Soft Lithography is a microfabrication process on which a soft polymer, such as poly(dimethylsiloxane) or other elastomers, is cast on a mold that contains a microfabricated relief or engraved pattern.

The un-crosslinked liquid polymer is poured over the mold and allowed to cure until it is crosslinked. After crosslinking, the polymer is peeled off the mold, and the surface of the polymer that was in contact with the mold is left with an imprint of the mold topography. Such topography typically defines channels and chambers that will form part of a microfluidic system. Several layers of elastomer, all of them with different patterns, can be stacked and bonded together to form a complete microfluidic device, thus the name "Multilayer Soft Lithography."

The molds used for casting the polymer are usually made of plain silicon wafers on which a photoresist pattern has been created using a conventional photolithographic process (identical to the one used for the manufacture of integrated circuits). The masks that define the patterns are made of transparencies on which the pattern is printed using a commercial laser printer with 20,000dpi resolution.

A basic microfluidic device is composed of two layers elastomer layers. One layer contains channels for flowing liquids (flow layer), and the other layer contains channels that when pressurized with air or nitrogen serve as valves and pumps for the flow channels (control layer). The following figure shows the basic fabrication process for a two-layer device (courtesy of Dr. Carl Hansen):

When a control channel and a flow channel cross, if the area of the intersection is large enough, a valve is created. The thin membrane separating the two channels deflects into the flow channel when the control channel is pressurized, creating a complete seal. The following picture shows a typical valve in the closed state (courtesy of Dr. Carl Hansen):

A peristaltic pump is created by at least three consecutive valves that are actuated in sequence, as shown in the following figure (courtesy of Dr. Carl Hansen):

More detailed information about Multilayer Soft Lithography and Soft Lithography can be found in the following references:

Marc A. Unger, Hou-Pu Chou, Todd Thorsen, Axel Scherer, and Stephen Quake, Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography, Science, vol. 288, no. 7, pp. 113-116, April 2000.

David C. Duffy, J. Cooper McDonald, Olivier J.A. Schueller, and George Whitesides, Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane), Analytical Chemistry, vol. 70, no. 23, pp. 4974-4984, December 1998.