Cement from nanocrystalline hydroxyapatite: Effect of calcium phosphate ratio

Nanocrystalline hydroxyapatite (nHA) can be mixed with phosphoric acid to form a brushite cement; a degradable inorganic bone filling material. nHA was precipitated from reactants of calcium to phosphate (Ca/P) ratio 0.8 to 2.0 and mixed with phosphoric acid, which resulted in the formation of a brushite cement. Cement was also formed by mixing microcrystalline calcium phosphates, β-tricalcium phosphate, hydroxyapatite and tetracalcium phosphate with phosphoric acid solution. Cement produced with nHA was stronger in compression than that formed with crystalline calcium phosphate phases. Setting time, strength and composition of cement produced with nHA was dependant on both the Ca/P ratio of nHA and the concentration of phosphoric acid in cement slurry. Increasing phosphoric acid concentration increased compressive strength whilst reducing the initial setting time of cement. Reducing the Ca/P ratio of nHA precipitation reactants retarded the setting and increased the extent of reaction of cements. This finding was unexpected and suggests that Ca/P ratio may strongly affect dissolution behaviour and this parameter is more important than stoichiometry in determining extent of reaction in this system. This study demonstrated that the wide variation in stoichiometry that may be attained in nanocrystalline apatite may be utilised to change cement performance and setting behaviour.     

Preparation and characterization of selenite substituted hydroxyapatite

Selenite-substituted hydroxyapatite (Se-HA) with different Se/P ratios was synthesized by a co-precipitation method, using sodium selenite (Na₂SeO₃) as a Se source. Selenium has been incorporated into the hydroxyapatite lattice by partially replacing phosphate (PO₄^3 ?) groups with selenite (SeO₃^2 ?) groups. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) techniques reveal that substitutions of phosphate groups by selenite groups cause lower carbonate groups occupying at phosphate sites and change the lattice parameters of hydroxyapatite. The powders obtained are nano-crystalline hydroxyapatite when the Se/P ratios are not more than 0.1. The particle shape of Se-HA has not been altered compared with selenite-free hydroxyapatite but Se-incorporation reduces the crystallite size. The crystallinity was reduced as the Se/P ratios increased until amorphous phase (Se/P = 0.3) appeared in the Se-HA powder obtained, and then another crystal phase presented as calcium selenite hydrate (Se/P = 10). In addition, the sintering tests show that the Se-HA powders with the Se/P ratio of 0.1 have thermal stability at 900 °C for 2 h; hence they have great potential in the fabrication of bone repair scaffolds.     

A comparative study on the in vivo behavior of hydroxyapatite and silicon substituted hydroxyapatite granules

Phase pure hydroxyapatite (HA) and a 0.8 wt % silicon substituted hydroxyapatite (SiHA) were prepared by aqueous precipitation methods. Both HA and SiHA were processed into granules 0.5–1.0 mm in diameter and sintered at 1200 °C for 2 h. The sintered granules underwent full structural characterization, prior to implantation into the femoral condyle of New Zealand White rabbits for a period of 23 days. The results show that both the HA and SiHA granules were well accepted by the host tissue, with no presence of any inflammatory cells. New bone formation was observed directly on the surfaces and in the spaces between both HA and SiHA granular implants. The quantitative histomorphometry results indicate that the percentage of bone ingrowth for SiHA (37.5%±5.9) was significantly greater than that for phase pure HA (22.0%±6.5), in addition the percentage of bone/implant coverage was significantly greater for SiHA (59.8%±7.3) compared to HA (47.1%±3.6). These findings indicate that the early in vivo bioactivity of hydroxyapatite was significantly improved with the incorporation of silicate ions into the HA structure, making SiHA an attractive alternative to conventional HA materials for use as bone substitute ceramics.     

