Atomic-scale compositional mapping reveals Mg-rich amorphous calcium phosphate in human dental enamel

Advancing dentin remineralization: Exploring amorphous calcium phosphate and its stabilizers in biomimetic approaches

OBJECTIVE

This review elucidates the mechanisms underpinning intrafibrillar mineralization, examines various amorphous calcium phosphate (ACP) stabilizers employed in dentin's intrafibrillar mineralization, and addresses the challenges encountered in clinical applications of ACP-based bioactive materials.

METHODS

The literature search for this review was conducted using three electronic databases: PubMed, Web of Science, and Google Scholar, with specific keywords. Articles were selected based on inclusion and exclusion criteria, allowing for a detailed examination and summary of current research on dentin remineralization facilitated by ACP under the influence of various types of stabilizers.

RESULTS

This review underscores the latest advancements in the role of ACP in promoting dentin remineralization, particularly intrafibrillar mineralization, under the regulation of various stabilizers. These stabilizers predominantly comprise non-collagenous proteins, their analogs, and polymers. Despite the diversity of stabilizers, the mechanisms they employ to enhance intrafibrillar remineralization are found to be interrelated, indicating multiple driving forces behind this process. However, challenges remain in effectively designing clinically viable products using stabilized ACP and maximizing intrafibrillar mineralization with limited materials in practical applications.

SIGNIFICANCE

The role of ACP in remineralization has gained significant attention in dental research, with substantial progress made in the study of dentin biomimetic mineralization. Given ACP's instability without additives, the presence of ACP stabilizers is crucial for achieving in vitro intrafibrillar mineralization. However, there is a lack of comprehensive and exhaustive reviews on ACP bioactive materials under the regulation of stabilizers. A detailed summary of these stabilizers is also instrumental in better understanding the complex process of intrafibrillar mineralization. Compared to traditional remineralization methods, bioactive materials capable of regulating ACP stability and controlling release demonstrate immense potential in enhancing clinical treatment standards.

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.

Remineralizing effects of hydroxypropyl methylcellulose film-loaded amorphous calcium phosphate nanoprecursors on enamel artificial caries lesions

OBJECTIVES

This study was to investigate hydroxypropyl methylcellulose (HPMC) film as a carrier for amorphous fluorinated calcium phosphate (AFCP) nanoprecursors to continuously deliver biomimetic remineralization of enamel artificial caries lesions (ACL).

MATERIALS AND METHODS

The AFCP/HPMC films were comprised of 25 wt% AFCP nanoparticles and 75 wt% HPMC. They were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and biocompatibility tests. Forty enamel ACL were prepared and randomly divided into four groups (n = 10): The enamel surfaces were covered with a pure HPMC film, Tooth Mousse Plus (contains 10% CPP-ACP and 0.2% NaF), and AFCP/HPMC film, or without any things (serving as negative control). Subsequently, all samples were alternatively kept in artificial saliva and a modified pH-cycling before they were characterized by Micro-CT, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), attenuated total reflectance (ATR)-FTIR, XRD, and nanoindentation.

RESULTS

After the enamel ACL was challenged by pH cycling, Tooth Mousse Plus and AFCP/HPMC film groups exhibited less lesion depth and mineral loss than the negative control and pure HPMC film groups. Additionally, the AFCP/HPMC film group revealed a highest remineralization rate of 55.34 ± 3.10 % among the all groups (p < 0.001). The SEM findings showed that the enamel ACL were densely deposited with minerals in the AFCP/HPMC film group, and the EDX results suggested a higher content of fluorine in the remineralized tissues. In particular, the AFCP/HPMC film group exhibited the best nanomechanical performance after 2 weeks of pH cycling (p < 0.05), with the hardness (H) restored from 0.29 ± 0.19 to 2.69 ± 0.70 GPa, and elastic modulus (Er) restored from 10.77 ± 5.30 to 68.83 ± 12.72 GPa.

CONCLUSION

The AFCP/HPMC film might be used as a promising strategy for arresting or reversing incipient enamel caries lesions.

Contribution to Knowledge on Bioapatites: Does Mg Level Reflect the Organic Matter and Water Contents of Enamel?

The matter constituting the enamels of four types of organisms was studied. The variability of the ions was presented in molar units. It was proven that the changes in water contents of the enamel are significantly positively related to changes in Mg; inversely, there is also a strong connection with changes in Ca and P, the main components of bioapatite. The variability in the organic matter has the same strong and positive characteristics and is also coupled with changes in Mg contents. Amelogenins in organic matter, which synthesize enamel rods, likely have a role in adjusting the amount of Mg, thus establishing the amount of organic matter and water in the whole enamel; this adjustment occurs through an unknown mechanism. Ca, P, Mg, and Cl ions, as well as organic matter and water, participate in the main circulation cycle of bioapatites. The selection of variations in the composition of bioapatite occurs only along particular trajectories, where the energy of transformation linearly depends on the following factors: changes in the crystallographic d parameter; the increase in the volume, V, of the crystallographic cell; the momentum transfer, which is indirectly expressed by ΔsinΘ value. To our knowledge, these findings are novel in the literature. The obtained results indicate the different chemical and crystallographic affinities of the enamels of selected animals to the human ones. This is essential when animal bioapatites are transformed into dentistic or medical substitutes for the hard tissues. Moreover, the role of Mg is shown to control the amount of water in the apatite and in detecting organic matter in the enamels.

How Amorphous Nanomaterials Enhanced Electrocatalytic, SERS, and Mechanical Properties

There is ever-growing research interest in nanomaterials because of the unique properties that emerge on the nanometer scale. While crystalline nanomaterials have received a surge of attention for exhibiting state-of-the-art properties in various fields, their amorphous counterparts have also attracted attention in recent years owing to their unique structural features that crystalline materials lack. In short, amorphous nanomaterials only have short-range order at the atomic scale, and their atomic packing lacks long-range periodic arrangement, in which the coordinatively unsaturated environment, isotropic atomic structure, and modulated electron state all contribute to their outstanding performance in various applications. Given their intriguing characteristics, we herein present a series of representative works to elaborate on the structural advantages of amorphous nanomaterials as well as their enhanced electrocatalytic, surface-enhanced Raman scattering (SERS), and mechanical properties, thereby elucidating the underlying structure-function relationship. We hope that this proposed relationship will be universally applicable, thus encouraging future work in the design of amorphous materials that show promising performance in a wide range of fields.

