Hydroxylapatite Coatings Research Articles

Plasma-Sprayed Osseoconductive Hydroxylapatite Coatings for Endoprosthetic Hip Implants: Phase Composition, Microstructure, Properties, and Biomedical Functions

This contribution attempts to provide a state-of-the-art account of the physicochemical and biomedical properties of the plasma-sprayed hydroxylapatite (HAp) coatings that are routinely applied to the surfaces of metallic endoprosthetic and dental root implants designed to replace or restore the lost functions of diseased or damaged tissues of the human body. Even though the residence time of powder particles of HAp in the plasma jet is extremely short, the high temperature applied induces compositional and structural changes in the precursor HAp that severely affect its chemical and physical properties and in turn its biomedical performance. These changes are based on the incongruent melting behavior of HAp and can be traced, among many other analytical techniques, by high resolution synchrotron X-ray diffraction, vibrational (Raman) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. In vivo reactions of the plasma-sprayed coatings to extracellular fluid (ECF) can be assessed and predicted by in vitro testing using simulated body fluids (SBFs) as proxy agents. Ways to safeguard the appropriate biological performance of HAp coatings in long-term service by controlling their phase content, porosity, surface roughness, residual stress distribution, and adhesion to the implant surface are being discussed.

Structural and morphological properties of hydroxylapatite coatings obtained by gas-detonation deposition on polymer and titanium substrates

The gas-detonation technique was used for the synthesis of biocompatible hydroxylapatite-based protective coatings on polymer and titanium substrates. Hydroxylapatite powder of high purity with a grain diameter of 50 µm was used as the raw material. The obtained coatings have the thickness close to 200 µm. It has been shown that the offered method enables to create of non-destructive hydroxylapatite-based coatings on polymer by varying the distance between the polymer target and the gun nozzle of gas- detonation setup. Using the data of Raman and X-ray measurements, it was ascertained that gas-detonation deposition doesn’t change the composition of the deposited material. The SEM investigation testifies that the formed hydroxylapatite-based coatings are porous. EPR studies have shown that there are no paramagnetic defects in the obtained coatings, and the coating itself has a higher radiation hardness as compared to the raw powder.

PREPARATION AND CHARACTERIZATION OF Sr-DOPED HAp BIOMEDICAL COATINGS ON POLYDOPAMINE-TREATED Ti6Al4V SUBSTRATES

Ti6Al4V alloy of titanium is a significant biomaterial due to its biocompatible nature, but it lacks required bioactivity to make it mimic properties to a human bone. Thus, hydroxyl-apatite (HAp), an inorganic compound found in human bones, is generally coated onto Ti6Al4V substrates to improve their bio-characteristics. But, HAp itself lacks certain bio-functionalities, such as allowing tissue bone regeneration and poor binding to the Ti6Al4V substrate, which results in osteoporosis and reduced bioactivity of the bio-implant, respectively. The proposed way out for this is the further doping of HAp with Strontium (Sr) for enabling tissue bone regeneration as well as addition of Polydopamine (PDA) for improved adhesion of HAp-based coatings with the substrate. Moreover, PDA results in increased drug delivery area and thus can be used as a material for enhancing resistance to bacterial growth. The present study demonstrates an experimental work on deposition of HAp, HAp with PDA and HAp with PDA and Sr coatings deposited onto Ti6Al4V alloy by means of biomimetic coating technique. Initially the pure HAp coatings were deposited using 10 SBF (simulated body fluid) solution and optimized in terms of time duration for desired coating uniformity. Then, for the optimized coating duration, the PDA pretreated Ti6Al4V substrates were coated, utilizing HAp, and Sr (at two different compositions) combinations were deposited through modified 10 SBF solutions. The characterization involving microstructural analysis and phase detection was performed for all these coatings using Scanned Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) of the coated substrates and adhesion strength was calculated using a standard pull out adhesion test ISO 13779–4. The study showed an effective and comparatively cheap method of depositing organic coatings using biomimetic technique to obtain improved bio-functionalities in metallic implants at low temperatures.

