Journal of Biomaterials and Nanobiotechnology, 2010, 1, 7-16
doi:10.4236/jbnb.2010.11002 Published Online October 2010 (
Copyright © 2010 SciRes. JBNB
Changes of Surface Composition and Morphology
after Incorporation of Ions into Biomimetic
Apatite Coatings
Wei Xia1,2*, Carl Lindahl1,2, Cecilia Persson1, Peter Thomsen2,3, Jukka Lausmaa2,4, Håkan Engqvist1,2*
1Materials in Medicine, Applied Materials Science, the Ångstrom Laboratory, Uppsala University, Uppsala, Sweden; 2Biomatcell,
VINN Excellence Center of Biomaterials and Cell Therapy, Gothenburg, Sweden; 3Institute for Clinical Sciences, Sahlgrenska
Academy at University of Gothenburg, Gothenburg, Sweden; 4Department of Chemistry and Materials Technology, SP Technical
Research Institute of Sweden, Borås, Sweden.
Email: {wei.xia@, hakan.engqvist}
Received July 28th, 2010; revised August 19th, 2010; accepted September 20th, 2010.
Incorporating trace elements into apatite coatings may permit the combination of several pharmaceutical effects due to
the different ions. In this study, strontium, silicon, and fluoride ions have been incorporated into apatite coatings
through a biomineralization method, which mimics an in vitro mineralization process. The surface composition was
tested with X-ray diffraction and X-ray pho to electron sp ectrosco py, and the surface morp holog y was ch arac terized with
scanning electron microscopy. Compar ed with pure hydroxyapatite coating, the stron tium, silicon, and fluoride substi-
tuted apatite coatings showed different morphologies, such as spherical, needle-like, and nano-flake-like, respectively.
The crystal size of these biomimetic hydroxya patite coa tings decreased after ion sub stitutio n. The results of th e analysis
of surface composition showed an increased amount of the ion substitutions with an increase of ion concentrations in
the soaking so lution. That means th e ion inco rporation into the apa tite structure ba sed on the biomin eraliza tion method
could not only vary th e ion content but a lso change the morpholog y of the apatite coatings. Herein, the ro le of ion sub-
stitution is considered from the point of view of materials science at the micro structural and surface chemistry levels.
Keywords: Biomineralization, Hyd r oxyapatite, Coating, Substitution, Biomimetic
1. Introduction
Surface composition and morphology are two key factors
in implant coatings. The outermost layer of a biomate-
rial’s surface possesses different properties depending on
its elemental compositions and functionalities (such as
-PO4, -SiOH, -TiOH, -OH, -COOH, and -NH2 groups).
Bioactive silicate glasses can chemically bond with the
bone tissue because they can form a SiOH layer on the
surface in water solution and induce, further, the forma-
tion of hydroxyapatite [1]. Kokubo et al. reported that the
bioactivity and bone-bonding ability of titanium implants
treated with an alkali hydroxide solution have been
improved because of the existence of TiOH groups on
the surface [2]. Surface topography also affects the bio-
activity, biocompatibility, and tissue in-growth [3]. Os-
teoblast-like cells adhere more readily to and appear
more differentiated on rough surfaces [3-5]. Recent stud-
ies report that surfaces with specific nano- and micro-
topographies can improve the cell adhesion, change the
cell spreading, and enhance the gene expression [6-8].
Therefore, the fabrication of bioactive materials present-
ing surfaces with these types of topographies is of great
Hydroxyapatite (HA), Ca10(PO4)6(OH)2, a calcium
phosphate found in bone and teeth, presents good osteo-
conductivity and osteoinductivity and is commonly used
in dental and orthopedic surgery [9]. The high biocom-
patibility of HA has also led to its use as a coating mate-
rial on metallic implants such as titanium, stainless steel
and Co-Cr alloys [10-12]. However, the synthetic HA is
still needs to be improved. For example, it has been
shown to induce apatite formation at a slower rate than
other materials, such as silicate based bioactive glasses
[13]. Furthermore, the low resorbability rate of synthetic,
stoichiometric hydroxyapatite limits the rate of in-growth
for new bone formation [14,15]. These factors may
result in an increased risk of implant failure due to the
Changes of Surface Composition and Morphology after Incorporation of Ions into Biomimetic Apatite Coating
deficient fixation.
