Structural, morphological and surface characteristics of two types of octacalcium phosphate-derived fluoride-containing apatitic calcium phosphates

Calcium Phosphate-Based Biomaterials for Bone Repair

Traumatic, tumoral, and infectious bone defects are common in clinics, and create a big burden on patient's families and society. Calcium phosphate (CaP)-based biomaterials have superior properties and have been widely used for bone defect repair, due to their similarities to the inorganic components of human bones. The biological performance of CaPs, as a determining factor for their applications, are dependent on their physicochemical properties. Hydroxyapatite (HAP) as the most thermally stable crystalline phase of CaP is mostly used in the form of ceramics or composites scaffolds with polymers. Nanostructured CaPs with large surface areas are suitable for drug/gene delivery systems. Additionally, CaP scaffolds with hierarchical nano-/microstructures have demonstrated excellent ability in promoting bone regeneration. This review focuses on the relationships and interactions between the physicochemical/biological properties of CaP biomaterials and their species, sizes, and morphologies in bone regeneration, including synthesis strategies, structure control, biological behavior, and the mechanisms of CaP in promoting osteogenesis. This review will be helpful for scientists and engineers to further understand CaP-based biomaterials (CaPs), and be useful in developing new high-performance biomaterials for bone repair.

Preparation of antimicrobial calcium phosphate/protamine composite powders with fluoride ions using octacalcium phosphate

Calcium phosphates are key biomaterials in dental treatment and bone regeneration. Biomaterials must exhibit antibacterial properties to prevent microbial infection in implantation frameworks. Previously, we developed various types of calcium phosphate powders (amorphous calcium phosphate, octacalcium phosphate (OCP), dicalcium phosphate anhydrate, and hydroxyapatite) with adsorbed protamine (which is a protein with antibacterial property) and confirmed their antibacterial property. In this study, as foundational research for the development of novel oral care materials, we synthesized calcium phosphate composite powders from three starting materials: i) OCP, which intercalates organic compounds, ii) protamine, which has antibacterial properties, and iii) F- ion, which promotes the formation of apatite crystals. Through investigating the preparation concentration of the F- ions and their loading into OCP, it was found that more F- ion could be loaded at higher concentrations regardless of the loading method. It was also observed that the higher the preparation concentration, the more the OCP converted to fluorapatite. The synthesized calcium phosphate composite powders were evaluated for biocompatibility through proliferation of MG-63 cells, with none of the powders exhibiting any growth inhibition. Antimicrobial tests showed that the calcium phosphate composite powders synthesized with protamine and F- ion by precipitation had enhanced antimicrobial properties than those synthesized by protamine adsorption. Thus, the calcium phosphate composite powder prepared from OCP, protamine, and F- ion forms the basis for promising antimicrobial biomaterials. Graphical abstract.

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.

A thermostability perspective on enhancing physicochemical and cytological characteristics of octacalcium phosphate by doping iron and strontium

Investigation of thermostability will lead the groundbreaking of unraveling the mechanism of influence of ion-doping on the properties of calcium phosphates. In this work, octacalcium phosphate (OCP), a metastable precursor of biological apatite, was used as a stability model for doping ions (Fe³⁺ and Sr²⁺) with different ionic charges and radii. After treated under hot air at different temperatures (110–200 °C), the phase, morphology, structure, physicochemical properties, protein affinity, ions release, and cytological responses of the ion-doped OCPs were investigated comparatively. The results showed that the collapse of OCP crystals gradually occurred, accompanying with the dehydration of hydrated layers and the disintegration of plate-like crystals as the temperature increased. The collapsed crystals still retained the typical properties of OCP and the potential of conversion into hydroxyapatite. Compared to the undoped OCP, Fe-OCP, and Sr-OCP had lower and higher thermostability respectively, leading to different material surface properties and ions release. The adjusted thermostability of Fe-OCP and Sr-OCP significantly enhanced the adsorption of proteins (BSA and LSZ) and the cytological behavior (adhesion, spreading, proliferation, and osteogenic differentiation) of bone marrow mesenchymal stem cells to a varying extent under the synergistic effects of corresponding surface characteristics and early active ions release. This work paves the way for understanding the modification mechanism of calcium phosphates utilizing ion doping strategy and developing bioactive OCP-based materials for tissue repair.

