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.

We have developed an integrated microfabricated cell sorter using multilayer soft lithography. This integrated cell sorter is incorporated with various microfluidic functionalities, including peristaltic pumps, dampers, switch valves, and input and output wells, to perform cell sorting in a coordinated and automated fashion. The active volume of an actuated valve on this integrated cell sorter can be as small as 1 pL, and the volume of optical interrogation is approximately 100 fL. Different algorithms of cell manipulation, including cell trapping, were implemented in these devices. We have also demonstrated sorting and recovery of Escherichia coli cells on the chip.

We have demonstrated a microfabricated single-molecule DNA sizing device. This device does not depend on mobility to measure molecule size, is 100 times faster than pulsed-field gel electrophoresis, and has a resolution that improves with increasing DNA length. It also requires a million times less sample than pulsed-field gel electrophoresis and has comparable resolution for large molecules. Here we describe the fabrication and use of the single-molecule DNA sizing device for sizing and sorting DNA restriction digests and ladders spanning 2-200 kbp.Many assays in biology require measurement of the length distribution of DNA molecules in a heterogeneous solution. This measurement is commonly done with gel electrophoresis; the molecules are separated by mobility, from which the lengths are inferred. This method is powerful, yet has some drawbacks. For medium to large DNA molecules the resolution is limited to approximately 10%. Gel electrophoresis is time consuming. It generally takes at least an hour to run the gel, not including the setup time to cast the gel. Furthermore, for large molecules the procedure fails. This problem has been alleviated to some extent by the development of pulsed-field gel electrophoresis (1), but running times can be days.With the development of high affinity intercalating DNA stains (2), it has become possible to directly measure the length of single molecules by quantitating fluorescence. The amount of intercalated dye is proportional to the length of the molecule, so measuring the total fluorescent intensity from a single molecule gives a direct measurement of its length. This method in principle allows the measurement of extremely long DNA molecules because the signal increases with the length of the molecule. This technique has been used with traditional methods of flow cytometry to measure length distributions of DNA molecules (3, 4). Other groups have imaged restriction enzymes digesting extended single DNA molecules for ''optical mapping'' (5, 6).We have developed microfabricated devices to size and sort microscopic objects based on measurement of fluorescent properties. The devices have a network of microfluidic channels and are fabricated from a silicone elastomer by using a replica technique (7). Master molds are made from silicon wafers by using standard micromachining techniques. Because the molds can be reused indefinitely, this method of fabrication allows economical mass production of the devices. The devices were patterned as shown in Fig. 1. This fabrication technique is one of a new set of technologies known as soft lithography. Previous work has demonstrated that elastomers can replicate gratings and other test patterns with high (Ϸ50 nm) resolution and fidelity (8, 9). Although some groups have made large (Ϸ30 m) elastomer structures for capillary electrophoresis (10), there is only one other example of a micron scale fluidic network with the elastomer (11). MATERIALS AND METHODSDevice Fabrication. Negative master devices were fabricated in silicon and use...

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Polarizing beam splitters that use the anisotropic spectral reflectivity (ASR) characteristic of high-spatialfrequency multilayer binary gratings have been designed, fabricated, and characterized. Using the ASR effect with rigorous coupled-wave analysis, we design an optical element that is transparent for TM polarization and reflective for TE polarization at an arbitrary incidence angle and operational wavelength. The experiments with the fabricated element demonstrate a high efficiency (Ͼ97%), with polarization extinction ratios higher than 220:1 at a wavelength of 1.523 m over a 20°angular bandwidth by means of the ASR characteristics of the device. These ASR devices combine many useful characteristics, such as compactness, low insertion loss, high efficiency, and broad angular and spectral bandwidth operations.