Octacalcium Phosphate Research Articles

Customized in-vivo tissue engineering for bone grafting

Relevance. Jaw bone volume restoration during dental implantation and reconstructive oral surgery is a relevant problem in modern dentistry. In recent years, the needs of daily dental practice determined the search for new osteoplastic materials with desired properties, including cellular technologies, to stimulate bone regeneration and accelerate bone repair processes.Materials and methods. The study used third molar area gingival specimens to create tissue-engineered constructs for bone matrix colonization, subject to in vitro expansion. Octacalcium-phosphate-based materials (OCP), used as the carrier matrix, were characterized by a larger particle surface area for a more developed microrelief, a bioresorption rate, and a hydrophilic surface. The finished tissue-engineered construct, consisting of multipotent mesenchymal stromal cells colonized on the matrix, was implanted into an artificially created tibial defect in 8 Chinchilla male rabbits. Animal experiments were conducted according to ethical standards. Rabbits were sacrificed on days 8 and 12 for histological testing.Results. In the early follow-up period (8 weeks), there were areas of mature bone with incorporated osteoblasts. Besides, there were areas of primary bone with adhesion lines. Later (12 weeks), such granules fully integrated into the diaphysis cortical part. The results showed the preservation of the low-mineralized bone girdle, osteoid - a bone substance formation precursor, between the octacalcium phosphate granule and the bone.Conclusion. The results of the experimental study allow us to conclude that the customized tissue-engineered construct developed by us contributes to bone grafting.

Properties of Calcium Phosphate/Hydrogel Bone Grafting Composite on the Model of Diaphyseal Rat Femur’s Defect: Experimental Study

Background. The problem of bone defects replacement is relevant nowadays, that is why many scientists create new synthetic bone substitutes, but the ideal material has not been found so far.
 The aims of the study: 1) to determine the suitability of the monocortical defect model in the rat femur diaphysis with additional prophylactic reinforcement with a bone plate for assessing the biological properties of implanted materials using the commercially available ChronOS material as an example; 2) to assess of the osteoconductive properties of composite materials based on poly(ethylene glycol)diacrylate and octacalcium phosphate with architecture Kelvin and gyroid types on the developed model.
 Methods. A prospective study, level of evidence II. A monocortical defect of the rat femoral diaphysis (length 7 mm) was produced under anaesthesia in aseptic conditions and fixed with a polyetheretherketone plate and six titanium screws. In the control group, the defect was left empty. In other groups, blocks of one of three materials were implanted сhronOS and composites of poly(ethylene glycol)diacrylate and octacalcium phosphate with 3D-printed Kelvin and gyroid architectures. After 3 and 6 weeks, the rats were sacrificed, and histological examination of the defect zone was performed. The amount of newly formed bone tissue was histometricly assessed, followed by statistical processing of the results.
 Results. All rats have reached the planned endpoint, and there were no infectious complications or loss of fixation. Histological examination of the defect zone revealed minimal bone growth in the Control group, rather slow bone formation in the Gyroid group, and statistically significantly more pronounced bone formation in the pores of the materials in the Kelvin and Chronos groups.
 Conclusions. Bone defect in this model was not spontaneously filled with bone tissue and allowed us to study the biological properties of bone substitutes (the ability to biodegrade and osteoconductive properties). The osteoconductive properties of a composite material based on poly(ethylene glycol)diacrylate and octacalcium phosphate with a Kelvin architecture are higher than with a gyroid architecture and are comparable to that of the сhronOS.

