Geological Aragonite Research Articles

Aragonite-Polylysine: Neuro-Regenerative Scaffolds with Diverse Effects on Astrogliosis.

Biomaterials, especially when coated with adhesive polymers, are a key tool for restorative medicine, being biocompatible and supportive for cell adherence, growth, and function. Aragonite skeletons of corals are biomaterials that support survival and growth of a range of cell types, including neurons and glia. However, it is not known if this scaffold affects neural cell migration or elongation of neuronal and astrocytic processes, prerequisites for initiating repair of damage in the nervous system. To address this, hippocampal cells were aggregated into neurospheres and cultivated on aragonite skeleton of the coral Trachyphyllia geoffroyi (Coral Skeleton (CS)), on naturally occurring aragonite (Geological Aragonite (GA)), and on glass, all pre-coated with the oligomer poly-D-lysine (PDL). The two aragonite matrices promoted equally strong cell migration (4.8 and 4.3-fold above glass-PDL, respectively) and axonal sprouting (1.96 and 1.95-fold above glass-PDL, respectively). However, CS-PDL had a stronger effect than GA-PDL on the promotion of astrocytic processes elongation (1.7 vs. 1.2-fold above glass-PDL, respectively) and expression of the glial fibrillary acidic protein (3.8 vs. and 1.8-fold above glass-PDL, respectively). These differences are likely to emerge from a reaction of astrocytes to the degree of roughness of the surface of the scaffold, which is higher on CS than on GA. Hence, CS-PDL and GA-PDL are scaffolds of strong capacity to derive neural cell movements and growth required for regeneration, while controlling the extent of astrocytic involvement. As such, implants of PDL-aragonites have significant potential as tools for damage repair and the reduction of scar formation in the brain following trauma or disease.

Mechanical behavior of mother-of-pearl and pearl with flat and spherical laminations

Laminated structure reduces the common inverse relationship of strength and toughness in many biological materials. Here the mechanical behavior of pearl and nacre with spherical and flat laminations was investigated and compared with the geological aragonite counterpart. The biological ceramics demonstrate higher strength, better reliability, and improved damage resistance owing to their laminated arrangement. Kinking and delamination occur in pearl to resist damage in addition to the crack-tip shielding mechanisms as in nacre, such as crack deflection, bridging, and platelet pull-out. The fracture mechanisms were interpreted in terms of the stress state using finite element simulation. This study may help clarify the compressive mechanics of laminated sphere between platens and advance the understanding on the mechanical behavior of biological and bio-inspired laminated materials.

Atomic structure and ultrastructure of the Murex troscheli shell

The structure of the gastropod Murex troscheli shell was investigated by using high-resolution scanning and transmission electron microscopies and high-resolution synchrotron X-ray powder diffraction. Rietveld refinement of X-ray diffraction patterns revealed anisotropic distortions of the orthorhombic aragonite unit cell, reaching a maximum of about 0.2% (along the c-direction) with respect to geological aragonite. Lattice distortions are effectively relieved under mild annealing at temperatures between 150 °C and 250 °C. This relaxation correlated well with the loss of organic substance during annealing, measured by thermal gravimetric analysis. The shell ultrastructure consists of three orders of hierarchy, which is typical for crossed-lamellar layers. The third-order fiber-like lamellae, nearly 50 × 250 nm² in cross-section, are heavily twinned, producing specific contrast features in transmission electron microscopy micrographs. So, higher orders of hierarchy are, in fact, twins, which boundaries are free of organic matter.

Crystallographic relationships in the crossed lamellar microstructure of the shell of the gastropod Conus marmoreus

