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scienceapr00 - REPORTS Monolithic Microfabricated Valves...

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Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography Marc A. Unger, Hou-Pu Chou, Todd Thorsen, Axel Scherer, Stephen R. Quake* Soft lithography is an alternative to silicon-based micromachining that uses replica molding of nontraditional elastomeric materials to fabricate stamps and microfluidic channels. We describe here an extension to the soft lithography paradigm, multilayer soft lithography, with which devices consisting of multiple layers may be fabricated from soft materials. We used this technique to build active microfluidic systems containing on-off valves, switching valves, and pumps entirely out of elastomer. The softness of these materials allows the device areas to be reduced by more than two orders of magnitude compared with silicon-based devices. The other advantages of soft lithography, such as rapid prototyping, ease of fabrication, and biocompatibility, are retained. The application of micromachining tech- niques is growing rapidly, driven by the dra- matic success of a few key applications such as microfabricated accelerometers ( 1 , 2 ), pres- sure sensors ( 3 ), and ink-jet print heads ( 4 ). New applications are appearing in other fields, in particular fiber optic communica- tions ( 5 , 6 ), displays ( 7 ), and microfluidics ( 8–12 ). The two most widespread methods for the production of microelectromechanical structures (MEMS) are bulk micromachining and surface micromachining. Bulk microma- chining is a subtractive fabrication method whereby single-crystal silicon is lithographi- cally patterned and then etched to form three- dimensional (3D) structures. Surface micro- machining, in contrast, is an additive method where layers of semiconductor-type materials (polysilicon, metals, silicon nitride, silicon dioxide, and so forth) are sequentially added and patterned to make 3D structures. Bulk and surface micromachining meth- ods are limited by the materials used. The semiconductor-type materials typically used in bulk and surface micromachining are stiff materials with Young’s modulus ; 100 GPa ( 13 ). Because the forces generated by micro- machined actuators are limited, the stiffness of the materials limits the minimum size of many devices. Furthermore, because multiple layers must be built up to make active devic- es, adhesion between layers is a problem of great practical concern. For bulk micromach- ining, wafer-bonding techniques must be used to create multilayer structures. For sur- face micromachining, thermal stress between layers limits the total device thickness to ; 20 m m. Clean-room fabrication and careful con- trol of process conditions are required to realize acceptable device yields. An alternative microfabrication technique based on replication molding is gaining pop- ularity. Typically, an elastomer is patterned by curing on a micromachined mold. Loosely termed soft lithography, this technique has been used to make blazed grating optics ( 14 ), stamps for chemical patterning ( 15 ), and mi- crofluidic devices ( 16–20 ). Soft lithogra- phy’s advantages include the capacity for
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scienceapr00 - REPORTS Monolithic Microfabricated Valves...

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