Crystallization behavior of silica-calcium phosphate biocomposites: XRD and FTIR studies

Effect of Different Phosphates on Pyrolysis Temperature-Dependent Carbon Sequestration and Phosphorus Release Performance in Biochar

Carbon sequestration is the primary function of biochar. Hence, it is necessary to design biochar with high carbon (C) retention and low C loss. In this study, three P compounds, including KH₂PO₄, Ca(H₂PO₄)₂, and NH₄H₂PO₄, were premixed with corn stalk (1:4, w/w), aiming to produce biochars (CSB+K, CSB+Ca, and CSB+N) with high C sequestration and slow release of P at three temperatures (300, 500, and 700 °C). The addition of all P sources obviously increased C retention, with the order of NH₄H₂PO₄ (65.6-83.5%) > Ca(H₂PO₄)₂ (60.4-78.2%) > KH₂PO₄ (50.1-76.1%), compared with the pristine biochar (47.8-73.6%). The addition of Ca(H₂PO₄)₂ and KH₂PO₄ led to an increase in aromaticity and graphitization, as evidenced by H/C, FTIR, Raman and XPS analysis, whereas an opposite result occurred on CSB+N. Furthermore, all three phosphates reduced C loss of biochars with H₂O₂ oxidation, and CSB+Ca showed the best effect. Ca(H₂PO₄)₂ and KH₂PO₄ pretreated biochars had higher resistance to K₂Cr₂O₇ oxidation and thermal treatment. In contrast, the C loss of NH₄H₂PO₄-added biochar at 500 and 700 °C with K₂Cr₂O₇ oxidation was increased by 54% and 36%, respectively. During the pyrolysis process, Ca(H₂PO₄)₂ was transformed into insoluble Ca₂P₂O₇, leading to the lowest P release rate of CSB+Ca. This study indicates that co-pyrolysis of corn stalk and Ca(H₂PO₄)₂ is optimal for increasing C retention, enhancing C stability and improving slow-release performance of P regardless of pyrolysis temperature.

Fiber-Templated 3D Calcium-Phosphate Scaffolds for Biomedical Applications: The Role of the Thermal Treatment Ambient on Physico-Chemical Properties

A successful bone-graft-controlled healing entails the development of novel products with tunable compositional and architectural features and mechanical performances and is, thereby, able to accommodate fast bone in-growth and remodeling. To this effect, graphene nanoplatelets and Luffa-fibers were chosen as mechanical reinforcement phase and sacrificial template, respectively, and incorporated into a hydroxyapatite and brushite matrix derived by marble conversion with the help of a reproducible technology. The bio-products, framed by a one-stage-addition polymer-free fabrication route, were thoroughly physico-chemically investigated (by XRD, FTIR spectroscopy, SEM, and nano-computed tomography analysis, as well as surface energy measurements and mechanical performance assessments) after sintering in air or nitrogen ambient. The experiments exposed that the coupling of a nitrogen ambient with the graphene admixing triggers, in both compact and porous samples, important structural (i.e., decomposition of β-Ca3(PO4)2 into α-Ca3(PO4)2 and α-Ca2P2O7) and morphological modifications. Certain restrictions and benefits were outlined with respect to the spatial porosity and global mechanical features of the derived bone scaffolds. Specifically, in nitrogen ambient, the graphene amount should be set to a maximum 0.25 wt.% in the case of compact products, while for the porous ones, significantly augmented compressive strengths were revealed at all graphene amounts. The sintering ambient or the graphene addition did not interfere with the Luffa ability to generate 3D-channels-arrays at high temperatures. It can be concluded that both Luffa and graphene agents act as adjuvants under nitrogen ambient, and that their incorporation-ratio can be modulated to favorably fit certain foreseeable biomedical applications.

Inorganic process for wet silica-doping of calcium phosphate

Silica is not only a biocompatible trace element but also an essential element for bone formation and metabolism. Therefore, it is often doped into bioceramics such as calcium phosphate and calcium carbonate for enhancing biomaterial ability. Heretofore, organic silica materials are employed as silica sources, but the residual organic matter is a significant drawback in biomaterial applications. Therefore, in this study, we introduce a one-pot inorganic synthesis method for the formation of silica-doped octacalcium phosphate (OCP) using Na₂SiO₃ as the silica source. Silica was intercalated into the OCP unit lattice, replacing its hydrous layer structure, and then a layer-by-layer structure of apatite and silica was formed. Furthermore, by immersing the fabricated silica-doped OCP into suitable solutions, both silica-doped hydroxyapatite and carbonate apatite were fabricated through a one-step inorganic processes.

