Direct morphological comparison of vacuum plasma sprayed and detonation gun sprayed hydroxyapatite coatings for orthopaedic applications

Creation of Bioceramic Coatings on the Surface of Ti–6Al–4V Alloy by Plasma Electrolytic Oxidation Followed by Gas Detonation Spraying

In this work, bioceramic coatings were formed on Ti6Al4V titanium alloy using a combined technique of plasma electrolytic oxidation followed by gas detonation spraying of calcium phosphate ceramics, based on hydroxyapatite. Plasma electrolytic oxidation was carried out in electrolytes with various chemical compositions, and the effect of electrolytes on the macro and microstructure, pore size and phase composition of coatings was estimated. Three types of electrolytes based on sodium compounds were used: phosphate, hydroxide, and silicate. Plasma electrolytic oxidation of the Ti–6Al–4V titanium alloy was carried out at a fixed DC voltage (270 V) for 5 min. The sample morphology and phase composition were studied with a scanning electron microscope and an X-ray diffractometer. According to the results, the most homogeneous structure with lower porousness and many crystalline anatase phases was obtained in the coating prepared in the silicate-based electrolyte. A hydroxyapatite layer was obtained on the surface of the oxide layer using detonation spraying. It was determined that the appearance of α-tricalcium phosphate phases is characteristic for detonation spraying of hydroxyapatite, but the hydroxyapatite phase is retained in the coating composition. Raman spectroscopy results indicate that hydroxyapatite is the main phase in the coatings.

Detonation Spraying of Hydroxyapatite on a Titanium Alloy Implant

Hydroxyapatite (HA), the major mineral component of tooth enamel and natural bones, is a good candidate for bone tissue engineering. Synthetic HA is used for making coatings on metallic implants intended for medical applications. A HA coating renders the implant biocompatible and osteoinductive. In addition, it improves fixation and the overall performance of the implanted object. In the present work, HA coatings were deposited on a medical titanium alloy implant with mesh geometry and a developed surface by detonation spraying. The feedstock powder was HA obtained by the dry mechanochemical method. Single-phase HA coatings were obtained. The coatings were formed not only on the surfaces normal to the particle flow direction, but also on the sides of the mesh elements. Despite partial melting of the powder, no decomposition of HA occurred. This work demonstrates the prospects of detonation spraying for the production of HA coatings on metallic implants with complex geometries.

Substituted hydroxyapatite coatings of bone implants

Surface modification of orthopedic and dental implants has been demonstrated to be an effective strategy to accelerate bone healing at early implantation times. Among the different alternatives, coating implants with a layer of hydroxyapatite (HAp) is one of the most used techniques, due to its excellent biocompatibility and osteoconductive behavior. The composition and crystalline structure of HAp allow for numerous ionic substitutions that provide added value, such as antibiotic properties or osteoinduction. In this article, we will review and critically analyze the most important advances in the field of substituted hydroxyapatite coatings. In recent years substituted HAp coatings have been deposited not only on orthopedic prostheses and dental implants, but also on macroporous scaffolds, thus expanding their applications towards bone regeneration therapies. Besides, the capability of substituted HAps to immobilize proteins and growth factors by non-covalent interactions has opened new possibilities for preparing hybrid coatings that foster bone healing processes. Finally, the most important in vivo outcomes will be discussed to understand the prospects of substituted HAp coatings from a clinical point of view.

X-ray diffraction analysis of hydroxyapatite-coated in different plasma gas atmosphere on Ti and Ti-6Al-4V

Objective:

The aim is to study the effect of plasma working gas on composition, crystallinity, and microstructure of hydroxyapatite (HA) coated on Ti and Ti-6Al-4V metal substrates.

Materials and Methods:

Ti and Ti-6Al-4V metal substrates were coated with HA by plasma spray using four plasma gas atmospheres of argon, argon/hydrogen, nitrogen, and nitrogen/hydrogen. The degree of crystallinity, the phases present, and microstructure of HA coating were characterized using X-ray diffraction and scanning electron microscopy.

Results:

Variation in crystallinity and the microstructure of HA coating on plasma gas atmosphere was observed. Micro-cracks due to thermal stresses and shift in the 2θ angle of HA compared to feedstock was seen.

Conclusion:

Plasma gas atmosphere has a significant influence on composition, crystallinity, and micro-cracks of HA-coated dental implants.

