* Corresponding author. Tel.: #[phone]; fax: #[phone].
E-mail address: [e-mail] (N.H. Loh).

Biomaterials 22 (2001) 1225}1232

Processing of HA-coated Ti}6Al}4V by a ceramic slurry approach:

an in vitro study

E.S. Thian, K.A. Khor, N.H. Loh*, S.B. Tor

School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

Received 17 November 1999; accepted 2 August 2000

Abstract

Hydroxyapatite-coated titanium alloy composite powders (Ti}6Al}4V/HA) was produced by a ceramic slurry approach. The aim

was to evaluate the stability of the coating when subjected to a physiological medium

simulated body #uid (SBF). Three

consolidation conditions (7003C for 5 h, 7003C for 8 h and 7003C for 11 h) were used in the production of the Ti}6Al}4V/HA
composite powders. Results showed that biodissolution followed by apatite precipitation had taken place after soaking in SBF. In
addition, it was found that consolidation at 7003C for 5 h resulted in a weak mechanical locking of calcium phosphate on the
Ti}6Al}4V surfaces; and the formation of small crystallites, which would increase the dissolution rate.

2001 Elsevier Science Ltd.

All rights reserved.

Keywords: Ceramic slurry approach; Simulated body #uid (SBF); Consolidation; Dissolution; Precipitation; Apatite

1. Introduction

Hydroxyapatite

(HA)-coated

titanium

alloy

(Ti}6Al}4V) has been used extensively in biomedical

"elds due to its excellent biocompatibility, osteoconduc-

tivity, and mechanical properties. Various coating tech-
niques, such as dipping method [1], electrodeposition
[2}6], magnetron sputtering deposition [7], pulsed-laser
deposition [8], plasma spraying [9}13], and sol}gel tech-
nique [14,15] have been used. These composite implants
have been used in most of the load bearing applications.
Recently, there has been an increasing interest in the
fabrication and properties of porous HA ceramics. These
porous bioactive ceramics promote bone or tissue in-
growth into the open pores of the implants, thereby
allowing a rapid return to the physiologically acceptable
state of function [16}18]. They are mostly applicable in
non-load bearing applications. However, due to their
extremely weak mechanical properties, the porous HA
ceramics might fracture if a sudden force is applied to
them during the healing stage. Thus, it is necessary to
increase their mechanical properties and at the same
time, maintain their porous nature. The mechanical

properties can be increased by using HA-coated
TI}6Al}4V composite powders.

In the current investigation, HA-coated Ti}6Al}4V

composite powders were produced by a ceramic slurry
approach. It involved the use of polyvinyl alcohol (PVA)
binder to coat HA powders onto acid-etched Ti}6Al}4V
powders, followed by binder removal and consolidation.
The HA-coated Ti}6Al}4V composite powders represent
a good combination for biomaterials, in the way that the
outer core is HA and therefore exhibits excellent biocom-
patibility while excellent mechanical strength is preser-
ved by Ti}6Al}4V. An in vitro dissolution study was then
conducted on these composite powders (Ti}6Al}4V/HA)
produced by the ceramic slurry approach to study the
stability of the coating when subjected to a physiological
medium.

2. Materials and methods

2.1. Production of Ti}6Al}4V/HA

Ti}6Al}4V powders (Sumitomo Sitix of Amagasaki

Inc., Japan) were progressively etched for 90 s in fuming
hydro#uoric acid, 40% aqueous HF solution (Merck,
Germany), diluted with distilled water. The residue
(acid-etched Ti}6Al}4V powders) was then dried in a

0142-9612/01/$ - see front matter

2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 4 2 - 9 6 1 2 (0 0 ) 0 0 2 7 2 - 6

Table 1
Salts used to make up 3 l of SBF solution

Salt

Mass (g)

NaCl

24.16

NaHCO

1.06

KCl

0.67

KHPO )3HO

0.52

MgCl ) 6HO

0.91

CaCl ) 2HO

1.10

(CHOH)CNH

18.15

Fig. 1. XRD patterns of the starting HA, and Ti}6Al}4V/HA at vary-
ing consolidation times before and after soaking in SBF for 1 and
2 weeks.

