Page 72 - The Mechatronics Handbook

In a molecular device a discrete number of molecular components are combined into a supramolecular
structure where each discrete molecular component performs a single function. The combined action of
these individual molecules causes the device to operate and perform its various functions. Molecular
devices require an energy source to operate. This energy must ultimately be used to activate the compo-
nent molecules in the device, and so the energy must be chemical in nature. The chemical energy can
be obtained by adding hydrogen ions, oxidants, etc., by inducing chemical reactions by the impingement
of light, or by the actions of electrical current. The latter two means of energy activation, photochemical
and electrochemical energy sources, are preferred since they not only provide energy for the operation
of the device, but they can also be used to locate and control the device. Additionally, such energy
transduction can be used to transmit data to report on the performance and status of the device. Another
reason for the preference for photochemical- and electrochemical-based molecular devices is that, as
these devices are required to operate in a cyclic manner, the chemical reactions that drive the system
must be reversible. Since photochemical and electrochemical processes do not lead to the accumulation
of products of reaction, they readily lend themselves to application in nanodevices.
Molecular devices have recently been designed that are capable of motion and control by photochemical
methods. One device is a molecular plug and socket system, and another is a piston-cylinder system [51].
The construction of such supramolecular devices belongs to the realm of the chemist who is adept at
manipulating molecules.
As one proceeds upwards in size to the next level of nanomachines, one arrives at devices assembled
from (or with) single-walled carbon nanotubes (SWNTs) and/or multi-walled carbon nanotubes
(MWNTs) that are a few nanometers in diameter. We will restrict our discussion to carbon nanotubes
(CNTs) even though there is an expanding database on nanotubes made from other materials, especially
bismuth. The strength and versatility of CNTs make them superior tools for the nanomachine design
engineer. They have high electrical conductivity with current carrying capacity of a billion amperes per
square centimeter. They are excellent field emitters at low operating voltages. Moreover, CNTs emit light
coherently and this provides for an entire new area of holographic applications. The elastic modulus of
CNTs is the highest of all materials known today [52]. These electrical properties and extremely high
mechanical strength make MWNTs the ultimate atomic force microscope probe tips. CNTs have the
potential to be used as efficient molecular assembly devices for manufacturing nanomachines one atom
at a time.
Two obvious nanotechnological applications of CNTs are nanobearings and nanosprings. Zettl and
Cumings [53] have created MWNT-based linear bearings and constant force nanosprings. CNTs may
potentially form the ultimate set of nanometer-sized building blocks, out of which nanomachines of all
kinds can be built. These nanomachines can be used in the assembly of nanomachines, which can then
be used to construct machines of all types and sizes. These machines can be competitive with, or perhaps
surpass existing devices of all kinds.
SWNTs can also be used as electromechanical actuators. Baughman et al. [54] have demonstrated that
sheets of SWNTs generate larger forces than natural muscle and larger strains than high-modulus ferro-
electrics. They have predicted that actuators using optimized SWNT sheets may provide substantially
higher work densities per cycle than any other known actuator. Kim and Lieber [55] have built SWNT
and MWNT nanotweezers. These nanoscale electromechanical devices were used to manipulate and
interrogate nanostructures. Electrically conducting CNTs were attached to electrodes on pulled glass
micropipettes. Voltages applied to the electrodes opened and closed the free ends of the CNTs. Kim and
Lieber demonstrated the capability of the nanotweezers by grabbing and manipulating submicron clusters
and nanowires. This device could be used to manipulate biological cells or even manipulate organelles
and clusters within human cells. Perhaps, more importantly, these tweezers can potentially be used to as-
semble other nanomachines.
A wide variety of nanoscale manipulators have been proposed [56] including pneumatic manipulators
that can be configured to make tentacle, snake, or multi-chambered devices. Drexler has proposed
telescoping nanomanipulators for precision molecular positioning and assembly work. His manipulator
has a cylindrical shape with a diameter of 35 nm and an extensible length of 100 nm. A number of six

©2002 CRC Press LLC