Sintering effects in a glass reinforced hydroxyapatite

Effect of Thermal Treatments and Ion Substitution on Sintering and Crystallization of Bioactive Glasses: A Review

Bioactive glasses (BGs) are promising materials for bone regeneration due to their ability to bond with living bone tissue. However, thermal stability and mechanical properties of BGs need improvement for better clinical performance. In this paper, we present an overview of the influence of different ions on the sintering and crystallization of BGs. Specifically, this review focuses on the impact of thermal treatments on the crystallization of 45S5 and other significant BG compositions. Potential applications of these thermally treated BGs, such as scaffolds, BG-based composites, and thermally sprayed coatings, are explored. Moreover, the substitution of ions has been investigated as a method to enhance the thermal properties of BGs. Notably, zinc, potassium, and strontium have been studied extensively and have demonstrated promising effects on both the thermal and the mechanical properties of BGs. However, it is important to note that research on ion inclusion in BGs is still in its early stages, and further investigation is necessary to fully comprehend the effects of different ions on sintering and crystallization. Therefore, future studies should focus on optimizing the ion substitution method to improve the thermal, mechanical, and even biological properties of BGs, thereby enhancing their potential for various biomedical applications.

In-vitroassessment of β-tricalcium phosphate/bredigite-ciprofloxacin (CPFX) scaffolds for bone treatment applications

In the present study, β-tricalcium phosphate (β-TCP) scaffolds with various amounts of bredigite (Bre) were fabricated by the space holder method. The effect of bredigite content on the structure, mechanical properties,in vitrobioactivity, and cell viability was investigated. The structural assessment of the composite scaffolds presented interconnected pores with diameter of 300-500 μm with around 78%-82% porosity. The results indicated that the compressive strength of the scaffolds with 20% bredigite (1.91 MPa) was improved in comparison with scaffolds with 10% bredigite (0.52 MPa), due to the reduction of the average pore and grain sizes. Also, the results showed that the bioactivity and biodegradability of β-TCP/20Bre were better than that of β-TCP/10Bre. Besides, in this study, the release kinetics of ciprofloxacin (CPFX) loaded β-TCP/Bre composites as well as the ability of scaffolds to function as a sustained release drug carrier was investigated. Drug release pattern of β-TCP/bredigite-5CPFX scaffolds exhibited the rapid burst release of 43% for 3 h along with sustained release (82%) for 32 h which is favorable for bone infection treatment. Antibacterial tests revealed that the antibacterial properties of β-TCP/bredigite scaffolds are strongly related to the CPFX concentration, wherein the scaffold containing 5% CPFX showed the most significant zone of inhibition (33 ± 0.5 mm) againstStaphylococcus aureus. The higher specific surface areas of nanostructure β-TCP/bredigite scaffolds containing CPFX lead to an initial rapid release followed by constant drug delivery. MTT assay showed that the cell viability of β-TCP/bredigite scaffold loading with up to 1%-3% CPFX (95 ± 2%), is greater than for scaffolds containing 5% CPFX (84 ± 2%). In Overall, it may suggested that β-TCP/bredigite containing 1%-3% CPFX possesses great cell viability and antibacterial activity and be employed as bactericidal biomaterials and bone infection treatment.

Model of dissolution in the framework of tissue engineering and drug delivery

Dissolution phenomena are ubiquitously present in biomaterials in many different fields. Despite the advantages of simulation-based design of biomaterials in medical applications, additional efforts are needed to derive reliable models which describe the process of dissolution. A phenomenologically based model, available for simulation of dissolution in biomaterials, is introduced in this paper. The model turns into a set of reaction-diffusion equations implemented in a finite element numerical framework. First, a parametric analysis is conducted in order to explore the role of model parameters on the overall dissolution process. Then, the model is calibrated and validated versus a straightforward but rigorous experimental setup. Results show that the mathematical model macroscopically reproduces the main physicochemical phenomena that take place in the tests, corroborating its usefulness for design of biomaterials in the tissue engineering and drug delivery research areas.

