Biomedical Engineering Reference
In-Depth Information
TABLE 2.1
Desired Properties of Implantable Bioceramics
1. Nontoxic
2. Noncarcinogenic
3. Nonallergic
4. Noninflammatory
5. Biocompatible
6. Biofunctional for its lifetime in the host
especially for blood interfacing applications such as heart valves. Due to their high specific strength as
fibers and their biocompatibility, ceramics are also being used as reinforcing components of composite
implant materials and for tensile loading applications such as artificial tendon and ligaments (Park and
Lakes, 1992).
Unlike metals and polymers, ceramics are difficult to shear plastically due to the (ionic) nature
of the bonding and minimum number of slip systems. These characteristics make the ceramics
nonductile and are responsible for almost zero creep at room temperature (Park and Lakes, 1992).
Consequently, ceramics are very susceptible to notches or microcracks because instead of undergoing
plastic deformation (or yield) they will fracture elastically on initiation of a crack. At the crack tip,
the stress could be many times higher than the stress in the material away from the tip, resulting in a
stress concentration
which weakens the material considerably. The latter makes it difficult to predict
the tensile strength of the material (ceramic). This is also the reason why ceramics have low tensile
strength compared to compressive strength. If a ceramic is flawless, it is very strong even when sub-
jected to tension. Flawless glass fibers have twice the tensile strengths of high strength steel (~7 GPa)
(Park and Lakes, 1992).
Ceramics are generally hard; in fact, the measurement of hardness is calibrated against ceramic mate-
rials. Diamond is the hardest, with a hardness index of 10 on Moh's scale, and talc (Mg
3
Si
3
O
10
COH)
is the softest ceramic (Moh's hardness 1), while ceramics such as
alumina
(Al
2
O
3
; hardness 9), quartz
(SiO
2
; hardness 8), and apatite (Ca
5
P
3
O
12
F; hardness 5) are in the middle range. Other characteristics
of ceramic materials are: (1) high melting temperatures and (2) low conductivity of electricity and heat.
These characteristics are due to the chemical bonding within ceramics.
In order to be classified as a bioceramic, the ceramic material must meet or exceed the properties
listed in Table 2.1. The number of specific ceramics currently in use or under investigation cannot be
accounted for in the space available for bioceramics in this topic. hus, this chapter will focus on a
general overview of the relatively bioinert, bioactive or surface-reactive ceramics, and biodegradable or
resorbable bioceramics.
Ceramics used in fabricating implants can be classified as nonabsorbable (relatively inert), bioac-
tive or surface reactive (semi-inert) (Hench, 1991, 1993), and biodegradable or resorbable (non-inert)
(Hentrich et al., 1971; Graves et al., 1972). Alumina, zirconia, silicone nitrides, and carbons are inert
bioceramics. Certain
glass ceramics
and dense
hydroxyapatites
are semi-inert (bioreactive) and
calcium
phosphates
and calcium aluminates are resorbable ceramics (Park and Lakes, 1992).
2.2 Nonabsorbable or Relatively Bioinert Bioceramics
2.2.1 Relatively Bioinert Ceramics
Relatively bioinert ceramics maintain their physical and mechanical properties while in the host. They
resist corrosion and wear and have all the properties listed for bioceramics in Table 2.1. Examples of
relatively bioinert ceramics are dense and porous aluminum oxides, zirconia ceramics, and single-phase
calcium aluminates (Table 2.2). Relatively bioinert ceramics are typically used as structural-support
implants. Some of these are bone plates, bone screws, and femoral heads (Table 2.3). Examples of
