E. BARTONICKOVA et al.: POROUS HA/ALUMINA COMPOSITES INTENDED FOR BONE-TISSUE ENGINEERING
631–636
POROUS HA/ALUMINA COMPOSITES INTENDED FOR
BONE-TISSUE ENGINEERING
POROZNI HA/ALUMINIJEVI KOMPOZITI, NAMENJENI ZA
NADOMESTNO UPORABO PRI KOSTNEM TKIVU
Eva Bartonickova, Jan Vojtisek, Jakub Tkacz, Jaromir Porizka, Jiri Masilko,
Miroslava Moncekova, Ladislav Parizek
Brno University of Technology, Faculty of Chemistry, Materials Research Centre, Purkynova 464/118, 612 00 Brno, Czech Republic
bartonickova@fch.vutbr.cz
Prejem rokopisa – received: 2016-07-15; sprejem za objavo – accepted for publication: 2016-11-24
doi:10.17222/mit.2016.191
Ceramic biomaterials based on hydroxyapatite (HA) or alumina have been intensively studied due to their load-bearing
applications in the bone-tissue replacement/reconstruction and dental applications. Here we present a study of the preparation
and properties of HA/alumina (HA/Al) composites with a targeted porosity. The HA powder used for the composite’s
preparation was synthetized via a precipitation method under a variety of pH values. The resulting powders were verified with
XRD, Raman and FTIR analyses. The particle size was assessed via SEM and laser diffraction. The as-prepared HA nano-
powder and alumina powder (median 3 μm) were homogenously mixed having a composition of HA/Alumina = 90/10 (w/w). A
suspension with 65 % mass fraction of the powders was properly mixed and, with the help of foaming agents, it was foamed in
situ. The behavior under an increasing temperature was studied, using a heating microscope and dried foams were sintered
under determined temperatures. The final sintered foams were examined in vitro in a synthetic body fluid, which predicted the
behavior of bone implants in vivo. The behavior of the treated samples was studied with SEM. The newly formed HA
composites were confronted with Ca2+ and PO43– contents in the applied body-fluid solution.
Keywords: hydroxyapatite/Al2O3 composite, ceramic scaffolds, in-vitro bioactivity, SEM
Kerami~ni biomateriali na osnovi hidroksiapatitov (HA) ali aluminijevega oksida so bili sistematsko preiskovani zaradi njihove
najbolj{e nadomestne vloge pri nadome{~anju kostnega tkiva/rekonstrukciji in pri zobnih vsadkih. Predstavljena je {tudija
priprave in lastnosti kompozitov HA / aluminijevega oksida (HA / Al) s ciljem preizkusa poroznosti. HA prah, ki se uporablja za
pripravo kompozitov, smo sintetizirali po postopku obarjanja pri razli~nih pH-vrednostih. Dobljene pra{ke smo preverili z
XRD-, Raman in FTIR-analizo. Velikost delcev je bila ocenjena s pomo~jo SEM in z lasersko difrakcijo. Pripravljen nanopra{ek
(HA) in aluminijev oksid v prahu (median 3 μm) sta bila homogeno preme{ana v me{anico HA/aliminij 90/10 (w/w). Suspenzija
s 65 masnimi % pra{kov je bila ustrezno me{ana in s pomo~jo penil spenjena in situ. Raziskovali smo obna{anje v okviru
nara{~ajo~e temperature, uporabo pod ogrevanim mikroskopom in suhe pene so bile sintrane pri dolo~enih temperaturah.
Kon~ne sintrane pene so bile pregledane in vitro kot sinteti~ne teko~ine, ki so napovedale obna{anje kostnih vsadkov in vivo.
Obna{anje obdelanih vzorcev smo preu~evali s SEM. Pri vstavitvi oz. stiku s telesno teko~ino so bili na novo oblikovani HA v
stiku z Ca2+ in vsebino PO43–.
