Cymbiola nobilis shell: Toughening mechanisms in a crossed-lamellar structure

A Novel Triad of Bio-Inspired Design, Digital Fabrication, and Bio-Derived Materials for Personalised Bone Repair

The emerging paradigm of personalised bone repair embodies a transformative triad comprising bio-inspired design, digital fabrication, and the exploration of innovative materials. The increasing average age of the population, alongside the rising incidence of fractures associated with age-related conditions such as osteoporosis, necessitates the development of customised, efficient, and minimally invasive treatment modalities as alternatives to conventional methods (e.g., autografts, allografts, Ilizarov distraction, and bone fixators) typically employed to promote bone regeneration. A promising innovative technique involves the use of cellularised scaffolds incorporating mesenchymal stem cells (MSCs). The selection of materials-ranging from metals and ceramics to synthetic or natural bio-derived polymers-combined with a design inspired by natural sources (including bone, corals, algae, shells, silk, and plants) facilitates the replication of geometries, architectures, porosities, biodegradation capabilities, and mechanical properties conducive to physiological bone regeneration. To mimic internal structures and geometries for construct customisation, scaffolds can be designed using Computer-aided Design (CAD) and fabricated via 3D-printing techniques. This approach not only enables precise control over external shapes and internal architectures but also accommodates the use of diverse materials that improve biological performance and provide economic advantages. Finally, advanced numerical models are employed to simulate, analyse, and optimise the complex processes involved in personalised bone regeneration, with computational predictions validated against experimental data and in vivo studies to ascertain the model's ability to predict the recovery of bone shape and function.

Micro-structural and micro-mechanical characterization of rock-boring angelwing clams

Rock-boring behavior is a common phenomenon among certain bivalve clams, yet the mechanisms enabling this capability remain elusive. This study delves into the microstructural and micromechanical properties of the shells and denticles of angelwing (Cyrtopleura costata), a rock-boring clam. X-ray Diffraction Analysis and Energy-dispersive Spectroscopy identify that angelwing shells are made of pure aragonite. Scanning Electron Microscope images reveal that angelwing shells are mostly made of submicrometer-thick lamellar sheets, which are packed closely forming crossed-lamellar groups. Nanoindentation tests yield Young's Moduli of 30-70GPa and hardness of 3-10GPa at different parts of the shells, making angelwing clam shells among the hardest biological materials. Further numerical simulations validate that the crossed-lamellar microstructure excels in withstanding external loads and safeguarding the integrity of the shell through minimized stress concentration. STATEMENT OF SIGNIFICANCE: Boring and drilling in rocks are important for construction, energy, and scientific exploration. Nature offers ideas for improving these techniques, as seen in the rock-boring angelwing clam. Our study focuses on the mechanical and micro-structural properties of the clam's shell, which help it bore into rocks. Through nanoindentation, we found that the clam's shell is one of the hardest and stiffest biological shells, a key factor in its boring ability. We also identified intricate shell structures that likely enhance its strength and resistance to mechanical stress. These findings highlight important bio-material traits that could inspire new, more efficient drilling technologies for human use.

Organization and Formation of the Crossed-Foliated Biomineral Microstructure of Limpet Shells

To construct their shells, molluscs are able to produce a large array of calcified materials including granular, prismatic, lamellar, fibrous, foliated, and plywood-like microstructures. The latter includes an aragonitic (the crossed-lamellar) and a calcitic (the crossed-foliated) variety, whose modes of formation are particularly enigmatic. We studied the crossed-foliated calcitic layers secreted solely by members of the limpet family Patellidae using scanning and transmission electron microscopy and electron backscatter diffraction. From the exterior to the interior, the material becomes progressively organized into commarginal first-order lamellae, with second and third order lamellae dipping in opposite directions in alternating lamellae. At the same time, the crystallographic texture becomes stronger because each set of the first order lamellae develops a particular orientation for the c-axis, while both sets maintain common orientations for one {104} face (parallel to the growth surface) and one a-axis (perpendicular to the planes of the first order lamellae). Each first order lamella shows a progressive migration of its crystallographic axes with growth in order to adapt to the orientation of the set of first order lamellae to which it belongs. To explain the progressive organization of the material, we hypothesize that a secretional zebra pattern, mirrored by the first order lamellae on the shell growth surface, is developed on the shell-secreting mantle surface. Cells belonging to alternating stripes behave differently to determine the growth orientation of the laths composing the first order lamellae. In this way, we provide an explanation as to how plywood-like materials can be fabricated, which is based mainly on the activity of mantle cells.

