Silicon-substituted Hydroxyapatite Research Articles

Deposition of substituted apatite coatings at different coating patterns via drop-on-demand micro-dispensing technique

Silicon-substituted hydroxyapatite (SiHA) and silver-substituted hydroxyapatite (AgHA) potentiated their usages as bone graft materials with favourable bioactivity and enhanced antibacterial properties, respectively. To facilitate bone growth and reduce bacterial adhesion simultaneously, multi-material coatings composed of SiHA and AgHA at ratios of 1:1, 1:2, 1:3 (1SiHA-1AgHA, 1SiHA-2AgHA, 1SiHA-3AgHA), were deposited by different coating patterns via a Drop-on-Demand (DoD) micro-dispensing technique. As an additive manufacturing technique, placement, size, and numbers of droplets could be controlled precisely, and this gave rise to an even distribution of SiHA and AgHA, which integrated functional properties to a thin coating layer of 9·0 ± 0·5 μm. From the antibacterial study, SiHA-AgHA coatings, especially 1SiHA-3AgHA coatings demonstrated an inhibition on the growth of Escherichia coli (E. coli) up to 12 h as compared to pure HA and SiHA coatings.

Polyethylene Oxide and Silicon-Substituted Hydroxyapatite Composite: A Biomaterial for Hard Tissue Engineering in Orthopedic and Spine Surgery.

Background:Tissue engineering and biomaterials have made it possible to innovate bone treatments for orthopedic and spine problems. The aim of this study is to develop a novel polyethylene oxide (PEO)/silicon-substituted hydroxyapatite (Si-HA) composite to be used as a scaffold for hard tissue engineering in orthopedic and spine procedures.Materials and Methods:The composite was fabricated through the electrospinning technique. The applied voltage (5 kV) and PEO concentration (5%) were fixed. Processing parameters such as the flow rates (20 μl/min and 50 μl/min), distances from capillary tube to the collector (130 mm and 180 mm), spinning time (10 min and 20 min), and concentration of Si-HA (0.2% and 0.6%) were explored to find the optimum conditions to produce fine composite fibers.Results:Scanning electron microscope images showed that 5% PEO, 5% PEO/0.2% Si-HA, and 5% PEO/0.6% Si-HA fibers were successively produced. Flow rates and working distances showed significant influence on the morphology of the polymeric and composite fibers. A high flow rate (50 μl/min) and a larger working distance (180 mm) resulted in larger fibers. The comparison between the mean fiber diameter of 5% PEO/0.2% Si-HA and 5% PEO/0.6% Si-HA showed to be significantly different. As the Si-HA concentration increased, certain fibers were having particles of Si-HA that were not properly integrated into the polymer matrix.Conclusions:Synthesis of a novel biomaterial for hard tissue scaffold through electrospinning was successful. In general, PEO/Si-HA fibers produced have the desired characteristics to mimic the extracellular matrix of bone.

Anti-inflammatory effect of new calcium hydroxide paste containing silicon-substituted hydroxyapatite in lipopolysaccharide-stimulated macrophages

Anti-inflammatory effect of new calcium hydroxide paste containing silicon-substituted hydroxyapatite in lipopolysaccharide-stimulated macrophages

Solid-state NMR and computational insights into the crystal structure of silicocarnotite-based bioceramic materials synthesized mechanochemically

In this work, we report the results of a detailed structural study of a promising bioceramic material silicocarnotite Ca5(PO4)2SiO4 (SC) synthesized from mechanochemically treated nanosized silicon-substituted hydroxyapatite by annealing at 1000°C. This novel synthetic approach represents an attractive and efficient route towards large-scale manufacturing of the silicocarnotite-based bioceramics. A combination of solid-state nuclear magnetic resonance (NMR), powder X-ray crystallography and density function theory (DFT) calculations has been implemented to characterize the phase composition of the prepared composite materials and to gain insight into the crystal structure of silicocarnotite. The phase composition analysis based on the multinuclear solid-state NMR has been found in agreement with X-ray powder diffraction indicating the minority phases of CaO (5–6wt%) and residual silicon-apatite (7–8wt%), while the rest of the material being a fairly crystalline silicocarnotite phase (86–88wt%). A combination of computational (CASTEP) and experimental methods was used to address the anionic site disorder in the silicocarnotite crystal structure. Distorted [OPO3] pyramids have appeared as an important structural motif in the SC crystal structure. The ratio between regular [PO4] and distorted [OPO3] tetrahedra is found between 2:1 and 3:1 based on XRD experiments and CASTEP calculations. The natural abundance 43Ca magic angle spinning NMR spectra of silicocarnotite are reported for the first time.