A previous study has shown that the bioactivity and
resorbability of hydroxyapatite can be improved by de-
creasing the crystallinity and introducing ion substitu-
tions in its structure [16]. In fact, it is well known that
natural bone mineral is a calcium deficient hydroxyapatite
with low crystallinity. Various ionic substitutions, cati-
onic as well as anionic, exist in bone mineral, such as Sr,
Si, Mg, CO32- and F. Anions can be incorporated into the
sites of OH- (type A) and PO43- (type B) [17], and cations
can be incorporated into the sites Ca2+ I and Ca2+ II [18].
These ions also have important functions in the bone. For
instance, a reduction in bone resorption and an increase
in the mechanical strength may be obtained through Sr
substitutions [19,20]. Silicon can increase the bone min-
eralization rate and enhance the osteopath proliferation,
differentiation and collagen production [21]. Fluoride,
which is good for teeth, can stabilize apatite and also
stimulate the osteoblast activity [22,23]. Finally, previous
research has shown that ion substituted apatite materials
may improve the interaction between bone and synthetic
apatite materials [24].
Methods to fabricate HA and ion substituted coatings
include plasma spraying [25], sol-gel [26], magnetron co-
sputtering [27], pulsed-laser deposition [28] and
micro-arc oxidation techniques [29]. Although these
techniques allow the preparation of HA coatings com-
bined with a tailoring of the chemical composition, the
draw- backs of synthetic HA have not been resolved.
Recently, a solution method termed the biomineralization
method has been used to fabricate biomimetic HA coat-
ings on implants [3, 30-32]. This biomimetic method
maintains low deposition temperatures and also provides
a good surface coverage of complex geometrical shapes.
The obtained coating is calcium deficient, not completely
crystallized and the chemical composition can be tailored
using different ions. In vivo studies show that this apatite
coating provides faster new bone in-growth and coating
resorbability in early bone formation [33]. However, a
histological study found that rough titanium implants had
a more positive effect on new bone formation than the
biomimetic apatite coating [34].
Crystalline titanium dioxides have been proven to
strengthen the physical–chemical bonding between the
implants and living bones because of their ability to in-
duce a bone-like apatite in the body’s environment [35].
Therefore, such oxidized Ti surface could be considered
as a good transition layer on the Ti-based implant’s sur-
face. In this study, a titanium oxide layer was created on
the titanium plates in the form of a rutile film via a ther-
mal oxidation process. Ti plates were treated at 800
for 1 hour with a ramping rate of 5/min. Thereafter the
plates were treated with an alkaline solution (1M NaOH)
and ethanol in an ultrasonic bath before use.
In brief, there is room for improvement of this biomi-
metic coating in order to strengthen the implant and bone
bonding and to have a stronger positive effect on new
bone formation. Previous work suggests that the surface
composition and the topography of the coatings are key
parameters affecting the bioactivity, biocompatibility,
and further material/tissue interactions such as tissue in-
growth. In this work, we prepared ion substituted (Sr, Si,
F) apatite (iHA) coatings with varied compositions and
surface morphology on pre-treated titanium substrates
through a biomineralization method.
2. Materials and Methods
2.1. Materials
Pure titanium (Grade 2, 99.4% pure, Edstraco AB, Swe-
den) cut as squares (10x10mm, with a 1mm thickness)
was used as a substrate for the biomimetic coatings. The
base soaking medium was phosphate buffer solution
(Dulbecco’s PBS, Aldrich, USA). The ion composition
of this PBS is: Na+ (145mM), K+ (4.3mM), Mg2+
(0.49mM), Ca2+ (0.91mM), Cl- (143mM), and HPO42-
2.2. Surface Treatment
2.3. Biomimetic Ion Substituted Apatite Coating
The preparation of the biomimetic ion substituted apatite
coating was performed in an ion modified PBS solution.
