Journal of Biomaterials and Nanobiotechnology, 2013, 4, 327-333
http://dx.doi.org/10.4236/jbnb.2013.44041 Published Online October 2013 (http://www.scirp.org/journal/jbnb)
327
Synthesis and Structural Characterization of
Hydroxyapatite-Wollastonite Biocomposites,
Produced by an Alternative Sol-Gel Route
Martín A. Encinas-Romero1*, Jesús Peralta-Haley2, Jesús L. Valenzuela-García1,
Felipe F. Castillón-Barraza3
1Departamento de Ingeniería Química y Metalurgia, Universidad de Sonora, Hermosillo, México; 2Posgrado en Ciencias de la
Ingeniería: Ingeniería Química, Universidad de Sonora, Hermosillo, México; 3Centro de Nanociencias y Nanotecnología, Uni-
versidad Nacional Autónoma de México, Ensenada B.C., México.
Received July 20th, 2013; revised August 18th, 2013; accepted September 3rd, 2013
Copyright © 2013 Martín A. Encinas-Romero et al. This is an open access article distributed under the Creative Commons Attribu-
tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
ABSTRACT
Hydroxyapatite is a type of calcium phosphate-based material with great interest for biomedical applications, due to the
chemical similarity between this material and the mineral part of human bone. However, synthetic hydroxyapatite is
essentially brittle; the practice indicates that the use of hydroxyapatite without additives for implant production is not
efficient, due to its low strength parameters. In the present work, biocomposites of hydroxyapatite-wollastonite were
synthesized by an alternative sol-gel route, using calcium nitrate and ammonium phosphate as precursors of hydroxya-
patite, and high purity natural wollastonite was added in ratios of 20, 50 and 80 percent by weight immersed in aqueous
medium. Formation of hydroxyapatite occurs at a relatively low temperature of about 350˚C, while the wollastonite re-
mains unreacted. After that, these biocomposites were sintered at 1200˚C for 5 h to produce dense materials. The char-
acterization techniques demonstrated the presence of hydroxyapatite and wollastonite as unique phases in all products.
Keywords: Hydroxyapatite; Wollastonite; Bioceramics; Biocomposites; Sol-Gel
1. Introduction
Hydroxyapatite (Ca10(PO4)6(OH)2), is the predominant
mineral component of vertebrate bones and dental tissue:
teeth and enamel. Its clinical applications are of great
importance, because it is the calcium phosphate ceramic
chemically more similar to the biological apatite crystals.
For this reason, many processing routes have been de-
veloped for synthesizing fine hydroxyapatite and sinter-
ing the reactive powders to form a dense bioceramic. The
most common approaches reported include precipitation,
solid state reaction, sol-gel methods, hydrothermal route,
emulsion and microemulsion techniques, mechanochemi-
cal reactions, and a combination of mechanochemical,
hydrothermal, and ultrasonically assisted reactions [1-4].
The sol-gel process is one of the most important methods
for the production of biomaterials, its advantages include
the use of inexpensive and readily available reagents, and
it is an effective method for the preparation of highly
pure powder due to the possibility of a strict control of
the process parameters. This method offers a molecular
mixing of calcium and phosphorus, capable of improving
chemical homogeneity. Moreover, the high reactivity of
the sol-gel powders allows a reduction of the processing
temperature and of any degradation phenomena during
sintering [5].
However, the mechanical properties of hydroxyapatite
are not good enough to be used as an implant in load
bearing situations, like artificial teeth or bones. One ap-
proach to solve this problem is to combine it with a suit-
able reinforcement phase, producing a biocomposite which
provides optimum mechanical properties, by overcoming
mechanical limitations. The ideal biocomposite should be
the one that who combines their biological and mechani-
cal properties, to provide adequate support for the hard
tissues [6,7].
Wollastonite (CaSiO3), is a calcium silicate which has
*Corresponding author.