Preparation of micro-porous bioceramic containing silicon-substituted hydroxyapatite and beta-tricalcium phosphate

Dimensional instability caused by sintering shrinkage is an inevitable drawback for conventional processing of hydroxyapatite (HA). A new preparation method for biphasic calcium phosphates was developed to increase micro pores and biodegradation without significant dimensional change. Powder pressed HA discs, under 100MPa, were immersed in a colloidal mixture of tetraethoxysilane (TEOS) and ammonium hydroxide for 10min, followed by drying, and then were sintered at 900°C, 1050°C, and 1200°C, respectively. Comparing with pure HA discs, the newly prepared product sintered up to 1200°C contained silicon substituted HA, beta-tricalcium phosphate, and calcium silicate with better micro-porosity, high specific surface area, less sintering shrinkage and the strength maintained. The cytocompatibility test demonstrated a better viability for D1 mice stem cells cultured on TEOS treated HA for 14days compared to the pure HA. This simple TEOS sol-gel pretreatment has the potential to be applied to any existing manufacturing process of HA scaffold for better control of sintering shrinkage, create micropores, and increase biodegradation.

A multi-material coating containing chemically-modified apatites for combined enhanced bioactivity and reduced infection via a drop-on-demand micro-dispensing technique.

Prevention of infection and enhanced osseointegration are closely related, and required for a successful orthopaedic implant, which necessitate implant designs to consider both criteria in tandem. A multi-material coating containing 1:1 ratio of silicon-substituted hydroxyapatite and silver-substituted hydroxyapatite as the top functional layer, and hydroxyapatite as the base layer, was produced via the drop-on-demand micro-dispensing technique, as a strategic approach in the fight against infection along with the promotion of bone tissue regeneration. The homogeneous distribution of silicon-substituted hydroxyapatite and silver-substituted hydroxyapatite micro-droplets at alternate position in silicon-substituted hydroxyapatite-silver-substituted hydroxyapatite/hydroxyapatite coating delayed the exponential growth of Staphylococcus aureus for up to 24 h, and gave rise to up-regulated expression of alkaline phosphatase activity, type I collagen and osteocalcin as compared to hydroxyapatite and silver-substituted hydroxyapatite coatings. Despite containing reduced amounts of silicon-substituted hydroxyapatite and silver-substituted hydroxyapatite micro-droplets over the coated area than silicon-substituted hydroxyapatite and silver-substituted hydroxyapatite coatings, silicon-substituted hydroxyapatite-silver-substituted hydroxyapatite/hydroxyapatite coating exhibited effective antibacterial property with enhanced bioactivity. By exhibiting good controllability of distributing silicon-substituted hydroxyapatite, silver-substituted hydroxyapatite and hydroxyapatite micro-droplets, it was demonstrated that drop-on-demand micro-dispensing technique was capable in harnessing the advantages of silver-substituted hydroxyapatite, silicon-substituted hydroxyapatite and hydroxyapatite to produce a multi-material coating along with enhanced bioactivity and reduced infection.

Synthesis and characterization of Si-substituted hydroxyapatite bioactive coating for SiC-coated carbon/carbon composites

A silicon-substituted hydroxyapatite (Si-HA) coating for SiC-coated carbon/carbon composites has been prepared by pack cementation and electrochemical deposition. The morphology, microstructure and in-vitro bioactivity of the Si-HA/SiC double-layer coatings were investigated. Results showed that the SiC inner-layer exhibited high crystallinity. The Si-HA outer-layer had a smaller crystal size than pure hydroxyapatite (HA). Simulated body fluid (SBF) immersion tests showed that the Si-HA coating was uniformly covered by a layer of apatite after 7 days of immersion, while the apatite layer formed on the HA coating was inhomogeneous. This work demonstrates that the Si-HA coating may have better bioactivity than a HA coating and shows promise for the potential application as bioactive coating.