Fluorapatite nanorod arrays with enamel-like bundle structure regulated by iron ions

Pigmented rodent tooth enamel is mainly composed of parallel hydroxyapatite nanorods and a small amount of organic matrix. These hydroxyapatite nanorods tend to be carbonated and contain traces of iron, fluorine, and magnesium. The pigmented rodent tooth enamel which contains trace iron is stronger and more resistant to acid corrosion than unpigmented rodent enamel, which could provide inspiration for the preparation and synthesis of high performance and corrosion resistant artificial materials. However, the regulatory role and mechanical enhancement of iron ions in enamel growth are unclear. Here, we synthesized enamel-like fluorapatite nanorod arrays in vitro using a mineralization technique at room-temperature. To investigate the regulatory effect of iron ions on the fluorapatite nanorod arrays (FAP-Fe), the phosphate solution is slowly transfused dropwise in the calcium ion solution, and different concentrations of iron ions are added to the calcium ion solution in advance. We demonstrated that fluorapatite nanorod arrays (FAP) can be epitaxially grown from amorphous calcium phosphate nanoparticles and iron ions can improve the microstructure of FAP nanorod arrays and obtain the same enamel bundle structure as the natural enamel. Moreover, high concentration of iron ions can inhibit the crystallization of fluorapatite. The FAP-Fe nanorod arrays controlled by 0.02 mM Fe3+ have good mechanical properties. Their hardness is 1.34 ± 0.02 GPa and Young's modulus is 65.3 ± 0.4 GPa, respectively. This work is helpful to understand the role of trace elements in natural enamel in the regulation of enamel formation and to provide a theoretical foundation for the preparation of high strength artificial composites, which can play a greater role in the fields of biological alternative materials, anti-oil coating, oil/water separation, anti-bioadhesion and so on.

Formation of Amorphous Iron-Calcium Phosphate with High Stability

Amorphous iron-calcium phosphate (Fe-ACP) plays a vital role in the mechanical properties of teeth of some rodents, which are very hard, but its formation process and synthetic route remain unknown. Here, the synthesis and characterization of an iron-bearing amorphous calcium phosphate in the presence of ammonium iron citrate (AIC) are reported. The iron is distributed homogeneously on the nanometer scale in the resulting particles. The prepared Fe-ACP particles can be highly stable in aqueous media, including water, simulated body fluid, and acetate buffer solution (pH 4). In vitro study demonstrates that these particles have good biocompatibility and osteogenic properties. Subsequently, Spark Plasma Sintering (SPS) is utilized to consolidate the initial Fe-ACP powders. The results show that the hardness of the ceramics increases with the increase of iron content, but an excess of iron leads to a rapid decline in hardness. Calcium iron phosphate ceramics with a hardness of 4 GPa can be achieved, which is higher than that of human enamel. Furthermore, the ceramics composed of iron-calcium phosphates show enhanced acid resistance. This study provides a novel route to prepare Fe-ACP, and presents the potential role of Fe-ACP in biomineralization and as starting material to fabricate acid-resistant high-performance bioceramics.

High-resolution Raman spectroscopy reveals compositional differences between pigmented incisor enamel and unpigmented molar enamel in Rattus norvegicus

Dental enamel is a peculiar biological tissue devoid of any self-renewal capacity as opposed to bone. Thus, a thorough understanding of enamel composition is essential to develop novel strategies for dental enamel repair. While the mineral found in bone and dental enamel is generally viewed as the biologically-produced equivalent of hydroxy(l)apatite, the formation of these bioapatites is controlled by different organic matrix frameworks-mainly type-I collagen in bone and amelogenin in enamel. In lower vertebrates, such as rodents, two distinct types of enamel are produced. Iron-containing pigmented enamel protects the continuously growing incisor teeth while magnesium-rich unpigmented enamel covers the molar teeth. Using high-resolution Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy, this work explores the differences in acid phosphate (HPO₄^2-), carbonate (CO₃^2-), hydroxyl (OH^-), iron, and magnesium content of pigmented incisor enamel and unpigmented molar enamel of Sprague Dawley rats. Bundles of hydroxy(l)apatite nanowires comprise the enamel prisms, where prisms in pigmented enamel are wider and longer than those in unpigmented molars. In contrast to magnesium-rich unpigmented enamel, higher mineral crystallinity, and higher HPO₄^2- and OH^- levels are hallmark features of iron-rich pigmented enamel. Furthermore, the apparent absence of iron oxides or oxy(hydroxides) indicates that iron is introduced into the apatite lattice at the expense of calcium, albeit in amounts that do not alter the Raman signatures of the PO₄^3- internal modes. Compositional idiosyncrasies of iron-rich pigmented and nominally iron-free unpigmented enamel offer new insights into enamel biomineralisation supporting the notion that, in rodents, ameloblast function differs significantly between the incisors and the molars.

Cementum is key to periodontal tissue regeneration: A review on apatite microstructures for creation of novel cementum-based dental implants

The cementum is the outermost layer of hard tissue covering the dentin within the root portion of the teeth. It is the only hard tissue with a specialized structure and function that forms a part of both the teeth and periodontal tissue. As such, cementum is believed to be critical for periodontal tissue regeneration. In this review, we discuss the function and histological structure of the cementum to promote crystal engineering with a biochemical approach in cementum regenerative medicine. We review the microstructure of enamel and bone while discussing the mechanism underlying apatite crystal formation to infer the morphology of cementum apatite crystals and their complex structure with collagen fibers. Finally, the limitations of the current dental implant treatments in clinical practice are explored from the perspective of periodontal tissue regeneration. We anticipate the possibility of advancing periodontal tissue regenerative medicine via cementum regeneration using a combination of material science and biochemical methods.

Testing the Influence of Laser Pulse Energy and Rate in the Atom Probe Tomography Analysis of Minerals

The use of atom probe tomography (APT) for mineral analysis is contributing to fundamental studies in Earth Sciences. Meanwhile, the need for standardization of this technique is becoming evident. Pending the use of mineral standards, the optimization of analysis parameters is needed to facilitate the study of different mineral groups in terms of data collection and quality. The laser pulse rate and energy are variables that highly affect the atom evaporation process occurring during APT analysis, and their testing is important to forecast mineral behavior and obtain the best possible data. In this study, five minerals representative of major groups (albite, As-pyrite, barite, olivine, and monazite) were analyzed over a range of laser pulse energies (10-50 pJ) and rates (100-250 kHz) to assess output parameter quality and evaluate compositional estimate stoichiometry. Among the studied minerals, As-pyrite, with the higher thermal conductivity and lower band gap, was the most affected by the laser pulse variation. Chemical composition estimates equal or close to the general chemical formula were achieved for monazite and As-pyrite. The analysis of multihit events has proved to be the best strategy to verify the efficacy of the evaporation process and to evaluate the best laser pulse setting for minerals.

