Octacalcium phosphate (OCP)-based bone substitute materials

Octacalcium phosphate phase forming cements as an injectable bone substitute materials: Preparation and in vitro structural study

In the realm of regenerating damaged or degenerated bones through minimally invasive techniques, injectable materials have emerged as exceptionally promising. Among these, calcium phosphate bone cements (CPCs) have garnered significant interest due to their remarkable bioactivity, setting it apart from non-degradable alternatives such as polymethyl methacrylate cements. α-Tricalcium phosphate (α-TCP) is a widely used solid phase component in CPCs. It can transform into calcium-deficient hydroxyapatite (CDHAp) when it comes in contact with water. In this study, we aimed to create an injectable, self-setting bone cement using low-temperature synthesized α-TCP powder as a single precursor of the powder phase. We found that changes in the pH of the liquid phase (pH 6.0, pH 6.2, pH 7.0 and pH 7.4) significantly altered the cement's setting, handling, and mechanical properties. The formation of the octacalcium phosphate (OCP) phase was identified in our study, which positively affects the osteoblastic cell response. Hardened OCP-forming bone cements prepared using a liquid phase with pH 7.0 and 7.4 showed better osteogenic cell attachment and proliferation than those prepared with pH 6.0 and 6.2. Our study suggests that changes in the pH of the liquid phase can significantly affect the properties of α-TCP-based bone cement, and the presence of the OCP phase is crucial for optimal cement performance.

Octacalcium Phosphate-Laden Hydrogels on 3D-Printed Titanium Biomaterials Improve Corrosion Resistance in Simulated Biological Media

The inflammatory-associated corrosion of metallic dental and orthopedic implants causes significant complications, which may result in the implant's failure. The corrosion resistance can be improved with coatings and surface treatments, but at the same time, it might affect the ability of metallic implants to undergo proper osteointegration. In this work, alginate hydrogels with and without octacalcium phosphate (OCP) were made on 3D-printed (patterned) titanium alloys (Ti Group 2 and Ti-Al-V Group 23) to enhance their anticorrosion properties in simulated normal, inflammatory, and severe inflammatory conditions in vitro. Alginate (Alg) and OCP-laden alginate (Alg/OCP) hydrogels were manufactured on the surface of 3D-printed Ti substrates and were characterized with wettability analysis, XRD, and FTIR. The electrochemical characterization of the samples was carried out with open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). It was observed that the hydrophilicity of Alg/OCP coatings was higher than that of pure Alg and that OCP phase crystallinity was increased when samples were subjected to simulated biological media. The corrosion resistance of uncoated and coated samples was lower in inflammatory and severe inflammatory environments vs. normal media, but the hydrogel coatings on 3D-printed Ti layers moved the corrosion potential towards more nobler values, reducing the corrosion current density in all simulated solutions. These measurements revealed that OCP particles in the Alg hydrogel matrix noticeably increased the electrical charge transfer resistance at the substrate and coating interface more than with Alg hydrogel alone.

Biomimetic Hydroxyapatite Crystals Growth on Phosphorylated Chitosan Films by In Vitro Mineralization Used as Dental Substitute Materials

Chitosan (CS) films exhibit great potential as a substrate for the in vitro mineralization process. In this study, to mimic the formation of nanohydroxyapatite (HAP) as natural tissue, CS films coated with a porous calcium phosphate were investigated using scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDX), Fourier transforms infrared spectroscopy (FTIR), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Calcium phosphate coating deposited on phosphorylated derivatives of CS was obtained by a process based on phosphorylation, Ca(OH)₂ treatment and artificial saliva solution (ASS) immersion. The phosphorylated CS films (PCS) were obtained by partial hydrolysis of the PO₄ functionalities. It was demonstrated that this precursor phase could induce the growth and the nucleation of the porous calcium phosphate coating when immersed in ASS. Moreover, oriented crystals and qualitative control of calcium phosphate phases on CS matrices are obtained in a biomimetic mode. Furthermore, in vitro antimicrobial activity of PCS was evaluated against three species of oral bacteria and fungi. It revealed an increase in antimicrobial activity with minimum inhibition concentration (MIC) values of 0.10% (Candida albicans), 0.05% (Staphylococcus aureus) and 0.025% (Escherichia coli) which proves their possible use as dental substitute materials.

Exploring the Formation Kinetics of Octacalcium Phosphate from Alpha-Tricalcium Phosphate: Synthesis Scale-Up, Determination of Transient Phases, Their Morphology and Biocompatibility

Even with decades of research studies behind octacalcium phosphate (OCP), determination of OCP phase formation has proved to be a cumbersome challenge. Even though obtaining a large quantity of OCP is important for potential clinical uses, it still remains a hindrance to obtain high yields of pure OCP. Taking that into consideration, the purpose of this study was to scale-up OCP synthesis for the first time and to use a multi-technique approach to follow the phase transformation pathway at multiple time points. In the present study, OCP has been synthesized from α-tricalcium phosphate (α-TCP), and subsequently scaled-up tenfold and hundredfold (100 mg → 10 g). The hydrolysis mechanism has been followed and described by using XRD and FTIR spectroscopy, as well as Raman and SEM. Gradual transformation into the OCP phase transpired through dicalcium phosphate dihydrate (brushite, DCPD, up to ~36%) as an intermediary phase. Furthermore, the obtained transitional phases and final OCP phases (across all scale-up levels) were tested with human bone marrow-derived mesenchymal stem cells (hBMSCs), in order to see how different phase mixtures affect the cell viability, and also to corroborate the safety of the scaled-up product. Twelve out of seventeen specimens showed satisfactory percentages of cell viability and confirmed the prospective use of scaled-up OCP in further in vitro studies. The present study, therefore, provides the first scale-up process of OCP synthesis, an in depth understanding of the formation pathway, and investigation of the parameters able to contribute in the OCP phase formation.

Acceleration of bone formation by octacalcium phosphate composite in a rat tibia critical-sized defect

Background

The osteogenic capabilities and biodegradability of octacalcium phosphate (OCP) composites make them unique. Despite the excellent characteristics of OCP, their use is limited due to handling difficulties. In this study, we aimed to evaluate and compare three types of OCPs (cemented OCP (C-OCP), C-OCP with collagen (OCP/Col), and synthetic OCP (S-OCP) with alginate (OCP/Alg)) versus commercially available β-tricalcium phosphate (β-TCP) regarding their potential to accelerate bone formation in defective rat tibias.

