Microfluidic Channel Fabrication in Dry Film Resist for Droplet Generation
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Microfluidic Channel Fabrication in Dry Film Resist for Droplet Generation Patrick W. Leech1, Nan Wu2 and Yonggang Zhu2 1 CSIRO Materials Science and Engineering, Clayton, 3168, Victoria, Australia 2 CSIRO Materials Science and Engineering, Highett, 3190, Victoria, Australia
ABSTRACT Dry film resist has been used in the fabrication of Masters in microfluidic devices for droplet generation. The minimum feature size in the resist was controlled by the type of mask (transparency or electron beam Cr mask), the resolution of the pattern in transparency masks (2400 or 5080 dpi) and thickness of resist in the range from 35 to 140 µm. The Master patterns formed in dry resist were replicated as a Ni shim and then hot embossed into Plexiglas 99524. These devices were used to generate water-in-oil droplets with a well defined dependence of diameter and frequency on flow parameters. The application of dry laminar resist and transparency masks has allowed the rapid fabrication of prototype devices.
INTRODUCTION Microfluidics systems have in recent years been used as a tool for the generation of microdroplets [1]. In particular, the incorporation of a flow focusing orifice in microfluidics systems has provided a means of forming a uniform, continuous stream of droplets of a specified size [1]. The function of the orifice has been to create an instability in flow within two immiscible fluids by hydrodynamic focusing. The selection of the channel widths and design of the flow focusing nozzle in these structures has allowed the control of droplet size within a narrow range (~3%) at frequencies of >1 kHz [1,2]. A promising application of such highly uniform droplets has been in the synthesis of solid microparticles containing a controlled structure of layers [3]. Droplets with an outer shell structure of this type have also been used to enclose a suspension of micro- or nanoparticles [4]. Furthermore, the incorporation of minute quantities of DNA, protein or biological cells in microdroplets has provided a large number of separate reactor compartments for experimentation. These characteristics have recently been applied by Tawfig and Griffiths at Cambridge, UK in an investigation of the concept of in-vitro compartmentalisation (IVC) [5]. The flow focusing devices used in droplet generation have predominantly been constructed in polydimethylsiloxane (PDMS) (for example, [3, 6-8]). The fabrication of the master pattern for the molding of channel structures in PDMS has typically been based on lithographic definition in a layer of SU8 resist. In this paper, we have examined the alternate application of dry film resist in fabrication of the Master pattern. Intrinsic properties of dry film resists have included low cost, excellent adhesion to a range of substrates, no edge bead, fast prototyping and easy removal. Dry film resists have been most commonly used in the patterning of printed circuit boards (PCBs) but have also recently been applied as moulds for electroplating in a LIGA type process [9] and soft lithography [10]
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