Cantilevers are miniature diving boards that measure 200 micrometers long and 40 micrometers wide, about half the width of a human hair. Two cantilevers are placed in a sensor and liquid is passed through them. When the molecule or microbe that is being looked for binds to its surface, the board bends and its electrical resistance is altered. Detection is achieved by measuring the change in resistance.
The device can be designed to search for specific things, for example, if the organism to be detected was E. coli, the cantilever could be coated in antibodies specific to E. coli cells. Many different molecules or organisms can also be recognized simultaneously. “The sensor can be expanded to contain several cantilevers, each coated with a specific detector molecule” says Professor Anja Boisen.
Lid devices also have a flexible board or ‘lid’ but it is placed on top of a tiny box that contains marker molecules, which produce colour visible to the naked eye. An organism, for example, binds to the lid, which then opens and releases the colour, indicating the presence of the organism. This can also be achieved by coating the board with ‘food’ for bacteria instead of binding molecules, so deflection occurs when the coating is removed. It can therefore be used to measure bacterial activity. The device is contained in a 1cm plastic box so, like the cantilever, it is portable.
Cantilevers and lid devices may soon be available to consumers. “We use processes where the cantilevers are fabricated by etching a thin silicon wafer three-dimensionally” says Professor Anja Boisen. “The procedure is suitable for mass production and it might be possible to make sensors so cheaply that they can be disposable.”
The applications for this new technology are abundant. The sensors can detect DNA, so may be used to test for human genetic diseases. They are also extremely sensitive and can measure deflections of just 1 nanometre, so are able to detect the presence of very small molecules. Conversely, whole bacteria and even parts of bacteria can be identified, making the sensors ideal for testing the quality of water and food samples.
“The lid device could be included in food packaging since it requires no external energy and is cheap to make. When a food is infected, the control unit in the plastic wrapping becomes coloured. Thus a simple colour indicator can show the quality of the food.”
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