Hydroxyapatite Grains Research Articles

3D-Printed Bioceramic Scaffolds Reinforced by the In Situ Oriented Growth of Grains for Supercritical Bone Defect Reconstruction.

Porous calcium phosphate ceramics have attracted widespread attention owing to their excellent bioactivity. However, their poor mechanical properties severely limit their clinical applications. Significantly improving the mechanical strength of porous CaP ceramics while maintaining their bioactivity remains a major challenge. To address this issue, calcium sulfate is used to regulate the directional growth of hydroxyapatite grains during ceramic sintering. The in situ oriented grains can not only alleviate the stress concentration but also strengthen the bonding force between the ceramic grain boundaries. Calcium sulfate improves the release of active calcium ions from calcium phosphate ceramics, further enhancing their bioactivity and osteoinductivity in vivo. Transcriptome and proteome sequencing reveals that the in situ whisker-reinforced ceramics increase the expression of proteins related to calcium ion binding and promote the expression of osteogenesis-related proteins. In the supercritical bone defect repair model, repair of the defect is achieved within 3 months, with mechanical recovery reaching more than 70% of the autologous bone.

Scaffold microstructure evolution via freeze‐casting and hydrothermal phase transformation of calcium phosphate

AbstractExtensive research efforts have been focused on customizing the microstructure, macrostructure, and phase composition of calcium phosphate for enhanced biocompatibility and bioactivity in scaffolds for bone substitutes. Despite significant progress, achieving precise phase composition and microstructure remains a challenge, primarily due to the necessity of scaffold sintering. This study addresses the challenges in developing customized patient‐specific bone substitutes by proposing a sequential approach that reduces processing steps while providing control over the phase and morphology of the scaffolds’ structure. The methodology utilizes freeze‐casting and sintering for highly porous the scaffolds’ preparation, followed by hydrothermal treatment to modify the microstructure. The introduction of CaCO3 induces a phase transformation of tricalcium phosphate, increasing the hydroxyapatite content, while the overall macrostructure retains the characteristics of freeze‐casting. The surface morphology undergoes a transition from equiaxial grains to whiskers‐like structures and hexagonal rods, impacting compressive strength. Following hydrothermal treatment, the formation of whiskers‐like hydroxyapatite grains leads to a notable strength increase from 2.8 to 5.7 MPa. Remarkably, the scaffolds undergo nearly complete phase transformation, shifting from 100% tricalcium phosphate to 99% hydroxyapatite, all while conserving the macrostructure.

Radiological properties of nano-hydroxyapatite compared to natural equine hydroxyapatite quantified using dual-energy CT and high-field MR

In equine medicine, assisted bone regeneration, including use of biomaterial substitutes like hydroxyapatite (HAP), is crucial for addressing bone defects. To follow-up on the outcome of HAP-based bone defect treatment, the advancement in quantified diagnostic imaging protocols is needed. This study aimed to quantify and compare the radiological properties of the HAP graft and natural equine bone using Magnetic Resonance (MR) and Computed Tomography (CT), both Single (SECT) and Dual Energy (DECT). SECT and DECT, allow for the differentiation of three HAP grain sizes, by progressive increase in relative density (RD). SECT, DECT, and MR enable the differentiation between natural cortical bone and synthetic HAP graft by augmentation in Effective Z and material density (MD) in HAP/Water, Calcium/Water, and Water/Calcium reconstructions, alongside the reduction in T2 relaxation time. The proposed quantification provided valuable radiological insights into the composition of HAP grafts, which may be useful in follow-up bone defect treatment.

Development of hybrid 3D printing approach for fabrication of high-strength hydroxyapatite bioscaffold using FDM and DLP techniques

