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Soft Lithography



Soft lithography is a set of rapid, simple, and inexpensive fabrication processes that have been successfully used to pattern the complex channels of microfluidic systems. In the electronics industry, lithography refers to the process of microfabrication using light and light-sensitive polymers to pattern parts of a thin film or the bulk of a substrate. The term soft lithography refers to the use of soft elastomeric materials like polydimethylsiloxane or PDMS to perform these techniques. In this video, we will illustrate the different types of soft lithography techniques followed by a protocol demonstrating the fabrication of a microfluidic device. Lastly, we will see how researchers in different fields are using soft lithography to their advantage.

First, let's review the most common soft lithography techniques. The first step in all these techniques is the fabrication of the master mold. This is done using traditional photolithography which uses light and a light-sensitive material called photoresist to create the desired pattern on a substrate like silicon. To learn about photolithography in detail, please see a previous video in this Jove collection. The second step is pouring an elastomer onto this master mold and then curing it. This creates the flexible elastomeric stamp bearing the relief features which is used in different ways in the various soft lithography techniques. The principle modes of transfer using the cast stamp are printing, molding, phase-shift optical lithography, mechanical sectioning, and casting. In printing, the stamp is first coated with a transferable ink like octadecanethiol or ODT which is then placed on the substrate, like gold. When the stamp is removed, just the ink from the raised stamp surface is imprinted on the substrate surface. Thus, printing directly replicates nano-scale features onto the substrate. In another technique called molding, the stamp itself is used as a mold. Here, the stamp is pressed into an uncured polymer and then cured. Then the mold is peeled off to reveal the pattern from the stamp. Like printing, molding also results in direct replication of nano-scale features onto the substrate. In the third technique, called phase shift-edge lithography, first, the substrate is coated with the photoresist material. Then the stamp is placed on the coated substrate and light is shown through the stamp. This results in the edges of the features being transferred to the film of photoresist as observed in traditional lithography techniques. In mechanical sectioning, A.K.A. nanoskiving, the stamp is used to mold uncured epoxy prepolymer, like in molding. This molded prepolymer is cured which is then coated with a thin film of a material of choice, for example, gold. This film is then embedded in more epoxy and cured after which it is sectioned using an ultramicrotome to form a slab of epoxy with the pattern. Finally, in casting, a polymer is poured into a master mold to make a stamp. It can then be punched for inlets and outlets and bonded to a substrate. In the following section, we review the protocol for fabricating a simple microfluidic device.

First, prepare the master mold using traditional lithography techniques. For protocol details refer to a previous video in this collection. The master mold is typically fabricated on a silicon substrate. To fabricate the stamp, first prepare a mixture of about 25 grams of PDMS and curing agent in the ratio of 10:1. Then degas the mixture to remove any air bubbles. Next, place the master mold in a flat-base container and pour the degassed PDMS mixture on it. Now bake the PDMS at 60 degrees Celsius for about one hour, followed by a natural cool down of the oven to room temperature for another hour. Next, cut the PDMS off the mold and place it in with the pattern side up to avoid contamination. Then cut out the individual patterns. Punch any inlets and outlets using a dermatological hole-punch of the right size to allow for fluid flow into and out of the device. Then place the PDMS device into an oxygen plasma cleaner and treat it for about one minute. Adhere the two layers of the device together and align the pattern under a microscope. Finally, bond the completed device to a substrate using PDMS and bake it to cure. Prior to use, test for any leaks by flowing water through the microfluidic channels.

Soft lithography has found application in fields ranging from molecular analysis to clinical diagnostics and drug development. Let's take a look at some of these examples. This technique can be used to create unconventional structures like flexible microposts for mechano-profiling of single cells. Mechano-profiling refers to the study of the mechanical parameters like forces applied by microorganisms on their environment. After the microposts are fabricated, cultured cells are allowed to grow on them. This results in the bending of the small flexible pillars, which can then be measured in order to calculate the forces exerted by the different types of cells. Multilayer, multifluidic systems can be used to study and understand the effects of different microenvironments on mammalian cells. These systems are fabricated by making each individual PDMS layer using different master molds. Then the various PDMS casts are cleaned, aligned, and layered on top of each other and baked. The multiple layers of the PDMS device allows for the efficient separation of the fluid from the cells via a semipermeable PDMS membrane. This setup allows researchers to study and characterize the effects of new microenvironments on the cells by enabling controlled amounts of fluids, like oxygen or a new media, to diffuse from the top test fluid layer to the mammalian cells at the bottom microfluidic channel.

You've just watched Jove's video on soft lithography. Here, we discussed the core techniques of soft lithography along with the detailed protocol of fabricating a PDMS microfluidic device as an example. Thanks for watching.

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