Micro cell counter using photoconductance of boron diffused resistor (BDR)

This paper describes a micro-cell counter using a boron diffused resistor (BDR) which shows photoconductive property at near infrared wavelength. The device consists of an infrared emitting diode (IRED), a micro-channel for cell flow, a window of silicon nitride membrane, and the BDR. When the cells pass in the micro-channel, they shade the window and counting is performed due to resistance change (ΔR) of the BDR. In the method the cells can be preserved intact because they are free from experiencing electric field as usual in the impedance spectroscopy method, or attachment of fluorescent markers in the fluorescent activation method. The experiment with the BDR shows peaks of 20 mV and counting capability of about 7×104 events per second with high signal-to-noise ratio (SNR) under periodic illumination by the IRED (950 nm wavelength) and a function generator. The experiments with SupT1 cells of 10 μm and Sephadex G-25 beads of 70 μm in diameter show peaks of 8 mV with relatively low SNR. The performance can be improved with the enhanced resistance ratio (ΔR/R) by optimizing of the area ratio of the window and the cell, the intensity of illumination, and the shape and diffusion of the BDR.
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