This study develops a hybrid 3D printing approach that combines fused deposition modeling (FDM) and digital light processing (DLP) techniques for fabricating bioscaffolds, enabling rapid mass production. The FDM technique fabricates outer molds, while DLP prints struts for creating penetrating channels. By combining these components, hydroxyapatite (HA) bioscaffolds with different channel sizes (600, 800, and 1000 μm) and designed porosities (10%, 12.5%, and 15%) are fabricated using the slurry casting method with centrifugal vacuum defoaming for significant densification. This innovative method produces high-strength bioscaffolds with an overall porosity of 32%–37%, featuring tightly bound HA grains and a layered surface structure, resulting in remarkable cell viability and adhesion, along with minimal degradation rates and superior calcium phosphate deposition. The HA scaffolds show hardness ranging from 1.43 to 1.87 GPa, with increasing compressive strength as the designed porosity and channel size decrease. Compared to human cancellous bone at a similar porosity range of 30%–40%, exhibiting compressive strengths of 13–70 MPa and moduli of 0.8–8 GPa, the HA scaffolds demonstrate robust strengths ranging from 40 to 73 MPa, paired with lower moduli of 0.7–1.23 GPa. These attributes make them well-suited for cancellous bone repair, effectively mitigating issues like stress shielding and bone atrophy.

EFFECT OF PREPARATION ACETONE ON FISH BONES SYNTHESIZED THROUGH SINTERING METHOD TO IMPROVE HYDROXYAPATITE CHARACTERISTICS

In the development of advanced materials and various technological applications, the preparation and sintering processes have become two important factors in determining material characteristics. This research focuses on two main aspects, namely the effect of fish bone preparation by soaking in acetone and the surface area of the material in the sintering process as part of the process of developing better materials. This research aims to determine the effect of soaking fish bone powder with acetone and the effect of the surface area of sintered fish bones to produce hydroxyapatite (HA). The immersion process with acetone is included in the sample preparation stage, while the sintering process is included in the material synthesis stage. These two things can affect the characteristics of the HA produced after analysis from the X-ray diffraction test. The HA structure obtained from all samples is hexagonal with cell parameter values a = b ≠ c and space group P 63 / m, where all samples have a value range of a = b = 9,42 Å and c = 6,88 Å. HA crystallinity was identified through the XRD peak at 2θ = 25,8 (002); 31,7 (211); 32,1 (112); 32,8 (300); 34,0 (202); 39,7 (310); 46,6 (222); 49,4 (213); 50,4 (321). The PAF-900 and CAF-900 samples are similar to HA in JCPDS 01-089-4405 whose compound formula is Ca5(PO4)3(OH) while the PWAF-900 sample is similar to HA in JCPDS 01-075-3727 whose compound formula is Ca5(PO4)3(CO3)0.01(OH)1.3. The percentage of crystallinity of PAF-900, CAF-900, and PWAF-900 respectively was 84,767; 73,506; and 71,962% with HA grain sizes of 0,8964; 0,6808, and 0,7398 nm. The HA density of PAF-900 and CAF-900 samples is 3,149 g/cm3 while PWAF-900 is 3,146 g/cm3. Based on this description, it can be concluded that the soaking preparation stage with acetone produces HA with the chemical formula Ca5(PO4)3OH with a higher percentage of crystallization and is denser compared to HA obtained without going through the soaking preparation stage with acetone. The sintering stage also plays an important role in increasing the crystallization percentage. The surface area of the material being sintered also influences the percentage of crystallization and the grain size of the resulting HA. Sintered fish bone powder produces a greater percentage of crystallization and grain size than fish bone chunks

Micro- and macro-scale strength properties of c-axis aligned hydroxyapatite ceramics

In this study we first investigated the strength of grains, i.e., single crystals, and grain boundaries (micro-scale strength) of hydroxyapatite ceramics in different crystal orientations, using microcantilever bending tests. The specimens were prepared from two types of hydroxyapatite ceramics; one is that with uniaxially aligned c-axes of grains and the other with randomly oriented ones. Both the grain and grain boundary strengths were slightly higher in tensile stress application parallel to c-axis than in the perpendicular in the aligned material. These strengths, however, were substantially higher than those of the randomly oriented one. Then, three-point bending tests were conducted to determine the polycrystalline strength (macro-scale strength) for the above two types of hydroxyapatite ceramics. While the strength values were comparable in tensile stress applications parallel and perpendicular to c-axis in the aligned material, that of the randomly oriented one is again significantly lower than them. In particular, the strength of the grain boundary was higher when the angle between the c-axis of the two hydroxyapatite grains divided by the boundary was close to 0 or 90°. The effects of the grain orientation and alignment on the mechanical properties were discussed on the basis of both the micro-scale and macro-scale results. These results indicate that selective formation of high-strength grain boundaries due to c-axis alignment of hydroxyapatite grains is effective in improving the mechanical properties of bulk ceramics, such as macroscopic strength and fracture toughness.

