First-principles study of substitutional magnesium and zinc in hydroxyapatite and octacalcium phosphate

Efficacy of sintered Zinc-doped fluorapatite scaffold as an antimicrobial regenerative bone filler for dental applications

OBJECTIVES

The objective of this study was to assess whether zinc-doped fluorapatite (ZnFA) could serve as an effective antimicrobial dental bone filler for bone regeneration compared to autografts.

METHODS

FA and 2 % zinc-doped FA (2ZnFA) were synthesized and characterized in-house. Compressed and sintered FA and 2ZnFA disks were incubated with bacteria to assess antimicrobial properties. Adipose-derived stem cells were cultured on these discs to evaluate the surfaces' ability to support cell growth and promote osteogenic differentiation. Surfaces exhibiting the highest expressions of the bone markers osteopontin and osteocalcin were selected for an in vivo study in a rat mandibular defect model. Twenty rats were divided into 5 groups, equally, and a 5 mm surgical defect of the jaw was left untreated or filled with 2ZnFA, FA, autograft, or demineralized bone matrix (DBM). At 12 weeks, the defects and surrounding tissues were harvested and subjected to microCT and histological evaluations.

RESULTS

Standard techniques such as FTIR, ICP-MS, fluoride probe, and XRD revealed the sintered FA and ZnFA's chemical compositions and structures. Bacterial studies revealed no significant differences in surface bacterial adhesion properties between FA and 2ZnFA, but significantly fewer bacterial loads than control titanium discs (p < 0.05). Cell culture data confirmed that both surfaces could support cell growth and promote the osteogenic differentiation of stem cells. MicroCT analysis confirmed statistical similarities in bone regeneration within FA, 2ZnFA, and autograft groups.

CONCLUSION

The data suggests that both FA and 2ZnFA could serve as alternatives to autograft materials, which are the current gold standard. Moreover, these bone fillers outperformed DBM, an allograft material commonly used as a dental bone void filler.

CLINICAL SIGNIFICANCE

The use of FA or 2ZnFA for treating mandibular defects led to bone regeneration statistically similar to autograft repair and significantly outperformed the widely used dental bone filler, DBM. Additional translational research may confirm FA-based materials as superior substitutes for existing synthetic bone fillers, ultimately enhancing patient outcomes.

Enhanced near-infrared optical transmission in zinc germanium phosphide crystals via precise magnesium doping

A zinc germanium phosphorus (ZnGeP2) crystal with a chalcopyrite structure is an efficient frequency converter in the mid-infrared region. However, point defect-induced optical absorption at the pumping wavelength (near infrared region) blocked the further application of ZnGeP2. To alleviate the absorption losses caused by point defects, in situ magnesium doping compensation was presented during the ZnGeP2 bulk crystal growth process via the vertical Bridgman method. Combined with theoretical calculations, the structural distortion of the magnesium-doped ZnGeP2 crystals in different orientations was illustrated. The thermodynamic and kinetic stability of the magnesium-doped ZnGeP2 structure were demonstrated. The transmission results indicated the improvement of transmittance within a wavelength range of 1.8-2.4 μm when doped with magnesium, which revealed the powerful ability of the appropriate dopant in optimizing near-infrared optical properties. Thus, the introduction of magnesium is a practical approach to improve the transmittance performance and extend the pumping source wavelengths of ZnGeP2 crystals.

In Vitro Evaluation of Ag- and Sr-Doped Hydroxyapatite Coatings for Medical Applications

Osseointegration plays the most important role in the success of an implant. One of the applications of hydroxyapatite (HAp) is as a coating for metallic implants due to its bioactive nature, which improves osteoconduction. The purpose of this research was to assess the in vitro behavior of HAp undoped and doped with Ag and/or Sr obtained by galvanostatic pulsed electrochemical deposition. The coatings were investigated in terms of chemical bonds, contact angle and surface free energy, electrochemical behavior, in vitro biomineralization in acellular media (SBF and PBS), and biocompatibility with preosteoblasts cells (MC3T3-E1 cell line). The obtained results highlighted the beneficial impact of Ag and/or Sr on the HAp. The FTIR spectra confirmed the presence of hydroxyapatite within all coatings, while in terms of wettability, the contact angle and surface free energy investigations showed that all surfaces were hydrophilic. The in vitro behavior of MC3T3-E1 indicated that the presence of Sr in the HAp coatings as a unique doping agent or in combination with Ag elicited improved cytocompatibility in terms of cell proliferation and osteogenic differentiation. Therefore, the composite HAp-based coatings showed promising potential for bone regeneration applications.

