Biomedical Engineering Reference
In-Depth Information
3.1
Introduction
The production of crystals or amorphous materials resulting from the interactions of
organic and inorganic substances that accompany biological activity within living
organisms is known as biomineralization. These living organisms can be both
lower and higher organisms, including humans. The production of calcium salt
crystals (calcium oxalate) within plants was first described in the 1600s. Therefore,
viewed from a historical perspective, biomineralization is an academic field with an
extremely long life span.
Narrowly defined, biomineralization is the process by which inorganic sub-
stances are produced on organic templates within an organism. Representative
examples include the formation of bone and teeth within the human body and
the formation of sea urchin spines and shells. Single crystals of apatite, a calcium
phosphate, are arranged in human bone possesses an organization in which single
crystals of apatite, a species of calcium phosphate with their crystal orientations
aligned. This arrangement produces a fine structure possessing both strength and
flexibility. Teeth are also assemblies of single apatite crystals, and enamel in
particular is a high-hardness tissue comprised of densely packed fine apatite with
an extremely high degree of crystallinity. Another example of biomineralization is
the pearl layer of the Akoya pearl oyster, in which single crystals of aragonite, a
calcium carbonate, are packed in an orderly fashion within a microscopic space.
The proteins that fill in the surrounding gaps contribute to the creation of a beautiful
sheen.
While biomineralization can occur in mild environments under normal tem-
peratures and pressures, it is difficult to directly form the groups of inorganic
crystals seen in hard tissues such as teeth, bone, and shells from simple solutions
under normal temperature and pressure while maintaining high crystallinity. The
mechanism used by organisms to build the tissues necessary for survival—the use
of organic substances as templates—enables crystallization with lower activation
energy. The application of this mechanism to industrial crystallization, which is
totally unrelated to living organisms, is a promising area of research. Crystallization
that utilizes organic-inorganic interaction processes in a mild environmental field
not related to living organisms is commonly called “biomineralization-mimicking”
or “biomineralization-based” crystallization.
Although biomineralization beneficially forms tissues necessary for maintain-
ing normal organism function, it also produces substances that adversely affect
organism health. For example, (broadly defined) biomineralization can cause gout,
gallstones, kidney stones, urinary tract stones, vein wall deposit, and cholesterol
accumulation in humans. Stones contain organic substances such as uric acid
and urate salts, in addition to inorganic substances such as calcium phosphate
and calcium oxalate salts. A cholesterol crystal, for example, is a natural organic
substance. This means that the production of organic crystals on organic substance
templates can be regarded as a type of biomineralization. Many of the crystals
formed by biomineralization inside organisms, especially within the human body,
