Some investigators describe a point-of-care system, with a microfluidic paper device and a handheld optical colorimeter, for quantifying analyte concentrations in biological fluids. First came paper-based microfluidic devices. Then came the idea of using camera phones to relay data from paper-based assays. an inexpensive handheld colorimeter that generates quantitative data in a point-of-care analytical system. It’s the dream that the latest step toward the goal of making medical diagnostics cheaper, easier, and more accessible to people in places with limited resources.
The system starts with a patterned, paper-based microfluidic device that runs colorimetric assays for albumin, a protein found in urine. The colorimeter—which is assembled by available electronics and operates on a 9 V battery—quantifies the protein concentration by measuring the optical absorbance of the colors. The assay requires only a couple of microliters of biological fluid and is rugged and self-contained. Transmission colorimetry in paper-based analytical devices is one way to achieve quantitative, cheap, point-of-care assays in places where traditional clinical tests aren’t practical. The yellow color is tetrabromophenol blue; the green-blue color indicates that albumin has been detected in a urine sample.
Because of pursued transmission-based, rather than reflectance-based, color detection because the former is more accurate in quantifying analyses present in the low micro- to molar concentration range. The aluminum-cased colorimeter—an improvement over an earlier prototype that quantified assays in PDMS devices—has a tricolor LED for detecting colors and a manifold to hold the paper device in proper alignment with the LED for measurements. It produces pulsed light so that by kilohertz modulation and narrow-band pass, frequency-accuracy detection, accuracy measurements can be carried out in all kinds of lighting conditions.
To measure colors through the thickness of the paper, it is necessary to wet the device with a handheld color meter that has a similar index of refraction. (Index matching diminishes scattering in wet paper.) In keeping with the theme of globally ubiquitous items, we should fill a small disposable plastic sleeve with the oil and pop the paper device into it—after the device had been dipped in an artificial urine sample spiked with BSA—for wetting and measuring. The result is that developing new dyes that have more pronounced color changes and are more stable would be very useful for point-of-care diagnostics.
The ultimate goal of this research direction is to develop entities that are detached from electronic devices. “Even though the device may be very cheap, it’s still an electronic device. Ultimately, what we need is a piece of paper that detects markers of disease or pathogens, like litmus paper.