Methods

The specimens with OCP composite were manufactured into 5 ​mm cubes and inserted into the segmental defects of rat tibias fixed with an external fixator. In addition, 3 ​mm-hole defects in rat tibias were evaluated to compare the graft material properties in different clinical situations. Serial X-ray studies were evaluated weekly and the tibias were harvested at postoperative 6 weeks or 8 weeks for radiologic evaluation. Histological and histomorphometric analyses were performed to evaluate the acceleration of bone formation.

Results

In the critical-defect model, OCP/Alg showed bone bridges between segmentally resected bone ends that were comparable to those of β-TCP. However, differences were observed in the residual graft materials. Most β-TCP was maintained until 8 weeks postoperatively; however, OCP/Alg was more biodegradable. In addition calcification in the β-TCP occurred at the directly contacted area between graft particles and bony ingrowth was observed in the region adjacent resected surface of tibia. In contrast, no direct bony ingrowth was observed in OCP-based materials, but osteogenesis induced from resected surface of tibia was more active. In the hole-defect model, OCP/Col accelerated bone formation. β-TCP and OCP/Alg showed similar patterns with relatively higher biodegradability. In histology, among the OCP-based materials, directly contacted new bone was formed only in OCP/Alg group. The new bone formation in the periphery area of graft materials was much more active in the OCP-based materials, and the newly formed bone showed a thicker trabecular and more mature appearance than the β-TCP group.

Conclusions

In this study, OCP/Alg was equivalent to β-TCP in the acceleration of bone formation with better biodegradability appropriate for clinical situations in different circumstances. Our OCP/Col composite showed fast degradation, which makes it unsuitable for use in mechanical stress conditions in clinical orthopedic settings.

The Translational Potential of this Article

In our research, we compared our various manufactured OCP composites to commercially available β-TCP in critical-defect rat tibia model. OCP/Col showed acceleration in hole-defect model as previous studies in dental field but in our critical-sized defect model it resorbed fast without acceleration of bony union. OCP/Alg showed matched results compared to β-TCP and relatively fast resorption so we showed market value in special clinical indication depending on treatment strategy. This is the first OCP composite study in orthopaedics with animal critical-sized tibia bone study and further study should be considered for clinical application based on this study.

Fabrication of interconnected porous Ag substituted octacalcium phosphate blocks based on a dissolution-precipitation reaction

Here, we introduce Ag substituted octacalcium phosphate (OCP-Ag) blocks with interconnected porous structure and sufficient mechanical strength as bone substitute (i.e., foam). We employed a two-step process for fabrication, which includes a setting reaction for acidic calcium phosphate granules using an acidic phosphate solution and a phase conversion process via dissolution-precipitation method in cocktail ((NH₄)₂HPO₄-NH₄NO₃-NaNO₃-AgNO₃) solutions. The Ag contents in the fabricated OCP-Ag foams were 0.08-0.15 at%, which were sufficient in exhibiting contact antibacterial ability. The mechanical strength and porosity of the OCP-Ag foams were about 0.5 MPa and 70%, respectively. These values were sufficient for the application of the OCP-Ag foams as bone substitute. Graphical abstract.

Dual setting brushite—gelatin cement with increased ductility and sustained drug release

A novel dual setting brushite-gelatin cement was achieved by genip ininitiated cross-linking of gelatin during cement setting. Although the combination of an inorganic and organic phase resulted in a decrease of the compressive strength from about 10 MPa without polymeric phase to 3–6–MPa for gelatin modified composites, an increase in elastic properties due to the gelatin hydrogel with a concentration of 10.0 w/v% was achieved. For a powder-to-liquid ratio of 2.5 g*mL−1, a shift of initial maximum stress value during compression testing was observed up to 5% deformation and tested samples showed a pseudo-ductile fracture behavior. The obtained composites of the different formulations were characterized regarding phase composition, porosity as well as drug loading capacity with rifampicin and vancomycin. For the latter, a sustained and prolonged release was realized with a drug release profile according to the Higuchi model and a release exponent of n = 0.5 for the formulation with a PLR of 2.5 g*mL−1 and an incorporation of 10.0 w/v% gelatin.

In Vitro Study of Degradation and Cytocompatibility of Ceramics/PLA Composite Coating on Pure Zinc for Orthopedic Application

Zinc and its alloys are considered to be next-generation materials for fabricating absorbable biomedical devices. However, cytotoxicity has been reported to be associated with rapid degradation. To address these issues, a composite coating (PLA/Li-OCP) consisting of CaHPO4 conversion coating (Ca-P) and polylactic acid (PLA) decorated with Li-octacalcium phosphate particles was constructed on pure zinc. The immersion tests showed that the presence of Ca-P coating and PLA/Li-OCP coating on pure zinc could reduce the pH value. Compared with Ca-P coating, the introduction of the PLA/Li-OCP film on the Ca-P-coated samples could enhance the corrosion resistance, and there was one order of magnitude decrease in the corrosion current density. The cytocompatibility assay suggested that the PLA/Li-OCP coating favored the cell viability and upregulated the expression of related osteogenic-genes including RUNX2, OCN, and BMP. Therefore, the presence of the PLA/Li-OCP coating on pure zinc could effectively improve the degradation rate and cytocompatibility of pure zinc.

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.

Bone Tissue Response to Different Grown Crystal Batches of Octacalcium Phosphate in Rat Long Bone Intramedullary Canal Area

The microstructure of biomaterials influences the cellular and biological responses in the bone. Octacalcium phosphate (OCP) exhibits higher biodegradability and osteoconductivity than hydroxyapatite (HA) during the conversion process from OCP to HA. However, the effect of the microstructure of OCP crystals on long tubular bones has not been clarified. In this study, two types of OCPs with different microstructures, fine-OCP (F-OCP) and coarse-OCP (C-OCP), were implanted in rat tibia for 4 weeks. F-OCP promoted cortical bone regeneration compared with C-OCP. The osteoclasts appearance was significantly higher in the C-OCP group than in the control group (defect only) at 1-week post-implantation. To investigate whether the solubility equilibrium depends on the different particle sizes of OCPs, Nano-OCP, which consisted of nanometer-sized OCPs, was prepared. The degree of supersaturation (DS) tended to decrease modestly in the order of C-OCP, F-OCP, and Nano-OCP with respect to HA and OCP in Tris-HCl buffer. F-OCP showed a higher phosphate ion concentration and lower calcium ion concentration after immersion in the buffer than C-OCP. The crystal structures of both OCPs tended to be converted to HA by rat abdominal implantation. These results suggest that differences in the microstructure of OCPs may affect osteoclastogenesis and result in osteoconductivity of this material in long tubular bone by altering dissolution behavior.

