Soft Lithography is a microfabrication process in which a soft polymer (such as polydimethylsiloxane (PDMS) ) is cast onto a mold that contains a microfabricated relief or engraved pattern. Using this technique, membrane microvalves can be produced. This membrane microvalve is the fundamental component which enables liquids to be controlled on-chip and is the key to realizing microfluidic large scale integration.

A basic microfluidic device is composed of two elastomer layers. One layer contains channels for flowing liquids (flow layer), and the other layer contains channels that deflect the membrane valve into the flow channel and stop liquid flow when pressurized with air/liquid (control layer). The following outlines the basic procedure used to fabricate these elegant membrane microvalves:

Creating the Molds

Molds containing the relief of the desired microfluidic circuit are made using conventional photolithography. This entails first designing your desired microfluidic network in a CAD program and printing it onto a transparency film using a very high resolution printer. Next, an appropriate photosensitive polymer (photoresist) is spun onto a silicon wafer and ultraviolet light is exposed to the wafer through the overlaying mask. Finally, the wafers are developed to reveal the transferred microfluidic network pattern on the silicon wafer. Note: one mold is made for the control layer and one mold is made for the flow layer.

Creating the Control Layer

The control layer is formed by pouring PDMS (liquid) onto the control mold, curing this layer until it cross-links, and peeling it from the mold. The surface of the polymer that was in contact with the mold is left with an imprint of the control mold topography.

Creating the Flow Layer

The flow layer is formed by spinning a thin layer of PDMS onto the flow mold and curing it until it cross-links.

Finishing the Device

The PDMS control layer is aligned and contacted with the PDMS covered flow mold. The PDMS control and flow layers are bonded to each other. The two-layer stack of PDMS is peeled from the flow mold and bonded to a substrate such as a glass slide and is ready for testing.

Process Overview

The following figure shows the basic fabrication process for this two-layer device (courtesy 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 Dr. Carl Hansen):

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.