Indian Journal of Chemistry

Vol. 47A, November 2008, pp. 1626-1631

Synthesis of nano sized hydroxyapatite powder using sol-gel technique and its

conversion to dense and porous bodies

Iis Sopyana,*, Ramesh Singh

b & Mohammed Hamdi

c

aDepartment of Manufacturing & Materials Engineering, Faculty of Engineering, International Islamic University Malaysia,

P.O. Box 10, 50728 Kuala Lumpur, Malaysia

Email: sopyan@iiu.edu.my bDepartment of Mechanical Engineering, College of Engineering, University Tenaga Nasional, Malaysia

cDepartment of Engineering Design and Manufacture, Faculty of Engineering, University Malaya, Malaysia

Received 13 May 2008; revised 20 October 2008

Hydroxyapatite powder has been prepared via sol-gel procedure using calcium nitrate tetrahydrate and diammonium

hydrogen phosphate as the precursors for calcium and phosphorus, respectively. XRD measurement shows that the powder

contains hydroxyapatite crystals with β-TCP and calcium oxide as secondary phases. Hydroxyapatite powder of higher

purity, i. e., the correct Ca/P ratio, has been obtained by adding an appropriate amount of diammonium hydrogen phosphate

and heating with stirring. Morphological evaluation by SEM measurement shows that the particles of the HA are tightly

agglomerated and globular in shape with an average size of 1-2 µm. The primary particulates have average diameters of

50-200 nm, as detected by SEM and nanoparticle sizer. Purity (almost 100%) of the obtained hydroxyapatite has been

confirmed by XRD analysis. Its performance has been tested by making dense and porous samples.

IPC Code: Int Cl.8 B82B3/00; C01B25/45; C01F11/00

Hydroxyapatites (HA) are particularly attractive

materials for bone and tooth implants since they

closely resembles human tooth and bone mineral and

have proved to be biologically compatible with these

tissues.1

Since the use of hydroxyapatite for the first

time in 1981 for periodontal lesion filling, its use in

the medical field has extended to solid blocks, solid

components, and films for dental implants. Many

studies have shown that HA ceramics show no

toxicity, inflammatory response, pyrogenetic

response, or fibrous tissue formation between implant

and bone. Also, these materials have the ability to

bond directly to the host bone.2

The main limitation of HA ceramics as well as all

other bioactive materials is that they have poor

mechanical properties. Basically, all bioceramics

which have good mechanical properties and suitable

for load bearing applications should be bioinert.

Hydroxyapatite, on the other hand, has high

bioactivity, with many medical applications in the

form of porous, dense, granules, and, as coatings.2,3

Several research groups have developed prepara-

tive procedures for hydroxyapatite. Traditionally, two

main methods are employed for preparation of HA

powders: wet (chemical) method (including

precipitation method,4 hydrothermal technique,

5 and

hydrolysis6) and dry (solid state reaction) method

(refs cited in Ref. 7). Differences in the preparative

routes leads to variations in morphology,

stoichiometry, and level of crystallinity. Other

methods, such as sol-gel,8,9

spray pyrolysis,10,11

mechano-chemical method,12

have also been

developed recently and are well documented.7

Sol-gel procedure was first employed for the

preparation of HA by Sakka and co-workers.8 They

used calcium diethoxide and phosphorus triethoxide

as starting materials. Hydrolysis – polycondensation

of the monomers in neutral and acidic solutions gave

HA powders of high purity. Extraordinarily fine

amorphous particulates with less than 10 nm in

diameter were obtained from precipitation of the

solutions, which increased to only ca. 100 nm after

calcination at 900°C.