Review of titanium surface modification techniques and coatings for antibacterial applications

Implanted biomaterials play a key role in the current success of orthopedic and dental procedures. Pure titanium and its alloys are the most commonly used materials for permanent implants in contact with bone. However, implant-related infections remain among the leading reasons for failure. The most critical pathogenic event in the development of infection on biomaterials is biofilm formation, which starts immediately after bacterial adhesion. In the last decade, numerous studies reported the ability of titanium surface modifications and coatings to minimize bacterial adhesion, inhibit biofilm formation and provide effective bacterial killing to protect implanted biomaterials. In the present review, the different strategies to prevent infection onto titanium surfaces are reported: surface modification and coatings by antibiotics, antimicrobial peptides, inorganic antibacterial metal elements and antibacterial polymers. STATEMENT OF SIGNIFICANCE: Implanted biomaterials play a key role in the current success of orthopedic and dental procedures. Pure titanium and its alloys are the most commonly used materials for permanent implants in contact with bone. Microbial infection is one of the main causes of implant failure. Currently, the global infection risk is 2-5% in orthopedic surgery. Numerous solutions exist to render titanium surfaces antibacterial. The LBPS team is an expert on the functionalization of titanium surfaces by using bioactive polymers to improve the biologiocal response. In this review, the different strategies to prevent infection are reported onto titanium and titanium alloy surfaces such as surface modification by antibiotics, antimicrobial peptides, inorganic antibacterial metal elements and antibacterial polymers.

Characterization and mechanical behaviour of reinforced hydroxyapatite coatings deposited by vacuum plasma spray on SS-316L alloy

Hydroxyapatite powder reinforced individually with 10wt percentage (wt%) of Al₂O₃ and ZrO₂ (HA + 10wt% Al₂O₃ and HA + 10wt% ZrO₂) was thermally sprayed onto SS-316L substrate with a bond coat of Zirconia by vacuum plasma spray (VPS) technique. The resulted coatings were heat treated at 700°C for 1h to study its effects on microstructural and mechanical properties of the deposited coatings. The characterization of the coatings was carried out using scanning electron microscope, x-ray diffraction, porosity, surface roughness and crystallinity using Rietveld analysis. The results indicated that after post coating heat treatment substantial decrease in porosity was witnessed along with significant improvement in crystallinity. Besides, the hardness across the cross-section of the coatings and bond strength was considerably improved; however the hardness of top coat was reduced owing to the loosening of un-melted and partially melted particles by diffusion process which takes place during heat treatment.

Calcium orthophosphate deposits: Preparation, properties and biomedical applications

Since various interactions among cells, surrounding tissues and implanted biomaterials always occur at their interfaces, the surface properties of potential implants appear to be of paramount importance for the clinical success. In view of the fact that a limited amount of materials appear to be tolerated by living organisms, a special discipline called surface engineering was developed to initiate the desirable changes to the exterior properties of various materials but still maintaining their useful bulk performances. In 1975, this approach resulted in the introduction of a special class of artificial bone grafts, composed of various mechanically stable (consequently, suitable for load bearing applications) implantable biomaterials and/or bio-devices covered by calcium orthophosphates (CaPO4) to both improve biocompatibility and provide an adequate bonding to the adjacent bones. Over 5000 publications on this topic were published since then. Therefore, a thorough analysis of the available literature has been performed and about 50 (this number is doubled, if all possible modifications are counted) deposition techniques of CaPO4 have been revealed, systematized and described. These CaPO4 deposits (coatings, films and layers) used to improve the surface properties of various types of artificial implants are the topic of this review.

Significance of calcium phosphate coatings for the enhancement of new bone osteogenesis--a review

A systematic analysis of results available from in vitro, in vivo and clinical trials on the effects of biocompatible calcium phosphate (CaP) coatings is presented. An overview of the most frequently used methods to prepare CaP-based coatings was conducted. Dense, homogeneous, highly adherent and biocompatible CaP or hybrid organic/inorganic CaP coatings with tailored properties can be deposited. It has been demonstrated that CaP coatings have a significant effect on the bone regeneration process. In vitro experiments using different cells (e.g. SaOS-2, human mesenchymal stem cells and osteoblast-like cells) have revealed that CaP coatings enhance cellular adhesion, proliferation and differentiation to promote bone regeneration. However, in vivo, the exact mechanism of osteogenesis in response to CaP coatings is unclear; indeed, there are conflicting reports of the effectiveness of CaP coatings, with results ranging from highly effective to no significant or even negative effects. This review therefore highlights progress in CaP coatings for orthopaedic implants and discusses the future research and use of these devices. Currently, an exciting area of research is in bioactive hybrid composite CaP-based coatings containing both inorganic (CaP coating) and organic (collagen, bone morphogenetic proteins, arginylglycylaspartic acid etc.) components with the aim of promoting tissue ingrowth and vascularization. Further investigations are necessary to reveal the relative influences of implant design, surgical procedure, and coating characteristics (thickness, structure, topography, porosity, wettability etc.) on the long-term clinical effects of hybrid CaP coatings. In addition to commercially available plasma spraying, other effective routes for the fabrication of hybrid CaP coatings for clinical use still need to be determined and current progress is discussed.

