On the desk of Harvard University professor George Whitesides are white sheets the size of stamps. Each is inlaid with a different pattern: a four-leaf flower, concentric circles surrounding four dots, a six-pointed burst. These prototypes are diagnostic tests made from a cheap, universally available substrate: paper. With the simple addition of tape, Whitesides and his group have been able to turn their paper tests into three-dimensional devices, allowing for more-complicated analysis. Such paper “lab on a chip” tests may lead to a cost-effective, portable, and accurate method for diagnosing diseases in countries lacking reliable health care.
Whitesides holds up his group’s latest development: a square slightly thicker than the other samples, covered in a grid of yellow, greed, red, and blue dots. This prototype is made up of layers of patterned paper, between which are waterproof double-sided tape. By layering the squares and connecting them with tape adhesive, the group is able to channel liquid horizontally and vertically in a very small area, as detailed in this week’s Proceedings of the National Academy of Sciences.
Each square of paper–which naturally soaks up liquid in its fibers–has been treated with photoresist material, to create channels that funnel liquid to tiny wells coated with proteins or antibodies. The fluid interacts with that area of the paper and turns the well a certain color, changing hues, for example, with varying concentrations of glucose.
“You can effectively make three-dimensional systems,” says Whitesides. “You can distribute fluid from a single point of origin to multiple points elsewhere.” Most microfluidic systems, he adds, require sophisticated piping to channel the fluid, but just by drawing water-resistant channels in the paper and using waterproof tape, the researchers have been able to control the movement of the liquid.
A sample of biological fluid is pressed onto one side of the paper chip. Photoresist channels guide the fluid to certain wells and ultimately through a tiny hole in the waterproof tape, which funnels the liquid on to the next piece of paper. This process continues for each layer of the chip, and the results appear in a dotted grid on the final layer.