In this present study, we have developed sol-gel

procedure for preparing HA powder. It is well known

that sol-gel techniques have several advantages for

producing ceramic particulates of high purity, high

crystallinity, nano sizes, and high reactivity. Sol-gel

process, however, has some drawbacks such as

expensive raw materials and low homogenity of the

final product. We report herein a novel sol-gel method

for preparing extraordinarily fine hydroxyapatite

SOPYAN et al.: SYNTHESIS OF NANO SIZED HYDROXYAPATITE POWDER USING SOL-GEL TECHNIQUE

1627

powder, utilizing easily obtainable raw materials of

relatively low cost. Simplicity of experimental

execution, in respect of methods employing wet

chemical reaction, is one of the most important

advantages offered by this method. Physico-chemical

characterization of the hydroxyapatite powder

obtained from the sol-gel procedure has been carried

out. Morphology as well as the mechanical properties

of dense and porous bodies prepared using the powder

have also been investigated.

Materials and Methods For preparation of hydroxyapatite powder, calcium

nitrate tetrahydrate and diammonium hydrogen

phosphate (reagent grade) were used as calcium and

phosphorus precursors, respectively. Both reagents

were purchased from Merck KGaA, Germany. Urea

(R&M Chemicals, UK) was used as gelling and

ammonium donor agent. EDTA (Merck KGaA) was

used as chelating agent to prevent an immediate

precipitate formation calcium ions in the course of gel

formation. The reaction was conducted in basic

solution using ammonium solution (R&M Chemicals,

UK) as solvent. Preparation of the stoichiometric hydroxyapatite powder

Ammoniun solution was heated at 60°C, and

EDTA (181 g) was added while stirring until it

dissolved. Into this, 200 mL aqueous solution of 129 g

calcium nitrate tetrahydrate was poured.

Diammonium hydrogen phosphate (39.83 g) and urea

(45.20 g) were subsequently added. The mixture was

heated at 100°C for 3-4 h. The obtained gel was dried

at 350°C under ambient static air and subsequently

subjected to an 820°C calcination under flowing air.

The powder was examined by X-ray diffraction

techniques to determine the phases formed. It was

observed that Ca/P molar ratio is ca. 1.8.

Accordingly, to compensate the upward deviation

from the stoichiometric ratio (1.667), the powder was

mixed with an appropriate amount of diammonium

hydrogen phosphate, followed by suspending in water

and heating at 90°C with rigorous stirring. This

procedure restored the Ca/P ratio of hydroxyapatite

powder to 1.67. After drying, pure hydroxyapatite

powder was obtained.

Preparation of dense and porous samples

For preparation of dense bodies, the as-prepared

hydroxyapatite powder (50 g), was mixed with

poly(vinyl alcohol) of 15.000 MW(2.5 g) and 100 ml

distilled water. The suspension was homogenized

using a magnetic stirrer followed by spray-drying on a

Buchi mini spray dryer (B-290 type). Well dispersed

powders obtained were compressed uniaxially using

cold pressing technique at 800 kg/cm2. The pellets

thus obtained were sintered in air at 1250°C for 1h.

The same procedure was applied for a commercial

hydroxyapatite powder (Sigma Aldrich).

For preparation of porous bodies, slurries were

prepared with the as-prepared HA powder, and

adjusted loading of HA (ca. 25 wt%), using Duramax

of D-3021 or D-3005 type (Rohm and Haas, USA) as

the dispersant. Commercial cellulosic sponges were

used and an initial impregnation procedure with a

considerable fluidity slurry was employed. After

soaking into the slurry, the sponges were dried in

ambient air for 72 h and then subjected to heat

treatment at 600°C for 1 h to eliminate organic

matrix. Sintering was carried out at 1250°C for 1h.

Characterization

Scanning electron microscopic measurement for

morphology evaluation of the powder was performed

on a Jeol FESEM instrument (model JSM 6700F).