Salvis vacuum oven at 1003C for 1 h. The coating process
was started by dissolving PVA binder in distilled water
on a Labquip magnetic stirrer. Acid-etched Ti}6Al}4V
powders were then added in small quantities at an inter-
val of 5 min into the PVA solution before the mixture was
allowed to mix homogeneously for 2 h. HA powder (Tai
Hei Chemical Co. Ltd., Japan) was then added slowly, in
small quantity at an interval of 5 min into the solution
containing PVA-coated acid-etched Ti}6Al}4V. Again,
the mixture was allowed to mix homogeneously for an-
other 2 h. The PVA binder was removed by heating at
4503C for 2 h in a Lenton high-temperature furnace,
followed by consolidation at 7003C for three varying
times: 5, 8 and 11 h. The consolidated Ti}6Al}4V/HA, in

"nal composition of 50 wt% Ti}6Al}4V and 50 wt% HA,

was then loosely crushed into individual particles.

2.2. Preparation of simulated body yuid (SBF)

SBF with ion composition nearly equal to the human

blood plasma was prepared by dissolving reagent-grade
chemicals: sodium chloride, NaCl; sodium hydrogen car-
bonate, NaHCO; potassium chloride, KCl; di-potassi-

um hydrogen phosphate trihydrate, KHPO ) 3HO;

magnesium chloride hexahydrate, MgCl ) 6HO; cal-

cium chloride dihydrate, CaCl ) 2HO; and tri-hydroxy-

methyl

aminomethane,

(CHOH)CNH (Merck,

Germany) in distilled water, and bu!ering at pH 7.25
with fuming hydrochloric acid, 37% aqueous HCl solu-
tion (Merck, Germany). Table 1 shows the required mass
of each salt to prepare 3 l of SBF.

2.3. Soaking of Ti}6Al}4V/HA in SBF

Two samples of Ti}6Al}4V/HA weighing approxi-

mately 2 g from each consolidation condition were
soaked in test tubes containing 25 ml of SBF each; main-
tained at 36.53C in a water bath for 1 and 2 weeks. One
sample from each consolidation condition was then "l-
tered, washed gently with distilled water and dried in
a Salvis vacuum oven at 1003C for 0.5 h after periods of
1 and 2 weeks.

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E.S. Thian et al. / Biomaterials 22 (2001) 1225}1232

Fig. 1. (Continued ).

2.4. Characterization of Ti}6Al}4V/HA

Surface morphology of the as-prepared Ti}6Al}4V/

HA and subsequent soaking in SBF for 1 and 2 weeks
were examined by X-ray di!raction (XRD: Philips APD
1700, The Netherlands) in the range 2

"20

}703

at

a step size of 0.02 per minute, operating at 40 kV and
30 mA; scanning electron microscope (SEM: JEOL
JSM-5600 LV), equipped with energy dispersive analyti-
cal system (EDS: Link ISIS); and inductively-coupled
plasma emission spectrometer (ICP: Perkin Elmer P400,
USA). Peak areas of HA,

-TCP, -TCP and TTCP

phases in the Ti}6Al}4V/HA were determined by
integrating the main peak of these phases using a pro"le

"tting software in the XRD system.

3. Results and discussion

3.1. XRD analysis

Fig. 1 shows the XRD patterns of the as-received HA,

and Ti}6Al}4V/HA at varying consolidation times be-
fore and after soaking in SBF for 1 and 2 weeks. Fig. 2

E.S. Thian et al. / Biomaterials 22 (2001) 1225}1232

1227

Fig. 2. Peak area of HA,

-TCP, -TCP, and TTCP phases in

Ti}6Al}4V/HA at varying consolidation times before and after soaking
in SBF for 1 and 2 weeks.

Fig. 3. Surface morphologies of Ti}6Al}4V/HA at varying consolida-
tion times before and after soaking in SBF for 1 and 2 weeks.

shows the peak area of HA,

-TCP, -TCP and TTCP

phases in Ti}6Al}4V/HA at varying consolidation times
before and after soaking in SBF for 1 and 2 weeks.

