The electric flash generated by an electric field aided the sintering of hydroxyapatite (HA) in relatively less time and at lower furnace temperatures than the conventional sintering method. The current work has reported a comparative study to explain thermodynamically the effects of the increasing DC electric field strength at 1000°C and 1100°C sintering furnace temperatures during flash sintering in terms of the grain growth and densification of the HA in air. The abrupt change in current density and conductivity of HA during the flash peak correlates the high grain growth and rapid densification with increasing electric field strength. The increased electric field strength reduced the power dissipation from the HA. The abrupt high-rise grain growth phenomenon during flash sintering might consist of the thermal effect and enhancement in ionic conductivity due to the application of an electric field.

Flash sintering (FS) is becoming a popular densification route for high-temperature oxide ceramics because of its rapid sintering performance. In the current study, bar-shaped hydroxyapatite (HA) specimens were hung between two Pt wire electrodes and consolidated by FS through a direct electric field at a constant furnace temperature (1000°C or 1100°C) in air. The electric field facilitated the sintering of HA at relatively lower furnace temperatures than that obtained using the conventional sintering method. The voltage required for the onset of the flash decreased with increasing furnace temperature. The effects of the DC electric field strength at different sintering furnace temperatures were examined in terms of the phase evolution and stability of HA. HA phase dissociation was not observed regardless of the electric field strength or furnace temperature because of the very short FS time. Higher grain growth with fast densification in the flashed-sintered HA samples occurred with increasing electric field strength at each furnace temperature.

Low toughness and wear resistance have limited application of many bioceramics in biomedical applications requiring load bearing capability. Spark plasma sintering (SPS) has widened the envelope of processing conditions available to produce bioceramics with new microstructural architectures. SPS has enabled realisation of transparent hydroxyapatite (HA) by providing the means to consolidate fully dense nanostructured HA. Recently, low-dimensional carbon nanomaterials, including carbon nanotubes (CNTs) and graphene/graphene nanoplatelets (GNP) have gained increasing attention as reinforcements due to their providing superior mechanical properties, favourable biocompatibility, and large specific surface area. Processing of these nanocomposites is done using SPS in order to consolidate the ceramics to full density in short time periods, while retaining the structure and properties of the nanomaterial reinforcements. This review focuses on recent progress on GNP/CNT reinforced HA and alumina nanocomposites, including mechanical properties, tribological behaviour, processing conditions, and mechanisms. Biocompatibility of these promising bioceramics with various cells/tissues are discussed.

Calcium-phosphate based hydroxyapatite (HA) biomaterials are widely being investigated due to their osseointegration property, but those inherently lack antibacterial property. In the present work, wet-chemically synthesized rod-shaped zinc oxide (ZnO) is embedded in HA and its role as antibacterial agent without compromising cytocompatability property is established. In particular, HA-xZnO (x=0, 5, 10 wt.%) composites were consolidated by spark plasma sintering at 950 degrees C in vacuum. A host of the spectroscopy and microscopy techniques were utilized to analyze the nature of distribution of ZnO as well as surface chemical characteristics. While the bactericidal property is expectedly induced, the elastic modulus as well as cell adhesion property remains uncompromised.

The surface properties of any biological implant are imperative in a cell-material interaction point of view. The current study compared three different surface morphologies of titanium (Ti) samples in terms of adhesion, proliferation and differentiation of cells. A net-porous nanostructure, randomly arranged nano-wires and micron/submicron-crater like morphologies were on the Ti samples by single step etching with H2O2, NaOH, and HCl solutions, respectively. Although cell proliferation was decreased by the surface modifications described above, osteogenic differentiation was induced in the cells grown on the etched-Ti samples compared to the polished-Ti samples. In particular, osteocalcin (OC) and osteopontin (OPN) osteogenic phenotype markers induced significantly in the HCl-etched-Ti samples, whereas only OC was induced on the H2O2-etched-Ti and OPN was expressed significantly in the NaOH-etched-Ti sample compared to polished-Ti. In addition, the NaOH-etched-Ti samples showed faster apatite deposition compared to the HCl- and H2O2-etched samples in the simulated body fluid.