HA晶须的制备及在HA/Mg生物复合材料中的应用

利用水热法合成羟基磷灰石晶须(HA_W), 采用浆料-粉末冶金法制备羟基磷灰石晶须/镁金属基复合材料(HA_W/Mg)。用XRD、 FTIR、 FESEM和TG对材料进行表征。分析了尿素含量对HA物相和晶体结构的影响, 以及HA_W/Mg复合材料的热稳定性和显微结构。结果表明: 在本实验条件下, 得到的产物均为带状、 含碳酸根的HA_W; 随着尿素添加量的增加, HA_W逐渐变长且结晶度增加; HA_W与Mg之间在600 ℃以下无化学反应, 且HA不会加剧Mg的氧化; HA_W在HA_W/Mg复合材料中分散均匀, 不易折断。      

Novel deposition of nano-sized silicon substituted hydroxyapatite by electrostatic spraying

Suspensions containing nano-sized silicon substituted hydroxyaptite (nSiHA) particles were produced and processed for electrostatic spray deposition. No secondary phases were detected by X-ray diffraction, which indicated that the nSiHA was phase pure. Electrostatic spraying of nSiHA in cone-jet mode was achieved at flow rate of 10⁻⁹ m³s⁻¹ with an applied voltage between the needle and the ring-shaped ground electrode set at 6 to 8 kV. Micrometer- and submicrometer-scaled islands of nSiHA have been deposited on glass and titanium substrates. The surface roughness of such nHA and nSiHA islands was in the range 60 to 80 nm, as measured from atomic force microscopy in tapping mode. The growth of primary human osteoblast (HOB) cells on the nSiHA deposited substrates increased with time during the 4 days of culture, and the increase was related with the Si content in substituted HA, indicating that nSiHA was able to promote and support the growth of HOB cells. Scanning electron microscopy (SEM) revealed that extracellular matrix (ECM) produced by the HOB cells on these nSiHA deposits was well organized. In addition, the presence of Ca and P containing nodules in the ECM were also confirmed by Energy Dispersive X-ray (EDX) analysis, indicating early signs of calcification fronts. The results showed that nSiHA produced by electrostatic spray deposition was able to promote the attachment and the growth of HOB cells. Therefore, electrostatic spray deposition offers great potential for the creation of bioactive surfaces to provide improved interfacial bonding with host tissues.     

Fine structure study on low concentration zinc substituted hydroxyapatite nanoparticles

The fine structure of zinc substituted hydroxyapatite was studied using experimental analysis and first-principles calculations. The synthetic hydroxyapatite nanoparticles containing low Zn concentration show rod-like morphology. The crystallite sizes and unit-cell volumes tended to decrease with the increased Zn concentration according to X-ray diffraction patterns. The Zn K-edge X-ray absorption spectra and fitting results suggest that the hydroxyapatite doped with 0.1 mole% zinc is different in the zinc coordination environments compared with that containing more zinc. The density function theory calculations were performed on zinc substituted hydroxyapatite. Two mechanisms included replacing calcium by zinc and inserting zinc along the hydroxyl column and were investigated, and the related substitution energies were calculated separately. It is found that the substitution energies are negative and lowest for inserting zinc between the two oxygen atoms along the hydroxyl column (c-axis). Combined with the spectral analysis, it is suggested that the inserting mechanism is favored for low concentration zinc substituted hydroxyapatite.     

Chemical synthesis and stabilization of magnesium substituted brushite

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is the most ubiquitous calcium phosphate phase used in implant coatings and more recently in gene/drug delivery applications due to its chemical stability under normal physiological conditions (37 °C, pH ～ 7.5, 1 atm.). However, different calcium phosphate phases, such as brushite (CaH(PO₄)?2(H₂O)) and tricalcium phosphate (Ca₃(PO₄)₂) which are thermodynamically unstable under physiological conditions are also being explored for biomedical applications. One way of stabilizing these phases under physiological conditions is to introduce magnesium to substitute for calcium in the brushite lattice. The role of magnesium as a stabilizing agent for synthesizing brushite under physiological conditions at room temperature has been studied. Chemical analysis, Fourier transform infrared spectroscopy and X-ray diffraction have also been conducted to validate the formation of magnesium substituted brushite under physiological conditions.     

