Biomedical Engineering Reference
In-Depth Information
them, porous bioceramics made of calcium orthophosphates appear
to be very prominent due to both the excellent biocompatibility and
bonding ability to living bone in the body. This is directly related to
the fact that the inorganic material of mammalian calcified tissues, i.e
.
of bone and teeth, consists of calcium orthophosphates [25-27]. Due
to this reason, other artificial materials are normally encapsulated
by fibrous tissue, when implanted in body defects, while calcium
orthophosphates are not [28]. Many types of calcium orthophosphate-
based bioceramics with different chemical composition are already
on the market (Tables 4.2 and 5.2). Unfortunately, as for any ceramic
material, calcium orthophosphate bioceramics by itself lack the
mechanical and elastic properties of the calcified tissues. Namely,
scaffolds made of calcium orthophosphates only suffer from a low
elasticity, a high brittleness, a poor tensile strength, a low mechanical
reliability and fracture toughness, which leads to various concerns
about their mechanical performance after implantation [29-31].
Besides, in many cases, it is difficult to form calcium orthophosphate
bioceramics into the desired shapes.
The superior strength and partial elasticity of biological calcified
tissues (e.g., bones) are due to the presence of bioorganic polymers
(mainly, collagen type I fibers [32]) rather than to a natural ceramic
(mainly, a poorly crystalline ion-substituted CDHA, often referred to
as “biological apatite”) phase [34, 35]. The elastic collagen fibers are
aligned in bone along the main stress directions. The biochemical
composition of bones is given in Table 6.1 [36]. A decalcified bone
becomes very flexible being easily twisted, whereas a bone without
collagen is very brittle; thus, the inorganic nano-sized crystals of
biological apatite provide with the hardness and stiffness, while the
bioorganic fibers are responsible for the elasticity and toughness
[26, 37]. In bones, both types of materials integrate each other
into a nanometric scale in such a way that the crystallite size,
fibers orientation, short-range order between the components,
etc., determine its nanostructure and therefore the function and
mechanical properties of the entire composite [33, 38-42]. From
the mechanical point of view, bone is a tough material at low strain
rates but fractures more like a brittle material at high strain rates;
generally, it is rather weak in tension and shear, particularly along
the longitudinal plane. Besides, bone is an anisotropic material
because its properties are directionally dependent [25, 26, 31]. For
further details, see section
1.4
of this topic.
Search WWH ::
Custom Search