Residual Strain and Stress in Biocrystals

Constructing Multifunctional Composite Single Crystals via Polymer Gel Incorporation

The non-uniformity of a single crystal can sometimes be found in biominerals, where surrounding biomacromolecules are incorporated into the growing crystals. This unique composite structure, combining heterogeneity and long-range ordering, enables the functionalization of single crystals. Polymer gel media are often used to prepare composite single crystals, in which the growing crystals incorporate gel networks and form a bi-continuous interpenetrating structure without any disruption to single crystallinity. Moreover, dyes and many kinds of nanoparticles can be occluded into single crystals under the guidance of gel incorporation. On this basis, the bio-inspired method has been applied in crystal morphology control, crystal dyeing, mechanical reinforcement, and organic bulk heterojunction-based optoelectronics. In this paper, the composite structure, the incorporation mechanisms, and the multiple functions of gel-incorporated single crystals are reviewed.

Long-lived enhanced magnetization-A practical metabolic MRI contrast material

Noninvasive tracking of biochemical processes in the body is paramount in diagnostic medicine. Among the leading techniques is spectroscopic magnetic resonance imaging (MRI), which tracks metabolites with an amplified (hyperpolarized) magnetization signal injected into the subject just before scanning. Traditionally, the brief enhanced magnetization period of these agents limited clinical imaging. We propose a solution based on amalgamating two materials-one having diagnostic-metabolic activity and the other characterized by robust magnetization retention. This combination slows the magnetization decay in the diagnostic metabolic probe, which receives continuously replenished magnetization from the companion material. Thus, it extends the magnetization lifetime in some of our measurements to beyond 4 min, with net magnetization enhanced by more than four orders of magnitude. This could allow the metabolic probes to remain magnetized from injection until they reach the targeted organ, improving tissue signatures in clinical imaging. Upon validation, this metabolic MRI technique promises wide-ranging clinical applications, including diagnostic imaging, therapeutic monitoring, and posttreatment surveillance.

Implications of intracrystalline OC17 on the protection of lattice incorporated proteins

Biogenic CaCO3 formation is regulated by crystallization proteins during crystal growth. Interactions of proteins with nascent mineral surfaces trigger proteins to be incorporated into the crystal lattice. As a result of incorporation, these intracrystalline proteins are protected in the lattice, an example of which is ancient eggshell proteins that have persisted in CaCO3 for thousands of years even under harsh environmental conditions. OC17 is an eggshell protein known to interact with CaCO3 during eggshell formation during which OC17 becomes incorporated into the lattice. Understanding protein incorporation into CaCO3 could offer insights into protein stability inside crystals. Here, we study the protection of OC17 in the CaCO3 lattice. Using thermogravimetric analysis we show that the effect of temperature on intracrystalline proteins of eggshells is negligible below 250 °C. Next, we show that lattice incorporation protects the OC17 structure despite a heat-treatment step that is shown to denature the protein. Because incorporated proteins need to be released from crystals, we verify metal chelation as a safe crystal dissolution method to avoid protein denaturation during reconstitution. Finally, we optimize the recombinant expression of OC17 which could allow engineering OC17 for engineered intracrystalline entrapment studies.

Guidelines derived from biomineralized tissues for design and construction of high-performance biomimetic materials: from weak to strong

Living organisms in nature have undergone continuous evolution over billions of years, resulting in the formation of high-performance fracture-resistant biomineralized tissues such as bones and teeth to fulfill mechanical and biological functions, despite the fact that most inorganic biominerals that constitute biomineralized tissues are weak and brittle. During the long-period evolution process, nature has evolved a number of highly effective and smart strategies to design chemical compositions and structures of biomineralized tissues to enable superior properties and to adapt to surrounding environments. Most biomineralized tissues have hierarchically ordered structures consisting of very small building blocks on the nanometer scale (nanoparticles, nanofibers or nanoflakes) to reduce the inherent weaknesses and brittleness of corresponding inorganic biominerals, to prevent crack initiation and propagation, and to allow high defect tolerance. The bioinspired principles derived from biomineralized tissues are indispensable for designing and constructing high-performance biomimetic materials. In recent years, a large number of high-performance biomimetic materials have been prepared based on these bioinspired principles with a large volume of literature covering this topic. Therefore, a timely and comprehensive review on this hot topic is highly important and contributes to the future development of this rapidly evolving research field. This review article aims to be comprehensive, authoritative, and critical with wide general interest to the science community, summarizing recent advances in revealing the formation processes, composition, and structures of biomineralized tissues, providing in-depth insights into guidelines derived from biomineralized tissues for the design and construction of high-performance biomimetic materials, and discussing recent progress, current research trends, key problems, future main research directions and challenges, and future perspectives in this exciting and rapidly evolving research field.

