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.