An in situ study of the deposition of a calcium phosphate mineralized layer on a silicon-substituted hydroxyapatite sensor modulated by bovine serum albumin using QCM-D technology

The deposition of a calcium phosphate (Ca-P) mineralized layer on Si-HA surface (the blank group, Au and the control group, HA) with and without BSA preadsorption was observed in real time using a quartz crystal microbalance with dissipation (QCM-D) technique. The effect of silicon doping on the adsorption of BSA onto a HA sensor was investigated. Approximately 1.5-fold more BSA adsorbed onto HA than Si-HA, indicating the inhibition of BSA adsorption onto the HA sensor by silicon doping. The decreasing ΔDsat/Δfsat value in the plots of ΔD–Δf versus adsorption time was predominantly attributed to BSA adsorption, and the decreasing ΔD value with adsorption time during adsorption equilibrium could be explained by the structural rearrangement of BSA on different substrates. Specifically, BSA preadsorption could be a trigger for the formation of nucleation sites for the mineralized layer, promoting nucleation but inhibiting growth. The growth rate was greater on bare substrates than on BSA adsorption substrates. The growth rate of the mineralized layer on the adsorbed BSA layer depended on the substrates in the following manner: Au>Si-HA>HA. The morphology of the mineralized layer depended on the surfaces, and a greater number and a more uniform distribution of smaller nanoparticles were observed on surfaces with preadsorbed BSA. Therefore, BSA could modulate the growth of the mineralized layer at the interface, which is advantageous for fabricating advanced biomaterials.

Quantitative analysis of vascular colonisation and angio-conduction in porous silicon-substituted hydroxyapatite with various pore shapes in a chick chorioallantoic membrane (CAM) model

The development of scaffolds for bone filling of large defects requires an understanding of angiogenesis and vascular guidance, which are crucial processes for bone formation and healing. There are few investigations on the ability of a scaffold to support blood vessel guidance and it this is of great importance because it relates to the quality and dispersion of the blood vessel network. This work reports an analysis of vascularisation of porous silicon-substituted hydroxyapatite (SiHA) bioceramics and the effects of pore shape on vascular guidance using an expedient ex ovo model, the chick embryo chorioallantoic membrane (CAM) assay. Image analysis of vascularised implants assessed the vascular density, fractal dimension and diameter of blood vessels at two different scales (the whole ceramic and pores alone) and was performed on model SiHA ceramics harbouring pores of various cross-sectional geometries (circles, square, rhombus, triangles and stars). SiHA is a biocompatible material which allows the conduction of blood vessels on its surface. The presence of pores did not influence angiogenesis related-parameters (arborisation, fractal dimension) but pore geometry affected the blood vessel guidance and angio-conductive potential (diameter and number of the blood vessels converging toward the pores). The measured angles of pore cross-section modulated the number and diameter of blood vessels converging to pores, with triangular pores appearing of particular interest. This result will be used for shaping ceramic scaffolds with specific porous architecture to promote vascular colonisation and osteointegration. Statement of SignificanceAn expedient and efficient method, using chick embryo chorioallantoic membrane (CAM) assays, has been set up to characterise quantitatively the angiogenesis and the vascular conduction in scaffolds. This approach complements the usual cell culture assays and could replace to a certain extent in vivo experiments. It was applied to silicon-substituted hydroxyapatite porous bioceramics with various pore shapes. The material was found to be biocompatible, allowing the conduction of blood vessels on its surface. The presence of pores does not influence the angiogenesis but the pore shape affects the blood vessel guidance and angio-conductive potential. Pores with triangular cross-section appear particularly attractive for the further design of scaffolds in order to promote their vascular colonisation and osteointegration and improve their performances.

Mechanochemical Synthesis of SiO44–‐Substituted Hydroxyapatite, Part III – Thermal Stability

AbstractThermal stability of mechanochemically synthesized silicon‐substituted hydroxyapatite containing 0.6, 0.8, 1.0, and 1.2 mol of silicon per mol of apatite unit cell has been studied for the first time. The powders of the silicon‐substituted hydroxyapatite were annealed within the temperature range 700–1300 °C. It was found that the substituted hydroxyapatites remain single‐phase after annealing at temperatures of up to 900 °C for all concentrations of added silicon. The second phase – silicocarnotite Ca5(PO4)2SiO4 – appears in the materials containing 1.0 and 1.2 mol of added silicon that have been annealed at 1000 °C. In the powder containing 0.8 mol of added silicon, silicocarnotite was detected after annealing at 1100 °C. The formation of silicocarnotite is likely to occur in the nanoparticles of the silicon‐substituted hydroxyapatite with silicon concentrations much higher than the average value. It was found that the silicon‐substituted hydroxyapatite containing 0.6 mol of silicon per mol of apatite unit cell has the highest thermal stability and remains single‐phase up to a temperature of 1200 °C, similarly to pure hydroxyapatite obtained by the same synthesis method.