Functional plasma-sprayed hydroxylapatite coatings for medical application: Clinical performance requirements and key property enhancement

Functional hydroxylapatite (HAp) coatings deposited by atmospheric plasma spraying on parts of hip and knee endoprosthetic implants, artificial dental roots, spinal implants, and other medical devices serve to aid in osseointegration by providing a biocompatible and osseoconductive/osseoinductive template for bone growth-supporting actions of cytokines and noncollagenous proteins and proteoglycans, mediated by transmembrane proteins such as integrins. To be successful in this task, HAp coatings need to be carefully designed and optimized by closely controlling key properties such as phase composition, degree of crystallinity, crystallographic texture, thickness, adhesion strength to the implant surface, porosity, pore size distribution, surface nanostructure and roughness, residual coating stresses, and dissolution kinetics during in vivo contact with extracellular fluid. As this contribution discusses salient aspects of design, properties, and application of HAp coatings, it attempts to chart ways toward improving their in vivo performance and, thus, may be considered a helping hand and guiding manual of instruction for their successful deposition. In this review, much contextual recourse has been taken to the work of the present author and his research group during the last two and a half decades.

Plasma-Sprayed Bioactive Ceramic Coatings with High Resorption Resistance Based on Transition Metal-Substituted Calcium Hexaorthophosphates.

Calcium (titanium, zirconium) hexaorthophosphates with a [NZP] (sodium zirconium phosphate) structure belonging to the NaSiCon (Na Superionic Conductor) family were deposited by atmospheric plasma spraying onto the surfaces of Ti6Al4V substrates. (NaSiCon sensu strictu refers to solids with the chemical formula Na1+xZr2SixP3−xO12, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements). The microstructure of the coatings revealed the incongruent melting of the precursor material as ascertained by electron probe microanalysis (EPMA). The adhesion of the coatings to the substrate surface was within the limits specified for biomedical coatings. The solubility of the coatings was tested by immersion in 0.2 molar tris–hydroxymethyl–amino–methane–HCl (TRIS–HCl) buffer and found to be at least one order of magnitude lower than that of conventional hydroxylapatite coatings deposited under comparable conditions. In vitro biocompatibility tests with primary rat bone marrow cells (BMCs) showed a substantial cell proliferation in the presence of fetal bovine serum. Animal tests confirmed that coatings based on calcium (titanium, zirconium) hexaorthophosphates applied to Ti6Al4V rods implanted in the femoral medulla of sheep led to the strong neoformation of dense bone at a stable interface implant-bioceramic coating without coating delamination. Hence, based on their multifarious advantageous properties in the biomedical context, CaTi4-xZrx(PO4)6 ceramics may be considered the ‘Sleeping Beauty’ of osseoconductive coatings for the stem of hip endoprostheses and dental root implants, osteosynthetic fixation devices, and bioelectric devices including bone growth stimulators.

Plasma-Sprayed Hydroxylapatite Coatings as Biocompatible Intermediaries Between Inorganic Implant Surfaces and Living Tissue

The present contribution discusses critical aspects of the thermal alteration that HAp particles undergo when passing along the extremely hot plasma jet. This heat treatment leads to dehydroxylated phases such as oxyhydroxylapatite/oxyapatite as well as thermal decomposition products such as tri- and tetracalcium phosphates, and quenched phases in the form of amorphous calcium phosphate (ACP) of variable composition. The contribution also includes studying the influence bioinert TiO2 bond coats have on adhesion, crystallinity, and composition of HAp coatings. Moreover, the question is being addressed whether oxyapatite might exist as a (meta)stable phase or whether its occurrence is merely an ephemeral event. In addition, the article deals with the role that HAp coatings are playing during in vitro interaction with simulated body fluid (SBF) resembling the composition of extracellular fluid (ECF). The biological and biomechanical advantages of using HAp coatings for medical implants as well as salient aspects of their biomineralization and osseointegration will be discussed in some detail.