We introduced a one-pot synthesis method for silica doping of calcium phosphate. Silica easily incorporated into OCP interlayer optimizing Na₂SiO₃ concentrations.

Regulation of Osteogenic Markers at Late Stage of Osteoblast Differentiation in Silicon and Zinc Doped Porous TCP

Calcium phosphates (CaPs) are one of the most widely used synthetic materials for bone grafting applications in the orthopedic industry. Recent trends in synthetic bone graft applications have shifted towards the incorporation of metal trace elements that extend the performance of CaPs to have osteoinductive properties. The objective of this study is to investigate the effects of silicon (Si) and zinc (Zn) dopants in highly porous tricalcium phosphate (TCP) scaffolds on late-stage osteoblast cell differentiation markers. In this study, an oil emulsion method is utilized to fabricate highly porous SiO₂ doped β-TCP (Si-TCP) and ZnO doped β-TCP (Zn-TCP) scaffolds through the incorporation of 0.5 wt.% SiO₂ and 0.25 wt.% ZnO, respectively, to the β-TCP scaffold. Reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) is utilized to analyze the mRNA expression of osteoprotegerin (OPG), receptor activator of nuclear kappa beta ligand (RANKL), bone morphogenetic protein 2 (BMP2), and runt-related transcription factor 2 (Runx2) at the later stage of osteoblast differentiation, day 21 and day 28. Results show that the addition of Si and Zn to the β-TCP structure inhibited the β to α-TCP phase transformation and enhance the density without affecting the dissolution properties. Normal BMP-2 and Runx2 transcriptions are observed in both Si-TCP and Zn-TCP scaffolds at the initial time point, as demonstrated by RT-qPCR. Moreover, the addition of both Si and Zn positively regulate the osteoprotegerin: receptor activator of nuclear factor k-β ligand (OPG:RANKL) ratio at 21-days for Si-TCP and Zn-TCP scaffolds. These results demonstrate the effects of Si and Zn doped porous β-TCP scaffolds on the upregulation of osteoblast marker gene expression including OPG, RANKL, BMP-2, and Runx2, indicating the role of trace elements on the effective regulation of late-stage osteoblast cell differentiation markers.

Enhanced osteogenesis and angiogenesis by PCL/chitosan/Sr-doped calcium phosphate electrospun nanocomposite membrane for guided bone regeneration

Membranes play pivotal role in guided bone regeneration (GBR) technique for reconstruction alveolar bone. GBR membrane that is able to stimulate both osteogenic and angiogenic differentiation of cells may be more effective in clinic practice. Herein, we fabricated the Sr-doped calcium phosphate/polycaprolactone/chitosan (Sr-CaP/PCL/CS) nanohybrid fibrous membrane by incorporating 20 wt% bioactive Sr-CaP nanoparticles into PCL/CS matrix via one-step electrospinning method, in order to endow the membrane with stimulation of osteogenesis and angiogenesis. The physicochemical properties, mechanical properties, Sr²⁺ release behavior, and the membrane stimulate bone mesenchymal stem cell (BMSCs) differentiation were evaluated in comparison with PCL/CS and CaP/PCL/CS membranes. The SEM images revealed that the nanocomposite membrane mimicked the extracellular matrix structure. The release curve presented a 28-day long continuous release of Sr²⁺ and concentration which was certified in an optimal range for positive biological effects at each timepoint. The in vitro cell culture experiments certified that the Sr-CaP/PCL/CS membrane enjoyed excellent biocompatibility and remarkably promoted rat bone mesenchymal stem cell (BMSCs) adhesion and proliferation. In terms of osteogenic differentiation, BMSCs seeded on the Sr-CaP/PCL/CS membrane showed a higher ALP activity level and a better matrix mineralization. What's more, the synergism of the Sr²⁺ and CaP from the Sr-CaP/PCL/CS membrane enhanced BMSCs angiogenic differentiation, herein resulting in the largest VEGF secretion amount. Consequently, the Sr-CaP/PCL/CS nanohybrid electrospun membrane has promising applications in GBR.

