Processing of hydroxyapatite reinforced ultrahigh molecular weight polyethylene for biomedical applications

A new method for processing hydroxyapatite/ultrahigh molecular weight polyethylene (HA/UHMWPE) composite has been developed by combining wet ball milling and swelling. Sintered HA particles were ground in ethanol to approximately 50 nm in diameter. The nano-sized HA particles were mechanically mixed with UHMWPE in the ball mill and then compression molded into solid slabs. The slabs were then swollen in a pharmaceutical grade paraffin oil to enhance the UHMWPE chain mobility and HA/UHMWPE interface adhesion before final hot press. The resultant composite exhibits a two-zone network structure formed by a homogeneous HA-rich phase and a UHMWPE-rich phase. This process resulted in a 90% increase in Young's modulus and a 50% increase in the yield strength of HA/UHMWPE composite, comparing with those of unfilled UHMWPE.

A composite coating by electrolysis-induced collagen self-assembly and calcium phosphate mineralization

A composite coating that is composed of collagen protein and calcium phosphate minerals is considered to be bioactive and may enhance bone growth and fixation of metallic orthopedic implants. In this study, we have successfully developed a uniform collagen fibril/octacalcium phosphate composite coating on silicon substrate by electrolytic deposition (ELD). The coating deposition was done through applying a constant potential to the cathode in a three-electrode electrochemistry cell that contain a mild acidic (pH 4.8-5.3) aqueous solution of collagen molecules, calcium and phosphate ions. The coating process involved self-assembly of collagen fibrils and the deposition of calcium phosphate minerals as a result of cathode reaction and local pH increase. The two steps could be synchronized to form a bone-like composite at nanometer scale through proper adjustment of the solution and deposition parameters. Coating morphology, crystal structure and compositions were analyzed by optical and fluorescence microscopy, scanning and transmission electron microscopy, energy dispersive X-ray analysis, inductively coupled argon plasma optical emission spectrophotometry, and Fourier-transformed infrared spectroscopy. Under typical deposition conditions, the cathode (Si) surface formed a thin (100 nm) layer of calcium phosphate coating, on top of which a thick (approximately 100 microm) composite layer formed. The porous composite layer consists of a collagen fibril network on which clusters of octacalcium phosphate crystals nucleate and grow. By combining photolithography and ELD, we were also able to pattern the composite coating into regular arrays of squares. Preliminary results by nanoindentation tests showed that properly prepared composite coating may have higher elastic modulus and scratch resistance than monolithic porous calcium phosphate coating. The results not only provide a novel bioactive coating for biomedical implants, but also establish a new experimental protocol for studying biomineralization mechanisms of collagen based biological tissues.

Synthesis and structure of cerium-substituted hydroxyapatite

The aim of this study was to explore the effect of cerium ions on the formation and structure of hydroxyapatite (HAP). All particles, prepared by hydrothermal method, were synthesized at varied X(Ce) = Ce/(Ca + Ce) (from 0 to 10%) with the atomic ratio (Ce + Ca)/P fixed at 1.67. Their morphology, composition and crystal structure were characterized by TEM, EPMA, XRD and FTIR. The results showed that in this composition range the apatite structure is maintained, Ce3+ ions could enter the crystal lattice of apatite and substitute Ca2+ ions. The doping of Ce3+ ions resulted in the decrease of the crystallite size with increase in X(Ce). The HAP particles without doping were short rods having a diameter from 10 to 20 nm and a length from 30 to 50 nm. They grew into long needles upon increasing X(Ce).