The crossed lamellar microstructure of mollusk shells shows a very complex hierarchical architecture constituted of long rod-shaped aragonite crystals stacked parallel to each other inside each first order lamella, which are almost perpendicular to the ones contained in parallel neighboring lamellae. To better understand the construction and properties of the crossed lamellar microstructure we have performed a detailed study to determine the crystallographic characteristics and their evolution during shell growth using scanning electron microscopy, transmission electron microscopy and X-ray diffraction texture analysis. The arrangement of crystals is rationalized by a set of twin law relationships between aragonite crystals. Specifically, the aragonite rods, or third order lamellae within each first order lamella, internally consist of polysynthetic twins bounded by {1 1 0} mirror planes. In turn, the polysynthetically twinned aragonite crystals also show a constant crystallographic orientation with respect to aragonite crystals in adjacent first order lamellae. It can be seen as another twin law in which crystals from adjacent lamellae are bounded by (1 1 0) planes but with their c-axes rotated within this plane by 30°. Thus there are two sets of twin laws that relate crystal units at lower (third order lamellae) and higher (first order lamellae) length scales. These hierarchical relationships play a crucial role in the construction, organization and properties of this complex microstructure. The later orientational relationships have never been described in geological aragonite and are only found in biogenic materials with a crossed lamellar microstructure. Their occurrence is probably determined by the presence of shell organic components which regulate crystal growth and may favor unusual crystallographic relationships.

Internal pressure in annealed biogenic aragonite

The thermal expansion of biomineral aragonite (CaCO3) was studied by in-situsynchrotron radiation (SR) diffraction in both pulverized and non-pulverized samples. The thermal expansion of biogenic aragonite depends on the sample form. Pulverized biomineral aragonite samples show the same thermal expansion as the reference geological aragonite. Biomineral aragonite samples studied en-bloc (non-pulverized) show smaller thermal expansion coefficients as compared with the reference geological aragonite. The observed difference is attributed to the pressure exerted by the heated organic inclusions trapped in the lattice of the biomineral samples studied en-bloc. The estimated pressure values are of the order of 200–350 MPa at temperatures ranging from 400 to 500 °C. The pressure values are different for the main crystallographic directions. The structural phase transition from aragonite to calcite is observed at different temperatures for pulverized biomineral 280 °C, biomineral en-bloc 360 °C and geological 450 °C aragonite samples.

Structure of Biogenic Aragonite (CaCO3)

By using high-resolution X-ray powder diffraction at a dedicated synchrotron beam line, ID-31, of the European Synchrotron Radiation Facility (ESRF, Grenoble, France), we studied structural and microstructural modifications in biogenic aragonite crystals, obtained from mollusk shells, and subjected to heat treatments at elevated temperatures. All investigated shells revealed anisotropic lattice distortions of the orthorhombic unit cell as compared to geological aragonite. Annealing at temperatures above 150−200 °C led to pronounced lattice relaxation which is accompanied by a substantial reduction of crystallite sizes and related growth of microstrain fluctuations. These findings indicate that biogenic aragonite crystals are strained, apparently, as a result of the amorphous/crystalline phase transformation, which proceeds within the supporting network of oriented biomacromolecules at early stages of biomineralization.

Atomic Structure of Biogenic Aragonite

Atomic positions in geological aragonite and biogenic aragonite, obtained from several mollusk shells, were precisely measured by high-resolution neutron diffraction, which is more sensitive than X-ray diffraction to the positions of light atoms, C and O, in the CaCO3 structure. Comparable analysis of atomic positions revealed, for the first time, the changes in some bond lengths and atomic configurations in biogenic specimens with respect to geological ones. Most pronounced are modifications in the aplanarity of the carbonate groups. These modifications correlate with the observed shifts of the ν2 frequency in the IR absorption spectra. The changes in atomic positions as well as the changes in lattice parameters detected by high-resolution X-ray diffraction are attributed to the interaction between organic macromolecules and growing crystallites during biomineralization. The obtained results allow us deeper understanding of the specific routes used by nature for growing bio-composites with superior chara...

