a qualitative description of preferred orientation in porous carbonate matrices of marine origin
TRANSCRIPT
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Materials Science and Engineering C 23 (2003) 593–595
A qualitative description of preferred orientation in porous carbonate
matrices of marine origin
Yuval Golana,*, David Dahana, Razi Vagob
aDepartment of Materials Engineering, Ben-Gurion University of the Negev, Beersheba 84105, IsraelbThe Institute for Applied Biosciences and the Department of Biotechnology Engineering, Ben-Gurion University of the Negev, Beersheba 84105, Israel
Accepted 26 June 2003
Abstract
Porous aragonite matrices of marine origin exhibit a prominent preferred orientation in which the (221) crystal planes are aligned
perpendicular to the principal growth direction of the organism. Since the aragonite crystallites that compose the matrix appear to be
organized perpendicular to the spherical pore surfaces, these apparently conflicting findings can be explained by a bimodal distribution of the
crystallites into pore and bulk crystallite populations. Analysis of X-ray diffraction data obtained from matrices taken from eight different
organisms was carried out. The validity of the bimodal distribution model was confirmed by correlation with porosity data.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Microstructure; Porosity; X-ray diffraction; Aragonite
Recently, there has been considerable interest in porous
carbonate matrices of marine origin for biomedical applica-
tions such as bone replacement [1]. These materials, which
are biofabricated by marine organisms such as corals, were
reported to be highly biocompatible under in-vitro and in-
vivo conditions and are easily recognized and colonized by
various cells and tissues [2,3]. One of the main reasons for
the biocompatibility is the morphological similarity of the
porous structure of the biomatrices to the porous structure of
bone [4]. We have conducted comprehensive microstruc-
tural studies of a variety of coral skeletons and found similar
microstructural guidelines: (i) All matrices were crystalline
according to X-ray diffraction measurements (XRD) and
showed the aragonite structure. (ii) The crystals were found
to be organized perpendicular to the matrix surface, and
were thus viewed edge-on when imaged by scanning
electron microscopy (SEM) as seen in Fig. 1. While the
full details of these studies will be published elsewhere, it is
important to portray another common feature in these
matrices, which is the prominent preferred crystallographic
orientation in which the (221) planes are preferentially
aligned perpendicular to the principal growth axis of the
0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0928-4931(03)00054-7
* Corresponding author. Tel.: +972-7-6461474; fax: +972-7-6472944.
E-mail address: [email protected] (Y. Golan).
organism. Thus, while the (111) reflection is the strongest
reflection in the aragonite powder diffraction file [5], X-ray
diffraction spectra taken from cross-sections (samples cut
perpendicular to the principal growth axis) of matrices
obtained from eight different organisms showed that the
(221) reflections were clearly stronger than the (111) reflec-
tions. This preferred orientation was initially surprising
since if all crystallites are organized perpendicular to the
spherical pore surfaces (as seen in Fig. 1), this should result
in the averaging of all crystal orientations and subsequent
loss of preferred orientation in the X-ray diffractograms. We
hypothesized that this discrepancy can be explained by a
bimodal distribution of the crystallites into bulk and surface
crystallites. The oriented bulk crystallites facilitate the
growth of the organism along the principal growth axis,
while at the same time, exposure of well-defined aragonite
c-planes at the pore surface can be advantageous for surface
recognition and adhesion of cells [6]. If proven true, this can
be another manifestation of nature’s remarkable ability to
construct ‘‘smart’’ complex materials.
Therefore, in this work, we have tested the assumption
that the crystallites in these matrices are composed of two
major populations. A population fraction of crystallites that
are organized normal to the pore surface, Nav, and hence
with averaged orientations, and a second population fraction
of crystallites, Nb, which are in the bulk of the matrix and
Fig. 1. Scanning electron micrographs of a porous carbonate matrix
obtained from the organism P. lutea. (a) Low magnification, showing the
porous structure of the matrix. (b) Higher magnification, showing the
organization of the aragonite crystallites normal to the pore surface.
Table 1
True porosity and anisotropy parameter (a) values calculated from XRD for
cross-sections obtained from the eight marine organisms studied in this
work
Species Direction a True porosity (%)
P. lutea cross-section 0.57 60.7
P. digitata cross-section 0.47 57.4
M. dichotoma cross-section 0.54 54.3
Acropora sp. cross-section 0.58 51.5
S. pistillata cross-section 0.59 47.2
A. palifera cross-section 0.58 44.7
T. reniformis cross-section 0.64 41.2
F. simplex septal 0.75 27.5
Fig. 2. (a) Photograph showing the multiseptal structure of F. simplex. (b)
Scanning electron micrograph showing the organization of the crystallites
parallel to the septal surface.
Y. Golan et al. / Materials Science and Engineering C 23 (2003) 593–595594
oriented with respect to the principal growth axis of the
organism. We assume that the bimodal distribution accounts
for the entire population of the samples, so that Nav +Nb = 1.