Investigating the addition of SiO2–CaO–ZnO–Na2O–TiO2 bioactive glass to hydroxyapatite: Characterization, mechanical properties and bioactivity

Hydroxyapatite (Ca10(PO4)6(OH)2) is widely investigated as an implantable material for hard tissue restoration due to its osteoconductive properties. However, hydroxyapatite in bulk form is limited as its mechanical properties are insufficient for load-bearing orthopedic applications. Attempts have been made to improve the mechanical properties of hydroxyapatite, by incorporating ceramic fillers, but the resultant composite materials require high sintering temperatures to facilitate densification, leading to the decomposition of hydroxyapatite into tricalcium phosphate, tetra-calcium phosphate and CaO phases. One method of improving the properties of hydroxyapatite is to incorporate bioactive glass particles as a second phase. These typically have lower softening points which could possibly facilitate sintering at lower temperatures. In this work, a bioactive glass (SiO2–CaO–ZnO–Na2O–TiO2) is incorporated (10, 20 and 30 wt%) into hydroxyapatite as a reinforcing phase. X-ray diffraction confirmed that no additional phases (other than hydroxyapatite) were formed at a sintering temperature of 560 ℃ with up to 30 wt% glass addition. The addition of the glass phase increased the % crystallinity and the relative density of the composites. The biaxial flexural strength increased to 36 MPa with glass addition, and there was no significant change in hardness as a function of maturation. The pH of the incubation media increased to pH 10 or 11 through glass addition, and ion release profiles determined that Si, Na and P were released from the composites. Calcium phosphate precipitation was encouraged in simulated body fluid with the incorporation of the bioactive glass phase, and cell culture testing in MC-3T3 osteoblasts determined that the composite materials did not significantly reduce cell viability.

Samarium doped glass-reinforced hydroxyapatite with enhanced osteoblastic performance and antibacterial properties for bone tissue regeneration

Cell adhesion of MG63 osteoblastic cells seeded over GR-HA_control and Sm doped composites, at day 1 of culture. Low (A) and high (B) CLSM images of cells stained for F-actin cytoskeleton (green) and nuclei (red); (C) SEM images.

Phase stability and rapid consolidation of hydroxyapatite–zirconia nano-coprecipitates made using continuous hydrothermal flow synthesis

A rapid and continuous hydrothermal route for the synthesis of nano-sized hydroxyapatite rods co-precipitated with calcium-doped zirconia nanoparticles using a superheated water flow at 450°C and 24.1 MPa as a crystallizing medium is described. Hydroxyapatite and calcium-doped zirconia phases in the powder mixtures could be clearly identified based on particle size and morphology under transmission electron microscopy. Retention of a nanostructure after sintering is crucial to load-bearing applications of hydroxyapatite-based ceramics. Therefore, rapid consolidation of the co-precipitates was investigated using a spark plasma sintering furnace under a range of processing conditions. Samples nominally containing 5 and 10 wt% calcium-doped zirconia and hydroxyapatite made with Ca:P solution molar ratio 2.5 showed excellent thermal stability (investigated using in situ variable temperature X-ray diffraction) and were sintered via spark plasma sintering to >96% sintered densities at 1000°C resulting in hydroxyapatite and calcium-doped zirconia as the only two phases. Mechanical tests of spark plasma sintering sintered samples (containing 10 wt% calcium-doped zirconia) revealed a three-pt flexural strength of 107.7 MPa and Weibull modulus of 9.9. The complementary nature of the spark plasma sintering technique and continuous hydrothermal flow synthesis (which results in retention of a nanostructure even after sintering at elevated temperatures) was hence showcased.

