Layered,adhesively bonded hydroxyapatite coatings for orthopaedic implants

A method of producing layered, adhesively bonded hydroxyapatite coatings is described. The coatings have a flexible polymeric interlayer which can improve load transfer from the implant to the bone, and a hydroxyapatite-rich surface which can provide mechanical interlocking with the bone. The coatings have been characterized using a number of methods including optical and electron microscopy, mechanical testing and finite element analysis. Some ways in which the coatings could be used to produce orthopaedic implants with improved mechanical and biological compatibility are discussed.     

Carbon nanotubes for orthopaedic implants

The physical and biological limitations of current orthopaedic implant materials are a major challenge for bone tissue engineering. Nanotechnology has introduced new materials and methods for meeting this challenge. The application of nanotechnology to engineering new bone substitutes finds a model in the nanoscale components of natural bone tissue. Carbon nanotubes are a macromolecular form of carbon with exceptional properties and similar morphology and dimensions to the nanoscale collagen fibers of natural bone tissue. Carbon nanotubes have been used in two main areas of bone tissue engineering: for structural and electrical enhancement of polymer and ceramic composites and for nanostructured coatings to improve the bioactivity of implant surfaces. By incorporating carbon nanotubes into the design and engineering of bone tissue substitutes, researchers have attempted to overcome limitations in the structural and biological compatibility of traditional orthopaedic implant materials.     

A novel combinatorial approach to the development of beta titanium alloys for orthopaedic implants

In recent years there has been a significant thrust directed towards the development of novel implant alloys based on β-Ti. Two recently developed and promising biocompatible β-Ti alloys are Ti–35Nb–7Zr–5Ta and Ti–29Nb–4.6Zr–13Ta. While both these alloy compositions, based on the quaternary Ti–Nb–Zr–Ta system, are promising, there is still a tremendous scope for improvement in terms of alloy design in this and other systems via optimization of alloy composition and thermo-mechanical treatments. Here a novel combinatorial approach has been used for the development of implant alloys with optimized compositions and microstructures. Using directed laser deposition, compositionally graded alloy samples based on the Ti–Nb–Zr–Ta system have been fabricated. These samples have been heat-treated to affect different microstructures in terms of the volume fraction and distribution of the α phase in the β matrix as a function of composition. Subsequently, composition-specific indentation-based hardness and modulus information has been obtained from these samples to construct a database relating the composition and microstructure to the mechanical properties. These databases have been used to train and test fuzzy-logic based neural-network models for predicting the mechanical properties. The trained models have also been used to predict the influence of different alloying additions on the hardness and modulus. These predictions have subsequently been verified by detailed experimental characterization, shedding light on the factors influencing the strength and modulus in these alloys. Such modeling approaches for the development of novel implant alloys can be highly beneficial since they offer the possibility of identifying promising compositions without the necessity for extensive experimental test cycles.     

Morphology and composition of hydroxyapatite coatings prepared by hydrothermal treatment on electrodeposited brushite coatings

Highly pure brushite CaHPO₄ · 2H₂O) coatings on porous Ti6Al4V substrates were prepared by electrodeposition from aqueous electrolytes. The influence of hydrothermal treatment parameters on brushite-to-hydroxyapatite conversion and the morphology and phase composition of hydroxyapatite (HAP) coatings was studied. It was found that the content, Ca/P atomic ratio, grain size and pore size of HAP in coatings increase with increasing hydrothermal treatment temperature, and that increasing the pH value can promote brushite-to-HAP conversion and reduce the grain size of HAP. Under optimal conditions, highly pure HAP coatings with needle-like crystals and non-stoichiometric form, which are similar to those of calcium phosphate in human bone, can be obtained. © 1999 Kluwer Academic Publishers     

Biocompatible solid-phase microextraction coatings based on polyacrylonitrile and solid-phase extraction phases

The applications of solid-phase microextraction (SPME) are continuously expanding, and one of the most interesting current aspects consists of applying SPME for fast analysis of biological fluids. The goal of this study is to develop biocompatible SPME coatings that can be utilized for in vivo and in vitro extractions, in direct contact with a biological matrix such as blood or tissue. The biocompatibility of the proposed new coatings is confirmed by X-ray photoelectron spectroscopy, and their performance is tested by developing an SPME/HPLC method for analysis of verapamil, loperamide, diazepam, nordiazepam, and warfarin in buffer solutions and in human plasma. The coatings prove to be biocompatible by not adsorbing proteins and are successfully applied for fast drug analysis and assay of drug plasma protein binding.     

Thermo-mechanical aspects of adiabatic shear failure of AM50 and Ti6Al4V alloys

The thermo-mechanical aspects of adiabatic shear band (ASB) formation are studied for two commercial alloys: Mg AM50 and Ti6Al4V. Tests are carried out on shear compression specimens (SCS). The evolution of the temperature in the deforming gauge section is monitored in real-time, using an array of high-speed infrared detectors synchronized with a Kolsky apparatus (split Hopkinson pressure bar). The evolution of the gage temperature is found to comprise three basic stages, in agreement with Marchand and Duffy’s simultaneous observations of mechanical data and gauge deformation patterns (1988). The onset and full formation stages of ASB are identified by combining the collected thermal and mechanical data. Full development of the ASB is identified as the point at which the measured and calculated temperature curves intersect and diverge thereon. At that stage, the homogeneous strain assumption used in calculating the maximum temperature rise is no longer valid.     