Lanthanides-Substituted Hydroxyapatite for Biomedical Applications

Lately, there has been an increasing demand for materials that could improve tissue regenerative therapies and provide antimicrobial effects. Similarly, there is a growing need to develop or modify biomaterials for the diagnosis and treatment of different pathologies. In this scenario, hydroxyapatite (HAp) appears as a bioceramic with extended functionalities. Nevertheless, there are certain disadvantages related to the mechanical properties and lack of antimicrobial capacity. To circumvent them, the doping of HAp with a variety of cationic ions is emerging as a good alterative due to the different biological roles of each ion. Among many elements, lanthanides are understudied despite their great potential in the biomedical field. For this reason, the present review focuses on the biological benefits of lanthanides and how their incorporation into HAp can alter its morphology and physical properties. A comprehensive section of the applications of lanthanides-substituted HAp nanoparticles (HAp NPs) is presented to unveil the potential biomedical uses of these systems. Finally, the need to study the tolerable and non-toxic percentages of substitution with these elements is highlighted.

In situ synthesis of magnesium-doped hydroxyapatite aerogel for highly efficient U(VI) separation with ultra high adsorption capacity and excellent recyclability

Mg-doped HAP aerogel (MHAPA) was firstly in situ prepared via freeze-drying-calcination technology to capture U(VI). The U(VI) removal capacity by MHAPA even arrived 2685.6 mg g^-1, which was about 2 times over purchased HAP, illustrating that the incorporation of Mg ions could greatly enhance the U(VI) removal capacity. Compared with HAP, MHAPA also showed better anti-ion interference ability and dynamic removal performances. In comparison with other HAP-based adsorbents, MHAPA possessed good recyclability and its desorption rate was up to 93.4% in the first cycle. The excellent U(VI) removal performances of MHAPA might be owing to its low crystallinity and grain size, fast ion exchange rate and partial ionization under acidic conditions, which would accelerate the process of electrostatic attraction, ion-exchange, and complexation to immobilize U(VI). To sum up, the prepared MHAPA was expected to be an environmentally friendly, recyclable and effective adsorbent to immobilize U(VI) in actual wastewater.

The Mutual Incorporation of Mg2+ and CO32- into Hydroxyapatite: A DFT Study

Hydroxyapatite (HA) with a stoichiometry composition of Ca₁₀(PO₄)₆(OH)₂ is widely applied for various biomedical issues, first of all for bone defect substitution, as a catalyst, and as an adsorbent for soil and water purification. The incorporation of foreign ions changes the acid-base relation, microstructure, porosity, and other properties of the HA materials. Here, we report the results of calculations of the density functional theory and analyze the possibility of two foreign ions, CO₃^2- and Mg²⁺, to be co-localized in the HA structure. The Na⁺ was taken into account for charge balance preservation. The analysis revealed the favorable incorporation of CO₃^2- and Mg²⁺ as a complex when they interact with each other. The energy gain over the sole ion incorporation was pronounced when CO₃^2- occupied the A position and Mg²⁺ was in the Ca(2) position and amounted to -0.31 eV. In the most energy-favorable complex, the distance between Mg²⁺ and the O atom of carbonate ion decreased compared to Mg…O distances to the surrounding phosphate or hydroxide ions, and amounted to 1.98 Å. The theoretical calculations agree well with the experimental data reported earlier. Understating the structure-properties relationship in HA materials varying in terms of composition, stoichiometry, and morphology paves the way to rational designs of efficient bio-based catalytic systems.

A New, Biomimetic Collagen-Apatite Wound-Healing Composite with a Potential Regenerative and Anti-Hemorrhagic Effect in Dental Surgery

The aim of this work was to obtain and characterize composite biomaterials containing two components, namely carbonated hydroxyapatite, which was substituted with Mg²⁺ and Zn²⁺ ions, and natural polymer-collagen protein. The following two different types of collagen were used: lyophilized powder of telocollagen from bovine Achilles tendon and atelocollagen solution from bovine dermis. The obtained 3D materials were used as potential matrices for the targeted delivery of tranexamic acid for potential use in wound healing after tooth extractions. Tranexamic acid (TXA) was introduced into composites by two different methods. The physicochemical analyses of the obtained composites included Fourier-transform infrared spectroscopy (FT-IR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), transmission electron microscopy (TEM), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), release kinetics tests, swelling test, and cytotoxicity assays. The studies showed that the proposed synthetic methods yielded biomaterials with favorable physicochemical properties, as well as the expected release profile of the drug and ions from the matrices.

