The structural characteristics and mechanical behaviors of nonstoichiometric apatite coatings sintered in air atmosphere

There Are over 60 Ways to Produce Biocompatible Calcium Orthophosphate (CaPO4) Deposits on Various Substrates

A The present overview describes various production techniques for biocompatible calcium orthophosphate (abbreviated as CaPO4) deposits (coatings, films and layers) on the surfaces of various types of substrates to impart the biocompatible properties for artificial bone grafts. Since, after being implanted, the grafts always interact with the surrounding biological tissues at the interfaces, their surface properties are considered critical to clinical success. Due to the limited number of materials that can be tolerated in vivo, a new specialty of surface engineering has been developed to desirably modify any unacceptable material surface characteristics while maintaining the useful bulk performance. In 1975, the development of this approach led to the emergence of a special class of artificial bone grafts, in which various mechanically stable (and thus suitable for load-bearing applications) implantable biomaterials and artificial devices were coated with CaPO4. Since then, more than 7500 papers have been published on this subject and more than 500 new publications are added annually. In this review, a comprehensive analysis of the available literature has been performed with the main goal of finding as many deposition techniques as possible and more than 60 methods (double that if all known modifications are counted) for producing CaPO4 deposits on various substrates have been systematically described. Thus, besides the introduction, general knowledge and terminology, this review consists of two unequal parts. The first (bigger) part is a comprehensive summary of the known CaPO4 deposition techniques both currently used and discontinued/underdeveloped ones with brief descriptions of their major physical and chemical principles coupled with the key process parameters (when possible) to inform readers of their existence and remind them of the unused ones. The second (smaller) part includes fleeting essays on the most important properties and current biomedical applications of the CaPO4 deposits with an indication of possible future developments.

Ion-substituted calcium phosphate coatings by physical vapor deposition magnetron sputtering for biomedical applications: A review

Coatings based on ion-substituted calcium phosphate (Ca-P) have attracted great attention in the scientific community over the past decade for the development of biomedical applications. Among such Ca-P based structures, hydroxyapatite (HA) has shown significant influence on cell behaviors including cell proliferation, adhesion, and differentiation. These cell behaviors determine the osseointegration between the implant and host bone and the biocompatibility of implants. This review presents a critical analysis on the physical vapor deposition magnetron sputtering (PVDMS) technique that has been used for ion-substituted Ca-P based coatings on implants materials. The effect of PVDMS processing parameters such as discharge power, bias voltage, deposition time, substrate temperature, and post-heat treatment on the surface properties of ion-substituted Ca-P coatings is elucidated. Moreover, the advantages, short comings and future research directions of Ca-P coatings by PVDMS have been comprehensively analyzed. It is revealed that the topography and surface chemistry of amorphous HA coatings influence the cell behavior, and ion-substituted HA coatings significantly increase cell attachment but may result in a cytotoxic effect that reduces the growth of the cells attached to the coating surface areas. Meanwhile, low-crystalline HA coatings exhibit lower rates of osteogenic cell proliferation as compared to highly crystalline HA coatings developed on Ti based surfaces. PVDMS allows a close reproduction of bioapatite characteristics with high adhesion strength and substitution of therapeutic ions. It can also be used for processing nanostructured Ca-P coatings on polymeric biomaterials and biodegradable metals and alloys with enhanced corrosion resistance and biocompatibility. STATEMENT OF SIGNIFICANCE: Recent studies have utilized the physical vapor deposition magnetron sputtering (PVDMS) for the deposition of Ca-P and ion-substituted Ca-P thin film coatings on orthopedic and dental implants. This review explains the effect of PVDMS processing parameters, such as discharge power, bias voltage, deposition time, substrate temperature, and post-heat treatment, on the surface morphology and crystal structure of ion-substituted Ca-P and ion-substituted Ca-P thin coatings. It is revealed that coating thickness, surface morphology and crystal structure of ion-substituted Ca-P coatings via PVDMS directly affect the biocompatibility and cell responses of such structures. The cell responses determine the osseointegration between the implant and host bone and eventually the success of the implants.

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.

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.