Resolving protein-mineral interfacial interactions during in vitro mineralization by atom probe tomography

Organic macromolecules exert remarkable control over the nucleation and growth of inorganic crystallites during (bio)mineralization, as exemplified during enamel formation where the protein amelogenin regulates the formation of hydroxyapatite (HAP). However, it is poorly understood how fundamental processes at the organic-inorganic interface, such as protein adsorption and/or incorporation into minerals, regulates nucleation and crystal growth due to technical challenges in observing and characterizing mineral-bound organics at high-resolution. Here, atom probe tomography techniques were developed and applied to characterize amelogenin-mineralized HAP particles in vitro, revealing distinct organic-inorganic interfacial structures and processes at the nanoscale. Specifically, visualization of amelogenin across the mineralized particulate demonstrates protein can become entrapped during HAP crystal aggregation and fusion. Identification of protein signatures and structural interpretations were further supported by standards analyses, i.e., defined HAP surfaces with and without amelogenin adsorbed. These findings represent a significant advance in the characterization of interfacial structures and, more so, interpretation of fundamental organic-inorganic processes and mechanisms influencing crystal growth. Ultimately, this approach can be broadly applied to inform how potentially unique and diverse organic-inorganic interactions at different stages regulates the growth and evolution of various biominerals.

Gradient Magnesium Content Affects Nanomechanics via Decreasing the Size and Crystallinity of Nanoparticles of Pseudoosteodentine of the Pacific Cutlassfish, Trichiurus lepturus Teeth

The formation of biomaterials such as enamel, dentin, and bone is important for many organisms, and the mechanical properties of biomaterials are affected by a wide range of structural and chemical factors. Special dentins exist in extant aquatic gnathostomes, and many more are present in fossils. When a layer of compact orthodentine surrounds the porous osteodentine core in the crown, the composite dentin is called pseudoosteodentine. Using various high-resolution analytical techniques, including micro-computed tomography (micro-CT), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and nanoindentation, we analyzed the micro- and nanostructures, chemical composition, and mechanical properties of pseudoosteodentine in the Pacific cutlassfish, Trichiurus lepturus teeth. Nanoscale oval-shaped hydroxyapatite (HA) crystals were distributed in a disordered manner in the pseudoosteodentine, and a cross-sectional analysis showed that the mineral crystallinity and crystalline particle size of the outer orthodentine were greater than those of middle and inner osteodentine. Moreover, the outer orthodentine comprised a mixture of smaller crystals and larger, more mature crystals. The nano-hardness and nano-stiffness of outer orthodentine were significantly higher than those of middle and inner osteodentine along a radical direction. The hardness and stiffness of pseudoosteodentine were inversely proportional to its magnesium (Mg) content. These data are consistent with the concept that Mg delays crystal maturation. The crystal size, crystallinity, nano-hardness, and nano-stiffness of pseudoosteodentine all decreased commensurately with the increase of its Mg concentration. The pseudoosteodentine of T. lepturus also can be regarded as a functional gradient material (FGM) because its mechanical properties are closely related to its chemical composition and nanostructure. Special pseudoosteodentine may therefore serve as a design standard for biomimetic synthetic mineral composites.

Nanoscale analysis of frozen honey by atom probe tomography

Three-dimensional reconstruction of the analysed volume is one of the main goals of atom probe tomography (APT) and can deliver nearly atomic resolution (~ 0.2 nm spatial resolution) and chemical information with a mass sensitivity down to the ppm range. Extending this technique to frozen biological systems would have an enormous impact on the structural analysis of biomolecules. In previous works, we have shown that it is possible to measure frozen liquids with APT. In this paper, we demonstrate the ability of APT to trace nanoscale precipitation in frozen natural honey. While the mass signals of the common sugar fragments C_xH_y and C_xO_yH_z overlap with (H₂O)_nH from water, we achieved correct stoichiometric values via different interpretation approaches for the peaks and thus determined the water content reliably. Next, we use honey to investigate the spatial resolution capabilities as a step toward the measurement of biological molecules in solution in 3D with sub-nanometer resolution. This may take analytical techniques to a new level, since methods of chemical characterization for cryogenic samples, especially biological samples, are still limited.

Advanced materials for enamel remineralization

Dental caries, a chronic and irreversible disease caused by caries-causing bacteria, has been listed as one of the three major human diseases to be prevented and treated. Therefore, it is critical to effectively stop the development of enamel caries. Remineralization treatment can control the progression of caries by inhibiting and reversing enamel demineralization at an early stage. In this process, functional materials guide the deposition of minerals on the damaged enamel, and the structure and hardness of the enamel are then restored. These remineralization materials have great potential for clinical application. In this review, advanced materials for enamel remineralization were briefly summarized, furthermore, an outlook on the perspective of remineralization materials were addressed.

Atomic-scale 3D imaging of individual dopant atoms in an oxide semiconductor

The physical properties of semiconductors are controlled by chemical doping. In oxide semiconductors, small variations in the density of dopant atoms can completely change the local electric and magnetic responses caused by their strongly correlated electrons. In lightly doped systems, however, such variations are difficult to determine as quantitative 3D imaging of individual dopant atoms is a major challenge. We apply atom probe tomography to resolve the atomic sites that donors occupy in the small band gap semiconductor Er(Mn,Ti)O₃ with a nominal Ti concentration of 0.04 at. %, map their 3D lattice positions, and quantify spatial variations. Our work enables atomic-level 3D studies of structure-property relations in lightly doped complex oxides, which is crucial to understand and control emergent dopant-driven quantum phenomena.

Small variations in the density of dopants change the physical properties of complex oxides. Here, the authors resolve doping levels in three dimension, imaging the atomic sites that donors occupy in the small band gap semiconductor Er(Mn,Ti)O₃.

Mechanically Reinforced Artificial Enamel by Mg2+-Induced Amorphous Intergranular Phases

Amorphous intergranular phases in mature natural tooth enamel are found to provide better adhesion and could dramatically affect their mechanical performance as a structure reinforcing phase. This study successfully synthesized an amorphous intergranular phase enhanced fluorapatite array controlled by Mg²⁺ (FAP-M) at room temperature. Furthermore, atom probe tomography (APT) observation presents that Mg²⁺ is enriched at grain boundaries during the assembly of enamel-like fluorapatite arrays, leading to the formation of intergranular phases of Mg-rich amorphous calcium phosphate (Mg-ACP). APT results also demonstrated that the segregation of Mg²⁺ caused the chemical gradient in nanocrystalline attachment and realignment under the drive of inherent surface stress. These results indicate that the amorphous intergranular phases served like glue to connect each nanorod to reinforce the enamel-like arrays. Therefore, the as-received FAP-M artificial enamel exhibits excellent mechanical properties, with hardness and Young's modulus of 2.90 ± 0.13 GPa and 67.9 ± 3.4 GPa, which were ∼8.3 and 2.2 times higher than those of FAP arrays without controlled by Mg²⁺, respectively.