The initial pH value is 7.4, adjusted by NaOH and HCl
solutions. In order to investigate which factors that in-
fluence the biomimetic coating deposition, a series of ion
concentrations and soaking times were studied. Sr, Si and
F ion concentrations were varied between 0.06mM and
0.6 mM, 0.075 mM and 0.15 mM, 0.04 mM and 0.2 mM,
respectively, and soaking time from 12 hours to 2 weeks.
The maximum concentrations used for the different ions
were limited by the precipitation of the same at higher
concentrations. All specimens were soaked in 40 ml pre-
heated solution in sealed plastic bottles, which were then
stored in an oven at 37. The solution was exchanged
every 3 days to avoid depletion of calcium and phosphate
ions. All samples were rinsed with de-ionized water and
dried at 37 prior to characterization.
2.4. Morphological and Chemical
The morphology of the specimens before and after coat-
ing was evaluated using field emission scanning electron
microscopy (FE-SEM, LEO 1550). An SE2 detector was
used with an accelerating voltage of 10 kV. The crystal-
linity of the specimens was analyzed using X-ray
Copyright © 2010 SciRes. JBNB
Changes of Surface Composition and Morphology after Incorporation of Ions into Biomimetic Apatite Coating
Copyright © 2010 SciRes. JBNB
diffractometry (TF-XRD, Siemens Diffractometer 5000)
using Cu Ka radiation (k = 1.5418 Å). The diffractometer
was operated at 45 kV and 40 mA at a 2 h range of
10–80°, with a fixed incidence angle of 1°. The chemical
composition was analyzed by X-ray photoelectron spec-
troscopy (XPS, Physical Electronics Quantum 2000, Al
Kα X-ray source).
Where d is the crystal size, λ is the wavelength of Cu
Kα radiation (λ = 1.5418Å), and k is the broadening con-
stant varying with crystal habit and chosen as 0.9 for the
elongated apatite crystallites.
Xc, the crystallinity, corresponds to the fraction of
crystalline apatite phase in the investigated volume of
powdered samples. An empirical relation between Xc and
the FWHM was deduced, according to the equation:
2.5. Estimation of Crystal size and Crystallinity 3
c (2)
This estimation method was referred from Li et al [18].
The size of HA and Sr, Si and F-HA crystals were calcu-
lated from XRD data using the Scherrer equation. The
peak of (002) was fit to define the full width at half
maximum intensity (FWHM):
where Xc is the crystallinity degree, KA is a constant set
at 0.24.
3. Results
d (1) Figure 1 shows the SEM images of hydroxyapatite (HA)
(a) (b)
(c) (d)
(e) (f)
Changes of Surface Composition and Morphology after Incorporation of Ions into Biomimetic Apatite Coating
(g) (h)
Figure 1. Surface morphology of the TiO2/Ti substrates after soaking for 2 weeks in PBS containing different ion concentra-
tions. (a) and (b) no additional ion doped, (c) and (d) 0.6 mM strontium ion doped, (e) and (f) 0.2 mM fluor ide ion doped, (g)
and (h) 0.15 mM silicate ion doped.
without substituted ions, strontium substituted hydroxyl-
apatite (Sr-HA), fluoride substituted hydroxyapatite (F-
HA) and silicon substituted hydroxyapatite (Si-HA)
coatings on TiO2/Ti substrates after soaking in PBS for 2
weeks. It is clear that the morphology strongly depends
on the substituted ions. For the pure HA coating (Figure
1(a) and (b)), it is flake-like, with flakes of approxi-
mately 100 nm-1 μm in length and width. The Sr-HA
coating (Figure 1(c) and (d)) has an entirely different
morphology with sphere-like particles of approximately
200-800 nm diameter. For the F-HA coating (Figure 1(e)
and (f)), the morphology was again different, presenting
a needle-like structure. The morphology of the Si-HA
coating (Figure 1(g) and (h)) was relatively similar to
the pure HA coating. It was also flake-like, but the size
of the Si-HA particles was much smaller than pure HA;
less than 100 nm in length and width. The XRD analysis
(Figure 2) showed diffraction peaks of apatite in all
cases, asdetected around 31º related to the overlapping of
planes (211), (112), and (300), and around 26° related to
plane (002).