Klju~ne besede: hidroksiapatit/AI2O3 kompozit, kerami~na ogrodja, bioaktivnost in vitro, SEM
1 INTRODUCTION
The load-bearing system of humans, although it is
very durable, can sometimes lose a vital part due to
injuries, illnesses or wear. In this case, it is necessary to
replace the original tissue with suitable implants. The
first attempts to replace bone tissues is dated back to the
early years before Christ, but the intensive development
of this research area underwent a boom in the second
half of the 20th century. Nowadays, the 2nd and 3rd gene-
rations of biomaterials are in progress. Hydroxyapatite
belongs to biocompatible materials with osteoconductive
and osteointegrating properties and exhibits chemical
and physical analogies to the minerals present in human
bones and teeth.1–2 Although HA exhibits excellent
bio-properties, its clinical application failed due to poor
mechanical properties, a long remodeling time and a
relatively slow rate of osteointegration.3 So, the rein-
forcement of HA become part of the research. In recent
works, HA ceramics were reinforced with metals, poly-
mers or toughened zirconia ceramics.4–6
A synthesis of HA at the nanoscale has been studied
very often. There are many ways of how to prepare a
powder with targeted properties (i.e., at least one dimen-
sion under 100 nm). The wet procedures under the
conventional or non-conventional conditions, especially
precipitation, hydrothermal, sonochemical or microwave
treatments, are used most commonly.7–9 The sol-gel
technique of an HA synthesis exhibits a high level of
HA-powder homogeneity; nevertheless, the secondary
formed calcium oxide represents extraction and
biocompatibility problems.8 The variability of the
preparation of a ceramic foam is extremely wide. An
in-situ foaming procedure favors scaffold-preparation
procedures due to the possibility of controlling the poro-
sity with an exact diameter, in comparison with the
Materiali in tehnologije / Materials and technology 51 (2017) 4, 631–636 631
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 669.71:666.3-1:616-008.66 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(4)631(2017)
widely used replica method using a polymeric
sponge.10–11
The composite preparation was based on the same
procedures. The HA matrix was reinforced with cera-
mic-oxide constituents such as zirconia, titania, silica,
yttria, bioglass or alumina.3,6,12–18 Alumina oxide is a
cost-effective and well-accessible option for enhancing
HA properties, especially the mechanical properties. The
mechanical strength of a porous human bone is in a
range of 2–12 MPa.19 The reported values for pure HA
are in a range of 1.2–16 MPa, depending on the foaming
process.20 A positive alumina contribution to the mecha-
nical strength was recently reported by I. Sopyan et al.21
and L. L. Wang et al.17,22
The presented paper introduces an overall scaffold
preparation based on an hydroxyapatite matrix rein-
forced with an alumina-oxide addition. The physical
properties and in-vitro behavior of the final porous
scaffolds are presented.
2 EXPERIMENTAL PART
2.1 Synthesis of the hydroxyapatite powder
The hydroxyapatite powder was synthetized via a
simple precipitation method from calcium nitrate
(Ca(NO3)2.4H2O; puriss; LachNer; the Czech Republic)
and phosphoric acid (H3PO4; puriss; Penta; Czech Re-
public) precursors. The syntheses was carried out under
laboratory conditions and via the reaction mechanism
given in Equation (1):
10Ca(NO3)2 + 6H3PO4 + 20NH4OH 
 Ca10(PO4)6(OH)2 + 20NH4NO3 + 18H2O (1)
Aqueous solutions of precursors (Ca2+ concentration
– 1.3 mol/L; PO43– concentration – 1 mol/L) were
simultaneously added dropwise into an aqueous solution
with a controlled pH value. The pH value was varied
from 1 to 9.6 and adjusted with an addition of ammo-
nium hydroxide (NH4OH; puriss; LachNer, the Czech
Republic). To assess the reaction kinetics, turbidimetric
measurements of the formed precipitates (2100Q Hach)
combined with the reaction-yield determination were
conducted. The precipitated products were centrifugally
separated, washed and dried at 80 °C. The prepared
powders were analyzed in the terms of phase and che-
mical compositions and morphology (an EMPYREAN
Panalytical diffractometer in the central focusing
arrangement using Co-K radiation, the Netherlands; a
FTIR spectrometer Nicolet iS50 Thermo Fisher Scien-
tific, U.S.A.; and an EVO CS10 ZEISS electron micro-
scope equipped with an energy-dispersive analyzer with
an Oxford X-Max 80 mm2 detector in the back-scattering
mode, respectively).
