We have developed a method for rapid prototyping of hard polymer microfluidic systems using solvent imprinting and bonding. We investigated the applicability of patterned SU-8 photoresist on glass as an easily fabricated template for solvent imprinting. Poly(methyl methacrylate) (PMMA) exposed to acetonitrile for 2 min then had an SU-8 template pressed into the surface for 10 min, which provided appropriately imprinted channels and a suitable surface for bonding. After a PMMA cover plate had also been exposed to acetonitrile for 2 min, the imprinted and top PMMA pieces could be bonded together at room temperature with appropriate pressure. The total fabrication time was less than 15 min. Under the optimized fabrication conditions, nearly 30 PMMA chips could be replicated using a single patterned SU-8 master with high chip-to-chip reproducibility. Relative standard deviations were 2.3% and 5.4% for the widths and depths of the replicated channels, respectively. Fluorescently labeled amino acid and peptide mixtures were baseline separated using these PMMA microchips in <15s. Theoretical plate numbers in excess of 5000 were obtained for a approximately 3 cm separation distance, and the migration time relative standard deviation for an amino acid peak was 1.5% for intra-day and 2.2% for inter-day analysis. This new solvent imprinting and bonding approach significantly simplifies the process for fabricating microfluidic structures in hard polymers such as PMMA.
Over the past fifteen years, research in the field of microfluidics has experienced rapid growth due to significant potential advantages such as low cost, short analysis times and elimination of sources of contamination. Whereas etched and thermally bonded glass substrates have seen widespread use and offer solid performance, device fabrication remains cumbersome. Recent advances in sacrificial layer microfabrication methods for microfluidics should overcome many disadvantages of conventional fabrication approaches. Phase-changing sacrificial layers have been implemented in making inexpensive and high-performance polymer microchips for electrophoretic analysis, protein focusing and sample preconcentration. In addition, novel channel fabrication methods based on standard thin-film processes, which are readily integratable with microfabrication techniques used for electrical components, are being applied increasingly for the creation of microfluidic devices. These new sacrificial layer fabrication approaches will be instrumental in making low-cost and highquality polymer microchips, and in interfacing electrical and fluidic systems on glass or semiconductor substrates.
Hollow tubular microfluidic channels were fabricated on quartz substrates using sacrificial layer, planar micromachining processes. The channels were created using a bottom-up fabrication technique, namely patterning a photoresist/aluminum sacrificial layer and depositing SiO(2) over the substrate. The photoresist/aluminum layer was removed by etching first with HCl/HNO(3), followed by etching in Nano-Strip, a more stable form of piranha (H(2)SO(4)/H(2)O(2)) stripper. Rapid separation of fluorescently labeled amino acids was performed on a device made with these channels. The fabrication process presented here provides unique control over channel composition and geometry. Future work should allow the fabrication of highly complex and precise devices with integrated analytical capabilities essential for the development of micro-total analysis systems.
Electro-osmotic flow (EOF) pumps are attractive for fluid manipulation in microfluidic channels. Open channel EOF pumps can produce high pressures and flow rates, and are relatively easy to fabricate on-chip or integrate with other microfluidic or electrical components. An EOF pump design that is conducive to on-chip fabrication consists of multiple small channel arms feeding into a larger flow channel. We have fabricated this type of pump design using a thin film deposition process that avoids wafer bonding. We have evaluated pumps fabricated on both silicon and glass substrates. Consistent flow rate versus electric field were obtained. For the range of 40-400 V, flow rates of 0.19-2.30 muLmin were measured. Theoretical calculations of pump efficiency were made, as well as calculations of the mechanical power generated by various pump shapes, to investigate design parameters that should improve future pumps.
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