Synthesis and Structural Characterization of Hydroxyapatite-Wollastonite Biocomposites,
Produced by an Alternative Sol-Gel Route
328
been widely used as a filler to fabricate composites with
improved mechanical properties. Moreover, it has also
been used as a medical material for artificial bones and
dental roots because of its good bioactivity and biocom-
patibility. Natural bone is a bioceramic composite made
up of small hydroxyapatite crystal particles reinforced by
organic collagen fibers. Because of its outstanding me-
chanical properties, researchers sought ways of duplicat-
ing its mechanical properties [8,9]. Kokubo and col-
leagues attempted to prepare a similar composite by the
crystallization process, developing a clinically important
glass ceramic (A-W glass-ceramic) [8,10]. A-W glass-
ceramic is an assembly of small apatite particles, effec-
tively reinforced by wollastonite in a glassy matrix. The
bending strength, fracture toughness, and Young’s mo-
dulus of A-W glass-ceramic are the highest among bio-
active glass and glass-ceramics [11]. Many studies have
involved the wollastonite phase in natural or synthetic
forms to produce different composites in which its pres-
ence improved some mechanical properties as well as the
bioactivity and porosity of the composites [12-14]. The
application of extra-pure natural materials instead of
synthetic materials reduces the cost of implant produc-
tion. Also, as distinct from synthesized materials, natural
wollastonite has a needle-shaped habit, with a ratio be-
tween the needle length and their diameter equal to 15 -
20 or more. This will presumably facilitate the produc-
tion of an interwoven reinforcing mesh of wollastonite
needles in the composite [6].
The aim of the present work was to synthesize and
characterize hydroxyapatite-wollastonite biocomposite
powders by a sol-gel route, using calcium nitrate and
ammonium phosphate as precursors of hydroxyapatite,
and high-purity natural wollastonite as a reinforcement
element. These powders were processed by sintering to
produce dense materials for the evaluation of their struc-
tural characteristics, whereas their bioactive and me-
chanical properties will be evaluated in a parallel study.
2. Experimental Procedure
2.1. Sol-Gel Synthesis of
Hydroxyapatite-Wollastonite
Biocomposite Powders
The biocomposites of hydroxyapatite-wollastonite were
prepared by sol-gel processing from calcium nitrate
(Ca(NO3)2·4H2O, Sigma-Aldrich USA) and ammonium
phosphate ((NH4)2HPO4, Sigma, Japan) and high-purity
natural wollastonite, NYAD M200 (CaSiO3, from NYCO’s
Pilares deposit in Hermosillo, Sonora, México), with
98.25% purity. Table 1 shows the chemical analysis of
the natural wollastonite used in this study [15]. The
amounts of the precursors reagents were chosen in order
Table 1. Chemical composition of wollastonite NYAD® M200,
produced by Minera NYCO S.A. de C.V. [15].
Chemical composition CaSiO3
Component Typical value (%)
CaO 46.25
SiO2 52.00
Fe2O3 0.25
Al2O3 0.40
MnO 0.025
MgO 0.50
TiO2 0.025
K2O 0.15
wt% loss (1000˚C) 0.40
to maintain the Ca/P molar ratio at 1.67 of stoichiometric
hydroxyapatite, and the amounts of wollastonite were
chosen in order to obtain 20, 50, and 80 wt%.
First, to produce about 1.0 g of pure hydroxyapatite
powder, 0.1639 mol of calcium nitrate was dissolved in
10 mL of deionized water by ultrasonic stirring during 15
minutes, and then 0.0979 mol of ammonium phosphate
was dissolved by ultrasonic stirring during 30 minutes.
The mixture of precursors was stirred at room tem-
perature by magnetic stirring, while the pH of the mix-
ture was controlled between 6 and 7 with liquid ammonia
for approximately 2 h, until the gelation was observed.
The gel was then dried at 120˚C for 12 h.
To produce the hydroxiapatite-wollastonite biocompo-
sites, a suitable amount of wollastonite to obtain 20 wt%
(0.25 g), 50 wt% (1.0 g), and 80 wt% (4.0 g) was sus-
pended, by ultrasonic stirring, in a proper volume of de-
ionized water to keep the solid: liquid ratio at 1:2 (w/v)
for all experiments. Then the same procedure, as de-
scribed previously, was followed until the dried gels
mixed with natural wollastonite were obtained.