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OCP was used as a stability model for doping ions with different charges and radii.

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Collapse of OCP crystals occurred with structural dehydration after heat treatment.

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Fe and Sr doping altered the thermostability of OCP crystals in an opposite way.

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The thermostable difference affected the surface properties and ion release of OCP.

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Active surface and ion release of OCP synergistically mediated its biocompatibility.

Comparative analysis of bovine serum albumin adsorption onto octacalcium phosphate crystals prepared using different methods

This study compared bovine serum albumin (BSA) adsorption onto octacalcium phosphate (OCP) materials prepared from two wet preparations in the absence (w-OCP) and presence (c-OCP) of gelatin. Raman spectroscopy was used to analyze the BSA adsorption onto OCPs in a 150 mM Tris-HCl buffer containing 0.5 mM calcium and inorganic phosphate (Pi) ions at pH 7.4 and at 37°C. The degree of supersaturation of the supernatants after the adsorption was determined by measuring the ion composition. The results showed that BSA adsorption onto w-OCP was higher than that for c-OCP. The calcium ion concentration of the supernatant decreased for both w-OCP and c-OCP, whereas the Pi ion concentration increased, approaching OCP equilibria at different saturation levels. BSA adsorbed even onto c-OCP, which included a small amount of gelatin during c-OCP preparation. These results indicate that the biodegradability of w-OCP and c-OCP may be modulated through interactions with serum proteins.

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.

Effect of calcium phosphate phases affecting the crosstalk between osteoblasts and osteoclasts in vitro

Previous studies have reported that octacalcium phosphate (OCP) enhances osteoblast differentiation and osteoclast formation during the hydrolysis process to hydroxyapatite (HA). However, the crystal phases that affect the crosstalk between osteoclasts and osteoblasts are unknown, which should determine the bone substitute material's property of OCP. The present study was designed to investigate whether the chemical composition and crystal structure of calcium phosphates affect osteoclast formation and the osteoclast-osteoblast crosstalk. Biodegradable β-tricalcium phosphate (β-TCP) was used as the control material. Osteoclasts were cultured on HA/OCP or HA/TCP disks and their cellular responses were assessed. Both OCP and β-TCP had a similar ability to create multinucleated osteoclasts. However, OCP promoted the expression of complement component 3a (C3a), a positive coupling factor, in osteoclasts, whereas β-TCP enhanced that of EphrinB2 (EfnB2) and collagen triple helix repeat containing 1 (Cthrc1). During osteoclast culture, phosphate ions were released from the crystals, and OCP-HA conversion was advanced in HA/OCP mixtures and OCP. X-ray diffraction analysis revealed no remarkable changes in the crystal structures of HA/TCP mixtures and β-TCP before and after osteoclast culture. These results indicate that the distinct chemical environment induced by the calcium phosphate phases affects the crosstalk between osteoclasts and osteoblasts. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1001-1013, 2019.

Formation and stability of well-crystallized metastable octacalcium phosphate at high temperature by regulating the reaction environment with carbamide

The formation and stability of pure well-crystallized metastable OCP were regulated under carbamide-mediated reaction conditions through the co-existing conversion mechanisms.