Cracking caused by cutting of plasma-sprayed hydroxyapatite coatings and its relation to the structural features of coatings deposited at different initial substrate temperatures

The present study estimated the cracking phenomenon in as-plasma-sprayed hydroxylapatite coatings (HACs) after they were being subjected to the severe cutting conditions in the direction perpendicular to the coating/substrate interface. In order to evaluate the effects of substrate preheating on the occurrence of micro-cracks, the HACs were deposited at different initial substrate temperatures (TS = 20, 100 and 200?C). The changes in phase composition and HA splat morphology with TS were observed and were correlated with the cracking occurrence. The results showed that severe cutting conditions introduced a localized cracking in the regions of HACs dominantly attributed to the brittle hydroxyl-deficient amorphous calcium phosphate (ACP) phase. This effect was particularly observable in the HACs deposited without preheating of substrate. On the other hand, the preheating of substrate reduced the presence of micro-cracks and caused insignificant changes in the average local phase composition. In HACs deposited with preheating of substrate, the HA splats (of which HACs are composed) were thinner and recrystallized HA regions seemed smaller in size and more evenly distributed. These results implied potentially important roles of the HA splat formation mechanism on the distribution of ACP and recrystallized HA regions in the as-plasma-sprayed HACs and the cracking resistance of HACs.

Tracking the thermal decomposition of plasma-sprayed hydroxylapatite

In modern orthopedics, plasma-sprayed hydroxylapatite coatings are applied routinely to metallic parts of hip and knee prostheses, as well as to dental root implants to render them osseoconductive, that is, able to assist the body in creating new bone by ingrowth of bone cells, blood capillaries, and soft tissue. In this work, hydroxylapatite coatings were deposited by atmospheric plasma spraying on titanium alloy substrates, and characterized by synchrotron radiation X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy. The deposition parameters were varied using a statistical design of experiments methodology. Depending on the degree of heat input and heat transfer rates, (1) dehydroxylated hydroxylapatite phases such as oxy- and oxyhydroxylapatite, (2) thermal decomposition phases (tri- and tetracalcium phosphates, CaO), and (3) amorphous calcium phosphate were formed. Implications of this research are that oxyapatite appears to be unstable at ambient conditions, and that proper selection of intrinsic plasma-spray parameters is key to the chemical stability and mechanical performance of the coating.

Formation of hydroxylapatite on CVD deposited titania layers

AbstractBioactive coatings on medical implants can be prepared from titanium dioxide due to its ability to induce the formation of hydroxylapatite (HA) from physiological solutions. In this study TiO2 layers were formed by chemical vapor deposition at different substrate temperatures in order to controllably change their structure from amorphous to consisting of preferentially oriented anatase crystallites. To evaluate the influence of phase composition and surface morphology of the deposited layers on their ability to form HA titania samples were immersed in Dulbecco's phosphate‐buffered saline (DPBS). The obtained results showed that structure of TiO2 layers determines the structure of HA formed on their surface. Polycrystalline titania layers with random orientation of anatase crystallites are the most favorable for intensive nucleation and formation of continuous HA coatings in DPBS solution. The textured TiO2 layers with smooth surface enhanced formation of HA deposits which consisted of sparsely distributed large separate grains. On the basis of the experimental results it was proposed that formation of hydroxylapatite nuclei on the surface of titania layers occurs by a mechanism of epitaxial growth. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Biosynthesis of Calcium Hydroxylapatite Coating on Sputtered Ti/TiN Nano Multilayers and their Corrosion Behavior in Simulated Body Solution

Titanium/titanium nitride (Ti/TiN) nanoscale multilayered films were deposited onto 316L stainless steel substrates by reactive magnetron sputtering using a Ti target. Coatings characterized by X-ray diffraction showed that the stack possesses centered cubic structure. The X-ray photoelectron spectroscopy survey spectra on the etched surfaces of the stack film on steel exhibited the characteristic Ti2p, N1s, and O1s peaks at the corresponding binding energies 454.5, 397.0, and 530.6 eV, respectively. Platelet adhesion experiments were carried out to examine the interaction between blood and the materials in vitro. The results indicated that the smoothness and lower isoelectric point contribute to better hemocompatibility of the Ti/TiN nanoscale multilayered coating. The biomediated synthesis of calcium hydroxylapatite (HA) was carried out on coated substrates using calcium-depositing bacteria. The observation of low corrosion current density (I(corr)) for the calcium HA-coated Ti/TiN specimens in simulated body fluid confirmed their highly resistive nature under the testing condition.