Sintering behavior and properties of reinforced hydroxyapatite/TCP biphasic bioceramics with ZnO-whiskers

Composite materials of hydroxyapatite/β-tricalcium phosphate (HA/β-TCP) biphasic bioceramics and ZnO whiskers, added in amounts up to 3 wt% to an HA/β-TCP matrix, were prepared by sintering. The sintering behavior, the mechanical reinforcement (due to ZnO), and the mineralization ability of the resulting materials in simulated body fluid (SBF) were experimentally investigated. The experimental results showed that the densification and mechanical properties were improved with increasing amounts of ZnO. The presence of ZnO appeared to affect the proportion of HA/β-TCP after sintering. Doping with a high amount of ZnO favored the dissolution of the materials in SBF.

Synthesis of novel tricalcium phosphate-bioactive glass composite and functionalization with rhBMP-2

A functionalization is required for calcium phosphate-based bone substitute materials to achieve an entire bone remodeling. In this study it was hypothesized that a tailored composite of tricalcium phosphate and a bioactive glass can be loaded sufficiently with rhBMP-2 for functionalization. A composite of 40 wt% tricalcium phosphate and 60 wt% bioactive glass resulted in two crystalline phases, wollastonite and rhenanite after sintering. SEM analysis of the composite's surface revealed a spongious bone-like morphology after treatment with different acids. RhBMP-2 was immobilized non-covalently by treating with chrome sulfuric acid (CSA) and 3-aminopropyltriethoxysilane (APS) and covalently by treating with CSA/APS, and additionally with 1,1'-carbonyldiimidazole. It was proved that samples containing non-covalently immobilized rhBMP-2 on the surface exhibit significant biological activity in contrast to the samples with covalently bound protein on the surface. We conclude that a tailored composite of tricalcium phosphate and bioactive glass can be loaded sufficiently with BMP-2.

High-strength nanograined and translucent hydroxyapatite monoliths via continuous hydrothermal synthesis and optimized spark plasma sintering

The synthesis of high-strength, completely dense nanograined hydroxyapatite (bioceramic) monoliths is a challenge as high temperatures or long sintering times are often required. In this study, nanorods of hydroxyapatite (HA) and calcium-deficient HA (made using a novel continuous hydrothermal flow synthesis method) were consolidated using spark plasma sintering (SPS) up to full theoretical density in ∼5 min at temperatures up to 1000°C. After significant optimization of the SPS heating and loading cycles, fully dense HA discs were obtained which were translucent, suggesting very high densities. Significantly high three-point flexural strength values for such materials (up to 158 MPa) were measured. Freeze-fracturing of disks followed by scanning electron microscopy investigation revealed selected samples possessed sub-200 nm sized grains and no visible pores, suggesting they were fully dense.

Phase development and sintering behaviour of biphasic HA-TCP calcium phosphate materials prepared from hydroxyapatite and bioactive glass

The composites of hydroxyapatite (HA) with 2.5 and 5 wt% of a double oxide (50 mol% CaO and 50 mol% P(2)O(5)) glass were prepared using the conventional powder mixing and sintering method. The addition of the glass significantly enhanced the decomposition process of HA into alpha tricalcium phosphate (alpha-TCP) for bodies sintered at 1,300 and 1,350 degrees C and beta-TCP phases for the ones sintered at 1,200, 1,250 and 1,300 degrees C. Microstructural characteristics, phase development and thermal behaviour were studied by SEM, XRD and STA. The effects of TCP phase content and phase transformation from beta-TCP to alpha-TCP on the sintering are discussed. The characterizations revealed considerable content of TCP in the form of large semi-islands due to important reactions between the fine HA and the glass mixed powders.

Influence of oxide-based sintering additives on densification and mechanical behavior of tricalcium phosphate (TCP)

In this research, we studied and analyzed the effects of four different oxide-based sintering additives on densification, mechanical behavior, biodegradation and biocompatibility of tricalcium phosphate (TCP) bioceramics. Selective sintering additives were introduced into pure TCP ceramics, in small quantities, through homogeneous mixing, using a mortar and pestle. The consequent powders of different compositions were pressed into cylindrical compacts, uniaxially and sintered at elevated temperatures, 1150 degrees C and 1250 degrees C, separately in a muffle furnace. X-ray powder diffraction technique was used to analyze the phase-purity of TCP after sintering. Hardness of these sintered specimens was evaluated using a Vickers hardness tester. Sintered cylindrical samples were tested under uniaxial compressive loading, as a function of composition to determine their failure strength. Biodegradation studies conducted using simulated body fluid under dynamic environment, revealed that these additives could control the rate of resorption and hardness degradation of TCP ceramics.