Physicochemical and biological properties of composite bone cement based on octacalcium phosphate and tricalcium silicate

Biocompatible tricalcium silicate (C3S) bone cement is widely used as dental and bone repair material; however, its long setting time, poor injectability and low initial mechanical properties limit clinical applications. In order to improve C3S silicate bone cement and its derivatives to play a more important role in tooth restoration, bone defect repair, implant coating and tissue engineering scaffolds, a novel C3S and octacalcium phosphate (OCP) composite bone cement (OCP/C3S) was prepared and evaluated for setting time, injectability, anti-flocculation, pH, microstructure, bioactivity and cytotoxicity. The setting time of the OCP/C3S composite bone cement was controlled within the clinically operable time range (8.3–13.7 min); the cement exhibited good compressive strength, injectability (93.54%), and anti-collapse performance. The 20% OCP/C3S composite bone cement had a compressive strength of 28.94 MPa, 93% stronger than pure C3S (14.98 MPa). An in vitro immersion test showed that the composite bone cement had excellent hydroxyapatite forming ability, proper degradation rate, and a low pH value. Cellular experiments confirmed the low cytotoxicity of the composite bone cement and its great capacity for cell proliferation. These results indicate that 20% OCP/C3S composite bone cement is a promising biomaterial.

Exploring the Formation Kinetics of Octacalcium Phosphate from Alpha-Tricalcium Phosphate: Synthesis Scale-Up, Determination of Transient Phases, Their Morphology and Biocompatibility.

Even with decades of research studies behind octacalcium phosphate (OCP), determination of OCP phase formation has proved to be a cumbersome challenge. Even though obtaining a large quantity of OCP is important for potential clinical uses, it still remains a hindrance to obtain high yields of pure OCP. Taking that into consideration, the purpose of this study was to scale-up OCP synthesis for the first time and to use a multi-technique approach to follow the phase transformation pathway at multiple time points. In the present study, OCP has been synthesized from α-tricalcium phosphate (α-TCP), and subsequently scaled-up tenfold and hundredfold (100 mg → 10 g). The hydrolysis mechanism has been followed and described by using XRD and FTIR spectroscopy, as well as Raman and SEM. Gradual transformation into the OCP phase transpired through dicalcium phosphate dihydrate (brushite, DCPD, up to ~36%) as an intermediary phase. Furthermore, the obtained transitional phases and final OCP phases (across all scale-up levels) were tested with human bone marrow-derived mesenchymal stem cells (hBMSCs), in order to see how different phase mixtures affect the cell viability, and also to corroborate the safety of the scaled-up product. Twelve out of seventeen specimens showed satisfactory percentages of cell viability and confirmed the prospective use of scaled-up OCP in further in vitro studies. The present study, therefore, provides the first scale-up process of OCP synthesis, an in depth understanding of the formation pathway, and investigation of the parameters able to contribute in the OCP phase formation.

The material design of octacalcium phosphate bone substitute: increased dissolution and osteogenecity.

Octacalcium phosphate (OCP) has been advocated as a precursor of bone apatite crystals. Recent studies have shown that synthetic OCP exhibits highly osteoconductive properties as a bone substitute material that stems from its ability to activate bone tissue-related cells, such as osteoblasts, osteocytes, and osteoclasts. Accumulated experimental evidence supports the proposition that the OCP-apatite phase conversion under physiological conditions increases the stimulatory capacity of OCP. The conversion of OCP progresses by hydrolysis toward Ca-deficient hydroxyapatite with Ca2+ ion incorporation and inorganic phosphate ion release with concomitant increases in the solid Ca/P molar ratio, specific surface area, and serum protein adsorption affinity. The ionic dissolution rate during the hydrolysis reaction was controlled by introducing a high-density edge dislocation within the OCP lattice by preparing it through co-precipitation with gelatin. The enhanced dissolution intensifies the material biodegradation rate and degree of osteogenecity of OCP. Controlling the biodegradation rate relative to the dissolution acceleration may be vital for controlling the osteogenecity of OCP materials. This study investigates the effects of the ionic dissolution of OCP, focusing on the structural defects in OCP, as the enhanced metastability of the OCP phase modulates biodegradability followed by new bone formation. STATEMENT OF SIGNIFICANCE: Octacalcium phosphate (OCP) is recognized as a highly osteoconductive material that is biodegradable by osteoclastic resorption, followed by new bone formation by osteoblasts. However, if the degradation rate of OCP is increased by maintaining the original osteoconductivity or acquiring a bioactivity better than its current properties, then early replacement with new bone can be expected. Although cell introduction or growth factor addition by scaffold materials is the standard method for tissue engineering, material activity can be augmented by introducing dislocations into the lattice of the OCP. This review article summarizes the effects of introducing structural defects on activating OCP, which was obtained by co-precipitation with gelatin, as a bone substitute material and the mechanism of improved bone replacement performance.

