Low-cost device exploits birefringence for point-of-care diagnostics

Researchers at the Swiss Federal Institute of Technology (ETH Zurich; Switzerland) have developed a new diagnostic method based on birefringence, the ability of substances to change the polarization state of light. With the method, doctors around the world can easily, rapidly, and reliably detect malaria, Ebola, or HIV, among other infectious diseases.

Related: Infectious disease control with portable CMOS-based diagnostics

With the method, a drop of blood is placed on a special carrier substance and after a few minutes, the slide is placed on a device that emits polarized light, thanks to an inexpensive polarization filter. It is covered with a lid that contains a second polarization filter, which blocks the light from all materials except crystalline or materials with directional properties. If light is visible through the cross-polarizer filter, a positive diagnosis is made. Through this, an immediate "yes or no" screening is possible. It is also possible to measure the light intensity and, thus, the amount of the pathogen through a simple light meter plugged into a smartphone and controlled via an app.

The detection methods are extremely fast, as well as considerably less expensive when compared with other detection methods. The polarization device costs around $20, says Jijo Vallooran, first author of study.

With a light meter and an app on his smartphone, researcher Jijo Vallooran is measuring the intensity of the birefringence signal of a sample. (Image: Laboratory Prof. R. Mezzenga/ETH Zurich)

The researchers use the phenomenon of birefringence of polarized light from the lipid-based lyotropic liquid crystals, which consist of self-assembled structures of fat molecules in water. The research team, led by Raffaele Mezzenga, professor of Food and Soft Materials at ETH Zurich, has been working with these liquid crystals for a long time and exploits their utility for other applications, such as drug delivery and protein crystallization.

Lyotropic liquid crystals organize themselves into special networks with unique symmetry, which means that their basic motif repeats itself periodically. In the case of liquid crystal cubic phases, the channels are made of lipid bilayer membranes in water and have a diameter of just a few nanometers, so only few free water molecules are available in the liquid crystal, whereas the majority is bound to the channel walls. These liquid crystal cubic phases are isotropic (do not have any birefringent properties), which means that if a slide with a layer of lyotropic liquid crystal films is placed under a light source that allows polarized light to pass through, it appears black when observed through another polarizer tilted at 90°.

Birefringence pattern of a sample positive to Ebola infection. (Image: ETH Zurich/Jijo Vallooran)

To achieve birefringence and thus receive a signal, the researchers added certain enzymes to the liquid crystal to allow a chemical reaction to take place in the nanotubes. Since only a small amount of water is freely available in the nanotubes, the products of the reactions precipitated together to form crystals, which are themselves birefringent. A closer look at the sample through a second polarization filter placed above it and perpendicular to the first shows a light pattern in instances where the enzyme has reacted with the substance tested. “This birefringence pattern is the only signal that we need to use for diagnostics and analysis,” Mezzenga says.

At the beginning of their research, the scientists tested their system with chemical compounds that can be enzymatically converted. They then refined their method and adapted it for medically relevant substances, such as glucose and cholesterol. In further steps, they broadened the scope to tests on bacteria and viruses, starting with the HIV virus. Eventually, Mezzenga, Vallooran, and colleagues were able to show that their method could also be adapted to diagnose malaria caused by Plasmodium parasites. “Plasmodium parasite invades erythrocytes and digests hemoglobin. The heme component, which is toxic to the parasites, is crystallized and thus has inherently birefringent surfaces. So it’s not necessary to mark it with antibodies and no enzymatic reaction is required,” Mezzenga explains.

Normally, viruses and bacteria must be made visible and chemically active with specific antibodies with enzymes coupled to them before they can be detected by birefringence.

Prototype of the cross-polarization device. (Image: ETH Zurich/Jijo Vallooran)

“Our test system can be extended to a large number of different viruses or bacteria. It is totally flexible,” stresses Vallooran. As it is so easy to use and because only one set of antibody-enzyme conjugates is necessary to detect viruses, the ETH researchers believe it can be used particularly in areas where expensive laboratory equipment is otherwise unaffordable. “Other than a refrigerator to store the antibodies and enzymes, the user needs only the box to detect the polarized light and the lipid carrier substance. This is very inexpensive,” says Vallooran. Pathogens such as HIV or even Ebola can be detected very rapidly, and a reliable result is received within less than an hour. “Our technology is very suitable for use in the field and the early detection of diseases.”

Vallooran received an ETH Pioneer Fellowship for his research in this technology. The researchers have also filed a patent application for it, but more funding is still needed to bring the development to market.

Full details of the work appear in the journal Advanced Functional Materials; for more information, please visit http://dx.doi.org/10.1002/adfm.201503428.

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