The effect of simple heat treatment on apatite formation on grit-blasted/acid-etched dental Ti implants already in clinical use

Grit-blasted/acid-etched titanium dental implants have a moderately roughened surface that is suitable for cell adhesion and exhibits faster osseointegration. However, the roughened surface does not always maintain stable fixation over a long period. In this study, a simple heat treatment at 600°C was performed on a commercially available dental Ti implant with grit-blasting/acid-etching, and its effect on mineralization capacity was assessed by examining apatite formation in a simulated body fluid (SBF). The as-purchased implant displayed a moderately roughened surface at the micrometer scale. Its surface was composed of titanium hydride accompanied by a small amount of alumina particles derived from the grit-blasting. Heat treatment transformed the titanium hydride into rutile without evidently changing the surface morphology. The immersion in SBF revealed that apatite formed on the heated implant at 7 days. Furthermore, apatite formed on the Ti rod surface within 1 day when the metal was subjected to acid and heat treatment without blasting. These indicate that apatite formation was conferred on the commercially available dental implant by simple heat treatment, although its induction period was slightly affected by alumina particles remaining on the implant surface. The heat-treated implant should achieve stronger and more stable bone bonding due to its apatite formation.

Monetite, an important calcium phosphate compound-Its synthesis, properties and applications in orthopedics

This review recognizes a unique calcium phosphate (CaP) phase known as monetite or dicalcium phosphate anhydrous (DCPA, CaHPO₄), and presents an overview of its properties, processing, and applications in orthopedics. The motivation for the present effort is to highlight the state-of-the-art research and development of monetite and propel the research community to explore more of its potentials in orthopedics. After a brief introduction of monetite, we provide a summary of its various synthesis routes like dehydration, solvent-based, energy-assisted processes and also discuss the formation of different crystal structures with respect to the synthesis conditions. Subsequently, we discuss the material's noteworthy physico-chemical properties including the crystal structure, vibrational spectra, solubility, thermal decomposition, and conversion to other phases. Of note, we focus on the biological (in vitro and in vivo) properties of monetite, given its ever-increasing popularity as a biomaterial for medical implants. Appropriately, we discuss various orthopedic applications of monetite as bone cement, implant coatings, granules for defect fillers, and scaffolds. Many in vitro and in vivo studies confirmed the favorable osteointegration and osteoconduction properties of monetite products, along with a better balance between implant resorption and new bone formation as compared to other CaP phases. The review ends with translational aspects of monetite and presents thoughts about its possible future research directions. Further research may explore but not limited to improvements in mechanical strength of monetite-based scaffolds, using monetite particles as a therapeutic agent delivery, and tissue engineering strategies where monetite serves as the biomaterial. STATEMENT OF SIGNIFICANCE: This is the first review that focusses on the favorable potential of monetite for hard tissue repair and regeneration. The article accurately covers the "Synthesis-Structure-Property-Applications" correlations elaborating on monetite's diverse material properties. Special focus is put on the in vitro and in vivo properties of the material highlighting monetite as an orthopedic material-of-choice. The synthesis techniques are discussed which provide important information about the different fabrication routes for monetite. Most importantly, the review provides comprehensive knowledge about the diverse biomedical applications of monetite as granules, defect--specific scaffolds, bone cements and implant coatings. This review will help to highlight monetite's potential as an effective regenerative medicine and catalyze the continuing translation of this bioceramic from the laboratory to clinics.

Mutual chemical effect of autograft and octacalcium phosphate implantation on enhancing intramembranous bone regeneration

This study examined the effect of a mixture of octacalcium phosphate (OCP) and autologous bone on bone regeneration in rat calvaria critical-sized defect (CSD). Mechanically mixed OCP and autologous bone granules (OCP+Auto), approximately 500 to 1000 μm in diameter, and each individual material were implanted in rat CSD for 8 weeks, and subjected to X-ray micro-computed tomography (micro-CT), histology, tartrate-resistant acid phosphatase (TRAP) staining, and histomorphometry for bone regeneration. Osteoblastic differentiation from mesenchymal stem cells (D1 cells) was examined in the presence of non-contacting materials by alkaline phosphatase (ALP) activity for 21 days. The material properties and medium composition before and after the incubation were determined by selected area electron diffraction (SAED) under transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and chemical analysis. The results showed that while bone formation coupled with TRAP-positive osteoclastic resorption and cellular ALP activity were the highest in the Auto group, a positive effect per OCP weight or per autologous bone weight on ALP activity was found. Although the OCP structure was maintained even after the incubation (SAED), micro-deposits were grown on OCP surfaces (TEM). Fibrous tissue was also exposed on the autologous bone surfaces (SEM). Through FT-IR absorption, it was determined that bone mineral-like characteristics of the phosphate group increased in the OCP + Auto group. These findings were interpreted as a structural change from OCP to the apatitic phase, a conclusion supported by the medium degree of saturation changes. The results demonstrate the mutual chemical effect of mixing OCP with autologous bone as an active bone substitute material.

Mineralogical-geochemical study of the anionic competition effect on the octacalcium phosphate reaction into fluorapatite

The unstable compound octacalcium phosphate (OCP) is one of the crystalline precursors of the apatite mineral series composed by hydroxyapatite, fluorapatite and chlorapatite. The feature of OCP to react into apatite, depending on the media conditions, has been mainly exploited for biomedical applications as bone and tooth substitute material. Recently, some important applications of OCP have been documented: e.g. as electrode material for supercapacitors and as fluoride remover reagent for environmental purposes. With the aim of deepening the property of OCP to be the crystalline precursor of apatite and assessing if and how the anionic competition can influence the formation of the different apatite end-members, the OCP → apatite reaction has been here investigated placing 0.223 mmol of OCP in 50 mL aqueous solution with 0.368 mmol of dissolved fluoride, chloride, hydroxyl and carbonate anions (fluoride alone, fluoride with each of the other anions, and all the anions together) at room temperature. The post-experiment analyses of solid and liquid phases, conducted by using XRD, ESEM and ICP-OES, show that fluoride is always the main anion removed from solution during the OCP transformation reaction. The precise mineralogical characterization of solid phases formed, performed using the Rietveld algorithm, shows that fluorapatite is always the main resulting apatitic phase, followed by hydroxyapatite. Taking into account the different application fields of OCP, these results could be significant in better defining the OCP → apatite reaction in aqueous solutions where different competing anions are involved.

