Nacre Tablets Research Articles

HcN57, A Novel Unusual Acidic Silk-Like Matrix Protein from Hyriopsis cumingii, Participates in Framework Construction and Nacre Nucleation During Nacreous Layer Formation.

In the classic molecular model of nacreous layer formation, unusual acidic matrix proteins rich in aspartic acid (Asp) residues are essential for nacre nucleation due to their great affinity for binding calcium. However, the acidic matrix proteins discovered in the nacreous layer so far have been weakly acidic with a high proportion of glutamate. In the present study, several silk-like matrix proteins, including the novel matrix protein HcN57, were identified in the ethylenediaminetetraacetic acid-soluble extracts of the nacreous layer of Hyriopsis cumingii. HcN57 is a highly repetitive protein that consists of a high proportion of alanine (Ala, 34.4%), glycine (Gly, 22.5%), and serine (Ser, 11.4%). It forms poly Ala blocks, GlynX repeats, an Ala-Gly repeat, and a Ser-Ala-rich region, exhibiting significant similarity to silk proteins found in spider species. The expression of HcN57 was specifically located in the dorsal epithelial cells of the mantle pallium and mantle center. Notably, expression of HcN57 was relatively high during nacreous layer regeneration and pearl nacre deposition, suggesting HcN57 is a silk matrix protein in the nacreous layer. Importantly, HcN57 also contains a certain content of Asp residues, making it an unusual acidic matrix protein present in the nacreous layer. These Asp residues are mainly distributed in three large hydrophilic acidic regions, which showed inhibitory activity against aragonite deposition and morphological regulation of calcite in vitro. Moreover, HcN57-dsRNA injection resulted in failure of nacre nucleation in vivo. Taken together, our results show that HcN57 is a bifunctional silk protein with poly Ala blocks and Gly-rich regions that serve as space fillers within the chitinous framework to prevent crystallization at unnecessary nucleation sites and Asp-rich regions that create a calcium ion supersaturated microenvironment for nucleation in the center of nacre tablets. These observations contribute to a better understanding of the mechanism by which silk proteins regulate framework construction and nacre nucleation during nacreous layer formation.

A Novel Kunitz-Type Serine Protease Inhibitor (HcKuSPI) is Involved in Antibacterial Defense in Innate Immunity and Participates in Shell Formation of Hyriopsis cumingii.

Serine protease inhibitors (SPIs) are abundantly reported for its inhibition against specific proteases involved in the immune responses, but SPI data related to calcareous shells are scarce. Previously, our research group has reported the proteome analysis of non-nucleated pearl powder, and a candidate matrix protein containing two Kunitz domains in the acid soluble fraction caught our attention. In the present study, the full-length cDNA sequence of HcKuSPI was obtained from Hyriopsis cumingii. HcKuSPI was specifically expressed in the mantle, with hybridization signals mainly concentrated to dorsal epithelial cells at the mantle edge and weak signals at the mantle pallium, suggesting HcKuSPI was involved in shell formation. HcKuSPI expression in the mantle was upregulated after Aeromonas hydrophila and Staphylococcus aureus challenge to extrapallial fluids (EPFs). A glutathione S transferase (GST)-HcKuSPI recombinant protein showed strong inhibitory activity against the proteases, trypsin and chymotrypsin. Moreover, HcKuSPI expression in an experimental group was significantly higher when compared with a control group during pellicle growth and crystal deposition in shell regeneration processes, while the organic shell framework of newborn prisms and nacre tablets was completely destroyed after HcKuSPI RNA interference (RNAi). Therefore, HcKuSPI secreted by the mantle may effectively neutralize excess proteases and bacterial proteases in the EPF during bacterial infection and could prevent matrix protein extracellular degradation by suppressing protease proteolytic activity, thereby ensuring a smooth shell biomineralization. In addition, GST-HcKuSPI was also crucial for crystal morphology regulation. These results have important implications for our understanding of the potential roles of SPIs during shell biomineralization.