Hydroxylapatite nanoparticles: fabrication methods and medical applications

Hydroxylapatite (or hydroxyapatite, HAp) exhibits excellent biocompatibility with various kinds of cells and tissues, making it an ideal candidate for tissue engineering, orthopedic and dental applications. Nanosized materials offer improved performances compared with conventional materials due to their large surface-to-volume ratios. This review summarizes existing knowledge and recent progress in fabrication methods of nanosized (or nanostructured) HAp particles, as well as their recent applications in medical and dental fields. In section 1, we provide a brief overview of HAp and nanoparticles. In section 2, fabrication methods of HAp nanoparticles are described based on the particle formation mechanisms. Recent applications of HAp nanoparticles are summarized in section 3. The future perspectives in this active research area are given in section 4.

Calcium orthophosphate coatings, films and layers

In surgical disciplines, where bones have to be repaired, augmented or improved, bone substitutes are essential. Therefore, an interest has dramatically increased in application of synthetic bone grafts. As various interactions among cells, surrounding tissues and implanted biomaterials always occur at the interfaces, the surface properties of the implants are of the paramount importance in determining both the biological response to implants and the material response to the physiological conditions. Hence, a surface engineering is aimed to modify both the biomaterials, themselves, and biological responses through introducing desirable changes to the surface properties of the implants but still maintaining their bulk mechanical properties. To fulfill these requirements, a special class of artificial bone grafts has been introduced in 1976. It is composed of various mechanically stable (therefore, suitable for load bearing applications) biomaterials and/or bio-devices with calcium orthophosphate coatings, films and layers on their surfaces to both improve interactions with the surrounding tissues and provide an adequate bonding to bones. Many production techniques of calcium orthophosphate coatings, films and layers have been already invented and new promising techniques are continuously investigated. These specialized coatings, films and layers used to improve the surface properties of various types of artificial implants are the topic of this review.

Osteoclast resorption of thermal spray hydoxyapatite coatings is influenced by surface topography

Coating characteristics such as composition, crystallite features and topography collectively impact the cell response. The influence from splats has not yet been assessed for hydroxyapatite (HAp) thermal spray coatings. The objective of this work is to (a) survey the topography on commercial implants, (b) ascertain topography formation from single splats, and (c) determine the osteoclast resorption pattern on a topographically refined coating compared to dentine. Coatings on dental implants, an orthopedic screw, a femoral stem and a knee implant were studied for reference. The effects of substrate pre-heat, roughness, spray distance and particle size on the coating roughness and topography were studied. Human-derived osteoclasts were placed on a coating with refined topography and compared to dentine, a polished coating and polished sintered HAp. A pre-heat of at least 200°C on titanium was required to form rounded splats. The greatest influence on coating roughness and topography arose from particle size. A 2-fold increase in the mean particle size from 30 to 72 μm produced a significant difference (P<0.001) in roughness from 4.8 and 9.7 μm. A model is shown to illustrate topography formation, nanostructure evolution on single splats, and the topography as seen in commercial implants. Osteoclasts showed a clear preference for activity on coatings with refined topography. A one-way ANOVA test revealed a significantly greater pit depth (P=0.022) for dentine (14 μm) compared to the as-sprayed and polished coating (5 μm). Coatings with topography display a similar number of resorption pits with dentine, but a 10-fold greater number than polished coatings, emphasizing the importance of flattened droplet topography on implant surfaces.

Calcium phosphate-based coatings on titanium and its alloys

Use of titanium as biomaterial is possible because of its very favorable biocompatibility with living tissue. Titanium implants having calcium phosphate coatings on their surface show good fixation to the bone. This review covers briefly the requirements of typical biomaterials and narrowly focuses on the works on titanium. Calcium phosphate ceramics for use in implants are introduced and various methods of producing calcium phosphate coating on titanium substrates are elaborated. Advantages and disadvantages of each type of coating from the view point of process simplicity, cost-effectiveness, stability of the coatings, coating integration with the bone, cell behavior, and so forth are highlighted. Taking into account all these factors, the efficient method(s) of producing these coatings are indicated finally.