Sintered Hydroxyapatite / Bioactive Glass Composites: Thermal Analysis and Bioactivity

Three-dimensionally ordered materials containing hydroxyapatite (HAp) as well as Bioglass® (BG) have been prepared in form of pellets and have been examined the effects of BG addition -in combination with a heat treatment process- on biological properties of composite HAp/BG. The investigation of the bioactive behavior of sintered and unheated biphasic mixtures and the deviations from pure HAp was performed by FTIR spectroscopy and Differential Thermal Analysis. It was observed the appearance of new phases on sintered composite pellets, while a faster biological HAp layer formation was detected, on the composite unheated pellets’ surface than on pure HAp and sintered composite pellets, currently leading to increased bioactivity.

Effects of Sintering Temperature Over 1,300.DEG.C. on the Physical and Compositional Properties of Porous Hydroxyapatite Foam

Porous hydroxyapatite (HAP) foam permits three-dimensional (3D) structure with fully interconnecting pores as well as excellent tissue response and good osteoconductivity. It is therefore thought to be a good candidate as scaffold material for bone regeneration and as a synthetic bone substitute material. To fabricate better porous HAP foam, improved physical and structural properties as well as higher osteoconductivity are desired. In the present study, the effects of sintering temperature on the physical and compositional properties of porous HAP foam were evaluated by employing high sintering temperature starting at 1,300 degrees C up to 1,550 degrees C. The mechanical strength of porous HAP foam increased with sintering temperature to reach the maximum value at 1,525 degrees C, then decreased slightly when sintering temperature was further increased to 1,550 degrees C. Alpha tricalcium phosphate (alpha-TCP) was formed, and thus the porous HAP foam became biphasic calcium phosphate. Biphasic calcium phosphate consisting of both alpha-TCP and HAP had been reported to show higher osteoconductivity than HAP alone. We therefore recommend 1,500-1,550 degrees C as the sintering temperature for porous HAP foam since this condition provided the most desirable physical properties with biphasic calcium phosphate composition.

Constitutive modeling of the densification and the grain growth of hydroxyapatite ceramics

In the present work, constitutive models for densification and grain growth were employed to investigate the sintering behavior of pure hydroxyapatite ceramics. For densification study, lattice diffusion, grain-boundary diffusion, and interface reaction mechanisms, and for grain-growth study, surface diffusion mechanism, were considered respectively. Hydroxyapatite ceramics were pressurelessly sintered. The sintering results were discussed and compared with the modeling results. Based on the constitutive models employed and the experimental results obtained, grain-boundary diffusion was identified as the dominant mechanism for the densification of the investigated hydroxyapatite. The grain-growth model provided a good prediction to the grain growth of the investigated hydroxyapatite. The activation energies for densification and grain growth of hydroxyapatite ceramics were evaluated as 1150+/-40 and 1020+/-40 KJ mol(-1), respectively.

Effect of Calcium Phosphate Glass on Compressive Strength of Macroporous Hydroxyapatite Scaffold

This study presents the manufacture of macroporous hydroxyapatite scaffolds with a small amount of calcium phosphate glass powder as sintering additives. Hydroxyapatite slurry was prepared by suspending the hydroxyapatite and glass powder in water. Polyurethane sponge was used to produce highly macroporous scaffolds. The rheological characteristic of the slurry was measured to identify the effect of adding calcium phosphate glass powder into hydroxyapatite slurry. Sintered scaffolds could be repeatedly coated to improve mechanical properties. Scaffolds prepared by single and double replication cycle process were characterized by density, porosity and compressive strength measurements by increasing amount of the calcium phosphate glass powder to the hydroxyapatite slurry, viscosity by increased more than same amount of pure hydroxyapatite, and the capillary force was similar to pure hydroxyapatite slurry. The compressive strength of the macroporous hydroxyxapatite scaffold containing the calcium phosphate glass powder showed higher value than that of pure hydroxyapatite at single replication cycle. SEM demonstrated that the microstructure of the scaffold became denser with the introduction of the calcium phosphate glass powder. The pore struts were thicker as replication cycle was increased.

