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

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

6.3.4.3 Artificial Chemomechanical Power Conversion

Chemomechanical gel actuators have been reviewed by Tatara,⁵⁸⁷ who describes a variety of polyelectrolyte gels and ion-exchange resins used to make centimeter-scale "mechanochemical pistons" and 100-micron polyelectrolyte fibers used to make chemomechanical "finger" actuators; unfortunately, these electrically-stimulated transducers may fatigue after only ~500 cycles. More interesting is the Sussmann-Katchalsky chemomechanical turbine (Fig. 6.3) -- a continuous collagen fiber contracts as it enters a 10M LiBr solution, producing a contractile force that is converted into a torque to rotate the shaft of the output pulley.⁵⁹⁷ The device has been patented, built, and operated with an energy conversion efficiency of ~40%. While it would be difficult to scale down this turbine to submicron dimensions, it nonetheless constitutes another useful proof of principle that direct chemomechanical power transduction may need only surprisingly simple mechanisms.

Astumian⁶⁹⁶ has examined the use of nonequilibrium chemical drivers for micron and submicron Brownian motors. For instance, when ATP (negatively charged in solution) binds to a protein, it changes the protein's net charge. Thus two proteins that react with ATP at different rates would feel different electrostatic forces; this force differential can drive a motor. Astumian estimates that even a crudely designed chemically-driven Brownian motor could move in 10-nm steps at ~3 microns/sec, developing ~0.5 pN of force, ~3 x 10^-6 pW of power, and a power density of ~10⁶ watts/m³. Many types of cellular protrusions such as filopodia, lamellipodia, and acrosomal extension do not involve molecular motors but rather the transduction of chemical bond energy into mechanical energy using Brownian ratchets to rectify random thermal motions by expending chemical energy in actin or tubulin polymerization.¹²⁰³

Last updated on 18 February 2003