Plasma Sprayed Calcium Phosphate Coatings Research Articles

Osseoconductive and Corrosion-Inhibiting Plasma-Sprayed Calcium Phosphate Coatings for Metallic Medical Implants

During the last several decades, research into bioceramic coatings for medical implants has emerged as a hot topic among materials scientists and clinical practitioners alike. In particular, today, calcium phosphate-based bioceramic materials are ubiquitously used in clinical applications to coat the stems of metallic endoprosthetic hips as well as the surfaces of dental root implants. Such implants frequently consist of titanium alloys, CoCrMo alloy, or austenitic surgical stainless steels, and aim at replacing lost body parts or restoring functions to diseased or damaged tissues of the human body. In addition, besides such inherently corrosion-resistant metals, increasingly, biodegradable metals such as magnesium alloys are being researched for osseosynthetic devices and coronary stents both of which are intended to remain in the human body for only a short time. Biocompatible coatings provide not only vital biological functions by supporting osseoconductivity but may serve also to protect the metallic parts of implants from corrosion in the aggressive metabolic environment. Moreover, the essential properties of hydroxylapatite-based bioceramic coatings including their in vitro alteration in contact with simulated body fluids will be addressed in this current review paper. In addition, a paradigmatic shift is suggested towards the development of transition metal-substituted calcium hexa-orthophosphates with the NaSiCON (Na superionic conductor) structure to be used for implant coatings with superior degradation resistance in the corrosive body environment and with pronounced ionic conductivity that might be utilized in novel devices for electrical bone growth stimulation.

Structural and mechanical characterizations of nano calcium phosphate coatings prepared by induction suspension plasma spray (rf-SPS)

Event Abstract Back to Event Structural and mechanical characterizations of nano calcium phosphate coatings prepared by induction suspension plasma spray (rf-SPS) Frédéric Faivre1, 2, Max Loszach2, Ghislaine Bertrand1, François Gitzhofer2, Joel Alexis3, Jade Pecune3, David Grossin1 and Christian Rey1 1 CIRIMAT, Université de Toulouse, France 2 Université de Sherbrooke, Département de génie chimique et génie biotechnologique, Canada 3 LGP, Université de Toulouse, France Introduction: Among the variety of coating technologies developed to deposit stoichiometric hydroxyapatite, plasma spraying is the most widely used technique implemented by industry to treat prosthetic devices.[1] The good biocompatibility and osteoconductivity of this material have been found to enhance bone healing at early implantation time but long-term stability is still controversial. The mechanisms which are responsible for this change can be related to instability of the coating/bone tissue interface (dissolution at neutral pH, resorption by osteoclasts) or of the coating/implant interface (delamination due to stress, preferential dissolution of amorphous phase, poor initial adhesion).[2] To find an optimal and sustainable integration, a nanostructured substituted hydroxyapatite coating mimicking the complex surface topography (roughness, grain size, porosity) and structure (Ca/P ratio, stoichiometry, crystallinity) of the mineral part of the natural bone was produced implementing the suspension/solution inductively rf coupled plasma spray system.[3] The objective of the present study is to assess the influence of operating conditions (suspension composition, power, pressure, gas composition, spraying distance) on coating/TA6V substrate adherence and coating physical and structural characteristics. Materials and Methods: Hydroxyapatite and substituted hydroxyapatite (Mg,Na,K)xCa10-x(PO4)6(F,Cl)y(OH)2-y coatings have been produced using a 50 kW inductively coupled RF plasma spray system operating at 3 MHz and equipped with a suspension feeder nozzle (Tekna Plasma System Inc.). The structural characteristics were determined by XRD, FTIR and Raman spectroscopies, the chemical compositions by ICP or SAAF and the morphology by SEM. Qualitative and quantitative determination of the phases, including the crystallinity of the coatings has been carried using Rietveld refinement. Micro and nano hardness tests were performed to evaluate the Young modulus and coating cohesion. 3-points bending tests with stiffener were applied to determine the coating adhesion. Results and Discussion: The spraying chamber pressure, the power, the liquid/solid phase ratio in the suspension and the spraying distance have a strong influence on the CaP phases. It was demonstrated that the optimized coating was mainly constituted of HA (91 vol.%) with nanometric sizes (below 100 nm). The secondary phases were (a-TCP, β-TCP, CaO and TTCP) as usually in plasma sprayed calcium phosphate coatings. Furthermore, it was showed that multisubstituted hydroxyapatite coating is achievable by suspension plasma spray. Coating roughness was around 5 µm and could be adapted thanks to substrate surface treatment. Finally mechanical properties of optimized coatings exhibited similar values (bending test Fc=140N, hardness test E=7±2 GPa, H=2.5±1 GPa) to the ones of a commercial HA coating (Fc=100 to 160 N, E=10 GPa, H=1.5 to 3 GPa). Conclusion: Both physical and structural characteristics can be tuned by adjusting the operating plasma spray parameters in order to favor the nanometric grain size, crystallinity (targeted phase HA), adhesion and cohesion as well as complex compositions. We would like to thank the CPCFQ-biennum 2013-2014 for providing financial support to this project during two years.

