Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
6.3.5 Electrical Energy Conversion Processes
Microscale electrostatic motors have been fabricated since the late 1980s and are the subject of great current research interest.556,557 Commercially available micromachined rotors ~1 micron thick and ~100 microns in radius respond to electric field intensities >108 volts/m producing motive torques of ~10 pN-m by converting electrical to mechanical energy, achieving power densities in the 104-105 watts/m3 range with device lifetimes approaching 107 rotations.556
Drexler10 has described a class of submicron direct-current (DC) electrostatic motors capable of converting between mechanical and electrical power in either direction at a power density of ~4 x 1014 watts/m3 and an estimated efficiency >99%. (Typical macroscale brush DC motors can have 55%-75% efficiencies, while DC brushless motors can have efficiencies as high as 95%.)
In one implementation (Fig. 6.6), electric charge is placed on the rim of a rotor as the rim passes within a dee electrode. This charge is then transported across a ~3 nm gap to the interior of the opposite dee electrode, where it is removed and replaced by a charge of opposite sign. Applying a voltage of proper sign across the dees causes the charges in transit to apply a torque to the rotor (like a Van de Graaff generator operating in reverse), converting electrical into mechanical power.10 As a specific example, a motor 390 nm in diameter and 25 nm thick driven by a electric field strength of ~0.2 x 109 volts/m spins with a near-maximum rim velocity of ~1000 m/sec; a current of ~110 nanoamperes and an electric potential of ~10 volts yields a power output of ~1.1 microwatts. Single electron-transfer events at nanometer electrodes have been generated and measured experimentally,878 and macroscale "Dirod" electrostatic generators have been operated as electrostatic motors in a student laboratory.3098
Electrical energy is also converted into mechanical energy via the inverse piezoelectric effect.553 Piezoelectric materials such as lead-zirconate-titanate ceramics (PZT) and natural mechanical resonators such as quartz, when subjected to an electric field, expand or contract depending on the orientation of the field to the piezoelectric polarization. For example, applying 100 volts across a 750 micron thick piezoelectric crystal causes it to deform by ~37 nm, or ~0.4 nm/volt.628 There are also piezoelectric rotary actuators, with rotations measured in ~arcsec. The feasibility of fabricating ~1 micron thick quartz resonators by etching techniques was demonstrated decades ago.554,555 PZT crystals are commonly used to provide movement of samples being examined in scanning probe microscopes such as STMs and AFMs.1093 Piezoelectric drivers used in bimorph configurations as electromechanical transducers can produce large mechanical displacements but relatively small forces.545 Dielectric induction micromotors 2 microns thick and 100 microns in diameter operating in water can be driven at 250 rev/sec at 110 volts applied potential, converting electrical to mechanical motion and producing a torque of 0.3 pN-m.552 Electroactive polymers (EAPs)3013,3124 and electrostrictive polymers such as iPMMA are being investigated for use in "artificial muscles";1301 an exceptionally high electrostrictive response (~4%) has been observed in electron-irradiated P(VDF-TrFE) copolymer.1594 Electromechanical actuators based on sheets of single-walled carbon nanotubes have been described.3238 Even bone can be made into an electret.1941
Electrostatic actuators can produce substantial mechanical forces. One electromechanical transducer described by Drexler10 is a capacitor with one plate fixed and connected to a signal source, with the other plate grounded via a tunneling junction, leaving it free to move within a small range of displacements. One design with a 1-nm stroke length and a (12 nm)2 plate area transduces an electrical voltage varying from 0-5 volts into a mechanical signal ranging from 0-1 nN.
A similar implementation for electromechanical transduction involves drawing a dielectric slab of area Aslab and dielectric constant kslab into a gap dgap between the plates of a charged capacitor at voltage Vcap producing a field-dependent force, making a simple mechanical dielectric drive. Applying a time-varying input voltage of frequency nd (Hz) produces a mechanical output power of:
For e0 = 8.85 x 10-12 coul/N-m2 (vacuum permittivity constant), Aslab = 100 nm2, dgap = 2 nm, Vcap = 1 volt, kslab = 5.7 for diamond at 300 K, and nd = 5000 Hz, Pn = 0.001 pW (power density 5 x 109 watts/m3) and the force applied on the slab during Dx = 1 nm displacements is Pn / (Dx np) ~ 1 nN.
Electrothermal energy conversion occurs during simple joule heating, or during Seebeck effect or Peltier effect cooling, when current passes through a bimetallic junction causing one side to get hot and the other side cold.1034,1035
Electrochemical transduction may occur in enzymes with electric field-sensitive conformational states,814 in voltage-gated ion channels and nanomembranes (Section 3.3.3), and in electrolytic cells which are chemoelectric cells (Section 18.104.22.168) operated in reverse.
Quantum well nanostructures can be useful in fabricating electrooptical transducers such as light-emitting diodes, electroluminescent displays and submicron-scale solid-state lasers (Section 5.3.7), and electrochromic glass with voltage-dependent transmittance permits optical gating. Electric current sent through carbon nanotubes causes both field emission and luminescence, about one photon per 106 electrons -- even a single nanotube makes a faint but visible glow,1995 another case of electrooptical energy transduction.
Magnetomechanical transduction has also been employed to remotely power centimeter-size simple legged robots using magnetostrictive alloys as magnetic field-driven actuators in an 80 Hz 1-tesla field,564 but this approach may find only limited utility in nanomedical systems because molecular-scale drives suffer from the adverse scaling properties of electromagnet fields.10 Magnetic sensors (Section 4.7.2), perhaps using molecular magnets,2598-2600 possibly could be employed as power transducers.
Last updated on 18 February 2003