Effect of phosphorus precursors on the structure of bioactive calcium phosphate silicate systems

The main aim of this work was to synthesize calcium phosphate silicate bioceramics by a low energy-consuming sol-gel method applying various phosphorous precursors (triethyl phosphate, phosphoric acid, and ammonium hydrogen phosphate). The investigations concentrated on the influence of phosphorous initial compounds on the bond and crystalline structures and the material quality. The application of the alkoxide and inorganic P-precursors results in considerably different textures. The inorganic PO₄-containing precursors lead to sol formations. The sol systems can be characterized by a randomly bonded aggregate structure. Monolith gel systems can only be prepared by using TEP. The alkoxide P-precursor more effectively furthers the connection between the phosphorous and silicon tetrahedra than the inorganic phosphate compounds. Over the P-precursors, the catalyst also affects the structure and properties. In the present work, a special attention was paid to identify the POSi bonds in the FTIR and ³¹P NMR spectra. The bond systems were investigated by FTIR, ³¹P and ²⁹Si MAS NMR spectroscopies, the morphology by SEM, WAXS, and XRD measurements, and the water solubility of the ceramic systems also was tested.

Cytocompatibility and osteogenic activity of a novel calcium phosphate silicate bioceramic: Silicocarnotite

In the present study, the effect of a novel bioceramic, silicon-containing calcium phosphate ceramic (silicocarnotite, Ca5 (PO4 )2 SiO4 , CPS) on attachment, proliferation, and osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (rBMSC) has been investigated in comparison to hydroxyapatite (HA). The CPS showed a similar cell attachment behavior to HA, while the proliferation of rBMSC on CPS was significantly higher than that on HA, which indicated that CPS had a good cytocompatibility. Moreover, the expression of alkaline phosphatase activity and osteogenic-related genes, including Runx-2, osteopontin (OPN), bone sialoprotein (BSP) and osteocalcin (OC), demonstrated that CPS enhanced the osteogenic differentiation of rBMSC and accelerated the differentiation process. The results suggest that CPS ceramic exhibits a good cytocompatibility and osteogenic activity, which might be used as a potential candidate material for bone tissue engineering.

Silicon-stabilized α-tricalcium phosphate and its use in a calcium phosphate cement: characterization and cell response

α-Tricalcium phosphate (α-TCP) is widely used as a reactant in calcium phosphate cements. This work aims at doping α-TCP with silicon with a twofold objective. On the one hand, to study the effect of Si addition on the stability and reactivity of this polymorph. On the other, to develop Si-doped cements and to evaluate the effect of Si on their in vitro cell response. For this purpose a calcium-deficient hydroxyapatite was sintered at 1250°C with different amounts of silicon oxide. The high temperature polymorph α-TCP was stabilized by the presence of silicon, which inhibited reversion of the β→α transformation, whereas in the Si-free sample α-TCP completely reverted to the β-polymorph. However, the β-α transformation temperature was not affected by the presence of Si. Si-α-TCP and its Si-free counterpart were used as reactants for a calcium phosphate cement. While Si-α-TCP showed faster hydrolysis to calcium-deficient hydroxyapatite, upon complete reaction the crystalline phases, morphology and mechanical properties of both cements were similar. An in vitro cell culture study, in which osteoblast-like cells were exposed to the ions released by both materials, showed a delay in cell proliferation in both cases and stimulation of cell differentiation, more marked for the Si-containing cement. These results can be attributed to strong modification of the ionic concentrations in the culture medium by both materials. Ca-depletion from the medium was observed for both cements, whereas continuous Si release was detected for the Si-containing cement.

Tissue regeneration and repair of goat segmental femur defect with bioactive triphasic ceramic-coated hydroxyapatite scaffold

Bone tissue engineering which is a developing and challenging field of science, is expected to enhance the regeneration and repair of bone lost from injury or disease and ultimately to gain its aesthetic contour. The objective of this study was to fabricate a tissue-engineered construct in vitro using a triphasic ceramic-coated hydroxypatite (HASi) in combination with stem cells and to investigate its potential in healing segmental defect in goat model. To accomplish this attempt, mesenchymal stem cells isolated from goat bone marrow were seeded onto HASi scaffolds and induced to differentiate into the osteogenic lineage in vitro. Scanning electron microscopy and light microscopy revealed adhesion and spread-out cells, which eventually formed a cell-sheet like canopy over the scaffold. Cells migrated and distributed themselves within the internal voids of the porous ceramic. Concurrently, the neo-osteogenesis of the tissue-engineered construct was validated in vivo in comparison with bare HASi (without cells) in goat femoral diaphyseal segmental defect (2 cm) at 4 months postimplantation through radiography, computed tomography, histology, histomorphometry, scanning electron microscopy and inductively coupled plasma spectrometry. Good osteointegration and osteoconduction was observed in bare and tissue-engineered HASi. The performance of tissue-engineered HASi was better and faster which was evident by the lamellar bone organization of newly formed bone throughout the defect together with the degradation of the material. On the contrary with bare HASi, immature woven bony bridges still intermingled with scattered small remnants of the material was observed in the mid region of the defect at 4 months. Encouraging results from this preclinical study has proved the capability of the tissue-engineered HASi as a promising candidate for the reconstruction of similar bony defects in humans.

