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Short communication

Porosities and pore sizes in coralline calcium carbonate

Jorge Laine⁎, Mary Labady, Alberto Albornoz, Simon Yunes1

Laboratorio de Fisicoquímica de Superficie, Centro de Química, Instituto Venezolano de Investigaciones Científicas, Apt 20632,Caracas 1020-A, Venezuela

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +58 212 504 1777E-mail addresses: jlaine@ivic.ve (J. Laine), y

1 Present address: Yuma Consulting, Carac

1044-5803/$ – see front matter © 2007 Elsevidoi:10.1016/j.matchar.2007.12.002

A B S T R A C T

Article history:Received 12 September 2007Received in revised form30 November 2007Accepted 11 December 2007

Coral is a material that recently has gained increased attention as a potential bone graftsubstitute material. The porosity and pore size distribution of the exoskeleton of eightdifferent coral species were investigated by mercury intrusion and microscopy. Aclassification was established comprising two groups according to porosity: L-type, havinglow porosity (b20 vol.%), and H-type, having high porosity (N20 vol.%). According toliterature, this value of 20 vol.% seems to be a lower porosity limit for successful surgicalapplications as bone graft substitution material. Pore size distributions are well-defined inthree H-type species, each one having a different order of magnitude for the median porediameter: Porites (order 2), Millepora (order 1), and Manicina (order 0). Tubular and slit poregeometries were suggested after microscopy.

© 2007 Elsevier Inc. All rights reserved.

Keywords:CoralsCalcium carbonatePorosityPore sizeBone graft substitute

1. Introduction

Coralline calciumcarbonate is constituted by the exoskeleton ofmarine living organisms that form several colony rigid masseswhose common names are referred to by divers according tovisual shape: fire corals, brain corals, etc. Despite the geologicalimportance of corals as reef-building organisms, the processesinvolved in shaping its skeletal structure are not yet clearlyunderstood.

The porous structure of these exoskeletons has receivedattention respect to possible potential application, in particu-lar as bone graft substitution for surgery [1–3]. After submittedto rigorous preparation and purification, the natural coralimplanted into bony tissue is gradually resorbed and replaced

; fax: +58 212 504 1350.unessimon@cantv.net (Sas 1073, Venezuela.

er Inc. All rights reserved

by the newly formed bone. Hydroxyapatite produced fromcorals has also been used for implants. In this case, the overallporous structure of the coral is maintained during conversionto hydroxyapatite [4,5]. Resorption in implants appears to bedirectly related to the coral porosity, therefore,most promisingcorals for this application appear to be limited to high porositycorals [6,7]. However, the possible medical significance of thedimension and shape of the coral pores has not been clearlyestablished. For these purposes, the identification of highporosity corals with different pore sizes and shapes would benecessary.

Within the above scope, the objective of this work is tocontribute to the understanding of the complex architecture ofcorals, by means of the study of the pore structure of various

. Yunes).

.

Table 1 – Sample identification

Sample # Common name Taxonomic name

1 Branching fire Millepora alcicornis2 Blade fire Millepora coplanata3 Stag horn Acropora cerviconis4 Fingers Porites porites5 Lettuce Agaricia agaricites6 Elk horn Acropora palmata7 Brain Diploria strigosa8 Rose Manicina areolata

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different coral species, in order to establish a preliminaryclassification of their porosity, pore size, and pore shape.

Fig. 2 –Relationship between porosity and total mercuryintrusion volume.

2. Materials and Methods

2.1. Sample Origin and Identification

Samples of eight different corals were taken in shallowwaters(b10 m deep) near the Los Roques atoll islands (Venezuela). Ineach case, the sample material was acquired by breaking off abranch of the colony. Care was taken to select samples fromabundant colony formations. In order to clean the corals of theorganisms, they were treated with a solution of 5% calciumhypochlorite for several days. Afterwards the samples werewashed, dried and then crushed using a hammer with stackedpaper. Sievingwas carried out in a vibratory apparatus and thefraction between 16 and 60mesh standard testing sieve (about0.3–1.2 mm particle size) was chosen for further analysis.

Fig. 1 –Micrographs by optical (left) and s

Common and taxonomic names of the coral species sampledare listed in Table 1. It should be remarked that in some casesa common name refers to various different species.

2.2. Analytical Methods

The pore structure of cleaned but otherwise coral sampleswasstudied in a Zeiss stereomicroscope type IV, and in a scanning-electron microscope (Philips SEM 500). For the electronmicroscopy, sample materials were sputter coated with Au,then put under the instrument operated at high vacuum(10−7 Torr), with 5 kV acceleration voltage.

canning-electron (right) microscopy.

Fig. 3 –Pore size distributions.

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Samples were submitted to mercury (Hg) intrusion by pres-surization employing a Micromeritics model Autopore III appa-ratus, using pressures between about 7×103 Pa and 414×106 Pa,corresponding to pore diameters between about 200 µm and3×10−3 µm. After evacuating the sample (about 1 g) undervacuum at room temperature, the Hg pressure was increasedstepwise following an automatic programmed procedure. Datameasured by the apparatus: pressure vs. cumulative intrusionvolume of Hg, was processed by its software to obtain the valuesof bulk density, skeletal density, porosity,median pore diameter,and pore size distribution of each sample, according to dataassessmentdetailedelsewhere [8]. Theabsenceofmicroporosity(i.e., pores b50 nm width) was confirmed in all coral species(using 1 g samples pre-treated at 300 °C under vacuum), afterobserving negligible N2 adsorption at 77 K, employing a Micro-meritics ASAP 2000 apparatus.