Current Development in Biomaterials-Hydroxyapatite and Bioglass for Applications in Biomedical Field: A Review

Inorganic biomaterials, including different types of metals and ceramics are widely used in various fields due to their biocompatibility, bioactivity, and bioresorbable capacity. In recent years, biomaterials have been used in biomedical and biological applications. Calcium phosphate (CaPs) compounds are gaining importance in the field of biomaterials used as a standalone material or in more complex structures, especially for bone substitutes and drug delivery systems. The use of multiple dopants into the structure of CaPs compounds can significantly improve their in vivo and in vitro activity. Among the general information included in the Introduction section, in the first section of this review paper, the authors provided a background on the development of hydroxyapatite, methods of synthesis, and its applications. The advantages of using different ions and co-ions for substitution into the hydroxyapatite lattice and their influence on physicochemical, antibacterial, and biological properties of hydroxyapatite are also presented in this section of the review paper. Larry Hench's 45S5 Bioglass®, commercially named 45S5, was the first bioactive glass that revealed a chemical bond with bone, highlighting the potential of this biomaterial to be widely used in biomedicine for bone regeneration. The second section of this article is focused on the development and current products based on 45S5 Bioglass®, covering the historical evolution, importance of the sintering method, hybrid bioglass composites, and applications. To overcome the limitations of the original biomaterials, studies were performed to combine hydroxyapatite and 45S5 Bioglass® into new composites used for their high bioactivity and improved properties. This particular type of combined hydroxyapatite/bioglass biomaterial is discussed in the last section of this review paper.

Drug Molecular Immobilization and Photofunctionalization of Calcium Phosphates for Exploring Theranostic Functions

Theranostics (bifunction of therapeutics and diagnostics) has attracted increasing attention due to its efficiency that can reduce the physical and financial burden on patients. One of the promising materials for theranostics is calcium phosphate (CP) and it is biocompatible and can be functionalized not only with drug molecules but also with rare earth ions to show photoluminescence that is necessary for the diagnostic purpose. Such the CP-based hybrids are formed in vivo by interacting between functional groups of organic molecules and inorganic ions. It is of great importance to elucidate the interaction of CP with the photofunctional species and the drug molecules to clarify the relationship between the existing state and function. Well-designed photofunctional CPs will contribute to biomedical fields as highly-functional ormultifunctional theranostic materials at the nanoscales. In this review, we describe the hybridization between CPs and heterogeneous species, mainly focusing on europium(III) ion and methylene blue molecule as the representative photofunctional species for theranostics applications.

Sr and Mg Doped Bi-Phasic Calcium Phosphate Macroporous Bone Graft Substitutes Fabricated by Robocasting: A Structural and Cytocompatibility Assessment

Bi-phasic calcium phosphates (BCPs) are considered prominent candidate materials for the fabrication of bone graft substitutes. Currently, supplemental cation-doping is suggested as a powerful path to boost biofunctionality, however, there is still a lack of knowledge on the structural role of such substituents in BCPs, which in turn, could influence the intensity and extent of the biological effects. In this work, pure and Mg- and Sr-doped BCP scaffolds were fabricated by robocasting from hydrothermally synthesized powders, and then preliminarily tested in vitro and thoroughly investigated physically and chemically. Collectively, the osteoblast cell culture assays indicated that all types of BCP scaffolds (pure, Sr- or Sr–Mg-doped) delivered in vitro performances similar to the biological control, with emphasis on the Sr–Mg-doped ones. An important result was that double Mg–Sr doping obtained the ceramic with the highest β-tricalcium phosphate (β-TCP)/hydroxyapatite mass concentration ratio of ~1.8. Remarkably, Mg and Sr were found to be predominantly incorporated in the β-TCP lattice. These findings could be important for the future development of BCP-based bone graft substitutes since the higher dissolution rate of β-TCP enables an easier release of the therapeutic ions. This may pave the road toward medical devices with more predictable in vivo performance.

Angio-osteogenic capacity of octacalcium phosphate co-precipitated with copper gluconate in rat calvaria critical-sized defect

The objective of this study is to investigate the effects of octacalcium phosphate (OCP)-induced bone regeneration on angiogenesis regulated by the inclusion of copper ions in OCP in vitro and in vivo. Calcium (Ca)-deficient Cu-OCPs, containing 0.01 wt% Cu (low-Cu-OCP) and 0.12 wt% Cu (high-Cu-OCP), were synthesized with co7pper gluconate salt. The lattice parameters of Cu-OCPs tended to decrease slightly with Cu inclusion, as estimated by Rietveld analysis. Cu ions were released in OCP when the materials were incubated in the medium for human umbilical vein endothelial cells (HUVECs). The solubility of Cu-OCPs, estimated by the degree of supersaturation, was slightly higher than that of the original OCP. Cu-OCP tended to hydrolyze to an apatite structure while maintaining the crystal plate-like morphology when incubated with mesenchymal stem D1 cells in osteogenic media for 14 days. The specimens were characterized by selected area electron diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy. Low-Cu-OCP significantly enhanced the HUVEC capillary cross-linking density. D1 cell differentiation was inhibited with the inclusion of Cu, even at low concentrations. The composite of low-Cu-OCP with a gelatin sponge (low-Cu-OCP/Gel) significantly enhanced angiogenesis coupled with bone regeneration when implanted in a rat calvarial critical-sized defect for 4 weeks, compared with the corresponding amount of Cu-containing Gel (Cu/Gel) or OCP/Gel materials through angiography and tissue histomorphometry. These results support the proposition that angiogenesis stimulated by low-Cu-OCP is closely related with enhanced bone regeneration.

