Formation of calcium phosphate layer on ceramics with different reactivities

In vitro measurement of the chemical changes occurring within β-tricalcium phosphate bone graft substitutes

Several mechanisms proposed to explain the osteoinductive potential of calcium phosphates involve surface mineralization ("bioactivity") and mention the occurrence of concentration gradients between the inner and the outer part of the implanted material. Determining the evolution of the local chemical environment occurring inside the pores of an implanted bone graft substitute (BGS) is therefore highly relevant. A quantitative and fast method was developed to measure the chemical changes occurring within the pores of β-Tricalcium Phosphate (β-TCP) granules incubated in a simulated body fluid. A factorial design of experiment was used to test the effect of particle size, specific surface area, microporosity, and purity of the β-TCP granules. Large pH, calcium and phosphate concentration changes were observed inside the BGS and lasted for several days. The kinetics and magnitude of these changes (up to 2 pH units) largely depended on the processing and properties of the granules. Interestingly, processing parameters that increased the kinetics and magnitude of the local chemical changes are parameters considered to favor calcium phosphate osteoinduction, suggesting that the model might be useful to predict the osteoinductive potential of BGSs. STATEMENT OF SIGNIFICANCE: Recent results suggest that in situ mineralization of biomaterials (polymers, ceramics, metals) might be key in their ability to trigger ectopic bone formation. This is the reason why the effect on in situ mineralization of various synthesis parameters of β-tricalcium phosphate granules was studied (size, microporosity, specific surface area, and Ca/P molar ratio). To the best of our knowledge, this is the first article devoted to the chemical changes occurring within the pores of a bone graft substitute. We believe that the manuscript will prove to be highly important in the design and mechanistic understanding of drug-free osteoinductive biomaterials.

Microstructure Evolution and Mechanical Properties Improvement in Liquid-Phase-Sintered Hydroxyapatite by Laser Sintering

CaO-Al₂O₃-SiO₂ (CAS) as a liquid phase was introduced into hydroxyapatite (HAp) to prepare bone scaffolds. The effects of the CAS content (1, 2, 3, 4 and 5 wt%) on microstructure and mechanical properties of HAp ceramics were investigated. The optimal compression strength, fracture toughness and Vickers hardness reached 22.22 MPa, 1.68 MPa·m1/2 and 4.47 GPa when 3 wt% CAS was added, which were increased by 105%, 63% and 11% compared with those of HAp ceramics without CAS, respectively. The improvement of the mechanical properties was attributed to the improved densification, which was caused by the solid particle to rearrange during liquid phase sintering. Moreover, simulated body fluid (SBF) study indicated the HAp ceramics could maintain the mechanical properties and form a bone-like apatite layer when they were immersed in SBF. Cell culture was used to evaluate biocompatibility of the HAp ceramics. The results demonstrated MG-63 cells adhered and spread well.

Preparation of hollow bioactive glass nanofibers by a facile electrospinning method

In this communication, hollow bioactive glass (BG) nanofibers were fabricated via a single-nozzle electrospinning method. The morphology of the prepared hollow BG nanofibers was observed by SEM and TEM, and the results showed that BG nanofibers had a continuous hollow interior. The hollow BG nanofibers were incubated in simulated body fluid (SBF) to investigate their apatitemineralization ability, and the result showed that after incubation for 6 h a flower-like apatite was observed on the surface of hollowBGnanofibers, and the Fourier transform infrared (FTIR) result further confirmed the formation of apatite. The results suggested that hollow BG nanofibers could be used for drug delivery and bone regeneration applications due to their unique hollow structure and bioactivity.

Nano-beta-tricalcium phosphates synthesis and biodegradation: 2. Biodegradation and apatite layer formation on nabo-β-TCP synthesized via microwave treatment

The degradation and/or apatite layer precipitation ability of porous β-tricalcium phosphate(β-TCP) samples treated and untreated with microwave radiation during synthesis is investigated. Microwave heating was used to accelerate the formation of CDHA with the Ca/P ratio 1.5 in a shorter processing time which later forms β-TCP at around 650 ◦C. Soaking in simulated body fluid (SBF) for several periods (4, 8, 12, 24, 36, 48, 60 and 72 h) is performed in a cumulative manner. The deposition of an apatite layer is followed through diffuse reflected FT-IR, SEM and EDS. A microwave-treated sample having a smaller particle size than its parent induces the formation of a homogeneous carbonated apatite layer on its surface.On the other hand, the parent β-TCP sample exhibited less ability to induce Ca–P formation after being soaked in SBF. The formation of an apatite layer is attributed to the increase in surface area consequent to reduced particle and grain sizes besides the presence of a minor amount of hydroxyapatite phase in the microwave-treated β-TCP sample. The results prove that it is possible to control the biodegradation and apatite layer formation on sintered β-TCP porous disks through controlling the particle size.