Calcium phosphate (Ca–P) based scaffolds were found to be a favourable alternative for orthopaedic applications because of their similar chemical composition to natural bone. In this study, porous triphasic Ca–P scaffolds containing macropores (∽200 μm) interconnected with micropores (∽20 μm) were fabricated using an extrusion method. The hydroxyapatite/tricalcium phosphate ratio of the porous scaffolds was varied using different ratios of starting materials while keeping the Ca/P ratio fixed (1.5). A water glass coating on the porous Ca–P scaffolds increased the compressive strength by 45% without significantly decreasing the porosity of the H100D50 scaffold. The maximum compressive strength, ∼15 MPa, was achieved on the H100D50 scaffold. The ability for apatite formation in simulated body fluid was amplified by the water glass coating on the sintered Ca–P scaffolds. Therefore, a water glass coating can be used to enhance the mechanical properties as well as the biomineralisation of the porous ceramic sc...

Flash sintering is a novel and emerging route for sintering ceramics within a few seconds, even under pressure-less conditions. In the current study, hydroxyapatite (HA) was fully densified by flash sintering at a furnace temperature of 1020 degrees C. Flash sintering with constant electric fields of 750 and 1000 V cm(-1) reduced the grain growth rate significantly compared to that sintered in the absence of an electric field at 1400 degrees C. The microstructure of HA consolidated by flash sintering was compared with that of the without electric field sintered samples. The flashsintered samples showed smaller grains (160 similar to 320 nm) than the without electric field sintered samples (similar to 15 mu m). The samples with a higher applied electric field showed slightly better densification than those with the lower field by flash sintering. Overall, the electric flash reduces the sintering temperature effectively and decreases the holding time to densify highly insulating ceramics, such as HA.

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Ca-P and silicon based materials have become very popular as bone tissue engineering materials. In this study, water-glass (also known as sodium silicate glass) was coated on sintered hydroxyapatite (HA) and HA-TCP (TCP stands for tricalcium phosphate) samples and subsequently heat-treated at 600°C for 2 hrs. X-rays diffraction showed the presence of β- and α-TCP phases along with HA in the HA-TCP samples. Samples without coating, with water-glass coating, and heat-treated after water-glass coating were used to observe the adhesion and proliferation response of bone marrow derived-mesenchymal stem cells (MSCs). Cell culture was carried out for 4 hrs, 1 day, and 7 days. Interestingly, all samples showed similar response for cell adhesion and proliferation up to 7-day culture but fibronectin, E-cadherin, and osteogenic differentiation related genes (osteocalcin and osteopontin) were significantly induced in heat-treated water-glass coated HA-TCP samples. A water-glass coating on Ca-P samples was not found to influence the cell proliferation response significantly but activated some extracellular matrix genes and induced osteogenic differentiation in the MSCs.

In recent, the use of the sandblasted with large grit and acid-etched treated dental implants with alumina (Al2O3) grits is getting popular instead of the use of resorbable blasting media treated implants with hydroxyapatite (Ca-10(PO4)(6)(OH)(2)) due to the cost reason and the similar cell behaviors, although the residues of alumina grit might hamper the osteointegration. This study compared the adhesion and proliferation of human mesenchymal stem cells on Ti disc surface modified by two popular grit materials. Hydroxyapatite and alumina grits were used as a blasting materials, and HNO3 and H3PO4 acids were used as an etching agents to remove the residues of the blasting grits from the surface of Ti. For the comparison of cytocompatibility on the above both type of samples, the human nnesenchymal stem cells were incubated for 2 hours, 1 day, and 1 week. The Ti samples treated with hydroxyapatite grits showed slightly better cell adhesion and proliferation than that Al2O3 grits.