Since the preferred orientation of the crystallites is man-
ifested with respect to the (111) reflection that is the
strongest reflection in the aragonite crystal structure [5], it
is thus reasonable to assume that the (111) reflections are
effected only by the pore population, Nav, and therefore the
contribution of Nb to the measured X-ray diffraction inten-
sity of the (111) reflections can be neglected. Hence, we can
assume that the intensity of the (111) reflections, Imeas(111),
corresponds solely to Nav. On the other hand, the (221)
reflection is effected by crystallites from both the Nav and
the Nb populations. This can be used in order to separate
the diffraction intensities for each of the two populations in
the total intensity measured for the (221) reflection,
Imeas(221). The contribution of the intensity corresponding
to the pore crystallites in the (221) reflection, Iav(221), can
be given by normalization of the measured (111) intensity,
Imeas(111), according to the theoretical ratio expected for
these two reflections in the corresponding powder diffrac-
tion file (pdf) [5]:
Iavð221Þ ¼ Imeasð111ÞIpdf ð221ÞIpdf ð111Þ
ð1Þ
We can now subtract the contribution of the intensity
obtained from the pore crystallites from the total measured
Fig. 3. Anisotropy parameter values calculated from XRD data of cross-
sections (Table 1) plotted vs. the true porosity of the matrices.
Y. Golan et al. / Materials Science and Engineering C 23 (2003) 593–595 595
intensity of the (221) reflection to obtain the intensity
contribution of the bulk crystallites:
Ibð221Þ ¼ Imeasð221Þ � Iavð221Þ ð2Þ
Now we define the anisotropy parameter, a, which
represents the fraction of the ordered crystallites out of the
total population:
a ¼ Nb ¼Ibð221Þ
Imeasð221Þð3Þ
The analysis outlined above was carried out for cross-
sections of carbonate matrix samples obtained from eight
different organisms, and the results are presented in Table 1.
All spectra were obtained in the h/2h geometry using CuKa
radiation. The APD X-ray data analysis package was used
for all analyses. Prior to peak integration, the background
was subtracted for each spectrum. Note that cross-sections
were measured for all samples except for Fungia simplex, a
solitary coral with a multi-septal structure (see photograph
in Fig. 2a), since in this particular matrix, the crystallites are
organized parallel to the septal surface as shown in the
SEM image in Fig. 2b [7]. As expected, the (221) preferred
orientation in this particular matrix is observed in the septal
section rather than the cross-section, and therefore the
septal section was measured and analyzed. Note that from
Fig. 2b it is strongly suggested that the porosity of the F.
simplex matrix is expected to be relatively small (compare
with Fig. 1a).
The results of this simple model were tested by correlat-
ing the X-ray data with true porosity measurements carried
out using the Archimedes method.1 Table 1 shows the
anisotropy parameter (a) values that were calculated accord-
ing to the analysis detailed above, and the results are plotted
vs. the porosity data and showed in Fig. 3. A linear
correlation (R2) of 90% was obtained, which validated the
bimodal distribution of the crystallites in the eight matrices
studied in this work. This degree of linear correlation is
clearly significant keeping in mind the variability in bio-
logical systems, and that a sharp interface between the two
populations is rather unlikely. Moreover, note that signifi-
cant deviation from linearity is obtained only for the two
most porous matrices, Porites lutea and Psammocora
digitata. Due to the spherical geometry of these two coral
colonies, the preparation of well-aligned cross-sections
becomes much harder and the error in the alignment of
the specimen with respect to the principal growth axis
becomes consequently larger. Thus, as the matrix becomes
more porous, the anisotropy parameter decreases due to an
increased contribution of the averaged pore crystals to the
diffracted intensity of the (221) reflection. It is important to
1 For details on true porosity measurement using the Archimedes
method see, e.g., Ref. [8].
note that no simple quantitative relationship exists between
X-ray profile intensity ratios and the population fractions of
oriented crystals. For a fully quantitative analysis, a more
rigorous treatment must be carried out by using, e.g., the
March analysis [9,10].
In summary, we have investigated the prominent pre-
ferred orientation in which the (221) planes are aligned
perpendicular to the principal growth direction of the organ-
ism. This finding was initially surprising since the orienta-
tion of the crystallites is expected to be averaged as they were
seen to be aligned normal to the spherical pore surface. A
bimodal distribution of the crystallites into pore (isotropic)
and bulk (aligned) populations was confirmed by correlating
the results of the XRD analysis with porosity data. We note
that this approach is not limited to carbonate matrix bio-
materials, and can be generally applied for the study of other
cases of preferred orientation in complex microstructures.
Acknowledgements
We are grateful to Youli Li from the Materials Research
Laboratory, UC Santa Barbara for inspiring discussions.
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