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

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

5.4 Metamorphic Bumpers

Nanorobots may use circumferential metamorphic bumpers to achieve a continuously tight fit to neighboring devices while laboring cooperatively in nanotissues overlaying moving biological surfaces (e.g., see Chapter 22). Bumpers are crudely analogous to the cell-cell adherens junction found in human epithelial sheet cells.⁵³¹ In this junction, a continuous adhesion belt (the zonula adherens or belt desmosome) surrounds each of the interacting cells in the sheet, resulting in a narrow adhesive zone mediated by Ca⁺⁺dependent transmembrane linker glycoproteins. Within each cell, a contractile bundle of actin filaments runs adjacent to the adhesion belt and parallel to the cellular membrane, serving as control cables for this biological junctional "bumper" system. Oriented contraction of the actin filaments causes specific localized movement of the sheet, as for example the rolling of epithelial tissue into a closed tube. Nanorobots linked by narrow equatorial-band or wide whole-wall bumpers into nanotissues may perform similar feats, under continuous computer control.

The other principal class of anchoring junction is the spot desmosome, a buttonlike point of intercellular contact that rivets together certain types of epithelial tissue cells. Each desmosome is a spoon-shaped molecule about 30 nm long, spaced at ~8 nm intervals across the cell surface, with the straight tail of each spoon firmly embedded in a subsurface cytoskeletal plaque and the head of the spoon protruding ~10 nm outside of the cellular lipid bilayer membrane.^312,531 Matching spot desmosome heads on neighboring cells adhere, gluing the cells together while maintaining a 10-20 nm intercellular gap.

Tighter junctions (~2 nm) called occluding junctions form nearly watertight seals in certain critical areas such as between the cells lining the digestive system. Blood platelets are also intensely reactive cells that respond to a variety of stimuli to undergo shape change, adhesion, primary and secondary aggregation, and a process known as viscous metamorphosis in which membrane fusion occurs between adjacent platelets with the loss of membrane integrity.⁵³⁰

Last updated on 18 February 2003