Calcium phosphate ceramic as a model for enamel substitute material in dental applications

ObjectiveThis study aimed to develop enamel substitute material using a mechanochemical technique.Materials and MethodsHydroxyapatite was synthesized with and without tricalcium phosphate under uniaxial pressing of 10 and 17 MPa (HA10, HA17, BCP10, and BCP17), followed by sintering at 1250 °C for 2 h. Human enamel and dentin blocks were used as control groups. The mechanical properties were determined by compressive strength test and Vickers microhardness. The data were analyzed with one-way ANOVA and LSD post-hoc test (α = 0.05). The phase formation and morphology of the specimens were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).ResultsHA17 and HA10 had compressive strength values comparable to enamel and dentin, respectively (p > 0.05). The microhardness of all synthesized groups was significantly higher than that of tooth structures (p < 0.05). From the XRD graphs, only the hydroxyapatite peak was observed in the control and HA groups. SEM images showed homogeneous hydroxyapatite grains in all groups, while the BCP groups contained higher porosities.ConclusionsBoth HA10 and HA17 are suitable for use as the inorganic part of dentin and enamel substitutes.

Erosive potential of ice tea beverages and kombuchas

Objectives Kombuchas and other tea-based beverages are often perceived as healthy products despite the lack of knowledge on their effects on oral health. This in vitro study determined the erosive potential of commercial kombuchas, and ice teas compared to cola drinks. Materials and methods The pH and fluoride content of 7 kombuchas and 18 tea drinks were measured with ion-selective electrodes. Calcium dissolution from hydroxyapatite grains was quantified by atomic absorption spectroscopy after beverage exposure. The effect of beverages on the enamel surface was visualized by scanning electron microscopy (SEM). Distilled water, and cola drinks were used as negative and positive controls. Results The kombuchas exhibited lower pH values (2.82–3.66) than the ice teas (2.94–4.86), but still higher than the cola drinks (2.48–2.54). The fluoride concentration varied between 0.05 and 0.46 ppm and for 7 beverages the concentration was below the detection limit. The calcium release for kombuchas was 198–746 mg/l, for ice teas 16.1–507 mg/l, and for cola drinks 57.7–71.9 mg/l. Twenty-two beverages had a significantly greater calcium release than the cola drinks (p = .009–.014). The surface etching of the enamel was seen in the SEM analysis after beverage exposure. Conclusions Tea-based beverages have even higher erosive potential than cola drinks. Kombuchas especially, displayed a considerable erosive potential.

Physicochemical characterization tungsten oxide modified hydroxyapatite embedded into polylactic acid nanocomposite for biomedical applications

To compensate for the feeble antimicrobial potential of hydroxyapatite (HAp), the compound has been incorporated into different nanocomposites (NCs) in addition to powder and fibrous structure. Hydroxyapatite (HAp)/tungsten oxide/graphene oxide (GO) ternary nanocomposite introduces a significant enhance in the cell viability to 96.7 ± 3% towards the osteoblast cells. The chemical compositions were approved by various analytical techniques such as XRD, FTIR, XPS, and EDS. TEM shows well-defined, rod-like HAp grains. The triple ingredient HAp/WO3/GO nano-composite exhibits an 22.8 nm grain size. FESEM introduces grain shape deviation via changing HAp additives from semi-spherical to needle-like with macro-porous structure within the composite of HAp/WO3/GO. Moreover, compatibility of ternary nanocomposite inside the PLA environment led to decrease in the contact angle of the nanofibrous structure reached 16.5 ± 4.4°. The combination of HAp and organic/inorganic elements might represent a good characterization for the target biological applications. HAp/WO3/GO composite exhibits a significant improvement in anti-microbial activity to be around 85.6 ± 1.6 % and 110.1 ± 2.0 % in for E. coli and S. aureus.