Correlative chemical and elemental nano-imaging of morphology and disorder at the nacre-prismatic region interface in Pinctada margaritifera

Understanding biomineralization relies on imaging chemically heterogeneous organic–inorganic interfaces across a hierarchy of spatial scales. Further, organic minority phases are often responsible for emergent inorganic structures from the atomic arrangement of different polymorphs, to nano- and micrometer crystal dimensions, up to meter size mollusk shells. The desired simultaneous chemical and elemental imaging to identify sparse organic moieties across a large field-of-view with nanometer spatial resolution has not yet been achieved. Here, we combine nanoscale secondary ion mass spectroscopy (NanoSIMS) with spectroscopic IR s-SNOM imaging for simultaneous chemical, molecular, and elemental nanoimaging. At the example of Pinctada margaritifera mollusk shells we identify and resolve ~ 50 nm interlamellar protein sheets periodically arranged in regular ~ 600 nm intervals. The striations typically appear ~ 15 µm from the nacre-prism boundary at the interface between disordered neonacre to mature nacre. Using the polymorph distinctive IR-vibrational carbonate resonance, the nacre and prismatic regions are consistently identified as aragonite ({overline{nu }}_{a}=860 cm−1) and calcite ({overline{nu }}_{c}=880 cm−1), respectively. We observe previously unreported morphological features including aragonite subdomains encapsulated in extensions of the prism-covering organic membrane and regions of irregular nacre tablet formation coincident with dispersed organics. We also identify a ~ 200 nm region in the incipient nacre region with less well-defined crystal structure and integrated organics. These results show with the identification of the interlamellar protein layer how correlative nano-IR chemical and NanoSIMS elemental imaging can help distinguish different models proposed for shell growth in particular, and how organic function may relate to inorganic structure in other biomineralized systems in general.

Geochemical and mineralogical proxies beyond temperature: Autumn seasons trapped in freshwater nacre

Measurements of trace element chemistry, mineralogy, and isotope geochemistry are rarely combined with known environmental data to provide a more complete story about how environmental conditions are recorded in biomineral carbonates. Here, cultured (farmed) pearls serve as relatively pristine time capsules to study these geochemical and mineral-based proxies. Cathodoluminescence (CL) imaging and Raman spectroscopic mapping on the µm-scale reveals that heterogeneous crystal bonding environments, geochemistry, and organic contents across the growth history of a freshwater pearl reflect environmental shifts in Kentucky Lake, TN, USA. A major CL peak at 551 nm aligns with increased manganese and organic contents and correlates with lake conductivity and alkalinity data. These CL features are temporally offset from previous 10-µm-scale aragonite nacre oxygen isotope measurements (δ18OArg) that record periods of minimum lake temperatures in winter seasons. Thus, we suggest that these trace element and organic features represent autumn or spring rainy seasons that experience lake turnover events and more land runoff rich in Mn and nutrients, increasing aragonite-bound Mn and organic contents in the nacre. This 551 nm signal is absent in Mn-poor saltwater pearl nacre. A second CL peak at 444 nm shows different heterogeneous features likely due to crystal structure shifts, as evidenced by correlations to a Raman map of translational (T): librational (L) mode height ratios typically signaling changes in nacre tablet orientation. Thus, we show that µm-scale CL and Raman mapping may serve as complementary environmental proxies to novel SIMS-based δ18OArg temperature proxies in order to capture additional information about local lake environments at seasonal to sub-seasonal temporal resolutions.

Shell Microstructure as an Indicator of Changing Environmental Conditions in Coastal Oceans

