Purpose: Soft lithography using polydimethylsiloxane (PDMS) allows one to fabricate complex microfluidic devices that could be used in a variety of biomedical applications. While PDMS is an inexpensive material that can be readily molded into channels of desired geometries, they are chemically incompatible with a large number of organic solvents; they swell in the presence of many organic solvents significantly degrading the performance of the device. The sol-gel chemistry can be used to coat PDMS channels with a glass-like layer, which greatly increases chemical resistance of the channels (Abate, 2008) moreover, it can be functionalized with a wide range of chemicals to precisely control interfacial properties (Abate, 2008). This method combines the ease of fabrication afforded by soft-lithography with the precision control and chemical robustness afforded by glass, thus further diversifying the usage of PDMS in a variety of applications involving organic solvents.
Method of Fabrication/Use: The Lee group uses a sol–gel chemistry to coat the channels of PDMS microfluidic devices with a chemically resistant glass layer. A mixture of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) is oligomerized to form the precursors by catalyzing condensation and hydrolysis reactions of the alkoxy silanes. The resulting larger oligmer precursors reduce contraction and cracking during gelation, yielding more homogenous coatings. PDMS channels are treated with oxygen plasma to generate hydroxyl groups just before they are bonded to the bottom plate, which can either be a glass slide or another slab of PDMS. The bonded device is then immediately flushed with the oligomerized precursor mixture, and the gelation reaction is initiated by placing the device on a 100 °C hotplate. After 10 s, 10 mL of air is used to flush out the unreacted sol. The thickness of the coating can be varied by changing the time on the hotplate before the precursor mixture is flushed with the air. The chemical composition of the precursor mixture, preconversion time and temperature, and curing temperature are processing parameter that control sol–gel properties, enabling further control of the coating properties. In addition, photoinitiators can be directed grafted on these sol-gel coatings, enabling the control of the surface wettability and potentially the patterning of various biomolecules such as peptides, nucleotides and proteins.
Results: The Lee group has used the silica coating approach to coat PDMS channels with a broad range of geometries. The increased resistance of the channels to solvents were demonstrated as shown.. Beyond the increased resistance to solvent absorption, the silica coating
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Abate AR, Krummel AT, Lee D, Marquez M, Holtze C, Weitz DA. Photoreactive coating for high-contrast spatial patterning of microfluidic device wettability. Lab Chip Dec 2008; 8 (12): 2157-2160.
Abate AR, Lee D, Do T, Holtze C, Weitz DA. Glass coating for PDMS microfluidic channels by sol-gel methods. Lab Chip Apr 2008; 8 (4): 516-518.