Pb Removal Efficiency by Calcium Carbonates: Biogenic versus Abiogenic Materials

The sorption of heavy metals on mineral surfaces plays a key role in controlling the fate and bioavailability of harmful elements through dissolution-precipitation reactions. Here, we investigate the efficiency of Pb removal from highly contaminated waters by two calcium carbonate hard tissues, scallop shells (up to 99.9 mol %; -biocalcite) and cuttlefish bones (up to 90.0 mol %; bioaragonite), which template the precipitation of the highly insoluble mineral cerussite (PbCO3). The experiments show that both biomaterials are about five times more effective Pb scavengers (5 mmol of cerussite precipitated/g sample) than their inorganic counterparts (∼1 mmol/g). We relate this enhanced Pb scavenging capacity of biocarbonates to their composite organic-inorganic nature, which modulates their specific nano- and microstructural features and defines their larger surface areas, solubility, and reactivity compared to those of their inorganic counterparts. The oriented growth of cerussite progressively passivates the bioaragonite surface, reducing its long-term Pb scavenging capacity. In contrast, the randomly oriented growth of cerussite crystals on biocalcite prevents surface passivation and explains why biocalcite outperforms bioaragonite as a long-term Pb scavenger. The use of biocarbonates could be a key for designing more efficient decontamination strategies for heavy metal-polluted waters.

Avian obligate brood parasitic lineages evolved variable complex polycrystalline structures to build tougher eggshells

Avian brood parasites and their hosts display varied egg-puncture behaviors, exerting asymmetric co-evolutionary selection pressures on eggshells' breaking strength. We investigated eggshell structural and textural characteristics that may improve its mechanical performance. Parasitic eggshell calcified layers showed complex ultra- and microstructural patterns. However, stronger parasitic eggshells are not due to lower textural severity (characterized by lower preferred crystallographic orientation, larger local grain misorientation and smaller Kearns factor), but rather to grain boundary (GB) microstructure characteristics within the eggshell outermost layer (palisade layer, PL). Accordingly, the thicker the PL and the more complex the GB pathways are, the tougher the parasitic eggshells will be. These characteristics, which we can identify as a "GB Engineering" driven co-evolutionary process, further improve eggshell breaking strength in those parasitic species that suffer relatively high frequencies of egg-puncturing by congeneric or hosts. Overall, plain textural patterns are not suitable predictors for comparing mechanical performance of bioceramic materials.

A Molecular-Scale Understanding of Misorientation Toughening in Corals and Seashells

Biominerals are organic-mineral composites formed by living organisms. They are the hardest and toughest tissues in those organisms, are often polycrystalline, and their mesostructure (which includes nano- and microscale crystallite size, shape, arrangement, and orientation) can vary dramatically. Marine biominerals may be aragonite, vaterite, or calcite, all calcium carbonate (CaCO₃ ) polymorphs, differing in crystal structure. Unexpectedly, diverse CaCO₃ biominerals such as coral skeletons and nacre share a similar characteristic: Adjacent crystals are slightly misoriented. This observation is documented quantitatively at the micro- and nanoscales, using polarization-dependent imaging contrast mapping (PIC mapping), and the slight misorientations is consistently between 1° and 40°. Nanoindentation shows that both polycrystalline biominerals and abiotic synthetic spherulites are tougher than single-crystalline geologic aragonite, and molecular dynamics (MD) simulations of bicrystals at the molecular scale reveals that aragonite, vaterite, and calcite exhibit toughness maxima when the bicrystals are misoriented by 10°, 20°, and 30°, respectively, demonstrating that slight misorientation alone can increase fracture toughness. Slight-misorientation-toughening can be harnessed for synthesis of bioinspired materials that only require one material, are not limited to specific top-down architecture, and are easily achieved by self-assembly of organic molecules (e.g., aspirin, chocolate), polymers, metals, and ceramics well beyond biominerals.