Bioinspired Additive Manufacturing of Hierarchical Materials: From Biostructures to Functions

Throughout billions of years, biological systems have evolved sophisticated, multiscale hierarchical structures to adapt to changing environments. Biomaterials are synthesized under mild conditions through a bottom-up self-assembly process, utilizing substances from the surrounding environment, and meanwhile are regulated by genes and proteins. Additive manufacturing, which mimics this natural process, provides a promising approach to developing new materials with advantageous properties similar to natural biological materials. This review presents an overview of natural biomaterials, emphasizing their chemical and structural compositions at various scales, from the nanoscale to the macroscale, and the key mechanisms underlying their properties. Additionally, this review describes the designs, preparations, and applications of bioinspired multifunctional materials produced through additive manufacturing at different scales, including nano, micro, micro-macro, and macro levels. The review highlights the potential of bioinspired additive manufacturing to develop new functional materials and insights into future directions and prospects in this field. By summarizing the characteristics of natural biomaterials and their synthetic counterparts, this review inspires the development of new materials that can be utilized in various applications.

Biodiesel Production from Waste Plant Oil over a Novel Nano-Catalyst of Li-TiO2/Feldspar

A novel Li-impregnated TiO2 catalyst loaded on feldspar mineral (Li-TiO2/feldspar) was synthesized via a wet impregnation method and was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) analysis. Using these techniques, it was possible to confirm the catalyst’s structural organization with a high crystallinity. This catalyst was used in the transesterification of five waste plant oils of Citrullus colocynthis (bitter apple), Pongamia pinnata (karanja), Sinapis arvensis (wild mustard), Ricinus communis (castor) and Carthamus oxyacantha (wild safflower). The catalytic tests were performed at temperatures ranging from 40 to 80 °C, employing a variable methanol/ester molar ratio (5:1, 10:1, 15:1, 20:1 and 25:1) and different catalyst concentrations (0.5%, 1%, 1.5%, 2% and 2.5%) relative to the total reactants mass. Conversion of 98.4% of fatty acid methyl esters (FAMEs) was achieved for Pongamia pinnata (karanja). The main fatty acids present in bitter apple, karanja, wild mustard, castor and wild safflower oils were linoleic acid (70.71%), oleic acid (51.92%), erucic acid (41.43%), ricinoleic acid (80.54%) and linoleic acid (75.17%), respectively. Li-TiO2/feldspar produced more than 96% for all the feedstocks. Fuel properties such as iodine value (AV), cetane number (CN), cloud point (CP), iodine value (IV), pour point (PP) and density were within the ranges specified in ASTM D6751.

Experimental Study on the Tridacna squamosa Shell: Distinctive Structure and Mechanical Behavior

Tridacna squamosa, Lamarck, 1819 (Bivalvia Cardiida Cardiidae, known as the fluted giant clam) is one of the largest-sized bivalve shells, which is equipped with a strong and tough bioceramic shell to effectively protect itself from the attack of predators. To better understand the mechanical defense mechanism, the relationship between the microstructure, composition, and mechanical properties of the Tridacna squamosa shell was investigated. We find that the Tridacna squamosa shell is composed of aragonite CaCO₃ and a small portion of organic matter, which are well-arranged, assembling a multiscale, inhomogeneous, and anisotropic structure. Three levels of microstructure units are identified, including the smallest aragonite rods, medium sheets, and block-like lamellae. Such multiscale structures are the main contributor to creating abundant fracture surfaces much larger than the case for single mineral components, leading to multiple toughening mechanisms observed in Vickers indentation experiments, such as pulled-out of mineral platelet and crack deflection. The material inhomogeneity in the cross-sectional direction indicates that the material is stronger at the inner layer than that at the outer layer, which also facilitates an effective defense against the predator attack. This study may provide insights into the design of biomaterials with the desired mechanical properties.

