Ratchet patterns sort molecular shuttles

Molecular shuttles based on microtubules propelled by motor proteins can be guided on surfaces by adsorbing motors in chemical patterns or by using open guiding channels. While chemical patterns can guide microtubules based on a Brownian ratchet mechanism, the rigidity of the microtubules limits guiding to features with dimensions on the order of their persistence length (5 mm). To achieve guiding on micron-scale dimensions, physical barriers are required which can exploit the forces exerted by multiple motors to bend tubules into tight radii of curvature. Microtubule guiding is illustrated for the case of a special ratchet pattern that is capable of sorting microtubules on the basis of the direction of their motion.