Rapid Fabrication of Microfluidic Devices by Photocopying

An Irish collaborative group has described a very simple and rapid method for the fabrication of microfluidic devices based on the material poly-dimethylsiloxane (PDMS). The fabrication process is greatly simplified and accelerated by using a standard photocopying machine to make a master on a transparency instead of using lithographic equipment. The fabrication is said to require <1.5 hr from design to device production.

This work has been performed by Professor Jeremy Glennon and colleagues at the Microseparations Laboratory, Department of Chemistry, National University of Ireland (Cork, Ireland), in collaboration with Kenneth Rogers et al. of the National Microelectronics Research Centre (Lee Maltings, Cork, Ireland). The workers say that, using SEM characterization, any microchannel artwork with a width of >50 µm and depth of 8-14 µm can be made by using this method.

In making a conventional PDMS device, a master pattern is first fabricated and a PDMS pre-polymer mixture is cast over the master and cured. A replica in PDMS can be made quickly, but the master fabrication is relatively time-consuming because it requires a photoresist, mask aligner equipment and silicon etching facilities that are not widely available. According to the research group, the use of a photocopying machine minimizes the need for special equipment, thus greatly decreasing the total fabrication time and enabling the rapid fabrication of a master during the early stages of testing new concepts in microfluidic systems.

To fabricate the devices, the designs of the networks of microfluidic channels were generated by a computer. The pattern was printed and transferred onto a transparency to enable a master to be made with a photocopying machine. A 10:1 mixture of the PDMS pre-polymer and curing agent was cast over the master with a frame for holding the solution. It was cured for 1 hr at 65°C.

The cured PDMS replica was peeled from the master and holes were punched in this PDMS replica for reservoirs. The PDMS was sealed to a Pyrex glass plate by adhesion to make a microfluidic device with channel dimensions of 55 mm in length, 200 µm in width and 12 µm in depth. Two platinum electrodes were micromachined on the glass plate by photolithography and rf plasma sputtering before wires were glued to these electrodes.

	SEM image of the cross section of the channel on PDMS. (Source: National University of Ireland)

The microchannel width in the PDMS replica depends on the width in the design and the reduction or enlargement ratio in photocopying. A routine desktop printer — rather than a precision printer — with a 4× reduction during photocopying was found to readily give any channel width of >50 µm. The master is based on carbon particles deposited on a transparency, so the walls of the molded polymer are not as smooth as those from processes using photoresist or silicon-based masters. However, finer carbon particles could be used to reduce the channel roughness. Devices of this type are often used in the electro-osmotic or electrophoretic mode, where channel roughness does not significantly degrade performance. As one can quickly move from a design to a replica, this technique is very useful for the rapid testing and selection of new concepts in microfluidics.

The researchers say that PDMS is a very attractive material for microfluidic devices and has been widely used in several microfabrication or nanofabrication methods collectively known as soft lithography. It has some very desirable physical and chemical properties for use in this application. It provides high optical quality, including transparency above 230 nm, high electrical bulk resistivity, good chemical stability, support for electro-osmotic flow after plasma oxidation and good adhesion to various substrates. A preliminary test has proved the applicability of the device in the analytical field. This material has been used to fabricate microchip electrophoresis systems using laser-induced fluorescence and amperometric detection. Other uses include the fabrication of microfluidic devices for handling picoliter volumes of liquids, sample delivery in mass spectrometry and even the production of complex three-dimensional microfluidic systems.

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