Surface reactivity of octacalcium phosphate-derived fluoride-containing apatite in the presence of polyols and fluoride

The present study was designed to characterize co-precipitated fluoridated apatitic materials from octacalcium phosphate (OCP) precursor and to investigate their surface reactions with polyols including glycerol in the presence of fluoride ions. Laboratory-synthesized fluoridated apatite crystals (LS-FA) were obtained in a solution containing fluoride (F) from 25 to 500 ppm. LS-FAs and commercially available fluoroapatite (FA) and hydroxyapatite (HA) were characterized by physical techniques, such as X-ray diffraction. LS-FA obtained in the presence of 100 ppmF (100 ppm-LS-FA) had an apatitic structure, but its solubility was close to HA in a culture medium (α-MEM) despite the fact it contains over 3 wt % of F. 100 ppm-LS-FA, FA, and HA were then subjected to the human serum albumin (HSA) adsorption test at pH 7.4 (in a 150 mM Tris-HCl buffer) and the dissolution and re-mineralization experiments in the presence of xylitol, D-sorbitol, or glycerol, and F under acidic and neutral conditions. Adsorption affinity of HSA was estimated as highest for FA and lowest for LS-FA. LS-FA, FA, and HA were immersed in a lactic acid solution with the polyols and/or F ion-containing solution up to 200 ppm to analyze the dissolution behavior. LS-FA had the highest dissolution tendency in the conditions examined. Glycerol enhanced the dissolution of phosphate from apatite crystals in particular from LS-FA. The results suggest that the apatite crystals, obtained through the hydrolysis of OCP in the presence of F, provide a more reactive surface than FA or HA under physiological environments. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2235-2244, 2018.

Application of an indicator-immobilized-gel-sheet for measuring the pH surrounding a calcium phosphate-based biomaterial

The present study was designed to investigate the local microenvironment of octacalcium phosphate in a granule form upon biomolecules adsorption utilizing an indicator-immobilized-gel-sheet for measuring pH. We previously showed that octacalcium phosphate enhances bone regeneration during its progressive hydrolysis into hydroxyapatite if implanted in bone defects. The gel-sheet was made from a photocrosslinkable prepolymer solution, which can easily immobilize a pH indicator (bromothymol blue; BTB) in the hydrogel. The indicator-immobilized-gel-sheet was mounted on a biochip which was made of polydimethylsiloxane (PDMS) with a flow channel. The pH value was calculated by detecting the color changes in the gel-sheet and displayed as the pH distribution. After pre-adsorption of bovine albumin, β-lactoglobuline or cytochrome C onto octacalcium phosphate granules, the granules with the gel-sheet were further incubated in Tris-HCl buffer solution in the absence or presence of fluoride, known as an accelerator of octacalcium phosphate hydrolysis. pH values of the gel-sheet surrounding octacalcium phosphate granules showed a decrease from pH 7.4 to 6.6 in relation to the proteins adsorbed. Overall, the proposed pH-sensitive gel can be used to detect the pH around octacalcium phosphate granules with a high spatial resolution.

Changes in physicochemical and biological properties of porcine bone derived hydroxyapatite induced by the incorporation of fluoride

As the main inorganic component of xenogenic bone graft material, bone-derived biological apatite (BAp) has been widely used in implant dentistry, oral and maxillofacial surgery and orthopedics. However, BAp produced via calcination of animal bones still suffers from some drawbacks, such as insufficient mechanical strength and inadequate degradation rate, which impede its application. Fluoride is known to play important roles in both physiological and pathological processes of human hard tissues for its double effects on bones and teeth. In order to understand the effects of fluoride on the properties of BAp, as well as the mechanism behind them, porcine bone derived hydroxyapatite (PHAp) was prepared via thermal treatment, which was then fluoride incorporated at a series concentrations of sodium fluoride, and noted as 0.25-FPHAp, 0.50-FPHAp, and 0.75-FPHAp respectively. The physicochemical characteristics of the materials, including crystal morphology, crystallinity, functional groups, elemental composition, compressive strength, porosity and solubility, were then determined. The biological properties, such as protein adsorption and cell attachment, were also evaluated. It was found that the spheroid-like crystals of PHAp were changed into rod-like after fluoride substitution, resulting in a fluoride concentration-dependent increase in compressive strength, as well as a decreased porosity and solubility of the apatite. However, even though the addition of fluoride was demonstrated to enhance protein adsorption and cell attachment of the materials, the most favorable results were intriguingly achieved in FPHAp with the least fluoride content. Collectively, low level of fluoride incorporation is proposed promising for the modification of clinically used BAp based bone substitute materials, because of its being able to maintain a good balance between physicochemical and biological properties of the apatite.

