Topographical Micropatterning of Poly(dimethylsiloxane) using Laminar Flows of Liquids in Capillaries

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Original entry: Naveen Sinha, APPHY 226, Spring 2009

Topographical Micropatterning of Poly(dimethylsiloxane) using Laminar Flows of Liquids in Capillaries

Shuichi Takayama, Emauele Ostuni, Xiangping Qian, J. Cooper McDonald, Xingyu Jiang, Phil LeDuc, Ming-Hsien Wu, Donald E. Ingber, and George M. Whitesides

Advanced Materials, vol. 13, no. 8 (April 18 2001).

Keywords: Lamilar, microfluidic

Summary

Microfluidics are an ideal setup for studying biological systems due to the similar length scale of the liquid channels and the living cells. To increase the possible range of experiments, it is useful to be create more complicated topographies within a single channel. The preferred substrate material is Poly(dimehtylsiloxane) or PDMS, since it is transparent, non-toxic, and permeable to gases. It is easy to create channels in the PDBS using a chemical etchant. However, it is somewhat more difficult to find a solvent (tetrabutylammonium fluoride or TABF) that can etch away the products of the etching process without causing swelling in the PDMS. The authors found that N-Mehtylpyrrolidinone (NMP) works quite well. The operation of the setup is illustrated in the figure below:

TakayamaFig1.jpg

Two main principles from fluid mechanics are used to create various channel cross-sections. The first is operating in the region of laminar flow, in which the Reynolds number is so low that fluids introduced through adjacent inlets mix by only a negligible amount downstream. The second is the Hagen-Poiseuille law, which describes how the volumetric flow rate is inversely proportional to the length of a channel, for a given pressure drop. This way, by changing the length of the inlet channels, the rate of etching at different places along the cross-section can be controlled. Additional features can be created by setting up posts within the stream. Variations on the basic experimental setup are shown in the figure below:

TakayamaFig2.jpg


Soft matter aspects

One question about this process is whether it chemically alters the surface of the PDMS. The researchers found that the advancing contact angle remains relatively constant before and after (109 vs. 107, respectively), but the receding contact angle becomes much smaller (98 vs. 87 respectively). This observation still lacks an explanation. However, the authors were still able to coat the inner surface with proteins and grow mammalian cells. They demonstrated the possibility of fluorescent tagging, which opens up a new world of biological experiments. One topic of research specifically related to soft matter would be bio-interfaces, i.e. where cells and other surfaces comes into contact in an aqueous environment.

written by: Naveen Sinha