A University of Tokyo group of researchers, led by bioengineer Shoji Takeuchi, has made an electronic sensor capable of smelling gases. The sensor uses genetically engineered frog cells. Since previous sensors were not very accurate, the scientist decided to try a biological approach. The invention was revealed in a US scientific journal yesterday, and is supposed to be used to design better machines to detect polluting gases in the atmosphere.
Previous smell sensors were based on quartz rods, which vibrate when a substance binds to them. The gases are distinguished by their molar masses, which can be similar for molecules with different structure, thus relatively often triggering a false positive. Trying to find a more accurate solution, Takeuchi decided to follow an example from insect world. As he explained, "when you think about the mosquito, it is able to find people because of carbon dioxide from the human. So the mosquito has CO2 receptors. When we can (extract) DNA (from the mosquito) we can put this DNA into the frog eggs to detect CO2."
Genes of several insects (the silk moth, diamondback moth and fruit fly), injected into African clawed frog Xenopus laevis eggs, allowed them to produce relatively inexpensive and useful sensors. The choice of the species was caused by their widely studied and well-understood protein expression mechanism.
The modified cells responded to three kinds of pheromones and one odourant, which have similar chemical properties. When a molecule of an odorous substance adhered to the receptor on the membrane protein, ion channels opened for a certain period of time, and a current was generated. Its magnitude was clearly different for all four tested substances, allowing to distinguish between them accurately.
The colleagues embedded the sensor into a mannequin, so that it could shake its head when a gas was detected. It was easier to observe. Pheromones and molecules with quite similar molecule structure produced clearly distinguishable reaction, with higher accuracy than other biological or human-made sensors. As the research group said, the detection sensitivity of the odor sensor is several tens of parts per billion (ppb), and it is as high as the sensitivity of an existing odor sensor that uses an oxide semiconductor. The distinctive feature of the new sensor is its capability to selectively detect some odorous substances, rather than its sensitivity. Very few false positives were possible due to the biological mechanism involved.
At normal temperature, the sensor lifetime is about 12 hours, which can be extended by putting it into a refrigerator before first use.
Shoji Takeuchi says has a great hope for research use in future, since the frog eggs are very practical for genetic engineering, and can be conveniently used to develop smell sensors for a wide range of gases. He said, "The X. laevis oocyte has high versatility for the development of chemical sensors for various odorants. We believe that a shared ability to smell might open a new relationship between man and robot. .. The research will have wide implications... If the sensor is embedded in a nursing robot, it will be able to identify certain mouth odors or body odors. Also, it can be used for detecting CO2, air pollution, water pollution and food. It's very important for the environment."

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Japanese researchers create smell sensor using genetically engineered frog eggs