Ocean ecosystems are undergoing pronounced changes in temperature and chemistry. Biomineralizing animals, such as mollusks, that produce calcium carbonate shells can be sensitive to these changes, and this may be reflected in the microstructure of their shells. Shell microstructure potentially offers conservation paleobiologists an additional tool for assessing spatiotemporal changes in environmental conditions resulting from human activities. Previous work suggests that environmental factors, such as temperature and pH, can affect how mollusks build their shells. Certain mollusks continuously lay down tablets of nacre on the interior of their shells, and the thickness of these tablets may reflect the temperature of the environment in which the organism lived. We have been investigating spatial and temporal variation in tablet thickness in two groups of marine mollusks. Our first case study focuses on a variety of present-day abalone species along a temperature gradient. Data were collected from individuals raised in aquaculture at controlled temperatures, as well as wild-grown individuals from different latitudes in the Pacific Ocean. Our second case study focuses on the marine bivalve Nucula proxima from the northern Gulf of Mexico. Live and dead Nucula proxima specimens were collected from 20 meters water depth offshore Louisiana and Alabama; radiocarbon analyses indicate that these specimens represent both present-day and pre-Industrial populations. Our preliminary results show: 1) little variation in tablet thickness among abalone grown in aquaculture at different temperatures, suggesting that fine scale variation in temperature has little effect on abalone microstructure; and 2) an increase in tablet thickness during the past 250 years for N. proxima offshore Louisiana, but little change in N. proxima microstructure in coastal Alabama over past centuries.

Shell proteins and microstructural analysis identify the origin of shell arts with species resolution in pearl oysters

Molluscan shells are diverse, beautiful, and mechanically robust. They were commonly used by humans during ancient times as money, decorative objects, or functional tools. Luodian, a unique shell art in China dating back 3000 years, has served as a cultural carrier between China and Europe. To understand the production processes as well as underlying cultural contexts, it is important to identify the material origin of the art. This study combined proteomics and microstructural analysis to identify the raw material origins of three Luodian samples from the Qing Dynasty with species resolution and potential maturity levels. Shell proteomics and multiple sequence alignment have identified key shell matrix proteins (N66 and Pif) from Pinctada maxima, a pearl oyster distributed in the Indo-Pacific region. Although other factors (temperature, salinity, etc.) may contribute to the thickness of nacreous tablets in the shell, the thickness is inversely proportional to the shell size in the pearl oyster. We inferred the maturity of raw materials in different Luodian samples, which are tailored to different purposes. This study provides a framework and foundation for identifying the origin of various shell arts, especially those made from pearl oysters.

Disturbed by pH? Nacre tablet thickness of freshwater pearl mussels (Margaritifera margaritifera) is a poor temperature proxy

Context The ultrastructure of marine bivalve shells, specifically the thickness of nacre tablets has recently been identified to be related to water temperature. Aims This study investigates whether the nacre tablet thickness (NTT) of freshwater pearl mussel shells can serve as a proxy for stream water temperature. Methods Laboratory-grown juvenile and adult Margaritifera margaritifera from three streams were analysed using scanning electron microscopy to determine the relationship between NTT and water temperature. Key results Neither laboratory-grown juvenile nor adult specimens from the field revealed consistent NTT trends with temperature. Conclusion The NTT of M. margaritifera is largely uncoupled from stream water temperature. Presumably, fluctuations in stream water pH caused physiological stress and forced the animal to allocate more energy to homeostasis than to shell growth. The absence of a coupling between NTT of juvenile mussels and temperature is likely to be due to stronger physiological control on nacre tablet morphology during early stages of ontogeny, favouring thinner tablets with a larger proportion of organics v. CaCO3, which increases the fracture resistance of the shell. Implications Therefore, only if ambient pH remains stable, NTT of freshwater mussels beyond the age of five may serve as a temperature proxy, albeit being associated with a large error.

Structural and functional analyses of chitinolytic enzymes in the nacreous layer of Pinctada fucata