Unraveling U6+, Am3+&Eu3+ ion's distribution in Ca10(PO4)6F2for radioactive waste immobilization and the associated U6+→ Eu3+energy transfer dynamics for tunable emission characteristics

A combined photoluminescence (PL) and theoretical study has been performed on Ca₁₀(PO₄)₆F₂:U⁶⁺ and Ca₁₀(PO₄)₆F₂:U⁶⁺,Eu³⁺ compounds in order to explore Ca₁₀(PO₄)₆F₂ as potential host for radioactive waste immobilization by understanding the distribution U⁶⁺, Eu³⁺ and Am³⁺ ions among the lattice sites and the related radiation stability. DFT based calculations on various structures with different distribution of U⁶⁺, Eu³⁺ and Am³⁺ ions showed that Eu³⁺ and Am³⁺ ions prefer to occupy the Ca2 sites while the highly charged U⁶⁺ ions prefer Ca1 site. This is also supported by the PL lifetime study, which provided two lifetime components with different contribution for both U⁶⁺ and Eu³⁺ ions present at two different lattice sites. The PL study of U⁶⁺ doped compounds confirmed the existence of U in the UO₂²⁺ form, which makes it as a pure green emitter. Upon co-doping Eu³⁺ ion, the compounds were transformed to red emitter. Further, there is an energy transfer process from U⁶⁺to Eu³⁺, which shifted the CIE color coordinates towards pure red region while increasing doping level of U⁶⁺. This proves U⁶⁺ as a good sensitizer for Eu³⁺ ion. PL study on gamma irradiated U⁶⁺ doped Ca₁₀(PO₄)₆F₂ compound also showed excellent radiation stability at Ca2 site.

Understanding Zinc-Doped Hydroxyapatite Structures Using the First-Principles Calculations and Convolutional Neural Network Algorithm

Element doping is widely used to improve the performance of materials by changing their intrinsic properties. However, the lack of direct crystallographic structures for dopants has restricted the effective high-throughput...

Octacalcium phosphate: Innovative vehicle for the local biologically active substance delivery in bone regeneration

Disadvantages of conventional drug delivery systems (DDS), such as systemic circulation, interaction with physiochemical factors, reduced bioavailability, and insufficient drug concentration at bone defect site, have underlined the importance of developing efficacious local drug delivery systems. Octacalcium phosphate (OCP) is presumed to be the precursor of biologically formed apatite, owing to its similarity to hydroxyapatite (HAp) and readiness to convert to it. Specific crystal structure of OCP is constructed of compiled apatite layers and water layers, which make possible the incorporation of various ions in its structure, making it feasible to alter the overall effect OCP has in the system. Next to that intrinsic property, characteristics as high solubility, biodegradability and osteoconductivity have made it indispensable to tailor OCP as a carrier material. In this review, we present the main characteristics and progress done on utilizing OCP as an innovative vehicle and provide suggestions for possible research pathways and advantages for local drug delivery in bone tissue engineering. STATEMENT OF SIGNIFICANCE: Octacalcium phosphate (OCP), being a precursor to biologically formed apatite, has many assets when compared to other calcium phosphates. Owing to its highly pertinent structure, it is being used as a vehicle for biologically active substances or ions for bone regeneration. However, orchestrating drug delivery systems with OCP, in order to achieve the best possible outcome, is still a pioneering concept, and the all-encompassing data is still scarce. Although several articles have been published on this matter, to this date there is no systematic overview pointing out the benefits that OCP can bring in the field of drug delivery. Here we offer a comprehensive overview, starting from the OCP synthesis to its structure, morphology, and the biological significance OCP has.

Using DFT to Calculate the Parameters of the Crystal Field in Mn2+ Doped Hydroxyapatite Crystals

Crystal field parameters for two nonequivalent positions Ca (I) and Ca (II) for hydroxyapatite (HAp) crystals from the density functional theory (DFT) are calculated. Calculations are compared with the experimental electron paramagnetic resonance (EPR) spectra (registered at two microwave frequencies) for the synthesized Mn-HAp powders Ca9.995Mn0.005(PO4)6(OH)2. It is found that in the investigated species, the manganese is redistributed between both calcium sites with prevalence in Ca (I). Agreement between the calculated and experimental data proves that crystal field parameters in HAp can be calculated in the classical DFT model using the distributed electron density.

