High-aspect-ratio microstructures have been prepared using hot-embossing techniques in poly(methyl methacrylate) (PMMA) from Ni-based molding dies prepared using LIGA (Lithographie, Galvanoformung, Abformung). Due to the small amount of mask undercutting associated with X-ray lithography and the high energy X-ray beam used during photoresist patterning, deep structures with sharp and smooth sidewalls have been prepared. The Ni-electroforms produced devices with minimal replication errors using hot-embossing at a turn around time of approximately 5 min per device. In addition, several different polymers (with different glass transition temperatures) could be effectively molded with these Ni-electroforms and many devices (>300) molded with the same master without any noticeable degradation. The PMMA devices consisted of deep and narrow channels for insertion of a capillary for the automated electrokinetic loading of sample into the microfluidic device and also, a pair of optical fibers for shuttling laser light to the detection zone and collecting the resulting emission for fluorescence analysis. Electrophoretic separations of double-stranded DNA ladders Phi X174 digested with Hae III) were performed with fluorescence detection accomplished using near-IR excitation. It was found that the narrow width of the channels did not contribute significantly to electrophoretic zone broadening and the plate numbers generated in the extended length separation channel allowed sorting of the 271/281 base pair fragments associated with this sizing ladder when electrophoresed in methylcellulose entangled polymer solutions. The dual fiber detector produced sub-attomole detection limits with the entire detector, including laser source, electronics and photon transducer, situated in a single box measuring 3'' x 10" x 14".

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Planar micro-cross-flow heat exchangers, similar in concept to most automobile radiators, have been fabricated using two different processes. A process that was previously reported (Harris et al.[1]) to fabricate a polymer heat exchanger involved embossing two identical polymer parts using the LIGA process. Then the two parts were aligned and bonded together. In this paper, a process is described to fabricate a nickel micro-cross-flow heat exchanger by embossing a sacrificial polymer mandrel using a LIGAfabricated mold insert. The mandrel is coated with nickel (using either electroplating or electroless plating), then the sacrificial mandrel is dissolved.Experimental results are reported for both the polymer and nickel heat exchangers to determine the rates of heat transfer between the in-plane liquid (water) and the through-plane gas (air). Pressure drops of both fluid streams were also measured. The experimental results compare favorably with a modified version of the analytical model that was described previously [1]. The fabricated heat exchangers have values of heat transfer/volume that are more than five times higher than conventional scale counterparts (with characteristic dimensions at least one order of magnitude larger than those reported here) and values of heat transfer/mass that are 50% greater than their conventional scale counterparts.[777]

A high heat flux cooling system using an array of micro-jets is presented. The Micro-Jet Cooling Array (MJCA) is based on an array of small diameter micro-jets, with jet diameters as low as 300 microns. The return flow in MJCA is arranged so that the effect of neighboring jets is essentially eliminated and the high heat fluxes achievable in a single jet are reproduced over large areas. MJCA has been machined using the LIGA process and has been extensively tested. Extremely high heat fluxes have been achieved with MJCA, making is suitable for a variety of high heat flux applications.

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