Matrix-Directed Mineralization for Bulk Structural Materials

Mineral-based bulk structural materials (MBSMs) are known for their long history and extensive range of usage. The inherent brittleness of minerals poses a major problem to the performance of MBSMs. To overcome this problem, design principles have been extracted from natural biominerals, in which the extraordinary mechanical performance is achieved via the hierarchical organization of minerals and organics. Nevertheless, precise and efficient fabrication of MBSMs with bioinspired hierarchical structures under mild conditions has long been a big challenge. This Perspective provides a panoramic view of an emerging fabrication strategy, matrix-directed mineralization, which imitates the in vivo growth of some biominerals. The advantages of the strategy are revealed by comparatively analyzing the conventional fabrication techniques of artificial hierarchically structured MBSMs and the biomineral growth processes. By introducing recent advances, we demonstrate that this strategy can be used to fabricate artificial MBSMs with hierarchical structures. Particular attention is paid to the mass transport and the precursors that are involved in the mineralization process. We hope this Perspective can provide some inspiring viewpoints on the importance of biomimetic mineralization in material fabrication and thereby spur the biomimetic fabrication of high-performance MBSMs.

Tuning the Magnetization of Manganese (II) Carbonate by Intracrystalline Amino Acids

Incorporation of organic molecules into the lattice of inorganic crystalline hosts is a common phenomenon in biomineralization and is shown to alter various properties of the crystalline host. Taking this phenomenon as a source of inspiration, it is shown herein that incorporation of specific single amino acids into the lattice of manganese (II) carbonate strongly alters its inherent magnetic properties. At room temperature, the magnetic susceptibility of the amino-acid-incorporating paramagnetic MnCO₃ decreases, following a simple rule of mixtures. When cooled below the Néel temperature, however, the opposite trend is observed, namely an increase in magnetic susceptibility measured in a high magnetic field. Such an increase, accompanied by a drastic change in the Néel phase transformation temperature, results from a decrease in MnCO₃ orbital overlapping and the weakening of superexchange interactions. It may be that this is the first time that the magnetic properties of a host crystal are tuned via the incorporation of amino acids.

Untangling the Mechanisms of Lattice Distortions in Biogenic Crystals across Scales

Biomineralized structures are complex functional hierarchical assemblies composed of biomineral building blocks joined together by an organic phase. The formation of individual mineral units is governed by the cellular tissue component that orchestrates the process of biomineral nucleation, growth, and morphogenesis. These processes are imprinted in the structural, compositional, and crystallographic properties of the emerging biominerals on all scales. Measurement of these properties can provide crucial information on the mechanisms that are employed by the organism to form these complex 3D architectures and to unravel principles of their functionality. Nevertheless, so far, this has only been possible at the macroscopic scale, by averaging the properties of the entire composite assembly, or at the mesoscale, by looking at extremely small parts of the entire picture. In this study, the newly developed synchrotron-based dark-field X-ray microscopy method is employed to study the link between 3D crystallographic properties of relatively large calcitic prisms in the shell of the mollusc Pinna nobilis and their local lattice properties with extremely high angular resolution down to 0.001°. Mechanistic links between variations in local lattice parameters and spacing, crystal orientation, chemical composition, and the deposition process of the entire mineral unit are unraveled.