Mesoporous Iron(III)-Doped Hydroxyapatite Nanopowders Obtained via Iron Oxalate

Mesoporous hydroxyapatite (HA) and iron(III)-doped HA (Fe-HA) are attractive materials for biomedical, catalytic, and environmental applications. In the present study, the nanopowders of HA and Fe-HA with a specific surface area up to 194.5 m²/g were synthesized by a simple precipitation route using iron oxalate as a source of Fe³⁺ cations. The influence of Fe³⁺ amount on the phase composition, powders morphology, Brunauer–Emmett–Teller (BET) specific surface area (S), and pore size distribution were investigated, as well as electron paramagnetic resonance and Mössbauer spectroscopy analysis were performed. According to obtained data, the Fe³⁺ ions were incorporated in the HA lattice, and also amorphous Fe oxides were formed contributed to the gradual increase in the S and pore volume of the powders. The Density Functional Theory calculations supported these findings and revealed Fe³⁺ inclusion in the crystalline region with the hybridization among Fe-3d and O-2p orbitals and a partly covalent bond formation, whilst the inclusion of Fe oxides assumed crystallinity damage and rather occurred in amorphous regions of HA nanomaterial. In vitro tests based on the MG-63 cell line demonstrated that the introduction of Fe³⁺ does not cause cytotoxicity and led to the enhanced cytocompatibility of HA.

Octacalcium phosphate bone substitute materials: Comparison between properties of biomaterials and other calcium phosphate materials

Octacalcium phosphate (OCP) is a material that can be converted to hydroxyapatite (HA) under physiological environments and is considered a mineral precursor to bone apatite crystals. The structure of OCP consists of apatite layers stacked alternately with hydrated layers, and closely resembles the structure of HA. The performance of OCP as a bone substitute differs from that of HA materials in terms of their osteoconductivity and biodegradability. OCP manifests a cellular phagocytic response through osteoclast-like cells similar to that exhibited by the biodegradable material β-tricalcium phosphate (β-TCP). The use of OCP for human cranial bone defects involves using its granule or composite form with one of the natural polymers, viz., the reconstituted collagen. This review article discusses the differences and similarities in these calcium phosphate (Ca-P)-based materials from the viewpoint of the structure and their material chemistry, and attempts to elucidate why Ca-P materials, particularly OCP, display unique osteoconductive property.

Toward an efficient antibacterial agent: Zn- and Mg-doped hydroxyapatite nanopowders

The use of synthetic hydroxyapatites (HAps) in biomedical and environmental applications is well warranted given that they have been shown to behave as an excellent bio-compatible material in human teeth and bones. In this paper, a series of HAps doped and co-doped with two metal cations (zinc and magnesium) has been successfully synthesized by means of the precipitation method using CaCl₂, Na₂HPO₄, ZnCl₂ and MgCl₂ aqueous solutions as reagents. The synthesized samples have been characterized using Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray fluorescence spectroscopy (XRF). All samples prepared using over 10 mol% of Zn and Mg ions were identified as HAp. However, the presence of metal cations caused a significant increase in their crystallite sizes (30-50 nm) along with the appearance of a second phase (scholzite, whitlockite). The XRF spectra indicated the presence of Ca, P, Zn and Mg in the powders prepared with a high Metal/P ratio (1.7-2). The antimicrobial activity of these nanopowders has been tested in vitro against five bacteria (Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa as Gram-negative; Staphylococcus aureus and Bacillus subtilis as Gram-positive) and two fungal strains (Candida albicans and Aspergillus niger). The outcomes revealed that these nanopowders exhibited strong antimicrobial activity, starting at 15 mol% of Zn and/or Mg.

Production and Physicochemical Characterization of Cu-Doped Silicate Bioceramic Scaffolds

Development of ion-releasing implantable biomaterials is a valuable approach for advanced medical therapies. In the effort of tackling this challenge, we explored the feasibility of porous bioceramic scaffolds releasing copper ions, which are potentially able to elicit angiogenetic and antibacterial effects. First, small amounts of CuO were incorporated in the base silicate glass during melting and the obtained powders were further processed to fabricate glass–ceramic scaffolds by sponge replica method followed by sinter crystallization. As the release of copper ions from these foams in simulated body fluid (SBF) was very limited, a second processing strategy was developed. Silicate glass–ceramic scaffolds were coated with a layer of Cu-doped mesoporous glass, which exhibited favorable textural properties (ultrahigh specific surface area >200 m²/g, mesopore size about 5 nm) for modulating the release of copper. All the produced scaffolds, containing biocompatible crystals of wollastonite (CaSiO₃), revealed high stability in a biological environment. Furthermore, the materials had adequate compressive strength (>10 MPa) for allowing safe manipulation during surgery. Overall, the results achieved in the present work suggest that these Cu-doped glass-derived scaffolds show promise for biomedical application and motivate further investigation of their suitability from a biological viewpoint.