Adsorption of Amorphous Silica Nanoparticles onto Hydroxyapatite Surfaces Differentially Alters Surfaces Properties and Adhesion of Human Osteoblast Cells

Silicon (Si) is suggested to be an important/essential nutrient for bone and connective tissue health. Silicon-substituted hydroxyapatite (Si-HA) has silicate ions incorporated into its lattice structure and was developed to improve attachment to bone and increase new bone formation. Here we investigated the direct adsorption of silicate species onto an HA coated surface as a cost effective method of incorporating silicon on to HA surfaces for improved implant osseointegration, and determined changes in surface characteristics and osteoblast cell adhesion. Plasma-sprayed HA-coated stainless steel discs were incubated in silica dispersions of different concentrations (0–42 mM Si), at neutral pH for 12 h. Adsorbed Si was confirmed by XPS analysis and quantified by ICP-OES analysis following release from the HA surface. Changes in surface characteristics were determined by AFM and measurement of surface wettability. Osteoblast cell adhesion was determined by vinculin plaque staining. Maximum Si adsorption to the HA coated disc occurred after incubation in the 6 mM silica dispersion and decreased progressively with higher silica concentrations, while no adsorption was observed with dispersions below 6 mM Si. Comparison of the Si dispersions that produced the highest and lowest Si adsorption to the HA surface, by TEM-based analysis, revealed an abundance of small amorphous nanosilica species (NSP) of ~1.5 nm in diameter in the 6 mM Si dispersion, with much fewer and larger NSP in the 42 mM Si dispersions. 29Si-NMR confirmed that the NSPs in the 6 mM silica dispersion were polymeric and similar in composition to the larger NSPs in the 42 mM Si dispersion, suggesting that the latter were aggregates of the former. Amorphous NSP adsorbed from the 6 mM dispersion on to a HA-coated disc surface increased the surface’s water contact angle by 53°, whereas that adsorbed from the 42 mM dispersion decreased the contact angle by 18°, indicating increased and decreased hydrophobicity, respectively. AFM showed an increase in surface roughness of the 6 mM Si treated surface, which correlated well with an increase in number of vinculin plaques. These findings suggest that NSP of the right size (relative to charge) adsorb readily to the HA surface, changing the surface characteristics and, thus, improving osteoblast cell adhesion. This treatment provides a simple way to modify plasma-coated HA surfaces that may enable improved osseointegration of bone implants.

Shaping by microstereolithography and sintering of macro–micro-porous silicon substituted hydroxyapatite

Additive manufacturing of silicon substituted hydroxyapatite (SiHA) ceramics with controlled macro–micro-porous architecture by microstereolithography and sintering is reported. Due to the role of silicon in bone calcification, the incorporation of silicate in hydroxyapatite has become of interest for applications in bone tissue engineering. But, the shaping and the sintering of SiHA remain few studied. For the shaping process, the formulation of a photopolymerizable suspension and microstereolithography parameters were optimized. Adjustment of the sintering parameters allowed the production of ceramics with controlled open microporosity in a wide range of variation, while preserving phase pure SiHA. A dimensioning model that takes into account the overcure due to light scattering during photopolymerization and the shrinkages during sintering was established. Using this method, macropores of various cross-sections, within the size range of interest for bone ingrowth, were shaped demonstrating the efficiency of microstereolithography for the direct manufacturing of bioceramic scaffolds with accurate architecture.

