A team of researchers collaborating across countries has developed a portable microfluidic device that can analyse samples for the presence of DNA and RNA of microbes you usually don’t want in the samples in the first place.

While that in itself isn’t unusual—a lot of work has been going on for several years to develop cheap, fast and portable systems—this device pushes the boundaries of just those highly desirable features. Pavel Neužil, senior researcher at Brno University of Technology, Czech Republic, told EDN Asia that he sees the following as the key features of the device:
 

  • Simple system making it economical
  • Extremely low cost usage below RMB1 per reaction
  • Fast reaction time, down to 10 minutes for 40 PCR cycles

 
PavelAtBEBC Professor Pavel Neužil meeting students at the Bioinspired Engineering and Biomechanics Center in Xi’an, Shaanxi, China.  

“Processing four samples a time is in our opinion minimum number for practical point-of-care system,” he added in an email response. Neužil has been conducting research in this area for several years and some of his published papers go back to the days of SARS (avian flu). He is currently at the Northwestern Polytechnical University in Xi’an, China, where he is aiming to sell the device—hence the reaction cost in Renminbi (Chinese Yuan).

PCR primer for electronics engineers

PCR stands for polymerase chain reaction, which is a way to make many copies of sections of DNA. PCR is used in the early stages of processing DNA for detecting the presence or absence of a gene, such as when you want to identify the presence of pathogens. To setup a PCR, you need the DNA to be copied, primers or parts of DNA (made in a lab) that attach to either side of DNA to be copied, DNA bases (you’d recall A, C, G and T), an enzyme and a buffer.

In general, however, any PCR system needs to accomplish three key steps:
 

  1. Denaturing: The double-stranded template DNA is heated, usually to about 95°C, to separate the strands.
  2. Annealing: The temperature is lowered to the low 50’s range (°C) to enable the DNA primers to attach to the template DNA.
  3. Extending: The temperature is raised again but to a sligntly lower temperature (~72°C) and the new strand of DNA is made by the Taq polymerase enzyme.  

Interesting fact: The Taq polymerase is a thermostable DNA polymerase named after the thermophilic bacterium Thermus Aquaticus—hence Taq—from which it was first isolated in 1976.


Now, this process needs to be repeated 20-40 times; in the case of the new device it is done 40 times as mentioned above. With each cycle the number of DNA copies are doubled. Yes, you want many copies of the target DNA. Why? That’s because you are isolating a very specific segment of DNA in an environment of lots of DNA material—you need many copies of the target segment to be able to work with it.

The basic principle of a PCR device, therefore, involves heaters and sensors, tight control of heating and cooling, an optical sub-system of dichroic mirrors, a photodiode to detect fluorescence and, of course, a processor. Shown below is a block diagram of Neužil's device (more detail on next page).

PCR BlockDiagram Neuzil Figure 1: Block diagram of Neužil's PCR device. Concurrent processing of four samples as shown above significantly cuts down time required to complete the test.
 

The problem originally was that this process required rather sophisticated but unwieldy machines that would take hours to complete the number of PCR cycles required to take meaningful measurements to detect SARS or Ebola for instance.

The new device aims to change all that.

 
Next: PCR testing in your hands »