The as-received HA powder contained

-TCP and

-TCP phases in a phase ratio of 0.18 and 1.62, respec-
tively. XRD patterns of the as-prepared Ti}6Al}4V/HA
show a decrease in HA phase, but an increase in

-TCP

and

-TCP phases as the consolidation time increased.

The appearance of TTCP in small concentration at
7003C for 11 h is observed. The peak area results also
show that the HA peak area decreased with increasing
consolidation time. On the other hand, the

-TCP,

-TCP and TTCP peak areas increased correspondingly.

The aforementioned phenomenon is due to the phase
decomposition of HA during consolidation, based on the
chemical reaction stated [19]:

Ca(PO)(OH) &2Ca(PO)#CaPO#HO.

It is also apparent that the crystallites grow with the

process of consolidation treatment, since XRD patterns
indicate sharp peaks with narrow peak widths. The
growth of the crystallites in this study might prove to be
of importance owing to the decrease in dissolution rate.
Since HA,

-TCP, -TCP and TTCP are considered as

biocompatible materials, the existence of these phases is
not supposed to a!ect the stability of the coating signi"-
cantly. HA is considered to have an excellent chemical
bonding ability with natural bone since it has a structure
more similar to natural bone tissue apatite [20,21]; TCP

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E.S. Thian et al. / Biomaterials 22 (2001) 1225}1232

Fig. 3. (Continued ).

E.S. Thian et al. / Biomaterials 22 (2001) 1225}1232

1229

Fig. 4. An EDX spectrum of loose particles.

Fig. 5. Relationship between calcium ion concentration and period of
soaking.

is considered as bioresorbable ceramics which will grad-
ually dissolve so as to promote osseointegration [22].

After having been soaked in SBF for 1 week, the three

XRD patterns of Ti}6Al}4V/HA revealed a marked de-
crease in the calcium phosphate peaks. Comparing the
peak area results before and after soaking in SBF for
1 week, it also revealed that there was a signi"cant de-
crease in the HA,

-TCP, -TCP and TTCP peak areas.

However, they are more pronounced at the consolidation
time of 5 h. This might be attributed to the fact that it
revealed a weaker mechanical locking of the calcium phos-
phate onto the Ti}6Al}4V surfaces and the presence of
small crystallites which are more soluble in SBF. Peak
areas for

-TCP and -TCP phases are observed to de-

crease signi"cantly compared to the HA phase for all the
consolidation conditions, due to the higher solubility of
TCP phases. On the other hand, the disappearance of
TTCP at a consolidation time of 11 h is detected. This

"nding is consistent with Fernandez et al

. [23].

A slight increase in the HA,

-TCP and -TCP phases

is shown by the XRD patterns after having been soaked
in SBF for 2 weeks. Moreover, the presence of new peaks
for HA and

-TCP are noticed. Furthermore, the peak

area results show an increment in the HA,

-TCP, and

-TCP peak areas. It is apparent that the precipitation of
apatite by the reaction between the calcium and phos-
phate ions from the SBF might be occurring.

3.2. Surface morphologies of Ti}6Al}4V/HA

Fig. 3 shows the surface morphologies of the Ti}6Al}

4V/HA at varying consolidation times before and after
soaking in SBF for 1 and 2 weeks. Before soaking in SBF,
the coating surfaces appear to be relatively dense and
homogeneous in appearance at high magni"cation of
2000

;. At a low magni

"cation of

600

;, the coating

displays some loose particles. These loose particles are
identi"ed as calcium phosphate, since EDX spectrum
(Fig. 4) shows the presence of calcium (Ca) and
phosphorus (P) peaks only.

Following soaking in SBF, the surface morphologies of

the coating dissolved to di!erent degrees. As shown in Fig.
3, the degree of surface attack by the SBF is clearly
evident. At week 1, large amounts of the calcium phos-
phate coating was dissolved in SBF after consolidation for

5 h, exposing the Ti}6Al}4V surfaces to the surrounding

#uids

which is undesirable. On the other hand, large

number of holes appeared on the coating after consolida-
tion for 8 and 11 h. This "nding is consistent with the XRD
analysis, which shows a decrease in the peak areas of HA,
-TCP and -TCP phases. Therefore, it can be concluded
that di!usion of ions from the coating surface and reaction
between the coating surface and SBF are taking place.