Properties of sintered zinc hydroxyapatite bioceramic prepared using waste chicken eggshells as calcium precursor

In the recent years, research on the development of hydroxyapatite (HA) using calcium from natural resources such as limestone, mammalian bones, marine shells and avian eggshells have been extensively studied. However, many studies focused on the properties of prestine HA without incorporation of dopants for strengthening effect. In this work, HA bioceramic was prepared using waste chicken eggshells calcium with addition of various concentrations of zinc dopant (1, 3 and 5 mol% Zn). In this work, the zinc-doped HA (ZnHA) was synthesized using a wet-chemical precipitation technique followed by oven drying and unixial pressing to formed green compacts. Pressureless sintering was carried out at 1200, 1250 and 1300 °C. The results showed that 5 mol% ZnHA (5ZnHA) exhibited the overall best properties after sintering at 1250 °C. The improvement in the fracture toughness was attributed to the formation of β-TCP phase when zinc ion was incorporated into HA, combined with enhanced densification. It was observed that the HA grains were coarser and more densely when sintered at 1250 °C, indicating that there was strong interaction between pores and grain boundaries. However, fracture toughness slightly declined after sintering at 1300 °C due to rapid and abnormal HA grain growth.

Effect of wet‐jet milling on the c‐axis orientation of hydroxyapatite ceramics fabricated using a strong magnetic field

AbstractControlling the crystal orientation of hydroxyapatite, an inorganic material that is a major component of human hard tissues is important to fabricate better artificial bones and artificial tooth roots. To obtain highly oriented hydroxyapatite ceramics under a strong magnetic field, a good dispersion of the raw materials in the slurry must be obtained. This study investigates the effect of wet‐jet milling of a slurry on the orientation of hydroxyapatite ceramics fabricated using a strong magnetic field. Although the prolonged ball milling with ZrO2 balls of the raw powder fractures the primary particles of hydroxyapatite, wet‐jet milling is used to successfully pulverize agglomerated hydroxyapatite raw powder without changing the morphology of the primary particles. Evidently, ceramics with a highly oriented c‐axis of hydroxyapatite are obtained by molding the wet‐jet milled slurry in a strong rotating magnetic field. They exhibit anisotropy in fracture toughness, and the fracture toughness calculated from the crack length perpendicular to the rotating axis is higher than that calculated from the crack length parallel to it. Furthermore, these values are higher than those of randomly oriented hydroxyapatite ceramics, which may be owing to the crystal orientation dependence of the fracture toughness of the hydroxyapatite grains and grain boundaries.

Optimizing parameter for electrophoretic deposition of hydroxyapatite coating with superior corrosion resistance on pure titanium

Bioactive hydroxyapatite (HA) coating is applied on a commercially pure Ti by electrophoretic deposition (EPD). Optimizing the coating structure is necessary to obtain a stable layer and the best corrosion protection. The EPD was conducted at a constant voltage of 20, 30, and 40 V for 30 min in a HA/DMF (dimethylformamide) suspension. Uniform HA layers with a Ca/P ratio of 1.82 were successfully deposited on the Ti surface. The layers, which consisted of HA grains with the size of 1–5 μm, exhibited a gradual increase in thickness of 32, 50, and 60 μm with formation voltage. For the biomedical application, the suitable coating thickness was at least 50 μm. The high compaction of HA grains deposited at 30 V led to an order magnitude higher polarization resistance and ten times lower corrosion current density relative to the other specimens. The porous HA layer formed at 20 V, and the presence of cracks in the 40 V-coating led to a lower corrosion resistance relative to the 30-V coating. The 20 V- and 30 V-coatings remained intact and triggered the deposition of HA during immersion in simulated body fluid for 28 days, while the 40 V-coating dissolved into the solution. The optimum EPD voltage for depositing a stable HA coating with reasonable coating thickness and the best corrosion resistance was 30 V.

Impact of rGO-coated PEEK and lattice on bone implant

The composite coating can effectively inhibit bacterial proliferation and promote the expression of bone-building genes in-vitro. Therefore, a novel production was used to produce poly-ether-ether-ketone, and reduced graphene oxide (PEEK-rGO) scaffolds with ratios of 1–3%, combining a different lattice for a bone implant. An inexpensive method was developed to prepare the new coatings on the PEEK scaffold with reduced graphene oxide (rGO). Mechanical testing, data analysis and cell culture tests for in-vitro biocompatibility scaffold characterisation for the PEEK composite were conducted. Novel computation microanalysis of four-dimensional (4D) printing of microstructure of PEEK-rGO concerning the grain size and three dimensional (3D) morphology was influenced by furrow segmentation of grains cell growth on the composite, which was reduced from an average of 216–155 grains and increased to 253 grains on the last day. The proposed spherical nanoparticles cell grew with time after dispersed PEEK nanoparticles in calcium hydroxyapatite (cHAp) grains. Also, the mechanical tests were carried out to validate the strength of the new composites and compare them to that of a natural bone. The established 3D-printed PEEK composite scaffolds significantly exhibited the potential of bone implants for biomimetic heterogeneous bone repair.