Synthesis and structure of iron- and strontium-substituted octacalcium phosphate: effects of ionic charge and radius

Octacalcium phosphate (OCP) has received intensive research focus as a main component of bone substitute materials due to its highly osteoconductive and biodegradable characteristics. In this work, OCP was synthesized using chemical precipitation methods. Biologically relevant iron ions (Fe³⁺) and strontium ions (Sr²⁺) which have different ionic charges and radii were successfully introduced into OCP crystal structure, and their effects on the formation, phase components and structure of OCPs were investigated. The incorporation of Fe³⁺ and Sr²⁺ led to lattice expansion of OCP. Both ionic substitutions had slight effects on the morphology and microstructure of typical plate-like OCP crystals. In particular, nanosized particles containing rich Fe were deposited on the surface of plate-like Fe³⁺-substituted OCP crystals, which confirmed the influence of iron substitution on the corresponding crystal surface nature. This work highlights the different replacements of complex Ca sites by Fe and Sr in the apatite layers and hydrated layers of OCP crystal structure, which gives more possible accounts for foreign trivalent and divalent cations.

Calcium orthophosphates (CaPO4): occurrence and properties

The present overview is intended to point the readers' attention to the important subject of calcium orthophosphates (CaPO4). This type of materials is of the special significance for the human beings because they represent the inorganic part of major normal (bones, teeth and antlers) and pathological (i.e., those appearing due to various diseases) calcified tissues of mammals. For example, atherosclerosis results in blood vessel blockage caused by a solid composite of cholesterol with CaPO4, while dental caries and osteoporosis mean a partial decalcification of teeth and bones, respectively, that results in replacement of a less soluble and harder biological apatite by more soluble and softer calcium hydrogenorthophosphates. Therefore, the processes of both normal and pathological calcifications are just an in vivo crystallization of CaPO4. Similarly, dental caries and osteoporosis might be considered as in vivo dissolution of CaPO4. In addition, natural CaPO4 are the major source of phosphorus, which is used to produce agricultural fertilizers, detergents and various phosphorus-containing chemicals. Thus, there is a great significance of CaPO4 for the humankind and, in this paper, an overview on the current knowledge on this subject is provided.

Biomimetic preparation of trace element-codoped calcium phosphate for promoting osteoporotic bone defect repair

Specific implants to speedily regenerate critical-sized osteoporotic bone defects (COBDs) are a major clinical need. However, little progress in methods focusing on biological repair has been reported. We developed a biomimetic mineralization method to prepare trace element-codoped calcium phosphate (CaP) particles via hydrothermal treatment of modified simulated body fluid (SBF) with the addition of binary to quaternary trace elements. The morphology, structure, and composition of the particles were characterized by a combination of SEM, TEM, XRD, and FTIR measurements. The quantitative analysis shows that the dopant contents in the solid phase can be regulated by the trace ion concentrations in the aqueous medium. The conditioned cell culture medium from the quaternary Mg/Zn/Sr/Si-co-doped CaP (qCaP) could significantly enhance cell activity and osteogenic differentiation of ovariectomized rat-derived bone marrow mesenchymal stem cells. After injecting the qCaP-loaded chitosan/hyaluronic acid hydrogel into the COBDs, histology and computed tomography scanning revealed that the new bone regeneration was significantly enhanced, and the quantity of mature bone was substantially increased in the rats implanted with qCaP 12 weeks post-operatively in comparison with the defects filled with the CaP obtained from SBF. These results suggest that the biomimetic mineralization of the trace ion-added SBF allows the preparation of highly bioactive trace element-codoped CaP biomaterials and these materials are potential candidates for the biological repair of COBDs.

Substituted hydroxyapatites for bone repair

Calcium phosphates such as hydroxyapatite have a wide range of applications both in bone grafts and for the coating of metallic implants, largely as a result of their chemical similarity to the mineral component of bone. However, to more accurately mirror the chemistry, various substitutions, both cationic (substituting for the calcium) and anionic (substituting for the phosphate or hydroxyl groups) have been produced. Significant research has been carried out in the field of substituted apatites and this paper aims to summarise some of the key effect of substitutions including magnesium, zinc, strontium, silicon and carbonate on physical and biological characteristics. Even small substitutions have been shown to have very significant effects on thermal stability, solubility, osteoclastic and osteoblastic response in vitro and degradation and bone regeneration in vivo.

