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354 WHITESIDES ET AL APPLICATIONS OF SOFT LITHOGRAPHY IN CELL BIOLOGY Overview The & 8230; SOFT LITHOGRAPHY IN BIOLOGY 361 applicable to biological problems. And ( d ) the combination of laminar & 8230; Abstract Soft lithography, a set of techniques for microfabrication, is based on printing and molding using elastomeric stamps with the patterns of interest in bas- relief. As a technique for fabricating microstructures for biological applications, soft lithography overcomes many of the shortcomings of photolithography. In particular, soft lithography offers the ability to control the molecular structure of surfaces and to pattern the complex molecules relevant to biology, to fabricate channel structures ap- propriateformicrofluidics, and to pattern and manipulate cells. For the relatively large feature sizes used in biology (‚50„m), production of prototype patterns and structures is convenient, inexpensive, and rapid. Self-assembled monolayers ofalkanethiolates on gold are particularly easy to pattern by soft lithography, and they provide exquisite control over surface biochemistry.

We have developed a set of techniques called soft lithography that offers tools for micropatterning that complement and extend conventional fabrication methods (13). Two of the key features of soft lithography are the use of elastomeric (that is, mechanically soft) materials to fabricate the pattern transfer elements by molding, and the development of techniques that pattern complex biochemicals (14-18). Both features involve organic materials and polymers—& 8221;soft matter,& 8221;in the language of physics (19,20). This review describes the central techniques of soft lithography and their applications to biology. The first section introduces the major techniques of soft lithography and discusses their advantages. Understanding the principles behind soft lithography helps rationalize its applications to problems in biology. We focus on the application of soft lithography in the fabrication of microsystems useful in biology. The second section describes the components and systems for microflu- idicsthatcanbe fabricated using soft lithography. The third section discusses the extensive application of soft lithography to problems in cell biology and uses these applications to illustrate the characteristics of each of the variants of soft lithography. KEY FEATURES OF SOFT LITHOGRAPHY AND THEIR RELEVANCE TO APPLICATIONS IN BIOLOGY Surface Engineering As the size of devices decreases, their surface-to-volume ratios increase, and their surface properties become increasingly important in determining their performance. In microscopic systems, and especially those used in biology, it is often necessary to engineer the properties of the surfaces with molecular-level detail. Self-assembled monolayers (SAMs) of alkanethiolates on gold have become indispensable systems for modeling and, in some cases, controlling biologically relevant surfaces (Figure 1) (21,22). Soft lithography is well suited to pattern the composition, topography, and properties of surfaces (13). The combination of soft lithography andSAMshasproved invaluable in the formation of microsystems for use in biology. SELF-ASSEMBLED MONOLAYERS The surfaces of noble metals such as gold and silver react with organic thiol groups and form SAMs. The most ordered structures are formed from compounds with the structure HS(CH 2 ) n X, where n »16-18 and XD asmall, nonpolar, organic functional group (22-27). Selective reaction of the metal atoms on the surface with the thiol groups generates a dense array ofthiolate groups; the organic groups then order and reach densities approaching those ofa crystalline solid (Figure 1) (23,24,28). SAMsofalkanethiolates with more than 10 methylene (CH 2 ) units are essentially impermeable to water, and the properties of their surfaces are largely determined by the functional group X (22,29,30). SAMsalsofunction as etch resists in the fabrication of patterned structures (31,32).

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