Multiscale mechanical consequences of ocean acidification for cold-water corals

Ocean acidification is a threat to deep-sea corals and could lead to dramatic and rapid loss of the reef framework habitat they build. Weakening of structurally critical parts of the coral reef framework can lead to physical habitat collapse on an ecosystem scale, reducing the potential for biodiversity support. The mechanism underpinning crumbling and collapse of corals can be described via a combination of laboratory-scale experiments and mathematical and computational models. We synthesise data from electron back-scatter diffraction, micro-computed tomography, and micromechanical experiments, supplemented by molecular dynamics and continuum micromechanics simulations to predict failure of coral structures under increasing porosity and dissolution. Results reveal remarkable mechanical properties of the building material of cold-water coral skeletons of 462 MPa compressive strength and 45-67 GPa stiffness. This is 10 times stronger than concrete, twice as strong as ultrahigh performance fibre reinforced concrete, or nacre. Contrary to what would be expected, CWCs retain the strength of their skeletal building material despite a loss of its stiffness even when synthesised under future oceanic conditions. As this is on the material length-scale, it is independent of increasing porosity from exposure to corrosive water or bioerosion. Our models then illustrate how small increases in porosity lead to significantly increased risk of crumbling coral habitat. This new understanding, combined with projections of how seawater chemistry will change over the coming decades, will help support future conservation and management efforts of these vulnerable marine ecosystems by identifying which ecosystems are at risk and when they will be at risk, allowing assessment of the impact upon associated biodiversity.

Biomineralized Materials as Model Systems for Structural Composites: Intracrystalline Structural Features and Their Strengthening and Toughening Mechanisms

Biomineralized composites, which are usually composed of microscopic mineral building blocks organized in 3D intercrystalline organic matrices, have evolved unique structural designs to fulfill mechanical and other biological functionalities. While it has been well recognized that the intricate architectural designs of biomineralized composites contribute to their remarkable mechanical performance, the structural features within and corresponding mechanical properties of individual mineral building blocks are often less appreciated in the context of bio-inspired structural composites. The mineral building blocks in biomineralized composites exhibit a variety of salient intracrystalline structural features, such as, organic inclusions, inorganic impurities (or trace elements), crystalline features (e.g., amorphous phases, single crystals, splitting crystals, polycrystals, and nanograins), residual stress/strain, and twinning, which significantly modify the mechanical properties of biogenic minerals. In this review, recent progress in elucidating the intracrystalline structural features of three most common biomineral systems (calcite, aragonite, and hydroxyapatite) and their corresponding mechanical significance are discussed. Future research directions and corresponding challenges are proposed and discussed, such as the advanced structural characterizations and formation mechanisms of intracrystalline structures in biominerals, amorphous biominerals, and bio-inspired synthesis.

High-Mg calcite nanoparticles within a low-Mg calcite matrix: A widespread phenomenon in biomineralization

Biominerals are extraordinarily intricate and possess superior mechanical properties compared with their synthetic counterparts. In this study, we show that the presence of high-Mg calcite nanoparticles within a low-Mg calcite matrix is a widespread phenomenon among marine organisms whose skeletons are composed of high-Mg calcite. It seems most likely that formation of such a complex structure is possible because of the phase separation that occurs as a result of spinodal decomposition of an amorphous Mg–calcium carbonate precursor and is followed by crystallization. We demonstrate that the basis of such phase separation stems from chemical composition rather than from biological similarities. The presence of high-Mg calcite nanoparticles increases the skeletons’ toughness and hardness.

During the process of biomineralization, organisms utilize various biostrategies to enhance the mechanical durability of their skeletons. In this work, we establish that the presence of high-Mg nanoparticles embedded within lower-Mg calcite matrices is a widespread strategy utilized by various organisms from different kingdoms and phyla to improve the mechanical properties of their high-Mg calcite skeletons. We show that such phase separation and the formation of high-Mg nanoparticles are most probably achieved through spinodal decomposition of an amorphous Mg-calcite precursor. Such decomposition is independent of the biological characteristics of the studied organisms belonging to different phyla and even kingdoms but rather, originates from their similar chemical composition and a specific Mg content within their skeletons, which generally ranges from 14 to 48 mol % of Mg. We show evidence of high-Mg calcite nanoparticles in the cases of six biologically different organisms all demonstrating more than 14 mol % Mg-calcite and consider it likely that this phenomenon is immeasurably more prevalent in nature. We also establish the absence of these high-Mg nanoparticles in organisms whose Mg content is lower than 14 mol %, providing further evidence that whether or not spinodal decomposition of an amorphous Mg-calcite precursor takes place is determined by the amount of Mg it contains. The valuable knowledge gained from this biostrategy significantly impacts the understanding of how biominerals, although composed of intrinsically brittle materials, can effectively resist fracture. Moreover, our theoretical calculations clearly suggest that formation of Mg-rich nanoprecipitates greatly enhances the hardness of the biomineralized tissue as well.

