© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.

Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.

B.4 Acoustic Transducer for Power and Control

The proposed molecular assembler receives all power and control signals acoustically. Externally generated ultrasonic pressure waves travel through the solvent solution to the assembler device, whereupon a piston on the device is driven back and forth in a well-defined manner, passing energy and information simultaneously into the device.

Although an acoustically-actuated nanoscale piston has not yet been demonstrated experimentally, microfluidic actuators are well known [3122] and there are many reasons to expect that such small pistons will work as theory [208] predicts. For example, microscale pressure sensors have already been built using conventional MEMS fabrication techniques – e.g., a piezoresistive pressure microsensor diaphragm [3123], a 250-micron medical pressure sensor that fits inside catheters [3124], a 27-micron circular capacitive pressure sensor [3125] and an optical pressure sensor 7 microns thick [3126]. Micromachined flaps 45 microns in size have been raised from horizontal to vertical position by ultrasonic pulsing [3127], demonstrating microscale acoustic actuation. Neutrophils ~8 microns in diameter that have phagocytosed a gas-filled microbubble exhibit large volume oscillations when insonated, with the trapped microbubbles experiencing viscous damping approximately sevenfold greater than for free microbubbles [3128]. HeLa cells acoustically irradiated at 1.5 MHz for 10 minutes lost microvilli and became smooth at 41.5 ^oC, but developed a heavily pitted and porous surface at 45 ^oC; careful monitoring suggested the damage was caused by bubble surface oscillations – that is, by acoustic energy transfer – not by collapse cavitation [3129]. Paclitaxel-carrying 2.9-micron lipospheres are “acoustically active,” releasing drug when ultrasound irradiation induces surface oscillations of sufficient amplitude to rupture the artificial cell [3130]. Gas-filled 2-4 micron micropores insonated at 1-2 MHz may exhibit “pistonlike” or “membranelike” vibration modes [3131-3133]. At the nanoscale, pressure applied, then released, on carbon nanotubes causes fully reversible compression [3134], and experiments have shown very low frictional resistance between nested nanotubes that are externally forced in and out like pistons [3082]. Masako Yudasaka, who studies C₆₀ molecules trapped inside carbon nanotubes or “peapods” at NEC, expects that “the buckyball can act like a piston” [3135]. Although microscale tympanic membranes appear absent in living systems, nanotube arrays are being investigated as directional acoustic sensors because these stereocilia-like structures are known to mechanically respond to acoustic energy [3136-3138].

In this Section, we will look at the choice of acoustic frequency (Section B.4.1), the physical description of the acoustic transducer and pressure bands (Section B.4.2), piston fluid flow dynamics (Section B.4.3), thermal expansion, acoustic cavitation and resonance (Section B.4.4), and finally the energy efficiency and energy cost of molecular manufacturing using the proposed molecular assembler device (Section B.4.5).

Last updated on 1 August 2005