RF Plasma Sputtering Deposition of Hydroxyapatite Bioceramics: Synthesis, Performance, and Biocompatibility

Abstract

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.

Plasmas meet nanoparticles—where synergies can advance the frontier of medicine

“…This bioceramic material is the main mineral constituent of bone and tooth tissues, and HA coatings have revealed inspiring clinical advantages in promoting efficient implant fixation and implant-to-bone adhesion shortly after the implantation, as well as faster bone remodeling due to enhanced bidirectional growth and formation of a bonding interlayer between bone and implant [149]- [151]. Recently, nanoparticle HA dental enamel paste has been used for rapid repair of early caries lesions without the need of removing of healthy tooth material, commonly practiced in dentistry to ensure the filling stick [152].…”

Reactive Plasma‐Aided RF Sputtering Deposition of Hydroxyapatite Bio‐implant Coatings

“…This structure is not common to calcium phosphate bioceramic films synthesized by thermal plasmabased powder spraying and is also quite rare for RF magnetron sputtering techniques. [1,2] Thirdly, from the FESEM image in Figure 1, one can clearly see that the interface between the bioceramic and the implant alloy is smooth and clear of major defects. The texture of the thin (70-100 nm) layer adjacent to the interface suggests that the main properties (e.g., phase and elemental composition) of this layer can differ from those in the bulk of the coating.…”

“…[15,16] Moreover, several important criteria, such as crystallinity, elemental composition, chemical stability, bioactivity, and biocompatibility have to be met before the coating can be tested in vivo. [1][2][3][4] The properties of calcium phosphate-coated metal implants exhibit a strong dependence on fabrication techniques and process parameters. Recently, plasma-assisted RF sputtering of multiple targets (e.g., hydroxyapatite tablets, calcium-rich powders, natural bones, titanium-based alloys, and some others) in various gas mixtures has been shown to be an efficient control tool of the most important characteristics of bioceramic-coated bio-implants.…”

“…[1,2] Hydroxyapatite, the main mineral component in natural bone materials, is able to form direct bonds with bone tissues shortly after implantation into the human body. However, these compounds cannot be used as load-bearing components due to their poor bulk mechanical properties.…”

“…Recently, plasma-assisted RF sputtering of multiple targets (e.g., hydroxyapatite tablets, calcium-rich powders, natural bones, titanium-based alloys, and some others) in various gas mixtures has been shown to be an efficient control tool of the most important characteristics of bioceramic-coated bio-implants. [1][2] In particular, co-sputtering of high-purity titanium targets significantly improves adhesion of the bioceramic film to the metal substrate. [17,18] However, the reason behind the significant improvement of the adhesive strength still remains unclear.…”