Multistep Diagnostics on Paper
A credit-card-sized diagnostic device can perform a task normally carried out by complex equipment.
Paper-based diagnostic tests represent an exciting opportunity for improving medical testing in poor countries. They are cheap to produce and don’t require complicated instruments to carry out a test or read the result, so they can be implemented in areas with few resources and little infrastructure. But paper diagnostic tests have thus far been limited to fairly simple reactions. Researchers at University of Washington in Seattle have now taken an important step toward enabling more complex chemical reactions on paper.
Paul Yager and collaborators have developed a way to control the timing of delivery of chemicals within a paper-based device, and demonstrated how this can be used to amplify the signal of a test antibody. The amplification step is an important part of routine technique called an enzyme-linked immonsorbent assay (ELISA) that is currently carried out on large, expensive instrumentation. “[ELISA is] the gold standard for sensitivity– the gold standard for many diagnostics where you’re detecting proteins, even small antibodies. It can be used to diagnose multiple diseases,” says Barry Lutz, a coauthor on the studies, and Research Assistant Professor at the University of Washington.
Existing clinical tests, involving trained laboratory technicians and large, expensive equipment are out of reach of clinics in remote areas of the developing world. A microfluidic device capable of controlling the movement of tiny amounts of fluid could reduce the amount of costly reagents and enzymes for the tests, and using paper as a material reduces the cost even further.
Pregnancy tests sold in drugstores are a simple example of a paper-based diagnostics. Yager and others are now creating much more complex paper-based tests. Other scientists have had some success developing a paper test of liver function. But most clinical tests are more complex, requiring multiple steps to isolate, label, and multiply a molecule of interest.
Yager’s team aims to transfer the microfluidics technology it has developed over the last two decades to a paper device. With traditional microfluidics, reagents flow through channels on a plastic chip to elicit controlled chemical reactions. These devices require tiny pumps and other mechanisms to manipulate the various chemicals. Paper-based microfluidic devices use the inherent flow properties of channels within paper, and so they do not need pumps. But this makes controlling the release of fluids in the device difficult.
Yager’s team designed a device in which they varied the lengths of paper that led from three reagent sources to a common reaction site, providing a way to control the timing of reactions.
The team demonstrated the amplification of a color indicator linked to a molecule associated with a disease. This allows the disease to be detected visually even if the tell-tale molecule is present only in very low concentrations. This is a crucial part of several clinical tests, but had not yet been carried out on paper because of its complexity–needing different reagents at different times. The team’s results were published this month in Sensors and Actuators and Microfluidics and Nanofluidics.
The new devices will be entirely self-sufficient and can be operated by without training. “What we’re doing is throwing away the pumps, throwing away the hardware, and keeping the full complexity of the microfluidics we’ve worked on in the last 15 years,” says Yager.
Bernhard Weigl, the director at the Center for Point-of-Care Diagnostics for Global Health, says that paper tests like the one Yager’s team is developing could be used to detect a range of diseases, with just one sample, “as samples can be guided to different reagent sites from a single distribution point,” he says. Weigl is collaborating with Yager’s team to develop and test the device.
Now that the team has shown that the amplification step is possible, they are working on ways to optimize and package the technology for distribution. The end result is expected to be a paper-based device, laminated in plastic for protection, with a lens to make the results visible.
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