A method for establishing stable concentration gradients in agar suitable for studying chemotaxis on a solid surface

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Abstract
A simple technique has been developed for establishing stable gradients of a substance in agar. The technique involves the creation of a spherically symmetric concentration profile in which concentration varies inversely with the distance from the source and is independent of the diffusion coefficient of the substance. It has been shown that the gradients established with this technique are stable forat least 190 h, and, on a theoretical basis, they can be kept stable for more than 1000 h. Time-variant gradients can also be established, if desired, using the same system and limiting either the source or the agar sink. It must be emphasized that a stable gradient cannot be obtained by using a shallow agar layer as a sink. The use of such conditions (e.g. the agar in a standard petri dish) can result only in time-variant gradients. The solution to the diffusion equation in a spherically symmetric system establishes the expected concentration profile, the basis for adjusting it, and the parameters that control the behavior of the system. Some useful applications for examining chemotaxis on a solid surface as well as possible further developments are discussed.

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