The control of hydrodynamic focusing in a microchannel has inspired new approaches for microfluidic mixing, separations, sensors, cell analysis and microfabrication. Achieving a flat interface between the focusing and focused fluids is dependent on Reynolds number and device geometry, and many hydrodynamic focusing systems can benefit from this understanding. For applications where a specific cross-sectional shape is desired for the focused flow, advection generated by grooved structures in the channel walls can be used to define the shape of the focused flow. Relative flow rates of the focused flow and focusing streams can be manipulated to control the crosssectional area of the focused flows. This manuscript discusses the principles for defining the shape of the interface between the focused and focusing fluids and provides examples from our lab that use hydrodynamic focusing for impedance-based sensors, flow cytometry, and microfabrication to illustrate the breadth of opportunities for introducing new capabilities into microfluidic systems. We evaluate each example for the advantages and limitations integral to utilization of hydrodynamic focusing for that particular application.

Electroconductive hydrogels (ECH) prepared as blends of UV-cross-linked poly(hydroxyethylmethacrylate) [p(HEMA)]-based hydrogels and electropolymerized polypyrrole (PPy) were synthesized as coatings on microlithographically fabricated interdigitated microsensor electrodes (IMEs) and microdisc electrode arrays (MDEAs). Hydrogels were synthesized from tetraethyleneglycol diacrylate (TEGDA), hydroxyethylmethacrylate (HEMA), polyethyleneglycol monomethacrylate (PEGMA), N-[tris(hydroxymethyl)methyl]-acrylamide (HMMA), and 3-sulfopropyl methacrylate potassium salt (SPMA) to produce p(HEMA-co-PEGMA-co-HMMA-co-SPMA) hydrogels. The conductive polymer was synthesized from pyrrole and 4-(3'-pyrrolyl)butyric acid by electropolymerization within the electrode-supported hydrogel. ECH films produced with different electropolymerization charge densities were investigated using cyclic voltammetry, electrical impedance spectroscopy, differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). Polymer morphology was studied by SEM. The ECH demonstrated the desired characteristics of high electrical conductivity (low impedance), as well as high thermal stability compared to pure hydrogel. Signal enhancement was achieved by modifying the surface of an MDEA biotransducer with the ECH, with a 10-fold increase in the voltammetric current response associated with the ferrocene monocarboxylic acid (FcCO(2)H) redox reaction.

Our interest is in the development of engineered microdevices for continuous remote monitoring of intramuscular lactate, glucose, pH and temperature during post-traumatic hemorrhaging. Two important design considerations in the development of such devices for in vivo diagnostics are discussed; the utility of micro-disc electrode arrays (MDEAs) for electrochemical biosensing and the application of biomimetic, bioactive poly(HEMA)-based hydrogel composites for implant biocompatibility. A poly(HEMA)-based hydrogel membrane containing polyethylene glycol (PEG) was UV cross-linked with tetraethyleneglycol diacrylate following application to MDEAs (50 mum discs) and to 250 mum diameter gold electrodes within 8-well culture ware. Cyclic voltammetry (CV) of the MDEAs revealed a reduction in the apparent diffusion coefficient of ferrocenemonocarboxylic acid (FcCO(2)H), from 6.68 x 10(-5) to 6.74 x 10(-6) cm(2)/s for the uncoated and 6 mum thick hydrogel coated devices, respectively. Single frequency (4 kHz) temporal impedance measurements of the hydrogels in the 8-well culture ware showed a reversible 5% change in the absolute impedance of the hydrogels when exposed to a pH change between 6.1 to 7.2 and a 20% drop between pH 6.1 and 8.8.

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