Trace element-incorporating octacalcium phosphate porous beads via polypeptide-assisted nanocrystal self-assembly for potential applications in osteogenesis

The promising future of calcium phosphates (CaP) as a group of biomedical materials with a wide range of functions, might ultimately depend on tuning their composition and microstructure. However, the disorderly growth and aggregation of CaP nanocrystals limit their practical application. This paper reports a strategy for designing polypeptide/trace elements (TE), dual mediating the self-assembly of octacalcium phosphate (OCP) nanocrystals, with multilayered porous cross section and TE dilute doping. Intriguing advantages such as bead morphology, mesoporous structure, tunable diameter (20-1,000 μm) and TE contents, biodegradability and bioactivity are obtained. The microcomputerized-tomography reconstruction reveals an interconnective macroporous architecture and a void volume of over 49.02% for the nearly close-packed bead scaffolds. The specific surface area and average mesopore size are 89.73 m(2)g(-1) and 2.75 nm for the 180 μm diameter bead group, and those of 500 μm diameter beads are 130.17 m(2)g(-1) and 3.69 nm, respectively. It is demonstrated that the bead production mechanism is a multistep process including liquid-like precursor formation, nanocrystal nucleation and aggregation, aggregate combination and bead growth. Such a multilayer structure of TE-OCP porous beads would have adequate physical strength to maintain their shape, in contrast to the physical weakness of pure OCP hollow shell. The beads exhibit good biocompatibility and degradability and encourage bone mineralization in the early stage in vivo. This study demonstrates the feasibility of developing highly porous calcium phosphate giant beads via biomimetic self-assembly for direct application in reconstructive surgery and other widespread applications such as tissue engineering and drug delivery.

On the effect of temperature on the insertion of zinc into hydroxyapatite

Rietveld analysis of X-ray powder diffraction patterns recorded from 28 hydroxyapatite (HAp) samples containing various amounts of zinc (0, 1.6, 3.2 and 6.1wt.% Zn) and heat treated at various temperatures (between 500°C and 1100°C) has enabled the Zn insertion mechanism into the HAp crystal structure to be finely characterized. The formation of Zn-doped HAp was achieved above 900°C only. Zn-doped HAp has the Ca(10)Zn(x)(PO(4))(6)(OH)(2-2)(x)O(2)(x) (0<x⩽0.25) chemical composition with a constant Ca/P ratio of 1.67 due to the insertion mechanism into the hexagonal channel (partial occupancy of the 2b Wyckoff site with the formation of linear O-Zn-O entities). Samples heat treated at 500°C were almost single phase, HAp did not incorporate Zn and about half of the Zn atoms incorporated during the synthesis are not observable by X-ray powder diffraction (contained in an amorphous compound or physisorbed at the HAp surface). The reversible formation of Zn-doped β-TCP phase was observed at 600°C, reached its maximum content at 900°C and had almost vanished at 1100°C. The results presented here strengthen the recently described mechanism of Zn insertion in the interstitial 2b Wyckoff position of the HAp structure, and explain the origin of the contradictory reports in the corresponding literature.

Zinc in the mouth, its interactions with dental enamel and possible effects on caries; a review of the literature

Zinc is an essential trace element. In the mouth, it is present naturally in plaque, saliva and enamel. Zinc is formulated into oral health products to control plaque, reduce malodour and inhibit calculus formation. It has good oral substantivity, and elevated concentrations can persist for many hours in plaque and saliva following delivery from mouthrinses and toothpastes. Although low concentrations of zinc can both reduce enamel demineralisation and modify remineralisation, during caries clinical trials, the addition of zinc to fluoride toothpastes has not affected their ability to reduce caries. Mechanistic studies may help explain this apparent contradiction. Zinc is readily desorbed from hydroxyapatite by calcium, which is plentiful in plaque and saliva. Where crystal-growth sites remain occupied by zinc despite this, they may simply be 'over-grown' by remineralisation initiated at unoccupied sites. Further, under certain conditions, low concentrations of zinc can enhance remineralisation of enamel lesions, by retarding lesion arrestment. Although this may help to explain the apparent lack of an overall zinc effect on caries, it seems unlikely that any negative effects would be countered exactly by positive effects. Further mechanistic studies, complementing well-designed in vitro and in situ caries studies, should lead to further understanding of the zinc-enamel interactions relevant to demineralisation and remineralisation.

Theoretical calculations of the thermodynamic stability of ionic substitutions in hydroxyapatite under an aqueous solution environment

Defect formation energies in materials generally depend on chemical potentials determined by a chemical equilibrium condition. In particular, an aqueous solution environment is important for biomaterials such as hydroxyapatite studied here. Therefore, a methodology to obtain ionic chemical potentials under chemical equilibrium between solid and aqueous solution was introduced, and was applied to substitutional divalent cations formed via ion exchange with Ca(2+) in hydroxyapatite. The calculated ranking of the stability of substitutional cations in HAp was in good agreement with the experimentally observed trend. The present theoretical approach would be useful to explore the thermodynamic stability of defects in materials subjected to an aqueous solution environment.

