Micromechanical properties of single crystal hydroxyapatite by nanoindentation

Nano-friction behavior and deformation study of hydroxyapatite in ultra-precision polishing process

CONTEXT

In order to study the effect of ultra-precision machining on the surface quality of hydroxyapatite semiconductor materials as well as the material removal mechanism of hydroxyapatite, the mechanical polishing behaviors of hydroxyapatite at different polishing depths were studied by molecular dynamics method. The results show that the subsurface damage of hydroxyapatite increases with increasing polishing depth. The polishing temperature and the polishing force showed a positive correlation with the polishing depth, and the variation of the polishing force was related to the accumulation-release effect of the potential energy of hydroxyapatite material. In addition, the variation of stresses in hydroxyapatite during polishing is mainly influenced by the thermal softening effect. With a smaller polishing depth, the hydroxyapatite semiconductor material has fewer structural defects, fewer atoms undergoing phase transitions, lower surface roughness, and better surface quality. Therefore, to ensure the long-lasting service life of hydroxyapatite semiconductor materials, a small polishing depth should be used in ultra-precision machining. Additionally, this study also provides a theoretical reference for future research on the mechanical properties of hydroxyapatite-based composites.

METHODS

A Large-Scale Atomic/Molecular Parallel Simulator (LAMMPS) was utilized to perform molecular dynamics simulations. The output was visualized and analyzed by the Open Visualization Tools (OVITO) software. The intermolecular interactions were described by the polymer consistent force-field and the 12/6 Lennard-Jones potential functions. The workpiece was polished under a micro-canonical ensemble with the temperature settled at 300 K. Periodic boundary conditions were adopted and the velocity-Verlet algorithm was used to integrate the atomic motion with a timestep of 0.1 femtoseconds (fs).

Spheroids as a 3D in vitro model to study bone and bone mineralization

Traumas, fractures, and diseases can severely influence bone tissue. Insight into bone mineralization is essential for the development of therapies and new strategies to enhance bone regeneration. 3D cell culture systems, in particular cellular spheroids, have gained a lot of interest as they can recapitulate crucial aspects of the in vivo tissue microenvironment, such as the extensive cell-cell and cell-extracellular matrix (ECM) interactions found in tissue. The potential of combining spheroids and various classes of biomaterials opens also new opportunities for research within bone tissue engineering. Characterizing cellular organization, ECM structure, and ECM mineralization is a fundamental step for understanding the biological processes involved in bone tissue formation in a spheroid-based model system. Still, many experimental techniques used in this field of research are optimized for use with monolayer cell cultures. There is thus a need to develop new and improving existing experimental techniques, for applications in 3D cell culture systems. In this review, bone composition and spheroids properties are described. This is followed by an insight into the techniques that are currently used in bone spheroids research and how these can be used to study bone mineralization. We discuss the application of staining techniques used with optical and confocal fluorescence microscopy, molecular biology techniques, second harmonic imaging microscopy, Raman spectroscopy and microscopy, as well as electron microscopy-based techniques, to evaluate osteogenic differentiation, collagen production and mineral deposition. Challenges in the applications of these methods in bone regeneration and bone tissue engineering are described. STATEMENT OF SIGNIFICANCE: 3D cell cultures have gained a lot of interest in the last decades as a possible technique that can be used to recreate in vitro in vivo biological process. The importance of 3D environment during bone mineralization led scientists to use this cell culture to study this biological process, to obtain a better understanding of the events involved. New and improved techniques are also required for a proper analysis of this cell model and the process under investigation. This review summarizes the state of the art of the techniques used to study bone mineralization and how 3D cell cultures, in particular spheroids, are tested and analysed to obtain better resolved results related to this complex biological process.

Bioinspired Design Rules from Highly Mineralized Natural Composites for Two-Dimensional Composite Design

Discoveries of two-dimensional (2D) materials, exemplified by the recent entry of MXene, have ushered in a new era of multifunctional materials for applications from electronics to biomedical sensors due to their superior combination of mechanical, chemical, and electrical properties. MXene, for example, can be designed for specialized applications using a plethora of element combinations and surface termination layers, making them attractive for highly optimized multifunctional composites. Although multiple critical engineering applications demand that such composites balance specialized functions with mechanical demands, the current knowledge of the mechanical performance and optimized traits necessary for such composite design is severely limited. In response to this pressing need, this paper critically reviews structure-function connections for highly mineralized 2D natural composites, such as nacre and exoskeletal of windowpane oysters, to extract fundamental bioinspired design principles that provide pathways for multifunctional 2D-based engineered systems. This paper highlights key bioinspired design features, including controlling flake geometry, enhancing interface interlocks, and utilizing polymer interphases, to address the limitations of the current design. Challenges in processing, such as flake size control and incorporating interlocking mechanisms of tablet stitching and nanotube forest, are discussed along with alternative potential solutions, such as roughened interfaces and surface waviness. Finally, this paper discusses future perspectives and opportunities, including bridging the gap between theory and practice with multiscale modeling and machine learning design approaches. Overall, this review underscores the potential of bioinspired design for engineered 2D composites while acknowledging the complexities involved and providing valuable insights for researchers and engineers in this rapidly evolving field.

A natural biomineral for enhancing the biomineralization and cell response of 3D printed polylactic acid bone scaffolds

Polylactic acid (PLA) has been extensively used as a bone scaffold material, but it still faces many problems including low biomineralization ability, weak cell response, low mechanical properties, etc. In this study, we proposed to utilize the distinctive physical, chemical and biological properties of a natural biomineral with organic matrix, pearl powder, to enhance the overall performance of PLA bone scaffolds. Porous PLA/pearl composite bone scaffolds were prepared using fused deposition modeling (FDM) 3D printing technology, and their comprehensive performance was investigated. Macro- and micro- morphological observation by optical camera and scanning electron microscopy (SEM) showed the 3D printed scaffolds have interconnected and ordered periodic porous structures. Phase analysis by X-ray diffraction (XRD) indicated pearl powder was well composited with PLA without impurity formation during the melt extrusion process. The mechanical test results indicated the tensile and compressive strength of PLA/pearl composite scaffolds with 10 % pearl powder content yielded the highest values, which were 15.5 % and 21.8 % greater than pure PLA, respectively. The water contact angle and water absorption tests indicated that PLA/pearl showed better hydrophilicity than PLA due to the presence of polar groups in the organic matrix of the pearl powder. The results of the simulated body fluid (SBF) soaking revealed that the addition of pearl powder effectively enhanced the formation and deposition of apatite, which was attributed to the release of Ca²⁺ from the dissolution of pearl powder. The cell culture of bone marrow mesenchymal stem cells (BMSCs) indicated that PLA/pearl scaffolds showed better cell proliferation and osteogenic differentiation than PLA due to the stimulation of the biological organic matrix in pearl powder. These outcomes signify the potential of pearl powder as a natural biomineral containing bio-signal factors to improve the mechanical and biological properties of polymers for better bone tissue engineering application.