An Investigation of Residual Stress of Porous Titania Layer by Micro-Arc Oxidation under Different Voltages

The surface modification of titanium by micro-arc oxidation under different voltages was processed to achieve good direct oseointegration. The new technique of two-dimensional X-ray diffraction was used to measure the residual stress of the layer. The results show that a porous titania layer containing Ca and P is obtained by micro-arc oxidation. The pore size and Ca/P of the layer are affected by the voltage. The high voltage can induce forming CaTiO3. The residual stress under different voltage is compressive stress and increases with the improvement of the voltage.

In vitro and in vivo tests of hydrothermally synthesised hydroxyapatite coating

High pure and crystalline Hydroxyapatite (HA) coatings on titanium alloy were prepared by hydrothermal synthesis (HS) of plasma-sprayed (PS) precursors from brushite powders (HS-HA). In vitro and in vivo tests were done to evaluate its biological property. The HS-HA coating was compared with the current PS-HA coating. Cultures of the primary osteoblasts on these two HA coatings showed similar cell attachment, proliferation and alkaline phosphatase (ALP) expression. The cell morphology on the coatings was demonstrated by scanning electron microscopy (SEM). The cell spread well at 1 day after seeding culture and the extracellular matrix was secreted after 14 days culture. Histomorphometric analysis was conducted on samples implanted in femoral bone of four dogs for 1 and 3 months, and bone-implant contact percentage was evaluated by light microscopy. The calcium and phosphate distribution on the interface of bone-implant was analysed by SEM and electron dispersive X-ray (EDX) analysis. The results show the osteoconduction of HS-HA coated implants.

Evaluation of nanostructured carbonated hydroxyapatite coatings formed by a hybrid process of plasma spraying and hydrothermal synthesis

Carbonated hydroxyapatite (CHA) coatings on a titanium alloy were prepared by hydrothermal synthesis of precursors plasma-sprayed with brushite as a raw powder. The structures, residual stresses, and bond strengths of the precursors and CHA coatings were investigated. The results showed that the sprayed precursors consisted of beta-Ca(2)P(2)O(7), alpha-Ca(3)(PO(4))(2), and CaHPO(4), whereas the CHA coatings exhibited a unique phase construction, nanostructured and needle-like crystals, and a fairly low tensile residual stress. The bond strength of a CHA coating 200 microm thick was 15 MPa, equivalent to that of a plasma-sprayed hydroxyapatite (HA) coating. The evaluation of the CHA coatings was performed together with that of plasma-sprayed HA coatings immersed in distilled water. The dissolution and bond-strength degradation of the CHA coatings were much lower than those of the plasma-sprayed HA coatings.

Dissolution response of hydroxyapatite coatings to residual stresses

The effect of residual stress on the dissolution of hydroxyapatite (HA) coatings was investigated. The examined coatings of 80-, 110-, and 200-microm thickness were prepared by a plasma-spraying technique under identical conditions. Residual stresses in the coatings were measured with a hole-drilling method. Dissolution of the coatings was monitored along with an examination of the phase composition. The results showed that both tensile residual stress and amorphous HA existed throughout the entire depth of the coatings and tended to increase from the surface to the interface of the coating and substrate. The thicker the coatings were, the higher the maximum residual stress was. Correspondingly, the pH value and calcium concentration of the solutions tended to increase with the coating thickness. On the basis of these phenomena and a thermodynamic analysis of the dissolution of the HA subjected to stresses, we concluded that besides structural effects, residual stress was also an important intrinsic factor influencing dissolution of HA coatings, and the dissolution can be delayed or even restrained by compressive residual stress.

Characterization and stability of hydroxyapatite coatings prepared by an electrodeposition and alkaline-treatment process

Hydroxyapatite (HA) coatings on titanium alloy substrates were prepared by an alkaline treatment of electrodeposited precursors. The structure, residual stress, and bond strength of the coatings were investigated. Test results showed that the coatings processed in this study exhibited fairly low tensile residual stress, high crystallinity, and were free of an amorphous phase. The bond strength of the coatings increased with the decrease of current density in the range of 0.2-15 mA/cm(2), and reached 14 MPa at 0.2 mA/cm(2). Evaluation of the coatings was performed together with the evaluation of the plasma-sprayed HA coatings immersed in distilled water. It was revealed that the dissolution and bond strength degradation of the coatings were much lower than those of the plasma-sprayed HA coatings.