Alyssa C. Henry

Surface modification of polymer-based microfluidic devices

Abstract We report the chemical modification of poly(methyl methacrylate) (PMMA), and poly(carbonate) (PC) surfaces for applications in microfluidic systems. For PMMA, a reaction of the surface methyl ester groups with a monoanion of α,ω-diaminoalkanes (aminolysis reaction) to yield amine-terminated PMMA surfaces will be described. Furthermore, it was found that the amine functionalities were tethered to the PMMA backbone through an alkane bridge to amide bonds formed during the aminolysis of the surface ester functionalities. The electro-osmotic flow (EOF) in aminated-PMMA microchannels was reversed when compared to that in unmodified channels. Finally, the availability of the surface amine groups was further demonstrated by their reaction with n -octadecane–1-isocyanate to form PMMA surfaces terminated with well ordered and highly crystalline octadecane chains, appropriate for performing reverse-phase separations. Examples of reverse-phase separations of ion-paired double-stranded DNAs in electric fields (capillary electrochromatography (CEC)) will be demonstrated using a PMMA-based fluidic chip. For PC, sulfonation of the surface with SO 3 will be described; this sulfonation makes the surface very hydrophilic. EOF studies of the sulfonated-PC surfaces indicated changes in the pH-dependent profile when compared to unmodified PC.

Microfluidic Analysis of Complex Samples with Minimal Sample Preparation Using Gradient Elution Moving Boundary Electrophoresis

Sample-in answer-out analytical tools remain the goal of much lab on a chip research, but miniaturized methods capable of examining minimally prepared samples have proven elusive. Complex samples, including whole milk, various types of dirt and leaves, coal fly ash, and blood serum, were analyzed quantitatively for dissolved potassium, calcium, sodium, magnesium, lithium, and melamine using gradient elution moving boundary electrophoresis (GEMBE) and contactless conductivity detection with the single preparatory step of dilution or suspension in sample buffer. GEMBE is a simple, robust analytical technique, well-suited to microfluidic analysis of complex samples containing material, such as particulates or proteins, that would confound the majority of other microfluidic techniques. GEMBE utilizes electrophoretic flow to drive electrically charged analytes into a microfluidic channel or capillary for detection, while opposing electro-osmotic and variable pressure-driven flows prevent the remainder of the sample from entering the channel. Contactless conductivity detection further simplifies device construction and operation, positioning GEMBE for inexpensive and facile multiplexed implementation outside laboratory settings.

Expanding the capabilities of microfluidic gradient elution moving boundary electrophoresis for complex samples.

Gradient elution moving boundary electrophoresis (GEMBE) is a robust, continuous injection separation technique that uses electrophoresis to drive electrically charged analytes into a capillary or microfluidic channel for detection, while opposing electroosmosis and controlled variable pressure-driven flow prevent other sample components-for example, cells, proteins, or particulates in complex samples that can interfere with analysis-from entering the channel. This work expands the sample-in/answer-out analytical capabilities of GEMBE for complex samples by demonstrating the quantitative analysis of anions, implementing aqueous background electrolyte (BGE) solutions at neutral pH, and introducing the use of additives to the sample solution to optimize performance. Dirt was analyzed quantitatively, with the sole preparatory step of suspension in an aqueous BGE solution at neutral pH, for dissolved chloride, nitrite, nitrate, sulfate, and oxalate using GEMBE with capacitively-coupled contactless conductivity detection. In addition to altering the pH of the BGE solution, optimization of the analysis of dirt and whole blood was achieved using various commercially available additives. These results, taken together with previous demonstrations of GEMBE for the analysis of complex samples, underscore the uncomplicated versatility of GEMBE, facilitate effective analysis of biological complex samples using BGE solutions at physiological pH, and offer a sufficient set of techniques and tools to build a foundation for the analysis of a broad range of complex samples.

Characteristics of Commercial PMMA Sheets Used in the Fabrication of Extreme High‐Aspect‐Ratio Microstructures

We describe the characterization of various commercial brands of poly(methyl methacrylate) (PMMA) sheets used in the LIGA process. LIGA, a German acronym for lithography, electroplating, and molding, is a relatively new method for construction of extreme high-aspect-ratio microstructures. Metal structures with millimeter-by-micron dimensions can be fabricated by electrodeposition of metals into developed features of the commonly used X-ray-sensitive resist, PMMA. Previous experiments from these laboratories have indicated that the adhesive strength of a given PMMA sheet to an electroplating substrate is a function of the source of the PMMA, thermal treatment of the PMMA, and the face of the PMMA sheet. We have used scanning force microscopy to inspect the topography of the faces of PMMA routinely used in the LIGA process. Depending on the thickness of the PMMA, or what face of a given PMMA sheet is inspected, the surface topography is found to vary from almost completely smooth to that where pits populate the surface. The dimensions and number of the pits depend on the thermal treatment of the PMMA sheets (annealing at an elevated temperature). Gas chromatography-mass spectrometry indicates that residual methyl methacrylate (MMA) monomer content in the PMMA decreases upon annealing. Sampling of the local atmosphere during annealing through use of solid phase microextraction confirms that residual MMA is released. The variation in pit number and size, the result of different amounts of expelled MMA, leads to the noted differences in adhesive strength of the various PMMA sheets to the substrate.

DNA purification from crude samples for human identification using gradient elution isotachophoresis

Gradient elution isotachophoresis (GEITP) was demonstrated for DNA purification, concentration, and quantification from crude samples, represented here by soiled buccal swabs, with minimal sample preparation prior to human identification using STR analysis. During GEITP, an electric field applied across leading and trailing electrolyte solutions resulted in isotachophoretic focusing of DNA at the interface between these solutions, while a pressure‐driven counterflow controlled the movement of the interface from the sample reservoir into a microfluidic capillary. This counterflow also prevented particulates from fouling or clogging the capillary and reduced or eliminated contamination of the delivered DNA by PCR inhibitors. On‐line DNA quantification using laser‐induced fluorescence compared favorably with quantitative PCR measurements and potentially eliminates the need for quantitative PCR prior to STR analysis. GEITP promises to address the need for a rapid and robust method to deliver DNA from crude samples to aid the forensic community in human identification.

Chemical Foundations for Plastic-Based Microdevices: Chemically Modified Poly(methyl Methacrylate) and Poly(carbonate) Substrates Used in the Fabrication of Microanalytical Devices

Polymer surfaces are modified so as to control the properties of microanalytical systems constructed from such polymers using hot embossing fabrication techniques. We demonstrate here the modification of poly(methyl metacrylate), PMMA, and poly(carbonate), PC, surfaces using solution- and gas-phase methods.