Indium-tin oxide coated microfabricated device for the injection of a single cell into a fused silica capillary for chemical cytometry

Cytometry uses instrumental means to characterize the composition of single cells. Classic techniques of image cytometry and flow cytometry use stains and dyes to visualize specific molecular species. Both techniques require a fluorescent probe to target an analyte. However, only a relatively few species can be monitored simultaneously because of the spectral overlap of the dyes.1 Chemical cytometry uses ultra-sensitive instrumentation to obtain qualitative and quantitative information on the contents of a single cell.2 The first report of single cell analysis analyzed ribosomal RNA using electrophoresis on a silk fiber.3 Jorgenson started the modern era of single cell analysis by using capillary chromatography to analyze amino acids in a single giant neuron from a snail.4 High throughput chemical cytometry is required to gain cellular information at a statistically relevant level. To be able to analyze hundreds-to-thousands of single cells each day, the cells must be easily trapped and integrated to the chemical cytometer. We have described a first-generation device that employed 150-μm diameter and 50-μm deep wells to trap cells for subsequent injection into a fused silica capillary.5 A cell suspension was added to the device and cells were allowed to settle into the wells. The wells were much larger than a single cell, and more than one cell can physically fit within the well. We employed a cell concentration that resulted in the introduction of an average of one cell per well. However, the number of cells was Poisson distributed; some wells contained no cells and other wells contained more than one cell. In this paper, we describe a new device that employs a small aperture at the bottom of a polydimethylsiloxane (PDMS) well. By application of vacuum to the aperture, a single cell is trapped at the bottom of the well. After washing excess cells, that cell is available for analysis. To assist transfer of cells from the device to the capillary for analysis, we employed an indium-tin oxide (ITO) coating at the glass substrate containing the device. ITO was chosen because it is both electrically conductive and transparent, which allows the visualization of the cell, and the subsequent application of a voltage to assist injection of the cell into the grounded analysis capillary.