Whitesides Conspectus ar-2009-00178k.R2 edited and approved
In this Account, we describe a strategy for fabricating multicomponent microsystems in which the structures of essentially all of the components are formed in a single step of micromolding. This strategy—which we call “co-fabrication”—is an alternative to multilayer microfabrication, in which multiple layers of components are sequentially aligned (“registered”) and deposited on a substrate by photolithography.
Co-fabrication has several characteristics that make it a particularly useful approach for building multicomponent microsystems. It rapidly and inexpensively generates correctly aligned components (for example, wires, heaters, magnetic field generators, optical waveguides, and microfluidic channels)—and over very large surface areas. By avoiding registration, the technique does not impose the size limitations of common registration tools, such as steppers and contact aligners, on substrates. We have demonstrated multicomponent microsystems with surface areas exceeding 100 cm2, but in principle device size is only limited by the requirements of generating the original master.
Co-fabrication can also serve as a low-cost and minimal-equipment strategy for building microsystems. The technique is amenable to a variety of laboratory settings and uses fabrication tools that are less expensive than those used for multistep microfabrication. The process also requires only small amounts of solvent and photoresist—a costly chemical required for photolithography. In co-fabrication, photoresist is applied and developed only once to produce a master, which is then used to produce multiple copies of molds containing the microfluidic channels.
Co-fabrication represents a new processing paradigm in which the exterior (or shell) of the desired structures are produced before the interior (or core). This approach—generating the insulation or packaging structure first, and injecting materials that provide function in channels in liquid phase—makes it possible to design and build microsystems with component materials that cannot be easily manipulated conventionally (such as solid materials with low melting points, liquid metals, liquid crystals, fused salts, foams, emulsions, gases, polymers, biomaterials, and fragile organics). Moreover, materials can be altered, removed, or replaced after the manufacturing stage. For example, co-fabrication allows one to build devices in which a liquid flows through the device during use (or is replaced before use). Metal wires can be melted and re-set by heating (in principle, repairing a break). This method leads to certain kinds of structures—such as integrated metallic wires with large cross-sectional areas, or optical waveguides aligned in the same plane as microfluidic channels—that would be difficult or impossible to make with techniques such as sputter deposition or evaporation.
This Account outlines the strategy of co-fabrication, describing several co-fabricated microsystems that combine microfluidics with (i) electrical wires for microheaters, electromagnets, and organic electrodes; (ii) fluidic optical components, such as optical waveguides, lenses, and light sources; (iii) gels for biological cell cultures; and (iv) droplets for compartmentalized chemical reactions, such as protein crystallization.