Mineralization generates megapascal contractile stresses in collagen fibrils

During bone formation, collagen fibrils mineralize with carbonated hydroxyapatite, leading to a hybrid material with excellent properties. Other minerals are also known to nucleate within collagen in vitro. For a series of strontium- and calcium-based minerals, we observed that their precipitation leads to a contraction of collagen fibrils, reaching stresses as large as several megapascals. The magnitude of the stress depends on the type and amount of mineral. Using in-operando synchrotron x-ray scattering, we analyzed the kinetics of mineral deposition. Whereas no contraction occurs when the mineral deposits outside fibrils only, intrafibrillar mineralization generates fibril contraction. This chemomechanical effect occurs with collagen fully immersed in water and generates a mineral-collagen composite with tensile fibers, reminiscent of the principle of reinforced concrete.

Uncovering the crystal defects within aragonite CaCO3

Knowledge of deformation mechanisms in aragonite, one of the three crystalline polymorphs of CaCO3, is essential to understand the overall excellent mechanical performance of nacres. Dislocation slip and deformation twinning were claimed previously as plasticity carriers in aragonite, but crystallographic features of dislocations and twins have been poorly understood. Here, utilizing various transmission electron microscopy techniques, we reveal the atomic structures of twins, partial dislocations, and associated stacking faults. Combining a topological model and density functional theory calculations, we identify complete twin elements, characters of twinning disconnection, and the corresponding twin shear angle (∼8.8°) and rationalize unique partial dislocations as well. Additionally, we reveal an unreported potential energy dissipation mode within aragonite, namely, the formation of nanograins via the pile-up of partial dislocations. Based on the microstructural comparisons of biogenic and abiotic aragonite, we find that the crystallographic features of twins are the same. However, the twin density is much lower in abiotic aragonite due to the vastly different crystallization conditions, which in turn are likely due to the absence of organics, high temperature and pressure differences, the variation in inorganic impurities, or a combination thereof. Our findings enrich the knowledge of intrinsic crystal defects that accommodate plastic deformation in aragonite and provide insights into designing bioengineering materials with better strength and toughness.

Amorphous-to-crystal transition in the layer-by-layer growth of bivalve shell prisms

Biomineralization integrates complex physical and chemical processes bio-controlled by the living organisms through ionic concentration regulation and organic molecules production. It allows tuning the structural, optical and mechanical properties of hard tissues during ambient-condition crystallisation, motivating a deeper understanding of the underlying processes. By combining state-of-the-art optical and X-ray microscopy methods, we investigated early-mineralized calcareous units from two bivalve species, Pinctada margaritifera and Pinna nobilis, revealing chemical and crystallographic structural insights. In these calcite units, we observed ring-like structural features correlated with a lack of calcite and an increase of amorphous calcium carbonate and proteins contents. The rings also correspond to a larger crystalline disorder and a larger strain level. Based on these observations, we propose a temporal biomineralization cycle, initiated by the production of an amorphous precursor layer, which further crystallizes with a transition front progressing radially from the unit centre, while the organics are expelled towards the prism edge. Simultaneously, along the shell thickness, the growth occurs following a layer-by-layer mode. These findings open biomimetic perspectives for the design of refined crystalline materials. STATEMENT OF SIGNIFICANCE: Calcareous biominerals are amongst the most present forms of biominerals. They exhibit astonishing structural, optical and mechanical properties while being formed at ambient synthesis conditions from ubiquitous ions, motivating the deep understanding of biomineralization. Here, we unveil the first formation steps involved in the biomineralization cycle of prismatic units of two bivalve species by applying a new multi-modal non-destructive characterization approach, sensitive to chemical and crystalline properties. The observations of structural features in mineralized units of different ages allowed the derivation of a temporal sequence for prism biomineralization, involving an amorphous precursor, a radial crystallisation front and a layer-by-layer sequence. Beyond these chemical and physical findings, the herein introduced multi-modal approach is highly relevant to other biominerals and bio-inspired studies.