Finally, the dried gels were ground to a fine powder
and heat treated in a furnace at 750˚C in air for 3 h; the
heating was done at a rate of 10˚C/min. The flow chart in
Figure 1 outlines the complete experimental procedure.
2.2. Sintering of Hydroxyapatite-Wollastonite
Biocomposites
For sintering experiments, the powders were ground us-
ing tungsten carbide milling balls in a 50 mL tungsten
carbide container with a Fritsch Pulverisette 6 planetary
mono mill (Idar-Oberstein, Germany). Cylindrical tablets
were produced by uniaxial pressing of powders (0.5 g)
into a 10 mm diameter die in a Carver press Hydraulic
Unit 3912 (Carver, Wabash, IN). Sintering tests were
carried out in a Lindberg/Blue M high temperature fur-

Synthesis and Structural Characterization of Hydroxyapatite-Wollastonite Biocomposites,
Produced by an Alternative Sol-Gel Route
329
Ultrasonicstirringfor15minutes
Ultrasonicstirringfor30minutes
pHcontrolbetween6and7byadding
NH
3(liq)
andmagn et icstirringfor2hours
Drygelat120°Cfor12hours
Heat‐treatmentat750°Cinairfor3hours
Characterization offinalproducts
SuitableamountofCaSiO3suspendedinH2O
0.1639 molof
Ca(NO
3
)
2
.4H
2
O
0.0979molof
(NH
4
)
2
HPO
4
Ca(NO3)2·4H2O
Figure 1. Schematic flow chart for the sol-gel procedure for
synthesizing hydroxyapatite- w o llastonite biocomposite s.
nace (ThermoScientific, Asheville, NC) at 1200˚C. The
powders were ground in isopropanol for 30 min at a rota-
tion speed of 200 rpm. The milled powder was dried at
120˚C to remove the isopropanol and produce a fine
powder. Cylindrical tablets were produced by pressing
the powders under a pressure of 220 MPa for a 5 min
dwell time and then sintered at 1200˚C. The sintering
procedure was performed at a heating rate of 2˚C/min
from room temperature to the sintering temperature, with
a dwell time of 5 h at the maximum temperature fol-
lowed by cooling at 2˚C/min. The tablets produced con-
tain 0, 20, 50, 80, and 100 wt% of wollastonite, coded as
100 H, 80 H - 20 W, 50 H - 50 W, 20 H - 80 W and 100
W, respectively.
2.3. Characterization Techniques
Thermal analysis was used to detect changes occurring
during the heat-treatment process. Thermogravimetric
(TGA) and differential thermal analyses (DTA) were
carried out using a TA Instruments SDT 2960 Simulta-
neous DSC-TGA (New Castle, DE). Heating to 1000˚C
was performed in an alumina crucible, with air flow (23
cm3/min) at a rate of 10˚C/min. The samples’ weight was
3 mg of dried xerogel. XRD analysis was carried out by
means of a Phillips X’PERT XRD diffractometer (Phil-
lips Electronics, Eindhoven, The Netherlands). CuK

radiation was used (40 mA, 40 kV). The 2  range was
from 10˚ to 80˚ at a scanning speed of 1.2˚/min. Identifi-
cation of the crystalline phases was performed by com-
parison with JCPDS files 09-0432 and 84-0654 for hy-
droxyapatite and wollastonite, respectively. For FT-IR
absorption analysis, the samples in KBr pellets were
analyzed in the transmission mode using a PerkinElmer
Spectrum GX System FT-IR spectrometer (Boston, MA)
over the range 4000 - 400 cm−1.
