Detonation Spraying of Hydroxyapatite on a Titanium Alloy Implant

Internal surface modification of additively manufactured macroporous TiAl6V4 biomimetic implants via a calciothermic reaction-based process and osteogenic in vivo responses

Three-dimensional macroporous titanium-aluminum-vanadium (TiAl6V4) implants produced by additive manufacturing (AM) can be grit blasted (GB) to yield microtextured exterior surfaces, with additional micro/nano-scale surface features provided by subsequent acid etching (AE). However, the line-of-sight nature of GB causes the topography of exterior GB + AE-modified surfaces to differ from internal GB-inaccessible surfaces. Previous in vitro studies using dense TiAl6V4 substrates indicated that a nonline-of-sight, calciothermic-reaction (CaR)-based process provided homogeneous osteogenic nanotextures on GB + AE surfaces, suggesting it could be used to achieve a homogeneous nanotopography on external and internal surfaces of macroporous AM constructs. Macroporous TiAl6V4 (3D) constructs were produced by direct laser melting and modified by GB + AE, with the CaR process then applied to 50% of constructs (3DCaR). The CaR process yielded nanoporous/nanorough internal surfaces throughout the macroporous constructs. Skeletally mature, male Sprague-Dawley rats were implanted with these constructs using a cranial on-lay model. Prior to implantation, a Cu++-free click hydrogel was applied to half of the constructs (3D + H, 3DCaR + H) to act as a challenge to osseointegration. Osseointegration was compared between the four implant groups (3D, 3DCaR, 3D + H, 3DCaR + H) at 4w. 3D + H implants exhibited lower bone volume (BV) and percent bone ingrowth (%BI) than the 3D implants. In contrast, osseointegrated 3DCaR + H implants had similar BV and %BI as the 3DCaR implants. Implant pull-off forces correlated with these results. In vitro analyses indicated that human bone marrow stromal cells (MSCs) exhibited enhanced production of osteoblast differentiation markers and factors associated with osteogenesis when grown on CaR-modified 3D substrates relative to control (TCPS) substrates. This work confirms that the CaR process provides osteogenic nanotextures on internal surfaces of macroporous 3D implants, and suggests that CaR-modified surfaces can promote osseointegration in cases where osteogenesis is impaired.

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.

Immobilization of Peroxo-Heteropoly Compound and Palladium on Hydroxyapatite for the Epoxidation of Propylene by Molecular Oxygen in Methanol

Peroxo-heteropoly compound PO₄[W(O)(O₂)₂] was synthesized on calcium-deficient hydroxyapatite using a reaction of surface [HPO₄]^2- groups on hydroxyapatite with a Na₂[W₂O₃(O₂)₄] aqueous solution. The vibration of [HPO₄]^2- at 875 cm^-1 became very weak, and the vibration of the peroxo-oxygen bond [O-O]^2- at 845 cm^-1 appeared in the FT-IR spectrum of the solid product, indicating that PO₄[W(O)(O₂)₂] was formed on the surface of hydroxyapatite. The formed solid sample was further reacted with PdCl₂(PhCN)₂ in an acetone solution to fix PdCl₂ between the O sites on the hydroxyapatite. Elemental analyses proved that the resultant solid contained 1.2 wt.% Pd, implying that PdCl₂ molecules were immobilized on the surface of hydroxyapatite. The hydroxyapatite-based hybrid compound containing Pd and PO₄[W(O)(O₂)₂] was used as a heterogeneous catalyst in a methanol solvent for propylene epoxidation by molecular oxygen in an autoclave batch reaction system. A propylene conversion of 53.4% and a selectivity for propylene oxide of 88.7% were obtained over the solid catalyst after reaction at 363 K for 8 h. The novel catalyst could be reused by a simple centrifugal separation, and the yield of propylene oxide did not decrease after the reaction for five runs. By prolonging the reaction time to 13 h, the highest yield of propylene oxide at 363 K over the solid catalyst was obtained as 53.8%, which was almost the same as that of the homogeneous catalyst containing PdCl₂(PhCN)₂ and [(C₆H₁₃)₄N]₂{HPO₄[W(O)(O₂)₂]₂} for the propylene epoxidation. Methanol was used as a solvent as well as a reducing agent in the propylene epoxidation by molecular oxygen. Small particles of Pd metal were formed on the surface of the hybrid solid catalyst during the reaction, and acted as active species to achieve the catalytic turnover of PO₄[W(O)(O₂)₂] in the propylene epoxidation by molecular oxygen in methanol.