CaO–P2O5–Na2O-based sintering additives for hydroxyapatite (HAp) ceramics

We have assessed the effect of CaO--P2O5--Na2O-based sintering additives on mechanical and biological properties of hydroxyapatite (HAp) ceramics. Five different compositions of sintering additives were selected and prepared by mixing of CaO, P2O5, and Na2CO3 powders. 2.5 wt% of each additive was combined with commercial HAp powder, separately, followed by ball milling, and sintering at 1250 degrees C and 1300 degrees C in a muffle furnace. Green and sintered densities of the compacts were analyzed for the influence of additives on densification of HAp. Phase analyses were carried out using an X-ray diffractometer. Vickers microhardness testing was used to evaluate hardness of sintered compacts of different compositions. A maximum microhardness of 4.6 (+/- 0.28) GPa was attained for a composition with 2.5 wt% addition of CaO:P2O5:Na2O in the ratio of 3:3:4. Results from mechanical property evaluation showed that some of these sintering additives improved failure strength of HAp under compressive loading. Maximum compressive strength was observed for samples with 2.5 wt% addition of CaO. Average failure strength for this set of samples was calculated to be 220 (+/- 50) MPa. Cytotoxicity, and cell attachment studies were carried out using a modified human osteoblast cell line called OPC-1. In vitro results showed that these compositions were non-toxic. Some sintering aids enhanced cell attachment and proliferation, which was revealed from SEM examination of the scaffolds seeded with OPC-1 cells.

Effects of MgO-CaO-P2O5-Na2O-based additives on mechanical and biological properties of hydroxyapatite

In this research, we improved densification, hardness, and compression strength of synthetic hydroxyapatite (HAp) ceramics by introducing small quantities of MgO-CaO-P(2)O(5)-Na(2)O-based sintering additives. Biological properties of HAp were not altered by this procedure. Phase analyses were performed by using a Philips Xpert fully automated diffractometer with Co K-alpha radiation to understand the influence of additives on phase purity in the final products. All compositions were characterized at green and sintered densities to understand the influence of additives on densification. Some of the compositions showed >40% increase in Vickers microhardness compared with pure HAp processed under the same conditions. Improvement in compression strength was also detected in some compositions. In vitro biological testing used a modified human osteoblast cell line to test biocompatibility, cell attachment, and cell proliferation. All these compositions were nontoxic and biocompatible. Our results indicate that MgO-CaO-P(2)O(5)-Na(2)O-based sintering additives can be used to improve both mechanical and biological properties of HAp ceramics.

Impact formation and microstructure characterization of thermal sprayed hydroxyapatite/titania composite coatings

Formation mechanism of hydroxyapatite (HA)/titania (TiO(2)) composite coating deposited by high velocity oxy-fuel (HVOF) thermal spray process was studied, and its structural characterization was conducted and elaborated in this paper. The impact theory was employed to analyze the formation procedure of the HA/titania composite coatings. Results revealed that the crater caused by the impact of entirely unmelted TiO(2) particles on the HA matrix during coating formation was of smaller dimensions than the original size of the reinforcements. It was found that chemical reaction between the mechanically blended HA and TiO(2) powder took place exclusively during the impingement stage, and calcium titanate, CaTiO(3), was one notable by-product. The bonding between the HA matrix and TiO(2) reinforcement might have been achieved predominantly through a chemical bond that resulted from the mutual chemical reactions among the components. Differential scanning calorimetry analyses showed that the chemical reaction between HA and TiO(2) was at approximately 1410 degrees C. The TiO(2) addition was found to exert particular effects on the thermal behavior of HA at elevated temperatures, during both heating and cooling cycles. Transmission electron microscopy observation identified the chemical reaction zone between HA and TiO(2), which revealed an improved splats' interface. The reaction zone demonstrated some influence on the grain size of HA nearby during resolidification of the melted portion. A structural model was proposed to illustrate the location of the different phases in the HA/titania composite coating.