Preparation of Graded Zirconia—Hydroxyapatite Composite Bioceramic and Its Immunocompatibility in vitro

To obtain immunocompatibility, graded zirconia—hydroxyapatite (ZrO 2-HA) composite bioceramic and simplex ZrO2-HA composite bioceramic are prepared. Peripheral blood mononuclear cells (PBMCs) harvested from healthy individuals are cultured with the two ceramic extracts to assess their effect on the transformation of lymphocytes, apoptotic ratio, CD69 expression, and expression of cytokine of TNFα and IL-6, with or without phytohemoagglutinin (PHA)-stimulated cells. Ceramic extracts did not activate the resting lymphocytes, whereas the response of the PHA-stimulated cells was significantly modified. The PBMCs activated by graded ZrO2-HA composite bioceramic is noticeably smaller than that by simplex ZrO2-HA composite bioceramic; these results, however can only be seen under the amplified effect of PHA-stimulation.

Residual Stress in Hydroxyapatite Coating: Nonlinear Analysis and High-Energy Synchrotron Measurements

The thermal deposition of hydroxyapatite (HA) on titanium alloy substrate (Ti-6A1-4V) leads to a structure that has very good osseointegration properties. However, clinical failures have been occasionally reported at the interface between substrate and coating. Lifetime is the main parameter in such prostheses; therefore, in order to improve their quality, it is necessary to evaluate the level of stresses near the interface. The high-energy synchrotron radiation combines the advantages of a bulk analysis and reduced volume of the gauge. The objective of our study was to calculate the residual stress using a nonlinear finite-element model and to measure residual stress level near the interface, in the hydroxyapatite coating and in titanium alloy substrate with a nondestructive and high-resolution experiment. The high-energy synchrotron radiation of the BM16 beam-line at ESRF (Grenoble-France) was used with a resolution of down to 10 micrometers. The experimental measurements validate the results found by means of nonlinear finite-element analysis of the plasma spraying induced stress.

Enhanced osteoblast adhesion on hydrothermally treated hydroxyapatite/titania/poly(lactide-co-glycolide) sol–gel titanium coatings

Sol-gel processing was used to coat titanium substrates with hydroxyapatite (HA), TiO2, and poly(DL-lactic-glycolic acid). Coating surface characteristics were analyzed with XRD, EDS, AFM, SEM, and water contact angle measurements which indicated that the coatings had a high degree of crystallinity and good resistance to cracking. Coatings were also evaluated by cytocompatibility testing with osteoblast-like cells (or bone-forming cells). The cytocompatibility of the HA composite coatings prepared in the present in vitro study was compared to that of a traditional plasma-sprayed HA coating. Results showed that osteoblast-like cell adhesion was promoted on the novel HA sol-gel coating compared to the traditional plasma-sprayed HA coating. In addition, hydrothermal treatment of the sol-gel coating improved osteoblast-like cell adhesion. Since osteoblast adhesion is a necessary prerequisite for subsequent formation of bone, these results provided evidence that hydrothermally sol-gel processed HA may improve bonding of titanium implants to juxtaposed bone and, thus, warrants further investigation.

Influence of fluorapatite on the properties of thermally sprayed hydroxyapatite coatings

Thermally sprayed hydroxyapatite has been the widely used on orthopaedic prosthesis to induce bone growth and facilitate bone attachment. However, hydroxyapatite has a greater affinity for the formation of an amorphous phase in the thermally sprayed coating that results in the release of excessive amount of mineral ions from the implant coating leading to a saturated environment in the immediate vicinity of the bone cells. Fluorapatite however is highly crystalline and offers the potential for lower mineral ion release by dissolution. Thus study investigates the influence of fluorapatite in a thermally sprayed hydroxyapatite coating. Mechanical blends of fluorapatite with hydroxyapatite were thermally sprayed, characterized with X-ray diffraction, SEM, FTIR, optical microscopy for microstructure, roughness and tested for solubility. Cathodoluminescence microscopy was used to examine the resorbed coating surface. Fluorapatite coatings crystallized more readily and produce a greater coating roughness. The roughness in fluorapatite coatings arises from less flattened droplets that show a tendency for finger formation. Addition of fluorapatite increases coating crystallinity. The use of slower resorbing fluorapatite produces less particle release which favors improved osseointegration. Less change in the surface topography during resorption can be used to an advantage to control the coating surface presented to cells and extra cellular matrix proteins.