Applications of scanning electron microscopy and focused ion beam milling in dental research

The abilities of scanning electron microscopy (SEM) and focused ion beam (FIB) milling for obtaining high-resolution images from top surfaces, cross-sectional surfaces, and even in three dimensions, are becoming increasingly important for imaging and analyzing tooth structures such as enamel and dentin. FIB was originally developed for material research in the semiconductor industry. However, use of SEM/FIB has been growing recently in dental research due to the versatility of dual platform instruments that can be used as a milling device to obtain low-artifact cross-sections of samples combined with high-resolution images. The advent of the SEM/FIB system and accessories may offer access to previously inaccessible length scales for characterizing tooth structures for dental research, opening exciting opportunities to address many central questions in dental research. New discoveries and fundamental breakthroughs in understanding are likely to follow. This review covers the applications, key findings, and future direction of SEM/FIB in dental research in morphology imaging, specimen preparation for transmission electron microscopy (TEM) analysis, and three-dimensional volume imaging using SEM/FIB tomography.

Multiscale engineered artificial tooth enamel

Tooth enamel, renowned for its high stiffness, hardness, and viscoelasticity, is an ideal model for designing biomimetic materials, but accurate replication of complex hierarchical organization of high-performance biomaterials in scalable abiological composites is challenging. We engineered an enamel analog with the essential hierarchical structure at multiple scales through assembly of amorphous intergranular phase (AIP)-coated hydroxyapatite nanowires intertwined with polyvinyl alcohol. The nanocomposite simultaneously exhibited high stiffness, hardness, strength, viscoelasticity, and toughness, exceeding the properties of enamel and previously manufactured bulk enamel-inspired materials. The presence of AIP, polymer confinement, and strong interfacial adhesion are all needed for high mechanical performance. This multiscale design is suitable for scalable production of high-performance materials.

Exploring the effect of Mg2+ substitution on amorphous calcium phosphate nanoparticles

HYPOTHESIS

The study of Amorphous Calcium Phosphate (ACP) has become a hot topic due to its relevance in living organisms and as a material for biomedical applications. The preparation and characterization of Mg-substituted ACP nanoparticles (AMCP) with tunable Ca/Mg ratio is reported in the present study to address the effect of Mg²⁺ on their structure and stability.

EXPERIMENTS

AMCPs particles were synthesized by precipitation of the precursors from aqueous solutions. The particles were analyzed in terms of morphology, crystallinity, and thermal stability, to get a complete overview of their physico-chemical characteristics. Computational methods were also employed to simulate the structure of ACP clusters at different levels of Mg²⁺ substitution.

FINDINGS

Our results demonstrate that AMCP particles with tunable composition and crystallinity can be obtained. The analysis of the heat-induced crystallization of AMCP shows that particles' stability depends on the degree of Mg²⁺ substitution in the cluster, as confirmed by computational analyses. The presented results shed light on the effect of Mg²⁺ on ACP features at different structural levels and may be useful guidelines for the preparation and design of AMCP particles with a specific Ca/Mg ratio.

Advances in biomineralization-inspired materials for hard tissue repair

Biomineralization is the process by which organisms form mineralized tissues with hierarchical structures and excellent properties, including the bones and teeth in vertebrates. The underlying mechanisms and pathways of biomineralization provide inspiration for designing and constructing materials to repair hard tissues. In particular, the formation processes of minerals can be partly replicated by utilizing bioinspired artificial materials to mimic the functions of biomolecules or stabilize intermediate mineral phases involved in biomineralization. Here, we review recent advances in biomineralization-inspired materials developed for hard tissue repair. Biomineralization-inspired materials are categorized into different types based on their specific applications, which include bone repair, dentin remineralization, and enamel remineralization. Finally, the advantages and limitations of these materials are summarized, and several perspectives on future directions are discussed.

Ultrastructural changes of bovine tooth surfaces under erosion in presence of biomimetic hydroxyapatite

Enamel and dentin are susceptible to acids from food sources leading to dental erosion, a global problem affecting millions of individuals. Particulate hydroxyapatite (HAP) on the tooth surface can influence the effects of acid attacks. Standardized bovine enamel and dentin samples with artificial saliva are used in an in vitro cyclic demineralization–remineralization protocol to analyze the structural changes experienced by tooth surfaces using high-resolution scanning electron microscopy and to evaluate the potential of a HAP-based oral care gel in the protection of teeth from erosive attacks. The interfaces between HAP particle and enamel HAP crystallites are investigated using focused ion beam preparation and transmission electron microscopy. The results show that erosion with phosphoric acid severely affects enamel crystallites and dentin tubules, while artificial saliva leads to remineralization effects. The HAP-gel forms a microscopic layer on both enamel and dentin surfaces. Upon acid exposure, this layer is sacrificed before the native tooth tissues are affected, leading to significantly lower degrees of demineralization compared to the controls. This demonstrates that the use of particulate HAP as a biomaterial in oral care formulations can help protect enamel and dentin surfaces from erosive attacks during meals using a simple and effective protection principle.

The Application of Chitosan Nanostructures in Stomatology

Chitosan (CS) is a natural polymer with a positive charge, a deacetylated derivative of chitin. Chitosan nanostructures (nano-CS) have received increasing interest due to their potential applications and remarkable properties. They offer advantages in stomatology due to their excellent biocompatibility, their antibacterial properties, and their biodegradability. Nano-CSs can be applied as drug carriers for soft tissue diseases, bone tissue engineering and dental hard tissue remineralization; furthermore, they have been used in endodontics due to their antibacterial properties; and, finally, nano-CS can improve the adhesion and mechanical properties of dental-restorative materials due to their physical blend and chemical combinations. In this review, recent developments in the application of nano-CS for stomatology are summarized, with an emphasis on nano-CS’s performance characteristics in different application fields. Moreover, the challenges posed by and the future trends in its application are assessed.

