Opto-fluidic detection system enabling sophisticated point-of-care diagnostics
Most biomedical tests are performed at large clinical laboratories because compact, robust, and inexpensive instruments for point-of-care (POC) testing are simply not available. Yet there is a need for POC testing. Optofluidic systems for fluorescence detection of bio-particles offer high performance, but they cannot meet POC specifications. We have demonstrated, prototyped, and benchmarked against commercial systems a new optical detection approach that delivers high signal-to-noise discrimination – without complex optics or bulky excitation sources. It therefore enables a truly compact, low-cost, high-performance microfluidic-based instrument that can be used for diagnostics on whole blood and for other complex fluids. The enabling technique is termed “spatially modulated emission” and generates a time-dependent signal as a continuously fluorescing bio-particle traverses a predefined pattern for optical transmission. Correlating the detected signal with the known pattern achieves high discrimination of the particle signal from background noise. In conventional flow cytometry, the size of the excitation area is restricted approximately to the size of the particle. Our method allows a large excitation area to increase the total flux of fluorescence light that originates from a particle. Despite the large excitation area, the mask pattern enables a high spatial resolution which permits independent detection and characterization of near-coincident particles, with a separation (in the flow direction) that can approach the dimension of individual particles. In addition, the concept is intrinsically tolerant to background fluorescence originating from fluorescent components in solution, fluorescing components of the chamber and contaminants on the surface. The detection technique has been extensively evaluated with measurements of absolute CD4+ and percentage CD4 counts in human blood, which is required for initiation and monitoring the treatment of HIV-infected patients. The technique has been benchmarked against a commercial instrument with a direct one-to-one comparison of measurements on the same labeled blood samples, with excellent agreement for both absolute CD4 and CD4%. More recent experiments demonstrate that the platform can address a large variety of diagnostic needs including multiplexed bead-based assays (ELISA on-the-flow) and identification and enumeration of pathogens (e.g., Giardia, Cryptosporidium and E. Coli) in fluids. We have assembled and tested a working prototype of a micro-fluidic based flow cytometer. The detection subsystem includes a basic pin photodiode rather than a PMT or APD. The prototype was assembled with off-the-shelf components (total cost of all active parts <$350). Measurements of the sensitivity and dynamic range were conducted with calibration particles and yielded a detection limit of ~200 MEPE, which meets the needs for a wide range of bio-particle-detection applications. By using an avalanche rather than a pin photodiode the sensitivity has been further improved to ~50 MEPE which is promising for detection of very dim objects, e.g., specifically tagged E-coli. Acknowledgments: This work is partially supported by a grant from the NIH and ARO.
Kiesel, P.; Martini, J.; Huck, M.; Johnson, N. M.; Recht, M. I.; Bern, M. W. Opto-fluidic detection system enabling sophisticated point-of-care diagnostics. Invited paper for the First Annual SLAS (Society for Laboratory Automation and Screening) Conference and Exhibition; 2012 February 4-8; San Diego, CA.