Change of properties of mononuclear leukocytes at patients with an osteogenesis imperfecta after operative treatment with using nanosized hydroxylapatite coatings

Change of properties of mononuclear leukocytes at patients with an osteogenesis imperfecta after operative treatment with using nanosized hydroxylapatite coatings

Study of the bones tissue reparation using nanostructured titanium implants with hydroxylapatite coatings by scanning electron microscopy

A method of medical implants (biocomposites) preparation based on nanostructured titanium with nanocrystalline bioactive hydroxylapatite coatings is developed. The operative treatment using these implants improves the regeneration of bone tissue for rats, as compared to the “false-operated” animals. The morphological data at 7, 14, 21, 45 days are obtained by means of scanning electron microscopy and discussed.

Preparation and Characterization of CaO-ZrO2-SiO2 Coating for Potential Application in Biomedicine

In this paper, chemically synthesized CaO-ZrO2-SiO2 (CZS) powder was plasma sprayed onto a Ti-6Al-4V alloy substrate for its potential applications in biomedicine. Surface morphology and phase composition were analyzed by scanning electron microscopy (SEM) and x-ray diffraction (XRD). The degradation behavior of the coating in biological environment was in vitro appraised in Tris-HCl buffer. The mass loss was much lower than that of the wollastonite coating (WC) and close to that of the hydroxylapatite coating (HC). The bond strength with titanium alloy substrate was 31.8 ± 4.7 MPa. In vitro bioactivity appraisement results showed that apatite could be formed on the surfaces after soaking in simulated body fluid (SBF) for 14 days. Canine marrow stem cells (MSCs) also showed well adhesion and proliferation on the coating surfaces. In summary, results suggest that the coating possesses well cytocompatibility and may be an appropriate candidate for application in biomedicine.

Bisphosphonate incorporation in surgical implant coatings by fast loading and co-precipitation at low drug concentrations

The objectives of the present work was to evaluate the possibility for fast loading by soaking of bisphosphonates (BPs) into hydroxylapatite (HA) implant coatings biomimetically grown on crystalline TiO(2) surfaces, and also investigate the influence of different BP loading concentrations in a buffer during co-precipitation of a calcium phosphate containing layer onto these surfaces. The co-precipitation method created coatings that contained BPs throughout most of the coating layer, but the presence of BPs in the buffer hindered the formation of a bulk HA-layer, thus resulting in very thin coatings most likely consisting of islands built up by a calcium phosphate containing BPs. The coatings biomimetically grown on TiO(2) surfaces, were shown to consist of crystalline HA. Soaking of these coatings during 15 min only in a low BPs concentration containing buffer yielded a concentration on the coating surface of the same order of magnitude as obtained with soaking during 60 min in significantly higher concentrated buffers. This could be of advantage during surgery since the operating surgeon could make a fast decision whether or not to include the drugs in the coating based on the need of the particular patient at hand. The BPs present on the surface of the fast-loaded HA coatings were found to be strongly bound, something which should be beneficial for in vivo use. Both the co-precipitation method and the fast loading by soaking method investigated here are promising techniques for loading of BPs onto surgical implants. The simplicity of both methods is an advantage since implants can have spatially complicated structures.

