Laminated and functionally graded hydroxyapatite/yttria stabilized tetragonal zirconia composites fabricated by spark plasma sintering

Hydroxyapatite (HA) ceramics have been conventionally strengthened and toughened in the form of composites and coatings. New microstructural designs and processing methodologies are still needed for the improvement of the mechanical properties of HA-based ceramics. This study was to prepare laminated and functionally graded HA/yttria stabilized tetragonal zirconia (Y-TZP) composites by the relatively new process of spark plasma sintering (SPS). The microstructure and the mechanical properties of the laminated and functionally graded composites were studied for possible orthopedic applications. It was found that the laminated and functionally graded HA/Y-TZP composites could be densified at 1200 degrees C within 5 min by the SPS process and the average HA grain size in the composite layers was reduced by half due to the well-dispersed Y-TZP second phase. The HA phase in the composite layers was stable up to 1200 degrees C and the Y-TZP second phase remained the tetragonal zirconia (t-ZrO(2)) phase after being processed at the highest temperature of 1250 degrees C. The laminated and functionally graded HA/Y-TZP composites exhibited much improved mechanical properties compared with the pure HA ceramics; the bending strength of the composites reached about 200 MPa, double the strength of the pure HA ceramics.     

Effect of TiO2-Ag2O additives on the formation of calcium phosphate based functionally graded bioceramics

The combined effect of titanium dioxide and silver oxide on the in situ formation of biphasic calcium phosphate ceramics was investigated. Titania (5-20 mol%) was mixed with pure hydroxyapatite (HA) or HA containing Ag2O (10-20 mol%) and was heated to 900 degrees C for 12 h. The sintered samples were found to contain tricalcium phosphate (beta-TCP) and other phases along with HA depending upon the amount as well as the type of the additives used as evidenced by X-ray powder diffraction (XRD) and fourier transform infrared (FT-IR) spectroscopic studies. Enhanced TCP formation with reduced impurity phases was observed with TiO2-Ag2O addition. In vitro solubility study in phosphate buffer at physiological conditions shows the resorbable nature of these materials. A functionally graded material (FGM) structure was formed by spreading TiO2-Ag2O mixture on the surface of the HA green compact and heating at 900 degrees C. The FGM shows gradient structure of TCP and HA from the surface to the interior of the pellet in addition to titania and silver phases.     

Differences in ceramic-bone interface between surface-active ceramics and resorbable ceramics: a study by scanning and transmission electron microscopy.

The interface between bioactive ceramics and bone was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The materials were apatite-wollastonite-containing glass ceramic (A-W.GC) as a representative surface-active ceramic, and calcite and beta-tricalcium phosphate (beta-TCP) as resorbable ceramics. Particles of these materials, ranging between about 100 microns and 300 microns in diameter, were implanted into rat tibiae, and specimens were prepared for observation at 8 weeks after implantation. Both SEM and TEM demonstrated that A-W.GC was bonded to bone through a thin Ca-P-rich layer consisting of fine apatite crystals apparently different from those of bone in shape, size, and orientation. Collagen fibers of the bone reached the surface of this layer, and chemical bonding between A-W.GC and the bone was speculated. Calcite and beta-TCP, on the other hand, made direct contact with the bone, and no apatite layer was present at the interface. The surfaces of the implants became rough due to degradation, and bone grew into the finest surface irregularities. However, we were unable to demonstrate any continuity of crystals between the resorbable implants and bone by high-resolution TEM. Accordingly, the bonding strength was considered to be mainly attributable to mechanical interlocking.     

A comparative study of ultrastructures of the interfaces between four kinds of surface-active ceramic and bone.