3. Results

Fig. 1 shows typical poremorphologies observed bymicroscopy.The micrographs reveal two main pore shapes: tubes, forminghoneycomb-like structures in samples 4 and 2; and slits,forming lettuce-like structure in sample 8. Similar structuresas those shown in Fig. 1 have been reported elsewhere: e.g.,tubular shape in Porites [5], Acropora [9], and Goniopora [4]; andslit shape in Fungia [10].

Fig. 2 confirms a linear correlation between the porosities ofthe coral samples and the total Hg intrusion volume filled after

Table 2 – Data from mercury intrusion and classification accord

Sample # Porosity (vol.%) Median pore diameter (µm) Bulk d

1 25.2 322 46.5 343 13.2 304 49.0 1255 12.4 316 21.8 607 9.5 18 33.9 2

a H:N20 vol.% porosity, L:b20 vol.% porosity; subscript indicate pore diam

pressurizing at the maximum pressure. Fig. 3 shows poresize distributions of the three samples that had the highestporosities: sample 4 (49.0 vol.%), sample 2 (46.5 vol.%), andsample 8 (33.9 vol.%). Data obtained from the Hg intrusion runsare summarized in Table 2. Samples were arbitrarily classifiedas low porosity when porosity is smaller than 20 vol.%, and ashighporosity forporosity larger than20vol.%,hereonreferred toas “L-type” and “H-type” porosities or species respectively.Notice that the calculated median pore diameters indicated inTable 2 are in good agreement with the peak values shown inFig. 3 and with the visual measures of the micrographs (Fig. 1).

4. Discussion

Calcium carbonate in corals is known to form the aragonitecrystalline structure (density: 2.93 g/mL). However, the deposi-tion of CaCO3 in corals as aragonite may depend on a numberof factors, among which temperature, salinity and presence ofcertain trace elements. Deviations of the skeletal densitiesshown in Table 2may be also consequence of several facts: forexample, the possible presence of non connected voidvolumes, and/or the possible particle compression andrupture as a result of the high Hg pressure. In addition, thepresence calcite, a less dense (2.71 g/mL) and more thermo-dynamically stable form of CaCO3, may also be possible,particularly in the oldest parts of the exoskeleton.

According to literature reviewed, the value chosen of 20 vol.% to distinguish between L-type and H-type porosities, seemsto be a lower porosity limit for successful surgical applicationas bone graft substitution material. Among the eight samplematerials investigated presently, two species with H-typeporosity have been cited previously as bone implants, theseare Porites genera [1–3,6] and Acropora palmate [9], reportingporosities around 45–50 vol.% and 20–30 vol.% respectively,values that are in good agreement with those obtained inpresent work (samples 4 and 6 respectively). Another AcroporaL-type species with 12 vol.% porosity has been experimentedwith as bone graft substitute (probably similar to sample 3),resulting in a low resorption rate as compared with a Poritesspecies [1]. Others H-type coral genera not included in presentwork has been studied as bone graft substitutes, e.g., Gonio-pora, Pocillopora, Montastrea, and Dichocoenia[3,4,6,9]. Conse-quently, the others H-type genera reported in Table 1 couldalso be promissory ones for bone implants due to their highporosity: i.e., samples 1, 2, and 8.

ing to porosity and pore diameter

ensity (g/mL) Skeletal density (g/mL) Classification typea

1.89 2.53 H1

1.44 2.69 H1

2.43 2.80 L11.48 2.89 H2

2.59 2.96 L12.18 2.79 H1

2.53 2.79 L01.80 2.73 H0

eter magnitude: 0: 1–9 µm, 1: 10–99 µm, 2: 100–999 µm.

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Sharp peaks in pore size distributions (Fig. 3) correspond tothree values of diameter having three different orders ofmagnitude: i.e., 1.25×102 (sample 4), 3.4×101 (sample 2), and2.5×100 (sample 8); accordingly, orders are 2, 1, and 0 respectively.

Finally, a classification is established including the porositytype (H or L) together with a subscript indicating the porediameter magnitude, as shown in Table 2.

5. Conclusions

The present work has revealed a variety of calcium carbonateporous structures in coral species. Twomain pore shapeswerefound by microscopy, notably tubular and slit. Mercury intru-sion analysis of samples from eight different coral speciesreveal porosity values ranging from about 10 vol.% (L-type) upto about 50 vol.% (H-type). The value of 20 vol.% seems to be alower porosity limit for successful application as bone graftmaterial. Furthermore, the corals present three types of poresdimensions (2, 1, and 0 order). Pores of orders 2 and 1 hadtubular shape, and pores of order 0 had slit shape.

Acknowledgement

To the Fundaciόn Científica Los Roques for their support.

R E F E R E N C E S

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[6] Roudier M, Bouchon C, Rouvillain JL, Amedee J, Rouais F,Fricain JC, et al. The resorption of bone-implanted coralsvaries with porosity but also with the host reaction. J BiomedMater Res 1995;29:909–15.

[7] Barbotteau Y, Irigaray JL, Mathiot JF. Modeling by percolationtheory of the behavior of natural coral used as bonesubstitute. Phys Med Biol 2003;48:3611–23.

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[10] Abramovitch-Gottlib L, Dahan D, Golan Y, Vago R. Effect oflight regimes on the microstructure of reef-building coralFungia simplex. Mater Sci Eng C 2005;25:81–5.