Investigation of boundary conditions for biomimetic HA deposition on titanium oxide surfaces

To improve the clinical outcome of metal implants, i.e. earlier loading and reduction of the incidence of revision surgery, better bone bonding ability is wanted. One method to achieve this is to change the surface chemistry to give a surface that facilitates bone bonding in vivo, i.e. a bioactive surface. Crystalline titanium oxide has recently been proven to be bioactive in vitro and is an interesting option to the more common hydroxylapatite (HA) coatings on implants. A materials possible in vitro bioactivity is tested through soaking in simulated body fluid and studies of possible HA formation on the surface. For bioactive materials, the formed HA layer can also be used as a coating. The aim of the current paper is to investigate some boundary conditions for HA formation on crystalline titanium oxide surfaces regarding influence from coating thickness, soaking time and soaking temperature. The influence from soaking time and temperature on the HA growth were investigated on oxidised Ti samples, (24 h at 800 degrees C) resulting in a rutile surface structure. The oxidised samples were tested at three temperatures (4, 37 and 65 degrees C) and four times (1 h, 1 day, 1 week and 4 weeks). The influence from titanium coating thickness on the HA growth was investigated via depositing thin films of crystalline titanium dioxide on Ti plates using a reactive magnetron sputtering process. Four different PVD runs with coating thicknesses between 19 and 74 nm were tested. The soaking temperature had an effect on the HA formation and growth on both rutile surfaces and native oxide on Ti substrates. Higher temperatures lead to easier formation of HA. It was even possible, at 65 degrees C, to grow HA on native titanium oxide from soaking in PBS. The coating quality was better for HA formed at 65 degrees C compared to 37 degrees C. All PVD-coatings showed HA growth after 1 week in PBS at 37 degrees C, thus even very thin coatings of crystalline titanium oxide coatings are bioactive.

Targeted Local Delivery of Bisphosphonate from Orthopaedic Implants

Two methods of loading bisphosphonate (BP) into hydroxylapatite (HA) coatings on crystalline TiO2 surfaces were investigated to improve bone ingrowth to implant surfaces. In the study the BP pamidronate was used. Ti-plates coated with crystalline TiO2 were soaked in phosphate buffer saline (PBS) for 7 days at 40° C and thereafter soaked in solutions of BP, 0.5 mg/ml for 15 or 60 minutes (fast loading method). In the second method BP was dissolved into PBS in different concentrations before immersion of the discs for 7 days (co-precipitation method). Surface and bulk were analysed using electron spectroscopy for chemical analysis, scanning electron microscope and x-ray diffraction. Release of BP was studied using alternating ionic current method. It was shown that fast loading by soaking in a BP solution for 15 minutes was sufficient to load BP into a HA coating. Co-precipitation showed that a thin layer of calcium phosphate crystals containing BP can be deposited directly onto a crystalline TiO2 surface.

Five-Year Prospective Clinical Evaluation of Highly Crystalline HA MP-1–Coated Dental Implants

Contemporary plasma-sprayed hydroxylapatite (HA) coatings with high crystalline content are much more resistant to in vivo degradation than HA coatings of a decade ago but reportedly exhibit reduced wettability, which could potentially negatively affect tissue adhesion and long-term clinical outcome. The present prospective study was undertaken to determine if highly crystalline HA MP-1-coated implants could meet a minimum 5-year implant success rate standard of 85% in view of their previously reported decreased wettability. Study subjects were consecutive patients with 1 or more missing teeth in the maxillary and/or mandibular jaw who presented in 3 university dental clinics and 1 private dental practice. A total of 120 patients were treated per protocol and successfully restored with implant-supported prostheses. Four implants failed in 3 patients and were withdrawn from the study. There were no other irresolvable adverse events. Cumulative implant success at 5 years was 97% (n = 184 implants in 88 patients), which exceeded the 85% standard for implant success after 5 years of clinical function.

Hydroxylapatite Coatings by Microplasma Spraying

HA coatings were deposited onto TiA16V4 substrate by microplasma spraying(MPS)as well as by atmospheric plasma spraying(APS).The morphology,phase compositions and crystallinity of the coatings were studied by means of optical,scanning electron microscopy and X-ray diffraction.The results show that the decomposition of HA can be significantly reduced during MPS than during APS. In addition to HA,onlyβ-TCP and amorphous phases form during MPS.MPS coating crystallinity is clearly influenced by spray distance.Shorter spray distances(80mm)result in high crystallinities, which are much higher than those of APS coatings.But with a spray distance of 130mm,the coating crystallinity is very low.β-TCP,α-TCP,TTCP,CaO and amorphous phases form during APS.Com- pared with APS coatings,MPS coatings are less decomposed during spraying,due to the less overheating of HA particles.