In this study, chitinolytic activity was confirmed in the protein extracted from the inner nacreous layer of Pinctada fucata shells. Allosamidin, a specific inhibitor of family 18 chitinase, was injected into pearl oysters, and an abnormal organic membrane was observed in the nacreous layer. When shell-notched individuals were injected with allosamidin, the organic fibers in the regenerated site were thicker than those without allosamidin treatment. To identify chitinases in the nacreous layer, proteins extracted from the nacreous layer were analyzed using LC-MS/MS. Two chitinases (PfCN1 and PfCN2) were identified, and their different spatial expression patterns were observed in mantle tissue. To understand the function of chitinolytic enzymes in nacre formation, we designed an in vivo gene knockdown experiment using PfCN1 and PfCN2 dsRNA, and in vitro calcium carbonate crystallization experiments using chitinolytic enzymes. In the knockdown experiment, abnormal aragonite tablets and organic films were observed in the nacreous layer. In the crystallization experiment, we found that the thickness of the chitin film decreased and the space between the chitin fibers increased when calcium carbonate was formed on the chitin film on the glass substrate with chitin-degrading enzyme treatment. These reactions induce the growth of calcium carbonate crystals through the chitin film. The penetration of chitin film by calcium carbonate is similar to that of mineral bridges that connect the nacre tablets vertically. These results showed the novel function of chitinolytic enzymes that will give a comprehension of nacre formation.

Interfacial Mechanical Behavior in Nacre of Red Abalone and Other Shells: A Review.

Interfaces between nacreous tablets are crucial to the outstanding mechanical properties of nacre in natural shells. Excellent research has been conducted to probe the effect of interfaces on strength and toughness of nacre, providing critical guidelines for the design of human-made laminated composites. This article reviews recent studies on interfacial mechanical behavior of nacre in red abalone and other shells, including experimental methods, analytical and numerical modeling. The discussions focus on the mechanical properties of dry and hydrated nacreous microstructures. The review concludes with discussions on representative studies of nacre-like composites with interfaces tuned using multiple approaches, and provides an outlook on improving the performance of composites with better interfacial controls.

Ballistic performance of a lightweight nacre-inspired armour panel – a numerical study

This paper provides a numerical investigation on the ballistic performance of ceramic composite armour panels with porous architectures inspired by nacre from mollusc shells. The development of such armour panels can lead to reduced weight and improve ballistic performance through projectile rotation induced by non-uniform contact stresses. The porous tablets of nacre have the potential to reduce the mass of armour panels without compromising their ballistic performance. Preliminary simulations were conducted to assess the performance of several porous bio-inspired structures that have the potential to survive projectile impact with less mass. Several bio-inspired panels composed of various porous designs were generated based on the different void architectures of nacre's tablets. Their performances were benchmarked against a monolithic panel under the same projectile impact condition and having the same 4.5 mm overall panel thickness but higher mass. It was found that for the cases considered, the biomimetic panels with porous architectures possess better ballistic performance compared to the corresponding monolithic panel by arresting the projectile having an initial impact velocity of 500 m/s, whilst reducing its mass by up to 18%. A further study was conducted on bio-inspired panels with a higher thickness but the same mass as the solid monolithic panel under the same projectile impact condition. It was found that the bio-inspired panel exhibits significantly improved ballistic performance when subjected to a fragment simulating projectile having an initial velocity chosen to be the ballistic limit velocity of the corresponding monolithic panel.

Color profile and microstructure of the nacre shell of an invasive freshwater mussel, Sinanodonta woodiana, at different elevations in West Java, Indonesia

The Chinese pond mussel Sinanodonta woodiana is an invasive freshwater mussel in Indonesia but has economic potential owing to the production of hemispherical pearls in their nacreous shell. To our knowledge, this study is the first to report on the species' shell nacre color profile and correlate it with microstructure and environmental factors at different elevations, from sea level to more than 1500 m. Samples from Lowland, Midland, and Highland elevations in West Java Province were observed and analyzed using the scanning electron microscope for shell microstructure. In addition, the nacre color was analyzed using spectrophotometry and estimated using the CIE Yxy color space and yellowness index. The nacre color ranged from gold to yellow and silvery. The microstructure at five positions on the shell was observed with a scanning electron microscope. The nacre with gold color had a thicker whole nacre layer, thinner nacre tablet thickness, and narrower surface nacre tablet area. In contrast, the nacre with silver color had a thinner whole nacre layer, thicker nacre tablet thickness, and greater surface nacre tablet area. The posterior position of shell nacre had a strong gold color with a high yellowness index, whereas the anterior and dorsal positions had a yellow color, and the central and ventral positions had a silver color. Mussels from the Midland locations displayed strong gold-colored nacre with a high yellowness index and high brightness. Based on these results, Midland areas are considered to have optimum environmental conditions for hemispherical pearl culture. S. woodiana shell with a thicker nacre layer and thinner nacre tablet shows a strong gold color, whereas a thinner nacre layer and thicker tablet nacre has a silvery color. We surmise that environments with high chlorophyll-a concentrations and low concentrations of total suspended solids result in the thicker nacre layer and the stronger gold color in the inner shell of S. woodiana.

