Ž .Materials Science and Engineering C 11 2000 35–40

www.elsevier.comrlocatermsec

Porous bioactive calcium carbonate implants processed by starch

consolidation

A.F. Lemos, J.M.F. Ferreira)

Department of Ceramics and Glass Engineering, UniÕersity of AÕeiro, UIMC, 3810-193 AÕeiro, Portugal

Accepted 20 September 1999

Abstract

Ž .Macroporous calcium carbonate CaCO materials with porous structures suitable for implantation purposes were prepared in the3

present work. A new ‘‘direct consolidation’’ technique that uses starch granules as consolidator agent and as pore formers, combined with

other larger organic inclusions, enabled us to tailor the porous microstructure for the intended application. Pore sizes as large as several

hundreds of micrometers could be generated without matrix cracking, due to the high solid loading of the starting suspensions.

The macroporous CaCO bodies fabricated exhibit an accentuated bioactivity even for short soaking time periods. The crystalline3

calcium phosphate phases precipitate preferentially in the pores, pore boundaries, or in other strained sites at the surface of the samples.

q 2000 Elsevier Science S.A. All rights reserved.

Keywords: Starch consolidation; Suspensions; Macroporous bioceramics; Calcium carbonate; Bioactivity

1. Introduction

Ž .Calcium carbonate CaCO is a material widely used3

for many applications in different industrial fields such asw x w xPortland cement 1 , metallurgy, 2 and soda–lime glass-

w xware 3 . CaCO can either be employed as a filler in3

paper and plastic industries and in the control of atmo-w xspheric pollution 2 . Recent applications as a bioceramic

material for implantation purposes have been proposed duew xto its biocompatibility and bioresorbability properties 4–6 .

Marine coral with 99% of CaCO and 1% of organic3

w xmaterials has been used as a bone graft substitute 4–10 .

An ideal bone grafting material should be replaceable by

the host bone. Therefore, the implant needs to be bothw xbiodegradable and osteoconductive 4–7 . The three di-

mensional macro-porous framework of coral mimics natu-

ral cancellous bone and facilitates tissue and vascular

invasion into the pore areas after the implantation of thew xcoral 7 . In fact, experimental and clinical data showed

excellent vascular invasion, biocompatibility and osteocon-

ductivity of the coral when used as a bone graft substitute

)

Corresponding author. Tel.: q351-34-370-242; fax: q351-34-425-

300.Ž .E-mail address: jmf@cv.ua.pt J.M.F. Ferreira .

and demonstrated that coral can represent an interestingw xalternative to bone auto-, allo- or xenografts 4,5 .

w xOhgushi et al. 6 showed that the bone forming re-

sponse of CaCO was comparable to that of the well-known3

bioactive hydroxyapatite; however, some degradation was

also shown. Degradation was dependent on the pore size

range and the total pore volume area. Larger pores enablew xa faster resorption and faster development of new bone 5 .

Bioactive ceramics show surface changes, including

dissolution and precipitation phenomena, that lead to car-

bonate containing apatite formation on their surface. This

apatite layer can be reproduced on their surfaces even inŽ .an acellular simulated body fluid SBF with chemical

Ž . w xcomposition close to human blood plasma Table 1 11 .

Such apatite layer is not formed on the surfaces of non-w xbioactive materials. Ohgushi 7 demonstrated that marine

coral with 99% of CaCO presents rapid carbonated ap-3

atite formation.

Bioactive materials with open porosity and pore diame-

ters greater than ;100 mm can allow bioactive fixation,

with direct attachment by chemical bonding with the bonew x12–14 . There are different techniques to obtain porous

ceramics such as the polymeric sponge method or foamingw xprocesses 15 , but a general problem is how to control the

processing and the ultimate material properties in terms of

pore structure and component dimensions. Some of the

techniques for the manufacture of porous ceramics are

0928-4931r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S0928-4931 00 00134-X

( )A.F. Lemos, J.M.F. FerreirarMaterials Science and Engineering C 11 2000 35–4036

