Biomedical Engineering Reference
In-Depth Information
biomaterials are bioinert and would release some undesired metal ions caused by corro-
sion in the human body's biological environment. Of the bioceramics, alumina, calcium
phosphate, and bioglass in use are mainly because of their superior biocompatibility and
bioactivity. However, their poor mechanical properties (e.g., tensile strength and fracture
toughness) are serious design limitations for these materials when used in load-bearing
implants (Table 1.1). Polymers, including polymethyl methacrylate (PMMA), polyethy-
lene, polyurethane, etc., are widely used in plastic surgery, cardiovascular surgery, and
other soft tissue surgery due to their properties of resilience and malleability, which are
not at all suitable for load-bearing implants. Therefore, none of the three kinds of above-
mentioned biomaterials meet all the requirements of implants for hard tissue repair and/
or replacement.
However, in view of the advantages and disadvantages of each kind of biomaterial,
an advisable and practicable solution is to develop bioceramic-coated metallic implants.
It is believed that such kinds of implants can do well in combining the desired biological
properties of bioceramics and the excellent mechanical properties of metallic substrates.
The bioceramic coating can also protect the metallic substrate from corrosion and serve
as a barrier for the possible release of toxic metal ions into the human body. Among all
these bioceramics, hydroxyapatite (HA) is the most suitable candidate to be used as a coat-
ing on the surface of metallic implants due to its chemical and biological similarity to
human hard tissues (Boretoes and Eden 1984). HA coating cannot only improve the rate of
osseointegration, but can also establish a high bone-implant interfacial strength by form-
ing strong chemical bonds with the surrounding tissues (Vedantam and Ruddlesdin 1996;
Roop, Kumar, and Wang 2002).
HAandHACoating
HA is the main mineral component of human hard tissues (mainly bones and teeth), and
provides storage for the control of calcium uptake and release for the human body (Aoki
1994). HA belongs to a class of calcium phosphate-based bioceramics with a chemical
formula of Ca
10
(PO
4
)
6
(OH)
2
. The word “hydroxyapatite” consists of “hydroxyl” ion and
“apatite,” which is the mineral name. The stoichiometric Ca/P molar ratio is 1.67, and the
calculated density is about 3.219 g/cm
3
. HA crystallite possesses a hexagonal structure
with the unit cell dimensions of:
a
=
b
= 0.9432 nm and
c
= 0.6881 nm (Hench and Wilson
1993; Kay, Young, and Posner 1964). The atomic structure of HA projected along the
c
-axis
on the basal plane is shown in Figure 1.1.
It is well known that HA can be used as bone substitutes in orthopedics and dental
treatment due to its biocompatibility and osteoconductive properties (Boretoes and Eden
1984). In order to meet the clinical requirements of different applications, HA has been
developed well to be fabricated into different forms, such as particulates (solid, porous,
or even hollow particles) in different sizes, blocks, and coatings (Wang et al. 2009). As for
load-bearing implants, bulk HA seems not suitable because of the limitation of its mechan-
ical properties (Hench and Wilson 1993; Bronzino 2000). There is a wide variation in the
reported mechanical properties of synthesized HA, which strongly depended on the pro-
cess applied with the HA sample preparation. For example, the compressive strength of
synthesized HA ranges from 294 to 917 MPa (Park 1984). Other related mechanical prop-
erties are listed in Table 1.1. The mechanical properties of HA strongly influence their
