MDS exploits the unique properties of flow in microfluidic channels – specifically laminar flow, where streams can flow alongside one another with no convective mixing.
Watch the animation explaining MDS below, or there is a pdf download available here.
In the Fluidity One a stream of your protein of interest is introduced alongside an auxiliary stream.
Because there is no convective mixing the only way your protein can migrate into the auxiliary stream is by diffusion, the rate of which will depend on the size of the protein. Small proteins will diffuse very rapidly, and large proteins more slowly.
Diffusion can occur at any point along the length of the diffusion chamber, but at the end the streams are re-split, and at this point the degree of diffusion is fixed.
The quantity of protein in each stream is then determined using an amine reactive fluorogenic dye.
The combined total fluorescence is used to determine the concentration of protein in solution, and the ratio of fluorescence between the two streams is used to determine the diffusion co-efficient of the protein, which in turn is used to determine the protein’s hydrodynamic radius.
Because diffusion occurs while the protein is in its native state, the reported size is that of the native protein in solution. And because this process all takes place in microfluidic volumes, the measurement can be performed quickly and with minimal sample input.