Low temperature bonding of PMMA and COC microfluidic substrates using UV/ozone surface treatment C. W. Tsao, L. Hromada, J. Liu, P. Kumar and D. L. DeVoe* Received 3rd January 2007, Accepted 13th February 2007 First published as an Advance Article on the web 7th March 2007 DOI: 10.1039/b618901f The use of UV/ozone surface treatments for achieving low temperature bonds between PMMA and COC microfluidic substrates is evaluated. Low temperature bond strengths, approaching those of native polymer substrates bonded above their glass transition temperatures, are demonstrated for both thermoplastics. To evaluate the effects of the UV/O 3 surface treatment on the operation of bonded microfluidic devices, the relationship between UV/O 3 exposure and polymer hydrophilicity and surface chemistry are measured. Post-treatment surface chemistry is evaluated by XPS (X-ray photoelectron spectroscopy) analysis, and the stability of the treated surfaces following solvent exposure is reported. Electroosmotic flow within fabricated microchannels with modified wall surfaces is also characterized. Overall, UV/O 3 treatment is found to enable strong low temperature bonds between thermoplastic microfluidic substrates using a simple, low cost, and high throughput fabrication technology. Introduction Rigid thermoplastic polymers have been extensively investi- gated over the past decade as substrates for the fabrication of microfluidic systems. Two particular thermoplastics, poly- (methyl methacrylate) (PMMA) and cyclic olefin copolymer (COC), have emerged as attractive materials for microfluidic applications, primarily due to their high transparency and low autofluorescence over a wide spectral range. 1 In a typical process flow, open microchannels are formed in a first thermoplastic substrate using one of several techniques such as hot 2 or cold 3 embossing, micro-injection molding, 4 or laser ablation. 5 A second plastic layer is then bonded to the first to enclose the microchannels. A variety of suitable bonding methods have been reported, including solvent bonding, 6–8 thermal bonding, 9,10 and thick film lamination employing either pressure or temperature sensitive adhesive layers. 11 Of these techniques, solvent and thermal bonding are of particular interest since they allow the same material to be used for both microfluidic substrate layers, ensuring homogeneity in surface properties for all microchannel walls. In thermal bonding, interlayer adhesion is achieved by heating the substrates near their glass transition temperature while applying a normal pressure, allowing polymer chains to diffuse between the mating surfaces for high bond strength. However, thermal bonding suffers from several disadvantages. Because the substrates must be heated at or slightly above their glass transition temperature to achieve a strong interfacial bond, microscale channels can readily become deformed or col- lapsed. Channel collapse is especially problematic for low aspect ratio channels and thin substrates. Furthermore, the resulting bond strength is often lower than desired, resulting in a significant limitation for applications such as liquid chromatography where high internal fluid pressures are required. Solvent bonding can also suffer from problems with dimensional stability, since the absorbed solvent softens the plastic and can lead to polymer flow during bonding. While recipes have been developed to minimize this problem in PMMA microfluidic chips by using specific solvent condi- tions 8,12 or sacrificial materials such as paraffin wax to prevent channel collapse, 13 the former recipes must be tuned for different polymer grades and types, and neither approach can entirely prevent deformation of channel geometries. Furthermore, solvents can embrittle thermoplastics and result in microcracking, particularly for microfluidic systems which require exposure to high or cyclical pressure loads. Because of these challenges, there remains a need for effective methods for low temperature thermoplastic bonding which are amenable to a wide range of microfluidic applications. The difficulty in realizing high bond strengths for polymer microfluidic chips is in large part due to the low surface energies of thermoplastics. Thermoplastic polymers, including PMMA and COC, are formed from hydrocarbons with additional atomic components such as oxygen and nitrogen. These surfaces possess low specific energy and thus tend to be hydrophobic or weakly hydrophilic, limiting the strength of bonds which may be formed between mating substrates. Increased surface energy serves to improve the wettability between the mating surfaces through enhanced mechanical interlocking and interdiffusion of chains. Bonding can also be enhanced through the generation of electrostatic interactions, and surfaces possessing high specific energy in the form of polar functional groups, which can produce hydrogen or covalent bonds across the interface, are capable of providing bond strengths exceeding that of the bulk polymer. 14 A variety of approaches for increasing the surface energy of polymers have been demonstrated and are routinely used in macroscale polymer engineering, including solvent or acid Department of Mechanical Engineering, University of Maryland, College Park, MD, USA. E-mail: ddev@umd.edu; Fax: +1 301 314 9477; Tel: +1 301 405 8125 PAPER www.rsc.org/loc | Lab on a Chip This journal is ß The Royal Society of Chemistry 2007 Lab Chip, 2007, 7, 499–505 | 499