A new device uses polarized light to quickly diagnose a host of diseases, including malaria, HIV, and Ebola.
The technology is based on birefringence, the ability of substances to change the polarization state of light.
“Our test system can be extended to a large number of different viruses or bacteria. It is totally flexible.”
Here’s how it works: Place a drop of blood on a special carrier substance. After a few minutes, place the slide on a device that emits polarized light—thanks to an inexpensive polarization filter. The device is covered with a lid containing a second polarization filter, which blocks the light from all materials except those that are crystalline, or materials with directional properties.
The diagnosis is positive if light is visible through the cross-polarizer filter. Scientists say 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 method is not only extremely fast, but also considerably less expensive when compared to other detection methods.
Although the device is simple to operate, the science behind it is fairly complex.
Birefringence and liquid crystals
It utilizes 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.
Raffaele Mezzenga, a professor of food and soft materials at ETH Zurich, and her team have been working with these liquid crystals for years and use them 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.
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These liquid crystal cubic phases are isotropic, which means they do not have any birefringent properties. If scientists place a slide with a layer of lyotropic liquid crystal films under a light source that allows polarized light to pass through, it will appear black when observed through another polarizer tilted at 90 degrees.
To achieve birefringence and thus receive a signal, the researchers added 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,” says Mezzenga.
HIV, malaria, and more
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 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 crystalized and thus has inherently birefringent surfaces. So it’s not necessary to mark it with antibodies and no enzymatic reaction is required,” explains Mezzenga.
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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.
“Our test system can be extended to a large number of different viruses or bacteria. It is totally flexible,” says Jijo Vallooran, first author of the paper in Advanced Functional Materials.
Because it is so easy to use and because only one set of antibody-enzyme conjugates is necessary to detect viruses, the researchers believe it could be particularly helpful 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 received within less than an hour. “Our technology is very suitable for use in the field and the early detection of diseases,” adds Vallooran.
Source: ETH Zurich