Fibrous growth of strontium substituted hydroxyapatite and its drug release

The effect of strontium on the crystallization of helical ribbon of hydroxyapatite (HAp) was investigated by single diffusion technique in silica gel matrix at 27 °C and physiological pH. Fibers of HAp were obtained on addition of strontium. The length of the HAp fibers, were found to decrease as the strontium substitution increases. The presence of strontium ion increased the crystallinity as well as crystallite size of HAp. The strontium substituted HAp (Sr-HAp) has similar stoichiometry to that of biological apatite. Sr-HAp was found to have increased surface area (35%) compared to control. Further, strontium substitution leads to an enhancement of in vitro bioactivity. The cumulative in-vitro amoxicillin drug release in phosphate buffer solution (PBS, pH 7.2) showed a prolonged release profile for Sr-HAp.     

Enhanced performance of fluorine substituted hydroxyapatite composites for hard tissue engineering

Dense fluorine (F) substituted hydroxyapatite composites with yttria-doped zirconia (Y-TZP) and/or alumina (Al₂O₃) were successfully fabricated without applying pressure at 1400 °C for 3 h. The suppression of decomposition via the formation of a fluor-hydroxyapatite (FHA) solid solution allowed the sintered body to reach full density. Such fully densified FHA-composites exhibited improved mechanical properties, such as strength, toughness, and hardness, having values of more than 2–4 times higher than those of pure HA or HA-composites. The proliferation behavior of osteoblast-like cells on the FHA-composites showed no cytotoxicity and comparable cell viability to that observed in pure HA for up to 10 days.     

锶磷灰石多孔陶瓷动物体内降解的实验研究

采用溶液反应方法制备含5%(原子分数)锶的羟磷灰石(Sr-HA)原料,经干压成型、烧结后制得孔隙率约35%的圆片状多孔体,植入新西兰大白兔脊柱两侧背脊肌中,以纯羟磷灰石试样作为实验对照;通过扫描电镜、四环素荧光标记法观察种植体试样表面生物降解性和与周围软组织之间界面状况以及四环素荧光分布,评价锶磷灰石多孔陶瓷在动物软组织内的生物降解性、对组织生长的引导性等生物学行为。结果显示,锶磷灰石种植试样3个月时表面可见由于晶体降解而呈现出大量凹陷结构,凹陷结构的边缘较中心点的降解更为明显和快速;6个月时可观察到凹陷样结构处外围有明显的从外向内吸收特征,并且可观察到大量的强亮色条索状、团状荧光带,连成一片。而羟磷灰石对照组材料表面仅见少量的降解,四环素荧光呈点状弥散样,荧光强度较低。说明锶磷灰石具有良好的组织亲和性和组织引导性;与羟磷灰石相比,锶磷灰石的降解速度快,降解程度高。锶磷灰石可能具有诱导骨基质形成的作用。     

掺锶羟磷灰石固溶体含锶量对骨组织融合能力的影响

评价不同掺锶量羟磷灰石固溶体材料锶含量与生物降解性和骨组织融合能力之间的影响关系.5组掺锶羟磷灰石固溶体材料锶克分子含量分别取0%,1%,5%,10%和100%,烧结体植入部位为家兔胫骨;手术后分不同时段处死动物,通过四环素荧光标记和电镜扫描方法考察植入材料与骨组织界面的生物反应状况.实验结果表明5组试样材料都显示了良好的组织相容性和骨结合性,其中锶含量为5%组和10%组试样显示较强的骨引导作用,呈现出很明亮的四环素荧光带以及明显的降解现象.这说明在羟磷灰石晶体中掺入适量的锶元素可以明显改善材料的生物相容性、骨引导性、生物降解性以及骨融合性;掺锶羟磷灰石固溶体不仅可以增强新骨的生成能力,而且可以延长新骨的生成时间和增加新骨的生成量.     

