A relatively simple DNA test can be used to diagnose infectious disease. The technique, called polymerase chain reaction (PCR), makes use of what we know about the genome sequences of disease-causing microbes.

In short, an appropriate patient sample -- e.g., blood, saliva, skin, etc. -- is taken. The sample is chemically processed to extract DNA. The DNA then undergoes "PCR," which is a process that amplifies a highly specific sequence of DNA. ("Amplifies" means that the target DNA sequence is increased from one molecule to billions of copies.) In the case of diagnostics, clinicians select a DNA sequence that is unique to the microbe of interest. A patient who has the disease would therefore provide a sample that, after PCR is performed on it, would yield billions of copies of a highly specific DNA fragment that is unique to a particular microbe -- indicating a positive reaction. The DNA is visualized by adding a dye, such as SYBR Green, which binds to double-stranded DNA.

This is a well-established biomedical technique. The trouble, however, is in developing countries, which often lack electricity or the equipment necessary to perform PCR. The amplification of DNA requires a machine capable of cycling high temperatures (203 degrees F, 140 degrees F, 162 degrees F) over and over again. This is an energy-intensive process.

Now, researchers from Cornell University have created a portable device that utilizes sunlight and an iPhone to amplify DNA and diagnose disease. (See image.)

Sunlight is focused onto a microfluidic chip that is designed in such a way that samples swirl around inside, being heated and cooled to the appropriate temperatures. After several cycles, the DNA has been amplified. It is then removed from the microfluidic device and mixed with SYBR Green dye. Finally, it is added to a small chip and illuminated with an iPhone-powered blue light. (The dye will absorb blue light and fluoresce green.) An iPhone app then detects the amount of green light emitted. (See image.)

As shown in Panel B, samples #1 and #2 glow green. Panel C depicts a screenshot of the iPhone app that detects the green light and declares the samples "positive" for the microbe of interest.

Even better, the device consumes a mere 80 milliwatts (mW) of power and could be very useful to clinicians who practice in poverty-stricken parts of the world. A very clever invention!

Source: Li Jiang et al. "Solar thermal polymerase chain reaction for smartphone-assisted molecular diagnostics." Scientific Reports 4, Article number: 4137. Published 20-February-2014. doi:10.1038/srep04137