Dear Colleagues,

We warmly invite you to submit your recent work in the field of synthesis and characterization techniques of hydroxyapatite (HA) coatings to the Special Issue “Synthetic and Biological-derived Hydroxyapatite Implant Coatings”.

In the last few decades, the field of bioactive materials for bone tissue engineering has emerged as one of the prominent approaches to tackling bone traumas and ailments, with an emphasis on the production of durable implants and bone substitutes capable of bypassing rejection complications.

HA is a well-known bioceramic which possesses a close chemical and structural resemblance to the mineral composition of bones and teeth of vertebrates. Despite their favorable bone regeneration properties, HA ceramics are very brittle, and this restricts their usage in high load-bearing applications. To overcome this drawback, HA can be applied as a coating on either metallic or polymeric implants, aiming to significantly improve the implant’s overall performance by harmoniously combining the excellent bioactivity of the ceramic with the mechanical advantages of the substrate implants.

Synthetic HA is commonly produced by several chemical routes, but recent methods to either extract or derive HA from natural sustainable resources (e.g., bones, sea-shells, various mineralogical resources) were reported. Biological-derived apatite is a carbonated HA, which contains various amounts of oligoelements and has a reduced crystallinity. Therefore, it differs from synthetic HA in terms of composition, stoichiometry, crystallinity degree, crystal size/morphology, and thus, degradation rate and overall biological performance.

Moreover, commercial thick plasma-sprayed HA coatings are susceptible to cracking and delamination and, owing to the high processing temperatures, could contain residual decomposition phases. In this respect, current research interests are rapidly advancing on two focused directions: (i) Increasing the biomimicry of HA coatings with respect to the composition and structure of bone apatite, and (ii) improving the present or even discovering alternative deposition techniques to allow for the fabrication of novel (doped) HA coatings with increased mechanical and biological performance.

In particular, the topics of interest of this Special Issue include, but are not limited to:

• Synthetic and biological-derived HA coatings for orthopedic and dental implants;
• Drug delivery;
• Composite HA-based coatings;
• Doping of synthetic and biological-derived HA coatings;
• Alternative methods for the deposition of synthetic and biological-derived HA coatings;
• Biofunctional assessment of synthetic and biological-derived HA coatings.

Dr. Liviu Duta
Prof. Dr. Faik Nuzhet Oktar
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Coatings is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2200 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

• Synthetic and biological-derived hydroxyapatite coatings
• Adherence
• Osseointegration
• Biocompatibility
• Medical applications

1. Title: Comparative structural and dielectric properties of pure and Li-doped synthetic and bovine bone-derived hydroxyapatite in bulk and thin film form
Authors: T. Tite, I.M. Bogdan, M. Enculescu, A.C. Galca, G. Boni, L. Hrib, G. Aldica, I. Pasuk, G.E. Stan *
Affiliation: National Institute of Materials Physics, RO-077125 Magurele, Romania
Abstract: The close resemblance of hydroxyapatite (HA) to the bone mineral, as well as the ease of fine tuning its functional response (e.g. osteoconductivity) by designed ion-substitutions, made it a material of choice for numerous biomedical applications. The prominent role of HA dielectric features on the biomedical performance (i.e. application of electromagnetic fields can accelerate the healing of bone fractures) has been stressed. HA’s piezoelectricity was advanced as explanation, but this is incompatible with its hexagonal structure. The engineering of HA dielectric properties by intentional compositional and structural alterations could be an alternative, but remained to date mostly unexplored. Lithium could constitute an interesting HA dopant, being able to allow not only the decrease solubility and boost of bone fracture healing, but also to change its dielectric response. In this work, the influence of physical-chemical features on the dielectric properties of simple and Li-doped synthetic and biological (BHA), in both bulk and sputtered film form, has been explored. SEM, EDXS, XRD, FTIR and impedance spectroscopy measurements were carried out. For the films, the structure and dielectric constant were strongly anisotropic and lower in value with respect to the bulk state. Their differences were explained by the different (a-axis or c-axis) preferred orientation. The results showed promises that the application horizon of HA-based ceramics could be expanded.

2. Title: Biogenic Hydroxyapatite Nanocoatings In The Orthopaedic Field: Features And Properties Obtained Through Application Of Ionized Jet Deposition Technique

Authors: M. Montesissa, M. Boi, E. Sassoni, E. Lopo, D. Ghezzi, N. Baldini, G. Graziani

Abstract: Biomedical devices, in orthopedics and dentistry, must favor osseointegration, which requires promoting host cells colonization, adhesion and differentiation. Lack of these characteristics would lead to mobilization of the prosthesis, resulting in failure. Therefore, in recent years, the attention has been focus on the interaction at the bio-interface, between biomaterials and host tissues. In this perspective, the surface properties of the biomaterial gain more importance.

Previous results of the research group have shown that biomimetic thin films can be successfully obtained by Ionized Jet Deposition (IJD) of bone apatite, directly from bovine bone targets. The so-obtained coatings have shown excellent behavior in vitro and in vivo, as they can promote cells adhesion, proliferation and differentiation towards an osteogenic lineage and can accelerate bone formation. However, the behavior of the coatings depend on their specific morphologic and compositional properties, which can be finely tuned and are also influenced by the substrate on which the deposition is performed.

For this reason, here, we systematically investigate the effect of the substrate on the coatings properties and, for the first time, we optimize the properties of the coatings by deposition in temperature.

Biogenic hydroxyapatite (bHA) films are deposited both on titanium alloy and on polymeric substrates, and their characteristics are compared, in terms of chemical composition (EDS and FT-IR), morphology and superficial topography (FEG-SEM), adhesion (scratch test), and crystallinity (XRD and FT-IR). Then, we investigate how different deposition parameters (time and substrate heating during deposition – at 100,200 and 400°C) can influence the coating properties.

Results show that IJD technique permit to obtain biomimetic coatings on different substrates, with properties that can be finely tuned, in terms of morphology, adhesion and crystallinity degree, all having important impact on the in vitro and in vivo behavior.

To conclude, the IJD technique allows the fabrication of biogenic hydroxyapatite coatings on substrates, and represents an innovative and promising technique for future applications in the orthopedic and dentistry fields.

3. Title: Effect of deposition temperature and Si addition on the structure, mechanical and degradation properties of Mg doped hydroxyapatite

Authors: Anca Constantina Parau^a, Mihaela Dinu^a, Cosmin Mihai Cotrut^b, Iulian Pana^a, Diana Maria Vranceanu^b, Lidia R.Constantin^a, Ioana Maria Marinescu^a, Catalin Vitelaru^a, Alina Vladescu (Dragomir)^a,c,*

Abstract: Degradable and non-degradable biomaterials are two categories that can be used to classify existing biomaterials, being a solution for eliminating a second operation for the implant when the tissue has properly recovered, which is one of the most significant advantages of these types of devices. In the present paper, the effect of deposition temperature and Si addition on the structure, mechanical and degradation properties of Mg doped hydroxyapatite was investigated. The coatings were obtained by RF magnetron sputtering technique at room temperature (RT) and 200^oC on AZ31B alloy substrate. By increasing the deposition temperature from room temperature up to 200 ^oC, the degradation rate of coatings was delayed. The Si addition into Mg doped hydroxyapatite coatings lead to improve of mechanical and degradation rate.