Biomedical Engineering Reference
In-Depth Information
also produced biocompatible scaffolds, with a high porosity and well-
interconnected microsized pores.
30
In addition to these solid preshaped materials, a bone filler based on nHA
has also been developed, that is, Ostim
®
(aap Biomaterials GmbH & Co,
Dieburg, Germany). This synthetic paste consists of nHA and 40% water
and is biocompatible and fully degradable after hardening. The possibility
to inject and shape the paste inside bony defects and to mix it with other
materials such as morselized bone grafts and tricalcium phosphate-HA to
provide better handling of the granules are the major advantages of this
material.
31
4.1.3 Nanohydroxyapatite Coatings
A common method to prepare nHA coatings on biomaterials is by spontane-
ous nucleation and growth of nanosized “bone-like” HA in metastable syn-
thetic body fluid (SBF). SBF contains an inorganic salt composition similar to
that of human blood plasma, which facilitates the growth of nHA at physio-
logical pH and temperature over time.
32-35
This biomimetic coating technique
can be used to evenly coat porous or smooth implants or scaffolds, and the
major advantages of this technique are the possibility to incorporate biologi-
cally active agents in the coating by coprecipitation and to control the release
of the factors. Potential incorporated biological agents are antibiotics to pre-
vent local infections after surgical procedures or osteoinductive (growth)
factors.
36-38
SBF with a modified composition containing calcium nitrate tet-
rahydrate and diammonium hydrogen phosphate salts can also be used in
a chemical precipitation technique to produce nHA powders. Precipitation
takes place at 37°C and pH 7.4, and the precipitate is transformed into HA
with submicron sizes after sintering at 1200°C for 6 h.
35
Electrocrystallization from dilute electrolytes (Ca
+
and
PO
3−
) is another
method to create ultrafine-grained nHA coatings. The electrolytes are pre-
pared by dissolving Ca(NO
3
)
2
and NH
4
H
2
PO
4
in deionized water, and the
deposition takes 2 h at a physiological pH and at a temperature of 85°C. With
this technique, the nHA is deposited directly onto the cathode when using
very low calcium and phosphate concentrations.
39
As a final example, the ESD technique can be used to create nHA coat-
ings. This technique requires a precursor solution containing nHA that
is pumped through a metal nozzle. Under the influence of a high voltage
between the nozzle and the substrate, a spray of highly charged droplets is
formed, which is attracted to the grounded substrate (Figure 4.2).
40-45
By this
technique, monodisperse and even porous coatings can be created. As an
example, Huang et al.
46
described the electrospraying of a nHA precursor
solution that resulted in a biocompatible coating composed of nHA particles
with a hexagonal shape and a particle size below 80 nm. The compositional
and morphological properties of the coating can be tailored by choosing the
appropriate deposition parameters. For instance, the flow rate and voltage