Table 1Ž .Ion concentration mM of SBF and human blood plasma

q q 2q 2q y y 2y 2yNa K Ca Mg Cl HCO HPO SO3 4 4

Plasma 142.0 5.0 2.5 1.5 103.0 2.0 1.0 0.5

SBF 142.0 5.0 2.5 1.5 148.8 4.2 1.0 0.5

closely related to the forming technique, e.g. the use of

polymeric sponge method. Other methods, namely those

using organic additives as pore formers, offer a higher

potential to design porous structures. Recently, a new

‘‘Direct-Consolidation’’ technique, called ‘‘starch consoli-

dation’’, that uses starch granules as pore formers and

consolidator agent has been developed, allowing a good

homogeneity of the consolidated bodies to be obtainedw x16 . This technique is based on the gelling ability of starch

in warm water. In fact, on heating the suspension contain-

ing the ceramic materialqstarch granules up to tempera-

tures in the range 55–808C, the starch granules start to

absorb water from the surroundings, undergoing a rapidw xand irreversible swelling 17 . However, the size of the

pores formed is closely related with the size of the starting

starch granules.

In the present work, the starch granules were used as

consolidator agent and pore formers. However, other larger

organic inclusions were also introduced in order to obtain

macroporous materials with pore diameters greater than

;100 mm to allow bioactive fixation, with direct attach-

ment by chemical bonding with the bone.

2. Experimental procedure

2.1. Materials

The materials used in this study were a natural CaCO3

Ž .Calcitec M1, Mineraria Sacilese, Italy with a relativelyŽ . Žhigh purity 99 wt.% and a very low particle size d s50

. Ž .1.8 mm , a potato starch TRECOMEX AET1 with an

average granule size of about 55 mm and wax spheres with

diameters less than 750 mm.ŽAn ammonium polycarbonate Targon 1128, BK

. w xLadenburg, Germany , which in a previous work 18 was

revealed to be a very efficient stabiliser, was used as

deflocculant to disperse the CaCO suspensions.3

2.2. Slip preparation

A 1000-cm3 volume of CaCO suspension was pre-3

pared with 55% solid loading and stabilised using 0.7

wt.% Targon 1128. Experimentally, the suspension was

prepared by first mixing water and dispersant for 1 min

with a mechanical stirrer. Then the powder was progres-

sively added, and kept under stirring for further 20 min.

Deagglomeration of the CaCO suspension was performed3

by ball milling in a plastic container for 24 h, using silicon

nitride and alumina balls as grinding media. Finally, the

Table 2

Volume fractions of CaCO , Trecomex and wax spheres in each sample3

Sample CaCO Trecomex Wax spheres3

Ž . Ž . Ž .code vol.% vol.% vol.%

550 50 50 0

523 50 20 30

514 50 10 40

532 50 30 20

424 40 20 40

433 40 30 30

balls were removed and the suspension left to homogenise

by rolling in the same plastic container.

To obtain bodies with different porous microstructures,

a variety of samples were prepared with different volume

proportions between CaCO , wax spheres and starch. These3

proportions are presented in Table 2.

Volumes of 15 cm3 were taken from the stock suspen-

sion and the required amounts of starch and wax spheres

were added together with additional water. Based on a

starch density of 1.43 grcm3, a CaCO density of 2.713

grcm3 and a wax sphere density of 0.89 grcm3, the

quantities added for each sample are listed in Table 3.

2.3. Green body preparation

From each suspension reported in Table 3, four cylin-Ž .drical samples diameters22 mm, thicknessf7 mm

were formed by pouring the slip into cylindrical plastic

boxes. The filled moulds were then put in an oven at 808C

for 1 h for consolidation. The moulds were covered to

minimise evaporation of water before and during the con-

solidation preventing segregation phenomena and avoid

uneven drying-shrinkage and subsequent deformations dur-

ing sintering. After consolidation, the coverings were re-

moved and the bodies were demoulded and left to dry for

12 h at 608C. Complete drying was performed at 1208C for

at least 24 h before the burn-out operation.