Studies of Peculiar Mg-Containing and Oscillating Bioapatites in Sheep and Horse Teeth

New types of biological apatites have been discovered in molar sheep and horse teeth and are divided in two types. In the first and more general type, the release of Mg ions is parallel to the changes in composition of apatite leading to a final stoichiometric ratio of Ca to P ions, going from dentin depth towards the boundary of enamel with air. Inside dentin, another apatite sub-types were discovered with alternating layers of Mg-rich and C-rich apatites. The approximate formal stoichiometric relationships for these peculiar types of bioapatites are suggested. We identified two kinds of ion-exchanges responsible for formation of peculiar apatites. Various combinations of main and minor elements lead to new versions of biological apatites.

Development of an Energy-Sensitive Detector for the Atom Probe Tomography

A position and energy-sensitive detector has been developed for atom probe tomography (APT) instruments in order to deal with some mass peak overlap issues encountered in APT experiments. Through this new type of detector, quantitative and qualitative improvements could be considered for critical materials with mass peak overlaps, such as nitrogen and silicon in TiSiN systems, or titanium and carbon in cemented carbide materials. This new detector is based on a thin carbon foil positioned on the front panel of a conventional MCP-DLD detector. According to several studies, it has been demonstrated that the impact of ions on thin carbon foils has the effect of generating a number of transmitted and reflected secondary electrons. The number generated mainly depends on both the kinetic energy and the mass of incident particles. Despite the fact that this phenomenon is well known and has been widely discussed for decades, no studies have been performed to date for using it as a means to discriminate particles energy. Therefore, this study introduces the first experiments on a potential new generation of APT detectors that would be able to resolve mass peak overlaps through the energy-sensitivity of thin carbon foils.

Enamel erosion control by strontium-containing TiO2- and/or MgO-doped phosphate bioactive glass

OBJECTIVES

To evaluate the effect of strontium-containing titanium- and/or magnesium-doped phosphate bioactive glass on the control of dental erosion.

MATERIALS AND METHODS

Fifty fragments of human enamel were divided into five groups: negative control, 45S5 bioglass, strontium-containing Ti-doped phosphate bioactive glass (PBG-Ti), strontium-containing Mg-doped phosphate bioactive glass (PBG-Mg), and strontium-containing Ti- and Mg-doped phosphate bioactive glass (PBG-TiMg). The specimens underwent cycles of erosive challenge twice daily for 5 days with 1 mL of citric acid for 2 min followed by 1 mL of the suspension with bioactive substances for 3 min. After the cycles, profilometry, roughness and microhardness testing, and scanning electron microscopy (SEM) were performed. The following statistical tests were used: one-way ANOVA (profile, roughness, and surface microhardness (%VMS) data variation), Tukey's HSD (%VMS), Games-Howell test (profilometry), Student's t test (roughness), and Pearson's correlation between the variables.

RESULTS

The lower loss of enamel surface and lower %VMS was observed in the PBG-Mg and PBG-TiMg groups, and only the PBG-Mg group showed similar roughness between baseline and eroded areas (p > 0.05). On SEM micrographs, PBG-Ti and PBG-Mg groups showed lower apparent demineralization.

CONCLUSION

All bioactive materials protected the enamel against erosion. However, strontium-containing phosphate bioactive glasses showed lower enamel loss, and the presence of Mg in these bioactive glasses provided a greater protective effect.

CLINICAL RELEVANCE

Experimental strontium-containing phosphate bioactive glasses are effective in controlling enamel erosion. The results obtained in this study will guide the development of new dental products.

The homogenous alternative to biomineralization: Zn- and Mn-rich materials enable sharp organismal "tools" that reduce force requirements

We measured hardness, modulus of elasticity, and, for the first time, loss tangent, energy of fracture, abrasion resistance, and impact resistance of zinc- and manganese-enriched materials from fangs, stings and other "tools" of an ant, spider, scorpion and nereid worm. The mechanical properties of the Zn- and Mn-materials tended to cluster together between plain and biomineralized "tool" materials, with the hardness reaching, and most abrasion resistance values exceeding, those of calcified salmon teeth and crab claws. Atom probe tomography indicated that Zn was distributed homogeneously on a nanometer scale and likely bound as individual atoms to more than ¼ of the protein residues in ant mandibular teeth. This homogeneity appears to enable sharper, more precisely sculpted "tools" than materials with biomineral inclusions do, and also eliminates interfaces with the inclusions that could be susceptible to fracture. Based on contact mechanics and simplified models, we hypothesize that, relative to plain materials, the higher elastic modulus, hardness and abrasion resistance minimize temporary or permanent tool blunting, resulting in a roughly 2/3 reduction in the force, energy, and muscle mass required to initiate puncture of stiff materials, and even greater force reductions when the cumulative effects of abrasion are considered. We suggest that the sharpness-related force reductions lead to significant energy savings, and can also enable organisms, especially smaller ones, to puncture, cut, and grasp objects that would not be accessible with plain or biomineralized "tools".

Biomimetic mineralisation systems for in situ enamel restoration inspired by amelogenesis

Caries and dental erosion are common oral diseases. Traditional treatments involve the mechanical removal of decay and filling but these methods are not suitable for cases involving large-scale enamel erosion, such as hypoplasia. To develop a noninvasive treatment, promoting remineralisation in the early stage of caries is of considerable clinical significance. Therefore, biomimetic mineralisation is an ideal approach for restoring enamel. Biomimetic mineralisation forms a new mineral layer that is tightly attached to the surface of the enamel. This review details the state-of-art achievements on the application of amelogenin and non-amelogenin, amorphous calcium phosphate, ions flow and other techniques in the biomimetic mineralisation of enamel. The ultimate goal of this review was to shed light on the requirements for enamel biomineralisation. Hence, herein, we summarise two strategies of biological minimisation systems for in situ enamel restoration inspired by amelogenesis that have been developed in recent years and compare their advantages and disadvantages.

Visualization of Collagen-Mineral Arrangement Using Atom Probe Tomography

Bone is a functional material comprised of mainly two phases: an organic collagenous phase and an inorganic mineral phase. Collagen-mineral arrangement has implications for bone function, aging, and disease. However, theories on collagen-mineral arrangement have been confined to studies with low spatial and/or compositional resolution resulting in an extensive debate over the location of mineral with respect to collagen. Herein, a strategy is developed to extract a single mineralized collagen fibril from bone and analyze its composition and structure atom-by-atom with 3D sub-nanometer accuracy and compositional clarity using atom probe tomography (APT). It is shown for the first time a method to probe fibril-level mineralization and collagen-mineral arrangement from an in vivo system with both the spatial and compositional precision required to comment on nanoscale collagen-mineral arrangement. APT of leporine bone shows distinct and helical collagen fibrils with mineralized deposits both encapsulating and incorporated into the collagenous structures. This study demonstrates a novel fibril-level detection method that can be used to probe the composition of bone and contribute new insights to the structure and organization of mineralized materials such as bones and teeth.