The effect of silicon doping on the transformation of amorphous calcium phosphate to silicon-substituted α-tricalcium phosphate by heat treatment

In the present study, the effects of silicon dopant contents on the phase composition of silicon-substituted hydroxyapatite (Si-HA) powders were investigated. Si-HA as-prepared samples, with varying silicon (Si) dopant contents, were prepared by an aqueous precipitation method. The phase transformation of the as-prepared samples during sintering was also discussed. The analyses of the as-prepared powders confirmed the formation of well-crystallized Si-HA at low Si dopant concentrations (≤1.5wt% Si). Contrarily, silicon-substituted amorphous calcium phosphate (Si-ACP) and amorphous silica formed at high Si dopant concentrations (>1.5wt% Si). Upon sintering at 1000°C, Si-HA (≤1.5wt% Si) underwent no phase transformations, whereas Si-ACP (>1.5wt% Si) transformed to biphasic silicon-substituted α-tricalcium phosphate (Si-α-TCP) and HA (no additional phases such as β-TCP, silicocarnotite or CaO were detected). Rietveld refinement on X-ray diffraction patterns was conducted to determine the percentage contents of ACP and differences of lattice parameters between pure α-TCP and Si-α-TCP. X-ray photoelectron spectroscopy measurements suggested the formation of amorphous silica and decomposition of the Si-ACP to Si-α-TCP were correlated. The results confirmed that Si doping led to formation of Si-ACP, which primarily transformed to Si-α-TCP upon sintering. The current study highlights a possible account of recrystallization of Si-ACP directly to Si-α-TCP at low sintering temperatures.

Enhanced bone regeneration by silicon-substituted hydroxyapatite derived from cuttlefish bone.

There is growing interest in the use of cuttlefish bone (CB) as a bone graft material. Silicon (Si) plays an important role in bone formation and calcification. This study aimed to prepare Si-substituted CB-derived hydroxyapatite (Si-CB-HAp) using a natural CB to improve the bioactivity for bone formation. We prepared Si-HAp from CB (Si-CB-HAp) using a hydrothermal and solvothermal method. The microstructure and chemical composition were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersive X-ray spectrometer (EDS). The bioactivity of the Si-CB-HAp was evaluated using human mesenchymal stem cells. Furthermore, the invivo bone regeneration efficiency was evaluated using a rabbit calvarial defect model. Our results show that the Si content was 0.77 wt% in Si-CB-HAp, and its original microstructure was conserved. The presence of Si was shown to enhance cell proliferation and early cellular attachment of human mesenchymal stem cells. Additionally, results of alkaline phosphatase activity and real-time PCR for osteoblast marker genes show that Si substitution into CB-HAp enhanced osteoblast differentiation. In addition, invivo bone defect healing experiments show that the formation of bone with Si-CB-HAp is higher than that with CB-HAp. These results indicate that Si-CB-HAp may potentially be used as a bone graft material to enhance bone healing.

Microstructure and in vitro Bioactivity of Silicon-Substituted Hydroxyapatite

Silicon-substituted hydroxyapatite has shown superior biological performance compared to its stoichiometric counterpart both in vitro and in vivo. In the present study, single-phase silicon-substituted hydroxyapatite was successfully synthesized by the precipitation method. Chemical composition, crystalline phase, microstructure, and morphology of the materials were characterized by XRF, XRD, FT-IR, solid-state NMR and SEM. The results showed that hydroxyapatite kept its original structure with silicon up to a level of 0.9 wt%. The precipitation method was proved to be an efficient way to synthesize single-phase silicon-substituted hydroxyapatite. Solid-state NMR combined with other techniques gave direct evidence for the isomorphous substitution of PO4 3- by SiO4 4- in the hydroxyapatite structure. Silicon-substituted hydroxyapatite showed better bioactivity than stoichiometric hydroxyapatite in the in vitro bioactivity experiment. The higher the silicon content in the hydroxyapatite structure, the better the in vitro bioactivity. The enhanced bioactivity of silicon-substituted hydroxyapatite over pure hydroxyapatite has been attributed to the effect of silicate ions in accelerating dissolution.