Microstructure, morphology and physicochemical properties of nanocomposites containing hydroxyapatite/vivianite/graphene oxide for biomedical applications.

Designing a nanocomposite that accumulates biocompatibility and antimicrobial behaviour is an essential requirement for biomedical applications. Hydroxyapatite (HAP), graphene oxide, and vivianite in one ternary nanocomposite with three phases and shapes led to an increase in cell viability to 97.6% ± 4 for the osteoblast cells in vitro. The obtained nanocomposites were investigated for their structural features using X-ray diffraction, while the microstructure features were analyzed using a scanning electron microscope (SEM) and a transmission electron microscope. The analysis showed a decrease in the crystal size to 13 nm, while the HAP grains reached 30 nm. The elongated shape of vivianite reached 200 nm on SEM micrographs. The monoclinic and hexagonal crystal systems of HAP and vivianite were presented in the ternary nanocomposite. The maximum roughness peak height reached 236.1 nm for the ternary nanocomposite from 203.3 nm, while the maximum height of the roughness parameter reached 440.7 nm for the di-nanocomposite of HAP/graphene oxide from 419.7 nm. The corrosion current density reached 0.004 μA/cm2 . The ferrous (Fe2+ ) and calcium (Ca2+ ) ions released were measured and confirmed. Therefore, the morphology of the nanocomposites affected bacterial activity. This was estimated as an inhibition zone and reached 14.5 ± 0.9 and 13.4 ± 1.1 mm for Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) after 24 h. The increase in viability and the antibacterial activity refer to the compatibility of the nanocomposite in different medical applications.

Characterization, in vitro bioactivity and biological studies of sol‐gel‐derived TiO2 substituted 58S bioactive glass

AbstractBioactive glasses (BGs) have been used for bone formation and bone repair processes in recent years. This study investigated the titanium substitution effect on 58S BGs (Ti‐BGs) 60SiO2‐(36 − X)CaO‐4P2O5‐XTiO2 (X = 0, 3, and 5 mol.%) prepared by the sol‐gel technique, and the main goal was to find the optimum amount of titanium in Ti‐BGs. Synthesized BGs, which were investigated after immersion in simulated body fluid (SBF), were tested by X‐ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy. Moreover alkaline phosphate (ALP) activity, 3‐(4,5dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) assay, and antibacterial studies were employed to investigate the biological properties of Ti‐BGs. According to the FTIR and XRD test results, hydroxyapatite (HA) formation on Ti‐BGs surfaces was confirmed. Meanwhile, the presence of 5 mol.% compared to 3 mol.% increased the HA grain distribution and their size on the Ti‐BGs surface. Additionally, MTT and ALP results confirmed that the optimal amount of titanium substitution in BG was 5 mol.%. Since 5 mol.% Ti incorporated BG (BG‐5) had the highest biocompatibility level, antibacterial properties, maximum cell proliferation, and ALP activity among the synthesized Ti‐BGs, it is presented as the best candidate for further in vivo investigations.

Batch preparation of homogeneous size hydroxyapatite based composite ceramic microspheres

A new and precise control printing method was developed for preparing homogeneous microspheres. Hydroxyapatite-sodium alginate (HA-SA) mixed suspension was used as printing material and CaCl2 solution was used as solidifying liquid to prepared ceramic microspheres by pneumatic extrusion printing technology. By adjusting the positive pressure, dwell time, and hydroxyapatite concentration, the influence of each parameter on the shape of printed microspheres was studied, and the suitable preparation parameters were determined. Then, the microspheres with uniform particle size distribution were successfully prepared in large quantities by selecting appropriate printing parameters. Then, the microspheres were dried and sintered at different temperatures. During the drying and sintering process, the microspheres showed obvious shrinkage, but still keep good sphericity. The phases of sintered microspheres are hydroxyapatite and calcium oxide. The microstructure shows typical hydroxyapatite grain characteristics.