Synchrotron-Based HR-Fluorescence and Mineralogical Mapping of the Initial Growth Stages of Polynesian Cultivated Pearls Disprove the ‘Reversed Shell’ Concept

In a series of Polynesian pearls collected after short cultivation periods, early post-grafting mineral deposits were characterized by high resolution synchrotron-based X-ray fluorescence with unprecedented accuracy. Morphological patterns and elemental composition are correlated through simultaneous imaging processes. Evidence that aragonite and calcite occur in neighboring units during the earliest biomineralization stages reveals that the grafting process can result in a greater degradation than usually admitted in the widely shared ‘reversed shell’ concept. Compared with ultrastructure of the pristine nacreous tablets, this method enables a precise evaluation of the possible biological changes in the biomineralization mechanism during grafting.

Coupling Mineralogy and Oxygen Isotopes to Seasonal Environmental Shifts Recorded in Modern Freshwater Pearl Nacre From Kentucky Lake

AbstractBiominerals that accrete shell or skeleton are commonly used as windows to past geochemical environments. Using biominerals as paleoproxies depends on the assumption that biominerals faithfully and predictably record environmental parameters, yet little has been done from a mineralogical perspective to understand how various environmental shifts impact shell and skeleton mineralogy and how mineralogy influences proxy calibrations. In this study, we correlate a suite of environmental data from the Kentucky Lake Long‐Term Monitoring Program with mineralogical and chemical signatures of modern cultured pearl nacre. Six transects were measured across three pearls from nucleus to edge for oxygen isotope ratio (δ18O) using SIMS (10‐μm spots) and for mineralogy via nacre tablet thickness using SEM and via carbonate bonding environments using Raman spectroscopy. After confirming a strong seasonal correlation between δ18O and temperature, we extend δ18O correlations to relative nacre OH/O ratios, lake dissolved oxygen, and lake light levels at 1 m depth. Although nacre tablet thicknesses are not clearly correlated with temperature, we observe correlative trends with lake conductivity and relative nacre OH/O ratios. For the Raman spectroscopy measurements, there was a modest inverse correlation between increasing carbonate ν1 and ν4 vibrational mode peak heights with δ18O and lake dissolved oxygen levels. Standard deviations in Raman T:L peak area ratios, as an estimation of angle spread of tablet crystallographic orientation, correlate with lake reduction potential. This approach of combining geochemical, isotopic, mineralogical, and environmental data lends a more holistic view to biomineralization in natural systems and to the decoding of environmental mineral signatures.

The mesoscale order of nacreous pearls

A pearl's distinguished beauty and toughness are attributable to the periodic stacking of aragonite tablets known as nacre. Nacre has naturally occurring mesoscale periodicity that remarkably arises in the absence of discrete translational symmetry. Gleaning the inspiring biomineral design of a pearl requires quantifying its structural coherence and understanding the stochastic processes that influence formation. By characterizing the entire structure of pearls (∼3 mm) in a cross-section at high resolution, we show that nacre has medium-range mesoscale periodicity. Self-correcting growth mechanisms actively remedy disorder and topological defects of the tablets and act as a countervailing process to long-range disorder. Nacre has a correlation length of roughly 16 tablets (∼5.5 µm) despite persistent fluctuations and topological defects. For longer distances (>25 tablets , ∼8.5 µm), the frequency spectrum of nacre tablets follows [Formula: see text] behavior, suggesting that growth is coupled to external stochastic processes-a universality found across disparate natural phenomena, which now includes pearls.