The interfaces between four kinds of surface-active ceramic and bone were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using undecalcified specimens. The materials were Bioglass-type glass (Bioglass), Ceravital-type glass-ceramic (KGS), apatite- and wollastonite-containing glass-ceramic (A-W.GC) and hydroxyapatite (HA). Particles of these materials, ranging between about 100 and 300 microns in diameter, were implanted into rat tibiae, and specimens were prepared for observation at 8 weeks after implantation. All materials were observed to bond to bone through a collagen-free layer consisting of fine apatite crystals distinct from those in bone. The crystals of this apatite layer and those of bone were intermingled at their interface, suggesting chemical bonding. In Bioglass, which had only a glassy phase, several tens of microns of the material surface had changed to such an apatite layer. In KGS and A-W.GC, which had macrocrystals in the glassy phase, an intervening apatite layer about 0.5 micron thick was observed between the materials and bone. Furthermore, fine apatite crystals were also observed among the macrocrystals near the surface of the materials. In HA, which had no glassy phase, an intervening apatite layer was much less distinct and sometimes absent. These differences were considered to be attributable to the differences in chemical composition, crystallization, and solubility of the materials.     

The influence of dispersant concentration on the pore morphology of hydroxyapatite ceramics for bone tissue engineering

There is a clinical need for synthetic scaffolds that will promote bone regeneration. Important factors include obtaining an optimal porosity and size of interconnecting windows whilst maintaining scaffold mechanical strength, enabling complete penetration of cells and nutrients throughout the scaffold, preventing the formation of necrotic tissue in the centre of the scaffold. To address this we investigated varying slip deflocculation in order to control the resulting porosity, pore size and interconnecting window size whilst maintaining mechanical strength. Hydroxyapatite (HA) porous ceramics were prepared using a modified slip casting process. Rheological measurements of the HA slips were used to identify deflocculation conditions which resulted in changes in the cell and window sizes of the resulting ceramics. Sintered ceramics were characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Pore and window size distribution was determined by SEM. XRD analysis confirmed that the crystal structure remained HA after the sintering process. SEM showed that HA porous ceramics presented a highly interconnected porous network with average pore sizes ranging from 391+/-39 to 495+/-25 microm. The average window size varied from 73+/-5 to 135+/-7 microm. Pore diameters obtained were controllable in the range 200-500 microm. Window sizes were in the range 30-250 microm. The use of dispersant concentration allows pore and window size to be modified whilst maintaining control over porosity demonstrated by a porosity of 85% for seven different dispersant concentrations. The advantage of this approach allows the correlation between the rheological conditions of the slip and the resultant sintered ceramic properties. In particular, optimising the ceramic strength by controlling the agglomeration during the casting process.     

Tensile strength of the interface between hydroxyapatite and bone.

Tensile strength of the interface between hydroxyapatite (HA) and bone was tested. Scanning electron microscopy was used to observe the tensile failure mode and the morphological change of hydroxyapatite ceramic surface in bone. The porosity of hydroxyapatite is 14% and pore size less than 2 microns. After 2 weeks of implantation, the tensile strength of the interface is 0.72 MPa. After 4, 8, and 16 weeks, the average tensile strength stayed at 1.5 MPa. SEM showed that tensile failure occurred at the HA-bone interface at the second week, but after 4 weeks, the failure occurred between HA particles within the bulk, and not at the HA-bone interface. Calcified tissue was directly deposited on the HA ceramic surface and exits also in the micropores. Near the interface, sintered necks among HA ceramic particles were subjected to biodegradation.     

Characterization of functionally graded hydroxyapatite/titanium composite coatings plasma-sprayed on Ti alloys

Bioceramic coatings like hydroxyapatite (HA) have shown promising bioactive properties in load-bearing implant applications. The aim of this work is to deposit functionally graded HA/Ti layers consisting of an underlying Ti bond coat, the alternating layer, and an HA top-layer on Ti6Al4V substrates using plasma spray to improve the coating-substrate interface properties. The alternating layers were created by means of changing the feeding rate and input power of Ti and HA powders, which gradually decrease Ti content with increasing depth from the Ti bond-coat. The major consideration is to examine the stability of the graded coatings. Experimental results indicated that surface chemistry and morphology of the graded coatings were similar to those of monolithic HA coatings. The bond strength values of the as-sprayed graded coatings were much superior to those of monolithic HA coatings. The cyclic fatigue did have a statistically significant effect on bond strength of monolithic HA coatings, with a decrease of 23%. However, the graded coatings were able to survive 1 million cycles of loading in air without significantly reduced bond strength. The in vitro electrochemical measurement results also indicated that the graded coatings had a more beneficial and desired behavior than monolithic HA coatings after fatigue.     