A bioactive triphasic ceramic-coated hydroxyapatite promotes proliferation and osteogenic differentiation of human bone marrow stromal cells

Hydroxyapatite (HA) ceramics are widely used as bone graft substitutes because of their biocompatibility and osteoconductivity. However, to enhance the success of therapeutic application, many efforts are undertaken to improve the bioactivity of HA. We have developed a triphasic, silica-containing ceramic-coated hydroxyapatite (HASi) and evaluated its performance as a scaffold for cell-based tissue engineering applications. Human bone marrow stromal cells (hBMSCs) were seeded on both HASi and HA scaffolds and cultured with and without osteogenic supplements for a period of 4 weeks. Cellular responses were determined in vitro in terms of cell adhesion, viability, proliferation, and osteogenic differentiation, where both materials exhibited excellent cytocompatibility. Nevertheless, an enhanced rate of cell proliferation and higher levels of both alkaline phosphatase expression and activity were observed for cells cultured on HASi with osteogenic supplements. These findings indicate that the bioactivity of HA endowed with a silica-containing coating has definitely influenced the cellular activity, projecting HASi as a suitable candidate material for bone regenerative therapy.

Nucleation and growth of apatite on NaOH-treated PEEK, HDPE and UHMWPE for artificial cornea materials

The skirt of an artificial cornea must integrate the implant to the host sclera, a major failure of present devices. Thus, it is highly desirable to encourage the metabolic activity of the cornea by using more bioactive, flexible skirt materials. Here we describe attempts to increase the bioactivity of polyether ether ketone (PEEK), high-density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE) films. The effectiveness of different strength NaOH pre-treatments to initiate apatite deposition on PEEK, HDPE and UHMWPE is investigated. We find that exposure of PEEK, HDPE and UHMWPE films to NaOH solutions induces the formation of potential nuclei for apatite (calcium phosphate), from which the growth of an apatite coating is stimulated when subsequently immersing the polymer films in 1.5 strength Simulated Body Fluid (SBF). As immersion time in SBF increases, further nucleation and growth produces a thicker and more compact apatite coating that can be expected to be highly bioactive. Interestingly, the apatite growth is found to also be dependent on both the concentration of NaOH solution and the structure of the polymer surface.

A triphasic ceramic-coated porous hydroxyapatite for tissue engineering application

Scaffolds which encourage the incorporation of a cell source for tissue engineering applications are critical determinants for clinical defects. Over the years, a number of biomaterials have emerged for cell support and growth, but only a few have demonstrated clinical efficacy. We therefore investigated an in-house-developed silica-based bioactive ceramic for its ability to support and sustain the growth of bone marrow-derived mesenchymal stem cells (BMSCs) in vitro. For this, MSCs aspirated from goat bone marrow were isolated and culture expanded on a novel triphasic ceramic composite coated hydroxyapatite (HASi) scaffold comprising hydroxyapatite, tricalcium phosphate and calcium silicate. The viability of cells that harbored on and within the material was ensured through fluorescence-activated cell sorting and confocal laser scanning microscope and for their anchorage sites by scanning electron microscopy. Interestingly, over the days in culture, cell-cell interactions gradually morphed into woven cell-sheets that spanned across the surface of the HASi, forming a canopy. To conclude, we have attempted to carry out the preliminary cytocompatibility studies of this novel ceramic to establish its appropriateness for bone tissue engineering application which is an important criterion in orthopaedic transplantation and regenerative surgery.

Bone engineering of the rabbit ulna

PURPOSE

The purpose of the present preliminary study is to show that a novel 3-dimensional porous silica-calcium phosphate nanocomposite (SCPC) can provide a controlled release of rhBMP-2 and regenerate bone in a load-bearing segmental defect.

