Biomedical Engineering Reference
In-Depth Information
Table 9.1
Requirements for Hydroxyapatite Coating Applied to Surgical
Implants, as Specified by ISO 13779-2
[63]
Property
Requirements
Calcium-to-phosphorus (Ca:P) atomic ratio
1.67e1.76
Total content of metals having adverse biological reactions
50 mg/kg
Content of arsenic (As)
3 mg/kg
Content of cadmium (Cd)
5 mg/kg
Content of mercury (Hg)
5 mg/kg
Content of lead (Pb)
30 mg/kg
Mass fraction of hydroxyapatite (HA)
50%
Mass fraction of alpha-tricalcium phosphate (
a
-TCP)
5%
Mass fraction of beta-tricalcium phosphate (
b
-TCP)
5%
Mass fraction of tetracalcium phosphate (TTCP)
5%
Mass fraction of calcium oxide (CaO)
5%
Crystallinity value of the HA phase
>
45%
Mean adhesion strength
15 MPa
ASTM F1609 specifies almost the same material
requirements for calcium phosphate coatings for
surgical implant applications
[64]
.
X-ray diffraction analysis (XRD) is the most
important method used for chemical characterization
[65,66]
. XRD is a nondestructive method for the
structural analysis of crystals and is based on
observing the scattered intensity of an X-ray beam
hitting a sample. The analysis is performed with an
X-ray diffractometer, which records the intensity of
diffracted X-rays as a function of diffraction angle,
usually indicated as
Q
. Because each type of crystal
has its distinctive scattering peaks, the presence of
a
ASTM F1185 specifies the allowed composition of
HA powder intended as raw material for the coating
of surgical implants
[67]
.
Further information regarding the surface topog-
raphy can be provided assessing the roughness as
described in ISO Standards 4287 and 4288
[68,69]
.
The most common roughness parameters are the
following (see also
Fig. 9.7
):
R
a
(arithmetic mean): the arithmetic mean of the
absolute departures of the roughness profile
from the mean line;
l
R
z
(maximum height of the profile): the mean
value of the maximum peak-to-valley height of
the profile within the sampling length;
l
-TCP, TTCP, and CaO can be detected by
the presence of peaks at precise angles on the dif-
fractogram, as illustrated, for example, in
Fig. 9.6
.
The crystallinity value and the calcium-to-phos-
phorus ratio can also be determined via XRD.
Element concentration can also be determined by
inductively coupled plasma atomic emission spec-
trometry (ICP-AES), which is a quite expensive but
very precise analytical atomic spectroscopy technique.
The coating adhesion can be determined in
accordance with the same ASTM Standard Test
Methods used for titanium coating
[55
e
57]
, whereas
-TCP,
b
R
t
(total height of the profile): the maximum
peak-to-valley height of the profile in the whole
evaluation length.
l
The CDRH Guidance, the ASTM Standards, and
the ISO Standards have been developed focusing on
metallic substrates but can also be applied to poly-
meric substrates such as PEEK, taking care of some
adjustments in the method for preparing the samples
because of
the different chemical and thermal