Combinatorial incorporation of fluoride and cobalt ions into calcium phosphates to stimulate osteogenesis and angiogenesis

Bone healing requires two critical mechanisms, angiogenesis and osteogenesis. In order to improve bone graft substitutes, both mechanisms should be addressed simultaneously. While the individual effects of various bioinorganics have been studied, an understanding of the combinatorial effects is lacking. Cobalt and fluoride ions, in appropriate concentrations, are known to individually favor the vascularization and mineralization processes, respectively. This study investigated the potential of using a combination of fluoride and cobalt ions to simultaneously promote osteogenesis and angiogenesis in human mesenchymal stromal cells (hMSCs). Using a two-step biomimetic method, wells of tissue culture plates were coated with a calcium phosphate (CaP) layer without or with the incorporation of cobalt, fluoride, or both. In parallel, hMSCs were cultured on uncoated well plates, and cultured with cobalt and/or fluoride ions within the media. The results revealed that cobalt ions increased the expression of angiogenic markers, with the effects being stronger when the ions were added as a dissolved salt in cell medium as compared to incorporation into CaP. Cobalt ions generally suppressed the ALP activity, the expression of osteogenic genes, and the level of mineralization, regardless of delivery method. Fluoride ions, individually or in combination with cobalt, significantly increased the expression of many of the selected osteogenic markers, as well as mineral deposition. This study demonstrates an approach to simultaneously target the two essential mechanisms in bone healing: angiogenesis and osteogenesis. The incorporation of cobalt and fluoride into CaPs is a promising method to improve the biological performance of fully synthetic bone graft substitutes.

Osteoconductive property of a mechanical mixture of octacalcium phosphate and amorphous calcium phosphate

The present study was designed to investigate the extent of osteoconductive property of a mechanical mixture of octacalcium phosphate (OCP) and amorphous calcium phosphate (ACP). OCP was mixed with ACP in granules that had a diameter of 300 and 500 μm, respectively, and at 25, 50, or 75 wt %. The physicochemical characteristics and the osteoconductive properties of the mixtures were compared with OCP alone or ACP alone through implantation into rat critical-sized calvaria defects for up to 12 weeks and simulated body fluid (SBF) immersion for 2 weeks. The mixtures of OCP and ACP, in particular the OCP 25 wt % and ACP 75 wt % (O25A75), had higher radiopacity compared to ACP and OCP alone. O25A75 induced greater enhancement of bone regeneration than ACP alone at 8 weeks and that than OCP alone at 12 weeks. X-ray diffraction and Fourier transform infrared (FTIR) analyses of the retrieved mixtures showed that ACP, OCP, and O25A75 tended to convert to hydroxyapatite (HA) after the implantation, while the structure of OCP remains without complete conversion after SBF immersion. Analyses by FTIR curve fitting of the solids and the degree of supersaturation of the SBF supported the observation that the existence of ACP enhances the kinetics of the conversion. Scanning electron microscopy found that the surface of O25A75 had distinct characteristics with OCP and ACP after SBF immersion. The results suggest that the extent of the osteoconduction of OCP could be controlled by the copresence of ACP most probably through the prevailing dissolution-precipitation of the surface of ACP crystals to form HA.

