Biomedical Engineering Reference
In-Depth Information
crystal within the material, or of a grain boundary, a domain, or a
molecule, or is it a parameter of a surface feature of the sample, or
perhaps of the resistivity or thermal conductivity of the material?
Clearly this is nonsense, but one has to accept that nanomaterials
are here to stay, with even some journal titles containing the
word” [97, p. 5898, left column]. Following this logic, such terms
as “nanocomposite,” “nanocoatings,” “nanopowders,” “nanofibers,”
and “nanocrystals” are senseless either and should be replaced, for
example, by “composites with nano-sized (or nanodimensional)
dispersed phase(s),” “coatings of nano-sized (or nanodimensional)
thickness,” “nano-sized (or nanodimensional) powders,” “fibers of
nano-sized (or nanodimensional) thickness,” and “nano-sized (or
nanodimensional) crystals,” respectively. At least, this has been done
in this topic.
According to their geometry, all nanodimensional materials can be
divided into three major categories: equiaxed, one-dimensional (or
fibrous), and two-dimensional (or lamellar) forms. Selected examples
and typical applications of each category of nanodimensional
materials and their use in biomedical applications are available in
literature [98]. It is important to note, that in literature on calcium
orthophosphates there are cases, when the prefix “nano” has been
applied for the structures, with the minimum dimensions exceeding
100 nm [42, 71, 99-108].
As a rule, nanodimensional materials can be manufactured
from nearly any substance. Of crucial importance, there are two
major characteristics conferring the special properties of any
nanodimensional material. These are the quantum effects associated
with the very small dimensions (currently, this is not applicable
to the biomaterials field) and a large surface-to-volume ratio that
is encountered at these dimensions. For instance, specific surface
areas for submicron-sized particles are typically 60-80 m
2
/g, while
decreasing particle diameter to tens of nanometers increases the
specific surface area up to 5 times more — an amazing amount of
surface area per mass! Furthermore, all nanophase materials have
the unique surface properties, such as an increased number of grain
boundaries and defects on the surface, huge surface area and altered
electronic structure, if compared to the micron-sized materials [89,
109]. While less than ~1% of a micron-sized particle's atoms occupy
the surface positions, over a tenth of the atoms in a 10 nm diameter
particle reside on its surface and ~60% in a 2 nm particle [110]. This
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