Page 44 - Nanotechnology an introduction

The ultimate in sophistication of the nano-object is the nanoscale robot or “nanobot”. Microscopic or nanoscopic robots are an extension of existing
ingestible devices that can move through the gastrointestinal tract and gather information (mainly images) during their passage. As pointed out by
Hogg [77], minimal capabilities of such devices are: (chemical) sensing; communication (receiving information from, and transmitting information
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to, outside the body, and communication with other nanobots); locomotion—operating at very low Reynolds numbers (around 10 ), implying that
viscosity dominates inertia; computation (e.g., recognizing a biomarker would typically involve comparing sensor output to some preset threshold
value; due to the tiny volumes available, highly miniaturized molecular electronics would be required for constructing on-board logic circuits of
practically useful data processing power); and of course power—it is estimated that picowatts would be necessary for propelling a nanobot at a
speed of around 1 mm/s. Such nanobots could also incorporate one or more drug reservoirs, or miniature devices for synthesizing therapeutic
substances using materials found in their environment. It is very likely that to be effective, these nanobots would have to operate in large swarms of
billions or trillions. Figure 4.6 sketches a putative nanobot.

Figure 4.6 Sketch of the likely form of a future nanobot (the small bacterium-sized cylinder in the upper left quadrant) drawn to scale in a blood vessel containing erythrocytes. Reproduced with permission from [77].
In vivo applications of nanotechnology also encompass the implantation of devices for sensing biochemicals (physiological markers). The smaller
the device, the more readily it can be implanted. The technology involved is basically the same as that being developed for nanobots.

It may well be that the most important future role for nanobots will be to carry out tricky repairs that at present require major surgery; for example,
clearing plaque from the walls of blood vessels or renewing calcified heart valves. The body has wonderful repair mechanisms of its own but for
some reason certain actions appear to be impossible.

4.2.2. Wider Implications
It is customary nowadays to take a global view of things, and in assessing the likely impact of nanotechnology on medicine this is very necessary.
Nanotechnology is often viewed to be the key to far-reaching social changes, and once we admit this link then we really have to consider the gamut
of major current challenges to human civilization, such as demographic trends (overpopulation, aging), climate change, pollution, exhaustion of
natural resources (including fuels), and so forth. Nanotechnology is likely to influence many of these, and all of them have some implications on
human health. Turning again to the dictionary, medicine is also defined as “the art of restoring and preserving health by means of remedial
substances and the regulation of diet, habits, etc.” With this in mind, it would be woefully inadequate if the impact of nanotechnology on medicine
were restricted to consideration of the development of the topics discussed in the preceding parts of this section.
4.3. Nanotoxicology

There is currently great concern about the potential adverse impacts of nano-objects on human health. Nanotoxicology is defined as the study of the
toxicology of nanomaterials; while allergic reactions to certain morphologies and combinations of atoms cannot be ruled out, overwhelming
attention is currently given to nano-objects, because they can be inhaled or ingested or otherwise enter the body, triggering inflammatory
responses. Clearly the field is part of nanomedicine; on the one hand the developers of nanoparticle-based therapeutics need to respect the
Hippocratic doctrine of “Primum nil nocere” (a major issue is what happens to the billions or trillions of systemically introduced nanoparticles after
they have accomplished their therapeutic task); and on the other hand the possible effects of exposure to nanoparticles on those engaged in
preparing products containing them is an important part of occupational medicine.
The toxicity of nano-objects falls into two categories: they can be considered as chemicals with enhanced reactivity due to their small size (see
Section 2.4), hence if the substance from which they are made is toxic or can become toxic in the body, the nano-object is likely to be toxic—as an
example consider mercury (Hg), which if ingested as millimeter-sized droplets typically passes through the body unmetabolized, but which in
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nanoparticulate form is likely to be readily oxidized to highly toxic Hg or Hg ; or they are not toxic per se, but initiate an immune response, ranging
from mild temporary inflammation to persistent chronic inflammation and may be ultimately carcinogenesis.
The mechanism of the latter category is denaturation of proteins adsorbed onto the nano-object from its environment (see Figure 4.5), which may
be the cytoplasm, intracellular space, or a fluid such as blood (whether denaturation actually takes place depends on the chemical nature of the
nano-object's surface [129]). The denatured protein-coated nano-object is then identified as nonself by the immune system, which initiates the
processes that should lead to elimination of the foreign object. One of the chief problems is caused by nanofibers. If insoluble and excessively
elongated, as is blue asbestos and most carbon nanotubes, they can be neither solubilized nor ingested by the macrophages (Figure 4.7), which
nevertheless persist indefinitely in a futile attempt to destroy them, engendering sickness.