The rate of diffusion of a particle dependent on its hydrodynamic radius – a relationship that is as described by the Stokes-Einstein equation. The Fluidity One exploits this observation by measuring the rate of diffusion of proteins in a microfluidic system under steady state laminar flow.
To achieve steady state laminar flow, the Fluidity One uses a microfluidic system in which microlitre volumes do not mix convectively. This means when a protein sample and an analyte solution are run as adjacent (non-mixing) streams, the only way protein molecules can migrate into the analyte stream is by diffusion – at a rate that is dependent on their size. This is the measurement step in the Fluidity One system, and occurs in the diffusion chamber of the chips without labelling.
Following measurement, the streams are split and a latent, fluorogenic dye is introduced to label the protein and enable detection. Fluorescence intensity is then measured in both streams, the total of which is used to determine concentration of protein, and the ratio of which can be used to determine the diffusion co-efficient, and consequently the hydrodynamic radius.