Biogenic Guanine Crystals Are Solid Solutions of Guanine and Other Purine Metabolites

Highly reflective crystals of the nucleotide base guanine are widely distributed in animal coloration and visual systems. Organisms precisely control the morphology and organization of the crystals to optimize different optical effects, but little is known about how this is achieved. Here we examine a fundamental question that has remained unanswered after over 100 years of research on guanine: what are the crystals made of? Using solution-state and solid-state chemical techniques coupled with structural analysis by powder XRD and solid-state NMR, we compare the purine compositions and the structures of seven biogenic guanine crystals with different crystal morphologies, testing the hypothesis that intracrystalline dopants influence the crystal shape. We find that biogenic "guanine" crystals are not pure crystals but molecular alloys (aka solid solutions and mixed crystals) of guanine, hypoxanthine, and sometimes xanthine. Guanine host crystals occlude homogeneous mixtures of other purines, sometimes in remarkably large amounts (up to 20% of hypoxanthine), without significantly altering the crystal structure of the guanine host. We find no correlation between the biogenic crystal morphology and dopant content and conclude that dopants do not dictate the crystal morphology of the guanine host. The ability of guanine crystals to host other molecules enables animals to build physiologically "cheaper" crystals from mixtures of metabolically available purines, without impeding optical functionality. The exceptional levels of doping in biogenic guanine offer inspiration for the design of mixed molecular crystals that incorporate multiple functionalities in a single material.

Artificial Nacre with High Toughness Amplification Factor: Residual Stress-Engineering Sparks Enhanced Extrinsic Toughening Mechanisms

The high fracture toughness of mollusk nacre is predominantly attributed to the structure-associated extrinsic mechanisms such as platelet sliding and crack deflection. While the nacre-mimetic structures are widely adopted in artificial ceramics, the extrinsic mechanisms are often weakened by the relatively low tensile strength of the platelets with a large aspect ratio, which makes the fracture toughness of these materials much lower than expected. Here, it is demonstrated that the fracture toughness of artificial nacre materials with high inorganic contents can be improved by residual stress-induced platelet strengthening, which can catalyze more effective extrinsic toughening mechanisms that are specific to the nacre-mimetic structures. Thereby, while the absolute fracture toughness of the materials is not comparable with advanced ceramic-based composites, the toughness amplification factor of the material reaches 16.1 ± 1.1, outperforming the state-of-the-art biomimetic ceramics. The results reveal that, with the merit of nacre-mimetic structural designs, the overall fracture toughness of the artificial nacre can be improved by the platelet strengthening through extrinsic toughening mechanisms, although the intrinsic fracture toughness may decrease at platelet level due to the strengthening. It is anticipated that advanced structural ceramics with exceeding performance can be fabricated through these unconventional strategies.

A Prestressing Strategy Enabled Synergistic Energy-Dissipation in Impact-Resistant Nacre-Like Structures

The application of prestresses is a valuable strategy for enhancing the overall mechanical performances of structural materials. Residual stresses, acting as prestresses, exist naturally in biological structural materials, such as the nacre with the 3D "brick-and-mortar" arrangement. Although regulation of the tablets sliding has recently been demonstrated to be vital to improve toughness in synthetic nacre-like structures, the effects of prestresses on the tablets-sliding mechanism in these nacre-like structures remain unclear. Here, by a combination of simulation, additive manufacturing, and drop tower testing the authors reveal that, at a critical prestress, synergistic effects between the prestress-enhanced tablets sliding and prestress-weakened structural integrality result in optimized impact resistance of nacre-like structures. Furthermore, the prestressing strategy is easily implemented to a designed nacre-inspired separator to enhance the impact resistance of lithium batteries. The findings demonstrate that the prestressing strategy combined with bioinspired architectures can be exploited for enhancing the impact resistance of engineering structural materials and energy storage devices.