Can Differently Stabilized Silver Nanoparticles Modify Calcium Phosphate Precipitation?

Calcium phosphates (CaPs) composites with silver nanoparticles (AgNPs) attract attention as a possible alternative to conventional approaches to combating orthopedic implant-associated infections. Although precipitation of calcium phosphates at room temperatures was pointed out as an advantageous method for the preparation of various CaP-based biomaterials, to the best of our knowledge, no such study exists for the preparation of CaPs/AgNP composites. Motivated by this lack of data in this study we investigated the influence of AgNPs stabilized with citrate (cit-AgNPs), poly(vinylpyrrolidone) (PVP-AgNPs), and sodium bis(2-ethylhexyl) sulfosuccinate (AOT-AgNPs) in the concentration range 5-25 mg dm-3 on the precipitation of CaPs. The first solid phase to precipitate in the investigated precipitation system was amorphous calcium phosphate (ACP). The effect of AgNPs on ACP stability was significant only in the presence of the highest concentration of AOT-AgNPs. However, in all precipitation systems containing AgNPs, the morphology of ACP was affected, as gel-like precipitates formed in addition to the typical chain-like aggregates of spherical particles. The exact effect depended on the type of AgNPs. After 60 min of reaction time, a mixture of calcium-deficient hydroxyapatite (CaDHA) and a smaller amount of octacalcium phosphate (OCP) formed. PXRD and EPR data point out that the amount of formed OCP decreases with increasing AgNPs concentration. The obtained results showed that AgNPs can modify the precipitation of CaPs and that CaPs properties can be fine-tuned by the choice of stabilizing agent. Furthermore, it was shown that precipitation can be used as a simple and fast method for CaP/AgNPs composites preparation which is of special interest for biomaterials preparation.

Effect of Octacalcium Phosphate Crystals on the Osteogenic Differentiation of Tendon Stem/Progenitor Cells In Vitro.

Synthetic octacalcium phosphate (OCP) activates bone tissue-related cells, such as osteoblasts, osteoclasts, and vascular endothelial cells. However, the effect of OCP on tendon-related cell activation remains unknown. This study examined the response of rat tendon stem/progenitor cells (TSPCs) to OCP and related calcium phosphate crystals in vitro. TSPCs were cultured with OCP and Ca-deficient hydroxyapatite (CDHA) obtained from the original OCP hydrolysis to assess the activity of alkaline phosphatase (ALP) and the expression of osteogenesis-related genes. Compared with CDHA, the effect of OCP on promoting the osteogenic differentiation of TSPCs was apparent: the ALP activity and mRNA expression of RUNX2, Col1a1, OCN, and OPN were higher in OCP than in CDHA. To estimate the changes in the chemical environment caused by OCP and CDHA, we measured the calcium ion (Ca2+) and inorganic phosphate (Pi) ion concentrations and pH values of the TSPCs medium. The results suggest that the difference in the osteogenic differentiation of the TSPCs is related to the ionic environment induced by OCP and CDHA, which could be related to the progress of OCP hydrolysis into CDHA. These results support the previous in vivo observation that OCP has the healing function of rabbit rotator cuff tendon in vivo.

Interlayer expansion of octacalcium phosphate via forced oxidation of the intercalated molecules within its interlayers.

Octacalcium phosphate (OCP), a precursor to apatite, has a layered structure that allows various molecules to be intercalated within its interlayers. Previous research on the phase conversion process of OCP to apatite indicated that the layered structures typically collapse due to the shrinking of the OCP layers. In contrast, this study presents a novel phenomenon involving OCP layer expansion during phase conversion. This expansion is based on a forced oxidation process of the intercalated molecules within the hydrous layers of OCP. By introducing NaClO to an OCP interlayer containing dithiodiglycolic acid (DSG), the OCP layers are expanded. This process involves DSG decomposition through its reaction with NaClO. Specifically, the process occurs when a DSG-substituted OCP (containing disulfide bonds (-S-S-)) is immersed in a NaClO solution. This is the first study to report the expansion phenomenon during the phase conversion process from OCP to apatite, providing a new perspective to the conventional understanding that these layers only shrink.