Bioceramics/Electrospun Polymeric Nanofibrous and Carbon Nanofibrous Scaffolds for Bone Tissue Engineering Applications

Bone tissue engineering integrates biomaterials, cells, and bioactive agents to propose sophisticated treatment options over conventional choices. Scaffolds have central roles in this scenario, and precisely designed and fabricated structures with the highest similarity to bone tissue have shown promising outcomes. On the other hand, using nanotechnology and nanomaterials as the enabling options confers fascinating properties to the scaffolds, such as precisely tailoring the physicochemical features and better interactions with cells and surrounding tissues. Among different nanomaterials, polymeric nanofibers and carbon nanofibers have attracted significant attention due to their similarity to bone extracellular matrix (ECM) and high surface-to-volume ratio. Moreover, bone ECM is a biocomposite of collagen fibers and hydroxyapatite crystals; accordingly, researchers have tried to mimic this biocomposite using the mineralization of various polymeric and carbon nanofibers and have shown that the mineralized nanofibers are promising structures to augment the bone healing process in the tissue engineering scenario. In this paper, we reviewed the bone structure, bone defects/fracture healing process, and various structures/cells/growth factors applicable to bone tissue engineering applications. Then, we highlighted the mineralized polymeric and carbon nanofibers and their fabrication methods.

The Effect of a Bioactive Oral System and CO2 Laser on Enamel Susceptibility to Acid Challenge

This study evaluated the structural changes of enamel treated by the Regenerate system and carbon dioxide (CO2) laser against acid challenge. Thirty human enamel slabs were prepared and assigned into three groups: Group I: untreated (control); Group II: treated with the Regenerate system; and Group III exposed to CO2 laser. All specimens were subjected to an acid challenge (pH 4.5–7.0) for 14 days. Specimens were evaluated and compared at 120 points using five Raman microspectroscopic peaks; the phosphate vibrations ν1, ν2, ν3, and ν4 at 960, 433, 1029, and 579 cm−1, respectively, and the carbonate at 1070 cm−1, followed by Vickers microhardness test. The ratio of carbonate to phosphate was correlated to the equivalent microhardness numbers. The intensities of phosphate peaks ν1, ν2, and ν4 were reduced in all groups post-acid challenge, while the carbonate and ν3 were significantly increased (p < 0.000). Surfaces treated by Regenerate exhibited higher peak intensity of phosphate and carbonate before and after pH-cycling (p < 0.05). The mineral content in enamel had a direct effect on tissue microhardness, and the CO2-lased surfaces showed a reduced carbonate content and higher microhardness values. Both approaches induced surface changes that can protect enamel against acid challenge resulting in a significant benefit for dental healthcare.

Surgical cotton microfibers loaded with proteins and apatite: A potential platform for bone tissue engineering

Tissue engineering has emerged as the best alternative to replacing damaged tissue/organs. However, the cost of scaffold materials continues to be a significant obstacle; thus, developing inexpensive scaffolds is strongly encouraged. In this study, cellulose microfibers (C), gelatin (G), egg white (EW), and nanohydroxyapatite (nHA) were assembled into a quaternary scaffold using EDC-NHS crosslinking, followed by freeze-drying method. Cellulose microfibers as a scaffold have only received a limited amount of research due to the absence of an intrinsic three-dimensional structure. Gelatin, more likely to interact chemically with collagen, was used to provide a stable structure to the cellulose microfibers. EW was supposed to provide the scaffold with numerous cell attachment sites. nHA was chosen to enhance the scaffold's bone-bonding properties. Physico-chemical, mechanical, and biological characterization of scaffolds were studied. In-vitro using MG-63 cells and in-ovo studies revealed that all scaffolds were biocompatible. The results of the DPPH assay demonstrate the ability of CGEWnHA to reduce free radicals. The CGEWnHA scaffold exhibits the best properties with 56.84 ± 28.45 μm average pore size, 75 ± 1.4 % porosity, 39.23 % weight loss, 109.19 ± 0.98 kPa compressive modulus, and 1.72 Ca/P ratio. As a result, the constructed CGEWnHA scaffold appears to be a viable choice for BTE applications.

Effect of Pore Defects on Uniaxial Mechanical Properties of Bulk Hexagonal Hydroxyapatite: A Molecular Dynamics Study

Hydroxyapatite (HAP) is a calcium apatite bioceramic used in various naturally-derived and synthetic forms for bone repair and regeneration. While useful for the regrowth of osseus tissue, the poor load-bearing capacity of this material relative to other biomaterials is worsened by the propensity for pore formation during the synthetic processing of scaffolds, blocks, and granules. Here we use molecular dynamics (MD) simulations to improve the current understanding of the defect-altered uniaxial mechanical response in hexagonal HAP single crystals relative to defect-free structures. The inclusion of a central spherical pore within a repeated lattice was found to reduce both the failure stress and failure strain in uniaxial tension and compression, with up to a 30% reduction in maximum stress at the point of failure compared to a perfect crystalline structure observed when a 30 Å diameter pore was included. The Z axis ([0 0 0 1] crystalline direction) was found to be the least susceptible to pore defects in tension but the most sensitive to pore inclusion in compression. The deformation mechanisms are discussed to explain the observed mechanical responses, for which charge imbalances and geometric stress concentration factor effects caused by pore inclusion play a significant role.

Role of the Mineral in the Self-Healing of Cracks in Human Enamel

Human enamel is an incredibly resilient biological material, withstanding repeated daily stresses for decades. The mechanisms behind this resilience remain an open question, with recent studies demonstrating a crack-deflection mechanism contributing to enamel toughness and other studies detailing the roles of the organic matrix and remineralization. Here, we focus on the mineral and hypothesize that self-healing of cracks in enamel nanocrystals may be an additional mechanism acting to prevent catastrophic failure. To test this hypothesis, we used a molecular dynamics (MD) approach to compare the fracture behavior of hydroxyapatite (HAP) and calcite, the main minerals in human enamel and sea urchin teeth, respectively. We find that cracks heal under pressures typical of mastication by fusion of crystals in HAP but not in calcite, which is consistent with the resilience of HAP enamel that calcite teeth lack. Scanning transmission electron microscopy (STEM) images of structurally intact ("sound") human enamel show dashed-line nanocracks that resemble and therefore might be the cracks healed by fusion of crystals produced in silico. The fast, self-healing mechanism shown here is common in soft materials and ceramics but has not been observed in single crystalline materials at room temperature. The crack self-healing in sound enamel nanocrystals, therefore, is unique in the human body and unique in materials science, with potential applications in designing bioinspired materials.