Understanding the Adhesion Mechanism of Hydroxyapatite-Binding Peptide

Understanding the interactions between the protein collagen and hydroxyapatite is of high importance for understanding biomineralization and bone formation. Here, we undertook a reductionist approach and studied the interactions between a short peptide and hydroxyapatite. The peptide was selected from a phage-display library for its high affinity to hydroxyapatite. To study its interactions with hydroxyapatite, we performed an alanine scan to determine the contribution of each residue. The interactions of the different peptide derivatives were studied using a quartz crystal microbalance with dissipation monitoring and with single-molecule force spectroscopy by atomic force microscopy. Our results suggest that the peptide binds via electrostatic interactions between cationic moieties of the peptide and the negatively charged groups on the crystal surface. Furthermore, our findings show that cationic residues have a crucial role in binding. Using molecular dynamics simulations, we show that the peptide structure is a contributing factor to the adhesion mechanism. These results suggest that even small conformational changes can have a significant effect on peptide adhesion. We suggest that a bent structure of the peptide allows it to strongly bind hydroxyapatite. The results presented in this study improve our understanding of peptide adhesion to hydroxyapatite. On top of physical interactions between the peptide and the surface, peptide structure contributes to adhesion. Unveiling these processes contributes to our understanding of more complex biological systems. Furthermore, it may help in the design of de novo peptides to be used as functional groups for modifying the surface of hydroxyapatite.

Prestressing Strategy for Strengthening Biocomposites: A Numerical Study

Natural materials developed in complex architectures that comprise hard and soft phases often display extraordinary mechanical properties, such as the combination of high strength and toughness. Besides the structural arrangements, residual stress is ubiquitous in those materials. Although evidence shows its significant role in the functionalities and properties of the composites, good or bad, residual stress is not fully understood and utilized. In this study, we show through extensive numerical simulations the role of the prestress in strengthening typical brick-and-mortar biocomposites. We investigate the influence of the prestressing modes, as well as the geometrical and material parameters. The results promise a deep understanding of the relation between the prestress and the material strength and may inspire a new dimension of material design.

Structural and chemical variations in Mg-calcite skeletal segments of coralline red algae lead to improved crack resistance

The calcareous alga Jania sp. is an articulated coralline red seaweed that is abundant in the shallow waters of oceans worldwide. We have previously demonstrated that its structure is highly intricate and exhibits hierarchical organization across multiple length scales from the macro to the nano scale. Moreover, we have proven that the inner pores of its structure are helical, conveying the alga greater compliance as compared to a cylindrical configuration. Herein, we reveal new insights into the structure of Jania sp., particularly, its crystallographic variations and the internal elemental distribution of Mg and Ca. We show that the high-Mg calcite cell wall nanocrystals of Jania sp. are arranged in layers with alternating Mg contents. Moreover, we show that this non-homogenous elemental distribution assists the alga in preventing fracture caused by crack propagation. We further reveal that each one of the cell wall nanocrystals in Jania sp. is not a single crystal as was previously thought, but rather comprises Mg-rich calcite nanoparticles demonstrating various crystallographic orientations, arranged periodically within the layered structure. We also show that these Mg-rich nanoparticles are present in yet another species of the coralline red algae, Corallina sp., pointing to the generality of this phenomenon. To the best of our knowledge this is a first report on the existence of Mg-rich nanoparticles in algal mineralized tissue. We envisage that our findings on the bio-strategy found in the algae to enhance their fracture toughness will have an impact on the design of structures with superior mechanical properties. STATEMENT OF SIGNIFICANCE: Understanding the structure-property relation in biomineralized tissues is of great importance in unveiling Nature's material design strategies, which form the basis for the development of novel structural materials. Crystallographic and elemental variations in the skeletal parts of the coralline red algae and their cumulative contribution to prevention of mechanical failure are yet poorly studied. Herein, we reveal that the high-Mg calcite cell wall nanocrystals of Jania sp. are arranged in layers with alternating Mg concentrations and that this organization facilitates crack deflection, thereby preventing catastrophic fracture. We further discovered that the nanocrystals contain incoherent Mg-rich nanoparticles and suggest that they form via spinodal decomposition of the Mg-ACC precursor and self-arrange periodically throughout the alga's mineralized cell wall, a phenomenon most likely to be widespread in high-Mg calcite biomineralization.

