North AmEricAN
60 | FEB 2013 | North AmEricAN | SPEctrUm.iEEE.orG
and easily dis-
posed of by incineration after one use.
Two of the pioneers in this field are George
Whitesides at Harvard University and Paul
Yager at the University of Washington. Both
have benefitted from early and sustained
support by the Bill & Melinda Gates Founda-
tion for developing simple and very-low-cost
diagnostic devices that do not require special
skills or facilities. Their groups have advanced
several simple and elegant approaches to
forming paper microfluidic devices. One ap-
proach relies on a wax-based patterning pro-
cess, which uses an inkjet printer to place
features on paper with a wax-based “ink.”
After printing, the paper is heated to drive the
wax though the entire thickness of the mate-
rial. Because the wax- impregnated regions
are hydrophobic, fluid flow is restricted to
wax-free regions. Wicking eventually guides
the liquid to be tested into dry “compart-
ments” containing chemical reagents or
bio markers. If the right combination of com-
pounds is present, a chemical reaction takes
place, producing a color change that the user
can then read. Three-dimensional versions of
these units have been developed in order to
create compact packages that can perform
multiple tests, in different layers, on the same
sample or else redundant tests to reduce the
chance of a false result. One recent innova-
tion comes from Richard Crooks’s group at
the University of Texas at Austin, which found
a way to construct many-layered paper fluidic
devices out of a single sheet of paper. The
team’s origami-like approach starts with a
two- dimensional hydrophobic pattern that is
formed using photoresist. After a few selec-
tive snips with scissors and some folding (think
paper doll construction), a multilayered square
can be assembled without tools. In the result-
ing assembly, overlaid openings allow fluid to
flow up through as many as nine layers.
These paper-based tests have already
spawned spin-off companies and nonprofits
and will probably be the first paper-based
technology to be commercialized. I expect
they could eventually be augmented with
communications circuitry and some logic
to make remote sensors. But there is still
much to learn about the actual fabrication
costs of these assays, as well as practical
properties like shelf life, sensitivity, and the
reproducibility of their results.
continued from page 52 |
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