A behavioural study of hydrothermally derived novel alumina/magnesia/hydroxyapatite (Al2O3/MgO/HA) bioceramic nanocomposite

Regeneration and regrowth of human bones is a gradual and time-consuming process. Therefore scaffolds are required to hold the bones in position and give them time to heal well. Though hydroxyapatite (HA) is a potential candidate, its weak mechanical nature has made it undesirable. To overcome this hurdle, a novel nanocomposite of HA with alumina (Al₂O₃) and magnesia (MgO) was prepared by hydrothermal method in this study. The constitution, composition and conformation of the Al₂O₃/MgO/HA nanocomposite were affirmed via X-ray Diffraction (XRD), Fourier-Transform Infra-Red (FTIR) and imaging tests. Vicker's hardness test evinces the mechanical calibre of the nanocomposite. A perspective load-displacement analysis was done by means of Onitsch and Kick's power laws. The nanocomposite excelled above par with a hardness of 5.19 GPa and fracture toughness of 1.27 MPam^1/2. This holistic progress makes this nanocomposite a very desirable bone implant material.

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.

Synthesis techniques, characterization and mechanical properties of natural derived hydroxyapatite scaffolds for bone implants: a review

Hydroxyapatite (HAp) with good mechanical properties is a promising material meant for a number of useful bids in dentistry and orthopedic for biomedical engineering applications for drug delivery, bone defect fillers, bone cements, etc. In this paper, a comprehensive review has been done, by reviewing different literatures related to synthesis techniques, mechanical properties and property testing, method of calcination and characterization of hydroxyapatite which are product of catfish and bovine bones. The discussion is in relations of the obligatory features vital to attain the best properties for the envisioned bid of bone graft. The process approaches that are capable of fabricating the essential microstructure and the ways to advance the mechanical properties of natural mined HAp are reviewed. The standard values for tensile strength were found to be within the range of 40–300 MPa, compressive strength was 400–900 MPa, while Elastic modulus was 80–120 GPa and fracture toughness was 0.6–1 MPa m1/2 (Ramesh et al. in Ceram Int 44(9):10525–10530, 2018; Landi et al. in J Eur Ceram Soc 20(14–15):2377–2387, 2000; Munar et al. in Dent Mater J 25(1):51–58, 2006). Also, the porosity range was 70–85% (Yang et al. in Am Ceram Soc Bull 89(2):24–32, 2010), density is 3.16 g/cm3 and relative density is 95–99.5% (Ramesh et al. 2018; Landi et al. 2000; Munar et al. 2006). The literature revealed that CaP ratio varies in relation to the source and sintering temperature. For example, for bovine bone, a CaP ratio of 1.7 (Mezahi et al. in J Therm Anal Calorim 95(1):21–29, 2009) and 1.65 (Barakat et al. in J Mater Process Technol 209(7):3408–3415, 2009) was obtained at 1100 °C and 750 °C respectively. Basic understanding on the effect of adding foreign material as a strengthening agent to the mechanical properties of HAp is ground factor for the development of new biomaterial (Natural hydroxyapatite, NHAp). Therefore, it is inferred that upon careful combination of main parameters such as compaction pressures, sintering temperatures, and sintering dwell times for production natural HAp (NHAp), mechanical properties can be enhanced.

Graphic abstract

Composite Inks for Extrusion Printing of Biological and Biomedical Constructs

Extrusion-based three-dimensional (3D) printing is an emerging technology for the fabrication of complex structures with various biological and biomedical applications. The method is based on the layer-by-layer construction of the product using a printable ink. The material used as the ink should possess proper rheological properties and desirable performances. Composite materials, which are extensively used in 3D printing applications, can improve the printability and offer superior performances for the printed constructs. Herein, we review composite inks with a focus on composite hydrogels. The properties of different additives including fibers and nanoparticles are discussed. The performances of various composite inks in biological and biomedical systems are delineated through analyzing the synergistic effects between the composite ink components. Different applications, including tissue engineering, tissue model engineering, soft robotics, and four-dimensional printing, are selected to demonstrate how 3D-printable composite inks are exploited to achieve various desired functionality. This review finally presents an outlook of future perspectives on the design of composite inks.

Methods for studying MLO-Y4 osteocytes in collagen-hydroxyapatite scaffolds in the rotary cell culture system

Purpose: The rotary cell culture system (RCCS) is a common clinorotation device for cell culture. It is also used as a low-shear suspension culture bioreactor to form functionalized 3D tissue constructs and to model microgravity. We sought to develop a 3D scaffold composed of type I collagen and hydroxyapatite (collagen-HA) to characterize MLO-Y4 osteocytes following suspension culture or clinorotation.Materials and Methods: MLO-Y4 cells were embedded in collagen-HA. The scaffold was formed into droplets for suspension culture or wall-adhered to the RCCS for clinorotation. AFM, rheometry, immunofluorescence and qRT-PCR were employed to measure the scaffold stiffness, cell viability and gene expression of cells in collagen-HA scaffolds. Dendritic cells were visualized and quantified and gene expression after suspension culture and clinorotation was compared to static controls.Results: The optimized scaffold for the RCCS consisted of collagen with 6 mg/mL HA which had a stiffness of < 1 kPa. MLO-Y4 cell viability was higher in collagen-HA scaffolds, compared to scaffolds without HA. Collagen-HA scaffolds induced higher osteocyte-specific gene expression compared to cells cultured on 2D plastic. Cells in the scaffold downregulated DMP1, E11, IL-6, and RANKL, and had fewer dendritic cells following suspension culture whereas clinorotation downregulated DMP1 and E11 genes, compared to static controls.Conclusions: Suspension culture for 3 days in collagen-HA stimulates growth of osteocytes but may also desensitize them to mechanical cues. Clinorotation for 3 days in collagen-HA does not stimulate proliferation or expression of mechanosensitive genes, indicating that it may be an effective mechanical unloading environment.

Investigation of the mechanical properties of a bony scaffold for comminuted distal radial fractures: Addition of akermanite nanoparticles and using a freeze-drying technique

One of the methods of repairing the damaged bone is the fabrication of porous scaffold using synergic methods like three-dimensional (3D) printing and freeze-drying technology. These techniques improve the damaged and fracture parts rapidly for better healing bone lesions using bioactive ceramic and polymer. This research, due to the need to increase the mechanical strength of 3D bone scaffolds for better mechanical performance. Akermanite bioceramic as a bioactive and calcium silicate bioceramic has been used besides the polymeric component. In this study, the porous scaffolds were designed using solid work with an appropriate porosity with a Gyroid shape. The prepared Gyroid scaffold was printed using a 3D printing machine with Electroconductive Polylactic Acid (EC-PLA) and then coated with a polymeric solution containing various amounts of akermanite bioceramic as reinforcement. The mechanical and biological properties were investigated according to the standard test. The mechanical properties of the porous-coated scaffold showed stress tolerance up to 30 MPa. The maximum strain obtained was 0.0008, the maximum stress was 32 MPa and the maximum displacement was 0.006 mm. Another problem of bone implants is the impossibility of controlling bone cancer and tumor size. To solve this problem, an electroconductive filament containing Magnetic Nanoparticles (MNPs) is used to release heat and control cancer cells. The mechanical feature of the porous scaffold containing 10 wt% akermanite was obtained as the highest stress tolerance of about 32 MPa with 46% porosity. Regarding the components and prepare the bony scaffold, the MNPs release heat when insert into the magnetic field and control the tumor size which helps the treatment of cancer. In general, it can be concluded that the produced porous scaffold using 3D printing and freeze-drying technology can be used to replace broken bones with the 3D printed EC-PLA coated with 10 wt% akermanite bioceramic with sufficient mechanical and biological behavior for the orthopedic application.

