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

Porous bioactive calcium carbonate implants processed by starchconsolidation

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 withother larger organic inclusions, enabled us to tailor the porous microstructure for the intended application. Pore sizes as large as severalhundreds 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 organic3w xmaterials has been used as a bone graft substitute 4–10 .

An ideal bone grafting material should be replaceable bythe host bone. Therefore, the implant needs to be both

w xbiodegradable and osteoconductive 4–7 . The three di-mensional macro-porous framework of coral mimics natu-ral cancellous bone and facilitates tissue and vascularinvasion into the pore areas after the implantation of the

w xcoral 7 . In fact, experimental and clinical data showedexcellent 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 wasalso shown. Degradation was dependent on the pore sizerange and the total pore volume area. Larger pores enable

w 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. Thisapatite layer can be reproduced on their surfaces even in

Ž .an acellular simulated body fluid SBF with chemicalŽ . 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 porousceramics such as the polymeric sponge method or foaming

w xprocesses 15 , but a general problem is how to control theprocessing and the ultimate material properties in terms ofpore structure and component dimensions. Some of thetechniques 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 0.5SBF 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 ofpolymeric sponge method. Other methods, namely thoseusing organic additives as pore formers, offer a higherpotential to design porous structures. Recently, a new‘‘Direct-Consolidation’’ technique, called ‘‘starch consoli-dation’’, that uses starch granules as pore formers andconsolidator agent has been developed, allowing a goodhomogeneity of the consolidated bodies to be obtainedw x16 . This technique is based on the gelling ability of starchin 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 toabsorb water from the surroundings, undergoing a rapid

w xand irreversible swelling 17 . However, the size of thepores formed is closely related with the size of the startingstarch granules.

In the present work, the starch granules were used asconsolidator agent and pore formers. However, other largerorganic inclusions were also introduced in order to obtainmacroporous 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 CaCO3Ž .Calcitec M1, Mineraria Sacilese, Italy with a relatively

Ž . 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 withdiameters 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 asdeflocculant 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 0.7wt.% Targon 1128. Experimentally, the suspension wasprepared by first mixing water and dispersant for 1 minwith 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 siliconnitride and alumina balls as grinding media. Finally, the

Table 2Volume fractions of CaCO , Trecomex and wax spheres in each sample3

Sample CaCO Trecomex Wax spheres3Ž . Ž . Ž .code vol.% vol.% vol.%

550 50 50 0523 50 20 30514 50 10 40532 50 30 20424 40 20 40433 40 30 30

balls were removed and the suspension left to homogeniseby rolling in the same plastic container.

To obtain bodies with different porous microstructures,a variety of samples were prepared with different volumeproportions 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 sphereswere added together with additional water. Based on astarch density of 1.43 grcm3, a CaCO density of 2.713

grcm3 and a wax sphere density of 0.89 grcm3, thequantities 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 plasticboxes. The filled moulds were then put in an oven at 808Cfor 1 h for consolidation. The moulds were covered tominimise evaporation of water before and during the con-solidation preventing segregation phenomena and avoiduneven drying-shrinkage and subsequent deformations dur-ing sintering. After consolidation, the coverings were re-moved and the bodies were demoulded and left to dry for12 h at 608C. Complete drying was performed at 1208C forat least 24 h before the burn-out operation.

2.4. Burn-out and sintering

The organic additives were burned out by heating themup to 5008C. The heating schedule was as follows: 58Crminup to 1208C, and then 0.58Crmin from 1208C up to 5008C,followed by a holding time of 4 h, and free cooling down.

Table 3Amounts of different components used in sample preparation and totalsolid 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 thefurnace to promote the oxidation of the organic matter.

The calcined bodies were then sintered at 9008C in ahorizontal tubular furnace, under 1 atm of CO . The2

heating rate was 108C up to 9008C, with 2 h holding timeat 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 measuredby this technique are not the true sizes of the pores but oftheir interconnections.

Microstructural observations of the overall porous struc-tures left by the starch granules and the wax spheres were

Žperformed 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 514samples. Three of these samples were heat-treated at 3008C

in a horizontal tubular furnace to remove all the impuritieseventually present at the surface. After that, each samplewas suspended into a small plastic flask, which was thenfilled with the required amounts of SBF solution. The SBFsolution with a pH of 7.34 was previously prepared andconserved into a refrigerator at about 58C. Considering thetotal surface area of the samples, 4 ml of SBF were pouredin the plastic flask for each mm2. During in vitro experi-ments, the plastic flasks were maintained at 378C and theSBF renewed every 24 h. The samples were definitelyremoved from the SBF after 2, 5 and 10 days, washedgently with distilled water and put again into the oven at378C to remove the water.

The surfaces of the tested samples were observed inSEM to evaluate the deposition of the apatite phasesformed.

3. Results and discussion

3.1. Density and porous microstructures

Although considerable high solid loading suspensionswere 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, theless dense wax spheres tended to concentrate in the upperpart of the samples. Although this phenomenon would beundesirable in most cases, it can also be exploited todesign functional gradient ceramics. Otherwise, when un-desirable, it can be significantly reduced by further increas-ing the viscosity of the suspension namely by improvingthe 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 thewax spheres migrate to the top part, while a homogeneousregion of the same sample is displayed in Fig. 1b. It can beseen that the porous microstructure is composed of smallerpore sizes corresponding approximately to the average sizeof the starch granules, and by larger pore sizes left by thewax spheres. The pores are interconnected, although theaverage diameter of these interconnections is significantlysmaller compared with the average size of the pores. Fig.1c is a magnification of one of part of Fig. 1b, showing aregion between three larger pores. The comparison be-tween the micrographs 1c and 1d enable us to concludethat the starch granules are responsible for the interconnec-tions among the larger pores and that the extension of theinterconnection depends on the amount of starch added.

Fig. 2a and b reports the pore size distributions of thesintered samples prepared with 50 and 40 vol.% CaCO ,3

respectively. No significant difference can be detectedamong all these samples. This might be partially due to thesegregation phenomenon referred above and the possibilityfor 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 datareported in Table 4. Moreover, the average pore sizes aresignificantly smaller than the pores observed in the micro-graphs of Fig. 1. This is not surprising since mercuryporosimetry 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 phosphatephases precipitated at the surface of the 514 samples

Ž .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, and3e have a crystal habit characteristic of a di-calciumphosphate di-hydrate, CaHPO P2H O, known as brusite4 2w 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 phasesappear in the samples soaked in the SBF for all differenttime 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 morestrained points at the surface of the samples seem to be theprivileged sites for the formation of the calcium phosphate

Ž .phases Fig. 3d and f . This can be understood since themost fine and strained points will dissolve more easily intothe SBF, supersaturating the solution in the close vicinityin the ionic active species, promoting the formation of thenew phases. It should be noticed that the amount of thecalcium phosphate phases precipitated increase with the

Table 4Density 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 thefollowing conclusions:

1. The starch consolidation technique was revealed to bean 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 starchgranules to consolidate the suspensions and to generateinterconnecting pores with other organic inclusions oflarger size, to generate the required macropores forbone ingrowth, offer great potentialities to design porousmicrostructures for implantation purposes.

3. The macroporous CaCO bodies fabricated exhibit an3

accentuated bioactivity even for short soaking timeperiods.

( )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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