Impact of Octacalcium Phosphate/Gelatin (OCP/Gel) Composite on Bone Repair in Refractory Bone Defects.

In clinical settings, bone grafting is frequently used to treat bone defects. Therefore, the development of bone graft substitutes with superior bone formation ability is expected, instead of autogenous bone grafting. Octacalcium phosphate (OCP) has been developed as a bone graft substitute, and preclinical studies using OCP have reported superior bone formation ability compared with β-tricalcium phosphate. Furthermore, OCP has been used in composite forms with natural polymers such as collagen and gelatin to improve the usability of OCP, and OCP/collagen composite forms have been clinically applied in the dental field because of their excellent usability and osteogenic potential. This review describes the development and preclinical results of OCP and OCP/gelatin (OCP/Gel) composites and prospects for future applications in orthopedics. The development of bone graft substitutes that achieve a high degree of biodegradability and strength will be needed for the clinical application of OCP composites in orthopedics in the future.

Building Osteogenic Microenvironments with a Double-Network Composite Hydrogel for Bone Repair.

The critical factor determining the in vivo effect of bone repair materials is the microenvironment, which greatly depends on their abilities to promote vascularization and bone formation. However, implant materials are far from ideal candidates for guiding bone regeneration due to their deficient angiogenic and osteogenic microenvironments. Herein, a double-network composite hydrogel combining vascular endothelial growth factor (VEGF)-mimetic peptide with hydroxyapatite (HA) precursor was developed to build an osteogenic microenvironment for bone repair. The hydrogel was prepared by mixing acrylated β-cyclodextrins and octacalcium phosphate (OCP), an HA precursor, with gelatin solution, followed by ultraviolet photo-crosslinking. To improve the angiogenic potential of the hydrogel, QK, a VEGF-mimicking peptide, was loaded in acrylated β-cyclodextrins. The QK-loaded hydrogel promoted tube formation of human umbilical vein endothelial cells and upregulated the expression of angiogenesis-related genes, such as Flt1, Kdr, and VEGF, in bone marrow mesenchymal stem cells. Moreover, QK could recruit bone marrow mesenchymal stem cells. Furthermore, OCP in the composite hydrogel could be transformed into HA and release calcium ions facilitating bone regeneration. The double-network composite hydrogel integrated QK and OCP showed obvious osteoinductive activity. The results of animal experiments showed that the composite hydrogel enhanced bone regeneration in skull defects of rats, due to perfect synergistic effects of QK and OCP on vascularized bone regeneration. In summary, improving the angiogenic and osteogenic microenvironments by our double-network composite hydrogel shows promising prospects for bone repair.

Substitution of dicarboxylate ions as succinate ion in lsquo;Apatite Worldrsquo;

In crystal chemistry, the apatite crystals is prepared by the dissolution precipitation reaction reaction, which led to HAP (hydroxyapatite; Ca10(PO4)6.(OH)2) nanocrystals formation under hydrothermal condition from OCP (octacalcium phosphate; Ca8(HPO4)2.(PO4)4.5H2O) at 180℃ for 3 hours with adjusted 5.5 pH range. The prepared solution is incorporating to dicarboxylate as succinate (OOC.(CH2)2.COO)2- ions. During incorporating of succinate ions in OCP crystals, the hydrogen phosphate (HPO42-) ions in the hydrated layers of OCP are being substituted by succinate ions. These organically modified OCP which generated to HAP have unique hexagonal nanostructure with micrometer thickness are characterized by reporting of scanning electron microscopy (SEM), X-ray diffraction analysis, Fourier transform infrared (FTIR) spectra and Scherrer’s equation as well.

Formation of octacalcium phosphate with incorporated dicarboxylate ions containing disulfide bonds.

