This article describes an integrated approach to tracking the end point of a time-based assay that is conducted on an analytical device made out of paper. The timing mechanism is built directly into a paper-based analytical device and does not require starting, stopping, reset buttons, batteries, or maintenance; the timer simply starts once the sample is added to the device. These "fluidic timers" are composed of paraffin wax and a signaling feature (e.g., a dye). The timing function is made possible by the specific time required for a liquid sample to wick through predefined regions in the device. This time period can be anywhere between 1 min and 2 h and is controlled by the quantity of wax present in the timer. Because both the fluidic timers and paper-based assays depend on the wicking rate of the sample, the fluidic timers automatically calibrate themselves (relative to the assay) to account for differences in wicking rates that are caused by variations in humidity. Fluidic timers are 97percent accurate (with respect to the time required for the assay) and provide slightly better accuracy than an external timer when used to track an assay that measured the level of glucose in a sample.
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