Morphological studies of the samples were performed
using a JEOL SEM 5300 scanning electron microscope
(JEOL, Tokyo, Japan). TEM studies of the samples were
done on a JEOL JEM-2010F instrument. Sample speci-
mens were prepared by dispersing the powders in dis-
tilled water to form very dilute suspensions. A drop of
suspension was transferred onto a carbon mesh supported
on a conventional copper microgrid.
3. Results and Discussion
3.1. Synthesis of Hydroxyapatite-Wollastonite
Biocomposite Powders
Figure 2 shows the thermogravimetric analysis (TGA)
combined with differential thermal analysis (DTA) for
the dried gel of calcium nitrate and ammonium phos-
phate in water from room temperature to 1000˚C. The
dried gel trace displays a thermal transition in the tem-
perature region 180˚C - 350˚C, which corresponds to a
weight loss of about 40 percent by weight with an exo-
thermic peak at about 280˚C. This weight loss could be
associated to the formation of hydroxyapatite in the early
stages of crystallization. A further thermal process between
500˚C and 700˚C, corresponding to a negligible weight
loss, and represented by an exothermic peak at about
0100 200 300 400 500 600 700 800 9001000
40
50
60
70
80
90
100
0100 200 300 400 500 600 700 800 9001000
ENDO
EXO
DTA Trace
TGA Trace
Tem
p
erature
(
oC
)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Temperature Difference (oC/mg)
Weight(%)
Figure 2. Thermogravimetric and differential thermal anal-
yses trace for dried gel of calcium nitrate and ammonium
phosphate in water from room temperature to 1000˚C at a
heating rate of 10˚C/min.

Synthesis and Structural Characterization of Hydroxyapatite-Wollastonite Biocomposites,
Produced by an Alternative Sol-Gel Route
330
650˚C, could be due to formation of hydroxyapatite in
advanced crystallization stages. Figure 3 demonstrated
that wollastonite does not exhibit any thermal transition
in the whole temperature range. This fact indicates that
synthesized hydroxyapatite and natural wollastonite added
to produce the biocomposites, both maintain their chemi-
cal integrity, even under severe thermal conditions.
Figure 4 shows the XRD patterns for dried gel of cal-
cium nitrate and ammonium phosphate in water upon
heat treatment at 350˚C and 750˚C. This figure indicates
that gel treated at 350˚C is transformed to hydroxyapatite
in an early crystallization stage; when gel was treated at
750˚C it is converted to perfectly crystallized hydroxya-
patite. In both cases the analysis reveals the presence of
hydroxyapatite as individual phase in the materials.
0200 400 600 80010001200
40
50
60
70
80
90
100
0200 400 600 80010001200
DTA T race
TGA Trace
ENDO
EXO
Temperature
(
oC
)
Weight(%)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Temperature Difference (oC/mg)
Figure 3. Thermogravimetric and differential thermal anal-
yses trace for natural wollastonite from room temperature
to 1200˚C at a heating rate of 10˚C/min.
20 30 40 50 60 70 80
750oC
350oC
Intensit y (a. u)
2-Theta (o)
002
102 210
211
112
300 202 310
222
213
004
304
432
*
*
*
*
*
*
*
***
*
*
*
*
* *
*
*
*
*
*
*
*
*
*
*
*
Relative Intensity (a.u)
Figure 4. X-ray diffraction patterns for dried gel of calcium
nitrate and ammonium phosphate in water upon he at treat-
ment at 350˚C and 750˚C. (*) Hydroxyapatite.
For the above, hydroxyapatite in the biocomposites of
this study, was synthesized at 750˚C to obtain this bioce-
ramic material completely crystallized.
