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Capillary-driven flow (CD-flow) in microchannels plays an important role in many microfluidic devices. These devices, the most popular being those based in lateral flow, are becoming increasingly used in health care and diagnostic applications. CD-flow can passively pump biological fluids as blood, serum or plasma, in microchannels and it can enhance the wall mass transfer by exploiting the convective effects of the flow behind the meniscus. The flow behind the meniscus has not been experimentally identified up to now because of the lack of high-resolution, non-invasive, cross-sectional imaging means. In this study, spectral-domain Doppler optical coherence tomography is used to visualize and measure the flow behind the meniscus in CD-flows of water and blood. Microchannels of polydimethylsiloxane and glass with different cross-sections are considered. The predictions of the flow behind the meniscus of numerical simulations using the power-law model for non-Newtonian fluids are in reasonable agreement with the measurements using blood as working fluid. The extension of the Lucas-Washburn equation to non-Newtonian power-law fluids predicts well the velocity of the meniscus of the experiments using blood.

Passive pumping for lab-on-a-chip using surface tension only is the most effective method because the effect of surface tension on flow is significant in microscale. The movement of the triple point at a meniscus is driven by surface tension, resisted by viscous stress, and balanced by inertial force. In previous studies, the meniscus motion has been predicted theoretically with a one-dimensional model. However, three-dimensional flow field around the meniscus and the effect of cross-sectional shapes on the flow have not yet been assessed. Here, we visualized and analyzed the surface-tension-driven blood flow using spectral-domain Doppler optical coherence tomography.

An amperometric enzyme sensor capable of operating at high temperatures was developed by utilizing a 'wired' thermostable glucose-6-phosphate dehydrogenase (tGtPDH) from the hyperthermophilic bacterium Aquifex aeolicus. The response of the system was monitored through detection of the product of the enzymatic reaction, NADH, which was electrocatalytically reoxidized to NAD by a thermostable redox mediator, osmium [(1,10-phenanthroline-5,6-dione).sub.2]-poly(4-vinylpyridine), at [E.sub.app]: + 150 mV vs Ag/AgCl/K[Cl.sub.sat]. The enzyme was 'wired' onto the surface of graphite electrodes by using an epoxy-based poly(ethylene glycol) diglycidyl ether cross-linker. The stability of the sensor at higher temperatures clearly surpassed the conventional system utilizing a mesophilic G6PDH (mG6PDH) from Leuconostoc mesenteroides. The mG6PDH-based system lost 26% of its response after 20 min at 50[degrees]C. The response of the tG6PDH-based system remained unchanged under the same conditions. The tGtPDH-based system demonstrated excellent stability up to a temperature of 83 [degrees]C.

A 0.29-mm-diameter flexible electrode designed for subcutaneous in vivo amperometric monitoring of glucose is described. The electrode was designed to allow 'one-point' in vivo calibration, i.e., to have zero output current at zero glucose concentration, even in the presence of other electroreactive species of serum or blood. A valid zero point, along with a measurement of the glucose concentration in a withdrawn sample of blood at which the current is known, defined the sensitivity in the linear response range. The electrode was four-layered, with the layers serially deposited within a 0.125-mm recess upon the tip of a polyimide-insulated 0.25-mm gold wire. The recessed structure reduced the sensitivity to movement and allowed, through control of the depth of the recess, control of the transport of glucose and thus the range of linearity. The recess contained the four polymeric layers, with a total mass less than 5 [[micro]gram] and no leachable components. The bottom glucose concentration-to-current transducing layer consisted of the enzyme 'wiring' redox polymer [poly[(vinylimidazole)Os[(bipyridine).sub.2]Cl].sup.+/2+], complexed with recombinant glucose oxidase and cross-linked with poly (allylamine), on which an immobilized interference-eliminating horseradish peroxidase based film was deposited. An outer biocompatible layer was formed by photo-cross-linked poly(allylamine), on which an immobilized interference-eliminating horseradish peroxidase based film was deposited. An outer biocompatible layer was formed by photo-cross-linking derivatized poly(ethylene oxide). The current output of a typical electrode at 10 mM, glucose and at 37 [degrees] C was 35 nA, the apparent [K.sub.m] was 20 mM, and the 10-90% response time was [approximately] 1 min. The sensitivity varied only by [+ or -] 5% over a 72-h test period. The electrode tracked the blood glucose concentration in a rat model before, during, and following intraperitoneal glucose infusion. Two failure modes were observed. The first, deactivation of hydrogen peroxide-producing lactate oxidase in the interference-eliminating layer, resulted in inadequate preoxidation of interferants. The second was an abrupt drop in the sensitivity of implanted electrodes, [approximately] 7 h after implantation. The failed electrodes promptly regained their sensitivity in buffer.

The possibility of electrochemical linkage between glycerophosphate oxidase and lactate oxidase to electrodes by cross-linked redox polymer-enzyme hydrophilic networks was investigated. Experiments conducted involved chronoamperometry, chronopotentiometry, electrochemical measurements and enzyme assays. The observed response time, sensitivites and linear ranges for all electrodes were satisfactory.