Carbonate substitution significantly affects the structure and mechanics of carbonated apatites

Bone mineral comprises nanoparticles of carbonate-substituted bioapatite similar to hydroxylapatite. Yet mechanical values of macroscopic-sized geological hydroxylapatite are often used to model bone properties due to a lack of experimental data for bioapatite. Here, we investigated the effects of carbonate substitution and hydration on biomimetic apatite response to load using in situ hydrostatic pressure loading and synchrotron x-ray diffraction. We find that increasing carbonate levels reduced the bulk modulus and elastic strain ratio. Elastic constants, determined using computational optimization techniques, revealed that compliance values and elastic moduli decreased with increasing carbonate content, likely a result of decreased bond strength due to CO₃^2- substitution and Ca²⁺ loss. Hydration environment had no clear effects on the elastic properties likely due to dissolution and reprecipitation processes modifying the crystal structure organization. These results reinforce the need to consider carbonate composition when selecting mechanical properties and provide robust data for carbonate-substituted apatite stiffness.

Crystal misorientation correlates with hardness in tooth enamels

The multi-scale hierarchical structure of tooth enamel enables it to withstand a lifetime of damage without catastrophic failure. While many previous studies have investigated structure-function relationships in enamel, the effects of crystal misorientation on mechanical performance have not been assessed. To address this issue, in the present study, we review previously published polarization-dependent imaging contrast (PIC) maps of mouse and human enamel, and parrotfish enameloid, in which crystal orientations were measured and displayed in every 60-nm-pixel. By combining those previous results with the PIC maps of sheep enamel presented here we discovered that, in all enamel(oid)s, adjacent crystals are slightly misoriented, with misorientation angles in the 0°-30° range, and mean 2°-8°. Within this limited range, misorientation is positively correlated with literature hardness values, demonstrating an important structure-property relation, not previously identified. At greater misorientation angles 8°30°, this correlation is expected to reverse direction, but data from different non-enamel systems, with more diverse crystal misorientations, are required to determine if and where this occurs. STATEMENT OF SIGNIFICANCE: We identify a structure-function relationship in tooth enamels from different species: crystal misorientation correlates with hardness, contributing to the remarkable mechanical properties of enamel in diverse animals.

Study of nanoscale deformation mechanisms in bulk hexagonal hydroxyapatite under uniaxial loading using molecular dynamics

Hydroxyapatite (HAP) is a natural bioceramic which is currently used in scaffolds and coatings for the regrowth of osseous tissue but offers poor load-bearing capacity compared to other biomaterials. The deformation mechanisms responsible for the mechanical behavior of HAP are not well understood, although the advent of multiscale modeling offers the promise of improvements in many materials through computational materials science. This work utilizes molecular dynamics to study the nanoscale deformation mechanisms of HAP in uniaxial tension and compression. It was found that deformation mechanisms vary with loading direction in tension and compression leading to significant compression/tension asymmetry and crystal anisotropy. Bond orientation and geometry relative to the loading direction was found to be an indicator of whether a specific bond was involved in the deformation of HAP in each loading case. Tensile failure mechanisms were attributed to stretching and failure in loading case-specific ionic bond groups. The compressive failure mechanisms were attributed to coulombic repulsion in each case, although loading case-specific bond group rotation and displacement were found to affect specific failure modes. The elastic modulus was the highest for both tension and compression along the Z direction (i.e. normal to the basal plane), followed by Y and X.

Surface and mechanical properties of a nanostructured citrate hydroxyapatite coating on pure titanium

The presence of a biomimetic HAP coating on titanium surface, which reduces the structural stiffness, is essential to improve implants biocompatibility and osteointegration. In this study, new citrate-HAP (cHAP) coatings were produced by a simple hydrothermal method on pure titanium (Ti) surface, without requiring any additional pretreatment on this metal surface. The formed cHAP coatings consisting of nanorod-like hydroxyapatite particles, conferred nanoroughness and wettability able to endow improved biological responses. Indeed, the presence of citrate species in the precipitate medium seems to be responsible for controlling the morphology of the new coatings. The presence of citrate groups on the surface of cHAP coatings, identified by chemical composition analysis, due to their implication in bone metabolism can additionally bring an add-value for bone implant applications. From a mechanical point of view, the Finite Element algorithm showing that cHAP coatings tend to decrease the mechanical stress at pure Ti, further favors these new coatings applicability. Overall, the simple and expedite strategy used to developed new biomimetic coatings of citrate-HAP resulted in improved physicochemical, morphological and mechanical properties of Ti, which can endeavor improved implantable materials in bone healing surgical procedures.

The geometrical structure of interfaces in dental enamel: A FIB-STEM investigation

In this study a high resolution structural analysis revealed that enamel prisms are surrounded by an interface that is discontinuous with frequent mineral to mineral contact separated by gaps. This contact manifests either by crystallites bridging the boundary between prismatic and interprismatic enamel or continuous crystallites curving and bridging the interprismatic enamel to the prisms. The geometrical resolution of this TEM investigation of the interfaces is ≤2 nm as a basis for micromechanical models. Within this resolution, contrary to existing structural descriptions of dental enamel structure in materials science literature, here the crystallites themselves are shown to be either in direct contact with each other, sometimes even fusing together, or are separated by gaps. Image analysis revealed that on average only 57 ± 15% of the interface consists of points of no contact between crystallites. This work reveals structural features of dental enamel that contribute important understanding to both the architecture and mechanical properties of this biological material. A new structural model is proposed and the implications for the mechanical properties of dental enamel are discussed. STATEMENT OF SIGNIFICANCE: In this study a high resolution structural analysis, employing focused ion beam and transmission electron microscopy revealed that enamel prisms are surrounded by interfaces that are discontinuous with frequent mineral to mineral contact separated by gaps. Although the interfaces in enamel have been investigated previously, existing studies are lacking in detail considering the geometry and morphology of the interfaces. We think that this result is of great importance when it comes to the understanding of the mechanical properties. In our opinion the concept of soft sheaths is no longer feasible. The resulting observations are included in a new structural model which provides new qualitative insights into the mechanical behavior. Existing analytical models were applied to simulate the new geometrical structure.