2.4. Burn-out and sintering

The organic additives were burned out by heating them

up to 5008C. The heating schedule was as follows: 58Crmin

up to 1208C, and then 0.58Crmin from 1208C up to 5008C,

followed by a holding time of 4 h, and free cooling down.

Table 3

Amounts of different components used in sample preparation and total

solid loading of the suspensions

Sample code 550 523 514 532 424 4333Ž .Suspension cm 15 15 15 15 15 15Ž .Trecomex AET 1 g 11.8 4.72 2.36 7.08 5.90 8.85

Ž .Wax spheres g – 4.36 5.85 2.93 7.32 5.49Ž .Additional water g 4.25 3.55 3.06 2.90 4.05 4.29

Total solid loading 60.00 61.57 62.71 63.10 65.63 65.13Ž .vol.%

( )A.F. Lemos, J.M.F. FerreirarMaterials Science and Engineering C 11 2000 35–40 37

During this burn-out operation, air was pumped into the

furnace to promote the oxidation of the organic matter.

The calcined bodies were then sintered at 9008C in a

horizontal tubular furnace, under 1 atm of CO . The2

heating rate was 108C up to 9008C, with 2 h holding time

at this temperature and free cooling down. For tempera-

tures G5008C, a continuous flow of CO along the fur-2

nace of about 40 bubblesrmin was maintained.

2.5. Sintered material eÕaluations

The densities and pore size distributions of sinteredŽbodies were determined by the mercury porosimetry Pore-

.Sizer 9320, Micromeritics, USA . The pore sizes measured

by this technique are not the true sizes of the pores but of

their interconnections.

Microstructural observations of the overall porous struc-

tures left by the starch granules and the wax spheres wereŽperformed on fracture surfaces by using a SEM S-4100,

.HITACHI, Japan .

2.6. Tests of bioactiÕity

The tests of bioactivity were carried out on the 514

samples. Three of these samples were heat-treated at 3008C

in a horizontal tubular furnace to remove all the impurities

eventually present at the surface. After that, each sample

was suspended into a small plastic flask, which was then

filled with the required amounts of SBF solution. The SBF

solution with a pH of 7.34 was previously prepared and

conserved into a refrigerator at about 58C. Considering the

total surface area of the samples, 4 ml of SBF were poured

in the plastic flask for each mm2. During in vitro experi-

ments, the plastic flasks were maintained at 378C and the

SBF renewed every 24 h. The samples were definitely

removed from the SBF after 2, 5 and 10 days, washed

gently with distilled water and put again into the oven at

378C to remove the water.

The surfaces of the tested samples were observed in

SEM to evaluate the deposition of the apatite phases

formed.

3. Results and discussion

3.1. Density and porous microstructures

Although considerable high solid loading suspensions

were used in the present work, the differences in density

Ž . Ž . Ž .Fig. 1. Porous microstructures of the sintered bodies: a segregated region of the sample 514; b homogeneous region of the same sample 514; cŽ .magnification of one of part of b; d general view of the sample 550.

( )A.F. Lemos, J.M.F. FerreirarMaterials Science and Engineering C 11 2000 35–4038

Fig. 2. Size distributions of the interconnection between the pores in theŽ . Ž .samples containing 50 a and 40 vol.% CaCO b .3

among the components would drive the composition to-

wards segregation by sizes andror densities. In fact, the

less dense wax spheres tended to concentrate in the upper

part of the samples. Although this phenomenon would be

undesirable in most cases, it can also be exploited to

design functional gradient ceramics. Otherwise, when un-

desirable, it can be significantly reduced by further increas-

ing the viscosity of the suspension namely by improving

the solid loading. Fig. 1 compares the porous microstruc-Ž . Ž .ture of the samples 514 Fig. 1a–c and 550 Fig. 1d . Fig.

1a shows a segregated region of the sample 514 where the

wax spheres migrate to the top part, while a homogeneous

region of the same sample is displayed in Fig. 1b. It can be

seen that the porous microstructure is composed of smaller

pore sizes corresponding approximately to the average size

of the starch granules, and by larger pore sizes left by the

wax spheres. The pores are interconnected, although the

average diameter of these interconnections is significantly

smaller compared with the average size of the pores. Fig.

