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Most of us are by now familiar with lab on a chip systems: That mix of fluidic channels and micro circuitry on a chip that enables chemical tests to be performed computationally and in parallel on any liquid source such as a drop of a patients' blood dropped onto it.
Many of us are also familiar with the concept of printing such lab on a chip systems onto ordinary paper, using a special ink for the channel, and a metallic ink for the circuitry. This has allowed otherwise prohibitively expensive biosensors to be used once and thrown away, even in places such as third world clinics, where both available funds and technology are limited.
The problem is, such biosensors are very specific: They can only find those specific conditions the circuitry was designed to test for. If you wish to test for another condition, you must print out the specific circuitry for that condition and apply another drop of blood or urine. So, they are faster and cheaper than traditional testing methods, but there is still a lot of room for improvement.
Wouldn't it be wonderful if we could test for a wide range of conditions, on a single sensor?
Well, we can, but it took a little lateral thinking to achieve this. Enter chemists at The University of Texas at Austin. USA. Specifically, Richard Crooks, the Robert A. Welch Professor of Chemistry and his doctoral student Hong Liu. They took the ancient Japanese art of Origami the art of paper folding, and applied it to these paper sensors.
Rather than trying to create one circuit that detects everything, put the basics of an incomplete circuit on one part of the paper, and other incomplete circuits on other sections of the paper, then include scored fold marks in specific places. To create one biosensor, fold the scores in a specific way, and the partial circuit diagrams line up to test for malaria, say. Fold the same sheet in a different pattern, and the circuitry lines up for HIV detection. A third configuration detects for complications in pregnancy. So on and so forth.
Because the sensor is already saturated in blood or urine from the initial droplet, no additional samples need to be taken. Just unfold the sensor, and refold it in the desired way. Connections are broken, new connections are made, and the liquid is drawn through new channels and into new testing gateways. It is simple, elegant, and ingenious. A single design sent to all the clinics in an area, will perform basic tests on all the common conditions encountered there.
The inspiration for the sensor came when Liu read a pioneering paper by Harvard University chemist George Whitesides. Whitesides was the first to build a three-dimensional microfluidic paper sensor that could test for biological targets. His sensor, however, was expensive and time-consuming to make, and was constructed in a way that limited its uses.
They had to pattern several pieces of paper using photolithography, cut them with lasers, and then tape them together with two-sided tape, says Liu, a member of Crooks lab. When I read the paper, I remembered when I was a child growing up in China, and our teacher taught us origami. I realized it didnt have to be so difficult. It can be very easy. Just fold the paper, and then apply pressure.
Within a few weeks of experiments, Liu had fabricated the sensor on one simple sheet using photolithography or simply an office printer they have in the lab. Folding it over into multiple layers takes less than a minute and requires no tools or special alignment techniques. Just fingers.
Crooks says that the principles underlying the sensor, which theyve successfully tested on glucose and a common protein, are related to the home pregnancy test. A hydrophobic material, such as wax or photoresist, is laid down into tiny canyons on chromatography paper. It channels the sample thats being tested urine, blood, or saliva, for instance to spots on the paper where test reagents have been embedded. If the sample has whatever targets the sensor is designed to detect, itll react in an easily detectable manner. It might turn a specific color, for instance, or fluoresce under a UV light. Then it can be read by eye.
Biomarkers for all kinds of diseases already exist, says Crooks. Basically you spot-test reagents for these markers on these paper fluidics. Theyre entrapped there. Then you introduce your sample. At the end you unfold this piece of paper, and if its one color, youve got a problem, and if not, then youre probably OK.
Crooks and Liu have also engineered a way to add a simple battery to their sensor so that it can run tests that require power. Their prototype uses aluminum foil and looks for glucose in urine. Crooks estimates that including such a battery would add only a few pennies to the cost of producing the sensor.
You just pee on it and it lights up, says Crooks. The urine has enough salt that it activates the battery. It acts as the electrolyte for the battery.
Origami-Inspired Paper Sensor Could Test for Malaria and HIV for Less than 10 Cents, Report Chemists