The particle size distribution and mean particle size

were measured using a back scattering method on a

Malvern Nanosizer (model Nano-S). Differential and

thermogravimetric analyses were performed on the

as-prepared of HA powders and dried gel in ambient

air using Perkin Elmer instrument (model PYRIS

Diamond) with a 10°C/min heating rate. The

crystalline phase compositions of the powders and of

the dense samples were evaluated in a Rigaku

diffractometer with copper Kα radiation and a scan

rate of 2° in 2θ min-1

. XRD patterns obtained were

utilized for quantitative phase analysis according

to literature.13

Density of porous bodies was measured as apparent

density (geometrical weight/volume measurement).

The compressive strength was measured on

cylindrical specimens (10 mm ht × 10 mm dia.) using

an Instron 1195 apparatus. The compressive strength

was calculated from the maximum load registered

during the test divided by the original area. Several

specimens were used for the testing.

Density of cylindrical dense bodies was measured

as apparent density (geometrical weight/volume

measurement). Bars of the polycrystalline

hydroxyapatite of dimension 25 mm × 2.0 mm ×

2.5 mm were cut from cylindrical plates with a

diamond saw. No further chemical treatment was

INDIAN J CHEM, SEC A, NOVEMBER 2008

1628

performed on bars before testing. The flexural tests

were conducted in four-point bending, using a Lloyd

LR10K plus mechanical tester.

Results and Discussion

After refluxing the reaction mixture at 100°C for

ca. 4 h, concentrated sol was formed. Subsequently it

was converted to white gel through in situ solvent

evaporation.

Reaction of hydroxyapatite formation can be

expressed as follows:

5Ca(NO3)2 + 3(NH4)2HPO4 + 4NH4OH

Ca5(PO4)3OH + 10NH4NO3 + 3H2O …(1)

The black dried gel obtained was then subjected to

TG-DTA for thermal characterization. DTA-TG

curves of the gel dried at 350°C shows the first weight

drop of 10% at 100°C due to water evaporation. A

subsequent decrease in weight of ca. 50% occurs until

700°C which is attributed to decomposition and

elimination of ammonia, nitrate, urea, organic

compounds, and carbon dioxide. There are two

exothermic peaks in the curve; the first ranging

from 350 – 420°C, is attributed to decomposition of

ammonia and organic compounds. The second one,

from 420 – 500°C is a large exothermic peak which

may be due to decomposition of urea and carbon

dioxide. Subsequently, calcination of dried gel at

820°C for 2h under flowing air converted it into

hydroxyapatite powder. The yields of the HA

powders were 90-95%.

XRD pattern of crystalline phase of the powder

after the heat treatment at 820°C (Fig. 1) shows that

hydroxyapatite is the main component in the powder

(ca. 75%). Calcium oxide (5%) and β-tricalcium

phosphate (20%) were present as secondary phases.

Generally, the powder mixture obtained at this stage

contained 75-85%, 15-20%, and 4-6% of HA, β-TCP,

and CaO, respectively. At this composition, the

Ca/P molar ratio is about 1.8. Since optimum

stoichiometric Ca/P molar ratio is 1.667, to

compensate for the upward deviation of Ca/P,

diammonium hydrogen phosphate was added to the

suspension of the mixture HA, and heated at 90°C

with rigorous stirring until the solvent was completely

removed.

In the XRD pattern of the pure powder obtained

after this treatment the peaks of β-TCP and CaO

disappear, proving 100% purity of the hydroxyapatite

powder. We also checked the possiblility of the

presence of calcium hydroxide in the powder. A

phenolphtalein test shows that no the hydroxide is

present in the powder.

SEM photograph of the as-prepared HA powder

(Fig. 2) shows that individual hydroxyapatite particles

formed in globular shape with an average size of

~ 50-200 nm in diameter. The nanometric primary

particles agglomerated tightly into micrometric

aggregates of various shape and size. On the other

hand, the particle size distribution of the

hydroxyapatite powder as measured by nanoparticle

sizer shows two separate size distributions; the lower

distribution ranging from ca. 50 – 500 nm may be

attributed to individual particles and the higher

distribution from 2000 – 7000 nm may be attributed

to tightly bonded particle agglomerates. Quite likely it

Fig. 1 — XRD pattern of HA powder mixed with impurities CaO

and β-TCP.