Mechanical properties, electronic structure and bonding of alpha- and beta-tricalcium phosphates with surface characterization

The mechanical properties and electronic structure of alpha- and beta-tricalcium phosphate (TCP) crystals are studied by using two ab initio density functional methods, the Vienna Ab initio Simulation Package (VASP) and the orthogonalized linear combination of atomic orbitals method. Based on the VASP optimized crystal structures, the elastic constants of alpha- and beta-TCP are obtained using an effective stress-strain computational scheme. From the calculated elastic constants, the bulk modulus, shear modulus, Young's modulus and Poisson's ratios are obtained. The results show that the mechanical properties of the two crystals are comparable and that alpha-TCP is somewhat softer than beta-TCP. Comparison with experimental extrapolations of the elastic constants shows significant differences, which attest to the difficulty of obtaining single crystal samples. The calculated electronic structure results show that both crystals are large gap insulators with a direct band gap of 4.89 eV for alpha-TCP and 5.25 eV for beta-TCP. Effective charge calculations show that, on average, beta-TCP has slightly less charge transfer per Ca than alpha-TCP. The (010) ((001)) surface model for alpha-TCP (beta-TCP) is studied using a supercell slab geometry and fully relaxed to obtain the optimized structures. The estimated surface formation energies are 0.777 and 0.842 J m(-2) for alpha-TCP and beta-TCP, respectively. The electronic structures of the two surface models are compared with the bulk models. Charge density analysis shows that the surfaces of both TCP crystals are positively charged overall owing to the presence of Ca ions near the surfaces.

Ionic substitutions in calcium phosphates synthesized at low temperature

Ionic substitutions have been proposed as a tool to improve the biological performance of calcium phosphate based materials. This review provides an overview of the recent results achieved on ion-substituted calcium phosphates prepared at low temperature, i.e. by direct synthesis in aqueous medium or through hydrolysis of more soluble calcium phosphates. Particular attention is focused on several ions, including Si, Sr, Mg, Zn and Mn, which are attracting increasing interest for their possible biological role, and on the recent trends and developments in the applications of ion-substituted calcium phosphates in the biomedical field.

Mechanism of incorporation of zinc into hydroxyapatite

The atomic level mechanism of incorporation of Zn(2+) into hydroxyapatite (HAp), which is a potential dopant to promote bone formation, was investigated, based on first principles total energy calculations and experimental X-ray absorption near edge structure (XANES) analyses. It was found that Zn(2+)-doped HAp tends to have a Ca-deficient chemical composition and substitutional Zn(2+) ions are associated with a defect complex with a Ca(2+) vacancy and two charge compensating protons. Moreover, first principles calculations demonstrated that Zn(2+) incorporation into HAp can take place by occupying the Ca(2+) vacancy of the defect complex. The Ca(2+) vacancy complex is not only the origin of the calcium deficiency in HAp, but also plays a key role in the uptake of trace elements during mineralization.

Mechanism of Zn stabilization in hydroxyapatite and hydrated (0 0 1) surfaces of hydroxyapatite

A basic understanding of Zn incorporation on bulk and hydrated (0 0 1) surfaces of hydroxyapatite (HA) is attained through electronic structure calculations which use a combined first principles density functional (DFT) and extended Hückel tight binding (EHTB) methodology. A Zn substituted hydroxyapatite relaxed structure is obtained through a periodic cell DFT geometry optimization method. Electronic structure properties are calculated by using both cluster DFT and periodic cell EHTB methods. Bond order calculations show that Zn preference for the Ca2 vacancy, near the OH channel and with greater structural flexibility, is associated with the formation of a four-fold (bulk) and nearly four-fold (surface) coordination, as in ZnO. When occupying the octahedral Ca1 vacancy, Zn remains six-fold in the bulk, but coordination decreases to five-fold in the surface. In the bulk and surface, Zn2 is found to be more covalent than Zn1, due to a decrease in bond lengths at the four-fold site, which approach the 1.99 Å ZnO value. Zn is however considerably less bound in the biomaterial than in the oxide, where calculated bond orders are twice as large as in HA. Surface phosphate groups (PO(4)) and hydroxide ions behave as compact individual units as in the bulk; no evidence is found for the presence of HPO(4). Ca-O bond orders decrease at the surface, with a consequent increase in ionicity. Comparison between DFT and EHTB results show that the latter method gives a good qualitative account of charge and bonding in these systems.