Influence of ferrous iron incorporation on the structure of hydroxyapatite

Iron is a vital element of cellular function within the body. High concentrations of iron can be found in the kidneys and the circulatory system. In bones and teeth it is present as a trace element. The use of iron-based compounds in combination with hydroxyapatite offers a new alternative for prosthetic devices. This work investigates the synthesis and processing of iron containing apatites as a possible new type of ceramic for biomedical devices. Stoichiometric and calcium deficient iron containing apatites were synthesized by a wet chemical reaction with di-ammonium-hydrogen-phosphate, calcium nitrate and a ferrous iron nitrate solution. A secondary phase of tri-calcium-phosphate (TCP) was observed after heat treatment of iron containing, calcium deficient, hydroxyapatite. The apatite structure was maintained after heat treatment of stoichiometric apatite, synthesized in the presence of iron. Sintering in air produced oxidation of Fe2+ to Fe3+, resulting in the formation of hematite as a secondary phase. The introduction of iron into the synthesis of hydroxyapatite causes: (i) an increase of the a-lattice parameter after synthesis and heat treatment in air; (ii) an increase in the c-lattice parameter after sintering in air.

Effect of pH and ionic strength on the reactivity of Bioglass® 45S5

Bioglass 45S5 is a silica-based melt-derived glass, used in medical field as a bone regenerative material because of the deposition of a layer of hydroxy carbonate apatite (HCA) on the surface of the glass when immersed in body fluid. The present paper studies the early steps of reaction of 2-microm sized particles of Bioglass, in solutions buffered with TRIS at different pH, by means of ICP-ES and FTIR spectroscopy. Only at pH 8 could a total reconstruction of the glass be observed, with the formation of both a silica and a calcium phosphate rich layers. At higher pH, selective dissolution of the glass was hindered by the immediate precipitation of a layer of calcium phosphate, whereas at lower pH a total breakdown of the glass occurred and no calcium phosphate precipitation was noted. The use of the ATR-liquid cell allowed the observation of the reaction in real time, and this showed that the process of silica formation is not separable from cation leaching from the glass, as well as the formation of the calcium phosphate rich layer.

Fluorapatite-mullite glass sputter coated Ti6Al4V for biomedical applications

A number of bioactive ceramics have been researched since the development of Bioglass in the 1970's. Fluorapatite mullite has been developed from the dental glass-ceramics used for more general hard tissue replacement. Being brittle in nature, glass-ceramics are currently used mainly as coatings. This paper shows that fluorapatite glass LG112 can be used as a sputtered glass coating on roughened surfaces of Ti6Al4V for possible future use for medical implants. An AFM was used to measure the roughness of the surface before and after coating to determine the change in the topography due to the coating process as this greatly affects cell attachment. The sputter coating partially filled in the artificially roughened surface, changing the prepared topography. Osteoblasts have been successfully grown on the surface of these coatings, showing biocompatibility with bone tissue and therefore potential use in hard tissue repair.

A new way of incorporating silicon in hydroxyapatite (Si-HA) as thin films

Bioactive silicon-containing hydroxyapatite (Si-HA) thin films that can be used as coatings for bone tissue replacement have been developed. A magnetron co-sputtering technique was used to deposit Si-HA films up to 700 nm thick on titanium substrates, with a silicon level up to 1.2 wt%. X-ray diffraction demonstrated that annealing transformed the as-deposited Si-HA films which were amorphous, into a crystalline HA structure. A human osteoblast-like (HOB) cell model was used to determine the biocompatibility of these films. HOB cells were seen to attach and grow well on the Si-HA films, and the metabolic activity of HOB cells on these films was observed to increase with culture time. Furthermore, mineralisation of the cell layers was observed after 8 weeks of culture. Based on the present findings, Si-HA of different film compositions demonstrate bioactive properties in-vitro, and indicate the potential as biocoatings for a wide variety of medical implants including load-bearing applications such as the femoral stem of hip replacement implants.

Dissolution and mineralization of sintered and thermally sprayed hydroxy-fluoroapatites

Hydroxyapatites are commonly used as bone cement, coatings on implants for dental and orthopaedic applications, but also as middle ear implants. These applications all require a different tissue healing response that can be attained by different manufacturing processes or by chemically modifying the composition. During implantation apatites undergo a process of dissolution and mineralization. The degree of dissolution is dependent upon the manufacturing process & is higher for thermally sprayed implant materials. This allows them to integrate to the natural bone. This study tests the dissolution and mineralization of fluoride containing hydroxyapatites through immersion in simulated body fluid. It shows that mineralization occurs more readily in hydroxyapatites than fluorapatites because of their higher dissolution rate. Mineralization was detected most readily by image analysis using scanning electron microscopy than by weight changes using a microbalance. Microscopy allowed small heterogeneous precipitates to be observed during the initial stages of mineralization.