Directional growth of octacalcium phosphate using micro-flow reactor mixing and subsequent aging

Well-defined belt-shaped particles of octacalcium phosphate were prepared by mixing aqueous solutions of calcium acetate and that of sodium phosphate monobasic with the aid of a micro-flow reactor. Higher crystallinity and narrower particle size distribution were achieved by the micro-flow reactor if compared with the results of the batch reaction using the same solutions. The width of the belt was controlled by the mixing temperature (0.8 and 2.3 μm for the preparation at 50 and 70 °C, respectively). Post mixing aging at 50 °C, resulted in the directional growth of belt-shaped particles to obtain particles with the length of 17 μm (aspect ratio of 53). XRD and TEM analysis indicated that the micro-flow reactor could separate nucleation and growth allowing preferential growth along the a-direction.

Well-defined octacalcium phosphate particles with varied size and aspect ratio were prepared by a micro-flow reactor mixing and subsequent aging in different temperature and aging time.

Evaluation of New Octacalcium Phosphate-Coated Xenograft in Rats Calvarial Defect Model on Bone Regeneration

Bone graft material is essential for satisfactory and sufficient bone growth which leads to a successful implant procedure. It is classified into autogenous bone, allobone, xenobone and alloplastic materials. Among them, it has been reported that heterogeneous bone graft material has a porous microstructure that increases blood vessels and bone formation, and shows faster bone formation than other types of bone graft materials. We observed new bone tissue formation and bone remodeling using Ti-oss^® (Chiyewon Co., Ltd., Guri, Korea), a heterologous bone graft material. Using a Sprague–Dawley rat calvarial defect model to evaluate the bone healing effect of biomaterials, the efficacy of the newly developed xenograft Ti-oss^® and Bio-Oss^® (Geistilch Pharma AG, Wolhusen, Switzerland). The experimental animals were sacrificed at 8 and 12 weeks after surgery for each group and the experimental site was extracted. The average new bone area for the Ti-oss^® experimental group at 8 weeks was 17.6%. The remaining graft material was 22.7% for the experimental group. The average new bone area for the Ti-oss^® group was 24.3% at 12 weeks. The remaining graft material was 22.8% for the experimental group. It can be evaluated that the new bone-forming ability of Ti-oss^® with octacalcium phosphate (OCP) has the bone-forming ability corresponding to the conventional products.

Tracking the interaction of drug molecules with individual mesoporous amorphous calcium phosphate/ATP nanocomposites - an X-ray spectromicroscopy study

Adenosine triphosphate (ATP) biomolecules play critial roles in the biomineralization process during the formation of amorphous calcium phosphate composites (ACPC), and ACPC is an important drug carrier due to its significant advantages of biocompatibility and biodegradability. Hence, studying the behavior of ACPC nanodrug carriers is crucial to investigate the structural regulation of biomimetic minerals and calcium phosphate (CaP)-based drug delivery systems. However, it is difficult to probe these interactions using traditional characterization methods. In this paper, XANES analysis together with STXM successfully provided a method to reveal the interaction of ATP and drug molecules with individual mesoporous ACPC. We found that the adenosine and phosphate groups of ATP biomolecules coordinated with Ca²⁺ and played critical roles in the formation of ACPC; drug molecules with the -COOH groups were linked to Ca²⁺via carboxylic acid groups primarily by electrostatic interactions, and the N-containing ring structures within the drug molecules also coordinated with Ca²⁺.

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.

Capacitive behaviour of nanocrystalline octacalcium phosphate (OCP) (Ca8H2(PO4)6·5H2O) as an electrode material for supercapacitors: biosupercaps

Octacalcium phosphate (OCP) is classified as a low-temperature phase of calcium phosphate (CaPs); it is a widely used ceramic material in biomedical applications. Interestingly, this study demonstrated the capacitive behavior of OCP as an electrode material in supercapacitors, alternatively named biosupercaps, for the first time in the literature. OCP powder was synthesized by solution precipitation at pH 5.5 at 60 °C in the presence of succinic acid. X-Ray diffraction (XRD) fully confirmed the OCP phase, with a crystallite size of around 40 nm, as calculated by the Scherrer equation. The FE-SEM micrographs of the OCP powder revealed plate-like morphology with a high surface area/thickness ratio. The surface widths of these layers ranged from about 2 to 100 microns, whereas the thickness of the layers was on the nanoscale (<100 nm). Raman spectroscopy was performed to confirm the microstructural formation of the OCP powder and electrodes according to the Raman spectra. Asymmetric and symmetric capacitors were prepared by various designs using OCP powder as a potential electrode material. The electrochemical performance of each biosupercap containing OCP was analyzed by a potentiostat in terms of current-voltage (CV) curves; each sample presented a typical pseudocapacitive behaviour. The electrochemical impedance spectra (EIS) of the OPC materials confirmed their significant capacitive performance, with up to 6 mA h g^-1 specific capacity (SCp); this may be valuable for future medical electronics such as biocompatible energy storage and harvesting microdevices.

Defluoridation of water through the transformation of octacalcium phosphate into fluorapatite

The consumption of water with fluoride concentration higher than 1.5 mg/L (WHO recommended limit) is recognized to cause serious diseases, and fluoride removal from natural contaminated waters is a health priority for more than 260 million people worldwide. The octacalcium phosphate (OCP), a mineralogical precursor of bio-apatite, is here tested as a fluoride remover. A new two-step method for the synthesis of OCP is proposed: 1) synthesis of brushite from calcium carbonate and phosphoric acid; 2) subsequent hydrolysis of brushite. Fluoride removal experiments are performed in batch-mode using different initial concentrations of fluoride (from 40 to 140 mg/L) and reaction times. Most of fluoride is removed within the first 2 h of all experiments, and the drinkable limit of 1.5 mg/L is reached within a minimum of 3 h for an initial fluoride concentration of 40 mg/L. The experimental fluoride removal capacity of OCP is 25.7 mg/g, and 4 g of OCP can effectively treat 1 L of water with fluoride concentration up to 50 times higher than the drinking limit of 1.5 mg/L. XRD and chemical characterization of the solid phases, before and after the removal experiments, indicate that OCP transforms into fluorapatite (FAP) uptaking fluoride from solution.

Advanced biomaterials for repairing and reconstruction of mandibular defects

Mandibles are the largest and strongest bone in the human face and are often severely compromised by mandibular defects, compromising the quality of life of patients. Mandibular defects may result from trauma, inflammatory disease and benign or malignant tumours. The reconstruction of mandibular defect has been a research hotspot in oral and maxillofacial surgery. Although the principles and techniques of mandibular reconstruction have made great progress in recent years, the development of biomedical materials is still facing technical bottleneck, and new materials directly affect technological breakthroughs in this field. This paper reviews the current status of research and application of various biomaterials in mandibular defects and systematically elaborates different allogeneic biomaterial-based approaches. It is expected that various biomaterials, in combination with new technologies such as digital navigation and 3D printing, could be tuned to build new types of scaffold with more precise structure and components, addressing needs of surgery and post-reconstruction. With the illustration and systematization of different solutions, aims to inspire the development of reconstruction biomaterials.