Bioprocess-Inspired Room-Temperature Synthesis of Enamel-like Fluorapatite/Polymer Nanocomposites Controlled by Magnesium Ions

Tooth enamel is composed of arrayed fluorapatite (FAP) or hydroxyapatite nanorods modified with Mg-rich amorphous layers. Although it is known that Mg²⁺ plays an important role in the formation of enamel, there is limited research on the regulatory role of Mg²⁺ in the synthesis of enamel-like materials. Therefore, we focus on the regulatory behavior of Mg²⁺ in the fabrication of biomimetic mineralized enamel-like structural materials. In the present study, we adopt a bioprocess-inspired room-temperature mineralization technique to synthesize a multilayered array of enamel-like columnar FAP/polymer nanocomposites controlled by Mg²⁺ (FPN-M). The results reveal that the presence of Mg²⁺ induced the compaction of the array and the formation of a unique Mg-rich amorphous-reinforced architecture. Therefore, the FPN-M array exhibits excellent mechanical properties. The hardness (2.42 ± 0.01 GPa) and Young's modulus (81.5 ± 0.6 GPa) of the as-prepared FPN-M array are comparable to those of its biological counterparts; furthermore, the enamel-like FPN-M array is translucent. The hardness and Young's modulus of the synthetic array of FAP/polymer nanocomposites without Mg²⁺ control (FPN) are 0.51 ± 0.04 and 43.5 ± 1.6 GPa, respectively. The present study demonstrates a reliable bioprocess-inspired room-temperature fabrication technique for the development of advanced high-performance composite materials.

Optimized Hierarchical Structure and Chemical Gradients Promote the Biomechanical Functions of the Spike of Mantis Shrimps

The tail spike of the mantis shrimp is the appendage for counteracting the enemy from behind. Here, we investigate the correlations between the chemical compositions, the microstructures, and the mechanical properties of the spike. We find that the spike is a hollow beam with a varying cross section along the length. The cross section comprises four different layers with distinct features of microstructures and chemical compositions. The local mechanical properties of these layers correlate well with the microstructures and chemical compositions, a combination of which effectively restricts the crack propagation while maximizing the release of strain energy during deformation. Finite element analysis and mechanics modeling demonstrate that the optimized structure of the spike confines the mechanical damage in the region near the tip and prevents catastrophic breakage at the base. Furthermore, we use a 3D printing technique to fabricate multiple hollow cylindrical samples consisting of biomimetic microstructures of the spike and confirm that the combination of the Bouligand structure with radially oriented parallel sheets greatly improves the toughness and strength during compression tests. The multiscale design strategy of the spike revealed here is expected to be of great interest for the development of novel bioinspired materials.

Atom Probe Tomography of Encapsulated Hydroxyapatite Nanoparticles

Hydroxyapatite nanoparticles (HAP NPs) are important for medicine, bioengineering, catalysis, and water treatment. However, current understanding of the nanoscale phenomena that confer HAP NPs their many useful properties is limited by a lack of information about the distribution of the atoms within the particles. Atom probe tomography (APT) has the spatial resolution and chemical sensitivity for HAP NP characterization, but difficulties in preparing the required needle-shaped samples make the design of these experiments challenging. Herein, two techniques are developed to encapsulate HAP NPs and prepare them into APT tips. By sputter-coating gold or the atomic layer deposition of alumina for encapsulation, partially fluoridated HAP NPs are successfully characterized by voltage- or laser-pulsing APT, respectively. Analyses reveal that significant tradeoffs exist between encapsulant methods/materials for HAP characterization and that selection of a more robust approach will require additional technique development. This work serves as an essential starting point for advancing knowledge about the nanoscale spatiochemistry of HAP NPs.

Biomimetic inorganic-organic hybrid nanoparticles from magnesium-substituted amorphous calcium phosphate clusters and polyacrylic acid molecules

Amorphous calcium phosphate (ACP) has been widely found during bone and tooth biomineralization, but the meta-stability and labile nature limit further biomedical applications. The present study found that the chelation of polyacrylic acid (PAA) molecules with Ca²⁺ ions in Mg-ACP clusters (~2.1 ± 0.5 nm) using a biomineralization strategy produced inorganic-organic Mg-ACP/PAA hybrid nanoparticles with better thermal stability. Mg-ACP/PAA hybrid nanoparticles (~24.0 ± 4.8 nm) were pH-responsive and could be efficiently digested under weak acidic conditions (pH 5.0–5.5). The internalization of assembled Mg-ACP/PAA nanoparticles by MC3T3-E1 cells occurred through endocytosis, indicated by laser scanning confocal microscopy and cryo-soft X-ray tomography. Our results showed that cellular lipid membranes remained intact without pore formation after Mg-ACP/PAA particle penetration. The assembled Mg-ACP/PAA particles could be digested in cell lysosomes within 24 h under weak acidic conditions, thereby indicating the potential to efficiently deliver encapsulated functional molecules. Both the in vitro and in vivo results preliminarily demonstrated good biosafety of the inorganic-organic Mg-ACP/PAA hybrid nanoparticles, which may have potential for biomedical applications.

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Mg-ACP/PAA hybrid nanoparticles have been synthesized following a biomineralization strategy.

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The chelation of PAA molecules in synergy with Mg²⁺ substitution improves thermal stability of Mg-ACP/PAA nanoparticles.

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The Mg-ACP/PAA nanoparticles are pH sensitive and can be digested in cell lysosomes within 24 h.

Magnesium incorporation into primary dental enamel and its effect on mechanical properties

Cross-sectional study of sound primary dental enamel revealed hardness zonation and, in parallel, significant change in the Mg content below the prismless layer. Mg content is known to play an important role in enamel apatite biomineralization, therefore, Mg ion exchange experiments were carried out on the outer surface of sound primary molars and on reference abiogenic Ca-phosphates using MgCl₂ solution. Effects of Mg incorporation on crystal/particle size, ionic ratio and morphology were compared and the observed changes were explained by parallel diffusion and dissolution/reprecipitation processes. Based on depth profile analysis and high resolution electron microscopy of the Mg-exchanged dental enamel, a poorly ordered surface layer of approximately 10-15 nanometer thickness was identified. This thin layer is strongly enriched in Mg and has non-apatitic structure. Below the surface layer, the Mg content increased only moderately (up to ~3 at%) and the apatite crystal structure of enamel was preserved. As a common effect of the Mg exchanged volume, primary dental enamel exhibited about 20% increase of nanohardness, which is intrepreted by strengthening of both the thin surface layer and the region below due to the decreased crystallite size and the effect of incorporated Mg, respectively. STATEMENT OF SIGNIFICANCE: Dental enamel is the most durable mineralized tissue in the human body, which, in spite to be exposed to extreme conditions like mastication and acidic dissolution, is able to fulfill its biological function during lifetime. In this study we show that minor component magnesium can affect hardness properties of human primary dental enamel. Then, through Mg incorporation experiments we provide an additional proof for the poorly ordered Mg-containing intergranular phase which has been recently observed. Also, we report that the hardness of dental enamel can be increased by ca. 20% by Mg incorporation. These results contribute to a deeper understanding of sound primary dental enamel structure and may inspire new pathways for assisted remineralization of enamel and regenerative dentistry.