Long-term stabilized amorphous calcium carbonate-an ink for bio-inspired 3D printing

Biominerals formed by organisms in the course of biomineralization often demonstrate complex morphologies despite their single-crystalline nature. This is achieved owing to the crystallization via a predeposited amorphous calcium carbonate (ACC) phase, a precursor that is particularly widespread in biominerals. Inspired by this natural strategy, we used robocasting, an additive manufacturing three-dimensional (3D) printing technique, for printing 3D objects from novel long-term, Mg-stabilized ACC pastes with high solids loading. We demonstrated, for the first time, that the ACC remains stable for at least a couple of months, even after printing. Crystallization, if desired, occurs only after the 3D object is already formed and at temperatures significantly lower than those of common postprinting sintering. We also examined the effects different organic binders have on the crystallization, the morphology, and the final amount of incorporated Mg. This novel bio-inspired method may pave the way for a new bio-inspired route to low-temperature 3D printing of ceramic materials for a multitude of applications.

Helical Microstructures of the Mineralized Coralline Red Algae Determine Their Mechanical Properties

Through controlled biomineralization, organisms yield complicated structures with specific functions. Here, Jania sp., an articulated coralline red alga that secretes high-Mg calcite as part of its skeleton, is in focus. It is shown that Jania sp. exhibits a remarkable structure, which is highly porous (with porosity as high as 64 vol%) and reveals several hierarchical orders from the nano to the macroscale. It is shown that the structure is helical, and proven that its helical configuration provides the alga with superior compliance that allows it to adapt to stresses in its natural environment. Thus, the combination of high porosity and a helical configuration result in a sophisticated, light-weight, compliant structure. It is anticipated that the findings on the advantages of such a structure are likely to be of value in the design or improvement of lightweight structures with superior mechanical properties.

From spinodal decomposition to alternating layered structure within single crystals of biogenic magnesium calcite

As organisms can form crystals only under ambient conditions, they demonstrate fascinating strategies to overcome this limitation. Recently, we reported a previously unknown biostrategy for toughening brittle calcite crystals, using coherently incorporated Mg-rich nanoprecipitates arranged in a layered manner in the lenses of a brittle star, Ophiocoma wendtii. Here we propose the mechanisms of formation of this functional hierarchical structure under conditions of ambient temperature and limited solid diffusion. We propose that formation proceeds via a spinodal decomposition of a liquid or gel-like magnesium amorphous calcium carbonate (Mg-ACC) precursor into Mg-rich nanoparticles and a Mg-depleted amorphous matrix. In a second step, crystallization of the decomposed amorphous precursor leads to the formation of high-Mg particle-rich layers. The model is supported by our experimental results in synthetic systems. These insights have significant implications for fundamental understanding of the role of Mg-ACC material transformation during crystallization and its subsequent stability.

Visualization of the effect of additives on the nanostructures of individual bio-inspired calcite crystals

Soluble additives provide a versatile strategy for controlling crystallization processes, enabling selection of properties including crystal sizes, morphologies, and structures. The additive species can also be incorporated within the crystal lattice, leading for example to enhanced mechanical properties. However, while many techniques are available for analyzing particle shape and structure, it remains challenging to characterize the structural inhomogeneities and defects introduced into individual crystals by these additives, where these govern many important material properties. Here, we exploit Bragg coherent diffraction imaging to visualize the effects of soluble additives on the internal structures of individual crystals on the nanoscale. Investigation of bio-inspired calcite crystals grown in the presence of lysine or magnesium ions reveals that while a single dislocation is observed in calcite crystals grown in the presence of lysine, magnesium ions generate complex strain patterns. Indeed, in addition to the expected homogeneous solid solution of Mg ions in the calcite lattice, we observe two zones comprising alternating lattice contractions and relaxation, where comparable alternating layers of high magnesium calcite have been observed in many magnesium calcite biominerals. Such insight into the structures of nanocomposite crystals will ultimately enable us to understand and control their properties.

Incorporation of organic and inorganic impurities into the lattice of metastable vaterite

Highly substituted Mg-vaterite and Ba-vaterite were synthesized in the presence of aspartic acid and characterized by means of synchrotron XRD.