Figure 5 shows the FT-IR spectra for the base materi-
als. Figure 5(a) shows the spectral characteristics of
natural wollastonite and Figure 5(b) shows the spectral
characteristics of dried gel of calcium nitrate and ammo-
nium phosphate in water upon heat treatment at 750˚C, in
the range of 4000 cm−1 to 400 cm−1. The broad band
around 1000 cm−1 in Figure 5(a) is mainly attributed to
the silicate IR absorption of wollastonite. The peaks ob-
served around 3571 cm−1 and 632 cm−1 in Figure 5(b)
arise from the stretching and librational modes, respec-
tively, of OH− ions. The peaks at 1048 cm−1 and 1090
cm−1 have been associated with the stretching 3 mode of
the P-O bonds, and the 962 cm−1 band arises from 1
symmetric P-O stretching vibrations mode of the 3
4
PO

group. Bands at 603 cm−1 and 571 cm−1 were caused by
the triply degenerate 4 bending vibration of the 3
4
PO

group. The clear presence of two peaks at 603 cm−1 and
571 cm−1, along the well-resolved peak at 632 cm−1
confirms the presence of hydroxyapatite as stoichiomet-
ric phase [6,7,16-18].
Figure 6 resumes the X-ray diffraction patterns for
natural wollastonite (100 W), and dried gels with diffe-
rent percentages by weight of wollastonite (20 H - 80 W)
80 wt%, (50 H - 50 W) 50 wt%, (80 H - 20 W) 20 wt%
(100 H) 0 wt%, upon heat treatment at 750˚C. This figure
indicates that hydroxyapatite and wollastonite appear as
unique phases in the biocomposites. In all cases well-
crystallized, hydroxyapatite was obtained. This fact de-
monstrates that the wollastonite remains unreacted,
throughout the whole in situ synthesis of hydroxyapatite.
Figure 7 shows the transmission electron micrographs
of natural wollastonite and dried gels with different per-
4000 3500 3000 2500 2000 1500 1000500
(b)
(a)
20%
% Transmittance (a.u.)
Wavenumber (cm−1)
Figure 5. FT-IR spectra: (a) Natural wollastonite, (b) Dried
gel of calcium nitrate and ammonium phosphate in water
upon heat treatment at 750˚C.

Synthesis and Structural Characterization of Hydroxyapatite-Wollastonite Biocomposites,
Produced by an Alternative Sol-Gel Route
331
10 20 30 40 50 6070 8
0
(*) Hydroxyapatite (+) Wollastonite
+
+
+
+
+
+ +
+
+
+
+
+
+
+
+
**
*
***
******
**
*
*
**
*
**
*
*100H
80H-20W
50H-50W
20H-80W
Relative Inten s ity (a.u.)
100W
2-Theta (θ)
Figure 6. X-ray diffraction patterns for natural wollastonite
(100 W), and gels of calcium nitrate, ammonium phosphate
in water with different wt% of wollastonite upon heat treat-
ment at 750˚C: (20 H - 80 W) 80 wt%, (50 H - 50 W) 50
wt%, (80 H - 20 W) 20 wt% (100 H) 0 wt%. (*) Hydroxya-
patite; (+) Wollastonite.
(100H) (100W)
(80H‐20W) (50H‐50W) (20H‐80W)
Figure 7. Transmission electron microscopic images of base
materials powders: hydroxyapatite (100 H) and wollastonite
(100 W), and hydroxyapatite-wollastonite biocomposites
powders (80 H - 20 W, 50 H - 50 W, 20 H - 80 W).
centages by weight of wollastonite upon heat treatment at
750˚C. This figure shows the typical lath shape and acic-
ular (needle like) morphology of wollastonite fibers,
while hydroxyapatite grains exhibit their typical shape
and its hexagonal growth morphology [6]. Almost all of
these grains are between 50 nm and 100 nm in size. In all
biocomposites the grains of hydroxyapatite appear in-
serted among the wollastonite fibers, with the same shape
and size than the observed in the base materials. The par-
ticles of synthesized hydroxyapatite agglomerate homo-
geneously on the surface of the wollastonite fibers, which
assures an efficient interaction of both phases in the bio-
composites. Additionally, the different amounts of wol-
lastonite involved in the formation of the biocomposites
do not affect the stages of synthesis and the final charac-
teristics, such as the structure and morphology of hy-
droxyapatite produced. These morphological characteris-
tics of these biocomposites can avoid direct fiber-to-fiber
interaction because the surfaces of wollastonite fibers are
covered thoroughly by hydroxyapatite. Any further treat-
ment such as pressing and sintering will produce materi-
als with mostly fiber-hydroxyapatite interactions, be-
cause when the fibers interacted with themselves, other
studies [12-14] have reported that these interactions af-
fect some of the physical and mechanical properties of
the materials.
