We present a fully integrated droplet-based microfluidic platform for the high-throughput assessment of photodynamic therapy photosensitizer (PDT) efficacy on Escherichia coli. The described platform is able to controllably encapsulate cells and photosensitizer within pL-volume droplets, incubate the droplets over the course of several days, add predetermined concentrations of viability assay agents, expose droplets to varying doses of electromagnetic radiation, and detect both live and dead cells online to score cell viability. The viability of cells after encapsulation and incubation is assessed in a direct fashion, and the viability scoring method is compared to model live/dead systems for calibration. Final results are validated against conventional colony forming unit assays. In addition, we show that the platform can be used to perform concurrent measurements of light and dark toxicity of the PDT agents and that the platform allows simultaneous measurement of experimental parameters that include dark toxicity, photosensitizer concentration, light dose, and oxygenation levels for the development and testing of PDT agents.

A newly developed conductivity detector, the floating resistivity detector (FRD), for microchip electrophoresis was introduced in this work. The detector design permits decoupling of the detection circuit from the high separation voltage without compromising separation efficiency. This greatly simplifies the integration of microchip electrophoresis systems. Its method of detection relies on platinum electrodes being dipped in two buffer-filled branched detection probe reservoirs on the microchip device. In this way, analytes passing through the detection window will not pass through and subsequently adsorb onto the electrodes, alleviating problems of electrode fouling due to analyte contamination and surface reactions. A customized microchip design was proposed and optimized stepwise for the new FRD system. Each branched detection probe was determined to be 4.50 mm long with a 0.075 mm detection window gap between them. The distance between the detection window and buffer waste reservoir was determined to be 1.50 mm. The optimized microchip design was subsequently used in the analysis of four groups of analytes - inorganic cations, amino acids, aminoglycosides antibiotics, and biomarkers. Based on the preliminary results obtained, the detection limits were in the range of 0.4-0.7 mg/L for the inorganic cations and 1.5-15 mg/L for the amino compounds.

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In this paper, silicon-based devices with a step structure integrated at the flow-focusing junction were designed, fabricated and characterized for droplet generation. A two-step silicon etching method was demonstrated to create the step structure. During fabrication, undesirable spikes encountered at the step edge were removed by oxygen plasma ashing and silicon isotropic etching. With this method, two types of step profile (flat and triangular profiles) were fabricated. These two profiles were compared for their differences in droplet-generation behavior. The device with the flat-step profile was found to make larger droplets and at a lower frequency compared to the device with the triangular-step profile. Additionally, polydimethylsiloxane and glass were tested as capping materials for the devices and the impact of their surface characteristics (hydrophobic and hydrophilic) on the type of droplets (water-in-oil or oil-in-water) formed was investigated.

A study has been made of the susceptibility of 20-μm oxide stripe GaAlAs lasers to current transients. The current amplitude (as a function of duration) required to cause increased threshold currents or degradation rates has been found. Damage caused by forward bias transients was found to occur at the facets and was probably due to high surface recombination, leading to local absorption and excessive heating. Even when the immediate damage was insufficient to cause a detectable increase in threshold current, increased degradation rates resulted. Damage during reverse bias transients resulted from local avalanche breakdown which, though usually observed at the facets, was occasionally found outside the stripe. Such damage did not always lead to increased degradation rates. The reverse bias breakdown voltage of the lasers was found to be a more sensitive indicator of transient damage than the threshold current. The results provide substantial evidence to confirm that precautions must be taken against electrostatic discharge whenever lasers are handled. In addition, all laser bias circuitry, including that in test and measurement equipment as well as in optical fiber systems, must be checked to ensure that dangerous transients cannot occur.