Low-Temperature Synthesis of Hollow β-Tricalcium Phosphate Particles for Bone Tissue Engineering Applications

β-Tricalcium phosphate (β-TCP) has been extensively used in bone tissue engineering in the form of scaffolds, granules, or as reinforcing phase in organic matrices. Solid-state reaction route at high temperatures (>1000 °C) is the most widely used method for the preparation of β-TCP. The high-temperature synthesis, however, results in the formation of hard agglomerates and fused particles which necessitates postprocessing steps such as milling and sieving operations. This, inadvertently, could lead to introducing unwanted trace elements, promoting particle shape irregularity as well as compromising the biodegradability and bioactivity of β-TCP because of the solid microstructure of particles. In this study, we introduce a one-pot wet-chemical method at low temperatures (between 160 and 170 °C) to synthesize hollow β-TCP (hβ-TCP) submicron particles of an average size of 300 nm with a uniform rhombohedral shape. We assessed the cytocompatibility of the hβ-TCP using primary human osteoblasts (HOB), adipose-derived stem cells (ADSC), and antigen-presenting cells (APCs). We demonstrate the bioactivity of the hβ-TCP when cultured with HOB, ADSC, and APCs at a range of particle concentrations (up to 1000 μg/mL) for up to 7 days. hβ-TCP significantly enhances osteogenic differentiation of ADSC without the addition of osteogenic supplements. These findings offer a new type of β-TCP particles prepared at low temperatures, which present various opportunities for developing β-TCP based biomaterials.

Fabrication and characterization of PHEMA–gelatin scaffold enriched with graphene oxide for bone tissue engineering

Background

Growing investigations demonstrate that graphene oxide (GO) has an undeniable impact on repairing damaged bone tissue. Moreover, it has been stated in the literatures that poly(2-hydroxyethyl methacrylate) (PHEMA) and gelatin could provide a biocompatible structure.

Methods

In this research, we fabricated a scaffold using freeze-drying method comprised of PHEMA and gelatin, combined with GO. The validation of the successful fabrication of the scaffolds was performed utilizing Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction assay (XRD). The microstructure of the scaffolds was observed using scanning electron microscopy (SEM). The structural properties of the scaffolds including mechanical strength, hydrophilicity, electrical conductivity, and degradation rate were also evaluated. Human bone marrow‐derived mesenchymal stem cells (hBM-MSCs) were used to evaluate the cytotoxicity of the prepared scaffolds. The osteogenic potential of the GO-containing scaffolds was studied by measuring the alkaline phosphatase (ALP) activity after 7, 14, and 21 days cell culturing.

Results

SEM assay showed a porous interconnected scaffold with approximate pore size of 50–300 μm, appropriate for bone regeneration. The increase in GO concentration from 0.25 to 0.75% w/v exhibited a significant improvement in scaffolds compressive modulus from 9.03 ± 0.36 to 42.82 ± 1.63 MPa. Conventional four-probe analysis confirmed the electrical conductivity of the scaffolds in the semiconductor range. The degradation rate of the samples appeared to be in compliance with bone healing process. The scaffolds exhibited no cytotoxicity using MTT assay against hBM-MSCs. ALP analysis indicated that the PHEMA–Gel–GO scaffolds could efficiently cause the differentiation of hBM-MSCs into osteoblasts after 21 days, even without the addition of the osteogenic differentiation medium.

Conclusion

Based on the results of this research, it can be stated that the PHEMA–Gel–GO composition is a promising platform for bone tissue engineering.

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

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

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.

Probing the combined additions of Ca2+ and PO43− in the stabilized ZrO2 polymorph: structural, morphological and mechanical analysis

Selective uptake of Ca²⁺ at c-ZrO₂ lattice: additions beyond critical limit lead to β-Ca₃(PO₄)₂ crystallization in the presence of PO₄³⁻.