Microsystem Technology in Chemistry and Life Sciences

Abstract

This paper describes a procedure for making topologically complex three-dimensional microfluidic channel systems in poly(dimethylsiloxane) (PDMS). This procedure is called the "membrane sandwich" method to suggest the structure of the final system: a thin membrane having channel structures molded on each face (and with connections between the faces) sandwiched between two thicker, flat slabs that provide structural support. Two "masters" are fabricated by rapid prototyping using two-level photolithography and replica molding. They are aligned face to face, under pressure, with PDMS prepoly-mer between them. The PDMS is cured thermally. The masters have complementary alignment tracks, so registration is straightforward. The resulting, thin PDMS membrane can be transferred and sealed to another membrane or slab of PDMS by a sequence of steps in which the two masters are removed one at a time; these steps take place without distortion of the features. This method can fabricate a membrane containing a channel that crosses over and under itself, but does not intersect itself and, therefore, can be fabricated in the form of any knot. It follows that this method can generate topologically complex microfluidic systems; this capability is demonstrated by the fabrication of a "basketweave" structure. By filling the channels and removing the membrane, complex microstructures can be made. Stacking and sealing more than one membrane allows even more complicated geometries than are possible in one membrane. A square coiled channel that surrounds, but does not connect to, a straight channel illustrates this type of complexity. The complexity of microfluidic systems is increasing rapidly as sophisticated functionsschemical reactions and analyses, bioassays, high-throughput screens, and sensorssare being integrated into single microfluidic devices. 1-7 Complex systems of channels require more complex connectivity than can be generated in a single level, since single-level design does not allow two channels to cross without connecting. Most methods for fabricating microfluidic channels are based on photolithographic procedures and yield two-dimensional (2D) systems. 8 There are a number of more specialized proceduressstereolithography, 9 laser-chemical three-dimensional (3D) writing, 10 and modular assembly 11 sthat yield 3D structures, but these methods are not ideally convenient either for prototyping or manufacturing and are not capable of making certain types of structures. Better methods for generating complex 3D microfluidic systems would accelerate the development of microfluidic technology. This report presents a procedure we call the "membrane sandwich" method for fabricating, transferring, registering, and fusing multiple elastomeric membranes of poly(dimethylsiloxane) (PDMS) that contain one or more in-plane systems of microfluidic channels. This procedure provides a convenient route to complex micro-fluidic systems. Representation and Analysis of Systems of Microfluidic Channels as Knots. We can extend the analogy between a microfluidic nonbranching channel and mathematical non-self-intersecting, curved line; the capability of the membrane sandwich method to fabricate the physical realization of knots is evidence that it is a general route to making topologically complex channel systems. In mathematical terms, a knot is a closed, non-self-intersecting, curved line in three dimensions (a homeomorphic image of the unit circle). 12 Knots are typically described in (1) Effenhauser, C. S.; Bruin, G. J. M.; Paulus, A.; Ehrat, M.