Fig. 2 — SEM photograph of the sol-gel derived HA powder.

SOPYAN et al.: SYNTHESIS OF NANO SIZED HYDROXYAPATITE POWDER USING SOL-GEL TECHNIQUE

1629

is difficult to disperse all agglomerates even after

rigorous stirring for hours. The specific surface area

measured by BET method gave a low value of 7 m2/g.

This value is unusual for particles as fine as in the

present case at nano levels, hence it is considered that

the surface area measured by BET is for agglomerates

and not of the particles. The particle size of the HA

powder obtained in this study is considerably fine, as

confirmed by SEM measurement, in respect of the

HA powder prepared by sol-gel technique. Earlier

studies have reported that sol-gel derived HA powders

have particle size of about 100 nm in diameter8.

Figure 3 shows the IR spectra of the dried gel

(a) and hydroxyapatite powder (b). The dried gel’s

spectrum clearly shows broad peaks, a characteristic

of amorphous products. Bands of carboxylic and

carbonate (1400-1600 cm-1

), phosphate (900-

1100 cm-1

), ammino (1400, 1600 and 3200 cm-1

) and

acetate (2800, 2300 cm-1

) groups were detected.

Obviously, CO32-

and HPO42-

groups are present,

partially substituting groups of PO43-

and/or OH

- in the

HA structure. In Fig. 3(b), FTIR spectrum of the final

powder shows the characteristic peaks corresponding

to OH- (630, 3560 cm

-1) and PO4

3- (960, 1050,

1090 cm-1

) vibrations, together with weak bands of

CO32-

group at 870 and 1540 cm-1

, which indicates

that the sol-gel HA powder is partially carbonated

hydroxyapatite, as commonly observed in synthesis

involving organic reagents.

Dense and porous samples

Dense and porous bodies were prepared to evaluate

the performance of the obtained sol-gel HA powder.

Dense samples were also prepared using commercial

HA powder. Table 1 lists the physical properties of

dense samples prepared using the sol-gel HA and

commercial HA powders after sintering at 1250°C.

The sintered dense bodies for the sol-gel derived HA

powder showed flexural strength of 57.7 MPa and an

apparent density of 2.855 g/cm3 at 90% density (i. e.,

10% porosity). Although the flexural strength is not

so high, it is still in the range of the flexural strength

of human cortical bones. The value is in fact much

better than that obtained using commercial powder

(36.0 MPa), at 89% relative density.

Figure 4 shows XRD pattern of the cylindrical

dense bodies. It is clear that α-TCP appeared at

2θ = 30.7° as the secondary phase after the sintering,

Fig. 3 — FTIR spectra of (a) dried gel HA and (b) sol-gel derived

HA powder.

Table 1 — Physical properties of dense samples prepared using

the sol-gel HA and commercial HA powders.

Type of HA

powder

Sintering

temp. (ºC)

Flexural

strength

(MPa)

Apparent

density

(g/cm3)

Rel.

density

(%)

Sol gel 1250 57.7 2.855 90

Commercial 1250 36.0 2.810 89

Fig. 4 — XRD pattern of the HA dense body after 1250°C

sintering.

INDIAN J CHEM, SEC A, NOVEMBER 2008

1630

although with an acceptable quantity (less than 3%).

The formation of α-TCP itself is unusual since

normally β-TCP will form first at temperatures

<1250°C, then α-TCP and tetracalcium phosphate

appear at higher temperature.14

This phenomenon was

observed in HA powders prepared by precipitation

method. It is deduced that the phenomena was

attributed to the high reactivity of the obtained HA

powder. However, this is an advantage since sintering

process at lower temperatures suffice to obtain

comparable particle densification level as indeed

proven by the much higher mechanical strengths of

the present HA dense bodies.