In order to study the influence of the ion concentration
on the structure of the HA coating, the results of using
lower concentrations of Sr (0.06 mM), Si (0.075 mM)
and F (0.02 mM) ions are shown in Figure 3. It can be
seen that the morphology of the Sr-HA coating still
appears sphere-like, although the interface is rougher,
porous and with a wider size distribution. However, the
decrease in Si and F ion concentration did not greatly
affect the morphology of F-HA and Si-HA coatings
(Figure 3(b) and (c)). The F-HA coating was still
needle-like, and the Si-HA coating was flake-like with
nano-sized particles.
The early formation of the HA coatings, after soaking
in PBS for 12 hours, is shown in Figure 4. All specimens
were soaked in the high concentration ion doped PBS
solution. It can be seen that the early Sr-HA coating
(Figure 4(a)) is a flat layer which is different to the
morphology of the coating after soaking for 2 weeks. A
few spherical nano-particles have formed on this early
layer. These spherical nanoparticles could be considered
to be the base of the formation of the second spherical
Sr-HA layer. Herein the Sr-HA coating prepared using
the biomineralization method is not a simple layer but a
hierarchical coating with a gradient structure from a flat
and dense early layer to a spherical and porous layer. The
early F-HA coating (Figure 4(b)) grows from the
substrate with needle-like particles. There seems to be no
hierarchical structure in the F-HA coating. The early
formation of the Si-HA coating (Figure 4(c)) shows a
nano-sized flake-like layer on the substrate. Again, the
structure is similar to the final structure after 2 weeks of
20 25 30 35 40 45
SiHA - 3 7-2 W
(002) *
Figure 2. XRD patterns of the HA coatings with and with-
out ion substitution. * and (002): HA. The substrates have
been soaked in pure, Sr (0.6 mM), F (0.2 mM) and Si (0.15
mM) PBS for 2 weeks.
Copyright © 2010 SciRes. JBNB
Changes of Surface Composition and Morphology after Incorporation of Ions into Biomimetic Apatite Coating 11
Figure 3. Surface morphology of the TiO2/Ti substrates
after soaking for 2 weeks in the lower concentration solu-
tions. (a) 0.06 mM strontium ion doped, (b) 0.04 mM fluo-
ride ion doped, (c) 0.075 mM silicate ion doped.
The changes of crystal size and crystallinity of differ-
ent apatite coating reflected from the XRD patterns were
calculated in order to observe the influence of the Sr, Si
and F ions. The (002) reflection was chosen to evaluate
the average crystal size because this peak was well re-
solved and showed no interferences. The d002 of HA, SrHA,
FHA and SiHA were 35.58, 16.27, 25.10 and 25.45 nm,
respectively. The crystal size decreased significantly
when hydroxyapatite had been substituted strontium,
fluoride and silicon ions. The crystallinity of all HA and
Sr, F, and Si substituted HA coatings were very low, just
0.0185, 0.0199, 0.0186 and 0.0185, respectively. That
Figure 4. Early growth of ion substituted HA coatings after
soaking for 12 hours. (a) stronti um (0.6 mM) substituted HA,
(b) fluoride (0.2 mM) substituted HA, (c) silicate (0.15 mM)
substituted HA.
