Throughout the world, reactive paper-based systems are used to test peoples’ blood, urine and other bodily fluids for biomarkers that indicate everything from diabetes to pregnancy. Such systems are also used to detect pollution in water. However, for many of these tests to be accurate, an exact amount of time must pass between the application of the fluid and the viewing of the paper – if the paper is observed any earlier or later, the perceived results could be inaccurate. People typically use stopwatches to avoid this problem, but not everyone in the world has access to such devices, or are proficient at using them. To that end, scientists from Pennsylvania State University (PSU) have developed a simple timer that can be built into the paper itself, that lets home users and clinicians know when the results are ready.
Scott Phillips and Hyeran Noh designed the timer, which involves treating part of the test paper with dye and paraffin wax. When a fluid sample is placed on the paper, some of it begins to react with the traditional reagents that will ultimately provide the results, while some of it begins to wick through microchannels in the wax, picking up the dye along the way. Once the fluid gets through the wax to the bottom layer of the paper, it causes a dot in one region of the paper to change color, indicating that the time is up. The amount of time that can be measured ranges from one minute to two hours, and is determined by how much wax is used. The system has also been modified to emit a sound when the time is up, or even to glow.
Colored dots in other areas of the paper provide the actual test results.
The PSU scientists tried the timer out on a CHEMCARD paper test, which is used for measuring blood sugar or cholesterol in a drop of blood. The test is 100 percent accurate if viewed exactly three minutes after the blood is introduced to the paper, but that accuracy drops by almost half if the timing is not followed. When the timer was used in a CHEMCARD test instead of a stopwatch, the results had an accuracy rate of 97 percent – that’s actually better than what was obtained using a stopwatch.
Besides the facts that the timer doesn’t require batteries or maintenance, and starts automatically, it also compensates for humidity. In conventional paper testing systems, the rate at which the fluid wicks into the paper and reacts with the reagent varies with environmental humidity, even though the time measured on a stopwatch remains the same regardless of the humidity – in other words, even though the watch indicates the correct amount of time has passed, the paper might or might not actually be ready. With the PSU system, the speed at which the fluid wicks into the reagent will be proportional to the speed at which it wicks through the wax, so the results and the timing mechanism will always be in sync.
Phillips and Noh hope that the system will lead to improved health care in developing nations, and reduced health care costs everywhere.