Figure 5 shows SEM pictures of the fractured

surface of dense samples. Micropores of 1-2 µm in

diameter are present on the surface, which are

responsible for high porosity (10%). In the dense

samples. Besides micropores, a macropore of ca.

20 µm in size was found, representing a less-

compacted region which was probably the origin of

the fracture.

Porous bodies were also prepared by the polymeric

sponge method as reported in literature.15,16

Figure 6

illustrates the resulting macrostructure of the porous

hydroxyapatite. It is clear that the sample has circular

open-cell pores of 500 microns - 2 mm in diameter.

Presumably, the pore structures are similar to that of

the original matrix.

A 44% sintering shrinkage was obtained after

1250°C sintering, with an apparent density of

1.290 g/cm3 and a relative porosity of 59%. On the

other hand, measurement of mechanical properties of

porous bodies provided a compressive strength of

1.96 MPa. It is important to note that conditions for

slurry preparation have not been optimised asyet. In

fact, large cavities were found in several samples as

the result of incomplete slurry penetration into the

pores. The slurry itself had to be ultrasonically treated

to prevent coagulation. So, the compressive strength

obtained, 1.96 MPa, was considered low for such

value of apparent density (1.290 g/cm3). It is also

reasonable to presume that such low compressive

strength is partly attributed to high macroporosity of

the sample.

Figure 7 shows the microporos structure of the wall

of the open pores in which micropores of 1-2 µm in

diameter were observed. Hing et al. reported that they

succceded in preparing porous HA with compressive

stress of 1 – 11 MPa for the increased apparent

density from 0.38 – 1.25 g/cm3.17

Even though it is

compared in respect of porosity, the value of

1.96 MPa is still low if we refer to the value reported Fig. 5 — SEM picture of the fractured surface of dense samples.

Fig. 6 — Macrostructure of the porous hydroxyapatite.

Fig. 7 — Microporosity structure of the wall of the open pores.

SOPYAN et al.: SYNTHESIS OF NANO SIZED HYDROXYAPATITE POWDER USING SOL-GEL TECHNIQUE

1631

by Guicciardi et al18

. (16.1 MPa) for the speciment

of 56% total porosity. The preliminary test on powder

performance shows that the sol-gel powders have

excellent physical properties which may be used to

produce dense and porous bioactive bone implants

with desired properties.

Conclusions Hydroxyapatite (HA) powders with particles

having nano- to submicro-metric size were

succesfully prepared via sol-gel procedure using

calcium nitrate tetrahydrate and diammonium

hydrogen phosphate as the precursors. The primary

particulates have globular shape with an average

diameter of 50-200 nm, as detected by SEM. The

results of the the nanoparticle sizer measurements are

in good agreement with those obtained by

morphological investigation. All the powders

obtained have good purity (nearly 100%).

Dense samples were prepared successfully from the

HA powder using cold pressing technique. Sintering

at 1250°C led to a total porosity of 10%, with

apparent density of 2.855 g/cm3. The mechanical test

of the dense samples provided a higher flexural

strength (57.7 MPa) than that using commercial HA

powder (36.0 MPa), thus showing its suitability for

load bearing bone implant applications.

Porous bodies produced via sponge impregnation

technique using the sol-gel derived HA powder have

apparent density of 1.290 g/cm3, with 59% total

porosity and 44% sintering shrinkage. The porous

bodies contained open macropores of 0.5-2 mm

diameter as the result of removal of the polymeric

matrix as well as micropores of 1-2 µm diameter. The

compressive strength measurement yielded a value of

1.96 MPa, proving that porous HA samples can be

used for human spongy bone substitutes. Thus,

physical characterization of the sol-gel HA powder

followed by the preliminary test on powder

performance in making porous and dense samples

shows that the powder has excellent physical

properties and may be to produce dense and porous

bioactive bone implants with desired properties.

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