Bond-coating in plasma-sprayed calcium-phosphate coatings

In the online and printed versions of the original article, there was a typographical error in I. Peker’s name. His name is spelled correctly above. This error occurred in both the html and pdf versions of the online article.

Bond-coating in plasma-sprayed calcium-phosphate coatings

The influence of bond-coating on the mechanical properties of plasma-spray coatings of hydroxyatite on Ti was investigated. Plasma-spray powder was produced from human teeth enamel and dentine. Before processing the main apatite coating, a very thin layer of Al2O3/TiO2 was applied on super clean and roughened, by Al2O3 blasting, Ti surface as bond-coating. The experimental results showed that bond-coating caused significant increase of the mechanical properties of the coating layer: In the case of the enamel powder from 6.66 MPa of the simple coating to 9.71 MPa for the bond-coating and in the case of the dentine powder from 6.27 MPa to 7.84 MPa, respectively. Both tooth derived powders feature high thermal stability likely due to their relatively high content of fluorine. Therefore, F-rich apatites, such those investigated in this study, emerge themselves as superior candidate materials for calcium phosphate coatings of producing medical devices. The methods of apatite powder production and shaping optimization of powder particles are both key factors of a successful coating. The methods used in this study can be adopted as handy, inexpensive and reliable ways to produce high quality of powders for plasma spray purposes.

Effects of Bond-Coatings on Plasma Sprayed Calcium Phosphate Coatings

Effects of Bond-Coatings on Plasma Sprayed Calcium Phosphate Coatings

Effect of spark plasma sintering on the microstructure and in vitro behavior of plasma sprayed HA coatings

The crystalline phases and degree of crystallinity in plasma sprayed calcium phosphate coatings on Ti substrates are crucial factors that influence the biological interactions of the materials in vivo. In this study, plasma sprayed hydroxyapatite (HA) coatings underwent post-spray treatment by the spark plasma sintering (SPS) technique at 500°C, 600°C, and 700°C for duration of 5 and 30 min. The activity of the HA coatings before and after SPS are evaluated in vitro in a simulated body fluid. The surface microstructure, crystallinity, and phase composition of each coating is characterized by scanning electron microscopy and X-ray diffractometry before, and after in vitro incubation. Results show that the plasma sprayed coatings treated for 5 min in SPS demonstrated increased proportion of β-TCP phase with a preferred-orientation in the (2 1 4) plane, and the content of β-TCP phase corresponded to SPS temperature, up to 700°C. SPS treatment at 700°C for 30 min enhanced the HA content in the plasma spray coating as well. The HA coatings treated in SPS for 5 min revealed rapid surface morphological changes during in vitro incubation (up to 12 days), indicating that the surface activity is enhanced by the SPS treatment. The thickest apatite layer was found in the coating treated by SPS at 700°C for 5 min.

