Surface & Coatings Technology 198 (2005) 223 – 226
www.elsevier.com/locate/surfcoat
Sol–gel preparation of Zn-doped fluoridated hydroxyapatite films
Shundong Miaoa, Wenjian Wenga,*, Kui Chengb, Piyi Dua, Ge Shena,
Gaorong Hana, Sam Zhangb
a
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027. P. R. China
School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
b
Available online 24 November 2004
Abstract
In comparison with hydroxyapatite (HA), fluoridated hydroxyapatite (FHA) films on metallic prostheses demonstrate better long-term
effectiveness since FHA is less soluble and possesses similar bioactivity. Zn is well established as an essential trace element known to have
positive effect on osteoblastic cell proliferation and bone formation. It is expected that the incorporation of Zn into FHA films could create
better physiochemical performance. In this work, FHA films with different Zn content on Ti6Al4V were prepared by sol–gel dip-coating
method, Ca(NO3)2, Zn(NO3)2, P2O5 and HPF6 were used as the precursors, and the F/Ca molar ratio was 1/15. The crystalline phase and the
surface morphology of the Zn-doped FHA films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM),
respectively, and the Zn content in the surface was determined by X-ray photoelectron spectroscopy (XPS) analysis. The results showed that
the concentrations of Zn in the surface were 10 to 30 times higher than the designed value. The Zn-doped FHA films were soaked into
simulated body fluid (SBF) solutions for bioactivity evaluation, and the morphology on the surface of the soaked films was examined using
SEM, the result showed that the Zn-doped FHA films had similar bioactivity as the FHA film.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Fluoridated hydroxyapatite; Films; Zinc; Segregation; Stimulating effect
1. Introduction
Biocompatible hydroxyapatite (HA) layers grown on
Ti6Al4V implants can well combine the mechanical
properties and bioactivity from both materials. Fluoridated
hydroxyapatite (Ca5(PO4)3(OH)x F1 x , FHA) could have
better long-term performance than HA films [1,2] because
the fluorine substitution can favor the crystallization of
calcium phosphate and decrease the mineral dissolution.
Therefore, the implants with FHA films have a good
integration of both strong fixation and long-term
effectiveness [2].
Although implants with HA or FHA films employed as
hard tissue replacements can bond directly with new bone
tissue, it is still a pursuing target that the films are capable of
enhancing bone formation and inhibiting bone resorption.
* Corresponding author. Fax: +86 571 87952321.
E-mail address: [email protected] (W. Weng).
0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2004.10.026
Zn is well established as an essential trace element known to
have stimulating effects on bone formation at low concentrations, and has a potent inhibitory effect on osteoclastic
bone resorption in vitro [3]. Many studies have been carried
out on Zn-containing tricalcium phosphate and apatite
cement [3–8]; these results showed that Zn had a stimulating
effect on osteoblastic cell proliferation and bone formation.
In this work, we prepared Zn-doped FHA films on
titanium alloy in order to integrate the stimulating effect of
Zn with the low soluble ability of FHA. The films with
different Zn contents were prepared by the sol–gel dipcoating method, and the influences on the films were
discussed of the Zn incorporation.
2. Experimental procedure
Zn(NO3)2d 6H2O (AR) was dissolved into absolute
ethanol to form 0.1 M/L solution. Ethanol solutions (2 M)
of Ca(NO3)2d 4H2O (AR) and P2O5 (AR) were prepared as
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S. Miao et al. / Surface & Coatings Technology 198 (2005) 223–226
Table 1
The compositions of the dipping sols
Sol no.
HPF6/Ca
(molar ratio)
Zn/Ca
(molar ratio)
Ca/P
(molar ratio)
Remark
SF-1
1/90
/
1.67
SZF-2
1/90
0.0046
1.67
SZF-3
1/90
0.0062
1.67
SZF-4
1/90
0.0077
1.67
SZF-5
1/90
0.0093
1.67
The derived film
called SF-1 film
The derived film
called SF-2 film
The derived film
called SF-3 film
The derived film
called SF-4 film
The derived film
called SF-5 film
Ca and P precursors, respectively. The Ca and P precursor
solutions were mixed and then HPF6 (AR) was added, the
mixture had a Ca/P molar ratio of 1.67 and was refluxed for
24 h to form an initial sol. Different designed amount of
Zn(NO3)2 solution was added into the initial sol to form a
series of dipping sols, then extra ethanol was added to adjust
the Ca ionic concentration to be the same in different
dipping sols. The compositions of the dipping sols were
listed in Table 1.
In preparation of Zn doped FHA films, cleaned Ti6Al4V
substrates were immersed into the dipping sols and withdrawn at a speed of 8 cm/min. The as-dipped films were
dried at 150 8C for 15 min and fired at 600 8C for 15 min in
each run. This deposition procedure was repeated 5 times
for a film thickness of about 1 Am.
For bioactivity evaluation, a simulated body fluid (SBF)
solution for high supersaturation with respect to apatite (1.5
SBF) was prepared [9]. The ion concentration in the
solution (Table 2) was 1.5 times of that of the SBF in
which the ionic concentrations were nearly equal to those of
the human body blood plasma. The films were soaked in
SBF for 10 days at 37 8C.
