Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
220.127.116.11 Macromolecular Cross-Interface Adhesion
Nanorobots may present to the cytoplasm a variety of working surfaces that must remain free to slide with respect to each other. Examples include telescoping manipulators with adjacent rotating tube segments (Section 18.104.22.168), screw drives with a rotating interface at a fixed housing (Section 22.214.171.124.2), or various metamorphic surfaces with adjacent motile plate segments (Section 126.96.36.199) or telescoping segments (Section 188.8.131.52). Noncovalent adhesion of a large macromolecule (e.g., a free-floating kinesin motor ) at two or more points on either side of the interface would produce an exogenous force that resists free rotation or translation of adjacent segments, leading to immobilization of the mechanism and possible device failure. Similar problems may arise for nanorobots negotiating the extracellular spaces or the vascular system.
A comprehensive analysis is beyond the scope of this book, but a simple example should suffice to illustrate how this situation may be resolved. Consider the 7-interface telescoping manipulator described in Section 184.108.40.206. With zero load at 1 cm/sec travel speed the total of all frictional losses amounts to ~0.1 pW , giving a minimum no-load power density of 109 W/m3 for the 100 nm long, 30 nm diameter tubular manipulator structure. The maximum power under heavy load was not estimated, but other nanomechanical systems such as the sliding diamondoid logic rods and registers of Drexler’s nanomechanical computer  assume power densities approximating 1012 W/m3. At this maximum power density, the telescoping manipulator could apply a total of ~70 pW of mechanical power, or ~10 pW per sliding interface or tube segment. A 1.5 nm diameter drive shaft with a tangential velocity of ~1 m/sec turns ~700 times to drive a tube segment through one complete rotation, hence the tube segment rotates at ~2.85 cm/sec and so each tube segment can provide a shearing force of ~350 pN, relative to its neighbor. This exceeds the noncovalent adhesion forces commonly encountered in protein-protein single-molecule interactions (Section 220.127.116.11) by roughly an order of magnitude, and exceeds single-molecule kinesin motor microtubule-microtubule binding forces  by almost two orders of magnitude. Additional protection may be afforded: (1) by constant fine motion (e.g., “jiggling”) of all moving segments to reduce cross-interface adhesion, (2) by providing self-scraping exterior housings, and (3) by using anti-adhesive exterior coatings (Section 18.104.22.168).
Last updated on 30 April 2004