Fast shuffle. Liquids of different colors stream from four inlets on the top of a microPAD into 64 predetermined reservoirs on the bottom.

Martinez et al., PNAS 105 (50) (16 December 2008)

A Dime-Store Dipstick for Diphtheria

To help doctors and health workers in developing countries diagnose diseases, researchers have developed simple detection devices made only of paper and adhesive tape that cost a fraction of conventional diagnostic equipment. The technology could also find uses in environmental monitoring and homeland security.

The lab work that hospitals and clinics routinely perform in developed countries is often unavailable in Third World countries. Two primary reasons are cost and lack of trained personnel. The situation isn't likely to change anytime soon, so some researchers have been looking for simpler and cheaper alternatives to the basic chemical-analysis tools--such as beakers and test tubes--that health workers use to detect diseases. That's what three chemists at Harvard University have attempted with a technology they call microPADs.

Their invention comprises bits of double-sided carpet tape, pieces of paper, and a mixture of cellulose powder and water that serves as a filler between the sheets. The trio of chemists--Andres Martinez, Scott Phillips, and George Whitesides--laser-cut channels in the paper, punched holes in the tape, and then stacked the two components in alternating layers. When drops of liquid are deposited in the holes on top of the microPADs, capillary action draws them through the separate channels and down into tiny reservoirs in the bottom layer. The scientists envision that the reservoirs will contain standard chemical markers, or assays, that change color if they detect the presence of bacteria, viruses, or hazardous substances in the human body--or in water supplies. For example, if a volunteer layperson wanted to test a suspect water sample for excess lead levels, all he or she would need to do is place some drops in the top holes, wait a few minutes, and then see how the special chemicals on the bottom react.

As the researchers describe online this week in The Proceedings of the National Academy of Sciences, the prototype microPADs transported four separate liquid samples to 64 designated reservoirs within 5 minutes. In 27 out of 30 tries, the devices moved the liquids without mixing them. That means the microPADs theoretically can simultaneously test for thousands of potentially harmful chemicals--such as dioxin, lead, or mercury--and for diseases such as diphtheria, malaria, or typhoid. And the results can be transmitted from fieldworkers to centralized laboratories by taking a cell-phone photo of the results on the colored dots (see photo). The researchers estimate that each microPAD could cost as little as 3 cents when manufactured in commercial quantities.

The technique "has broad implications, and I would venture to suggest that it will also find interest in the developed--as well as the developing--world" to produce cheap testing devices, says chemist Timothy Swager of the Massachusetts Institute of Technology in Cambridge. He also says the microPADs are compatible with enough standard assays that they could even "inspire the creation of new assays." Whitesides says he and his colleagues are exploring uses for the devices in environmental monitoring, agricultural testing, and military operations.