The film samples were characterized by X-ray diffraction
(XRD, RIGAKU, D-Max, RA, CuKa, 28/min, 0.058 per
step), field emission scanning electron microscopy (FESEM, Model FEI SIRION), and X-ray photoelectron
spectroscopy (XPS, AXIS HSi 165 Ultra). Three samples
were analyzed for each composition.
3. Results and discussion
The XRD patterns (Fig. 1) show that all the films have a
pure apatite phase. The film without Zn (SF-1 film) had
Concentration (mM/dm3)
1.5 SBF
213
Blood plasma 142
SBF
142
Ca2+ Mg2+ HCO-3 Cl-
7.5 3.8
25.0 2.5
5.0 2.5
2.3
1.5
1.5
6.30
27.0
4.20
stronger XRD intensities of apatite than those incorporated
with Zn, and the apatite diffraction peaks of the films
became wider with increasing Zn concentration. This
indicates that the incorporation of Zn into the FHA films
induces a decrease in the crystallinity of the apatite phase,
since Zn inhibits crystal growth of apatite by segregating in
the grain boundaries [10]. As shown in Fig. 1, there is no
indication that Zn substitutes for Ca into HA. It is believed
that the Zn in the present films exists most probably in grain
boundaries.
The Zn content in the surface (Table 3) was 10 to 30
times the amount added into the dipping sol (Table 1). Most
likely Zn segregated from bulk of the film during the
formation of apatite films. In contrast, the change in F/Ca
ratios was minimum: the ratios were close to the designed
value of 1/15 (0.067) in the dipping sols. Thus the
incorporation of Zn has no obvious influence on the
existence of F in the films.
Release of Zn can exert an in vivo stimulating effect on
bone formation [8], the segregation of Zn in grain
boundaries and precipitation in surface will therefore favor
this stimulation via enhanced Zn release.
Fig. 2 shows SEM micrographs of the Zn-doped FHA
films. The morphology (Fig. 2a) of FHA film was dense. As
Zn was incorporated, the film became porous, and more
porous with increasing Zn content. That was undesirable.
Table 3
The XPS results of the films
Table 2
Ion concentration of SBFs and human blood plasma
Na+ K+
Fig. 1. The XRD patterns of the FHA films doped with different Zn content.
HPO2SO24
4
223.0 1.5
103.0 1.0
147.8 1.0
0.80
0.50
0.50
Sample no.
Zn/Ca in the films
(molar ratio)
F/Ca in the films
(molar ratio)
SZF-2
SZF-3
SZF-4
SZF-5
0.130F0.006
0.140F0.004
0.070F0.003
0.098F0.008
0.044F0.005
0.036F0.004
0.055F0.007
0.064F0.006
film
film
film
film
S. Miao et al. / Surface & Coatings Technology 198 (2005) 223–226
Fig. 2. SEM micrographs of the 600 8C fired films with different Zn content. (a) SF-1 film, (b) SZF-2 film, (c) SZF-4 film, (d) SZF-5 film.
Fig. 3. SEM micrographs of the soaked films doped with different Zn content. (a) SF-1 film, (b) SZF-2 film, (c) SZF-4 film.
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S. Miao et al. / Surface & Coatings Technology 198 (2005) 223–226
turn, facilitates precipitation of apatite particles. Zn-doped
FHA has lower crystallinity than that without Zn, therefore
dissolution of film is more severe and so is precipitation of
the apatite particles.
4. Conclusion
Zn-doped fluoridated hydroxyapatite films on the
Ti6Al4V substrates are obtained by sol–gel method. Zn is
rich in the surface of the films and exists in grain
boundaries, which favors Zn in vivo release to act as an
agent for enhancing bone formation. However, large amount
of Zn incorporation renders undesirable porosity. The SBF
soaking confirms that apatite layer forms on Zn-doped FHA
films in the same way on that without Zn. Doping of Zn
lowers the film crystallinity and thus promotes precipitation
of apatite particles from SBF solution.
Fig. 4. The XRD patterns of the soaked films doped with different Zn
content.
The formation of voids in the film could result from the
decomposition of Zn(NO3)2. The further research on Zndoped fluoridated hydroxyapatite films with optimized
microstructure, physiochemical and physiological properties
is underway.
The SEM micrographs of the Zn doped-FHA films
soaked in SBF are presented in Fig. 3. It is obvious that a
new layer with embedded particles has formed on the
surface. A complete coverage of the surface is observed.
The cracks in the layer are resulted from drying from 100
8C for SEM observation. XRD studies confirm that this
layer was an apatite layer (Fig. 4). The observation that
the formation of the apatite layer is not affected shows
that inclusion of Zn does not affect the bioactivity of the
FHA film.
The density of the embedded particles in the apatite layer
increases with increasing Zn (Fig. 3). The formation of
apatite layers could be related to two processes: normal
chemical reaction on the interface and the precipitation of
small apatite particles from SBF solution. The 1.5 SBF
solution used is highly supersaturated with respect to apatite
[9], and the Zn-doped FHA has low crystallinity (Fig. 1),
thus is more soluble than higher crystallinity films [11,12].
The dissolution of the film further increases the level of
local supersaturation of Ca and P in the solution, which, in
Acknowledgement
This work is partially supported by grant 032-101-0005
of Agency for Science, Technology and Research (A*Star),
Singapore.
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