In vitroosteoclast formation and resorption of silicon-substituted hydroxyapatite ceramics

Materials that participate in bone remodeling at the implant/tissue interface represent a modern tissue engineering approach with the aim of balancing implant resorption and nascent tissue formation. Silicon-substituted hydroxyapatite (SiHA) ceramics are capable of stimulating new bone formation, but little is known about their interaction with osteoclasts (OC). The effects of soluble silicate and SiHA on OCs were investigated in this study. Soluble silicate below 500 μM did not stimulate cell metabolism at 4 days or alter resorption area at 7 days on calcium phosphate discs. On sintered ceramics, OC numbers were similar on HA, Si0.3 HA (0.5 wt % Si) and Si0.5 HA (1.2 wt % Si) after 21 days in vitro, but actin ring sealing zone morphology on SiHA resembled that commonly found on bone or on carbonate-substituted hydroxyapatite (CHA). Smaller and thicker actin rings on SiHA as compared to HA were probably the result of altered surface chemistry and solubility differences. The more stable sealing zones and increased lattice solubility likely contributed to increased individual pit volumes observed on Si0.5 HA. The delayed formation of OCs on Si0.5 HA (lower numbers at day 14) excludes earlier differentiation as a possible mechanism of increased individual OC pit volumes at later times (day 21). Materials characterization of Si containing biomaterials remains paramount as the Si type and amounts can subsequently impact downstream OC behaviour in a complex manner.

Luminescence variations in europium-doped silicon-substituted hydroxyapatite nanobiophosphor via three different methods

Abstract This paper reports the first attempt for the synthesis of europium-doped Si-substituted hydroxyapatite (HA) nanostructure to achieve strong and stable luminescence of nanobiophosphor, particularly, by addition of different Eu dopants, Si substitutions, and application of optimum annealing temperatures of up to 1000 °C. The nanobiophosphor was synthesized by the coprecipitation, microwave, and hydrothermal methods. The nanoparticles demonstrated a nanowire to a spindle-like morphology, which was dependent on the method of synthesis. The photoluminescence (PL) intensity of the sample increases with the increase in Si substitutions and Eu dopants. The luminescent nanoparticles also showed the typical luminescence of Eu 3+ centered at 610 nm, which was more efficient for the annealed Eu-doped Si-HA nanoparticles than for the as-synthesized nanoparticles. Among the different synthesis methods, the hydrothermal method reveals the best light emission represented by high PL intensity and narrow PL spectra. These results suggest the potential application of Eu-doped Si-HA in stable and biocompatible nanophosphors for light emission and nanomedicine.

Synthesis of silicon-substituted hydroxyapatite from the extracellular fluid model solution

The possibility of modifying hydroxyapatite by silicate ions under conditions similar to physiological ones (extracellular fluid solution prototype) has been investigated. The formation of silicon-structured hydroxyapatite with different degrees of substitution of phosphate groups by silicate ones has been established by means of the methods of chemical and X-ray diffraction analysis, IR Fourier-spectroscopy, and optical microscopy.

Bioactive, nanostructured Si-substituted hydroxyapatite coatings on titanium prepared by pulsed laser deposition.

The aim of this work was to deposit silicon-substituted hydroxyapatite (Si-HAp) coatings on titanium for biomedical applications, since it is known that Si-HAp is able to promote osteoblastic cells activity, resulting in the enhanced bone ingrowth. Pulsed laser deposition (PLD) method was used for coatings preparation. For depositions, Si-HAp targets (1.4 wt % of Si), made up from nanopowders synthesized by wet method, were used. Microstructural and mechanical properties of the produced coatings, as a function of substrate temperature, were investigated by scanning electron and atomic force microscopies, X-ray diffraction, Fourier transform infrared spectroscopy, and Vickers microhardness. In the temperature range of 400-600°C, 1.4-1.5 µm thick Si-HAp films, presenting composition similar to that of the used target, were deposited. The prepared coatings were dense, crystalline, and nanostructured, characterized by nanotopography of surface and enhanced hardness. Whereas the substrate temperature of 750°C was too high and led to the HAp decomposition. Moreover, the bioactivity of coatings was evaluated by in vitro tests in an osteoblastic/osteoclastic culture medium (α-Modified Eagle's Medium). The prepared bioactive Si-HAp coatings could be considered for applications in orthopedics and dentistry to improve the osteointegration of bone implants.

Synthesis and Photocatalytic Activity of Silicon-Substituted Hydroxyapatite

Series of silicon-substituted hydroxyapatite were prepared and characterized, and their ability to degrade methyl orange and Congo red in aqueous solutions were studied. The effect of substituted silicon amount on the photocatalytic degradation efficiency of the dyes was investigated. The substitution of SiO44− groups for PO43− groups caused OH− loss and difference in the microstructure of hydroxyapatite, and the elimination rates of silicon-substituted hydroxyapatite decreased with the increasing of substituted silicon amount compared to that of pure hydroxyapatite.