Immobilization of polyphosphoesters on poly(ether ether ketone) (PEEK) for facilitating mineral coating

Poly(ether ether ketone) (PEEK) is an alternative material to metals for orthopedic applications. However, the compatibility of PEEK with hard tissues needs to be improved. To address this issue, this study proposes a novel technique for PEEK surface modifications. A polyphosphodiester macromonomer (PEPMA·Na) was synthesized via the demethylation of polyphosphotriester macromonomer obtained via the ring-opening polymerization of cyclic phosphoesters using 2-hydroxypropyl methacrylamide as the initiator. The surface modification of PEEK was performed via photoinduced and self-initiated graft polymerization of PEPMA·Na without using any photoinitiators. The amount of phosphorus due to poly(PEPMA·Na) immobilized on PEEK increased with an increase in the photoirradiation time. The PEEK surface turned hydrophilic due to poly(PEPMA·Na) grafting, with almost similar advancing and receding contact angles, implying that the modified PEEK surface (PEEK-g-poly(PEPMA·Na)) was homogeneous. Specimens were mineral coated by simple static soaking in ×1.5 simulated body fluid (1.5SBF) and by an alternative process that included additional soaking steps in 200 mM CaCl₂ aq. and 200 mM K₂HPO₄ aq. before static soaking in 1.5SBF. Specimens were immersed in 1.5SBF for 28 days in simple static soaking, after which the PEEK-g-poly(PEPMA·Na) surface was completely covered with spherical cauliflower-like mineral deposits that resembled octacalcium phosphate (OCP). Their structural similarities were confirmed via X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDS), and X-ray fluorescence (XRF) analyses. However, these mineral deposits were not observed on the bare PEEK surface. Due to the additional soaking steps (alternative soaking) undertaken before the static soaking of the specimens in 1.5SBF, the mineral coating on the PEEK-g-poly(PEPMA·Na) was dramatically accelerated and the surface was fully covered with mineral deposits in only one day of soaking. The mineral deposits resulting from both the soaking processes had similar structures. Compared with bare PEEK, osteoblastic MC3T3-E1 cells proliferated more actively on mineral-coated PEEK-g-poly(PEPMA·Na). Thus, the surface immobilization of poly(PEPMA·Na) on a PEEK surface is effective for mineral coating and may be useful to provide hard-tissue compatibility on PEEK.

Vascularized Bone-Mimetic Hydrogel Constructs by 3D Bioprinting to Promote Osteogenesis and Angiogenesis

Bone is a highly vascularized tissue with a unique and complex structure. Long bone consists of a peripheral cortical shell containing a network of channels for vascular penetration and an inner highly vascularized bone marrow space. Bioprinting is a powerful tool to enable rapid and precise spatial patterning of cells and biomaterials. Here we developed a two-step digital light processing technique to fabricate a bone-mimetic 3D hydrogel construct based on octacalcium phosphate (OCP), spheroids of human umbilical vein endothelial cells (HUVEC), and gelatin methacrylate (GelMA) hydrogels. The bone-mimetic 3D hydrogel construct was designed to consist of a peripheral OCP-containing GelMA ring to mimic the cortical shell, and a central GelMA ring containing HUVEC spheroids to mimic the bone marrow space. We further demonstrate that OCP, which is evenly embedded in the GelMA, stimulates the osteoblastic differentiation of mesenchymal stem cells. We refined the design of a spheroid culture device to facilitate the rapid formation of a large number of HUVEC spheroids, which were embedded into different concentrations of GelMA hydrogels. It is shown that the concentration of GelMA modulates the extent of formation of the capillary-like structures originating from the HUVEC spheroids. This cell-loaded hydrogel-based bone construct with a biomimetic dual ring structure can be potentially used for bone tissue engineering.

Current state of fabrication technologies and materials for bone tissue engineering

A range of traditional and free-form fabrication technologies have been investigated and, in numerous occasions, commercialized for use in the field of regenerative tissue engineering (TE). The demand for technologies capable of treating bone defects inherently difficult to repair has been on the rise. This quest, accompanied by the advent of functionally tailored, biocompatible, and biodegradable materials, has garnered an enormous research interest in bone TE. As a result, different materials and fabrication methods have been investigated towards this end, leading to a deeper understanding of the geometrical, mechanical and biological requirements associated with bone scaffolds. As our understanding of the scaffold requirements expands, so do the capability requirements of the fabrication processes. The goal of this review is to provide a broad examination of existing scaffold fabrication processes and highlight future trends in their development. To appreciate the clinical requirements of bone scaffolds, a brief review of the biological process by which bone regenerates itself is presented first. This is followed by a summary and comparisons of commonly used implant techniques to highlight the advantages of TE-based approaches over traditional grafting methods. A detailed discussion on the clinical and mechanical requirements of bone scaffolds then follows. The remainder of the manuscript is dedicated to current scaffold fabrication methods, their unique capabilities and perceived shortcomings. The range of biomaterials employed in each fabrication method is summarized. Selected traditional and non-traditional fabrication methods are discussed with a highlight on their future potential from the authors' perspective. This study is motivated by the rapidly growing demand for effective scaffold fabrication processes capable of economically producing constructs with intricate and precisely controlled internal and external architectures. STATEMENT OF SIGNIFICANCE: The manuscript summarizes the current state of fabrication technologies and materials used for creating scaffolds in bone tissue engineering applications. A comprehensive analysis of different fabrication methods (traditional and free-form) were summarized in this review paper, with emphasis on recent developments in the field. The fabrication techniques suitable for creating scaffolds for tissue engineering was particularly targeted and their use in bone tissue engineering were articulated. Along with the fabrication techniques, we emphasized the choice of materials in these processes. Considering the limitations of each process, we highlighted the materials and the material properties critical in that particular process and provided a brief rational for the choice of the materials. The functional performance for bone tissue engineering are summarized for different fabrication processes and the choice of biomaterials. Finally, we provide a perspective on the future of the field, highlighting the knowledge gaps and promising avenues in pursuit of effective scaffolds for bone tissue engineering. This extensive review of the field will provide research community with a reference source for current approaches to scaffold preparation. We hope to encourage the researchers to generate next generation biomaterials to be used in these fabrication processes. By providing both advantages and disadvantage of each fabrication method in detail, new fabrication techniques might be devised that will overcome the limitations of the current approaches. These studies should facilitate the efforts of researchers interested in generating ideal scaffolds, and should have applications beyond the repair of bone tissue.