2.2 Scaffold fabrication and characterization
The synthetized HA powder and commercial alumina
powder (Nabalox 325; Nabaltec AG, Germany) were
properly mixed at a HA/alumina ratio of 90:10. The
aqueous suspensions containing 55 % mass fraction of
the HA solid loading and 65 % mass fraction of the
composite solid loading were homogenously mixed to
avoid the unfavorable agglomeration. The foaming agent
(Schäumungsmittel Zschimmer&Scharz GmbH & Co,
Germany) was added to the prepared suspensions with a
concentration of 0.25 % mass fraction. The given con-
centrations of the solid loading and the foaming agent
were previously experimentally determined. After the
agent addition, the mixing rate of the magnetic stirrer
was immediately accelerated to initialize the foaming
process (up to 1000 min–1). The foamed suspensions
were cast in an open aluminium-foil mould with dimen-
sions of (1 × 1 × 1) cm and (2 × 2 × 2) cm for apatite-
forming-ability tests and determination of the mechani-
cal properties, respectively. To avoid the formation of
unfavourable cracks, the drying process was carried out
in several steps – at 50 °C for 4 h; 80 °C for 10 h and,
finally, 105 °C for 3 h. Dried samples were demoulded
and heated to 600 °C for an organic-species removal and
to 1250 °C (the temperature determined with a heating
microscopy analysis (EM 201 Leitz, Germany) to carry
out the foam consolidation. The porosity evaluation was
examined using mercury intrusion porosimetry (Pore-
master Quantachrome, U.S.A.), an image analysis (Stemi
508 ZEISS, Austria; processed via ImageJ software) and
Archimedes’ bulk-density determination (ISO EN 5017
2013). The mechanical properties of porous scaffolds
were calculated from stress-strain curves using a univer-
sal mechanical testing machine (5985C INSTRON,
U.S.).
2.3 Apatite-forming-ability testing
The behavior of the samples in the human body was
simulated with a stability test. The samples were
thoroughly immersed in a modified simulated body
liquid (SBF) prepared via Kokubo’s procedure23 and
soaked for (7, 14 and 28) d in an incubator chamber at
37 °C. The surfaces of the treated samples were observed
E. BARTONICKOVA et al.: POROUS HA/ALUMINA COMPOSITES INTENDED FOR BONE-TISSUE ENGINEERING
632 Materiali in tehnologije / Materials and technology 51 (2017) 4, 631–636
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Turbidimetry and reaction yield as a function of pH
with SEM. The concentrations of Ca2+ and PO43– in pure
and treated SBFs were analyzed using ion chromato-
graphy (Metrosep C4 15/4.0 Metrohm, Switzerland) and
ICP-OES (Ultima 2 Jobin Yvon Technology, France).
3 RESULTS AND DISCUSSION
3.1 Hydroxyapatite characterization and in-situ foam-
ing
The hydroxyapatite formation was studied by means
of reaction kinetics using turbidity measurements.
Figure 1 describes the yield and turbidity of the reaction
product as a function of pH. An increase in pH from 1 to
6 increased the turbidity values and the product yield.
The maximum turbidity was reached at a pH of 4 and
corresponded to the colloidal character of the nucleated
and grown HA particles. After this value, the precipitated
particles started to flocculate and the suspension became
unstable. Large-sized particles started to agglomerate
and the sediment corresponded with lower turbidity
values, whereas the yield of the reaction was un-
changed.8,24 The high level of agglomeration was con-
firmed with a morphology analysis and laser-diffraction
measurements (Figure 2). The phase purity and crys-
tallinity of the prepared powders are given in Figure 3.