Atom probe tomography

Atom probe tomography (APT) provides three-dimensional compositional mapping with sub-nanometre resolution. The sensitivity of APT is in the range of parts per million for all elements, including light elements such as hydrogen, carbon or lithium, enabling unique insights into the composition of performance-enhancing or lifetime-limiting microstructural features and making APT ideally suited to complement electron-based or X-ray-based microscopies and spectroscopies. Here, we provide an introductory overview of APT ranging from its inception as an evolution of field ion microscopy to the most recent developments in specimen preparation, including for nanomaterials. We touch on data reconstruction, analysis and various applications, including in the geosciences and the burgeoning biological sciences. We review the underpinnings of APT performance and discuss both strengths and limitations of APT, including how the community can improve on current shortcomings. Finally, we look forwards to true atomic-scale tomography with the ability to measure the isotopic identity and spatial coordinates of every atom in an ever wider range of materials through new specimen preparation routes, novel laser pulsing and detector technologies, and full interoperability with complementary microscopy techniques.

Novel Approach to Tooth Chemistry: Quantification of Human Enamel Apatite in Context for New Biomaterials and Nanomaterials Development

A series of linear profiles of the elements of the enamel in human molar teeth were made with the use of an electron microprobe and a Raman microscope. It is postulated that the enamel can be treated as the superposition of variable "overbuilt" enamel on the stable "core" enamel at the macro-, micro- and nanoscale level. The excessive values characterize the "overbuilt enamel". All the profiles of excessive parameters along the enamel thickness from the enamel surface to the dentin enamel junction (DEJ) can be approximated very precisely with the use of exponential functions, where Ca, P, Cl and F spatial profiles are decaying while Mg, Na, K and CO32- ones are growing distributions. The "overbuilt" apatite formed on the boundary with DEJ, enriched in Na, Mg, OH and carbonates, reacts continuously with Ca, Cl and F, passing into an acid-resistant form of the "overbuilt" enamel. The apparent phases arriving in boundary regions of the "overbuilt enamel" were proposed. Microdiffraction measurements reveal relative variation of energy levels during enamel transformations. Our investigations are the milestones for a further new class of biomaterial and nanomaterial development for biomedical applications.

Field evaporation and atom probe tomography of pure water tips

Measuring biological samples by atom probe tomography (APT) in their natural environment, i.e. aqueous solution, would take this analytical method, which is currently well established for metals, semi-conductive materials and non-metals, to a new level. It would give information about the 3D chemical structure of biological systems, which could enable unprecedented insights into biological systems and processes, such as virus protein interactions. For this future aim, we present as a first essential step the APT analysis of pure water (Milli-Q) which is the main component of biological systems. After Cryo-preparation, nanometric water tips are field evaporated with assistance by short laser pulses. The obtained data sets of several tens of millions of atoms reveal a complex evaporation behavior. Understanding the field evaporation process of water is fundamental for the measurement of more complex biological systems. For the identification of the individual signals in the mass spectrum, DFT calculations were performed to prove the stability of the detected molecules.

Characterization of elemental distribution across human dentin-enamel junction by scanning electron microscopy with energy-dispersive X-ray spectroscopy

The human dentin-enamel junction (DEJ) is a natural junction that unites two dissimilar mineralized tissues in the human tooth: enamel and dentin. DEJ plays a critical role in maintaining structural and functional integrity of the tooth. However, its structure, chemical composition and function remain unclear and controversial. Systematic investigation of elemental distribution across human DEJ is still lacking in the literature. This study aimed to investigate the elemental distributions of Ca, P, O, C, N, Na, and Mg across the DEJ of human teeth using scanning electron microscope with energy dispersive spectroscopy of X-ray. The results revealed abrupt changes in the distributions of six elements (C, N, Ca, P, Na, and Mg) across the DEJ. Specifically, the four mineral elements showed similar level of change in distribution, with Ca, P, Na decreasing while Mg increasing by 21%-25% from enamel to dentin side of the DEJ. The two organic elements C and N showed much larger changes in distribution, with C increasing by ~150% and N increasing by ~270% from enamel to dentin side of the DEJ. The slope of the distribution curves across the DEJ was estimated to be ~2 μm in width and coincided with the phase intermixing of the micro-scallop structure of the DEJ.

Characterization of Enamel and Dentine about a White Spot Lesion: Mechanical Properties, Mineral Density, Microstructure and Molecular Composition

The study focuses on in vitro tracing of some fundamental changes that emerge in teeth at the initial stage of caries development using multiple approaches. The research was conducted on a mostly sound maxillary molar tooth but with a clearly visible natural proximal white spot lesion (WSL). Values of mineral density, reduced Young's modulus, indentation hardness and creep as well as the molecular composition and surface microstructure of the WSL and bordering dentine area were studied. The results obtained were compared to those of sound enamel and dentine on the same tooth. A decrease of mechanical properties and mineral density both for the WSL and bordering dentine was detected in comparison to the sound counterparts, as well as increase of creep for the enamel WSL. Differences in molecular composition and surface microstructure (including the indenter impressions) were found and described. WSL induces a serious change in the state of not only the visually affected enamel but also surrounding visually intact enamel and dentine in its vicinity. The results provide the basis for future studies of efficacy of minimal invasive treatments of caries.

New frontiers in atom probe tomography: a review of research enabled by cryo and/or vacuum transfer systems

There has been a recent surge in the use of cryo and/or vacuum specimen preparation and transfer systems to broaden the scope of research enabled by the microscopy technique of atom probe tomography. This is driven by the fact that, as for many microscopes, the application of atom probes to air- and temperature-sensitive materials or wet biological specimens has previously been limited by transfer through air at room temperature. Here we provide an overview of areas of research that benefit from these new transfer and analysis protocols, as well as a review of current advances in transfer devices, environmental cells, and glove boxes for controlled specimen manipulation. This includes the study of catalysis and corrosion, biological samples, liquid-solid interfaces, natural aging, and the distribution of hydrogen in materials.

