© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

9.4.3.4 Contact Event Cycling

How fast can ambulatory contact events be cycled? Molecular receptors ~10 nm² in size can bind large (5-10 nm), "common" (~10^-2/nm³) molecules in t_EQ ~ 0.2 microsec (Section 4.2.1). With ~10⁵/micron² glycocalyx strands and a similar number of integral proteins near the cell surface (Section 9.4.3.1), there are ~10 strands and ~10 proteins under each 100 nm² footpad. For either glycocalyx strands or integral proteins, mean separation is ~3 nm giving a concentration of ~4 x 10^-2/nm³ > 10^-2/nm³, thus qualifying as "common" molecules in the footpad environment. (There are also ~250 bilayer lipid heads directly beneath a 100 nm² footpad, of which ~100 are phospholipids, ~50 are cholesterols, 10-75 are glycolipids, and the remaining 25-90 are other lipids.) Alternatively, mechanical grippers that close and open on a 5-nm wide molecular target using a 1 cm/sec jaw speed require ~1 microsec per gripping cycle.

Finally, a mechanical manipulator appendage traveling at a conservative ~1 cm/sec (Section 9.3.1) can transit the 10-100 nm glycocalyx, reaching the plasma membrane surface, if necessary, in 1-10 microsec. A similar amount of time must be allowed for the limb to be retracted from the cell surface. These figures are all consistent with a maximum leg-swing frequency n_leg ~ 100 KHz.

Last updated on 21 February 2003