Glass reinforced hydroxyapatite for hard tissue surgery--part 1: Mechanical properties

Commercial hydroxyapatite (HA) was reinforced by adding 2.5 and 5 wt% of a Na2O-CaO-P2O5 glass and then sintered. The resulting composites have chemical compositions that are similar to the inorganic constituent of the mineral part of bone, and are closely related to the trace elements that are present, in this case Na. X-ray diffraction showed no decomposition of HA to secondary phases; however, the glass reinforced-HA composites contained a HA phase and variable amounts of tricalcium phosphate phase, depending on the sintering temperature and the amount of glass added. The HA-composite material exhibited higher flexural strength overall compared to sintered HA. The presence of secondary phases beta- and alpha-tricalcium phosphate in the microstructure of the composites has a major influence on the mechanical properties. Additionally, the presence of porosity also has a bearing on the mechanical properties of the material.

Microstructural dependence of Young's and shear moduli of P2O5 glass reinforced hydroxyapatite for biomedical applications

P2O5 glass reinforced hydroxyapatite composite materials were prepared through a liquid-phase sintering process. Secondary phases, beta- and alpha-tricalcium phosphates (beta-TCP and alpha-TCP), were formed in the microstructure of the composites, due to the reaction between the liquid glassy phase and the hydroxyapatite matrix. The dynamic Young's modulus (E) and shear modulus (G) of these composites were determined using an impulse excitation method. By applying the Duckworth-Knudsen equation, the elastic property results were correlated with the relative proportion of beta-TCP and alpha-TCP phases and with the porosity percentage present in the microstructure. Glass reinforced hydroxyapatite composites showed lower Young's and shear moduli than unmodified hydroxyapatite, mainly due to the presence of beta-TCP phase. The Duckworth-Knudsen model demonstrated an exponential dependence of E and G modulus with porosity and mathematical equations were derived for composite materials with porosity correction factors (b) of 4.04 and 4.11, respectively, indicating that porosity largely decreased both E and G moduli.

Glass-reinforced hydroxyapatite composites: fracture toughness and hardness dependence on microstructural characteristics

Fracture toughness and hardness properties of CaO-P2O5 glass-reinforced hydroxyapatite composites have been assessed using indentation techniques and results calculated according to Laugier and Evans' equations. Both properties showed to be dependent upon several microstructural characteristics, namely residual porosity and the percentage of secondary beta and alpha tricalcium phosphate phases in the structure of the composites. Composites presented a Palmqvist-type indentation crack system, which is the specific crack system addressed by Laugier's approach. Fracture toughness determinations according to Evan's equation, which is a universal one and adapted to both median and Palmqvist crack systems, did not correlate well with Laugier determinations.

CaO-P2O5 glass hydroxyapatite double-layer plasma-sprayed coating: in vitro bioactivity evaluation

Double-layer composite coatings composed of a P2O5-based glass/Ca10(PO4)6(OH)2 (HA) mixture top layer and a simple HA underlayer, on Ti-6Al-4V substrates, were prepared using a plasma-spraying technique. The in vitro bioactivity of these coatings was assessed by immersion testing in simulated body fluid. Both scanning electron microscopy (SEM) analysis and the ionic solution changes followed by atomic absorption spectroscopy and the molybdenum blue method demonstrated that these composite coatings induce a faster surface Ca-P layer formation than the simple HA coatings used as a control. X-ray photoelectron spectroscopy (XPS) analysis demonstrated that the Ca-P layer formed was apatite. The combination of SEM and XPS analyses showed that the apatite layer was a calcium-deficient hydroxyapatite with a Ca/P ranging from 1.3 to 1.4 with CO3(2-) groups contained in the structure.