Flexible embryonic shell allies large offspring size and anti-predatory protection in viviparous snails

The evolutionary conflicts between viviparous reproductive mode and skeleton shape may occur whenever the space available for embryo development or delivery is limited by hard inflexible structures of a parent (bones, shell, etc.). In tetrapods, offspring size is at odds with female locomotion efficiency, which results in obstetric selection. We suggest a similar relationship for viviparous gastropods, where spacious canal needed for embryo delivery may interfere with anti-predatory role of narrow and toothed shell aperture. We explored this hypothesis in the group of viviparous land snails (Clausiliidae, subfamily Phaedusinae), known for complex apertural barriers protecting the shell interior. Most of the shell structure modifications we recorded facilitate the delivery of embryos but simultaneously reduce the safeguard of a narrow shell opening. However, we also observed highly flexible embryonic shells that may withstand squeezing between apertural barriers during birth. We investigated the microstructure of these flexible embryonic shells, compared to the typical hard shells of clausiliid embryos, which are rigid and unpliable already in the genital tract of the parent. Our results suggest that the unusual flexibility, which is related to a low number of organomineral layers in the shell, evolved in two phylogenetically distant lineages of Phaedusinae. This adaptation reduces mechanical constraints for birth of the neonates but allows to maintain the protective function of the apertural barriers.

Study on the Fracture Toughness of Softwood and Hardwood Estimated by Boundary Effect Model

The tensile strength and fracture toughness of softwood and hardwood are measured by the Boundary Effect Model (BEM). The experimental results of single-edge notched three-point bending tests indicate that the BEM is an appropriate method to estimate the fracture toughness of the present fibrous and porous woods. In softwood with alternating earlywood and latewood layers, the variation in the volume percentage of different layers in a small range has no obvious influence on the mechanical properties of the materials. In contrast, the hardwood presents much higher tensile strength and fracture toughness simultaneously due to its complicated structure with crossed arrangement of the fibers and rays and big vessels diffused in the fibers. The present research findings are expected to provide a fundamental insight into the design of high-performance bionic materials with a highly fibrous and porous structure.

Microstructure Analysis and Chemical and Mechanical Characterization of the Shells of Three Freshwater Snails

The shells of freshwater snails are discarded as waste, which qualify as biological materials with prospective multiple uses. To substantiate this proposition, an attempt was made to elucidate the physical and chemical properties of the shells of three freshwater snails, namely, Bellamya bengalensis, Pila globosa, and Brotia costula. The shells were prepared for electron microscopy and assessment of the calcium carbonate content, apart from the Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and nanoindentation studies. The results indicated that the calcium carbonate content (y) of the shells ranged between 87 and 96% of the total weight (x) and complied with a power regression equation: y = 0.801x 1.016; R 2 = 0.994; r = +0.998; P < 0.001. Observations through SEM depicted different snail species-specific arrangement patterns of calcium carbonate crystals in the diverse layers of shells. The XRD, FTIR, and EDS observations revealed the dominance of the aragonite form of the calcium carbonate crystal in the microstructures of each snail shell with the occurrence of different shell surface functional groups. The Brunauer-Emmett-Teller analysis elucidated the surface textures of shell dust taken from each snail species; in addition, the nanohardness properties indicate the shells as a tough biocomposite exoskeleton. Species-specific variations in the shell morphology, microstructure, and calcium carbonate content were prominent for the three freshwater snails considered for the study. Nonetheless, the physical and chemical properties substantiate that the shells of B. bengalensis, P. globosa, and B. costula qualify as biological materials for sustainable use in various fields including bioremediation, biocatalyst, biomedical applications, and a source of lime. Since the shells of the freshwater snails are discarded as aquaculture waste, subsequent use as a biological material will support the "waste made useful" paradigm in sustainability, both from ecological and economic perspectives.

Foamy oysters: vesicular microstructure production in the Gryphaeidae via emulsification

The vesicular microstructure is a very distinctive arrangement of calcite, consisting of hollow cavities (vesicles) of diverse sizes and shapes, usually elongated in the direction of shell thickening. It is uniquely found among living bivalves in a single oyster family, Gryphaeidae. The vesicles are distributed in lenses interleaved with compact foliated layers. We have studied the morphology and distribution of vesicles within the lenses using optical and electron microscopy, and micro-computed tomography. At a small scale, vesicles do not follow a classical von Neumann-Mullins route typical of ideal foams. At a larger scale, the initiation and evolution of a vesicular layer statistically proceed like a foam, with vesicles becoming more numerous, larger and more even in size. In summary, the vesicular material follows a foam-like coarsening to reduce the number of energetically costly interfaces. However, a steady state is never reached because the animal permanently introduces energy in the system by creating new vesicles. The fabrication of the vesicular material is mediated by the production of an emulsion between the extrapallial fluid and the precursor PILP of the calcitic walls within the thin extrapallial space. For this mechanism to proceed, the mantle cells must perform highly sophisticated behaviours of contact recognition and secretion. Accordingly, the vesicular material is under mixed physical-biological control.

