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

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

9.4.2.5 Nanomechanisms for Natation

Swimming motions consisting purely of reciprocal deformations (e.g., shape A deforms into shape B, then retraces the same motion back to shape A) cannot produce forward progress in a viscosity-dominated environment.^389,1386 For example, in the macroscale world, a scallop may open its shell slowly, then close it rapidly to squirt out water, relying upon inertia and "coasting" to carry it forward a little bit each cycle. At the low Reynolds numbers of most medical nanorobots, such single-hinge deformations yield only back-and-forth motion. Indeed, Fukuda et al¹³⁸⁵ estimate that a vibrating-fin-driven water-swimming robot shorter than ~6 mm can no longer overcome viscous drag.* Purcell³⁸⁹ notes that the simplest possible mechanical swimmer may require at least two hinges and a cyclical deformation loop traced out in a two-dimensional configuration space.

* The smallest known swimming fish is the dwarf pygmy goby (Pandaka pygmaea), measuring 7.1-9.7 mm long in the adult form.⁷³⁹

At least four distinct classes of mechanisms for natation are readily distinguished, as described below.

Last updated on 21 February 2003