Engineered Ureolytic Microorganisms Can Tailor the Morphology and Nanomechanical Properties of Microbial-Precipitated Calcium Carbonate

We demonstrate for the first time that the morphology and nanomechanical properties of calcium carbonate (CaCO₃) can be tailored by modulating the precipitation kinetics of ureolytic microorganisms through genetic engineering. Many engineering applications employ microorganisms to produce CaCO₃. However, control over bacterial calcite morphology and material properties has not been demonstrated. We hypothesized that microorganisms genetically engineered for low urease activity would achieve larger calcite crystals with higher moduli. We compared precipitation kinetics, morphology, and nanomechanical properties for biogenic CaCO₃ produced by two Escherichia coli (E. coli) strains that were engineered to display either high or low urease activity and the native producer Sporosarcina pasteurii. While all three microorganisms produced calcite, lower urease activity was associated with both slower initial calcium depletion rate and increased average calcite crystal size. Both calcite crystal size and nanoindentation moduli were also significantly higher for the low-urease activity E. coli compared with the high-urease activity E. coli. The relative resistance to inelastic deformation, measured via the ratio of nanoindentation hardness to modulus, was similar across microorganisms. These findings may enable design of novel advanced engineering materials where modulus is tailored to the application while resistance to irreversible deformation is not compromised.

Multiscale Toughening Mechanisms in Biological Materials and Bioinspired Designs

Biological materials found in Nature such as nacre and bone are well recognized as light-weight, strong, and tough structural materials. The remarkable toughness and damage tolerance of such biological materials are conferred through hierarchical assembly of their multiscale (i.e., atomic- to macroscale) architectures and components. Herein, the toughening mechanisms of different organisms at multilength scales are identified and summarized: macromolecular deformation, chemical bond breakage, and biomineral crystal imperfections at the atomic scale; biopolymer fibril reconfiguration/deformation and biomineral nanoparticle/nanoplatelet/nanorod translation, and crack reorientation at the nanoscale; crack deflection and twisting by characteristic features such as tubules and lamellae at the microscale; and structure and morphology optimization at the macroscale. In addition, the actual loading conditions of the natural organisms are different, leading to energy dissipation occurring at different time scales. These toughening mechanisms are further illustrated by comparing the experimental results with computational modeling. Modeling methods at different length and time scales are reviewed. Examples of biomimetic designs that realize the multiscale toughening mechanisms in engineering materials are introduced. Indeed, there is still plenty of room mimicking the strong and tough biological designs at the multilength and time scale in Nature.

The hidden structure of human enamel

Enamel is the hardest and most resilient tissue in the human body. Enamel includes morphologically aligned, parallel, ∼50 nm wide, microns-long nanocrystals, bundled either into 5-μm-wide rods or their space-filling interrod. The orientation of enamel crystals, however, is poorly understood. Here we show that the crystalline c-axes are homogenously oriented in interrod crystals across most of the enamel layer thickness. Within each rod crystals are not co-oriented with one another or with the long axis of the rod, as previously assumed: the c-axes of adjacent nanocrystals are most frequently mis-oriented by 1°-30°, and this orientation within each rod gradually changes, with an overall angle spread that is never zero, but varies between 30°-90° within one rod. Molecular dynamics simulations demonstrate that the observed mis-orientations of adjacent crystals induce crack deflection. This toughening mechanism contributes to the unique resilience of enamel, which lasts a lifetime under extreme physical and chemical challenges.

A porous polymeric-hydroxyapatite scaffold used for femur fractures treatment: fabrication, analysis, and simulation

BACKGROUND

One of the most common fractures in the skeleton happens in the femur. One of the important reasons for this fracture is because it is the longest bone in the body and osteoporosis affect this part a lot. The geometric complexity and anisotropy properties of this bone have received a lot of attention in the orthopedic field.

METHODS

In this research, a femur designed using 3D printing machine using the middle part of the hip made of polylactic acid-hydroxyapatite (PLA-HA) nanocomposite containing 0, 5, 10, 15, and 25 wt% of ceramic nanoparticle. Three different types of loadings, including centralized loading, full-scale, and partially loaded, were applied to the designed femur bone. The finite element analysis was used to analyze biomechanical components.

RESULTS

The results of the analysis showed that it is possible to use the porous scaffold model for replacement in the femur having proper strength and mechanical stability. Stress-strain analysis on femoral implant with biometric HA and PLA after modeling was performed using the finite element method under static conditions in Abaqus software.

CONCLUSION

Three scaffold structures, i.e., mono-, hybrid, and zonal structures, that can be fabricated using current bioprinting techniques are also discussed with respect to scaffold design.

Strontium doped hydroxyapatite from Mercenaria clam shells: Synthesis, mechanical and bioactivity study

Synthesis of strontium-doped hydroxyapatite from Mercenaria clam shells has been carried out by hydrothermal method. The doping of bioceramic, processed from biogenic resources is mostly unexplored. The objective is to understand the effect of strontium (Sr) incorporation on phase stability, sintering behaviour, mechanical properties and cytotoxicity of hydroxyapatite (HAp) derived from clam shells. The different molar concentrations of Sr, varies from 10, 30, 50, 70% of Ca, were substituted into the HAp. The synthesized powders were sintered at 1200 °C in air. The as synthesized powders and sintered specimens were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and high resolution transmission electron microscopy. The crystallite size and cell parameters of sintered specimens were analyzed from XRD. The XRD of hydrothermally synthesized powders mostly matched with HAp with slight shifting due to Sr doping. However, some distinct Sr based compounds were also observed where Sr substitution is more that 50% of Ca. The XRD of sintered specimen showed increasing β-tricalcium phosphate (β-TCP) phase with Sr substitution. The sintered density of solid samples gradually increased from 3.04 g/cc to 3.50 g/cc and surface energy decreased with increasing Sr substitution. Similarly, microhardness, fracture toughness and nanohardness of solid samples found to be enhanced with Sr substitution. The elastic modulus gradually increased from 130 to 137 GPa for HAp and Sr substituted HAp (70% of Ca). The in vitro cytotoxicity of sintered specimen against mouse osteoblast cell line showed that all the samples were nontoxic. However cell proliferation found low for the solid samples containing more than 50% Sr substitution.

Nano-indentation of native phytoliths and dental tissues: implications for herbivore-plant combat and dental wear proxies

Tooth wear induced by abrasive particles is a key process affecting dental function and life expectancy in mammals. Abrasive particles may be plant endogenous opal phytoliths, exogene wind-blown quartz dust or rain borne mineral particles ingested by mammals. Nano-indentation hardness of abrasive particles and dental tissues is a significant yet not fully established parameter of this tribological system. We provide consistent nano-indentation hardness data for some of the major antagonists in the dental tribosystem (tooth enamel, tooth dentine and opaline phytoliths from silica controlled cultivation). All indentation data were gathered from native tissues under stable and controlled conditions and thus maximize comparability to natural systems. Here we show that native (hydrated) wild boar enamel exceeds any hardness measures known for dry herbivore tooth enamel by at least 3 GPa. The native tooth enamel is not necessarily softer then environmental quartz grit, although there is little overlap. The native hardness of the tooth enamel exceeds that of any silica phytolith hardness recently published. Further, we find that native reed phytoliths equal native suine dentine in hardness, but does not exceed native suine enamel. We also find that native suine enamel is significantly harder than dry enamel and dry phytoliths are harder than native phytoliths. Our data challenge the claim that the culprit of tooth wear may be the food we chew, but suggest instead that wear may relates more to exogenous than endogenous abrasives.

