RTC Magazine

Until recently, many inspection and control systems have required a human in the loop to provide the actual inspection. However, due to the inefficiency, and sometimes ineffectiveness, of manual inspection, automated inspection and control has become the most sought-after method. This has proven to be no easy task, especially in the complex world of flow cytometry and cell sorting. This is the process used to measure the physical or chemical characteristics of a biological cell and, using this information, sort cells from a sample. Such high-end applications require an intelligent system that can also provide a high level of flexibility and reprogrammability, so that the system is, to some extent, a general-purpose one that can be tailored to the needs of individual applications.

For years, DSPs have been the processor of choice for applications requiring the type of compute-intensive, high-speed calculations used in both medical and bioscience fields, as well as in manufacturing. FPGAs are now being touted as do-everything processors, especially when reconfigurability is a requirement. Often, the best solution is a combination of FPGAs and DSPs working in conjunction with each other (Figure 1).

Basic Inspection and Control System Needs

All systems used for inspection and control, regardless of industry, include two stages of processing. The first stage is the initial inspection during which the system acknowledges the presence of a sample. In the second stage, the required feature extraction is performed. This consists of gathering certain characteristics from the unit under test, often using digitized images for pattern recognition and feature extraction; providing the “intelligent” processing of these images; and ultimately determining if this is a “good” sample or a “bad” sample.

The processing required for the initial inspection, although fairly minimal, needs to be repeated indefinitely as the system consistently monitors for the next unit. Regardless of the type of control and inspection system, the second stage of processing is quite complex.

Flow Cytometry and Cell Sorting Systems

In high-end bioscience applications, automated inspection and control is often required to perform tasks that would otherwise be impossible, since they cannot be done manually. The sorting and classification of biological cells via flow cytometry and cell sorting is one such area. This type of control and inspection system is very similar to the basic system described above in that there are two stages: the initial constant inspection to determine the presence of a cell, and the intelligent processing. The end goal of the system is to divert a droplet containing a cell into the appropriate receptacle by analyzing the cell’s identifying characteristics.

Before each droplet is released from the sample, the system must identify that a cell is present and begin to analyze it. In order to analyze and isolate certain characteristics of a cell, a fluorescent compound is formulated specifically to highlight those characteristics. When added to the sample, this compound binds itself to the cells that possess the desired characteristic. The pressurized cell suspension is forced into a tube and then through a needle, making the cells form a single-file line. The line of cells is then forced through a small nozzle, emerging as a jetting stream. The jetting cells pass one at a time through a laser beam where the cells scatter laser light.

If the desired characteristic is present, the compound in the cell also emits fluorescent light. This fluorescence is detected by a photo sensor, which converts the fluorescent light into a momentary voltage pulse. This stream of voltage pulses is fed into an A/D converter that continuously samples the stream of pulses. Within microseconds, the system needs to perform the pulse detect, identifying that a cell is present, then extract the desired features from the pulse, analyze these features and ultimately determine whether or not the cell is to be sorted.

These tasks take place in parallel so that the system may, at any single point in time, simultaneously detect one cell, analyze another cell, and route yet another cell. This requires a microsecond-level timeframe. Within one second, there could be up to 100,000 cells shooting out of the system, all needing to be measured, classified and routed. Typically, the sorting decision must be made 100 microseconds after the cell passes through the laser beam.