means all of these biomimetic coatings were poorly crysta
lliz ed. XPS survey spectra identified calcium, phospho-
rus, oxygen, strontium, fluoride and silicon as the major
constituents of the strontium, fluoride and silicon substi-
tuted apatite coating on titanium plates, as expected, see
Figure 5. All of these spectrums were for the sample
prepared at 37 for 2 weeks using the higher ion con-
Copyright © 2010 SciRes. JBNB
Changes of Surface Composition and Morphology after Incorporation of Ions into Biomimetic Apatite Coating
Copyright © 2010 SciRes. JBNB
centration. The peak at 134eV (Figure 5 (a)) was an
overlap of Sr3d and P2p because Sr3d5/2 (133 ± 0.5 eV),
Sr3d3/2 (135 ± 0.5 eV), P2p (132 - 133 eV) lines were
closely located. The peak at 684eV (Figure 5 (b)) was
F1s. The weak peak 153eV (Figure 5 (c)) was Si2s. Si2p
(~99eV) could not be clearly observed. Table 1 gives the
atomic ratio of the ions in the coatings, obtained from
XPS. It can be noted that the composition of the coatings
can be varied by changing the ion concentrations in the
soaking medium. When the Sr concentration was in-
creased from 0.06 mM to 0.6 mM, the Sr content in the
newly formed biomimetic coating increased from 3.56%
to 7.74%. The F content in the coating increased from
1.24% to 1.53% after the concentration in PBS was in-
creased from 0.04 mM to 0.2 mM. For the lower Si con-
centration (0.075 mM), no data was available from the
XPS. After the Si concentration was increased to 0.15
mM, 0.56% silicon was found in the coating.
Changes of Surface Composition and Morphology after Incorporation of Ions into Biomimetic Apatite Coating 13
Figure 5. XPS spectra for strontium (a), fluoride (b), and silicon (c) substituted apatite coating on titanium plates (37 for 2
Table 1. Preparation parameters and atomic concentrations of the final coatings. (Error in brackets).
Samples Ion concentration
Substituted ion % of
final coating
006SrPBS-37-2W 0.06 37 2 3.56 (1.01)
06SrPBS-37-2W 0.6 37 2 7.74 (1.53)
004FPBS-37-2W 0.04 37 2 1.24 (0.24)
02FPBS-37-2W 0.2 37 2 1.53 (0.11)
0075SiPBS-37-2W0.075 37 2 -
015SiPBS-37-2W 0.15 37 2 0.56 (0.16)
4. Discussions like nanocrystals in highly ordered bundles. Compared
with the Sr and F substituted hydroxyapatite coatings, Si
substituted hydroxyapatite coating showed a similar
morphology to pure hydroxyapatite coating. However,
the particle size decreased, from the micrometer to the
nanometer scale. These results are different to previously
reported HA coatings prepared with solution methods,
especially for the Sr-HA and the F-HA coatings [37,38].
Li et al. [37] reported on a solution-derived strontium
doped apatite coating where the morphology was similar
to pure apatite coating even though the strontium con-
centration reached 1.5 mM. However, in our study the
morphology of the Sr-HA coating was completely
different to the pure HA coating, even at concentrations
as low as 0,06mM Sr. Also, a higher concentration of
strontium ions seemed to facilitate the formation of more
In this study it was shown that the topography of hy-
droxyapatite coatings obtained using a biomineralization
method can be varied by adding different ions of varied
concentrations to the soaking medium. Figure 6 gives a
schematic illustration of ion substituted hydroxyapatite
coatings with different morphologies on the TiO2/Ti sub-
strates. The non substituted hydroxyapatite coating was
composed of flake-like particles in the order of microns.
After some of the calcium ions had been replaced by
strontium ions, the flake-like coating transformed into a
spherical coating with a transitional flat layer. When the
OH- ions were partially replaced by F ions, the new
coating had a needle-like morphology. This is very simi-
lar to the morphology of tooth enamel [36]. Hydroxyapa-
tite incorporated with fluoride in enamel presents needle-
Copyright © 2010 SciRes. JBNB
Changes of Surface Composition and Morphology after Incorporation of Ions into Biomimetic Apatite Coating
Copyright © 2010 SciRes. JBNB
amount of substituted ions was highest using strontium,
followed by fluoride and finally silicon. This could be
due to the differences in initial concentrations of the ions.