Parrotfish Teeth: Stiff Biominerals Whose Microstructure Makes Them Tough and Abrasion-Resistant To Bite Stony Corals

Parrotfish (Scaridae) feed by biting stony corals. To investigate how their teeth endure the associated contact stresses, we examine the chemical composition, nano- and microscale structure, and the mechanical properties of the steephead parrotfish Chlorurus microrhinos tooth. Its enameloid is a fluorapatite (Ca₅(PO₄)₃F) biomineral with outstanding mechanical characteristics: the mean elastic modulus is 124 GPa, and the mean hardness near the biting surface is 7.3 GPa, making this one of the stiffest and hardest biominerals measured; the mean indentation yield strength is above 6 GPa, and the mean fracture toughness is ∼2.5 MPa·m^1/2, relatively high for a highly mineralized material. This combination of properties results in high abrasion resistance. Fluorapatite X-ray absorption spectroscopy exhibits linear dichroism at the Ca L-edge, an effect that makes peak intensities vary with crystal orientation, under linearly polarized X-ray illumination. This observation enables polarization-dependent imaging contrast mapping of apatite, a method to quantitatively measure and display nanocrystal orientations in large, pristine arrays of nano- and microcrystalline structures. Parrotfish enameloid consists of 100 nm-wide, microns long crystals co-oriented and assembled into bundles interwoven as the warp and the weave in fabric and therefore termed fibers here. These fibers gradually decrease in average diameter from 5 μm at the back to 2 μm at the tip of the tooth. Intriguingly, this size decrease is spatially correlated with an increase in hardness.

The Functional Response of Mesenchymal Stem Cells to Electron-Beam Patterned Elastomeric Surfaces Presenting Micrometer to Nanoscale Heterogeneous Rigidity

Cells directly probe and respond to the physicomechanical properties of their extracellular environment, a dynamic process which has been shown to play a key role in regulating both cellular adhesive processes and differential cellular function. Recent studies indicate that stem cells show lineage-specific differentiation when cultured on substrates approximating the stiffness profiles of specific tissues. Although tissues are associated with a range of Young's modulus values for bulk rigidity, at the subcellular level, tissues are comprised of heterogeneous distributions of rigidity. Lithographic processes have been widely explored in cell biology for the generation of analytical substrates to probe cellular physicomechanical responses. In this work, it is shown for the first time that that direct-write e-beam exposure can significantly alter the rigidity of elastomeric poly(dimethylsiloxane) substrates and a new class of 2D elastomeric substrates with controlled patterned rigidity ranging from the micrometer to the nanoscale is described. The mechanoresponse of human mesenchymal stem cells to e-beam patterned substrates was subsequently probed in vitro and significant modulation of focal adhesion formation and osteochondral lineage commitment was observed as a function of both feature diameter and rigidity, establishing the groundwork for a new generation of biomimetic material interfaces.

On the relationship between indentation hardness and modulus, and the damage resistance of biological materials

The remarkable mechanical performance of biological materials is based on intricate structure-function relationships. Nanoindentation has become the primary tool for characterising biological materials, as it allows to relate structural changes to variations in mechanical properties on small scales. However, the respective theoretical background and associated interpretation of the parameters measured via indentation derives largely from research on 'traditional' engineering materials such as metals or ceramics. Here, we discuss the functional relevance of indentation hardness in biological materials by presenting a meta-analysis of its relationship with indentation modulus. Across seven orders of magnitude, indentation hardness was directly proportional to indentation modulus. Using a lumped parameter model to deconvolute indentation hardness into components arising from reversible and irreversible deformation, we establish criteria which allow to interpret differences in indentation hardness across or within biological materials. The ratio between hardness and modulus arises as a key parameter, which is related to the ratio between irreversible and reversible deformation during indentation, the material's yield strength, and the resistance to irreversible deformation, a material property which represents the energy required to create a unit volume of purely irreversible deformation. Indentation hardness generally increases upon material dehydration, however to a larger extent than expected from accompanying changes in indentation modulus, indicating that water acts as a 'plasticiser'. A detailed discussion of the role of indentation hardness, modulus and toughness in damage control during sharp or blunt indentation yields comprehensive guidelines for a performance-based ranking of biological materials, and suggests that quasi-plastic deformation is a frequent yet poorly understood damage mode, highlighting an important area of future research.

STATEMENT OF SIGNIFICANCE

Instrumented indentation is a widespread tool for characterising the mechanical properties of biological materials. Here, we show that the ratio between indentation hardness and modulus is approximately constant in biological materials. A simple elastic-plastic series deformation model is employed to rationalise part of this correlation, and criteria for a meaningful comparison of indentation hardness across biological materials are proposed. The ratio between indentation hardness and modulus emerges as the key parameter characterising the relative amount of irreversible deformation during indentation. Despite their comparatively high hardness to modulus ratio, biological materials are susceptible to quasiplastic deformation, due to their high toughness: quasi-plastic deformation is hence hypothesised to be a frequent yet poorly understood phenomenon, highlighting an important area of future research.

Molecular dynamics simulations of the mechanical properties of monoclinic hydroxyapatite

Inorganic biomedical materials are of great interest in the biomedical field. One such material, hydroxyapatite (HAP), is the main inorganic substance in the hard tissue of bones and teeth in the human body. Until recently, studies of the mechanical properties of HAP by uniaxial tension testing and compression molecular dynamics (MD) simulation had remained difficult. In addition, electric charges used in such simulations alter the molecular structure of HAP. Here, we present the mechanical properties and new charges of the monoclinic form of HAP using the self-consistent charge equilibration (QEq) scheme proposed by Rappé and Goddard in 1991 (J Phys Chem 95:3358-3363), and a successful MD simulation of the uniaxially tensile and compressive properties of monoclinic HAP. Also presented is the change in potential energy in the cell and how temperature and strain rate affect the uniaxial tension and compressive properties of HAP. Additionally, we compare the mechanical properties of this substance along different directions, and some conclusions useful for further studies of the mechanical properties of HAP composite materials are discussed.

Nanoindentation creep behavior of enamel biological nanocomposites

Organic matrix and water are essential factors for enamel biological nanocomposite to resist external force as revealed by nanoindentation creep test.