Electrical characterization of hydroxyapatite-based bioceramics

This paper studies the AC conductivity and permittivity of hydroxyapatite (HA)-based ceramics from 0.1 Hz-1 MHz at temperatures from room temperature to 1000 degrees C. HA-based ceramics were prepared either as dense ceramics or in porous form with interconnected porosity and were sintered in either air or water vapour. Samples were thermally cycled to examine the influence of water desorption on AC conductivity and permittivity. Surface-bound water was thought to contribute to conductivity for both dense and porous materials at temperatures below 200 degrees C. At temperatures below 700 degrees C the permittivity and AC conductivity of HA was also influenced by the degree of dehydration and thermal history. At higher temperatures (700-1000 degrees C), bulk ionic conduction was dominant and activation energies were of the order of approximately 2 eV, indicating that hydroxyl ions are responsible for conductivity.     

Enhanced functions of osteoblasts on nanophase ceramics

Select functions of osteoblasts (bone-forming cells) on nanophase (materials with grain sizes less than 100 nm) alumina, titania, and hydroxyapatite (HA) were investigated using in vitro cellular models. Compared to conventional ceramics, surface occupancy of osteoblast colonies was significantly less on all nanophase ceramics tested in the present study after 4 and 6 days of culture. Osteoblast proliferation was significantly greater on nanophase alumina, titania, and HA than on conventional formulations of the same ceramic after 3 and 5 days. More importantly, compared to conventional ceramics, synthesis of alkaline phosphatase and deposition of calcium-containing mineral was significantly greater by osteoblasts cultured on nanophase than on conventional ceramics after 21 and 28 days. The results of the present study provided the first evidence of enhanced long-term (on the order of days to weeks) functions of osteoblasts cultured on nanophase ceramics; in this manner, nanophase ceramics clearly represent a unique and promising class of orthopaedic/dental implant formulations with improved osseointegrative properties.     

Preparation and properties of porous microspheres made from borate glass

Dysprosium lithium-borate glass microspheres and particles, ranging from 45 to 150 microm in diameter, were reacted with a 0.25 M phosphate solution at 37 degrees C, whose pH was either 3 or 8.8. The glass reacted nonuniformly and was converted into a porous, amorphous, hydrated, dysprosium phosphate reaction product. The amorphous product had the same volume and shape (pseudomorphic) as the unreacted glass, and could be dried without cracking. After heating at 300 degrees C for 1 h, the amorphous reaction product had a specific surface area of approximately 200 m(2)/g, a pore size of approximately 30 nm, and nominal crushing strength of approximately 10 MPa. When the reaction product was heated to 600 degrees C for 15 min, the specific surface area decreased to approximately 90 m(2)/g and the nominal crushing strength increased to 35 MPa. Heating above 615 degrees C converted the amorphous dysprosium phosphate product into crystalline DyPO(4), which contained open porosity until heated above 800 degrees C for 15 min. Highly porous materials of different chemical composition can be prepared by chemically reacting a borate-based glass with an aqueous solution at low-temperature (<100 degrees C). These highly porous materials are easy to process, and are considered candidates for controlled drug delivery, catalysis, chromatographic separation, filtration, and as bioactive materials.     