After 2 weeks of soaking, small globular crystals start

to precipitate on the coating surfaces after consolidation
for 5, 8 and 11 h. EDX spectrum has con"rmed the
precipitation of calcium phosphate crystals since only
Ca, P and oxygen (O) peaks are present. There is no
preferential location for these crystals and are found on
all the features of the coating. This "nding again supports
the XRD analysis, which shows an increase in the peak
areas of HA,

-TCP and -TCP phases. It was suspected

that more crystals would be formed after longer soaking
time until the coating surface is totally covered, with no
further dissolution taking place after that [24].

3.3. ICP analysis

Fig. 5 shows the variation of calcium ion concentration

after 1 and 2 weeks of soaking in SBF. It is apparent from
Fig. 5 that an increase in calcium ion concentration after
having been soaked in SBF for 1 week is observed after
consolidation for 5, 8 and 11 h. A plausible explanation
for this phenomenon is that the di!usion of ions from the
coating surface is actually taking place, whereby TCP
and TTCP undergo a transformation through reactions
1 and 2 stated as follows [25]:

4Ca(PO)#3HOPCa(PO)(OH)

#

2Ca

>#2HPO\

,

(1)

3CaPO#HOPCa(PO)(OH)

#

2Ca

>#4OH\.

(2)

Consolidation for 5 h results in the highest calcium ion
concentration release among the three. This is consistent
with the earlier "nding, and can be concluded that a low

1230

E.S. Thian et al. / Biomaterials 22 (2001) 1225}1232

mechanical locking of calcium phosphate onto the
Ti}6Al}4V surfaces existed and small crystallites are
more soluble.

On the other hand, the calcium ion concentration

decreased after having been soaked in SBF for 2 weeks
after consolidation for 5, 8 and 11 h. It implies that
apatite precipitation has occurred. This "nding is consis-
tent with the surface morphological analysis, which dem-
onstrates that small globular calcium phosphate crystals
appear on the coating surfaces after soaking in SBF for
2 weeks. A signi"cant drop in the calcium ion concentra-
tion release is observed after consolidation for 8 and 11 h,
and this rapid morphological change is expected to in#u-
ence the rate of bone bonding.

Ti}6Al}4V/HA composite powder produced by sim-

ilar ceramic slurry technique (on unetched Ti}6Al}4V
powder) has been fabricated by the powder injection
molding process to give mechanically strong, yet porous
Ti}6Al}4V/HA tensile specimens. The sintered tensile
specimens have an average relative sintered density of
51%, Vicker's hardness of 1.395 GPa at a load of 300 gf,
bending strength of 17.321 MPa, Young's modulus of
23.848 GPa and toughness of 700.716 J/m

[26]. The

mechanical properties achieved are higher than the por-
ous HA ceramics of Ref. [27] (approximately 43 and 39%
higher for bending strength and Young's Modulus, re-
spectively at 51% porosity level).

4. Conclusion

An in vitro dissolution study is used to investigate the

biological response of Ti}6Al}4V/HA produced by the
ceramic slurry approach. The most important point elu-
cidated in this study is the long-term stability of the
coating when the in#uence of a physiological medium
(SBF in this study) is considered. When exposed to SBF,
Ti}6Al}4V/HA would undergo two biointegration pro-
cesses:

1. biodissolution; followed by
2. apatite crystal precipitation.

Evidence has shown that a weak mechanical locking of

calcium phosphate onto the Ti}6Al}4V surfaces and
small crystallite sizes accounted for the instability of the
coating when subjected to SBF. Finally, small globular
calcium phosphate crystals start to precipitate after soak-
ing in SBF for 2 weeks, and this would be of paramount
importance since it is expected to in#uence the rate of
bone bonding.

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