Microporosity enhances bioactivity of synthetic bone graft substitutes

This paper describes an investigation into the influence of microporosity on early osseointegration and final bone volume within porous hydroxyapatite (HA) bone graft substitutes (BGS). Four paired grades of BGS were studied, two (HA70-1 and HA70-2) with a nominal total porosity of 70% and two (HA80-1 and HA80-2) with a total-porosity of 80%. Within each of the total-porosity paired grades the nominal volume fraction of microporosity within the HA struts was varied such that the strut porosity of HA70-1 and HA80-1 was 10% while the strut-porosity of HA70-2 and HA80-2 was 20%. Cylindrical specimens, 4.5 mm diameter x 6.5 mm length, were implanted in the femoral condyle of 6 month New Zealand White rabbits and retrieved for histological, histomorphometric, and mechanical analysis at 1, 3, 12 and 24 weeks. Histological observations demonstrated variation in the degree of capillary penetration at 1 week and bone morphology within scaffolds 3-24 weeks. Moreover, histomorphometry demonstrated a significant increase in bone volume within 20% strut-porosity scaffolds at 3 weeks and that the mineral apposition rate within these scaffolds over the 1-2 week period was significantly higher. However, an elevated level of bone volume was only maintained at 24 weeks in HA80-2 and there was no significant difference in bone volume at either 12 or 24 weeks for 70% total-porosity scaffolds. The results of mechanical testing suggested that this disparity in behaviour between 70 and 80% total-porosity scaffolds may have reflected variations in scaffold mechanics and the degree of reinforcement conferred to the bone-BGS composite once fully integrated. Together these results indicate that manipulation of the levels of microporosity within a BGS can be used to accelerate osseointegration and elevate the equilibrium volume of bone.

Concentrated hydroxyapatite inks for direct-write assembly of 3-D periodic scaffolds

Hydroxyapatite (HA) scaffolds with a 3-D periodic architecture and multiscale porosity have been fabricated by direct-write assembly. Concentrated HA inks with tailored viscoelastic properties were developed to enable the construction of complex 3-D architectures comprised of self-supporting cylindrical rods in a layer-by-layer patterning sequence. By controlling their lattice constant and sintering conditions, 3-D periodic HA scaffolds were produced with a bimodal pore size distribution. Mercury intrusion porosimetry (MIP) was used to determine the characteristic pore size and volume associated with the interconnected pore channels between HA rods and the finer pores within the partially sintered HA rods.

Formation of apatitic calcium phosphates in a Na-K-phosphate solution of pH 7.4

Poorly crystalline, apatitic calcium phosphate powders have been synthesized by slowly adding a Na- and K-containing reference phosphate solution with a pH value of 7.4 to an aqueous calcium nitrate solution at 37 degrees C. Nano-particulated apatitic powders obtained were shown to contain small amounts of Na and K, which render them more similar in chemical composition to that of the bone mineral. Precipitated and dried powders were found to exhibit self-hardening cement properties when kneaded in a mortar with a sodium citrate- and sodium phosphate-containing starter solution. The same phosphate solution used in powder synthesis was found to be able to partially convert natural, white and translucent marble pieces of calcite (CaCO3) into calcium-deficient hydroxyapatite upon aging the samples in that solution for 3 days at 60 degrees C. Sample characterization was performed by using scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, inductively-coupled plasma atomic emission spectroscopy, and simultaneous thermogravimetry and differential thermal analysis.