3D microenvironment as essential element for osteoinduction by biomaterials

In order to unravel the mechanism of osteoinduction by biomaterials, in this study we investigated the influence of the specific surface area on osteoinductive properties of two types of calcium phosphate ceramics. Different surface areas of the ceramics were obtained by varying their sintering temperatures. Hydroxyapatite (HA) ceramic was sintered at 1150 and 1250 degrees C. Biphasic calcium phosphate (BCP) ceramic, consisting of HA and beta-tricalcium phosphate (beta-TCP), was sintered at 1100, 1150 and 1200 degrees C. Changes in sintering temperature did not influence the chemistry of the ceramics; HA remained pure after sintering at different temperatures and the weight ratio of HA and beta-TCP in the BCP was independent of the temperature as well. Similarly, macroporosity of the ceramics was unaffected by the changes of the sintering temperature. However, microporosity (pore diameter <10 microm) significantly decreased with increasing sintering temperature. In addition to the decrease of the microporosity, the crystal size increased with increasing sintering temperature. These two effects resulted in a significant decrease of the specific surface area of the ceramics with increasing sintering temperatures. Samples of HA1150, HA1250, BCP1100, BCP1150 and BCP1200 were implanted in the back muscles of Dutch milk goats and harvested at 6 and 12 weeks post implantation. After explantation, histomorphometrical analysis was performed on all implants. All implanted materials except HA1250 induced bone. However, large variations in the amounts of induced bone were observed between different materials and between individual animals. Histomorphometrical results showed that the presence of micropores within macropore walls is necessary to make a material osteoinductive. We postulate that introduction of microporosity within macropores, and consequent increase of the specific surface area, affects the interface dynamics of the ceramic in such a way that relevant cells are triggered to differentiate into the osteogenic lineage.     

High release of antibiotic from a novel hydroxyapatite with bimodal pore size distribution

We developed a novel hydroxyapatite (HA) cylinder (HA-A) and compared the slow release of antibiotic in vitro as well as osteoconduction of the material in vivo to a commercially produced porous hydroxyapatite cylinder (HA-B). HA-A (4 x 4 mm) was synthesized by mixing HA powder, gelatin, and vegetable oil. The material had a bimodal pore size distribution, with intragranular (10 nm to 10 microm) and intergranular (100 microm) pores, and porosity of 40 vol %, while HA-B had pore sizes ranging from 50 to 300 microm and identical porosity. In vitro drug release was tested using antibiotics (isepamicin sulfate, vancomycin hydrochloride, and flomoxef sodium) soaked on the HA cylinders using a vacuum system. The mean adsorption efficiency was higher for HA-A (46%) than for HA-B (26%) and higher levels of antibiotic were released from HA-A. Of the antibiotics, ISP showed the longest release duration. Bone ingrowth into the pores was observed for both materials. Because the novel HA showed both the slower release of antibiotic (nanosize pores) and supported excellent osteoconduction (microsize pores), it could be useful for the treatment of osteomyelitis.     

结构仿生羟基磷灰石多孔陶瓷的制备及其性能研究*

目的结合浸渍法和注浆成型工艺制备具有内疏外密天然骨结构仿生羟基磷灰石(hydroxyapatite,HA)多孔陶瓷。方法观察多孔陶瓷的截面形貌,表征其相组成,并测试多孔陶瓷的孔隙率和压缩强度。结果结构仿生HA多孔陶瓷的外层是孔隙率为17%的陶瓷块体,芯部为孔隙率在44%~92%的三维连通多孔陶瓷,内外两部分的结合良好。结论通过浸渍次数可以控制芯部的孔隙率,其压缩强度主要依赖于外层的厚度,当外层厚度为4mm,芯部孔隙率为44%时,压缩强度可达40.3MPa;其生物降解性适中,14天时Ca2+的溶出速率开始减缓。     

Biodegradation and bioabsorption innovation of the functionally graded bovine bone-originated apatite with blood permeability

