The need for disposable diagnostic sensors in the health care industry has been a major driver in the development of low-cost polymer microfluidic devices. Of crucial importance to many of these devices is the incorporation of sieves and filters for the pretreatment of biological samples. Much of the previous work on integrating filtration systems in microdevices has focused on silicon and glass technologies. Of more difficulty, due to the different manufacturing methodology and lower mechanical strength, is the integration of filtration systems in polymer microfluidic chips. This paper presents a design and construction methodology to fabricate such integrated devices in polyethylene terepthalate (PET) and describes their characterization for particle filtration. To demonstrate the application of these systems, DNA extraction from whole blood was investigated. This currently represents a major stumbling block for point-of-care diagnostics. To this end two approaches were taken; the isolation of leucocytes for subsequent DNA extraction, and the trapping of silica microspheres for DNA adsorption. The polymer surfaces of the fluidic chips were modified by UV exposure and chemical etching to increase their surface energy for improved non-specific binding and electroosmotic flow characteristics. Integrated filtration devices were successfully fabricated with excimer laser machined membranes having pore dimensions down to 1μm, and contact angles from 75° down to less than 25° were achieved using UV modification, and from 75° down to 16° by chemical modification of PET. White blood cells were filtered from whole blood and silica particle retention was demonstrated successfully.
Two-dimensional control over the surface chemistry of substrate materials is of interest to a wide range of applications from microelectronics to biomedical diagnostics. Here, we describe a general principle for creating spatially controlled surface chemistries by subsequent deposition of thin plasma polymer coatings followed by laser ablation. The creation of surfaces with spatially controlled wettability was used as an example. Plasma polymerization of n-heptylamine produced a hydrophilic surface on silicon wafer substrates while subsequent plasma polymerization of perfluoro-1,3-dimethylcyclohexane produced a hydrophobic surface. Excimer laser ablation at an energy density of 125 mJ/cm<sup>2</sup> was used to remove a defined thickness of the two-layer coating, completely removing the upper layer without exposing the substrate material. Excimer laser ablation resulted in two-dimensional control over the surface chemistry with a resolution of ca. 1 μm. Surface modifications were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Contact angle measurements were used to estimate the wettability of modified surfaces. The method was shown to be suitable for the precise control over the location of droplets containing aqueous solutions. In conclusion, the method described here represents an effective tool for the production of substrates with spatially controlled surface chemistry and wettability.
The need for disposable diagnostic sensors in the health care industry has been a major factor in the development of low-cost microfluidic devices. Polymer materials have been the obvious choice due to their cost effectiveness. However, these materials often do not possess the desired properties for biochip operation, such as their high non-specific binding and poor electroosmotic flow characteristics. Various fabrication techniques have also been developed for polymeric chips over the past few years due to their incompatibility with the traditionally preferred micromachining technologies associated with glass and silicon. This paper presents a method for constructing microfluidic devices in Poly(ethylene terephthalate) (PET) using a direct-write Neodymium Yttrium Aluminium Garnet (Nd:YAG) laser system. Issues involving the operation and fabrication of such disposable devices, with particular emphasis on the development of a bio-chip for DNA diagnostics, are discussed.