Natural arrangement of fiber-like aragonites and its impact on mechanical behavior of mollusk shells: A review

During billions of years of evolution, creatures in nature have possessed nearly perfect structures and functions for survival. Multiscale structures in biological materials over several length scales play a pivotal role in achieving structural and functional integrity. Fiber, as a common principal structural element in nature, can be easily constructed in different ways, thus resulting in various natural structures. In this review, we summarized the decades of investigations on a typical biological structure constructed by fiber aragonites in mollusk shells. Crossed-lamellar structure, as one of the most widespread structures in mollusk shells, reconciles the strength-toughness trade-off dilemma successfully due to the presence of highly-hierarchical architectures. This distinctive structure includes several orders of sub-lamellae, and the different order lamellae present a cross-ply feature in one macro crossed-lamellar layer. When a mollusk shell has more than one macro-layer, the crossed-lamellar structure exhibits various forms of architectures including 0°/90°, 0°/90°/0° typical-sandwich, 15°/75°/0° quasi-sandwich, and 0°/90°/0°/90° arranged modes. The fracture resistance and the relevant toughening mechanisms are directly related to the highly-hierarchical crossed-lamellar structures on different length scales. This article is aimed to review the different arranged modes of crossed-lamellar structures existing in nature, with special attention to their impact on the mechanical behavior and salient toughening mechanisms over several length scales, for seeking the design guidelines for the fabrication of bio-inspired advanced engineering materials that are adaptive to different loading conditions.

Anisotropy of Mechanical Properties of Pinctada margaritifera Mollusk Shell

The mechanical properties such as compressive strength and nanohardness were investigated for Pinctada margaritifera mollusk shells. The compressive strength was evaluated through a uniaxial static compression test performed along the load directions parallel and perpendicular to the shell axis, respectively, while the hardness and Young modulus were measured using nanoindentation. In order to observe the crack propagation, for the first time for such material, the in-situ X-ray microscopy (nano-XCT) imaging (together with 3D reconstruction based on the acquired images) during the indentation tests was performed. The results were compared with these obtained during the micro-indentation test done with the help of conventional Vickers indenter and subsequent scanning electron microscopy observations. The results revealed that the cracks formed during the indentation start to propagate in the calcite prism until they reach a ductile organic matrix where most of them are stopped. The obtained results confirm a strong anisotropy of both crack propagation and the mechanical strength caused by the formation of the prismatic structure in the outer layer of P. margaritifera shell.

Designed for the enhancement of structure mechanostability and strength: Suture-serrate margins of bivalve shells

Through biological evolution, bivalve mollusks developed shells to improve the utilization of metabolic energy and provide protection against external threats. In addition to the mechanical optimization of the microstructure, the design of the macroscopic shape of a bivalve shell naturally becomes a potential approach to achieving the aforementioned purposes. While the functions of some features of mollusk shells have been studied, the role of the suture-serrate margins, a common morphology of bivalve shell edges, in the global mechanical behaviors of bivalve shells requires further exploration. Here, we present how the serrate margins contribute to the global mechanical properties of bivalve shells. The results of the compression tests employed on a typical bivalve, M. mercenaria, showed that the complete bivalve shells with suture-serrate margins perform better in terms of strength and work to fracture than those without the margins under the same conditions (dry and wet). The primary failure types observed during compression reveal that the failure mechanisms of valve shells are dependent on the suture-serrate margin morphology and water content. Using numerical simulations, the mechanical functions of the suture-serrate margins were demonstrated. Specifically, serrate margins provide mutual resistance by "locking" complementary valves to redistribute and eliminate stress concentrations around pre-existing defects, thereby enhancing the mechanostability and strength of the entire structure.