Formation and transformation of calcium phosphate phases under biologically relevant conditions: Experiments and modelling

The experimental data on calcium phosphates formation were collected in dilute solution at constant pH (7.40) and temperature (37.0 °C) at different levels of ionic strength (IS). The evolution of the solid phase formation is described in detail using a thermodynamic-kinetic model. The thermodynamic model takes into account all relevant chemical species as well as Posner's clusters; the kinetic model, based on the discretized population balance approach, accounts for the solid formation from solution. The experimental data are consistent with an initial formation of dicalcium phosphate dihydrate (DCPD, brushite), which dominates the nucleation rate, and its rapid transformation into octacalcium phosphate (OCP) or hydroxyapatite (HA), which dominates the growth rate. Depending on the experimental conditions and, including the influence of the IS level, OCP may be further transformed into apatite. The classical nucleation theory is able to describe the experimental results very well and the solid phase growth is limited by the diffusion of Ca²⁺ ions. The precipitation pathway described by a complete thermodynamic-kinetic model is expected to contribute to the understating of the in vivo osteogenesis.

STATEMENT OF SIGNIFICANCE

The formation mechanism of calcium phosphates under biomimetic conditions is unraveled. The formation pathway is mathematically described based on a thermodynamic-kinetic model in which (i) the nucleation stages (primary and secondary) are dominated by the formation of dicalcium phosphate dihydrate (DCPD) and (ii) the fast growth stage is limited by the diffusion of Ca²⁺ ions under the driving force of octacalcium phosphate (OCP), or hydroxyapatite (HA), solubility. The obtained solid phase seems correlated to the activity coefficient of phosphate ions, thus to the ionic strength and local phosphate speciation. The model, being able to highlight the details of the precipitation pathway, is expected to contribute to the understanding of the apatitic phase formation in the biomineralization-biodemineralization processes under in-vivo conditions.

Surface modification of porous alpha-tricalcium phosphate granules with heparin enhanced their early osteogenic capability in a rat calvarial defect model

Heparin binds to and modulates various growth factors, potentially augmenting the bone-forming capability of biomaterials. Here, α-tricalcium phosphate (α-TCP) granules were modified with peptide containing the marine mussel-derived adhesive sequence, which reacts with α-TCP surface, and cationic sequence, which binds to heparin (α-Ph). α-Ph retained the α-TCP phase and intergranule spaces after the surface modification. The existence of heparin on α-Ph granules was confirmed using X-ray photoelectron spectroscopy. Granules of α-TCP and α-Ph were implanted into critical-size defects in rat calvaria for 4 weeks. Micro-computed tomography, histological evaluation, and Alcian blue staining revealed that α-Ph induced superior bone formation compared with α-TCP. Newly formed bone on α-Ph was preferentially in contact with the Alcian blue-stained surfaces of granules. These results suggested that heparinization enhanced the early osteogenic capacity of α-TCP, possibly by modulating the secretion of Alcian blue-stained extracellular matrixes.

Capacity of octacalcium phosphate to promote osteoblastic differentiation toward osteocytes in vitro

Octacalcium phosphate (OCP) has been shown to act as a nucleus for initial bone deposition and enhancing the early stages of osteoblastic differentiation. However, the effect on differentiation at the late stage into osteocytes has not been elucidated. The present study was designed to investigate whether OCP can promote the differentiation lineage from osteoblasts to late osteocytes using a clonal cell line IDG-SW3 compared to commercially available sintered β-tricalcium phosphate (β-TCP) and hydroxyapatite (HA) in a transwell cell culture. Special attention was paid to detect the progress of OCP hydrolysis associated with ionic dissolution products from this material. OCP induced the appearance of an alkaline phosphatase (ALP) peak in the IDG-SW3 cells compared to β-TCP and HA and increased SOST/sclerostin and FGF23 gene expression after 35 days of incubation. Analyses by X-ray diffraction, curve fitting of Fourier transform infrared spectra, and acid phosphate inclusion of the materials showed that OCP tended to hydrolyze to an apatitic structure during the incubation. Since the hydrolysis enhanced inorganic phosphate ion (Pi) release from OCP in the media, IDG-SW3 cells were further incubated in the conditioned media with an increased concentration of Pi in the presence or absence of phosphonoformic acid (PFA), which is an inhibitor of Pi transport within the cells. An increase in Pi concentration up to 1.5 mM raised ALP activity, while its positive effect was eliminated in the presence of 0.1 to 0.5 mM PFA. Calcium ions did not show such an effect. These results indicate the stimulatory capacity of OCP on osteoblastic differentiation toward osteocytes.

STATEMENT OF SIGNIFICANCE

Octacalcium phosphate (OCP) has been shown to have a superior osteoconductivity due to its capacity to enhance initial stage of osteoblast differentiation. However, the effect of OCP on the late osteoblastic differentiation into osteocyte is unknown. This study showed the capacity associated with the structural change of OCP. The data show that OCP released inorganic phosphate (Pi) ions while the hydrolysis advanced if soaked in the media, determined by chemical and physical analyses, and enhanced osteocytes differentiation of IDG-SW3 cells more than hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP). Conditioned elevated Pi-containing media in the absence of OCP enhanced the osteocyte differentiation in the range of the concentration induced by OCP, the effect of which was cancelled by the inhibitor of Pi-transporters.

Understanding interactions between biomaterials and biological systems using proteomics

The role that biomaterials play in the clinical treatment of damaged organs and tissues is changing. While biomaterials used in permanent medical devices were required to passively take over the function of a damaged tissue in the long term, current biomaterials are expected to trigger and harness the self-regenerative potential of the body in situ and then to degrade, the foundation of regenerative medicine. To meet these different requirements, it is imperative to fully understand the interactions biomaterials have with biological systems, in space and in time. This knowledge will lead to a better understanding of the regenerative capabilities of biomaterials aiding their design with improved functionalities (e.g. biocompatibility, bioactivity). Proteins play a pivotal role in the interaction between biomaterials and cells or tissues. Protein adsorption on the material surface is the very first event of this interaction, which is determinant for the subsequent processes of cell growth, differentiation, and extracellular matrix formation. Against this background, the aim of the current review is to provide insight in the current knowledge of the role of proteins in cell-biomaterial and tissue-biomaterial interactions. In particular, the focus is on proteomics studies, mainly using mass spectrometry, and the knowledge they have generated on protein adsorption of biomaterials, protein production by cells cultured on materials, safety and efficacy of new materials based on nanoparticles and the analysis of extracellular matrices and extracellular matrix-derived products. In the outlook, the potential and limitations of this approach are discussed and mass spectrometry imaging is presented as a powerful technique that complements existing mass spectrometry techniques by providing spatial molecular information about the material-biological system interactions.

