10-10-2012, 05:20 PM
Bio-Nanorobotics: State of the Art and Future Challenges
Bio-Nanorobotics.pdf (Size: 2.23 MB / Downloads: 123)
Introduction
Nanotechnology can best be defined as a description of activities at the level of atoms and molecules that
have applications in the real world. A nanometer is a billionth of a meter, that is, about 1/80,000 of the
diameter of a human hair, or 10 times the diameter of a hydrogen atom. The size-related challenge is the
ability to measure, manipulate, and assemble matter with features on the scale of 1 to 100 nm. In order to
achieve cost-effectiveness in nanotechnology it will be necessary to automate molecular manufacturing.
The engineering of molecular products needs to be carried out by robotic devices, which have been termed
as nanorobots. A nanorobot is essentially a controllable machine at the nanometer or molecular scale that
is composed of nanoscale components. The field of nanorobotics studies the design, manufacturing,
programming, and control of the nanoscale robots.
This review chapter focuses on the state of the art in the emerging field of nanorobotics, its applications
and discusses in brief some of the essential properties and dynamical laws which make this field more
challenging and unique than its macroscale counterpart. This chapter is only reviewing nanoscale robotic
devices and does not include studies related to nanoprecision tasks with macrorobotic devices that are
usually included in the field of nanorobotics.
The Kinesin, Myosin, Dynein, and Flagella molecular motors
With modern microscopic tools, we view a cell as a set of many different moving components powered
by molecular machines rather than a static environment. Molecular motors that move unidirectionally
along protein polymers (actin or microtubules) drive the motions of muscles as well as much smaller
intracellular cargoes. In addition to the F0–F1-ATPase motors inside the cell, there are linear transport
motors present as tiny vehicles known as motor proteins that transport molecular cargoes [7] that also
require ATP for functioning. These minute cellular machines exist in three families — the kinesins, the
myosins, and the dyneins [8]. The cargoes can be organelles, lipids, or proteins etc. They play an important
role in cell division and motility. There are over 250 kinesin-like proteins, and they are involved in processes
as diverse as the movement of chromosomes and the dynamics of cell membranes. The only part they
have in common is the catalytic portion known as the motor domain. They have significant differences
in their location within cells, their structural organization, and the movement they generate [9]. Muscle
myosin, whose study dates back to 1864, has served as a model system for understanding motility for
decades. Kinesin, however, was discovered rather recently using in vitro motility assays in 1985 [10].
Conventional kinesin is a highly processive motor that can take several hundred steps on a microtubule
without detaching [11,12] whereas muscle myosin executes a single “stroke” and then dissociates [13].
A detailed analysis and modeling of these motors has been done [10,14].
The Myosin Linear Motor
Myosin is a diverse superfamily of motor proteins [17]. Myosin-based molecular machines transport
cargoes along actin filaments — the two stranded helical polymers of the protein actin, about 5 to 9 nm
in diameter. They do this by hydrolyzing ATP and utilizing the energy released [18]. In addition to
transport, they are also involved in the process of force generation during muscle contraction, wherein
thin actin filaments and thick myosin filaments slide past each other. Not all members of the myosin
superfamily have been characterized as of now.However,much is known about the structure and function.
Myosin molecules were first sighted through electron microscope protruding out fromthick filaments and
interacting with the thin actin filaments in late 1950s [19–21]. Since then it was known that ATP plays a
role in myosin related muscle movement along actin [22]. However, the exact mechanism was unknown,
which was explained later in 1971 by Lymn and Taylor [23].
The Kinesin Linear Motor
Kinesin [15] and dynein family of proteins are involved in cellular cargo transport along microtubules
as opposed to actin in the case of myosin [46]. Microtubules are 25 nm diameter tubes made of protein
tubulin and are present in the cells in an organized manner. Microtubules have polarity; one end being
the plus (fast growing) end while the other end is the minus (slow growing) end [47]. Kinesins move
from minus end to plus end, while dyneins move from plus end to the minus end of the microtubules.
Microtubule arrangement varies in different cell systems. In nerve axons, they are arranged longitudinally
in such a manner that their plus ends point away from the cell body and into the axon. In epithelial cells,
their plus end points towards the basement membrane. They deviate radially out of the cell center in
fibroblasts and macrophages with the plus end protruding outwards [48]. Like myosin, kinesin is also an
ATP-driven motor. One unique characteristic of kinesin family of proteins is their processivity — they
bind to microtubules and literally “walk” on it for many enzymatic cycles before detaching [49,50]. Also,
each of the globular heads/motor domains of kinesin is made of one single polypeptide unlike myosin
(heavy and light chains and dynein heavy, intermediate, and light chains).
DNA-Based Molecular Machines
As mentioned earlier, nature chose DNA mainly as an information carrier. There was no mechanical work
assigned to it. Energy conversion, trafficking, sensing, etc., were the tasks assigned mainly to proteins.
Probably for this reason, DNA turns out to be a simpler structure — with only four kinds of nucleotide
bases adenosine, thiamine, guanine, and cytosine (A, T, G, and C) attached in a linear fashion that take
a double helical conformation when paired with a complementary strand. Such structural simplicity
vis-à-vis proteins — made of 20 odd amino acids with complex folding patterns — results in a simpler
structure and predictable behavior. There are certain qualities that make DNA an attractive choice for the
construction of artificial nanomachines. In recent years,DNAhas found use in not only mechanochemical,
but also in nanoelectronic systems as well [107–110]. A DNA double-helical molecule is about 2 nm in
diameter and has 3.4–3.6nmhelical pitch no matter what its base composition is; a structural uniformity is
not achievable with protein structures if one changes their sequence. Furthermore, double-stranded DNA
(ds-DNA) has a respectable persistence length of about 50 nm [111] which provides it enough rigidity
to be a candidate component of molecular machinery. Single stranded DNA (ss-DNA) is very flexible
and cannot be used where rigidity is required, however, this flexibility allows its application in machine
components like hinges or nanoactuators [112]. Its persistence length is about 1 nm nm covering up to
three base pairs [113] at 1 M salt concentration.