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 scientific approach signaling pathways modeling microsystems microsystems in-depth soft lithography measurement scientists  |
Microsystems In-Depth: Soft LithographyHow devices are made using soft lithography
IntroductionThe development of micromachining techniques for the semiconductor industry has in turn made it possible to easily construct microsystems to enable new techniques for biology. The immense flexibility in designing channel layouts as well as other advances has made it possible to quickly fabricate nearly any two-dimensional channel geometry using photolithography. Three dimensional shapes require more difficult multilevel bonding, and remain the greatest limitation of microfabrication techniques. Until recently, microfluidic channels were made from etched silicon wafers. Duffy et al. demonstrated the use of polydimethylsiloxane (PDMS), a silicone elastomer as an alternative substrate for creating microfluidic channels. PDMS oligomer is a viscous, oily liquid that has very low surface tension. Because of its low surface tension, it can flow into the micron-sized features of a master mold and be polymerized to become a tough, transparent rubber. When treated with oxygen plasma this rubber will bind irreversibly to glass surfaces, thus enabling rapid fabrication of microfluidic devices. PDMS channels are compatible with most solvent systems and buffers that are used in biology. The surface lends itself to modification, either via silane functionality or with a type II photoinitiator. Such modifications can modify cell and protein adhesion, or incorporate other polymers to further enhance device performance. Fabrication ProcedureA summary of a typical PDMS fabrication procedure is shown below:
In the panels above, liquid SU-8 (a photocurable epoxy) is poured over a spinning silicon wafer (1), leaving a controllably thin layer of liquid monomer. Exposing this layer to ultraviolet light through a high-resolution transparency mask (2) and baking the wafer (3) to crosslink the SU-8 forms a permanent mold master. This master wafer can be used indefinitely to repeat the subsequent steps. Liquid PDMS is poured over the master mold and polymerized at 70 Celsius. Because PDMS is soft and can deform during device operation, a glass platform may be optionally used to reinforce the device. In this case, during curing two magnets (7) are used to hold the glass in place as a second layer of PDMS is cast (8-9) over the platform. To remove the molded PDMS, a scalpel is used to manually cut out the device from the master (10). Mold release agents or solvents such as ethanol can be used to reduce adhesion during this step. Once cut out, fluidic connections are punched through the rubber, and the devices as well as glass substrates are exposed to oxygen plasma to activate their surfaces (11). The PDMS and glass substrate are brought together to form a permanent bond, sealing the device (12). Once the device is sealed, it may be used as is, or the surface can be modified at will. Techniques such as soft lithography coupled with surface chemistry allow researchers to rapidly iterate and optimize device designs to explore the complexities of systems biology. References
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