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 (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 (tooth decay) and osteoporosis (a low bone mass with microarchitectural changes) 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. Due to the compositional similarities to the calcified tissues of mammals, CaPO₄ are widely used as biomaterials for bone grafting purposes. In addition, CaPO₄ have many other applications. 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.

Engineered electrospun polycaprolactone (PCL)/octacalcium phosphate (OCP) scaffold for bone tissue engineering

The main challenge in bone tissue engineering is to find suitable biological substitutes which act as scaffolds. Hence, in this work, a novel scaffold composed of octacalcium phosphate (OCP) particles fabricated by co-precipitation method and polycaprolactone (PCL) using electrospinning technique was introduced. The electrospun scaffolds were characterized by SEM, FTIR, XRD, DSC and TGA analysis. The mechanical properties of the composite scaffolds including maximum tensile stress, strain at break and Young modulus were measured. The bioactivity of the scaffolds was determined by soaking in simulated body fluid (SBF). The osteoblast human G-292 cells were seeded on the scaffold's surface for in vitro studies including cell culture and MTT assay. The FTIR and XRD results showed that OCP component has an appropriate incorporation into the polymeric PCL matrix. The SEM analysis exhibited a significant reduction in the fiber size thanks to the OCP. The results of tensile test confirmed that the PCL/OCP composite introduced suitable mechanical properties. Furthermore, the OCP particles led to form hydroxyapatite layer on the scaffold's surface in the vicinity of SBF solution. The obtained results from the MTT assay described that OCP particles have a positive impact on the growth of the osteoblast human G-292 cells on the scaffolds. Overall, aforesaid features of the PCL/OCP composite scaffold make it a great candidate for the bone tissue engineering application.

Structural modification of titanium surface by octacalcium phosphate via Pulsed Laser Deposition and chemical treatment

In the present study, the Pulsed Laser Deposition (PLD) technique was applied to coat titanium for orthopaedic and dental implant applications. Calcium carbonate (CC) was used as starting coating material. The deposited CC films were transformed into octacalcium phosphate (OCP) by chemical treatments. The results of X-ray diffraction (XRD), Raman, Fourier Transform Infrared Spectroscopy (FTIR) and scanning electron microscopy (SEM) studies revealed that the final OCP thin films are formed on the titanium surface. Human myofibroblasts from peripheral vessels and the primary bone marrow mesenchymal stromal cells (BMMSs) were cultured on the investigated materials. It was shown that all the investigated samples had no short-term toxic effects on cells. The rate of division of myofibroblast cells growing on the surface and saturated BMMSs concentration for the OCP coating were about two times faster than of cells growing on the CC films.

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.

Synthesis and enhanced bone regeneration of carbonate substituted octacalcium phosphate

Using a wet method, we have synthesized octacalcium phosphate carbonate, in which HPO42- in octacalcium phosphate is replaced with CO32-. The physical, crystal, and chemical properties of this new material were compared to octacalcium phosphate, Ca-deficient hydroxyapatite, and Ca-deficient carbonate apatite using X-ray diffraction, Fourier-transform infrared spectroscopy, inductively coupled plasma spectroscopy, and scanning electron microscopy. Surface roughness and morphology were also characterized, along with the ability to support proliferation and differentiation of MG63 cells, as measured by MTT and alkaline phosphatase assay. We found that octacalcium phosphate carbonate enhanced osteoblast proliferation more strongly than all other materials tested. Similarly, Ca-deficient carbonate apatite, a hydrolysate of octacalcium phosphate carbonate, stimulated osteoblast differentiation to a better extent than Ca-deficient hydroxyapatite, a carbonate-free hydrolysate of octacalcium phosphate. These results indicate that octacalcium phosphate carbonate has good biocompatibility and osteoconduction, and incorporation of carbonate into octacalcium phosphate and apatite enhances bone regeneration.

Cellular hydrogels based on pH-responsive chitosan-hydroxyapatite system

The development of bioactive injectable system as cell carrier with minimal impact on viability of encapsulated cells represents a great challenge. In the present work, we propose a new pH-responsive chitosan-hydroxyapatite-based hydrogel with sodium bicarbonate (NaHCO₃) as the gelling agent. The in situ synthesis of hydroxyapatite phase has resulted in stable composite suspension and final homogeneous hydrogel. The application of sodium bicarbonate has allowed non-cytotoxic fast gelation of chitosan-hydroxyapatite within 4min, and without excess of sodium ions concentration. Rheological properties of crosslinked hydrogel have demonstrated possible behaviour as 'strong physical hydrogel'. The live dead staining has confirmed good viability and dispersion, as well as proliferation of encapsulated cells by the culture time. Presented preliminary results show good potential of chitosan-hydroxyapatite/NaHCO₃ as a cell carrier, whose impact on the cell differentiation need to be confirmed by encapsulation of other cell phenotypes.

In vivo erosion of orthopedic screws prepared from nacre (mother of pearl)

BACKGROUND

Biodegradable biomaterials have been proposed to prepare orthopedic devices. Nacre is a natural aragonitic material made of calcium carbonate and is bioerodible.

WORKING HYPOTHESIS

We postulated that nacre is biodegradable without provoking bone erosion and favors bone apposition.

MATERIAL AND METHODS

We prepared orthopedic screws from nacre of the giant oyster Pinctada maxima. Threaded screws (3.5mm diameter) were implanted in 6 ewes in the upper tibial metaphysis (3 to 4 screws per animal). Their trajectory was transcortical and intramedullary to the opposite cortex. Animals were kept for 3months (n=2) and 6 months (n=4). They did not develop local inflammation. Before euthanasia, they received a double calcein labeling. Bone samples were analyzed by X-ray nanotomography and histology after embedding in poly(methyl methacrylate). The fractal dimension of the screw profiles (measured by the box-counting method) was used to quantify surface erosion.

RESULTS

3D nanotomography showed a gradual erosion of the threads, which was confirmed by a decreased fractal dimension. Histologically, multinucleated cells (non-osteoclastic appearance) were visible at the surface of the screws. No ruffled border was seen in these cells but they had extensions creeping in the organic matter between the aragonite tablets. Bone apposition was noted in the transcortical path of the screws with limited osteoconduction at the endosteum. Mineralization rate was increased in these zones composed of woven bone in contact with the nacre.

