Built-In Timers For Paper Devices

Integrated timers for paper-based microfluidic devices have been developed by Scott T. Phillips and coworkers at Pennsylvania State University. The timers allow more precise measurements of time-based biomedical assays, for example glucose assays, than are possible with external timers such as stopwatches. The results were presented on Tuesday at the American Chemical Society meeting in Boston.

The devices are being developed as inexpensive options for assays in resource-limited settings, such as developing countries. Most quantitative assays carried out on such devices are time-dependent, Phillips noted. "It's rather expensive to have one timer per device" and inefficient to keep track of each timer and device, he said. "Logistically that's a real challenge."

To avoid the need for external timers, Phillips instead integrated into each device a timer that consists of a combination of paraffin wax and a signaling dye.

The wax slows the movement of fluid through the paper to a length of time dictated by the amount of added paraffin. As a fluid sample passes through the layer, it carries the dye with it. The arrival of the dye at the end of the channel indicates that a predetermined amount of time has elapsed and that the assay is complete.

"We can control the flow rate from one minute all the way up to two hours and anywhere in between," Phillips said. The integrated timers allow automatic calibration for humidity differences that change wicking rates and that would skew the results with external timers.

In time-dependent glucose assays, devices with integrated timers yielded more precise and more accurate measurements than did devices that required external timers.

The group's initial timers have simple colorimetric readouts, but other signaling methods are also possible. For example, a device presoaked with salts can make electrochemical connections that trigger a buzzer.

"I don't know if the timers will be useful in the short term—it depends on the application, which in practice is now focused on the simplest systems—but I think it will be interesting in the long term," said George M. Whitesides, a Harvard chemistry professor who develops paper-based devices.