Octacalcium phosphate (OCP) is a layered compound capable of incorporating carboxylate ions within its interlayer structure. In this study, we successfully synthesised OCP with incorporated 3,3'-dithiodipropionate ions. Our finding is beneficial for the development of novel OCP-based materials with dynamic properties derived from disulfide bonds.

Термодинамический расчет формирования керамических покрытий на основе дикальцийфосфат дигидрата и октакальцийфосфата

This research is focused on preliminary calculations of the thermodynamic model and total Gibbs energy for ceramic coatings based on dicalcium phosphate dihydrate (DCPD, CaHPO4×2H2O) and octacalcium phosphate (OCP, Ca8H2(PO4)6×5H2O) obtained by chemical modification of ceramics based on α-tricalcium phosphate (α-TCP, Ca3(PO4)2). The formation of thermodynamically favorable calcium phosphate (CP) phases during the formation of coatings is shown. The calculations were carried out using the Medusa software. Solutions of sodium acetate (CH3COONa) and phosphoric acid (H3PO4) were used as initial components of the system. According to the calculations, at a pH value of 5.1 and a concentration of Ca2+ ions = 10-2 M and PO4- = 0.45 M, a coating based on DCPD is formed, and at a pH value of 9 and a concentration of Ca2+ ions = 10-3 M - phase OCP .

Comparison of bovine serum albumin and chitosan effects on calcium phosphate formation in the presence of silver nanoparticles.

The precipitation of calcium phosphates (CaPs) in the presence of more than one type of additive is of interest both from a fundamental point of view and as a possible biomimetic route for the preparation of multicomponent composites in which the activity of the components is preserved. In this study, the effect of bovine serum albumin (BSA) and chitosan (Chi) on the precipitation of CaPs in the presence of silver nanoparticles (AgNPs) stabilized with sodium bis(2-ethylhexyl)sulfosuccinate (AOT-AgNPs), poly(vinylpyrrolidone) (PVP-AgNPs), and citrate (cit-AgNPs) was investigated. In the control system, the precipitation of CaPs occurred in two steps. Amorphous calcium phosphate (ACP) was the first precipitated solid, which transformed into a mixture of calcium-deficient hydroxyapatite (CaDHA) and a smaller amount of octacalcium phosphate (OCP) after 60 min of ageing. Both biomacromolecules inhibited ACP transformation, with Chi being a stronger inhibitor due to its flexible molecular structure. As the concentration of the biomacromolecules increased, the amount of OCP decreased both in the absence and presence of AgNPs. In the presence of cit-AgNPs and two highest BSA concentrations, a change in the composition of the crystalline phase was observed. Calcium hydrogen phosphate dihydrate was formed in the mixture with CaDHA. An effect on the morphology of both the amorphous and crystalline phases was observed. The effect depended on the specific combination of biomacromolecules and differently stabilized AgNP. The results obtained suggest a simple method for fine-tuning the properties of precipitates using different classes of additives. This could be of interest for the biomimetic preparation of multifunctional composites for bone tissue engineering.

Antibacterial Electrodeposited Copper-Doped Calcium Phosphate Coatings for Dental Implants.

Dental implants provide a good solution for the replacement of tooth roots. However, the full restoration of tooth functions relies on the bone-healing period before positioning the abutment and the crown on the implant, with the associated risk of post-operative infection. This study aimed at developing a homogeneous and adherent thin calcium phosphate antibacterial coating on titanium dental implants by electrodeposition to favor both implant osseointegration and to limit peri-implantitis. By combining global (XRD, FTIR-ATR, elemental titration) and local (SEM, Raman spectroscopy on the coating surface and thickness) characterization techniques, we determined the effect of electrodeposition time on the characteristics and phases content of the coating and the associated mechanism of its formation. The 1-min-electrodeposited CaP coating (thickness: 2 ± 1 μm) was mainly composed of nano-needles of octacalcium phosphate. We demonstrated its mechanical stability after screwing and unscrewing the dental implant in an artificial jawbone. Then, we showed that we can reach a high copper incorporation rate (up to a 27% Cu/(Cu+Ca) molar ratio) in this CaP coating by using an ionic exchange post-treatment with copper nitrate solution at different concentrations. The biological properties (antibiofilm activity and cytotoxicity) were tested in vitro using a model of mixed bacteria biofilm mimicking peri-implantitis and the EN 10993-5 standard (direct contact), respectively. An efficient copper-doping dose was determined, providing an antibiofilm property to the coating without cytotoxic side effects. By combining the electrodeposition and copper ionic exchange processes, we can develop an antibiofilm calcium phosphate coating on dental implants with a tunable thickness and phases content.