In vivo comparison of the osseointegration of vacuum plasma sprayed titanium- and hydroxyapatite-coated implants

For the last 15 years, orthopedic implants have been coated with hydroxyapatite (HA) to improve implant fixation. The osteoconductive effect of HA coatings has been demonstrated in experimental and clinical studies. However, there are ongoing developments to improve the quality of HA coatings. The objective of this study was to investigate whether a rough and highly crystalline HA coating applied by vacuum plasma spraying (VPS) had a positive effect on the osseointegration of special, high-grade titanium (Ti) implants with the same surface roughness. Ti alloy implants were coated (VPS) with special, high-grade Ti or HA. The osseointegration of the implants was evaluated by either light microscopy or pullout tests after 1, 2, and 4 weeks of unloaded implantation in the cancellous bone of 18 sheep. The interface shear strength increased significantly over all time intervals. By 4 weeks, values had reached approximately 10N/mm(2). However, the difference between the coatings was not significant at any time interval. Direct bone-implant contact was significantly different between the coatings after 2 and 4 weeks, and reached 46% for Ti and 68% for HA implants by 4 weeks. This study indicates that the use of a rough and highly crystalline HA coating, applied by VPS, enhances early osseointegration. Accelerated establishment of secondary implant fixation decreases the risk of early loosening.

High temperature in-situ XRD of plasma sprayed HA coatings

The control of phase transformations in plasma sprayed hydroxyapatite (HA) coatings are critical to the clinical performance of the material. This paper reports the use of high temperature X-ray diffraction (HT-XRD) to study, in-situ, the phase transformations occurring in plasma sprayed HA coatings. The coatings were prepared using different spray power levels (net plasma power of 12 and 15 kW) and different starting powder size ranges (20-45; 45-75 microm). The temperature range employed was room temperature (approximately 26 degrees C) to 900 degrees C in normal atmosphere and pressure. High temperature differential scanning calorimetry (DSC) was also employed to investigate and determine the precise onset temperature of phase transformations during the recrystallization process. Results showed that actual onset of thermal degradation in the coating into other metastable phases like TTCP, beta-TCP and CaO occurred at 638 degrees C. The aforementioned phase transitions were independent of the selected spraying parameters. The degree of melting and thermal dissociation of HA actually determines the amount of calcium phosphate phases that are formed. A high power level of 15 kW produced a greater degree of melting, resulting in more CaO, TTCP and beta-TCP being formed as a result.

In vitro dissolution behaviour of two morphologically different thermally sprayed hydroxyapatite coatings

Two types of hydroxyapatite coating on titanium substrates were produced by vacuum plasma spraying and detonation gun spraying. The response of the coatings to immersion in Ringer's solution at pH 7.2 and held at 37 degrees C for periods of one, two, four and eight weeks was compared. X-ray diffraction was used to measure changes in crystallinity and residual stresses. Porosity and coating thickness were measured using optical microscopy combined with image analysis. Surface morphology was characterised using scanning electron microscopy and Talysurf measurements. The DGUN coatings were found initially to have a lower crystallinity and porosity but higher surface roughness and residual stress level than the VPS coatings. This resulted in a higher rate of dissolution compared to the more stable VPS coatings.

Abrasive waterjet peening: a new method of surface preparation for metal orthopedic implants

Abrasive waterjet (AWJ) peening is a new mechanical surface treatment process envisioned for use on metal orthopedic implants. The process utilizes an abrasive waterjet to simultaneously texture and work harden the surface of a metal substrate through controlled hydrodynamic erosion. In this study, a titanium alloy (Ti6Al4V) was subjected to AWJ peening over a range of parametric conditions. The textured surfaces were quantified in terms of the apparent interdigitation volume (V(i)), the effective stress concentration factor (K(t)) posed by the surface topography, and the magnitude of residual stress (sigma(r) ). Topographical features of the prepared surfaces were determined using contact profilometry, and X-ray diffraction was used in evaluating the in-plane residual stress. It was found that a large range in V(i) (9.4-43.8 microm(3)/microm(2)) and K(t) (1.3-2.7) are available through selection of the AWJ peening process parameters. Furthermore, a compressive residual stress (-409 +/- sigma(r) +/- -33) was found to result within the surface of the Ti6Al4V substrates regardless of treatment conditions. When compared to a titanium plasma spray coating used for cementless fixation, the AWJ peened Ti6Al4V exhibited a surface topography with significantly lower effective stress concentration and higher compressive residual stress. Based on results from this study, AWJ peening may serve as a new method of surface treatment for metal orthopedic implants, which supports the development of stable primary fixation and simultaneously enhances the component fatigue strength.