Comparison between the in vitro surface transformations of AP40 and RKKP bioactive glasses

Two bioactive silica-phosphate glasses, AP40 and RKKP, were compared in their behaviour in simulated biological environment. Their chemical composition is practically identical, except that RKKP contains small amounts of amphoteric network-former oxides Ta2O5 and La2O3 (composition in wt% for AP40: beta-Ca3(PO4)2 24.50, SiO2 44.30, CaO 18.60, Na2O 4.60, K2O 0.19, MgO 2.82, CaF2 4.99; RKKP: beta-Ca3(PO4)2 24.23, SiO2 43.82, CaO 18.40, Na2O 4.55, K2O 0.19, MgO 2.79, CaF2 4.94, Ta2O5 0.99, La2O3 0.09). Previous investigations showed a better performance in osteopenic bone for RKKP. To gain more insight into these differences in biological behaviour, the in vitro bioactivity of the glasses was studied by treatment with a continuously replenished Hanks' Balanced Salt Solution (HBSS). The glasses were examined before and after HBSS treatment for 20 and 40 days by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Energy Dispersion (EDX), Raman and IR vibrational spectroscopies. Some slight but notable differences between the two glasses were observed after HBSS treatment. IR and EDX analyses showed that deposits formed on both glasses were composed of a calcium deficient carbonate-apatite; however, the layer formed on RKKP glass was found to be slightly more calcium deficient and thinner. EDX analysis evidenced the presence of a small percentage of F- ions only in the layers formed on the RKKP samples. The differences disclosed, although slight, can contribute to the understanding of the different biological behaviour previously observed.

Synthesis and characterization of fluorine-containing hydroxyapatite by a pH-cycling method

Different fluorine-containing hydroxyapatite (FHA) powders were synthesized through a pH-cycling method by varying sodium fluoride (NaF) concentration in hydroxyapatite (HA) suspensions. The powders were then calcined at 1200 ( composite function)C for 1 h. Both uncalcined and calcined powders were characterized using X-ray diffraction (XRD), Fourier transform infra-red (FTIR), and F-electrode. It was discovered that fluorine incorporation increased with the fluorine content in the initial solution and the number of pH cycles employed. A relatively low fluorine incorporation efficiency, approximately 60%, was attained for most of the FHA samples, and it did not vary significantly after calcination. It was also revealed that the FHA particles produced by the pH-cycling method were inhomogeneous. They were a mixture of hydroxyapatite and F-rich apatite (or FA) particles. After calcination, however, these FHA particles were homogenized and became single phased FHA.

Search for ideal biomaterials to cultivate human osteoblast-like cells for reconstructive surgery

In this study we cultured human osteoblast-like cells on 16 different biomaterials to find an optimal biomaterial for subsequent use in reconstructive surgery. The tested biomaterials can be divided into five groups: collagen-based membranes of bovine, equine or calf origin, tricalcium phosphate based membranes (alpha and beta), hyaluronic acid based, anorganic bovine bone and anorganic silicone-based membranes. Cell proliferation and cell colonization (Environmental Scanning Electron Microscope, ESEM) analysis were performed. The results of the study demonstrated that four of the examined biomaterial/cell constructs showed a very good proliferation rate and cell density: No. 3 (Tissue Vlies), No. 7 (Sepra film), No. 16 (Biobrane) and No. 17 (Biomend). No favourable group of biomaterials was noticeable. Moreover, the results indicate that these four biomaterials as a part of bone constructs are the best tools for engineering new bone tissue. In contrast, biomaterials No. 19a (Bio-Oss) and 19b (Bio-Oss Collagen) showed the lowest proliferation rates. The result of No. 19b was improved by treatment in the perfusion chamber for 48 h as well as by additional use of vacuum. The present study is an important base for further analysis of biomaterials and consequently for the development of tissue engineering.

Dynamic study of calcium phosphate formation on porous HA/TCP ceramics

Bone-like apatite formation on porous calcium phosphate ceramics was investigated in static simulated body fluid (SBF) and dynamic SBF at different flowing rates. The results of a 14-day immersion in static SBF showed that the formation of bone-like apatite occurred both on the surface and in the pores of the samples. When SBF flow at the physiological flow rate in muscle (2 ml/100 ml min1), bone-like apatite could be detected only in internal surface of the pores of samples. The result that bone-like apatite formation could only be found in the pores when SBF flown at physiological flow rate was consistent with that of porous calcium phosphate ceramics implanted in vivo: osteoinduction was only detected inside the pores of the porous calcium phosphate ceramics. This result implicates that the bone-like apatite may play an important role in the osteoinduction of Ca-P materials. The dynamic model used in this study may be better than usually used static immersion model in imitating the physiological condition of bone-like apatite formation. Dynamic SBF method is very useful to understand bone-like apatite formation in vivo and the mechanism of ectopic bone formation in calcium phosphate ceramics.