Chemical gradients in human enamel crystallites

Dental enamel is a principal component of teeth1, and has evolved to bear large chewing forces, resist mechanical fatigue and withstand wear over decades2. Functional impairment and loss of dental enamel, caused by developmental defects or tooth decay (caries), affect health and quality of life, with associated costs to society3. Although the past decade has seen progress in our understanding of enamel formation (amelogenesis) and the functional properties of mature enamel, attempts to repair lesions in this material or to synthesize it in vitro have had limited success4-6. This is partly due to the highly hierarchical structure of enamel and additional complexities arising from chemical gradients7-9. Here we show, using atomic-scale quantitative imaging and correlative spectroscopies, that the nanoscale crystallites of hydroxylapatite (Ca5(PO4)3(OH)), which are the fundamental building blocks of enamel, comprise two nanometric layers enriched in magnesium flanking a core rich in sodium, fluoride and carbonate ions; this sandwich core is surrounded by a shell with lower concentration of substitutional defects. A mechanical model based on density functional theory calculations and X-ray diffraction data predicts that residual stresses arise because of the chemical gradients, in agreement with preferential dissolution of the crystallite core in acidic media. Furthermore, stresses may affect the mechanical resilience of enamel. The two additional layers of hierarchy suggest a possible new model for biological control over crystal growth during amelogenesis, and hint at implications for the preservation of biomarkers during tooth development.

Amelotin is expressed in retinal pigment epithelium and localizes to hydroxyapatite deposits in dry age-related macular degeneration

Deposition of hydroxyapatite (HAP) basal to the retinal pigment epithelium (RPE) is linked to the progression of age-related macular degeneration (AMD). Serum-deprivation of RPE cells in culture mimics some features of AMD. We now show that serum-deprivation also leads to the induction of amelotin (AMTN), a protein involved in hydroxyapatite mineralization in enamel. HAP is formed in our culture model and is blocked by siRNA inhibition of AMTN expression. In situ hybridization and immunofluorescence imaging of human eye tissue show that AMTN is expressed in RPE of donor eyes with geographic atrophy ("dry" AMD) in regions with soft drusen containing HAP spherules or nodules. AMTN is not found in hard drusen, normal RPE, or donor eyes diagnosed with wet AMD. These findings suggest that AMTN is involved in formation of HAP spherules or nodules in AMD, and as such provides a new therapeutic target for slowing disease progression.

Nature's design solutions in dental enamel: Uniting high strength and extreme damage resistance

The most important demand of today's high-performance materials is to unite high strength with extreme fracture toughness. The combination of withstanding large forces (strength) and resistance to fracture (toughness), especially preventing catastrophic material failure by cracking, is of utmost importance when it comes to structural applications of these materials. However, these two properties are commonly found to be mutually exclusive: strong materials are brittle and tough materials are soft. In dental enamel, nature has combined both properties with outstanding success - despite a limited number of available constituents. Made up of brittle mineral crystals arranged in a sophisticated hierarchical microstructure, enamel exhibits high stiffness and excellent toughness. Different species exhibit a variety of structural adaptations on varying scales in their dental enamel which optimise not only fracture toughness, but also hardness and abrasion behaviour. Nature's materials still outperform their synthetic counterparts due to these complex structure-property relationships that are not yet fully understood. By analysing structure variations and the underlying mechanical mechanisms systematically, design principles which are the key for the development of advanced synthetic materials uniting high strength and toughness can be formulated. STATEMENT OF SIGNIFICANCE: Dental enamel is a hard protective tissue that combines high strength with an exceptional resistance to catastrophic fracture, properties that in classical materials are commonly found to be mutually exclusive. The biological material is able to outperform its synthetic counterparts due to a sophisticated hierarchical microstructure. Between different species, microstructural adaptations can vary significantly. In this contribution, the different types of dental enamel present in different species are reviewed and connections between microstructure and (mechanical) properties are drawn. By consolidating available information for various species and reviewing it from a materials science point of view, design principles for the development of advanced biomimetic materials uniting high strength and toughness can be formulated.

Introducing the crystalline phase of dicalcium phosphate monohydrate

Calcium orthophosphates (CaPs) are important in geology, biomineralization, animal metabolism and biomedicine, and constitute a structurally and chemically diverse class of minerals. In the case of dicalcium phosphates, ever since brushite (CaHPO₄·2H₂O, dicalcium phosphate dihydrate, DCPD) and monetite (CaHPO₄, dicalcium phosphate, DCP) were first described in 19^th century, the form with intermediary chemical formula CaHPO₄·H₂O (dicalcium phosphate monohydrate, DCPM) has remained elusive. Here, we report the synthesis and crystal structure determination of DCPM. This form of CaP is found to crystallize from amorphous calcium hydrogen phosphate (ACHP) in water-poor environments. The crystal structure of DCPM is determined to show a layered structure with a monoclinic symmetry. DCPM is metastable in water, but can be stabilized by organics, and has a higher alkalinity than DCP and DCPD. This study serves as an inspiration for the future exploration of DCPM's potential role in biomineralization, or biomedical applications.

Probing Heterogeneity in Bovine Enamel Composition through Nanoscale Chemical Imaging using Atom Probe Tomography

OBJECTIVE

The aim of this study was to determine the heterogeneity in chemical composition of bovine enamel using atom probe tomography, and thereby evaluate the suitability of bovine enamel as a substitute for human enamel in in vitro dental research.

DESIGN

Enamel samples from extracted bovine incisor teeth were first sectioned using a diamond saw and then milled into needle-like samples (<100 nm diameter) by focused ion beam (FIB) coupled with a scanning electron microscope (SEM). These samples were analyzed in the atom probe to acquire three-dimensional (3D) images and quantify the atomic chemistry and distribution in bovine enamel.

RESULTS

For the first time, the atomic-level composition and clustering of major constituents and impurities within bovine enamel were determined and imaged. We discovered that the chemical composition of bovine enamel is spatially inhomogeneous at the atomic scale. The average bulk Ca/P ratio, ∼1.4, was in agreement with previously reported literature values from alternative conventional methods. When assessed locally at the atomic scale, the Ca/P ratio varied between 1.1 and 2.03. We also discovered that the Mg impurities were significantly segregated throughout the enamel, and such clustering influenced the variation of Ca/P ratios. The increase in Mg concentrations, near the Mg clusters, correlated with increased Ca and decreased P concentrations.

CONCLUSION

The presented findings of variability in local composition should be taken into account when interpreting dental research results from bovine enamel.