Osteogenic capacity of octacalcium phosphate involving macrophage polarization.

Previous research has found that octacalcium phosphate (OCP) increases macrophage accumulation and alters the initial inflammatory response. However, the role of the immune response induced by OCP in osteogenesis remains unknown. This study investigated the behavior of macrophages and bone regeneration capacity during the early inflammatory stage of OCP-mediated osteogenesis. To assess the change in macrophage polarization and osteogenic capacity, we used a standardized rat defect model filled with OCP or calcium-deficient hydroxyapatite (CDHA)-a material obtained through the hydrolysis of the original OCP. OCP or CDHA granules were incubated with RAW264 cells for 5 days to investigate the effect of physicochemical characteristics on macrophage cytokine/chemokine expression in vitro. Our in vivo results show that due to the OCP implantation, macrophages in the rat tibial defect area tend to polarize to the M2 phenotype (anti-inflammatory) and inhibit the formation of the M1 phenotype (pro-inflammatory). In comparison to CDHA, OCP exhibited superior bone regeneration potential due to its rapid promotion of cortical bone healing and stimulation of macrophage-related growth factors. Furthermore, our in vitro results have shown that OCP regulates the expression of macrophage chemokines over time. Compared to incubation with CDHA, incubation with OCP caused changes in the ionic microenvironment. These findings suggest that the OCP-mediated macrophage polarization and secretion profile not only regulate immune function but also positively affect osteogenesis.

Synthesis of Octacalcium Phosphate Containing Glutarate Ions with a High Incorporation Fraction.

Octacalcium phosphate (OCP) has received considerable attention in the field of ceramic biomaterials as an advanced functional material. It exhibits a layered structure composed of apatitic and hydrated layers and can incorporate various dicarboxylate ions into the hydrated layer. Saturated dicarboxylic acids (HOOC(CH2)nCOOH) with an odd number of methylene groups (-CH2-) exhibit lower incorporation fractions than those with an even number of methylene groups, possibly owing to a compositional dependence on the synthetic method. In this study, calcium carbonate, phosphoric acid, and various amounts of glutaric acid were used to produce glutarate-ion-incorporated OCP by a wet chemical method, which is different from the conventional synthetic strategy. While utilising 1-20 mmol of glutaric acid during synthesis did not produce the desired product, using 25 mmol of glutaric acid resulted in the formation of single-phase glutarate-ion-incorporated OCP with a Ca/P molar ratio of 1.57 and a 90% incorporation fraction of glutarate ions. This glutarate-ion-incorporation fraction is significantly higher than that reported in the previous studies (35%). Thus, the synthetic procedure proposed herein was able to produce single-phase OCP containing glutarate ions with a high incorporation fraction. Our findings can contribute to development of novel functional ceramic biomaterials in the future.

Physicochemical and cytological properties of silicon-doping octacalcium phosphate

• Si was successfully doped in the metastable OCP crystals by a precipitation method. • Si doping led to the contraction of OCP structure with smaller lattice parameters. • Si doping preferred forming a cluster-like hierarchical structure of OCP crystals. • Si-OCP notably promoted the proliferation and osteogenic differentiation of BMSCs. • Si-OCPs enhanced the responses of BMSCs by ionic release and mineral deposits. Silicon was successfully doped into an intermediate precursor of biological apatite, octacalcium phosphate (OCP), accompanied with a lattice contraction. Si-doped OCP significantly enhanced the proliferation and osteogenic capacities of bone marrow mesenchymal stem cells by the synergistical effect of bioactive ions release and mineral deposits, providing a potential bone substitute.