Characterization and bond strength of electrolytic HA/TiO2 double layers for orthopedic applications

Insufficient bonding of juxtaposed bone to an orthopedic/dental implant could be caused by material surface properties that do not support new bone growth. For this reason, fabrication of biomaterials surface properties, which support osteointegration, should be one of the key objectives in the design of the next generation of orthopedic/dental implants. Titanium and titanium alloy have been widely used in several bioimplant applications, but when implanted into the human body, these still contain some disadvantages, such as poor osteointegration (forming a fibrous capsule), wear debris and metal ion release, which often lead to clinical failure. Electrolytic hydroxyapatite/titanium dioxide (HA/TiO2) double layers were successfully deposited on titanium substrates in TiCl4 solution and subsequently in the mixed solution of Ca(NO3)2 and NH4H2PO4, respectively. After annealing at 300 degrees C for 1 h in the air, the coated specimens were evaluated by dynamic cyclic polarization tests, immersion tests, tensile tests, surface morphology observations, XRD analyses and cells culture. The adhesion strength of the HA coating were improved by the intermediate coating of TiO2 from 11.3 to 46.7 MPa. From cell culture and immersion test results, the HA/TiO2 coated specimens promoted not only cells differentiation, but also appeared more bioactive while maintaining non-toxicity.

Crystallization behavior of silica-calcium phosphate biocomposites: XRD and FTIR studies

Silica and calcium phosphates (CaP) are the most important ingredients in bioactive materials that bond to bone and enhance bone tissue formation. In this study, silica-calcium phosphate (SiO2-CaP) composites were developed by powder metallurgy method, using silica (SiO2) and anhydrous dicalcium phosphate (CaHPO4) powders (CaP) in the ratios (wt%): 20/80, 40/60, 60/40 and 80/20. The effects of temperature and chemical composition on crystallization and phase transformation of the SiO2-CaP composites were evaluated by XRD and FTIR. Thermal treatment of the starting material suggested that CaHPO4 transforms into: gamma-Ca2P2O7 at 800 degrees C; beta-Ca2P2O7 at 1000 degrees C and alpha-Ca2P2O7 at 1200 degrees C. On the other hand, beta-quartz was the only detected phase after thermal treatment of silica in the temperature range 800-1200 degrees C. For all SiO2-CaP composites, SiO2 and CaP did not modify the crystallization behavior of each other when sintered in the temperature range 800-1000 degrees C. However, at 1200 degrees C, CaP promoted the transformation of gamma-quartz into alpha-cristobalite. Moreover, SiO2 stabilized beta-Ca2P2O7. The modifications in the crystallization behavior were related to ion substitution and formation of solid solutions.

The effect of partially stabilized zirconia on the mechanical properties of the hydroxyapatite-polyethylene composites

The effect of partially stabilized zirconia (PSZ) on the mechanical properties of the hydroxyapatite-high density polyethylene composites was studied by investigating the effect of hydroxyapatite and the simultaneous effect of hydroxyapatite and PSZ volume fractions on fracture strength, modulus of elasticity, and absorbed energy in the composite samples. The results showed a decrease in fracture strength, and absorbed energy with an increase in the volume fraction of hydroxyapatite content in the hydroxyapatite-polyethylene samples. Partial replacement of hydroxyapatite with PSZ particles was beneficial in the improvement of both the fracture strength and failure energy values in the composite samples. A transition from ductile to brittle behavior was observed as the volume fraction of ceramic filler particles increased in the samples.

Predicting graded young modulus values of Ti alloys modified by ion implantation

There has been, and is still, concern about the high elastic modulus of Ti alloys compared to bone. Any reduction in the Young's modulus value of the implant is expected to enhance stress redistribution to the adjacent bone tissues, minimize stress shielding and eventually prolong device lifetime. Dynamic Monte Carlo simulation is used to predict the gradual reduction in Young's modulus values between the bulk of Ti alloys and the modified surface layers due to Ca ion implantation. The simulation can be used as a screening step when applying new alloys and/or coatings.