Nanocrystalline hydroxyapatite doped with magnesium and zinc: Synthesis and characterization

During recent years, there have been efforts in developing nanocrystalline bioceramics, to enhance their mechanical and biological properties for use in tissue engineering applications. In this research, we made an attempt to synthesize nanocrystalline bioactive hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) ceramic powder in the lower-end of nano-range (2–10 nm), using a simple low-temperature sol–gel technique and studied its densification behavior. We further studied the effects of metal ion dopants during synthesis on powder morphology, and the properties of the sintered structures. Calcium nitrate and triethyl phosphite were used as precursors for calcium and phosphorous, respectively, for sol–gel synthesis. Calculated quantities of magnesium oxide and zinc oxide were incorporated as dopants into amorphous dried powder, prior to calcination at 250–550 °C. The synthesized powders were analyzed for their phases using X-ray diffraction technique and characterized for powder morphology and particle size using transmission electron microscopy (TEM). TEM analysis showed that the average particle size of the synthesized powders were in the range of 2–10 nm. The synthesized nano-powders were uniaxially compacted and then sintered at 1250 °C and 1300 °C for 6 h, separately, in air. A maximum average sintered density of 3.29 g/cm³ was achieved in structures sintered at 1300 °C, developed from nano-powder doped with magnesium. Vickers hardness testing was performed to determine the hardness of the sintered structures. Uniaxial compression tests were performed to evaluate the mechanical properties. Bioactivity and biodegradation behavior of the sintered structures were assessed in simulated body fluid (SBF) and maintained in a dynamic state.     

Long-term in vivo experimental investigations on magnesium doped hydroxyapatite bone substitutes

Despite several efforts to find suitable alternatives to autologous bone, no bone substitute currently available provides the same characteristics and properties. Nevertheless, among the wide range of materials proposed as bone substitutes, calcium phosphate materials represent the most promising category and the present study is aimed at improving the knowledge on non-stoichiometric magnesium-doped hydroxyapatite substitutes (Mg-HA), tested in two different formulations: Mg-HA Putty and Mg-HA Granules. These bone substitutes were implanted bilaterally into iliac crest bone defects in healthy sheep and comparative histological, histomorphometric, microhardness and ultrastructural assessments were performed 9, 12, 18 and 24 months after surgery to elucidate bone tissue apposition, mineralization and material degradation in vivo. The results confirmed that the biomimetic bone substitutes provide a histocompatible and osteoconductive structural support, during the bone formation process, and give essential information about the in vivo resorption process and biological behavior of biomimetic bone substitutes.     

Sol–gel synthesis and spectrometric structural evaluation of strontium substituted hydroxyapatite

Investigations of the electronic structures of substituted strontium apatites were carried out by using X-ray photo electron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction technique (XRD) has been used to determine the structural parameters. Electron microprobe microanalysis technique was used to estimate the elemental concentrations in each substituted apatite material. The present work aims at studying the changes in the electronic structure of Sr₅(PO₄)₃OH (Sr-HAP) upon isomorphic substitution by F and Cl at the OH site of apatite and Sr by Na at trans Sr-HAP. The ion exchange between Na⁺ in sodium alginate and aqueous Ca²⁺ was important for the preparation of calcium hydroxyapatite. In contrast, the reaction of sodium alginate with the mixture of Na₄P₂O₇ and aqueous Sr²⁺ afforded strontium hydroxyapatite at the specific ratio. The structure of calcium and strontium phosphates prepared from the sol–gel process evidently depended on the amount of sodium alginate introduced into the mixture of Na₄P₂O₇ and the corresponding divalent cations. The findings have ensured that substitution of Sr-HAP by Na enhances the binding energy of O and Sr core levels. It was also noticed that the same substitution decreases the binding energy of P 2s-level. These observations point out to a decrease in the electron density at P and an increase in the electron density at O in Sr atoms.     

Liquid phase epitaxial growth of indium substituted magnesium ferrite films

Single crystal films of Mg(Fe,In)₂O₄ have been grown by means of liquid-phase-epitaxy on (100)/1bMgO substrates using a PbO/1bB₂O₃/1bFe₂O₃ solvent. The final polishing of the substrates appeared to be very important in connection with the necessary large supersaturation of the flux. Electron microprobe analysis revealed Pt and high Pb contents in our films. The easy axis of magnetization is in-plane, probably due to the compressive strain of the films caused by the difference in thermal expansion of MgO and Mg(Fe,In)₂O₄.     