Bioabsorbable and functionally graded apatite (fg-HAp) ceramics were designed using bovine bone by the calcination and partial dissolution-precipitation methods. The fg-HAp ceramics that were developed had gradual distributions of the degree of crystallinity and the grain size of single-phase hydroxyapatite from the surface layer of the pore wall to the bulk structure region. Calcination at 1073 K gave a specific surface area of 30 m2 x g-1 and porosities of 60-80%. The pore structure of the fg-HAp was classified into two regions: a macro-pore region (100-600 microm) originating from spongy bone and a micro-pore region (10-160 nm) related to body fluid permeation and blood permeability. By implantation in subcutaneous tissue of rat, it was confirmed that body fluid permeated the bulk region of the fg-HAp ceramics through the micro-pores. The volumetric populations occupied by body fluid were 60% at 4 weeks and 68% at 8 weeks in the ceramics explants, indicating drastic bioabsorption, although the body fluid was found to be immunopositive for an albumin as the main serum protein in blood. On the fg-HAp ceramics developed here, the bioabsorption rate could be controlled by careful selection of the calcination temperature. These ceramics can be applied as new biomimetic ceramics exhibiting surface and bulk degradations and cellular absorption by giant cells.     

The role of hydroxyapatite as solid signal on performance of PCL porous scaffolds for bone tissue regeneration

Highly porous composites made up of biodegradable poly-epsilon-caprolactone (PCL) and stoichiometric hydroxyapatite (HA) particles have been developed as substrate for bone-tissue regeneration. The processing technique consists of phase inversion and particulate (salt crystals) leaching. Three different HA contents (13, 20 and 26 vol %) in PCL-based composite were considered in this study. Pore microstructure with fully interconnected network and pore sizes ranging around a few hundred of mum (macroporosity) was obtained as a result of salt particles removal by leaching process. Several microns (microporosity) porosity was also created through phase inversion of polymer solution. Total porosity up to 95% was achieved. Human marrow stromal cells (MSC) were seeded onto porous PCL-based composites for 1-5 weeks and cultured in osteogenic medium. MSC were able to adhere and grow on PCL-based substrates with a plateau at 3-4 weeks. However, the small effect of bioactive signals on the biological response evaluated in MSC cell culture suggests a prior role of topography on the biological response. Importantly, the presence of HA as a bioactive solid signal determines an increase of mechanical properties. On the overall, the results indicated that porous PCL-based composites are potential candidate for bone substitution with beneficial influence on structural characteristics by solid signal addition.     

Preparation and characterization of porous PDLLA/HA composite foams by supercritical carbon dioxide technology

A composite poly(D,L)lactic acid (PDLLA)/hydroxyapatite (HA) biomaterial was prepared by in situ polymerization of D,L-lactide monomer and HA. Supercritical CO2 (SC CO2) technology was developed to prepare the biodegradable composite foams for use in tissue regeneration. In this technology, NaCl particles were used as porogen to produce an open-pore structure. Organic solvents were not used and high temperature was not necessary. The problem with pore interconnectivity was resolved. High-porosity composite foams (up to 90% +/- 2% porosity) were obtained with pore sizes ranging from 100 to 300 microm suitable for cell seeding. The microstructure and morphology of the composite foams could be controlled by saturation pressure, saturation time, and temperature as well as amount of NaCl particles. The compressive strength and water absorbability of the composite foams were also determined. With an increase in HA amount, the molecular weight of PDLLA/HA composite foams decreased, but the mechanical strength and hydrophilicity increased slightly.     

Preparation and properties of inhomogeneous hydroxyapatite ceramics.

Inhomogeneous ceramics of hydroxyapatite (HA) were prepared by sintering briquettes in which an inhomogeneous distribution of density was made by pressing HA powder into a die with rough walls. The resulting sample of such a ceramic is a hard thin shell with a loose core, and it is characterized by an inhomogeneous macro- and microstructure. It is a sintered conglomerate from HA grains containing grain boundary macropores in contact with the surface and micropores located inside the grains, part of which are also associated with the free surface. The highest value of microhardness is fixed on the surface of the sample. The radial distribution of microhardness in the (cylindrical) sample has an axisymmetric, nonmonotonic character and, on the whole, shows the decrease of microhardness (the increase of porosity) from the surface to the center. The highest values of microhardness, crushing strength, and fracture strength are close to those known for ceramics of moderate strength.     