Effects of straw mulching and phosphorus application on forms and availability of soil phosphorus in hilly dryland of Sichuan, China

We conducted a two-factor split-plot experiment to examine the alteration of soil inorganic phosphorus forms and phosphorus availability under straw mulching and phosphorus fertilizer rates. The main factor was straw mulching and non-mulching, while the sub-factor was phosphorus supply rates, including 0, 75, and 120 kg·hm-2. We analyzed the characteristics of phosphorus adsorption-desorption, the content of inorganic phosphorus components and their relationship with available phosphorus in hilly upland purple soil in Sichuan. Results showed that compared with the non-mulching, the maximum phosphorus adsorption capacity of straw mulching was notably decreased by 7.7% and 7.4% in the two experimental years from 2018 to 2020. The degree of phosphorus saturation and readily desorbable phosphorus of straw mulching were remarkably increased by 35.4% and 21.6% in 2019 and 18.6% and 35.2% in 2020, respectively. The maximum buffer capacity of phosphorus was not different between straw mulching and non-mulching. The maximum phosphorus adsorption capacity and maximum buffer capacity of phosphorus were significantly lower, and the degree of phosphorus saturation was notably higher in the phosphorus application treatment than that under no phosphorus treatment. The readily desorbable phosphorus increased with the increases of phosphorus rates. The contents of dicalcium phosphate (Ca2-P), octa-calcium phosphate (Ca8-P) and iron phosphorus (Fe-P) in straw mulching treatment were notably higher than those in non-mulching treatment, whereas the content of aluminum phosphorus (Al-P) significantly lower under the straw mulching. Meanwhile, the contents of occluded phosphate (O-P) and apatite (Ca10-P) tended to decrease in the straw mulching compared with that under the non-mulching. Phosphorus application increased the content of different inorganic phosphorus components. Compared with the non-mulching, soil available phosphorus content and the phosphorus activation coefficient of straw mulching remarkably increased by 23.2% and 21.3% in 2019, and 9.6% and 8.9% in 2020, respectively. Soil available phosphorus content and phosphorus activation coefficient increased with the increases of phosphorus rate. Results of regression analysis showed that the contribution of inorganic phosphorus components to the availability of available phosphorus in purple soil was Ca2-P > Fe-P > Al-P > Ca8-P > Ca10-P > O-P. Therefore, straw mulching combined with a reasonable phosphorus fertilizer rate could promote the decomposition and transformation of insoluble soil phosphorus to moderately active or easily absorbed phosphorus forms, reduce soil phosphorus adsorption, stimulate soil phosphorus desorption, and improve soil phosphorus availability. Based on the economic benefits, phosphate fertilizer application at the rate of 75 kg·hm-2 combined with straw mulching was recommended in Sichuan hilly dryland, which would be more beneficial in improving soil phosphorus availability.

Synthesis and Characterization of Hydroxyapatite from Green Mussel Shell with Sol-Gel Method

Hydroxyapatite (HAp) is a major component of bones and teeth. HAp is widely used to repair, fill, extend, and reconstruct damaged bone tissue. HAp is used for bone and dental implants, so it is necessary to synthesize HAp. HAp synthesis can utilize green mussel shell waste as a calcium precursor. This research synthesized HAp from a green mussel shell using the sol-gel method. The controlled synthesis parameter was pH, and the variable being compared was the heating temperature at 900 and 1100 °C. The HAp products were characterized by an X-ray diffractometer (XRD), Fourier-transform infrared spectrophotometer, and scanning electron microscope. The results showed that HAp was formed at alkaline pH, namely at pH 11. The XRD pattern showed that Hap was formed along with type A apatite carbonate, octacalcium phosphate, α- and β-tricalcium phosphate. The crystallinity was increased by raising the temperature and prolonging the heating time. The quality of HAp will improve with increasing crystallinity, and increasing the temperature will also raise the amount of HAp formed.