Characterization and in vitro evaluation of biphasic calcium pyrophosphate–tricalciumphosphate radio frequency magnetron sputter coatings

The objective of this study was to characterize the physicochemical, dissolution, and osteogenic properties of radio frequency magnetron sputtered dicalcium pyrophosphate/tricalciumphosphate (Pyro/TCP) and hydroxylapatite (HA) coatings. Therefore Pyro/TCP and HA coatings were deposited on grit-blasted titanium discs. The results showed that the deposited coatings were amorphous and changed into a crystalline structure after IR heat-treatment of 550 degrees C for HA and 650 degrees C for Pyro/TCP. Heat-treated HA coatings appeared to be stable during immersion in simulated body fluid (SBF), that is no changes in the XRD pattern were observed. Also, no dissolution of the coating was observed by scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDS) revealed that the Ca/P ratio of the HA coatings remained constant during SBF immersion. On the other hand, the heat-treated Pyro/TCP coatings showed a surface reaction of calcium pyrophosphate into a beta-tricalcium phosphate phase during SBF immersion. This was confirmed by EDS analysis. Rat bone marrow-derived osteoblast-like cells cultured on the heat-treated substrates showed that cell proliferation and differentiation occurred on both types of bioceramic coatings. No significant differences for proliferation and early differentiation were observed between cells cultured on heat-treated Pyro/TCP and HA at individual time points. However, osteocalcin expression, a late marker for osteoblast-like cell differentiation, was significantly increased after 12 days of culture on HA-coatings. These results were confirmed by SEM observations and suggest increased osteogenic properties for HA-coatings over Pyro/TCP-coatings. Additional research is necessary to obtain conclusive evidence on the in vivo osteogenic capacity of Pyro/TCP coatings.

In vitro evaluation of different heat‐treated radio frequency magnetron sputtered calcium phosphate coatings

Surface chemical compositions, such as calcium/phosphorus ratio and phase content, have a strong influence on the bioactivity and biocompatibility of calcium phosphate (CaP) coatings as applied on orthopedic and dental implants. Hydroxylapatite (HA) and dicalcium pyrophosphate (DCPP) coatings were prepared on titanium substrates by RF magnetron sputter deposition. The surfaces were left as-prepared (amorphous HA coating; A-HA, amorphous DCPP coating; A-DCPP) or heat treated with: infrared (IR) at 550 degrees C (I-HA) or at 650 degrees C (I-DCPP), and a water steam at 140 degrees C (S-HA and S-DCPP). The surface changes of these coatings were determined after incubation in simulated body fluid (SBF). Also, the growth of rat bone marrow cells (RBM) was studied with scanning electron microscopy (SEM). Both IR and water steam heat treatment changed the sputter-deposited coatings from the amorphous into the crystalline phase. As-prepared amorphous coatings dissolved partially in SBF within 4 weeks of incubation, while heat-treated coatings supported the deposition of a precipitate, i.e., carbonated apatite on both I-HA and S-HA specimens, and tricalciumphosphate on the I-DCPP and S-DCPP specimens. The Ca/P ratio of the A-HA, I-HA, S-HA, A-DCPP, I-DCPP and S-DCPP coatings changed, respectively, from 1.98 to 1.12, 2.01 to 1.76, 1.91 to 1.68, 0.76 to 1.23, 0.76 to 1.26 and 1.62 to 1.55 after 4 weeks of incubation in SBF. Finally, the RBM cells grew well on all heat-treated coatings, but showed different mineralization morphology during cell culturing. The different heat-treatment procedures for the sputtered HA and DCPP coatings influenced the surface characteristics of these coatings, whereby a combination of crystallinity and specific phase composition (Ca/P ratio) strongly affected their in vitro bioactivity.