Article Commentary: A Commentary on “Evaluation of the in vitro Bioactivity of Bioceramics”

The interesting article reported in Bone and Tissue Regeneration Insights by Wu and Xiao entitled “Evaluation of the in vitro bioactivity of bioceramics” contrasts two methods (Simulated body fluid (SBF) and cell culture experiments) which have been commonly used to evaluate the in vitro bioactivity of bioceramics. Limitations in estimating the bioactivity of bioceramics using both methods have been reviewed. Therefore, Wu and Xiao suggest the combination of these two methods to evaluate the bioactivity of bioceramics can improve the screening efficiency for the selection of bioactive ceramics for bone regeneration.

Hydrothermal synthesis of porous triphasic hydroxyapatite/(alpha and beta) tricalcium phosphate

A novel, porous triphasic calcium phosphate composed of nonresorbable hydroxyapatite (HAp) and resorbable tricalcium phosphate (alpha- and beta-TCP) has been synthesized hydrothermally at a relatively low temperature. The calcium phosphate precursor for hydrothermal treatment was prepared by gel method in the presence of ascorbic acid. XRD, FT-IR, Raman analyses confirmed the presence of HAp/TCP. The surface area and average pore size of the samples were found to be 28 m2/g and 20 nm, respectively. The samples were found to be bioactive in simulated body fluid (SBF).

Effect of Ca/P Ratio on the Dissolution of Calcium Phosphate Powders

Calcium phosphate ceramics (CPC) have been attractively used in different areas of biomedical applications, such as substances of artificial hard tissue replacement implants, drug delivery system due to their biocompatibility and bioactivity. In this work, three calcium phosphate powders between Ca/P molar ratio 1.50-1.67 were synthesized by aqueous precipitation method, using the mixture of Ca(NO3)2·4H2O and H3PO4 solutions to ammonia solution. During the precipitation reaction, Ca/P molar ratio was adjusted by controlling pH of the solution between 8.0 and 10.5 by the addition of ammonium hydroxide solution. All powders were treated at 800 oC for 2 hrs. The calcined powders were immersed in pH 7.4 distilled water at 37°C for 3 and 7 days. Decomposition and related dissolution with the various Ca/P ratios were investigated by XRD, FTIR, and TEM observation.

Dissolution of Calcium Phosphate Powders with Different Compositions in Simulated Body Fluid

Dissolution of calcium phosphate powders with different compositions in simulated body fluid was investigated. Hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) powders were synthesized by a precipitation method and by controlling pH of the solution. Biphasic calcium phosphate (BCP) powders were prepared by mixing the HA and β-TCP powders with ratio of 80:20 and 60:40 in weight % in a planetary mill for 4 h. For dissolution test, 0.5g of each β-TCP, HA and BCP powders were immersed in 50 ml of SBF at 37°C for 3-28 days. HA powders with stoichiometric composition showed no significant morphological change regardless of the immersion time. β-TCP powders were not dissolved in SBF in spite of its high solubility. The surface of BCP powders became rough due to dissolution with increasing immersion time. This surface dissolution gradually extended into the core region of the particles resulting in disintegration of the dense particle into fine crystallites. It may be caused that the milling process during preparation of BCP powders may induce stress concentration or formation of dislocation on the surface of particles.

Bioactivity in in situ hydroxyapatite-polycaprolactone composites

In our previous work, hydroxyapatite (HAP) was synthesized under two conditions: one in the presence of polyacrylic acid (in situ HAP) and the other in the absence of polyacrylic acid (ex situ HAP). Composites of both HAPs with polycaprolactone (PCL) were investigated for their applicability as scaffolds for bone tissue engineering. In the current work, bioactivity of these composites has been investigated by soaking them in simulated body fluid for different intervals of time. Nucleation and growth mechanism of apatite on these composites has also been investigated. Fourier transform infrared spectroscopy study suggests that although apatite growth starts with an intermediate phase, it completely transforms to HAP after 4 days of soaking. Nanoindentation results suggest that the apatite growing on in situ HAP/PCL composites has much higher hardness and elastic modulus as compared to the apatite growing on ex situ HAP/PCL composites. The apatite grown on the ex situ composites has a net-like interconnected structure. The observed differences in mechanical properties and morphology of apatite have been described on the basis of nucleation mechanisms. The nucleation of apatite on the in situ HAP/PCL composites proceeds through the formation of a complex between Ca2+ and COO- groups; on the other hand, nucleation occurs because of dissolution reaction of apatite in ex situ HAP/PCL composites.