The Relationship between Osteoinduction and Vascularization: Comparing the Ectopic Bone Formation of Five Different Calcium Phosphate Biomaterials

Objective: The objective of this study is to compare the bone induction of five kinds of calcium phosphate (Ca-P) biomaterials implanted in mice and explore the vascularization and particle-size-related osteoinductive mechanism. Methods: The following five kinds of Ca-P biomaterials including hydroxyapatite (HA) and/or tricalcium phosphate (TCP) were implanted in the muscle of 30 BALB/c mice (n = 6): 20 nm HA (20HA), 60 nm HA (60HA), 12 µm HA (12HA), 100 nm TCP (100TCP) and 12 µm HA + 100 nm TCP (HATCP). Then, all animals were put on a treadmill to run 30 min at a 6 m/h speed each day. Five and ten weeks later, three mice of each group were killed, and the samples were harvested to assess the osteoinductive effects by hematoxylin eosin (HE), Masson’s trichrome and safranine−fast green stainings, and the immunohistochemistry of the angiogenesis and osteogenesis markers CD31 and type I collagen (ColI). Results: The numbers of blood vessels were 139 ± 29, 118 ± 25, 78 ± 15, 65 ± 14 in groups HATCP, 100TCP, 60HA and 20HA, respectively, which were significantly higher than that of group 12HA (12 ± 5) in week 5 (p < 0.05). The area percentages of new bone tissue were (7.33 ± 1.26)% and (8.49 ± 1.38)% in groups 100TCP and HATCP, respectively, which were significantly higher than those in groups 20HA (3.27 ± 0.38)% and 60HA (3.43 ± 0.27)% (p < 0.05); however, no bone tissue was found in group 12HA 10 weeks after transplantation. The expression of CD31 was positive in new blood vessels, and the expression of ColI was positive in new bone tissue. Conclusions: Nanoscale Ca-P biomaterials could induce osteogenesis in mice muscle, and the osteoinductive effects of TCP were about 124% higher than those of 20HA and 114% higher than those of 60HA. The particle size of the biomaterials affected angiogenesis and osteogenesis. There was a positive correlation between the number of blood vessels and the area percentage of new bone tissue; therefore, osteoinduction is closely related to vascularization. Our results provide an experimental basis for the synthesis of calcium−phosphorus matrix composites and for further exploration of the osteoinductive mechanism.

A Study of Thermal Stability of Hydroxyapatite

High-temperature powder sintering is an integral part of the dense ceramic manufacturing process. In order to find the optimal conditions for producing a ceramic product, the information about its behavior at high temperatures is required. However, the data available in the literature are very contradictory. In this work, the thermal stability of hydroxyapatite prepared by a solid-state mechanochemical method and structural changes occurring during sintering were studied. Stoichiometric hydroxyapatite was found to remain as a single-phase apatite structure with the space group P63/m up to 1300 °C inclusively. A further increase in the sintering temperature leads to its partial decomposition, a decrease in the crystallite size of the apatite phase, and the appearance of significant structural strains. It was shown that small deviations from stoichiometry in the Ca/P ratio upward or downward during the hydroxyapatite synthesis lead to a significant decrease in the thermal stability of hydroxyapatite. An apatite containing almost no hydroxyl groups, which is close to the composition of oxyapatite, was prepared. It was shown that the congruent melting of stoichiometric hydroxyapatite upon slow heating in a high-temperature furnace does not occur. At the same time, the fast heating of hydroxyapatite by laser radiation allows, under certain conditions, its congruent melting with the formation of a recrystallized monolayer of oxyhydroxyapatite. The data obtained in this study can be used when choosing sintering conditions to produce hydroxyapatite-based ceramics.