Monolithic and flexible polyimide film microreactors for organic microchemical applications fabricated by laser ablation.

There is a growing interest in innovative chemical synthesis in microreactors owing to the potential for high efficiency, selectivity, and yield. In microfluidic systems, the lowvolume spatial and temporal control of reactants and products offers a novel method for chemical manipulation and product generation. Glass, silicon, poly(dimethylsiloxane) (PDMS), and poly(methylmethacrylate) (PMMA) have been used for the fabrication of miniaturized devices. Fabrication with glass or silicon substrates requires relatively complex processes, and the fabrication costs are high. Relatively cheap polymers such as PDMS or PMMA are not suitable for application in organic chemical processes owing to their low chemical stability and easy swelling. Therefore, there is a strong demand for economical organicsolvent-resistant materials that can be used for easy fabrication of microfluidic systems with reliable durability. Our group has recently reported novel polymer microreactors for organic syntheses, which involve two kinds of inorganic polymers that are manipulated in relatively simple microfabrication processes. The improved solvent resistance allowed good performance in organic reactions, except in chlorinated solvents such as dichloromethane. Nevertheless, the bonding step was somewhat tricky and had a low rate of success. Alternatively, a monolithic thin-film microreactor can be used for applications in organic synthesis, which allows integration of electrodes, heaters, light-emitting diodes, and various electronics. There are many fabrication techniques available for microfluidic devices. Of these, laser ablation is convenient to use for polymer chips. The whole microfabrication process could be completed in a few minutes instead of the days that are needed for wet photolithography processes. It is well known that polyimide (PI) offers advantages over other polymers, such as excellent chemical and thermal stability and low water uptake. Recently Barrett et al. reported fabrication of PI-based microfluidic devices by laser ablation. The devices were used for X-ray scattering experiments to yield a better spatial resolution for structural measurements. However, there has been no attempt to demonstrate the potential advantages of PI-based microreactors for chemical syntheses. Herein, we introduce a monolithic and flexible PI film microreactor for organic synthesis. Mixing units, such as a staggered herringbone pattern on the channel surface, can easily be built into the reactor during the fabrication. The film microreactors were readily fabricated within several tens of minutes by ablation with either a 193 nm excimer laser (ArF) or a 355 nm UV laser. The excellent stability of the fabricated microreactors was successfully demonstrated by performing five chemical reactions under various harsh conditions. The simple and economical laser fabrication process and the facile adhesive sealing step facilitate mass production of the flexible PI film microfluidic devices for various microchemical applications. Furthermore, these devices could provide a platform for integrating microfluidic and electronic components that are necessary for a micro total analysis system (m-TAS). As described in the literature, photoablation could be performed either dynamically by continuous moving and shooting of laser pulses to generate the channel or statically by shooting on specific spots assigned by the controlling program. As shown in Figure 1b, two types of microchannel designs, ArF and UV, were fabricated using the ArF excimer laser with a metal mask and a UV laser with a 25 mm diameter beam spot. Finally, 4 cm long ArF-type microchannels were ablated on the smooth PI film (ArF-1 type) or on a grooved surface with a staggered herringbone (SH) pattern (ArF-SH type). UV-type microchannels 53 cm long were directly ablated along the channel (UV-1 type) or across the channel (UV-2 type) in a repeated scanning mode, finally generating the line-grooved microchannels. The SH pattern was also grooved statically along the beginning part of the channel (UV-SH type). In general, it is reported that laser irradiation ablates by photochemical decomposition of chemical bonds in the polymer as well as by photothermal evaporation or melt expulsion. It is also known that the laser is strong enough to [*] K. I. Min, T. H. Lee, Dr. C. P. Park, Prof. D. P. Kim National Creative Research Center of Applied Microfluidic Chemistry, Chungnam National University Daejeon, 305-764 (South Korea) Fax: (+82)42-823-6665 E-mail: [email protected]