Octacalcium Phosphate: A Potential Scaffold Material for Controlling Activity of Bone-Related Cells In Vitro

We have previously established a wet synthesis method of octacalcium phosphate (OCP) in a relatively large scale and found that OCP enhances bone formation more than synthetic hydroxyapatite (HA) if implanted onto bone surface and various bone defects. The present paper reviews, based on our studies, as to how OCP controls in vitro cellular activities of bone-related cells, such as bone marrow stromal cells, and how OCP enhances bone repair in critical sized bone defect experimentally created in animal models. OCP tends to progressively convert to HA in culture media and in rat calvaria defects. OCP is capable of enhancing in vitro osteoblast differentiation and osteoclast formation in the presence of osteoblasts. Recent our studies also indicated that OCP enhances odontoblast differentiation while suppresses chondrogenic differentiation. The physicochemical properties, such as chemical composition and adsorption affinity of serum proteins, vary depending on the advancement of conversion from OCP to HA, which suggests that the change on the surface property during the conversion of OCP may affect the cellular responses in vitro and tissue reaction in vivo. OCP could be used as a scaffold material that can control the activity of bone-related cells.

Effect of addition of hyaluronic acids on the osteoconductivity and biodegradability of synthetic octacalcium phosphate

The present study was designed to investigate whether three sodium hyaluronic acid (HyA) medical products, Artz(®), Suvenyl(®) and a chemically modified derivative of sodium HyA Synvisc(®), can be used as suitable vehicles for an osteoconductive octacalcium phosphate (OCP). OCP granules (300-500 μm diameter) were mixed with these sodium HyAs with molecular weights of 90 × 10(4) (Artz(®)), 190 × 10(4) (Suvenyl(®)) and 600 × 10(4) (Synvisc(®)) (referred to as HyA90, HyA190 and HyA600, respectively). OCP-HyA composites were injected using a syringe into a polytetrafluoroethylene ring, placed on the subperiosteal region of mouse calvaria for 3 and 6 weeks, and then bone formation was assessed by histomorphometry. The capacity of the HyAs for osteoclast formation from RAW264 cells with RANKL was examined by TRAP staining in vitro. Bone formation was enhanced by the OCP composites with HyA90 and HyA600, compared to OCP alone, through enhanced osteoclastic resorption of OCP. HyA90 and HyA600 facilitated in vitro osteoclast formation. The results suggest that the osteoconductive property of OCP was accelerated by the HyAs-associated osteoclastic resorption of OCP, and therefore that HyA/OCP composites are attractive bone substitutes which are injectable and bioactive materials.

Synthetic octacalcium phosphate: a possible carrier for mesenchymal stem cells in bone regeneration

The present paper reviews biomaterial studies of synthetic octacalcium phosphate (OCP) as a scaffold of osteoblastic cells. OCP crystals have been suggested to be one of precursor phases in hydroxyapatite (HA) crystal formation in bone and tooth. The recent intensive biomaterials and tissue engineering studies using synthetic OCP disclosed the potential function of OCP as a bioactive material as well as synthetic HA materials due to its highly osteoconductive and biodegradable properties. In vitro studies showed that OCP crystals exhibit a positive effect on osteoblastic cell differentiation. In vivo studies confirmed that the materials of OCP in a granule forms and OCP-based composite materials with natural polymers, such as gelatin and collagen, enhance bone regeneration if implanted in various model bone defects with critical-sized diameters, defined as a defect which does not heal spontaneously throughout the lifetime of the animals. One of particular characteristics of OCP, found as a mechanism to enhance bone regeneration in vivo, is a process of progressive conversion from OCP to HA at physiological conditions. The OCP-HA conversion is accompanied by progressive physicochemical changes of the material properties, which affects the tissue reaction around the crystals where osteoblastic cells are encountered. Mesenchymal stem cells (MSCs) seeded in an OCP-based material enhanced bone regeneration in the rat critical-sized calvaria defect more than that by the material alone. The overall results reveal that OCP crystals have an effect on osteoblastic cell differentiation including the differentiation of MSCs in vivo. The evidence collected experimentally in the laboratory was presented.