3.2. Sintering of Hydroxyapatite-Wollastonite
Biocomposites
Figures 8 and 9 show the scanning electron micrographs
for cylindrical tablets of natural wollastonite (100 W),
and synthesized hydroxyapatite (100 H), and biocompo-
sites of hydroxyapatite-wollastonite with different per-
centages by weight of wollastonite (20 H - 80 W) 80
wt%, (50 H - 50 W) 50 wt%, (80 H - 20 W) 20 wt%,
sintered at 1200˚C for 5 h. When the powders were sin-
tered, the surfaces in all materials show a compact micro-
crystalline appearance with clear crystal boundaries and
some micropores. This fact demonstrates that the sintered
hydroxyapatite-wollastonite biocomposites were not com-
pletely dense in the processing conditions of this study.
Although it could be thought that this porosity could
(100H) (100W)
10µm 10µm
4µm 4µm
1µm 1µm
Figure 8. Scanning electron micrographs of sintered base
materials: hydroxyapatite (100 H) and natural wollastonite
(100 W) sintered at 1200˚C for 5 h, at different magnifica-
tions.

Synthesis and Structural Characterization of Hydroxyapatite-Wollastonite Biocomposites,
Produced by an Alternative Sol-Gel Route
332
Figure 9. Scanning electron micrographs of sintered hy-
droxyapatite-wollastonite biocomposites (80 H - 20 W, 50 H
- 50 W, 20 H - 80 W) sintered at 1200˚C for 5 h, at different
magnifications.
have a negative effect on the mechanical properties, the
porosity actually enhances the bioactive properties, be-
cause the pores increase the penetration of physiological
solutions in the specimens and permit the crystal growth
of the bone-like apatite towards the surface, resulting in
more efficient bioactive properties [7,19]. Therefore, the
appropriate relationship between porosity and mechani-
cal behavior in these materials has important implications
in bone regeneration. Consequently, in a complementary
study the porosity effect will be analyzed, and its rela-
tionship with the microstructure and mechanical proper-
ties of the materials will be evaluated.
4. Conclusion
Hydroxyapatite-wollastonite biocomposites were synthe-
sized using an alternative sol-gel route using calcium
nitrate and ammonium phosphate as hydroxyapatite pre-
cursors, and high purity natural wollastonite as rein-
forcement phase in aqueous medium. Formation of hy-
droxyapatite in the biocomposites occurs at the relatively
low temperature of about 350˚C, in the early stages of
crystallization. A further thermal treatment of about
750˚C produces completely crystallized hydroxyapatite,
while wollastonite remains unreacted. The particles of
hydroxyapatite appear agglomerated on the surface of
wollastonite fibers. The hydroxyapatite-wollastonite bio-
composite powders sintered at 1200˚C for 5 h, experi-
ence no significant decomposition upon this heat treat-
ment. The hydroxyapatite and wollastonite remain as
single phases in the sintered biocomposites, exhibiting a
certain grade of porosity. Although this porosity could
have a negative effect on the mechanical properties, it is
important for ensuring the efficient integration of these
materials with bony tissues, which could be modulated to
vary the amounts of base materials in the biocomposites.
5. Acknowledgements
The authors are grateful to Francisco Brown Bojórquez
and Víctor Emmanuel Alvarez Montaño from DIPyM-
UNISON, Israel Gradilla, Eloisa Aparicio, Jesús Antonio
Díaz, David Dominguez, Eric Flores Aquino and Jaime
Mendoza López from CNyN-UNAM for assistance in the
characterization and discussions. This work was partially
support by PACAC UNISON-UNAM through project
Px-864 and DGAPA-PAPIIT-UNAM through grant
IN207511.
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