DISCUSSION AND CONCLUSION

Screws prepared from nacre have the advantage of an in vivo resorbability by macrophage-derived cells and an osteoconductive apposition in contact with the material without triggering a local inflammatory reaction.

[The impact of octacalcium phosphate on the dynamics of bone matrix formation in experimental bone defects]

The aim of the study was to assess the interaction of of octacalcium phosphate (OCP) with bone matrix and cells and its impact on the process of bone generation. The survey was conducted on animal model: critical hipbone defect was created in 12 230-250 g Wister rats. The animals were then divided in two groups. In group 1 (6 animals) defect was left to heal under blood clot and in group 2 (6 animals) it was filled with OCP. Three animals with no defect served as a control group. It was showed significant (p<0.05) increase of the area of the newly formed bone tissue and its direct correlation with duration of observation.

Cytotoxic effects of mineral trioxide aggregate, calcium enrichedmixture cement, Biodentine and octacalcium pohosphate onhuman gingival fibroblasts

Background. This in vitro study compared the effects of mineral trioxide aggregate (MTA), calcium enriched mixture(CEM) cement, Biodentine (BD) and octacalcium phosphate (OCP) on the viability of human gingival fibroblasts (HGFs). Methods. After completion of the setting time of the materials under study, fibroblasts were placed in 24-well insert platesand 1 mg of each material was added to the respective wells. The plates were then incubated at 37°C. The inserts were removedat 24, 48 and 168 hours and 2,5-diphenyltetrazolium bromide was added to assess cytotoxicity via the MTT colorimetricassay. Data were analyzed at different time intervals using repeated-measures ANOVA, followed by the Bonferronitest at three levels of significance of P < 0.05, P < 0.01 and P < 0.001. Results. Cytotoxicity of the materials under study was not significantly different at 24 and 48 hours compared to the controlgroup. However, at 168 hours, a significant difference was noted between MTA (P < 0.05) and Biodentine (P < 0.01)and the control group. Conclusion. Cytotoxicity of MTA, CEM, Biodentine and OCP against HGFs was similar to that of the control group at 24and 48 hours. Over time, MTA and Biodentine exhibited less cytotoxicity than other materials.

Evaluation of bioactivity of octacalcium phosphate using osteoblastic cell aggregates on a spheroid culture device

Much attention has been paid to three-dimensional cell culture systems in the field of regenerative medicine, since three-dimensional cellular aggregates, or spheroids, are thought to better mimic the in vivo microenvironments compared to conventional monolayer cultured cells. Synthetic calcium phosphate (CaP) materials are widely used as bone substitute materials in orthopedic and dental surgeries. Here we have developed a technique for constructing a hybrid spheroid consisting of mesenchymal stem cells (MSCs) and synthetic CaP materials using a spheroid culture device. We found that the device is able to generate uniform-sized CaP/cell hybrid spheroids rapidly and easily. The results showed that the extent of osteoblastic differentiation from MSCs was different when cells were grown on octacalcium phosphate (OCP), hydroxyapatite (HA), or β-tricalcium phosphate (β-TCP). OCP showed the greatest ability to increase the alkaline phosphatase activity of the spheroid cells. The results suggest that the spheroids with incorporated OCP may be an effective implantable hybrid consisting of scaffold material and cells for bone regeneration. It is also possible that this CaP-cell spheroid system may be used as an in vitro method for assessing the osteogenic induction ability of CaP materials.

Monolithic calcium phosphate/poly(lactic acid) composite versus calcium phosphate-coated poly(lactic acid) for support of osteogenic differentiation of human mesenchymal stromal cells

Calcium phosphates (CaPs), extensively used synthetic bone graft substitutes, are often combined with other materials with the aim to overcome issues related to poor mechanical properties of most CaP ceramics. Thin ceramic coatings on metallic implants and polymer-ceramic composites are examples of such hybrid materials. Both the properties of the CaP used and the method of incorporation into a hybrid structure are determinant for the bioactivity of the final construct. In the present study, a monolithic composite comprising nano-sized CaP and poly(lactic acid) (PLA) and a CaP-coated PLA were comparatively investigated for their ability to support proliferation and osteogenic differentiation of bone marrow-derived human mesenchymal stromal cells (hMSCs). Both, the PLA/CaP composite, produced using physical mixing and extrusion and CaP-coated PLA, resulting from a biomimetic coating process at near-physiological conditions, supported proliferation of hMSCs with highest rates at PLA/CaP composite. Enzymatic alkaline phosphatase activity as well as the mRNA expression of bone morphogenetic protein-2, osteopontin and osteocalcin were higher on the composite and coated polymer as compared to the PLA control, while no significant differences were observed between the two methods of combining CaP and PLA. The results of this study confirmed the importance of CaP in osteogenic differentiation while the exact properties and the method of incorporation into the hybrid material played a less prominent role.

Dose-dependent enhancement of octacalcium phosphate biodegradation with a gelatin matrix during bone regeneration in a rabbit tibial defect model

The present study was designed to investigate how the dose of granular octacalcium phosphate in a gelatin matrix affects its bone regenerative and biodegradable properties in a rabbit tibia defect.

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.

Calcium Orthophosphate-Containing Biocomposites and Hybrid Biomaterials for Biomedical Applications

The state-of-the-art on calcium orthophosphate (CaPO4)-containing biocomposites and hybrid biomaterials suitable for biomedical applications is presented. Since these types of biomaterials offer many significant and exciting possibilities for hard tissue regeneration, this subject belongs to a rapidly expanding area of biomedical research. Through the successful combinations of the desired properties of matrix materials with those of fillers (in such systems, CaPO4 might play either role), innovative bone graft biomaterials can be designed. Various types of CaPO4-based biocomposites and hybrid biomaterials those are either already in use or being investigated for biomedical applications are extensively discussed. Many different formulations in terms of the material constituents, fabrication technologies, structural and bioactive properties, as well as both in vitro and in vivo characteristics have been already proposed. Among the others, the nano-structurally controlled biocomposites, those containing nanodimensional compounds, biomimetically fabricated formulations with collagen, chitin and/or gelatin, as well as various functionally graded structures seem to be the most promising candidates for clinical applications. The specific advantages of using CaPO4-based biocomposites and hybrid biomaterials in the selected applications are highlighted. As the way from a laboratory to a hospital is a long one and the prospective biomedical candidates have to meet many different necessities, the critical issues and scientific challenges that require further research and development are also examined.