A microfluidic assay was developed for screening botulinum neurotoxin serotype A (BoNT-A) by using a fluorescent resonance energy transfer (FRET) assay. Molded silicone microdevices with integral valves, pumps, and reagent reservoirs were designed and fabricated. Electrical and pneumatic control hardware were constructed, and software was written to automate the assay protocol and data acquisition. Detection was accomplished by fluorescence microscopy. The system was validated with a peptide inhibitor, running 2 parallel assays, as a feasibility demonstration. The small footprint of each bioreactor cell (0.5 cm2) and scalable fluidic architecture enabled many parallel assays on a single chip. The chip is programmable to run a dilution series in each lane, generating concentration-response data for multiple inhibitors. The assay results showed good agreement with the corresponding experiments done at a macroscale level. Although the system has been developed for BoNT-A screening, a wide variety of assays can be performed on the microfluidic chip with little or no modification.

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Many of the applications that require the unique capabilities of Photoconductive Semiconductor Switches (PCSS) demand a compact package. We have been able to clemonstrate that GaAs switches operated in the high gain mode called "lock-on" meet the required electrical switching parameters of several such applications using small switch sizes. The only light source that has enough power to trigger a PCSS and is compatible with a small package is a laser diode. This paper will describe the progress that leads to the triggering of high power PCSS switches with laser diodes. As of this writing, we have used an 850 W optical pulse from a laser diode array Lo trigger a 1.5 em long switch that delivered 8.5 MW to a 38.3 0 load. Using 166 W arrays we have triggered a 2.5 mm long switch delivering 1.2 MW with 600 ps risetimes at pulse repetition frequencies of 1 kHz. These 2. 5 nun long switches were tested for pulse lifetime and survived 10 5 pulses at 1.0 MW levels. In single pulse operation we have switched up to 600 A with laser diode ax:x:ays. Our goal is to switch up to 5 kA in a single shot mode and up to 100 MW repetitively at up to 10 kHz. These goals are feasible since the switches can be used in parallel or in series. Low light level triggering became possible after the discovery of a high electric field, hi gh gain switching mode in GaAs (and lat er in InP). At electric fields below 3 kV/cm GaA s switches are activated by creation of, at most, only one conduc tion electron-valence hole pair per photon absorbed in the sample. This lineax: mode demands high laser power and, after thf'> 1 i ght i ~ P.Yt inguishecl. the Cr thP. switch is reduced by a factor of up to 500. The gain continues until the field across the sample dx:ops to a material dependent lock-on field. At this point the switch will cax:ry as much c urrent as, and for a s long as, the circuit can maintain the lock-on field. The gain in the switch allows for the use of laser diodes.

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