Definition of roughness structures for superhydrophobic and hydrophilic optical coatings on glass

With specific modeling, measurement, and analysis procedures, it is possible to predict, define, and control roughness structures for tailored wetting properties of optical coatings. Examples are given for superhydrophobic and hydrophilic sol-gel layers on glass substrate.     

Silicate-based Plasma Electrolytic Oxidation (PEO) coatings with incorporated CeO2 particles on AM50 magnesium alloy

Plasma Electrolytic Oxidation (PEO) coatings on AM50 were developed in electrolytes containing CeO₂ particles. The effect of particle concentration in the electrolyte on the final microstructure of the coatings was studied and correlated with the corrosion behavior. CeO₂ particles were incorporated into the coatings and located preferentially in pores and cracks. The process involves both uptake and reactive incorporation of CeO₂ under local melting caused by microdischarges.The coating with the lower concentration of particles has the best corrosion properties as measured with EIS. Higher concentration of particles increased the defect density in the coating which has a negative effect on the corrosion resistance.     

Properties of open-cell porous metals and alloys for orthopaedic applications

One shortcoming of metals and alloys used to fabricate various components of orthopaedic systems, such as the femoral stem of a total hip joint replacement and the tibial plate of a total knee joint replacement, is well-recognized. This is that the material modulus of elasticity (E′) is substantially larger than that of the contiguous cancellous bone, a consequence of which is stress shielding which, in turn, has been postulated to be implicated in a cascade of events that culminates in the principal life-limiting phenomenon of these systems, namely, aseptic loosening. Thus, over the years, a host of research programs have focused on the synthesis of metallic biomaterials whose E′ can be tailored to match that of cancellous bone. The present work is a review of the extant large volume of literature on these materials, which are called open-cell porous metals/alloys (or, sometimes, metal foams or cellular materials). As such, its range is wide, covering myriad aspects such as production methods, characterization studies, in vitro evaluations, and in vivo performance. The review also includes discussion of seven areas for future research, such as parametric studies of the influence of an assortment of process variables (such as the space holder material and the laser power in the space holder method and the laser-engineered net-shaping process, respectively) on various properties (notably, permeability, fatigue strength, and corrosion resistance) of a given porous metal/alloy, innovative methods of determining fatigue strength, and modeling of corrosion behavior.     

Endurance limit of die-cast magnesium alloys AM50hp and AZ91hp depending on type and size of internal cavities

The aim of this work was to investigate the influence of internal cavities on the fatigue properties of two of the technical most common die-cast magnesium alloys, AM50hp and AZ91hp. For this purpose the endurance limits of altogether three batches of S–N specimens, two conventional cast and one vacural cast, with varying internal defects have been measured. After fatigue failure the fracture surface of each sample has been analysed with respect to the site of crack initiation and, where appropriate, the size of the crack initiating cavity or pore. Moreover, on both alloys crack growth tests have been carried out and the thresholds ΔK_th of the stress intensity factor have been measured.Finally, the experimental data from both, the S–N tests and the crack propagation measurements, were depicted in a modified Kitagawa–Takahashi diagram. Using El Haddad’s and Topper’s approach the distribution function of the endurance limit has been proposed, whose parameters could be determined by fitting them to the experimental results. The knowledge of these parameters allows the calculation of the fracture probability as a function of an equivalent crack length and the stress amplitude.     

Biomimetic mineral coatings in dental and orthopaedic implantology

Biomimetic techniques are used to deposit coatings of calcium phosphate upon medical devices. The procedure is conducted under near-physiological, or “biomimetic”, conditions of temperature and pH primarily to improve their biocompatibility and biodegradability of the materials. The inorganic layers generated by biomimetic methods resemble bone mineral, and can be degraded within a biological milieu. The biomimetic coating technique involves the nucleation and growth of bone-like crystals upon a pretreated substrate by immersing this in a supersaturated solution of calcium phosphate under physiological conditions of temperature (37°C) and pH (7.4). The method, originally developed by Kokubo in 1990, has since undergone improvement and refinement by several groups of investigators. Biomimetic coatings are valuable in that they can serve as a vehicle for the slow and sustained release of osteogenic agents at the site of implantation. This attribute is rendered possible by the near-physiological conditions under which these coatings are prepared, which permits an incorporation of bioactive agents into the inorganic crystal latticework rather than their mere superficial adsorption onto preformed layers. In addition, the biomimetic coating technique can be applied to implants of an organic as well as of an inorganic nature and to those with irregular surface geometries, which is not possible using conventional methodologies.     