Mechanical properties of Chamelea gallina shells at different latitudes

In this work we evaluate the mechanical properties of Chamelea gallina shells, collected at various locations in the Adriatic Sea, through compression tests. We present an analytical model for the extraction of the material Young's modulus and ultimate strength, based on the approximation of the valves with a simpler geometry. The effect of porosity and the computation of the energy dissipated at fracture are also discussed. Results show a dependence of the mechanical performance on the location at which the samples were collected, i.e. latitude, and thus the environmental factors can affect the rigidity, strength and toughness of the shells. These findings integrate preliminary results published in a previous work.

Crack initiation and growth in a special quasi-sandwich crossed-lamellar structure in Cymbiola nobilis seashell

Sandwich structure consisting of three crossed-lamellar layers (inner, middle and outer) is one of the most common structures found in mollusk shells, and is normally arranged in a 0°/90°/0° or 90°/0°/90° mode. However, the Cymbiola nobilis seashell in the present study is observed to exhibit a unique quasi-sandwich structure, where the inner and middle layers have an ~15° rotation in comparison with those of typical sandwich structures, resulting in a 15°/75°/0° or 75°/15°/90° mode. This has been identified as the weak/tough/weak and tough/weak/tough modes, and the sample arranged in the 15°/75°/0° mode with a tough layer in the middle has a higher strength than that arranged in the 75°/15°/90° mode with a weak layer in the middle. The fracture resistances of these two types of structural arrangements depend mainly on crack propagation. The interfaces between the macrolayers can effectively arrest the crack propagation especially when the tough layer is positioned in the middle (15°/75°/0° mode), hence increasing the strength and toughness of materials. Salient toughening mechanisms involving crack deflection together with zig-zag crack propagation paths as well as the fiber pull-out of second-order lamellae are identified. Moreover, triangular fracture paths with a convex morphology are observed to form due to the concurrent occurrence of fiber fracturing and channel cracking in single 1st-order lamellae. These findings shed light on the superb crack propagation resistance of the unique quasi-sandwich structure in the C. nobilis shell, thus paving the way for the development of bio-inspired advanced structural materials.

The mechanical principles behind the golden ratio distribution of veins in plant leaves

Tree leaves are commonly composed of thin mesophyll, carrying out photosynthesis under sunlight, and thick veins. Although the role of leaf veins in water transportation has been known for a long time, their role in providing structural support and guaranteeing large sunlighted area was rarely studied and remains elusive. Here, with use of a novel inverse optimization approach, we aim for uncovering the material design principle behind the unique pattern of venation. It is intriguing to observe that an almost Golden Ratio (GR) distribution of leaf veins always provides optimized structural behavior. Specifically, our research reveals, for the first time, that this unique GR distribution of relatively strong vein material is helpful for maximizing the bending stiffness and leading to a large sunlighted area which is vital for the photosynthesis process of a leaf. Moreover, the GR distribution of leaf veins is also observed in a wide class of plant leaf geometries (i.e., shape, thickness), where experimental evidence is provided for the optimized results. Therefore, our findings can not only serve to explain the mystery of veins GR distribution but also provide widely applicable guidelines on designing soft structures with exceptional mechanical performances.

Mechanical properties of crossed-lamellar structures in biological shells: A review

The self-fabrication of materials in nature offers an alternate and powerful solution towards the grand challenge of designing advanced structural materials, where strength and toughness are always mutually exclusive. Crossed-lamellar structures are the most common microstructures in mollusks that are composed of aragonites and a small amount of organic materials. Such a distinctive composite structure has a fracture toughness being much higher than that of pure carbonate mineral. These structures exhibiting complex hierarchical microarchitectures that span several sub-level lamellae from microscale down to nanoscale, can be grouped into two types, i.e., platelet-like and fiber-like crossed-lamellar structures based on the shapes of basic building blocks. It has been demonstrated that these structures have a great potential to strengthen themselves during deformation. The observed underlying toughening mechanisms include microcracking, channel cracking, interlocking, uncracked-ligament bridging, aragonite fiber bridging, crack deflection and zig-zag, etc., which play vital roles in enhancing the fracture resistance of shells with the crossed-lamellar structures. The exploration and utilization of these important toughening mechanisms have attracted keen interests of materials scientists since they pave the way for the development of bio-inspired advanced composite materials for load-bearing structural applications. This article is aimed to review the characteristics of hierarchical structures and the mechanical properties of two kinds of crossed-lamellar structures, and further summarize the latest advances and biomimetic applications based on the unique crossed-lamellar structures.