Diagnostic tools that are cheap to make, simple to use, and rugged enough for rural areas could save thousands of lives in poor parts of the world. To make such devices, Harvard University professor George Whitesides is coupling advanced microfluidics with one of humankind's oldest technologies: paper. The result is a versatile, disposable test that can check a tiny amount of urine or blood for evidence of infectious diseases or chronic conditions.

The finished devices are squares of paper roughly the size of postage stamps. The edge of a square is dipped into a urine sample or pressed against a drop of blood, and the liquid moves through channels into testing wells. Depending on the chemicals present, different reactions occur in the wells, turning the paper blue, red, yellow, or green. A reference key is used to interpret the results.

Color change: Paper tests, such as those shown here, could make it possible to diagnose a range of diseases quickly and cheaply. A small drop of liquid, such as blood or urine, wicks in through the corner or back of the paper and passes through channels to special testing zones. Substances in these zones react with specific chemicals in the sample to indicate different conditions; results show up as varying colors. These tests are small, simple, and inexpensive.

Credit: Bruce Peterson

The squares take advantage of paper's natural ability to rapidly soak up liquid­s, thus circumventing the need for pumps and other mechanical components common in microfluidic devices. The first step in building the devices is to creat­e tiny channels, about a millimete­r in width, that direct the fluid to the test wells. Whiteside­s and his coworkers soak the paper with a light-­sensitive photo­resist; ultraviolet light causes polymers in the photoresist to cross-link and harden, creating long, waterproof walls wherever the light hits it. The researchers can even create the desired channels and wells by simply drawing on the paper with a black marker and laying it in sunlight. "What we do is structure the flow of fluid in a sheet, taking advantage of the fact that if the paper is the right kind, fluid wicks and hence pulls itself along the channels," says Whiteside­s. Each well is then brushed with a different solution that reacts with specific molecules in blood or urine to trigger a color change.