1c is a magnification of one of part of Fig. 1b, showing a

region between three larger pores. The comparison be-

tween the micrographs 1c and 1d enable us to conclude

that the starch granules are responsible for the interconnec-

tions among the larger pores and that the extension of the

interconnection depends on the amount of starch added.

Fig. 2a and b reports the pore size distributions of the

sintered samples prepared with 50 and 40 vol.% CaCO ,3

respectively. No significant difference can be detected

among all these samples. This might be partially due to the

segregation phenomenon referred above and the possibility

for the part of the sample used in the porosimetry measure-

ments do not represent the overall composition. This inter-

pretation is also supported by the density and porosity data

reported in Table 4. Moreover, the average pore sizes are

significantly smaller than the pores observed in the micro-

graphs of Fig. 1. This is not surprising since mercury

porosimetry only measures the sizes of the passages be-

tween the pores.

3.2. BioactiÕity

The in vitro experiments revealed that the porous CaCO3

bodies produced in the present work show good bioactiv-

ity. Fig. 3 shows some aspects of the calcium phosphate

phases precipitated at the surface of the 514 samplesŽ .suspended into SBF solutions for 2 days Fig. 3a and b ,

Ž . Ž .for 5 days Fig. 3c and d , and for 10 days Fig. 3e and f .

The crystalline phases shown in micrographs 3a, 3c, and

3e have a crystal habit characteristic of a di-calcium

phosphate di-hydrate, CaHPO P2H O, known as brusite4 2

w x w x19 . It can be formed according to the reaction 19 :

Ca2qqHPO2yq2H OzCaHPO P2H O,4 2 4 2

Kss10y25.2 , at 258C 1Ž . Ž .

The kinetics of this reaction will be higher at the testingŽ .temperature 378C .

Besides brusite, other acicular calcium phosphate phases

appear in the samples soaked in the SBF for all different

time periods. These acicular phases seem to come prefer-Ž .entially from the interior of the pores Fig. 3b . However,

the preference for the pores and pore borders or other more

strained points at the surface of the samples seem to be the

privileged sites for the formation of the calcium phosphateŽ .phases Fig. 3d and f . This can be understood since the

most fine and strained points will dissolve more easily into

the SBF, supersaturating the solution in the close vicinity

in the ionic active species, promoting the formation of the

new phases. It should be noticed that the amount of the

calcium phosphate phases precipitated increase with the

Table 4

Density and porosity data of the green and sintered bodies

Sample code 514 523 424 4333Ž .Green density grcm 1.29 1.19 1.20 1.22

3Ž .Sintered density grcm 1.50 1.31 1.39 1.28Ž .Average pore size mm 12.12 13.42 13.89 12.73

Ž .Porosity % 42.24 48.33 45.61 47.30

( )A.F. Lemos, J.M.F. FerreirarMaterials Science and Engineering C 11 2000 35–40 39

Ž .Fig. 3. Some aspects of the calcium phosphate phases precipitated at the surface of the 514 samples suspended into SBF solutions for: 2 days a and b ; 5Ž . Ž .days c and d ; 10 days e and f .

soaking time. This can be clearly seen from the compari-

son between micrographs 3a, 3c, and 3e.

4. Conclusions

The results presented in this work enable us to draw the

following conclusions:

1. The starch consolidation technique was revealed to be

an interesting method to fabricate porous bioceramic

materials, enabling the transformation of a fluid suspen-

sion into a rigid body without liquid removal.

2. A combination of the gelling properties of the starch

granules to consolidate the suspensions and to generate

interconnecting pores with other organic inclusions of

larger size, to generate the required macropores for

bone ingrowth, offer great potentialities to design porous

microstructures for implantation purposes.

3. The macroporous CaCO bodies fabricated exhibit an3

accentuated bioactivity even for short soaking time

periods.

( )A.F. Lemos, J.M.F. FerreirarMaterials Science and Engineering C 11 2000 35–4040

Acknowledgements

ŽThe authors acknowledge FCT Portuguese Foundation.for Science and Technology for the financial support.

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