Micro-Raman spectroscopy shows how the coating process affects the characteristics of hydroxylapatite

The diversity in the structural and chemical state of apatites allows implant manufacturers to fine-tune implant properties. This requires suitable manufacturing processes and characterization tools to adjust the amorphous phase and hydroxyl content from the source hydroxylapatite. Hydroxylapatite was processed by high-velocity oxy-fuel spraying, plasma spraying and flame spraying, and primarily analyzed by Raman spectroscopy. Investigation of rounded splats, the building blocks of thermal spray coatings, allowed correlation between the visual identity of the splat surface and the Raman spectra. Splats were heat-treated to crystallize any remaining amorphous phase. The ν1 PO4 stretching peak at 950-970 cm(-1) displayed the crystalline order, but the hydroxyl peak at 3572 cm(-1) followed the degree of dehydroxylation. Hydroxyl loss was greatest for flame-sprayed particles, which create the longest residence time for the melted particle. Higher-frequency hydroxyl peaks in flame- and plasma-sprayed splats indicated a lower structural order for the recrystallized hydroxylapatite within the splats. Crystallization at 700 °C has shown potential for revealing hydroxyl ions previously trapped in amorphous calcium phosphate. This work compares Fourier transform infrared and Raman spectroscopy to measure the hydroxyl content in rapidly solidified apatites and shows that Raman spectroscopy is more suitable.

Theoretical nonlinear response of complex single crystal under multi-axial tensile loading

The mechanical properties of single crystals are of interest as they represent the behavior of the basic building blocks. Using the density functional theory based ab initio technique we have devised an approach to analyze the behavior of single crystal so their mechanical properties can be studied beyond linear elasticity. Here we have applied the approach to investigate the mechanical properties of a single stoichiometric hydroxyapatite (HAP) crystal using a large supercell subjected to multi-axial tensile loading. The results reveal a complex nonlinear and loading-path dependent behavior with evolving anisotropy for the HAP crystal. Further, we have introduced a failure envelope index to quantify the strength behavior for comparison of similar materials. We have found that the complexities of the behavior of a single crystal originate from the local structural changes in these multi-component materials.

Contact nanofatigue shows crack growth in amorphous calcium phosphate on Ti, Co-Cr and Stainless steel

Fatigue testing of load-bearing coated implants is usually very time-consuming and so a new contact nanofatigue test using a nanoindenter has been evaluated. A cube corner indenter provided the fastest indication of failure, through crack formation, compared to a spherical indenter. Contact nanofatigue was performed on a sintered hydroxyapatite and then on amorphous calcium phosphate splats produced on titanium, stainless steel and Co-Cr surfaces, made either at room temperature or on 250°C preheated surfaces. Sintered hydroxyapatite showed continual plastic deformation, but this is not that apparent for splats on metal surfaces. Substrate preheating was found to induce cracking in splats, explained by greater thermal residual stresses. Endurance during contact nanofatigue, measured as time to crack formation, was the lowest for splats on titanium followed by Co-Cr and stainless steel. The splat on titanium showed both cracking and plastic deformation during testing. Good agreement has been reached with previous studies with cracking directed to the substrate without splat delamination. Contact nanofatigue with the nanoindenter easily and quickly identifies cracking events that previously required detection with acoustic emission, and shows good feasibility for mechanical testing of discs and splats produced by thermal spraying, spray forming, laser-ablation, aerosol jet and ink jet printing.

Fracture mechanics of hydroxyapatite single crystals under geometric confinement

Geometric confinement to the nanoscale, a concept that refers to the characteristic dimensions of structural features of materials at this length scale, has been shown to control the mechanical behavior of many biological materials or their building blocks, and such effects have also been suggested to play a crucial role in enhancing the strength and toughness of bone. Here we study the effect of geometric confinement on the fracture mechanism of hydroxyapatite (HAP) crystals that form the mineralized phase in bone. We report a series of molecular simulations of HAP crystals with an edge crack on the (001) plane under tensile loading, and we systematically vary the sample height whilst keeping the sample and the crack length constant. We find that by decreasing the sample height the stress concentration at the tip of the crack disappears for samples with a height smaller than 4.15nm, below which the material shows a different failure mode characterized by a more ductile mechanism with much larger failure strains, and the strength approaching that of a flaw-less crystal. This study directly confirms an earlier suggestion of a flaw-tolerant state that appears under geometric confinement and may explain the mechanical stability of the reinforcing HAP platelets in bone.

The Distribution of Carbonate in Enamel and its Correlation with Structure and Mechanical Properties

The correlation of carbonate content with enamel microstructure (chemical and crystal structure) and mechanical properties was evaluated via linear mapping analyses using Raman microspectroscopy and nanoindentation. Mappings started at the outer enamel surface and ended in the inner enamel near the dentin-enamel junction (DEJ) in lingual and buccal cervical and cuspal regions. The carbonate peak intensity at 1070 cm^-1 gradually increased from outer to inner enamel. Moreover, the phosphate peak width, as measured by the full width at half maximum (FWHM) of the peak at 960 cm^-1, also increased, going from ~9 cm^-1 in outer enamel to ~13 cm^-1 in enamel adjacent to the DEJ, indicating a decrease in the degree of crystallinity of hydroxyapatite from outer to inner enamel. In contrast, Young's modulus decreased from 119±12 to 80±19 GPa across outer to inner enamel with a concomitant decrease in enamel hardness from 5.9±1.4 to 3.5±1.3 GPa. There were also significant correlations between carbonate content and associated crystallinity with mechanical properties. As carbonate content increased, there was an associated decrease in crystallinity and both of these changes correlated with decreased modulus and hardness. Collectively, these results suggest that enamel carbonate content and the associated change in the crystal structure of hydroxyapatite, i.e. degree of crystallinity, may have a direct effect on enamel mechanical properties. The combination of Raman microspectroscopy and nanoindentation proved to be an effective approach for evaluating the microstructure of enamel and its associated properties.

Synthesis and characterisation of large chlorapatite single-crystals with controlled morphology and surface roughness

This work describes the synthesis of chlorapatite single crystals using the molten salt method with CaCl(2) as a flux. By manipulating the processing conditions (amount of flux, firing time and temperature, and cooling rates) it is possible to manipulate the crystal morphology from microscopic fibres to large crystals (up to few millimetre long and ~100 μm thick). The crystal roughness can be controlled to achieve very flat surfaces by changing the melt composition "in situ" at high temperature. The Young modulus and hardness of the crystals are 110 ± 15 and 6.6 ± 1.5 GPa respectively as measured by nanoindentation. Crystal dissolution in Hanks solution starts around the defects. Several in vitro assays were performed; ClAp crystals with different size and shape are biocompatible. Cell apoptosis was very low at 5, 10, and 15 days (Caspase-3) for all the samples. Proliferation (MTT) showed to be influenced by surface roughness and size of the crystals.

Cl–OH ion-exchanging process in chlorapatite (Ca5(PO4)3Cl x (OH)1 − x ) – a deep insight

We have synthesized large chlorapatite [ClAp, Ca5(PO4)3Cl x (OH)1 − x , where x = 1] single crystals using the molten salt method. We have corroborated that the hexagonal symmetry P63/m describes the crystal structure best, even though the crystals are synthetic and stoichiometric. Moreover, we have performed several thermal treatments on these ClAp crystals, generating new single crystals in the apatite system [Ca5(PO4)3Cl x (OH)1 − x , where x ≤ 1], where the chloride anions (Cl−) were systematically substituted by hydroxyl anions (OH−). These new single crystals were methodically characterized by powder and single-crystal X-ray diffraction (SXRD), scanning electron microscopy (SEM), Fourier transform–IR spectroscopy (FT–IR), and energy-dispersive X-ray spectroscopy (EDS). We have discovered a previously unreported OH− inclusion site substituting the Cl− anion during the ion-exchanging process. Finally, we evaluated the atomic rearrangements of the other species involved in the structure. These movements are associated with ionic exchange, which can be justified from an energetic point of view. We also found a novel phase transformation at high temperature. When the crystals are heated over 1753 K the apatite system evolves to a less ordered monoclinic structure, in which the complete loss of the species in the anionic channel (Cl−, OH−) has been confirmed.

