Biomedical Engineering Reference
In-Depth Information
dynamic behaviors of the cells, such as adhesion, proliferation, and differentiation, on bio-
material surface. This chapter also discusses the problems encountered during the coating
deposition and elucidates the overall fabrication procedure and applications of the thermal
sprayed biomedical coatings.
BackgroundandCurrentProblems
Since a biomaterial is a material in contact with fluids, cells, and tissues of the living
body to repair or replace any tissue or organ of the body, prerequisites of the biomaterial
include its favorable biocompatibility and sufficient mechanical properties that accom-
plish its quick fixation after surgery and long-term functional service. According to all the
prerequisites, attempts have been made to find suitable materials to repair or replace any
deficient tissue or body organ throughout the history of mankind. In orthopedic surgery,
biomaterials provide an alternative approach in the treatment of bone defects, with initial
emphasis placed on the replacement of the missing tissue with biomaterials designed to
induce minimal or no immune response. Among the materials for orthopedic surgery,
ceramics play important roles owing to their significant advantages, such as promis-
ing mechanical properties, compared to other materials. There are two major classes of
ceramics used as biomaterials: structural (or technical) and resorbable (or soluble), the
latter includes the so-called bioceramics. In recent decades, several kinds of bioceramics
have been used in orthopedic surgery, which can be clinically classified into (1) bioinert,
and (2) bioactive ceramics from the point of view of the reactions they elicit in the adjacent
tissues (Park 1984). Bioactive implants normally act as a scaffold for cells, enabling them
to anchor, attach, and differentiate. Among all the bioceramics explored, hydroxyapatite
(HA) ceramics are particularly suitable as bone graft substitutes owing to a chemical com-
position that is similar to the mineral nature of bone. HA, which is regarded as synthetic
bone and the most common calcium phosphate ceramic, is well established as a biocom-
patible ceramic capable of forming a good bond with natural bone (Jarcho 1981). The first
application of calcium phosphate materials as bone substitute or bone graft may be traced
to Albee, who reported that a “triple calcium phosphate” compound used in a bony defect
promoted osteogenesis or new bone formation (Albee 1920). Application of HA-based
bioceramics include dental implants, percutaneous devices, and use in periodontal treat-
ment, alveolar ridge augmentation, orthopedics, maxillofacial surgery, and spinal surgery
(Luo et al. 1995).
However, the principal limitation in the clinical use of HA as a load-bearing implant
material is its brittleness (tensile strength = ~200 MPa, fracture toughness = 0.5-1.2 MPa
m
1/2
). At the same time, ceramics are difficult to shape due to their high melting points
as well as limited ductility. Metal alloys such as Ti-6Al-4V have a long clinical history of
implantation (Williams 1981). Although metal implants have superior mechanical proper-
ties, concern over the toxic responses continues to increase. For example, studies showed
that during implantation, metal alloys, in particular titanium, released corrosion prod-
ucts into surrounding tissues and fluids (Healy et al. 1992). Advances in coating technol-
ogy have brought about a new dimension in the processing of biomaterials. Therefore,
there has been a significant interest in the use of HA-coated metal implants (Hardy et al.
1999; Yoshikawa et al. 1996). Figure 4.1 shows the formation of trabecular bone between
unsorbed HA coating and bony tissue after 3 years of implantation. Due to the high impact
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