Summary: Hydroxyapatite (HA) coatings have numerous applications in orthopedics and dentistry, owing to their excellent ability to promote stronger implant fixation and faster bone tissue ingrowth and remodeling. Thermal plasma spray and other plasma‐assisted techniques have recently been used to synthesize various calcium phosphate‐based bioceramics. Despite notable recent achievements in the desired stoichiometry, phase composition, mechanical, structural, and biocompatible properties, it is rather difficult to combine all of the above features in a single coating. For example, many existing plasma‐sprayed HA coatings fall short in meeting the requirements of grain size and crystallinity, and as such are subject to enhanced resorption in body fluid. On the other hand, relatively poor interfacial bonding and stability is an obstacle to the application of the HA coatings in high load bearing Ti6Al4V knee joint implants. Here, we report on an alternative: a plasma‐assisted, concurrent, sputtering deposition technique for high performance biocompatible HA coatings on Ti6Al4V implant alloy. The plasma‐assisted RF magnetron co‐sputtering deposition method allows one to simultaneously achieve most of the desired attributes of the biomimetic material and overcome the aforementioned problems. This article details the film synthesis process specifications, extensive analytical characterization of the material's properties, mechanical testing, simulated body fluid assessments, biocompatibility and cytocompatibility of the HA‐coated Ti6Al4V orthopedic alloy. The means of optimization of the plasma and deposition process parameters to achieve the desired attributes and performance of the HA coating, as well as future challenges in clinical applications are also discussed.SEM image of differentiated cells on HA bioceramics deposited by plasma sputtering, after a two‐day incubation period.magnified imageSEM image of differentiated cells on HA bioceramics deposited by plasma sputtering, after a two‐day incubation period.

The plasma-assisted RF sputtering deposition of a biocompatible, functionally graded calcium phosphate bioceramic on a Ti6Al4 V orthopedic alloy is reported. The chemical composition and presence of hydroxyapatite (HA), CaTiO 3 , and CaO mineral phases can be effectively controlled by the process parameters. At higher DC biases, the ratio [Ca]/[P] and the amount of CaO increase, whereas the HA content decreases. Optical emission spectroscopy suggests that CaO + is the dominant species that responds to negative DC bias and controls calcium content. Biocompatibility tests in simulated body fluid confirm a positive biomimetic response evidenced by in-growth of an apatite layer after 24 h of immersion.

Back Cover: The picture, based on the research results of the team of the Plasma Sources and Applications Center of Nanyang Technological University, Singapore, shows four steps of plasma‐aided fabrication and performance assessment of biocompatible HA coatings of Ti6A14V orthopedic implant alloy material. Top left: Plasma glow in the process of concurrent Rf magnetron sputtering of HA and Ti targets. Top right: Surface morphology of the HA coating under optimized process conditions. Bottom left: SEM showing that apatite regrowth after immersion of the HA‐coated Ti6A14V samples in the simulated body fluid is accompanied by the formation of a complex granular network. Bottom right image: Surface of the HA‐coated sample covered by differentiated osteoblast cells during the cell culture assessment. Further details can be found in the Feature Article by S. Xu,* J. D. Long, L. Sim, C. H. Diong, and K. Ostrikov on page 373.

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