Science Sunday: Reading the Paper–EDITED
Researchers Hong Liu and Richard Crooks at the University of Texas–Austin developed an innovative new biomedical test that could be used in the home and in areas without easy access to laboratories, such as in developing countries. Each test is made of paper, costs about 10 cents, and has the potential to test for diseases such as HIV and malaria. (It doesn’t test for Cylon or Time Lord ancestry . . . yet.)
This paper sensor test is based on the same principle as a home pregnancy test except that it can test for multiple biological markers of disease using a smaller surface area.
Oh, and you can print it out on an office printer.*
George Whitesides at Harvard created a 3-D paper sensor before Liu and Crooks, but it was complicated to make, involving photolithography to design patterns, lasers to cut the pieces, and—I kid you not—two-sided tape to put it all together. Kind of like a Christmas ornament I made for my mom one year (except I used safety scissors instead of lasers—I’m old school like that). The concept itself was brilliant but expensive (Whitesides’, not my ornament), so Liu came up with the idea of using origami, hence the name of Liu and Crooks’ sensor—the origami Paper Analytical Device, or oPAD.
This whole thing got me wondering how exactly paper sensors work. Just the idea that we can print something out, fold it up, and test ourselves for pregnancy or disease is amazing to me. So I looked into the most basic version to get the gist: the one-dimensional pregnancy test.
A pregnancy test is simple because it looks for only one substance: a hormone, called human chorionic gonadotropin (HCg), found in pregnant women’s urine and blood. An antibody to HCg is embedded in the paper, so that if the HCg (the antigen to the antibody) is in the urine, it will bind to the antibody in the paper.
When this binding happens, it triggers an enzyme to produce dye, which will then color that part of the paper. Most of the rest of the paper (usually chromatography paper) is coated with a water-resistant substance (such as wax or photoresist), with grooves that channel the urine to the uncoated area where the test reagents (in this case, the HCg antibodies) are.
That’s the basic idea behind the test, but there’s actually more to it to ensure accuracy—a control area in addition to the test area. I’m going to use what I think is a European test as an example because the areas are numbered and clearly delineated, but the concept is the same in all paper devices for determining pregnancy. In the photo below, area 1 is the urine collection spot. Just beyond this spot, before window 2, are mobile antibodies to HCg, meaning they are not bound to the paper as described above. Let’s call these antibodies 1 (A1).
An excess of A1 will be placed on the paper here so that HCg will start binding with it but both the HCg and the A1 will continue to go with the flow toward window 2, leaving enough unbound HCg to bind with the embedded (bound) antibodies in window 2 (the area I initially described above). Let’s call these antibodies 2 (or A2).
Some of A1 will be left unbound to the HCg, so it will keep on moving to window 3. In this area is yet another type of antibody (A3), but this antibody is actually an antibody to A1. So when the last of the A1 makes it to window 3, it binds with A3, triggering an enzyme to dye the area in window 3.
Window 3 is the control area. It doesn’t react with HCg, so it doesn’t tell a woman she’s pregnant. It simply shows that the test is working, that A1 successfully traveled down to window 3 and reacted. The dye in window 2 confirms whether or not the woman is pregnant. It could be a single line, or a plus, or the word “pregnant”—whatever shape the manufacturer leaves uncoated in that area so that the dye can seep through.
Some tests have only one window, which will show two lines if pregnant, one if not, and I guess there are digital ones out there now as well (although that’s getting beyond the point here, which is to compare to the 3-D paper sensor). Anyway, that’s the basic idea.
So this new kind of 3-D paper sensor takes that concept a step further, allowing people to test for multiple biomarkers instead of just one, which is necessary for tests such as those for HIV. You just print it out and fold it by hand, which takes less than a minute, according to Liu and Crooks (although I’m skeptical for those of us who are origami-challenged).
The grooves in the paper channel the urine, blood, or saliva to areas on the paper where test reagents are bound. If the sample has whatever substance being tested for, the paper will change color in that spot or glow under UV light.
So there you have it, where art meets biochemistry to produce an awesomely crafty science love child.
Even cooler? Crooks and Liu have also created a low-cost way to power the sensor for tests that require it, using the salt in the urine itself as an electrolyte for the battery.
What will they think of next? My money’s on something involving duct tape.
* A reader, pciszek, posed some excellent questions in the comments about how someone printing it on their printer would get the waterproofing and reagents on the paper. At the link to the abstract, there’s an image showing that the substances are injected, so I asked the researchers about the coating and the availability of the substances as well as the need for a specialist to work with them. Dr. Crooks responded immediately with this explanation:
The printer we use is a wax printer that costs about $700. The wax is available commercially: no modification is made to the printer or to the wax. We do use a special kind of paper usually, but it is also available commercially. We have used regular photocopier paper and it works OK too.
The only thing that really requires a bit of technical skill is adding the necessary reagents to the device (like the antibodies referred to in your blog), but this can be done by anyone with a little training. I think just about anyone could order the antibodies: we get them from a commercial supplier and as far as I know we have never had to have a license of any type to get them.
In the near future I believe these devices would be made in a regular manufacturing facility, simply to keep the cost down and the volume up. However, in the longer term there’s no reason why they couldn’t be manufactured in resource-limited environments like parts of Africa, Asia, and So. America.
Liu and Crooks, “Three-Dimensional Paper Microfluidic Devices Assembled Using the Principles of Origami,” Journal of the American Chemical Society 2011, 133 (44), pp 17564–1756.
Liu and Crooks, “Paper-Based Electrochemical Sensing Platform with Integral Battery and Electrochromic Read-Out,” Analytical Chemistry, 2012, 84 (5), pp 2528–2532.
“Origami-Inspired Paper Sensor Could Test for Malaria and HIV for Less than 10 Cents, Report Chemists,” Science Daily, March 8, 2012.
Image credits: Alex Wang (featured image), Flickr Creative Commons, and Wikimedia commons