Calcium phosphates (Ca-P) are used commonly as artificial bone substitutes to control the biodegradation rate of an implant in the body fluid. This study examined the in vitro proliferation of human bone marrow-derived mesenchymal stem cells (hBMSCs) on triphasic Ca-P samples. For this aspect, hydroxyapatite (HA), dicalcium phosphate dehydrate (DCPD), and calcium hydroxide (Ca(OH)2 ) were mixed at various ratios, cold compacted, and sintered at 1250°C in air. X-ray diffraction showed that the β-tricalcium phosphate (TCP) to α-TCP phase transformation increased with increasing DCPD/HA ratio. The micro-hardness deceased with increasing TCP content, whereas the mean grain size and porosity increased with increasing TCP concentration. To evaluate the in vitro degree of adhesion and proliferation on the HA/TCP samples, human BMSCs were incubated on the HA/TCP samples and analyzed by a cells proliferation assay, expression of the extracellular matrix (ECM) genes, such as α-smooth muscle actin (α-SMA) and fibronectin (FN), and FITC-phalloidin fluorescent staining. In terms of the interactions of human BMSCs with the triphasic Ca-P samples, H50T50 (Ca/P = 1.59) markedly enhanced cell spreading, proliferation, FN, and α-SMA compared with H100T0 (Ca/P = 1.67). Interestingly, these results show that among the five HA/TCP samples, H50T50 is the optimal Ca-P composition for in vitro cell proliferation. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 72-80, 2017.

Porous hydroxyapatite (HA)–tricalcium phosphate (TCP) ceramic scaffolds were prepared using a screw-type extrusion method with polymer beads. HA and dicalcium phosphate dehydrates (DCPD) were added at various ratios to obtain different HA/TCP ratios in sintered ceramic scaffolds. To further enhance the pore interconnectivity and porosity, the developed porous ceramic scaffolds were etched with acid solutions. The maximum porosity (~85%) was observed in the Ca-P scaffold with the lowest HA (~7%) content. On the other hand, the maximum compressive strength was noted in the scaffolds with the highest HA content (~85%). X-ray diffraction showed that the extent of the β-TCP to α-TCP phase transformation increased with decreasing HA/DCPD ratio. All HCl-etched scaffolds were observed to generate micropores, which improved the interconnectivity, while biomineralization was found to be the same for both the HCl-etched and non-etched scaffolds. In particular, hydrochloric acid etching is a promising method for improving the interconnectivity and porosity of the ceramic scaffolds.

Calcium phosphate (Ca-P) based scaffolds were found to be a favourable alternative for orthopaedic applications because of their similar chemical composition to natural bone. In this study, porous triphasic Ca-P scaffolds containing macropores (similar to 200 mu m) interconnected with micropores (similar to 20 mu m) were fabricated using an extrusion method. The hydroxyapatite/tricalcium phosphate ratio of the porous scaffolds was varied using different ratios of starting materials while keeping the Ca/P ratio fixed (1.5). A water glass coating on the porous Ca-P scaffolds increased the compressive strength by 45% without significantly decreasing the porosity of the H100D50 scaffold. The maximum compressive strength, similar to 15 MPa, was achieved on the H100D50 scaffold. The ability for apatite formation in simulated body fluid was amplified by the water glass coating on the sintered Ca-P scaffolds. Therefore, a water glass coating can be used to enhance the mechanical properties as well as the biomineralisation of the porous ceramic scaffolds.

Background Ti and its alloys have been widely used as orthopedic and dental implants due to their outstanding mechanical properties and biocompatibility. However, long time is required to form bond between Ti implant and surrounding tissues. Therefore, these implants necessitate surface treatment such as mechanical/chemical treatment and coating of bioactive materials for improving the osseointegration. Results This study was focused on the calcium-phosphate (Ca-P) coating on machined Ti, blasted-Ti (B-Ti), and blasted-NaOH-etched-Ti (BNH) surfaces by hydrothermal method to evaluate the ability of HA formation. Nanostructured morphology was created by NaOH etching on blasted-Ti surface. XRD analysis confirmed the existence of sodium titanate phase on such samples. Rutile and anatase phases along with hydroxyapatite were observed after hydrothermal treatment in Ca-P solution. Substantial hydroxyapatite together with TiO 2 was observed during hydrothermal treatment at 200°C for 12 hrs. Blasted-NaOH-etched samples (BNH-Ti) revealed appreciable bone-like apatite formation as compared to machined-Ti and blasted-Ti (B-Ti) surfaces. However, maximum HA formation was confirmed on Ca-P coated-BNH samples (BNHA-Ti-200-12) by XRD and ICP analysis. Conclusion Multistep surface treatment adopted in current study would be effective to enhance HA formation on Ti surface.