The influence of donor and recipient oyster red and blue shell nacre interference color on Pinctada fucata martensii pearl quality

The influence of shell body color on pearls has been studied extensively, but the effect of shell interference color selection on pearls has not been fully understood yet. Because interference color is an important factor determining pearl quality, it is important to study whether the shell interference color of the donor and recipient oysters has any influence on the resulting pearls' interference color to cope with the market demands. We report that selective breeding of red- and blue-line Pinctada fucata martensii pearl oysters produce high-quality pearls based on shape, flaw, color, size, and luster assessment categories. Saibo from red- and blue-line donors was implanted with a rounded nucleus into the red- and blue-line recipient oysters from the same batch in four treatments which we refer to as DRMR (red-donor, red-recipient), DRMB (red-donor, blue-recipient), DBMR (blue-donor, red-recipient), and DBMB (blue-donor, blue-recipient) oysters. Pearl shape and flaws, body yellowness index (YI) and interference value color categories, luster value, total nacre and tablet thickness, and surface microstructure for these treatments are reported. More DRMR pearls are high grade (H) than DBMB, DBMR, and DRMB pearls. Using electronic pearl quality measuring devices, DRMR pearls have the lowest YI and highest interference color values, rendering them superior in color category. Whole nacre thickness of pearls estimated by the SEM on the pearl section reveal DRMR pearls have significantly (p < .001) thicker nacre than DBMB, DBMR, and DRMB pearls, rendering them generally larger in size. DRMR pearls also have significantly (p < .001) thicker nacre tablets in basal, middle, and surface regions of the nacre layer. Whereas luster does not differ significantly among treatments (p > .05), DRMR pearls are superior to those of other treatments in their shape, reduced number of flaws, color, and size. We consider that P. fucata martensii oysters with red shell nacre interference color improve pearl quality, and for the purposes of pearl culture should be selected for. These results are of commercial importance, especially where P. fucata is cultured for pearl production.

Hyperspectral interference tomography of nacre

Significance We invented an optical technique—hyperspectral interference tomography—that rapidly and nondestructively extracts nanoscale structural information across large samples of nacre (mother-of-pearl) and other layered materials by combining multiangle and polarization-resolved hyperspectral imaging with optical-interference modeling. We investigated nacre in mollusk shells from two different species, red abalone and rainbow abalone, and discovered a previously unknown relationship between the age of the mollusk and the thickness of aragonite tablets in nacre. Hyperspectral interference tomography will have applications in climate science, since nacre tablet thickness in fossil shells is a proxy for ancient seawater temperature, and in bioinspired mechanics, because the layered structure of nacre inspires engineered materials with exceptional strength and toughness.

The effects of nacre microstructure on green and pink interference colors in Pinctada fucata martensii pearls

Despite the importance of both pearl body and interference colors in determining pearl quality, only body color has been studied extensively. We report novel information on relationships between pearl nacre microstructure and interference color saturation and hue in pearls produced by Pinctada fucata martensii. The interference color saturation and hue of 60 round pearls was subjectively graded by eye by an expert. Nacre optical characteristics and microstructure of pearls of 1) high (grade A) and low (grade B) interference color saturation, and 2) pink and green interference hues are reported. Using electronic pearl quality measuring devices, grade A pearls have higher interference color values (0.050 ± 0.061) than grade B pearls (0.008 ± 0.039), consistent with subjective grading by eye. SEMs of surface microstructure reveal the nacre tablet area of grade A pearls, 4.079 ± 2.173 μm2, to be generally greater than that of grade B pearls, 2.940 ± 1.213 μm2 (P < .001), with the nacre surface of grade A pearls smoother than grade B pearls. Regardless of grade, the basal and middle regions of the nacreous layer of pearls with a pink interference color hue differ significantly in nacre tablet thickness (p < .001), but nacre tablet thickness does not differ significantly between their central and surface regions. The nacre tablet thickness of pearls with green interference color hue changes more consistently from the basal to surface regions, with only the basal and surface regions differing significantly in thickness (p < .001). We report pearl interference color to be greatly affected by nacre tablet microstructure. The saturation of pearl interference color is affected by the smoothness of the nacre surface, which is a function of the surface nacre tablet area. The hue of pearl interference color is affected by differing thicknesses of nacre tablets through the nacreous layer, from surficial to basal regions. Results of this study might benefit the pearl industry, particularly where P. fucata is cultured. Additional study on how shell matrix protein genes are related to nacre tablet biomineralization would improve understanding of the mechanisms affecting pearl nacre microstructure, leading to saturation and hue of pearl interference colors.

