Biomedical Engineering Reference
In-Depth Information
their poor mechanical properties. Bioceramics have been used as an alterna-
tive to autografts and allografts. They include bioactive glass, calcium carbon-
ate, calcium sulfate, and calcium phosphates of biologic (derived from bovine
bone, coral, and marine algae) or synthetic origin. These bioceramics are avail-
able as granules or blocks (dense or porous), with various designed shapes and
sizes, cements, or as coatings on titanium implants. In the case of load-bearing
applications, bioceramic coatings are applied onto a metal substrate in order to
combine the strength of metal with the bioactivity of bioceramics.
Based on the nature of the attachment of the bioceramics to the living bone
tissue, bioceramics are described as either bioinert (makes contact with bone
without leading to low tissue reactions and formation of a fibrous layer and
with no chemical contacts between bone and materials) or bioactive (directly
attached to the bone by chemical strong bonding without interposition of a
fibrous layer) (Cao and Hench 1996). Commercial alumina (Al
2
O
3
) and zirco-
nia (Zr
2
O
3
) that are used for both dental and orthopedic applications are con-
sidered as stable bioinert bioceramics. In contrast, bioactive bioceramic can
be biostable (i.e., calcium phosphate) or bioresorbable (i.e., bioactive glasses
and glass-ceramics).
Bone is a dynamic, vascular, living tissue that is simply described as a
biocomposite consisting of organic and inorganic parts. The inorganic is
mainly crystalline mineral salts and calcium, which is present in the form of
hydroxyapatite, Ca
10
(PO
4
)
2
(OH)
2
(Beevers and McIntyre 1946). HAp in bone
is a multisubstituted calcium phosphate, including traces of CO
3
2-
, F
-
, Mg
2+
,
Sr
2+
, Si
4+
, and so on (Beevers and McIntyre 1946; Dorozhkin and Epple 2002;
Vallet-Regi and Gonzalez-Calbet 2004).
Critical physic-biochemical properties of bone include (1) interconnecting
porosities (macro- and microporosity), (2) biodegradability (bone remodel-
ing), (3) bioactivity, (4) osteoconductivity, and (5) osteoinductivity. These
bone properties have emulated in the bioceramics synthesis. For example,
osteoinductivity is introduced by mixing the bioceramics with osteogenic
molecules (e.g., growth factors, demineralized bone matrix). However, differ-
ences in composition and syntheses or processing methods affect the proper-
ties of the bioceramics.
Among the various techniques that have been used to coat Ti implants
with layers of hydroxyapatite (de Groot et al. 1987), calcium phosphate (de
Groot 1989), or mixtures of the two (Klein et al. 1994), the most common is
the plasma-spray technique. These coated implants are, moreover, charac-
terized by a rough surface profile, which further improves osteoconduction
and osseointegration. However, plasma-sprayed HAp coatings are approxi-
mately 30 to 50 mm in thickness and are prone to delaminate from the metal
substrate in certain situations owing to their poor bonding strength, which
creates a weak interface that may eventually lead to implant failure (Lemons
et al. 1988; Albrektsson et al. 1992; Gotfredsen et al. 1995).
To avoid the drawbacks of plasma-sprayed HAp coatings, scientists
have developed a new coating method inspired by the natural process of
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