Incorporation of tobramycin into biomimetic hydroxyapatite coating on titanium

Calcium phosphate coatings containing an antibiotic were produced on titanium alloy (Ti6Al4V) implants using a biomimetic approach. Thin, amorphous calcium phosphate (ACP) coatings were first deposited onto Ti6Al4V plates by immersion in 5 times concentrated simulated body fluid (SBF), for 24 h at 37°C. The ACP-coated implants were then immersed in a supersaturated calcium phosphate (SCP) solution containing 0, 100, 200, 400, 600 or 800 mg/l of tobramycin for 48 h at 37°C. A carbonated hydroxyapatite (CHA) layer, approximately 40 μm thick, was formed. Approximately 3 μg/mg of tobramycin was co-precipitated with the CHA crystals onto titanium alloy plates, using 800 mg/l tobramycin in the coating solution. For comparison, plasma-sprayed calcium phosphate coatings were also immersed in solutions containing 100, 200, 400 or 1000 mg/l of tobramycin for 10, 40 min, or 48 h. A maximum of about 0.3 μg/mg could be adsorbed onto the plasma-sprayed calcium phosphate coating with the comparable concentration of 800 mg/l in solution. The dissolution of coating and release of tobramycin were also measured in vitro using saline solution buffered at pH 5.0 or 7.3 at 37°C. The release rate of tobramycin was faster at pH 7.3 than at pH 5, with 50 and 4 μg/ml/min, respectively. Tobramycin released from the biomimetic-coated plates could inhibit growth of Staphylococcus aureus bacteria. The result of this study, therefore, indicates that the biomimetic CHA coatings containing antibiotics could be used to prevent post-surgical infections in orthopaedic or trauma.

Bone response to calcium phosphate-coated and bisphosphonate-immobilized titanium implants

Thin calcium phosphate (Ca-P) coatings have been introduced to overcome the shortcomings of plasma-sprayed Ca-P coatings. In our previous experiments, thin Ca-P coatings also enabled the immobilization of bisphosphonate, which is a drug used to treat osteoporosis. The present study was designed to evaluate the bone response to titanium implants treated with a thin Ca-P coating and bisphosphonate. Forty cylindrical commercially pure titanium implants with a length of 7 mm and a diameter of 3 mm were used as test implant fixtures. Three groups of surface-treated implants were prepared: (1) blasted with titanium powder and etched with a solution of 10% HF+5% HNO 3 (control); (2) modified with 0.5-μm thick Ca-P coatings and rapid heat-treating, and (3) immobilized with bisphosphonate by immersion in pamidronate disodium solution (10 −2 m) for 24 h at 37°C. These surface-treated implants were inserted into edentulous areas in the mandibular molar region of five beagle dogs. After implantation periods of 4 and 12 weeks, the bone–implant interface was evaluated histologically and histomorphometrically. All measurements were statistically evaluated using a one-way ANOVA and Fisher PLSD test for multiple comparisons among the means. Four weeks after the implantation, higher percentage of bone contact was found around the thin Ca-P-coated implants compared to that of the control group. The highest percentage of bone contact was found around the bisphosphonate-immobilized implants after 12 weeks of implantation. These data suggest that a thin coating of calcium phosphate followed by bisphosphonate-immobilization is effective in the promotion of osteogenesis on surfaces of dental implants.

