Biomedical Engineering Reference
In-Depth Information
Greek philosopher and scientist Aristotle of Stageira maintained that ''Nature,
like a good householder, is not in the habit of throwing away anything from which
it is possible to make anything useful. Now in a household the best part of the food
that comes in is set apart for the free men, the inferior and the residue of the best
for the slaves, and the worst is given to the animals that live with them. Just as the
intellect acts thus in the outside world with a view to the growth of the persons
concerned, so in the case of the embryo itself does Nature form from the purest
material the flesh and the body of the other sense-organs, and from the residues
thereof bones, sinews, hair, and also nails and hoofs and the like; hence these are
last to assume their form, for they have to wait till the time when Nature has some
residue to spare'' [1].
About two-thirds of the weight of a bone, or half of its volume, comes from
an inorganic material known as the
bone salt
that conforms, so to say, the bone
to a nonliving world. It is an example of biomineralization: the process by which
living organisms produce minerals. Owing to the inorganic architecture of the
bone its biological properties may often be assumed as time independent, and
the bone may be described by the methods of mathematics and mechanics devel-
oped for inanimate materials. However, by treating the bone as a live tissue, we
observe that its biological activity is essentially directed toward keeping the whole
organism in a state of well-being. The functionality of a bone is closely related
to that of a cartilage tissue. In embryogenesis, a skeletal system is derived from
a mesoderm, and chondrification (or chondrogenesis) is a process by which the
cartilage is formed from a condensed mesenchymal tissue, which differentiates
into chondrocytes and begins secreting the molecules that form an ECM. Carti-
lage is a dense CT and, along with collagen type 1, can be mineralized in the
bone.
The high stiffness and toughness of biomineralized tissues of a bone are explained
by the material deformation mechanisms at different levels of organization, from
trabeculae and osteons at the micrometer level to the mineralized collagen fibrils
at the nanometer length scale. Thus, inorganic crystals and organic molecules are
intertwined in the complex composite of the bone material [2].
Bone, like every living tissue, cannot be described completely in terms of a
nonanimate matter description. It breaks as a lifeless stick if overloaded, but if
set up it recovers after some time. Under some loads, microcracks can appear;
these are
ad hoc
healed, and the bone undergoes reinforcement. These properties
of a bone are due to a complicated but coordinated structure, as it is seen during
remodeling. Living bone could be treated as a solid-state fluid composite with
circulating blood and living cells, while a bone skeleton has hierarchical structure
and variable biomechanical properties. In addition, the blood flows through bones
according to the rhythm of the heart beat.
Following Erwin Schrodinger, one sees that most physical laws on a large scale
are due to stochasticity on a small scale (''order-from-disorder'' principle). For
example, the diffusion, in macroscopic description, is an ordered process, but in
microscopic view it is caused by random movement of particles. If the number
of atoms in the particle increases, the behavior of the system becomes less and