Marrow cell induced osteogenesis in porous hydroxyapatite and tricalcium phosphate: a comparative histomorphometric study of ectopic bone formation

To investigate the bone formation ability of porous hydroxyapatite (HA) and tricalcium phosphate (TCP), ceramic discs were implanted with or without rat marrow cells into subcutaneous sites in syngeneic rats. The discs of HA and TCP had identical microstructures: pore size was 190-230 microns, porosity was 50-60%, and they were fully interconnected. Implants without marrow cells (discs themselves) did not show bone formation, whereas implants with marrow cells showed bone formation in the pores of the ceramics. The bone formation of both HA and TCP occurred initially on the surface of the ceramic and progressed towards the center of the pore. The de novo bone was quantitated from decalcified serial sections of the implants. One month after implantation with marrow cells, the percentage fractions of the pore area filled with bone for implanted HA and TCP were 16.9 and 15.1, respectively. At 2 months after implantation with marrow cells, the fractions of bone were 34.3 and 30.9, respectively. These results indicate that both HA and TCP ceramics can show comparable osteogenic ability in the presence of marrow cells.     

Magnesia-doped HA/beta-TCP ceramics and evaluation of their biocompatibility

The sintering behavior, mechanical properties and biocompatibility of magnesia (MgO)-doped HA/TCP biphasic ceramics were studied. Pure HA/TCP ceramics showed poor sinterability due to the phase transformation of beta- to alpha-TCP. MgO-doped HA/TCP ceramics showed high density without any phase transformation of beta-TCP up to 1300 degrees C, for MgO dopants incorporated into the beta-TCP preferably and increased thermal stability of beta-TCP. However, the addition of MgO higher than a critical content, suppressed grain growth of HA/TCP ceramics and lowered sinterability. The optimum amount of MgO doping was 1 wt%, which lead 99% relative density and higher mechanical properties than HA or beta-TCP ceramics. From in vitro test and in vivo test, 1 wt% MgO-doped HA/TCP ceramics showed a good biocompatibility without cytotoxicity. After implantation under the muscle of rabbits, beta-TCP phase was dissolved from the surface and a biological apatite covered the surface. These results proved that MgO addition increased drastically the sintering and mechanical properties of HA/beta-TCP ceramics without altering the biological safety and biocompatibility of the original composite.     

In vitro flexural properties of hydroxyapatite and self-reinforced poly(L-lactic acid)

Poly(L-lactic acid) (PLLA) has been used for fracture fixation devices, but its use is limited because of its poor biocompatibility and mechanical properties. The effects of extrusion, incorporation of hydroxyapatite (HA) and self-reinforced composites (SRCs) on the resultant mechanical properties of PLLA were examined. Samples were conditioned for up to 52 weeks in PBS at 37 degrees C. Extrusion did not adversely affect the mechanical properties of PLLA. After in vitro conditioning, a slight but significant reduction in the strain to failure and modulus was seen. HA (10-40%) by weight was evenly distributed into PLLA using an intermeshing twin-screw extruder. As ceramic content increased, the initial modulus increased but flexural strength decreased. After immersion, the modulus of all HA-PLLA blends was lower than PLLA. HA particles did not form a strong bond with the PLLA, which promoted easier degradation of the HA-PLLA matrix. SRCs showed a higher modulus and strength when compared to all materials except the modulus of 30 and 40% HA-PLLA composites before immersion. Water preferentially attacked the matrix of the composite, leading to more fiber pullout, but the fiber orientation maintained the advantages in strength and modulus up to 24 weeks in vitro.