Regulation of enamel hardness by its crystallographic dimensions

Enamel is a composite biomaterial comprising a minor organic matrix (~2%) and a hierarchically organized inorganic ultrastructure (~96-98%). Surprisingly, to date there is no available information in the literature regarding the possible role of the enamel ultrastructure on the nanoscale level in tooth macroscopic properties. Understanding this relationship is of special interest for restorative purposes in dentistry. Accordingly, this study was designed to investigate how enamel nanocrystals regulate its hardness. We performed microindentation analysis on 100 extracted human teeth. The tooth enamel hardness was quantified and correlated with changes in enamel chemical composition and crystallographic dimensions obtained from Fourier transform infrared spectroscopy and X-ray diffraction, respectively. Enamel hardness was not related to the variability in organic content, but was associated with the size of apatite crystals along the c-axis. This association followed the Hall-Petch model for polycrystalline materials, indicating that the optimal size of apatite nanocrystals (larger than the critical size) provides enamel with the greatest hardness, which enables teeth to survive the heavy wear over a human lifetime.

Nano-indentation on amorphous calcium phosphate splats: effect of droplet size on mechanical properties

Droplet processing technologies and many biological processes use disk-like or hemispherical shapes for construction or the design of surfaces. The ability to tune the characteristics and properties of a surface is important at the micro- and nano-scale. The influence of size on the mechanical properties is presently unknown. This work set out to produce splats from different droplet sizes (20-40 μm, 40-60 μm and 60-80 μm), and then determine the deposit characteristics and mechanical properties. All splats produced by melting particles in a flame and depositing onto a polished titanium surface were amorphous, as determined by Raman micro-spectrometry. The topography shown in an optical and scanning electron microscope and topographically mapped using the scanning mode of the nano-indenter revealed a flattened hemispherical deposit. The critical nano-indentation load for determining the true hardness decreased with increasing splat size; for 20-40 μm, 40-60 μm and 60-80 μm splats the critical load was 19, 16, 11 mN respectively compared to 30 mN for sintered hydroxyapatite. Higher loads are required to cause cracking and delamination in smaller splats. A load between 40 and 60 mN was required for delamination of the splat. Delamination of the splats could offer a new means to determine the adhesion of splats on low roughness surfaces.

Structure, composition, and mechanical properties of shark teeth

The teeth of two different shark species (Isurus oxyrinchus and Galeocerdo cuvier) and a geological fluoroapatite single crystal were structurally and chemically characterized. In contrast to dentin, enameloid showed sharp diffraction peaks which indicated a high crystallinity of the enameloid. The lattice parameters of enameloid were close to those of the geological fluoroapatite single crystal. The inorganic part of shark teeth consisted of fluoroapatite with a fluoride content in the enameloid of 3.1 wt.%, i.e., close to the fluoride content of the geological fluoroapatite single crystal (3.64 wt.%). Scanning electron micrographs showed that the crystals in enameloid were highly ordered with a special topological orientation (perpendicular towards the outside surface and parallel towards the center). By thermogravimetry, water, organic matrix, and biomineral in dentin and enameloid of both shark species were determined. Dentin had a higher content of water, organic matrix, and carbonate than enameloid but contained less fluoride. Nanoindentation and Vicker's microhardness tests showed that the enameloid of the shark teeth was approximately six times harder than the dentin. The hardness of shark teeth and human teeth was comparable, both for dentin and enamel/enameloid. In contrast, the geological fluoroapatite single crystal was much harder than both kinds of teeth due to the absence of an organic matrix. In summary, the different biological functions of the shark teeth ("tearing" for Isurus and "cutting" for Galeocerdo) are controlled by the different geometry and not by the chemical or crystallographic composition.

Amorphous calcium phosphate offers improved crack resistance: a design feature from nature?

Amorphous calcium phosphate (ACP) is found in biological organisms and coated implants, used in calcium phosphate cements, and has been recently confirmed as a precursor in mineralized tissue; however, nothing is known about crack initiation in ACP or its fracture toughness. The objective of this study was to determine the crack resistance of ACP to help understand its role in biological organisms and assist in the design of calcium-phosphate-based implants. ACP was manufactured by quenching droplets to form a bulk sample and individual splats. Testing of Berkovich and cube-corner indenter types revealed that the Berkovich indenter was more suitable, providing ease of crack detection. Nanoindentation was performed on polished ACP and hydroxyapatite (HAp), and cracks were identified with scanning electron microscopy. Additional nanoindentation was done on splats to assess the suitability for testing microarrays used in high throughput discovery of new bioceramics. ACP required about three times more force to initiate a crack compared to sintered HAp, but about nine times more than a single crystal. Crack initiation resistance decreased with increasing grain size. The fracture toughness of ACP was comparable to a single crystal, but higher for nanograined HAp. The crack initiation load can be potentially used for evaluating microsized samples. ACP prevents crack formation, but requires the presence of nanograins to provide a greater toughness. The implications of the higher crack initiation load in ACP are discussed for biological organisms and thermally processed biomaterials such as thermally sprayed and sintered HAp.

In vitro cell performance on hydroxyapatite particles/poly(L-lactic acid) nanofibrous scaffolds with an excellent particle along nanofiber orientation

Highly porous hydroxyapatite (HA)/poly(L-lactide) (PLLA) nanofibrous scaffolds were prepared by incorporating needle-shaped nano- or micro-sized HA particles into PLLA nanofibers using electrospinning. The scaffolds had random or aligned fibrous assemblies and both types of HA particles were perfectly oriented along the fiber long axes. The biocompatibility and cell signaling properties of these scaffolds were evaluated by in vitro culture of rat osteosarcoma ROS17/2.8 cells on the scaffold surface. Cell morphology, viability and alkaline phosphatase (ALP) activity on each scaffold were examined at different time points. The HA/PLLA scaffolds exhibited higher cell viability and ALP activity than a pure PLLA scaffold. In addition, micro-sized HA particles supported cell proliferation and differentiation better than nano-sized ones in random scaffolds through a 10 day culture period and in aligned scaffolds at an early culture stage. The fibrous assembly of the scaffold had a pronounced impact on the morphology of the cells in direct contact with the scaffold surface, but not on cell proliferation and differentiation. Thus, HA/PLLA nanofibrous scaffolds could be good candidates for bone tissue engineering.