However, because of the similarity in radius between
Sr2+ and Ca2+ and F- and OH-, respectively, the replace-
ment of calcium and hydroxide ions by strontium and
fluoride ions is also easier than substituting phosphate by
silicate. Since the latter molecules are not single ions
they are more difficult to incorporate into hydroxyapatite.
Finally, one hydroxyapatite molecule has 10 calcium
ions and 2 hydroxide ions. This could allow for more Sr
than F substitutions. The resorption rate and the cell
response of these ion substituted hydroxyapatite coatings
remain to be evaluated and may be of interest for future
spherical particles. Wang et al. [38] prepared a F-HA
coating using an electrochemical deposition method.
They reported that the obtained F-HA particles became
sharper and smaller when the F concentration increased
in the solution. At certain F concentrations, they could
get spindle-like F-HA. Needle-like F-HA exists naturally
in tooth enamel [36]. In this study, a well organized nee-
dle-like FHA coating was obtained via a mineralization
method without an inductor. Table 2 shows a decrease of
the crystal size after hydroxyapatite had been substituted.
This reduction in the crystal size of synthetic ion substi-
tuted hydroxyapatite was also reported by Li et al [18].
The poor crystallinity of the biomimetic coatings has
been also reported Bracci and Bigi et al [39].
XPS analysis showed that the ion concentration in the
soaking solution also influenced the amount of ions sub-
stituted into the HA. A higher ion concentration tended
to give a higher amount of substituted ions. However, the
substitutions we obtained may not be the highest possible
substitutions of these ions in hydroxyapatite since, in
order to avoid an initial precipitation in PBS solution,
higher concentrations of Sr, F and Si ions could not be
used. Furthermore, from Table 1 it can be noted that the
5. Conclusions
A simple method for the preparation of ion substituted
hydroxyapatite coatings with controlled topography and
composition was explored in this study. The results
showed that the surface structure could be modified by
substituting different ions into the apatite structure. The
strontium, fluoride and silicon substituted hydroxyapatite
coatings gave spherical, needle-like, and nano-flake-like
morphologies which were very different to pure hy-
droxyapatite. The concentration of strontium ions was
also found to affect the morphology of the coatings. The
lower Sr concentration resulted in irregular and porous
spherical particles and the higher one resulted in regular
spherical particles. For the fluoride and silicon substi-
tuted coatings, there was no obvious difference between
a high ion concentration and a low one. After ion substi-
tution, the biomimetic hydroxyapatite coatings showed a
poor crystallinity, and the crystal size decreased. XPS
analysis showed that the atomic concentration of ions in
the final coating was affected by the ion concentration in
the PBS solution. A high ion concentration could result
in higher amounts of substitutions. The surface composi-
tion analysis also showed that the highest amount of sub-
stitutions was achieved using strontium ions, followed by
the fluoride ions and finally the silicon ions. In summary,
the surface composition and morphology of hydroxyapa-
tite produced via a biomineralization method can be con-
trolled using ion substitutions.
Figure 6. Schematic illustration of ion substituted hy-
droxyapatite coatings with different morphology on the
TiO2/Ti substrates.
Table 2. Crystal size and crystallinity of HA, SrHA, SiHA
and FHA reflected by XRD patterns.
6. Acknowledgements
Sample Line width (002)
FWHM (2θ)
Average crystal
size d (nm)
HA-37-2W 25.9963 35.58 0.0185
SrHA-37-2W 25.3722 16.27 0.199
FHA-37-2W 25.9395 25.10 0.0186
SiHA-37-2W 26.0015 25.45 0.0185
This work was supported by BIOMATCELL, VINN Ex-
cellence Center of Biomaterials and Cell Therapy.
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