Anisotropic lattice distortions in the mollusk-made aragonite: A widespread phenomenon

In this paper, we present experimental results demonstrating systematic structural distinctions between biogenic and non-biogenic calcium carbonate. Specifically we show, by high-resolution X-ray powder diffraction on dedicated synchrotron beam lines, that the orthorhombic unit cell of the mollusk-made aragonite is anisotropically distorted as compared with that one of geological aragonite. In all investigated shells, belonging to different classes (bivalve, gastropod, and cephalopod) and taken from different habitat origins (sea, fresh water, and land), the maximum elongation of about 0.1–0.2% was found along the c-axis. The lattice distortions along the a-axis were also of the positive sign (elongation) but lower than those along the c-axis, whereas lattice distortions along the b-axis were always negative (contraction). Supporting experiments, including structural analysis after a bleach procedure, measurements of temperature-dependent lattice relaxation, measurements of the CO 2 release at elevated temperatures, signify that the observed structural distinctions are most probably caused by the organic molecules intercalating into the aragonite lattice during biomineralization. Our findings show that in some sense organisms control the atomic structure of the crystals. Deeper understanding of this phenomenon will aid in the development of new approaches to grow biomimetic composites and tailor their properties on a molecular level.

On the structure of aragonite

High-resolution synchrotron powder diffraction measurements were carried out at the 32-ID beamline of the Advanced Photon Source of Argonne National Laboratory in order to clarify the structure of geological aragonite, a widely abundant polymorph of CaCO(3). The investigated crystals were practically free of impurity atoms, as measured by wavelength-dispersive X-ray spectroscopy in scanning electron microscopy. A superior quality of diffraction data was achieved by using the 11-channel 111 Si multi-analyzer of the diffracted beam. Applying the Rietveld refinement procedure to the high-resolution diffraction spectra, we were able to extract the aragonite lattice parameters with an accuracy of about 20 p.p.m. The data obtained unambiguously confirm that pure aragonite crystals have orthorhombic symmetry.

Nacre surface transformation to hydroxyapatite in a phosphate buffer solution

Nacre, also known as mother-of-pearl, constitutes the inner layer of mollusc shells. Nacre is a natural composite material consisting mostly of calcium carbonate in the aragonite crystal form and some organic matter. Previous studies have shown that geological aragonite, coral and nacre can convert hydrothermally to hydroxyapatite (HAP) in phosphate solution by a solid-state topotactic ion-exchange reaction. This conversion typically occurs within the range of 140–260°C, although higher temperatures are possible. In this work, we have found that nacre can transform to HAP in a phosphate buffer solution at room temperature via a surface reaction. The morphology of the nacre-transformed HAP surface was investigated by scanning electron microscopy (SEM). The HAP surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). A layer covered with packed particles was found in contrast to the tablet structure typical of nacre surfaces. XPS and SIMS indicated that the mineral phase of the nacre surface had converted from an aragonite phase to an HAP phase. Fourier transform infrared spectroscopy (FTIR) showed that phosphate (PO 4) bands appeared after nacre was soaked in a phosphate buffer and the intensity of the PO 4 bands increased with exposure time. The FTIR was consistent with XPS and SIMS results. We suggest that this surface reaction occurs by a dissolution–precipitation mechanism. Calcium ions are released from the nacre surface, react with phosphate ions in the buffer solution, and then precipitate as HAP on the nacre surface.

Low-loss Electron Energy-loss Spectroscopy and Dielectric Function of Biological and Geological Polymorphs of CaCO(3).

: Previous work on microstructural characterization has shown variations in terms of defects and organization of nanostructures in the two polymorphs of calcium carbonate, calcite, and aragonite in mollusc shells. Large variations in mechanical properties are observed between these sections which have been attributed to variations in composite microstructure as well as intrinsic properties of the inorganic phases. Here we present local low-loss electron energy-loss spectroscopic (EELS) study of calcitic and aragonitic regions of abalone shell that were compared to geological (single-crystal) counterpart polymorphs to reveal intrinsic differences that could be related to organismal effects in biomineralization. In both sets of samples, local dielectric function is computed using Kramer-Kronig analysis. The electronic structures of biogenic and geological calcitic materials are not significantly different. On the other hand, electronic structure of biogenic aragonite is remarkably different from that of geological aragonite. This difference is attributed to the increased contribution from single electron excitations in biogenic aragonite as compared to that of geological aragonite. Furthermore, an apparent bound characteristic of the Re(1/epsilon) is observed for biological samples which suggests a "quasi-plasmon"-like nature of the collective excitations. Implications of these changes are discussed in the context of macromolecular involvement in the making of the microstructures and properties in biogenic phases.