Analisis Scanning Electron Microscopy pada Nacre Sinanodonta (Anodonta) woodiana (Lea, 1834)

Chinese pond shell, Sinanodonta (Anodonta) woodiana (Lea, 1834), is a freshwater bivalve that has essential ecological and economic functions. The microstructure of the nacre is of great interest and is the main attraction for the development of pearl farming. This study aims to describe the microstructure and composition of biomineral elements of the nacre at several shell sizes of S. woodiana. The shell is cut with a small forcepon the ventral margin with a size of 3-5 mm for Scanning Electron Microscpy (SEM). SEM images display that a shell layer consists of periostracum, prismatic and nacre layers. The surface of the nacre layer is an irregular or labyrinth patterned. The nacre tablets are hexagonal, glued to each other, so the nacre tablets become polygonal. Moreover, the microstructure of the nacre tablets is like a brick wall, and the thickness of tablets from 0.43 μm to 0.59 μm. The composition of the biomineral elements are C, O, Ca, and the mineralization mechanism is under the control of aquatic environmental factors that help the process of microstructure formation in nacre.

A comparative study on the mechanical and structural design of nacre in gastropod and bivalve molluscs

The lesson learnt from nature is presented through the examination of shells from four mollusc species (two gastropods and two bivalves). Individual aragonite tablets together with the organic network have been studied in terms of tablet dimensions, tablet stacking sequence, texture and the weight fraction of the organic present in these bio-composite shells. Mechanical properties viz. elastic modulus and hardness at the macro (bulk), micro (tablet) and nano (aragonite nanograins) levels have been determined using the experimental technique of nanoindentation together with the theoretical formulations offered by the Mori-Tanaka method and other such mechanical models for composites. XRD studies confirm the strong c-axis crystallographic orientation - aligned parallel to the direction of growth and crystallization of nacreous tablets, and fractography performed on the shell specimens suggests a difference in the stacking of individual aragonite layers for the gastropod and bivalve molluscan class, respectively. DSC and TG-DTA data helped in characterizing the organic material present in the shell specimens and in calculating the organic weight and volume fractions across species. Further to this, the DSC data revealed that the nature of the organic in the brick and mortar assembly of nacre, was similar in nature across the two molluscan classes reported in this study. EBSD studies performed on our system revealed a difference in crystallographic texture for the two different tablet stacking (columnar and sheet nacre). It has been shown experimentally that the shells of gastropods (columnar nacre) are orthotropic in nature, while the bivalve mollusc shells (sheet nacre) are anisotropic in nature.

Sliding Avalanches Between Nacreous Tablets.

Nacre, also known as mother of pearl, possesses extraordinary mechanical properties resulting from its intriguing hierarchical brick-and-mortar microstructures. Despite prior studies, interactions between nanoasperities during sliding still need to be elucidated. In this study, we measure slip events between nanograins of microlayers at high temporal resolution during torsion-induced sliding. We model the slips as avalanches caused by interactions of atoms on nanograin surfaces, from which power laws and scaling functions describing statistics and dynamics of slip events are studied. The largest avalanche occurs when nanograins leave each other after the maximum contact. The agreement between measurements and predictions shows that avalanches act essentially in the inhomogeneous sliding of nacreous tablets. Further insights into nanofriction provided in this work may lead to the development of nanoscale tribological systems.