A strong dependence of the precipitation reaction on the
pH is obvious (Figure 3 and Table 1). A pH below 7
exhibits a heterogeneous system based on hydroxyapatite
and monetite (ICCD 01-089-6438 and ICCD 01-070-
0359, resp.). Similar kinetics was observed in several
works where a phase-pure HA powder with nanodimen-
sions, but agglomerated, was also synthetized.8,25–26
Based on the results summarized above, the optimum pH
of the reaction was set to 8.7. The HA powder used for
the in-situ foaming was successfully and repeatedly
prepared on large scale, with a 100 % phase purity and
the mean agglomerate size of about 4 μm.
Table 1: Phase analysis of the synthetized HA powders under
different pH values
pH Monetite – CaHPO4(w/%)
Hydroxyapatite –
Ca(PO4)3(OH) (w/%)
4.5 100 0
5.7 87 13
6.6 18 82
8.7 0 100
9,2 0 100
9.4 0 100
A significant factor for the successful foaming of the
prepared suspensions was the stability of the foam
E. BARTONICKOVA et al.: POROUS HA/ALUMINA COMPOSITES INTENDED FOR BONE-TISSUE ENGINEERING
Materiali in tehnologije / Materials and technology 51 (2017) 4, 631–636 633
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Foam-stability test in terms of time dependence
Figure 2: Synthetized hydroxyapatite powder: a) morphology, b) par-
ticle-size distribution
Figure 3: Dependence of the HA-product phase purity on the pH
value
prepared without a ceramic powder. This stability was
studied with respect to the content of the foaming agent
(Figure 4) and the time to the foam structure's collapse.
The 0.25 % mass-fraction content of the foaming agent
was determined as the optimum value. P. Ptá~ek et al.27
recently published a paper, in which an aqueous/deter-
gent-based foam was also studied in terms of the
time-stability dependence and similar experimental data
was observed.
The sintering behavior of the pure powder and
composite is shown in Figure 5. The area shrinkage was
found to be 38 % for HA and 24 % for the HA/Al com-
posite. The optimum sintering temperature for HA and
HA/Al sintering was determined, according to the
experimental data, to be 1250 °C. The consolidation of
the closely packed particles seemed to be finished
according to the sintering curve (Figure 5). The phase
analysis of the sintered samples showed a decomposition
of the HA structure to several apatite forms above the
temperature of 1150 °C18 (tricalcium phosphate, trical-
cium hydrogen diphosphate, tetracalcium tetraphosphate
and residual HA). In the case of the HA/Al composite
sample, the formation of tricalcium phosphate and the
Ca-Al phase was observed. It was reported that - and
-tricalcium phosphates are more biodegradable apatite
forms in comparison with the HA structure28, which is
closely connected with the chosen temperature of the
sintering discussed above. The newly formed Ca-Al
structure of the composite samples was positively iden-
tified as hibonite CaAl12O19 (ICCD 01-076-0665) with a
platelet hexagonal arrangement, which is shown, in
detail, in Figure 6d.
L.-P. Li et al.29, M. H. Ghazanfari et al.16 and B. Basar
et al.30 also observed the formation of the Ca-Al phase in
HA/Al composites with similar particle morphologies at
temperatures of 1350, 1250 and 1100 °C, respectively.