MATERIALS AND METHODS

A bone replica of the rabbit ulna was created from SCPC powder using rapid prototyping technology. The ceramic bone replica was coated with rhBMP-2 and then implanted into a 10-mm segmental defect created in a rabbit ulna and fixated with a 1-mm titanium adaptation plate. Bone healing was evaluated using computed tomography (CT) scan, histomorphometry, and biomechanical techniques. The release kinetics of rhBMP-2 and the dissolution kinetics were also determined in vitro. Statistical analysis was performed to compare the biomechanical strength of the grafted bone with the contralateral unoperated ulna.

RESULTS

After 4 weeks, CT scans showed that the critical size defect had been replaced by newly formed bone. Torsional testing of the ulna after 12 weeks showed restoration of maximum torque and angle at failure. Histological evaluation showed that the regenerated bone had the morphological characteristics of mature bone. SCPC provided a sustained release profile of an effective dose of rhBMP-2 for 14 days.

CONCLUSIONS

The SCPC-rhBMP-2 hybrid enhanced bone regeneration in a load-bearing segmental defect in a rabbit ulna. The regenerated bone acquired morphology and mechanical strength typical for natural bone. The enhanced bone formation correlates well with the surface bioactivity and effective release profile of rhBMP-2. The present preliminary study shows the proof of principles that porous, resorbable, bioactive SCPC-rhBMP-2 tissue engineering hybrid can serve as a substitute for autologous bone in load-bearing applications.

Dissolution kinetics of a Si-rich nanocomposite and its effect on osteoblast gene expression

Silica-calcium phosphate nanocomposite (SCPC) has recently been proposed as a novel resorbable, bioactive, and mechanically compatible template for bone reconstruction. The effect of the physicochemical properties on the surface reactivity and dissolution kinetics of SCPC immersed in simulated body fluid (SBF) was investigated and compared to that of bioactive glass (BG). Moreover, the stimulatory effect on osteoblast gene expression of SCPC was determined using quantitative real-time polymerase chain reaction (qRT-PCR), and compared to that of hydroxyapatite (HA-200). Mercury porosimetry revealed that surface areas of SCPC particles containing 10 (SCPC10), 30 (SCPC30), and 50 (SCPC50) wt % Si-content were 14-, 18-, and 32-times higher than that of BG. Inductively coupled plasma analysis showed that after 192 h of immersion, Si-rich SCPC50 exhibited controlled bulk-dissolution and released 43.1 ppm Si, which was sixfold higher than that released from BG (7.7 ppm). Moreover, SCPC50 showed a rapid Ca-uptake from SBF and developed a surface apatite layer after only 2 h, whereas a similar layer was detected on BG after 8 days of immersion under the same experimental conditions. qRT-PCR revealed that osteopontin and osteocalcin mRNA expression by osteoblast-like cells attached to Si-rich SCPC50 was significantly higher than that on HA-200 or polystyrene after 2 days in culture. This suggested a role of dissolved Si in stimulating the differentiation and mineralization of osteoblast precursor cells. The favorable physiochemical and bioactivity properties of Si-rich SCPC nanocomposite indicate that SCPC can have wide applications as a synthetic bone graft for cell delivery applications in tissue engineering.

Effects of silica on the bioactivity of calcium phosphate composites in vitro

In the present study, silica-calcium phosphate composites (SiO(2)-CaP composites) were developed by mixing the starting materials (SiO(2) and CaHPO(4)) in different ratios with the addition of 0.1% w/v NaOH solution. The phase composition of the SiO(2)-CaP composites was determined by XRD and FTIR. After thermal treatment at 350 degrees C/1 h and at 1000 degrees C/3.5 h; all SiO(2)-CaP composites composed of beta-quartz, alpha-cristobalite and beta-Ca2P2O7. The presence of calcium phosphate enhanced the transformation of beta-quartz into alpha-cristobalite at 1000 degrees C. SEM observation indicated favorable attachment and spreading of neonatal rat calvaria osteoblasts onto the surface of silica-rich SiO(2)-CaP composites. After attachment, these cells produced significantly higher amount of protein and expressed higher AP activity than cells attached to silica-poor samples. Results of the study suggested that the silica-based composites are more bioactive than calcium phosphate-based composites. Silica promoted the expression of osteoblast phenotype by both solution-mediated effect and direct interaction with the surface of the substrate.