Synthesis of nano-sized hydroxyapatite

In this work, nanometer HA crystals have been synthesized via wet chemical precipitation and characterized. This research studies how key synthesis parameters affect the size and phase purity of the produced HA. Characterization work was carried out using X-ray powder diffraction method and scanning electron microscopy for phase identification and particle sizing, respectively.

HAp physical investigation--the effect of sintering temperature

The paper presents the effect of sintering temperature on the physical properties of porous hydroxyapatite (HAp In this study, the HAp was prepared using polymeric sponge techniques with different binder concentrations. The sintering process was carried out in air for temperature ranging from 1200 degrees C to 1600 degrees C. Different physical properties namely density and porosity were observed at different sintering temperatures. The HAp prepared with higher PVP binder showed a slightly decreased in apparent density with increasing sintering temperature, while those HAp prepared with lower PVP showed a slightly increase in apparent density with increasing sintering temperature. The total porosity was found to be approximately constant in the whole sintering temperature range. However, closed porosity decreases with increasing sintering temperature for HAp prepared by lower binder concentration. On the other hand, the HAp prepared by higher binder concentrations showed increasing closed porosity with increasing sintering temperature. Other features such as the influence of sintering temperatures on grain and strut would also be presented in this paper.

Fabrication of synthetic apatites by solid-state reactions

The paper presents a method of producing synthetic Hydroxyapatite (HA) Ca10(PO4)6(OH)2 and other apatites for biological use by solid-state reaction. The solid-state reaction involves mix-grinding dry powders of beta-tricalcium phosphate powder (TCP) and either calcium hydroxide (Ca(OH)2) or calcium carbonate (CaCO3) or combination thereof, from pure commercial chemicals or derived from natural limestone or from seashells, of total calcium/phosphorus molar ratio between 1.5 to 2.0, to particle size of less than 10 microns, and firing the resultant powder to temperature between 600 degrees C - 1250 degrees C in atmosphere or in controlled atmospheric condition. The resultant apatites formed were characterised using XRD, SEM-EDX and FTIR. The presented reaction process was found to be much simpler compared to conventional methods of producing synthetic apatites since it involves only dry mix-grinding of the reactants before firing at high temperatures based on the required levels of purity. It can also produce synthetic apatites with good reproducibility in a shorter time. Thus the presented method has a great industrial value.

Comparative study between dense and porous hydroxyapatite using light and scanning electron microscopy

This in vivo study revealed that porous hydroxyapatite (PHA) and dense hydroxyapatite (DHA) are good implant materials that can accelerate bone healing and resorbed in acceptable time. But there were differences in the mechanism of the resorption of DHA and PHA due to variability in the physical properties and osteogenicity.

Comparison of hydroxyapatite powders derived from different resources

There is a great demand of Hydroxyapatite (HA) material in Orthopaedics and Dental applications due to its similarity to human bone. However, the lack of availability and due to high import cost of this material in Malaysia, research in producing synthetic HA locally is therefore timely. The use of local resources as the raw materials for the production of HA is also desirable in reducing the overall cost of HA. In this study, two HA materials were synthesised from different starting precursors, i.e. commercial pure Ca(OH)2 (HAS) and Ca(OH)2 directly from a local natural limestone deposit (HAL). Whereas a commercially available HA "Captal 60" (HAC) was used as reference. The synthesised powders obtained were fired at 1000 degrees C and at 1250 degrees C. Characterisation evaluations on bulk properties were carried out using XRD, SEM-EDX, ICP and FTIR. The results indicate that both HAS and HAL are comparable to HAC even at 1000 degrees C. Thus, the local natural limestone can be used to form HA. However, the overall appearance of these materials are quite different (HAC - blue, HAS - greenish and HAL - light green). The reasons for this and the subsequent mechanical and bioactive effects of these materials are currently being investigated.