Fatigue behavior of calcium phosphate coatings with different stability under dry and wet conditions

To obtain stable plasma sprayed calcium phosphate coatings, coatings with a high crystallinity and low solubility were developed. However, stability of ceramic coatings is also influenced by their fatigue resistance. Recently, fatigue failure was proposed to explain coating detachment from implants under loaded conditions. Therefore, plasma-sprayed calcium phosphate coatings with different crystallinity were investigated in vitro for fatigue failure. An amorphous and a crystalline hydroxylapatite coating (AHA and CHA) and a highly crystalline fluorapatite coating (FA) were subjected to cyclic load tests, both in dry conditions and in simulated body fluid (SBF). The results in SBF revealed that the crystalline CHA and FA coating detached completely at the highest stressed middle section of the bar. The FA coating delaminated earlier than the CHA coating. The amorphous AHA coating showed only partial coating loss at the completion of the test. Tests in dry conditions did not reveal any change in the coatings tested. These results suggest a relation between crystallinity of apatite coatings and their failure due to fatigue: high crystallinity coatings demonstrate earlier and more complete fatigue failure than the amorphous apatite coatings. It can be concluded that coating stability is not determined solely by static dissolution, but by fatigue failure as well.

Fatigue behavior of calcium phosphate coatings with different stability under dry and wet conditions.

To obtain stable plasma sprayed calcium phosphate coatings, coatings with a high crystallinity and low solubility were developed. However, stability of ceramic coatings is also influenced by their fatigue resistance. Recently, fatigue failure was proposed to explain coating detachment from implants under loaded conditions. Therefore, plasma-sprayed calcium phosphate coatings with different crystallinity were investigated in vitro for fatigue failure. An amorphous and a crystalline hydroxylapatite coating (AHA and CHA) and a highly crystalline fluorapatite coating (FA) were subjected to cyclic load tests, both in dry conditions and in simulated body fluid (SBF). The results in SBF revealed that the crystalline CHA and FA coating detached completely at the highest stressed middle section of the bar. The FA coating delaminated earlier than the CHA coating. The amorphous AHA coating showed only partial coating loss at the completion of the test. Tests in dry conditions did not reveal any change in the coatings tested. These results suggest a relation between crystallinity of apatite coatings and their failure due to fatigue: high crystallinity coatings demonstrate earlier and more complete fatigue failure than the amorphous apatite coatings. It can be concluded that coating stability is not determined solely by static dissolution, but by fatigue failure as well.

Analytical and mechanical testing of high velocity oxy-fuel thermal sprayed and plasma sprayed calcium phosphate coatings.

Plasma spraying (PS) is the most frequently used coating technique for implants; however, in other industries a cheaper, more efficient process, high-velocity oxy-fuel thermal spraying (HVOF), is in use. This process provides higher purity, denser, more adherent coatings than plasma spraying. The primary objective of this work was to determine if the use of HVOF could improve the mechanical properties of calcium phosphate coatings. Previous studies have shown that HVOF calcium phosphate coatings are more crystalline than plasma sprayed coatings. In addition, because the coatings are exposed to more complex loading profiles in vivo than standard ASTM tensile tests provide, a secondary objective of this study was to determine the applicability of four-point bend testing for these coatings. Coatings produced by HVOF and PS were analyzed by profilometry, diffuse reflectance Fourier transform infrared spectroscopy, X-ray diffraction, four-point bend, and ASTM C633 tensile testing. HVOF coatings were found to have lower amorphous calcium phosphate content, higher roughness values, and lower ASTM C633 bond strengths than PS coatings; however, both coatings had similar crystal unit cell sizes, phases present (including hydroxyapatite, beta-tricalcium phosphate, and tetracalcium phosphate), and four-point bend bond strengths. Thus, the chemical, structural, and mechanical results of this study, in general, indicate that the use of HVOF to produce calcium phosphate coatings is equivalent to those produced by plasma spraying.

Analytical and mechanical testing of high velocity oxy-fuel thermal sprayed and plasma sprayed calcium phosphate coatings

Analytical and mechanical testing of high velocity oxy-fuel thermal sprayed and plasma sprayed calcium phosphate coatings

Evaluation of plasma-spray and magnetron-sputter Ca-P-coated implants: an in vivo experiment using rabbits.