Cobalt-based alloys for orthopaedic applications studied by electrochemical and XPS analysis

The composition of the passive layers formed by electrochemical oxidation at different passivation potentials on Co-Cr-Mo and Co-Ni-Cr-Mo alloys in simulated physiological solution (SPS), with and without the complexing agent EDTA, was studied by X-ray photoelectron spectroscopy. Composition as a function of depth, cationic fraction and thickness of the passive film was determined. Chromium oxide is shown to be the major constituent of the passive layer on both Co-Cr-Mo and Co-Ni-Cr-Mo alloys. The minor constituents of the passive layers, Co- and Mo-oxide in the case of Co-Cr-Mo alloy and Ni-, Co- and Mo-oxides in the case of Co-Ni-Cr-Mo alloy, are also located in the outer part of the layer. EDTA affects the formation of the passive layer on each alloy. The content of Co-, Ni- and Mo-oxide in the passive layer is lower in the presence of EDTA, thus indicating increased solubility associated with higher stability constants for complexes of metal cations with EDTA.     

Bioactive borate glass coatings for titanium alloys

Bioactive borate glass coatings have been developed for titanium and titanium alloys. Glasses from the Na(2)O-CaO-B(2)O(3) system, modified by additions of SiO(2), Al(2)O(3), and P(2)O(5), were characterized and compositions with thermal expansion matches to titanium were identified. Infrared and X-ray diffraction analyses indicate that a hydroxyapatite surface layer forms on the borate glasses after exposure to a simulated body fluid for 2 weeks at 37 degrees C; similar layers form on 45S5 Bioglass((R)) exposed to the same conditions. Assays with MC3T3-E1 pre-osteoblastic cells show the borate glasses exhibit in vitro biocompatibility similar to that of the 45S5 Bioglass((R)). An enameling technique was developed to form adherent borate glass coatings on Ti6Al4V alloy, with adhesive strengths of 36 +/- 2 MPa on polished substrates. The results show these new borate glasses to be promising candidates for forming bioactive coatings on titanium substrates.     

Chrome-free neodymium-based protective coatings for magnesium alloys

The microstructure of chrome-free neodymium-based conversion coating on magnesium alloy was investigated and the corrosion resistance was evaluated as well. The micro-morphology, transverse section, crystal structure and composition of the coating were observed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), respectively. The corrosion resistance was evaluated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS).The results revealed that the morphology of neodymium conversion coating is of crack-mud structure. Tiny cracks distribute in the compact coating deposited by neodymium oxides. EDS results characterize that the coating is made of neodymium oxides. The potentiodynamic polarization curve, EIS and OCP indicate that the neodymium conversion coating can improve the corrosion resistance of magnesium alloys.     

Effects of surface coating on reducing friction and wear of orthopaedic implants

Coatings such as diamond-like carbon (DLC) and titanium nitride (TiN) are employed in joint implants due to their excellent tribological properties. Recently, graphite-like carbon (GLC) and tantalum (Ta) have been proven to have good potential as coating as they possess mechanical properties similar to bones—high hardness and high flexibility. The purpose of this systematic literature review is to summarize the coating techniques of these four materials in order to compare their mechanical properties and tribological outcomes. Eighteen studies published between January 2000 and February 2013 have met the inclusion criteria for this review. Details of their fabrication parameters, material and mechanical properties along with the tribological outcomes, such as friction and wear rate, were identified and are presented in a systematic way. Although experiment conditions varied, we conclude that Ta has the lowest wear rate compared to DLC, GLC and TiN because it has a lower wear rate with high contact pressure as well as higher hardness to elasticity ratio. However, a further tribology test is needed in an environment which replicates artificial joints to confirm the acceptability of these findings.     

Sr对AM50显微组织和力学性能的影响

研究了不同含量Sr对AM50合金微观组织和力学性能的影响.结果表明,添加Sr后,AM50合金铸体组织β-Mg17Al12变得细小,晶粒明显细化;Sr基本上与Al结合生成高熔点、高稳定的Al4Sr相,能够强化晶界并阻碍位错滑移,从而强化合金.适量的Sr明显提高了室温下合金的屈服强度和抗拉强度,而且不影响合金的延伸率,而过量的Sr会导致AM50延伸率和轻度的降低.含0.5%Sr的铸态合金可得到最高抗拉强度233MPa,屈服强度90MPa,延伸率16.3%的性能.     

Biomimetic coatings functionalized with adhesion peptides for dental implants

A complete biological integration into the surrounding tissues (bone, gingiva) is a critical step for clinical success of a dental implant. In this work biomimetic coatings consisting either of collagen type I (for the gingiva region) and hydroxyapatite (HAP) or mineralized collagen (for the bone interface) have been developed as suitable surfaces regarding the interfaces. Additionally, using these biomimetic coatings as a matrix, adhesion peptides were bound to further increase the specificity of titanium implant surfaces. To enhance cell attachment in the gingiva region, a linear adhesion peptide developed from a laminin sequence (TWYKIAFQRNRK) was bound to collagen, whereas for the bone interface, a cyclic RGD peptide was bound to HAP and mineralized collagen using adequate anchor systems. The biological potential of these coatings deduced from cell attachment experiments with HaCaT human keratinocytes and MC3T3-E1 mouse osteoblasts showed the best results for collagen and laminin sequence coating for the gingiva region and mineralized collagen and RGD peptide coatings for regions with bone contact. Our concept opens promising approaches to improve the biological integration of dental implants.