The structures of the scaffolds prepared from the pure
hydroxyapatite powder and composite powder using
in-situ foaming are given in Figure 6. The alumina
contribution is clearly visible; the composite foam has a
ball-shaped structure (Figure 6b) resulting from the
foaming process, with a pore diameter of around 60 μm
(obtained with the image analysis). The microstructure is
apparently similar to those presented by P. Ptá~ek et al.27,
where the aqueous-surfactant system was also used. On
the other hand, the observed microstructures of the HA
foams (Figure 6a and 6c) are more heterogeneous,
having even smaller pores (of around 28 μm according to
the image analysis) than the composite one. The pre-
sence of hibonite was also confirmed with the EDS and
SEM analysis (Figure 6d). The HA scaffolds provided
significantly higher mechanical-strength values (see the
embedded image in Figure 7). These findings indicate a
low degree of foam consolidation due to the alumina part
with a higher sintering-temperature region. Similar
values of the mechanical strength and the corresponding
E. BARTONICKOVA et al.: POROUS HA/ALUMINA COMPOSITES INTENDED FOR BONE-TISSUE ENGINEERING
634 Materiali in tehnologije / Materials and technology 51 (2017) 4, 631–636
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Sintering study provided with a heating microscope analysis
Figure 7: Porosity determination and mechanical strength (embedded)
of HA and HA/Alumina scaffolds
Figure 6: Microstructures of the HA and composite scaffolds: a) HA
scaffold, b) HA/Alumina scaffold, c) HA scaffold – a detail, d)
hibonite structure observed in the HA/Alumina scaffold
phase composition were also achieved by M. H. Ghazan-
fari et al.16
The total porosities determined with Archimedes’
method were (45.9 and 63.1) % of materials theorethical
density for HA and the HA/alumina composite, res-
pectively. Pore-size distributions for both of the scaffolds
are given in Figure 7, reflecting the effect of the alumina
addition on the pore-structure evolution. The performed
analysis showed a possible collapse of the HA/Al foam
structure during an Hg intrusion, visible as an increasing
tendency of the curve above 100 μm or the presence of
significantly large pores. Figure 7 (the embedded image)
shows the mechanical properties of the prepared
scaffolds. The measured values are in the equipment
limits, so the relative deviation is considerable. The
maximum mechanical strength reached for the pure HA
structure was 0.2 MPa. The expected enhancement of the
alumina addition in the mechanical properties was not
confirmed. The higher strength also reflected the ob-
tained porosity values, i.e., the HA/alumina composites
showed higher total porosities with a lower sintering
degree than the pure HA ones.
3.2 In-vitro behavior – the apatite-forming ability
Figures 8 and 9 summarize the in-vitro testing of the
prepared scaffolds. The synthetic body fluid, prepared by
T. Kokubo23, simulated the bloody-plasma environment
in the human body. In Figure 8, the concentrations of the
most important ion species, Ca2+, Al3+ and PO43–, are
given. The pure HA scaffolds exhibited the expected
behavior; the gradual consumption of both of the ions
corresponded to the formation of the new HA on the
scaffold surface (Figure 9). The ability of a Ca-apatite
formation on the surface was frequently reported in the
past.31–32 Otherwise the reactivity of the HA/Al compo-
site in the SBF environment was slightly different. As
could be seen from Figure 8, the PO43– ions were totally
consumed within 28 d; on the other hand, the Ca2+ and
Al3+ cation concentration had an increasing tendency.
The newly formed phase practically covered the compo-
site scaffold surfaces (Figure 9). The performed EDS
analysis could not exactly distinguish between the
formations of the Ca- or Al-bonded apatite structures.
The probability of the formation of both phases is almost
certain. X. Chatzistavrou et al.33 described the mecha-
nism of the Ca-Al apatite formation on a HA/Al compo-
site surface according to the reaction scheme in Equation
(2), which is in good agreement with our data:
Al-OH + H2PO4
2–  Al-O-PO3
– + OH– (2)
4 CONCLUSION
In this work, a synthesis of the nanosized phase of a
pure hydroxyapapatite powder was successfully per-
formed on a large scale. Porous scaffolds based on the
as-synthetized hydroxyapatite and supplied alumina
powders were fabricated using the in-situ foaming
process. The prepared HA scaffolds exhibited signifi-
cantly better mechanical properties, with the total poro-
sity of about 46 %, than the HA/Al scaffold with a
porosity of about 64 %. In-vitro testing of both scaffolds
confirmed an appropriate apatite-forming ability. The
HA/Al composite was significantly more covered with
the newly formed phase than the pure HA ones. A study
of the nucleation and growth mechanism of Ca-Al
apatite is in progress.
Acknowledgement
The paper was supported by the project Materials
Research Centre at FCH BUT – Sustainability and
Development, REG LO1211, with a financial support
from the National Programme for Sustainability I (the
Ministry of Education, Youth and Sports).
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Materiali in tehnologije / Materials and technology 51 (2017) 4, 631–636 635
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