diameter that can affect flow it is believed that the answer can be found in the data in fig6 coupled with the posited channel constriction mechanism illustrated at the bottom of fig1 at the edges of the microchannels where the curved regions meet the flatter bottom segment see fig1 the channel height is at or near the u02dc 10 nm molecular scale such that biotin streptavidin crosslinking of the top surface to the bottom is possible this interaction would pull the top and bottom surfaces incrementally closer enabling similar molecular scale crosslinking to occur moving inward toward the middle of the channel as this interaction progresses the edges of the channel would be constricted while the middle would remain open i e fig1 j k importantly the data in fig6 a b are indicative of exactly this type of change in cross sectional channel profile induced by streptavidin solution flow strongly supporting the hypothesized mechanism in further support of surface intermolecular interactions leading to channel constriction some simple force calculations were made a typical surface density of b bsa molecules is 6 u00d7 10 16 m 2 24 while the force needed to unbind one biotin streptavidin molecular pair has been measured as 200 300 pn 25 26 hence the force per area exerted by biotin streptavidin surface interactions would be 1 2 u00d7 10 7 n m 2 or 1740 psi which is at least a factor of 100 greater than the 5 10 psi needed to completely close similarly shaped pdms microfluidic valves 21 thus it is concluded that molecular scale interactions have sufficient force to induce channel constriction and that the occurrence of such interactions from the edges toward the.