Crystal imperfection studies of pure and silicon substituted hydroxyapatite using Raman and XRD

Hydroxyapatite (HA) is important in biomedical applications because of its chemical similarity to the mineral content of bone and its consequent bioactivity. Silicon substitution into the hydroxyapatite crystal lattice was found to enhance its bioactivity both in vitro and in vivo [1, 2]. However, the mechanism for the enhancement is still not well understood. In this paper, the crystal imperfections introduced by silicon substitution were studied using XRD and Raman spectroscopy. It was found that silicon substitution did not introduce microstrain, but deceased the crystal size in the hk0 direction. Three new vibration modes and peak broadening were observed in Raman spectra following silicon incorporation. The imperfections introduced by silicon substitution may play a role in enhancing bioactivity. A phenomenological relationship between the width of the PO₄ v1 peak and crystal size was established.     

Preparation and structure of carbonated calcium hydroxyapatite substituted with heavy rare earth ions

Calcium hydroxyapatite (CaHap) particles substituted five types of heavy rare earth ions (Ln: Y³⁺, Gd³⁺, Dy³⁺, Er³⁺ and Yb³⁺) were synthesized using a precipitation method and characterized using various means. These Ln ions strongly affected the crystal phases and the structures of the products. With increasing Ln/(Ln + Ca) in the starting solution ([X_Ln]), the length and the crystallinity of the particles first increased and then decreased. The rare earth metal-calcium hydroxyapatite (LnCaHap) solid solution particles were obtained at [X_Y] ≤ 0.10 for substituting Y system and at [X_Ln] ≤ 0.01–0.03 for substituting the other Ln systems. LnPO₄ was mixed with LnCaHap at higher [X_Ln] for all Ln systems. A series of yttrium-calcium hydroxyapatite (YCaHap) solid solutions with [X_Y] = 0–0.10 were investigated using XRD, TEM, ICP-AES, IR and TG–DTA in detail.     

掺锶羟磷灰石固溶体陶瓷不同孔径对成骨能力的影响

通过控制发泡剂粒径的方法制得两组孔隙率相同孔径尺寸不同的掺锶羟磷灰石多孔体陶瓷:S组平均孔径200μm;L组平均孔径750μm.植入兔桡骨缺损部位,植入后3、6和12周取材,行X线观察、大体标本观察、组织学图像观察和四环素荧光染色观察,来对比不同孔径的锶磷灰石陶瓷多孔体的骨融合能力.结果表明锶磷灰石陶瓷多孔体具有良好的生物相容性及骨传导性;两种孔径材料都有良好的成骨性能,大孔径组锶磷灰石陶瓷多孔体材料比小孔径组材料有更佳的骨修复效果;且大孔径组在12周就能观察到陶瓷材料的降解以及新生骨包绕降解陶瓷颗粒生长的现象.     

Cements from nanocrystalline hydroxyapatite

Calcium phosphate cements are used as bone substitute materials because they may be moulded to fill a void or defect in bone and are osteoconductive. Although apatite cements are stronger than brushite cements, they are potentially less resorbable in vivo. Brushite cements are three-component systems whereby phosphate ions and water react with a soluble calcium phosphate to form brushite (CaHPO4 x 2H2O). Previously reported brushite cement formulations set following the mixture of a calcium phosphate, such as beta-tricalcium phosphate (beta-TCP), with an acidic component such as H3PO4 or monocalcium phosphate monohydrate (MCPM). Due to its low solubility, hydroxyapatite (HA) is yet to be reported as a reactive component in calcium phosphate cement systems. Here we report a new cement system setting to form a matrix consisting predominantly of brushite following the mixture of phosphoric acid with nanocrystalline HA. As a result of the relative ease with which ionic substitutions may be made in apatite this route may offer a novel way to control cement composition or setting characteristics. Since kinetic solubility is dependent on particle size and precipitation temperature is known to affect precipitated HA crystal size, the phase composition and mechanical properties of cements made from HA precipitated at temperatures between 4 and 60 degrees C were investigated.