | Silicon Sense Inc. | 3P0110TEST | 3-inch diameter, P/Boron |
“Master” wafers containing SU-8 patterns | Supplies | Fabricated in house using standard photolithography procedures | ||
Desiccators (2) | Equipment | VWR | 24987-048 | One for silanization, one for PDMS de-bubbling. |
Balance | Equipment | OHAUS Corp. | SC6010 | |
Oven | Equipment | Sheldon Mfg. Inc. | 1330GM | |
MiniVortexer | Equipment | VWR | 58816-121 | |
Spinner | Equipment | Headway Research Inc. | PWM32 | |
Plasma etcher | Equipment | Plasmatic Systems Inc. | Plasma Preen II-973 | |
Hot Plate | Equipment | Torre Pines Scientific | HP30A | |
Stereoscope | Microscope | Nikon | TMZ1500 | |
CCD camera | Equipment | Diagnostic Instruments | SPOT RT | |
Solenoid valves | Equipment | Lee Company | LHDA0511111H | |
Data acquisition board | Hardware | National Instruments | PCI 6025E, CB-50LP | |
LabView | Software | National Instruments | Version 8.0 | |
Tridecafluoro-1,1,2,2,-tetrahydrooctyl)-1-trichlorosilane | Reagent | United Chemical Technologies | T2492 | Silanization must be done in a chemical fume hood. |
PDMS prepolymer and crosslinker | Reagent | Dow-Corning | Sylgard 184 | |
Hexane | Reagent | EMD | HX0295-6 | |
Color Dyes | Reagent | Spectrum Chemical Mfg. Corp. | FD&C 110, 135, 150 | Blue #1, Yellow #5, Red #3. |
3 ml disposable transfer pipets | Supplies | Fisher Scientific | 13-711-20 | |
Kimwipes | Supplies | Kimberly-Clark | 34155 | |
Weighing boats | Supplies | VWR | 12577-027 | |
Tongue depressor | Supplies | Fisher Scientific | 11-700-555 | |
P100 dishes | Supplies | Fisher Scientific | 08-772E | |
Silicone tubing (1.14 mm inner diameter (I.D.)) | Supplies | Cole-Palmer Instrument Co. | 07625-30 | |
Tygon tubing (O.D. 1/16 in; I.D. 1/32 in) | Supplies | Cole-Palmer Instrument Co. | 06418-02 | |
Duco Cement | Supplies | Devcon | 6245 | |
Razor blade | Tools | VWR | 55411-050 | |
Needles | Tools | Fisher Scientific | 0053482 (25 Gauge) | |
#5 Forceps | Tools | Fine Science Tools | 11251-20 | |
50 ml centrifuge tube | Supplies | Fisher Scientific | 05-526B | |
Seal wrap film | Supplies | AEP Industries Inc. | 0153877 | |
1.5 ml microcentrifuge tubes | Supplies | Fisher Scientific | 05-406-16 | |
15 ml centrifuge tubes | Supplies | BD Falcon | 352097 | |
Purple nitrile power-free gloves | Supplies | VWR | 40101-348 | |
1.2 mm Harris biopsy punch | Tools | Ted Pella, Inc. | 15074 |