In addressing the issue of prosthetic infection, we demonstrate herein how direct electric field (DC EF) stimulation can effectively inhibit biofilm formation, when pathogenic Staphylococcus aureus (MRSA, USA 300) are grown on HA-xZnO (x = 0, 5, 7.5, and 10 wt %) biocomposites in vitro. After bacterial preincubation for 4 h, a low intensity DC EF (1V/cm) was applied for different time periods (t = 6, 12, 18, and 24 h). The bacterial viability and biofilm maturation were evaluated by a combination of biochemical assays, fluorescence/confocal microscopy, and flow cytometry. The results confirm a time-dependent and composition-independent decrease in bacterial viability and biofilm formation on HA-xZnO composites w.r.t EF-treated HA. Flow cytometry analysis indicated that 12 h EF application resulted in membrane depolarization of ∼35% of S. aureus populations on HA-xZnO composites. The live/dead assay results revealed ∼60% decline in viable bacterial numbers with a concomitant 3.5-fold increase in the production of reactive oxygen species (ROS) after 18 h of EF. The loss in bacterial viability and biofilm instability is due to the synergistic bactericidal action of ZnO and EF. Taken together, the use of engineered biomaterial substrate with antimicrobial reinforcement coupled with continuous low intensity EF application can be adopted to treat prosthetic implant associated infection. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1064-1075, 2016.

Graphene nanoplatelets (GNPs)-reinforced hydroxyapatite composites were analyzed in two directions of the applied pressure (perpendicular and parallel). Platelet-shaped pores were observed in the cross-section normal to the applied pressure, whereas elongated or buttonhole type pores around the agglomerated GNPs were found in the parallel cross-section. The normal cross-section of the composite showed lower hardness and higher intensity of XRD peak of (002) plane than the parallel cross-section.

In addressing the issue of prosthetic infection, this work demonstrated the synergistic effect of the application of static magnetic field (SMF) and ferrimagnetic substrate properties on the bactericidal property in vitro. This aspect was studied using hydroxyapatite (HA)-xFe3 O4 (x=10, 20, and 40 wt.%) substrates, which have different saturation magnetization properties. During bacteria culture experiments, 100 mT SMF was applied to growth medium (with HA-xFe3 O4 substrate) in vitro for 30, 120, and 240 min. A combination of MTT assay, membrane rupture assays, live/dead assay, and fluorescence microscopic analysis showed that the bactericidal effect of SMF increases with the exposure duration as well as increasing Fe3 O4 content in biomaterial substrates. Importantly, the synergistic bactericidal effect was found to be independent of bacterial cell type, as similar qualitative trend is measured with both gram negative Escherichia coli (E. coli) and gram positive Staphylococcus aureus (S. aureus) strains. The reduction in E. coli viability was 83% higher on HA-40 Wt % Fe3 O4 composite after 4 h exposure to SMF as compared to nonexposed control. Interestingly, any statistically significant difference in ROS was not observed in bacterial growth medium after magnetic field exposure, indicating the absence of ROS enhancement due to magnetic field. Overall, this study illustrates significant role being played by magnetic substrate compositions towards bactericidal property than by magnetic field exposure alone.

Although HA is highly biocompatible, one of the major disadvantages of HA include the lack of antibacterial property. In an earlier study, we demonstrated the potential role of magnetic field stimulation on bactericidal property in vitro. Following this, it was hypothesized that antibacterial property can be realized if bacteria are grown on magnetic biocomposites in vitro. In addressing this issue, this study demonstrates the development of HA‐Fe3O4‐based magnetic substrate with multifunctional properties. For this purpose, HA‐xFe3O4 (x: 10, 20 and 40 wt%) powder compositions were sintered using uniquely designed spark plasma sintering conditions (three stage sintering with final holding temperature of 1050°C for 5 min). A saturation magnetization of 24 emu/g is measured with HA‐40%Fe3O4. Importantly, all the HA‐Fe3O4 composites demonstrated bactericidal property by rupturing the membrane of Escherichia coli bacteria, while supporting cell growth of metabolically active human fetal osteoblast cells over 8 d culture. A systematic decrease in bacterial viability with Fe3O4 addition is consistent with a commensurate increase in reactive oxygen species (ROS).