The Effects of an Experimental Green Tea Extract Gel on the Surface Roughness of Bleached Teeth with Carbamide Peroxide Gels

Esthetics of the enamel surface is an important factor for a good oral health care. The mastication process could damage the enamel surface by hydroxyapatite layer alteration via local demineralization and micro scratches. We developed an experimental green tea extract gel for enamel restoring treatment. Microstructure features were investigated by SEM microscopy and the surface details were revealed by AFM. The surface roughness and hydroxyapatite grains size and shape were the parameters followed into the investigation. The results show that the best restoring is obtained by the experimental gel leading to surface parameters similar to the healthy enamel better than the values obtained with opalescence carbamide gel.

Fabrication and properties of zirconia/hydroxyapatite composite scaffold based on digital light processing

Zirconia and hydroxyapatite(HA) are two typical implant materials, which have the advantages of excellent mechanical strength and good biological activity respectively. It was found that composite material had good biocompatibility and mechanical strength compared to the single material.In this paper, the porous scaffolds of ZrO2/HA composite were formed by digital light processing (DLP) technology and their performance were evaluated. Cell experiments showed that the addition of HA had a positive effect on cell proliferation and differentiation. Mechanical tests showed that the composite scaffold with 10 wt% HA had the best compressive capacity due to the pinning and bridging effect of a small amount of HA grains. When scaffolds were immersed in the simulated body fluid (SBF), the compressive strengths of the composite scaffolds decreased within the first 14 days and gradually increased after 14 days. The reason for this phenomenon was the degradation of calcium phosphate components and the deposition of apatite. By the 28th day, the compressive strengths of all the composite scaffolds increased to over 20 MPa, close to that of the zirconia scaffolds during the same period (25 MPa). The compressive strengths of all scaffolds met the requirement of cancellous bone during the entire soaking period, and the composite scaffolds have potential application value in bone repair.

Hydroxyapatite/Silver Nanoparticles Powders as Antimicrobial Agent for Bone Replacements

This paper reports a superficial morphological modification of the hydroxyapatite grains obtained by in situ deposition of Ag nanoparticles on natural origin calcium phosphate powder. Ceramic material was prepared in three stage bone treatment, including hydrolysis with a lactic acid, pre-calcination, and proper calcination. Subsequently, the Ag nanoparticles were synthesized by chemical reduction of Ag+ by sodium borohydride in a solution of polyvinylpyrrolidone and in presence of hydroxyapatite. Such-prepared materials were investigated with X-ray diffraction, Fourier-transformed infrared spectroscopy, atomic absorption spectrometry and scanning electron microscopy with energy dispersive spectroscopy. Furthermore, Ca/P molar ratio was calculated and microbiological tests were performed to investigate materials antimicrobial activity. The appearance of Ag nanoparticles located on phosphate surface was confirmed by SEM analysis, and no chemical bonding with hydroxyapatite was recorded by IR and XRD techniques. Additionally, the biological assessment revealed bactericidal effect on Escherichia coli and Staphylococcus aureus, while slightly affected on Enterococcus faecalis viability.

Intrinsic Orientation of Hydroxyapatite Grains on the Surface of Dense Pellets Produced by Uniaxial Pressing

Bioceramics synthesized in laboratories must undergo biological tests that consider the final format and all the processing steps involved in the production of the final device. However, in many cases, the synthesized materials are characterized in formats very different from those that will ultimately be implanted into the body. Once living cells are directly cultured on surfaces, a mismatch between bulk and surface microstructural characterizations often leads to the misinterpretation of biological responses. Therefore, our objective in this work was to determine whether and at what level significant microstructural changes can occur between the surface and the bulk of hydroxyapatite pellets produced by uniaxial pressing followed by calcination. Our results showed that the as-synthesized hydroxyapatite crystals had a preferential orientation along the [0 0 1] direction. The calcination process followed by grain growth inhibited this texture feature. However, on the pellet surfaces, the initial orientation was preserved. At the same time, crystallite growth was inhibited, and the cell unit volume of the crystals on the surface was significantly lower than that of the crystals in the bulk. These results demonstrated that the crystallographic features on the surface of hydroxyapatite pellets can be completely different from those observed for the bulk.