The bone response to different plasma-spray and magnetron-sputter calcium phosphate (Ca-P)-coated implants was evaluated in a rabbit animal model. Four types of Ca-P coatings have been investigated: a plasma-spray Ca-P coating (HA-PS), a heat-treated plasma-spray Ca-P coating (HA-PS/ht), an amorphous magnetron-sputter coating (Ca-P-a), and a crystalline magnetron-sputter coating (CA-P-c). Seventy-two specially designed cylindrical implants were inserted in the lateral and medial femoral condyles of 18 New Zealand White rabbits. The four differently coated implants were positioned in one animal according to a split-plot design. After implantation periods of 3, 6, and 9 weeks, the bone-implant interface was evaluated histologically. Besides descriptive light microscopical evaluation, quantitative histomorphometrical measurements were done to determine bone contact and the amount of bone surrounding the implant-bone interface. Light microscopical examination revealed that all types of coatings followed the same process of bone healing. Measurements of bone contact at 6 and 9 weeks did not reveal significant differences between the various coatings. For the amount of bone, in a circular region at a certain distance from the implant, the Ca-P-c-coated implants showed a significantly greater amount of bone after 6 weeks of implantation than did the other three Ca-P coatings. At 9 weeks this difference could no longer be measured. On the basis of these findings we concluded that magnetron-sputtered Ca-P coatings show the same process of bone healing as the plasma-sprayed Ca-P coatings when inserted into the trabecular femoral bone of rabbits.

Cytotoxicity investigations of plasma sprayed calcium phosphate coatings

One potential alternative material to replace hydroxyapatite (HAp) as a coating material for plasma-sprayed coatings on implants for hip replacement is fluorapatite (FAp). FAp has advantages over HAp regarding the capability of being chemically stable during the coating process. This leads to surface coatings containing high apatite rates with a mechanical stability (bond strength, microhardness) comparable to HAp. From the technical point of view the production of FAp coatings is well investigated, although studies on biocompatibility of FAp coatings are fewer. This paper reports the production of HAp and FAp coatings with varying solubilities by plasma spraying and their in vitro cytotoxicity. Varying solubilities were realized by using modified plasma-spray parameters in common with suitable apatite powders with different crystallinities. Coating solubilities were evaluated by immersing the plasma-sprayed coatings in deionized water and electrolyte solution. Afterwards, cytotoxicity tests were performed using a modified half-slide technique. Cell attachment and cell morphology were evaluated. Neither HAp nor FAp coatings exhibited cytoxic influence on cells in culture. Results suggest that HAp coatings stimulate cell growth and FAp coatings do not. This could be explained by a negative effect on cell growth of the dissolved fluoride ions.

Porous-Coated Uncemented Components in Experimental Total Hip Arthroplasty in Dogs

The effect of a thin plasma-sprayed, calcium phosphate ceramic coating on bone ingrowth into titanium fiber mesh porous-surfaced prostheses was examined in a controlled canine cementless total hip arthroplasty model. Bone ingrowth was quantified using backscattered scanning electron microscopy of undemineralized sections. When good contact between the bone and porous coating was present, the calcium phosphate-coated prostheses contained significantly higher amounts of bone ingrowth at three weeks postimplantation than the uncoated control prostheses. At six weeks, however, there was no significant difference in the amount of bone ingrowth between the coated prostheses and uncoated prostheses. The ingrown bone seemed to be more intimately associated with the calcium phosphate-coated porous surfaces than the uncoated porous surfaces. When gaps at the bone-porous coating interface occurred, the calcium phosphate coating did not enhance bone ingrowth across those gaps. Plasma-sprayed calcium phosphate coatings may be useful in enhancing the early ingrowth of bone into porous-surfaced joint replacement prostheses, but they may lack long-term effectiveness. The coatings were not effective in enhancing bone ingrowth across gaps between the porous surface and the bone bed prepared at surgery.