This topic will focus on the recent advances in design and functionalisation of ceramic, metallic and polymeric materials, from nano to macroscopic structures. This topic will include: -Fundamentals of deposition processes, growth modeling, plasma-surface interactions and in situ diagnostics -New deposition processes -Nanostructured coatings -Correlation of microstructure and functional properties -Hard, wear and oxidation resistant coatings -Characterization methods to determine the properties of the films in their conditions of use (high temperature, high pressure, irradiation, gaseous environment…) -New materials in thin film form: high entropy alloys, oxynitrides, intermetallics… -Design of novel thin films for energy conversion, save and storage -Coatings for biological applications

National Institute for Research and Development in Optoelectronics

Polytechnic University of Bucharest

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Frontiers in Materials

Thin Solid Films

Biomaterials

Frontiers in Bioengineering and Biotechnology

Frontiers in Molecular Biosciences

Functional surfaces and interfaces

Research dedicated to the most adequate biocompatible material for insertion in biological internal environment has led to the development of many alloys, starting with stainless steels or Mg based alloy (<a href="#B1">Froes and Qian, 2018</a>). The implant biocompatibility is directly related with its corrosion resistance in the surrounding environment and to measure of how safe and compatible is with a biological system. Titanium and its alloys and the austenitic stainless steels such as 316 L have in general a good corrosion resistance, being widely used for implants. Nevertheless, in strong mechanical stressed or non-oxygenated environments, all alloys are susceptible to corrosion. Moreover, all metallic materials have limitless bioactivity abilities which are related to low osseointegration capabilities and at the end will lead to a rejection of the implant by the human body (<a href="https://doi.org/10.3389/fmats.2016.00040">Monsees</a>). Material properties improvement by coating with suitable thin films represents a modern approach in material processing technologies, being recommendable to enhance qualities required also for biocompatibility. The recent rapid progress in biomedical requires materials of good mechanical properties and high resistance to wear and corrosion especially under the influence of dynamic loads along with high level of biocompatibility. These requirements can be satisfied by producing surface coatings of specified composition and structure (<a href="#B2">Vladescu et al., 2016</a>). The clinical advantage of these coatings is due to their biocompatible properties which are dependent upon their composition and structure such as stoichiometry, surface morphology, mechanical strength, corrosion and wear resistance.There is a wide variety of coating which could be used for biomedical applications such as oxides, carbides, oxynitrides or calcium phosphates (<a href="https://doi.org/10.3389/fmats.2018.00003">Prosolov et al.</a>; <a href="https://doi.org/10.3389/fmats.2016.00046">Birlik et al.</a>). Hydroxyapatite has been widely studied as coating for improving the fixation between bone and metallic implants, being the most biocompatible and bioactive coating due to its chemical similarity to the mineral component of human natural bone (<a href="https://doi.org/10.3389/fmats.2018.00003">Prosolov et al.</a>; <a href="#B2">Vladescu et al., 2016</a>). Various surface modification techniques are used to obtain biocompatible coatings, such as electrochemical deposition, laser deposition, plasma spraying, sol-gel, magnetron sputtering. By comparing all these methods, plasma spraying is currently the most commercially used method due to its high deposition rates and low cost (<a href="#B2">Vladescu et al., 2016</a>).In the other hand tissue engineering aims to bring together biomaterials, cells and signaling molecules within properly designed microenvironments in order to create viable treatment options for the lost or malfunctioning tissues (<a href="https://doi.org/10.3389/fmats.2016.00040">Monsees</a>). Design and production of scaffolds and cell-laden grafts that can mimic the complex structure and function of native tissues are among the most important elements of tissue engineering strategy (<a href="https://doi.org/10.3389/fmats.2018.00024">Bayrak and Yilgor Huri</a>). Therefore, tissue engineering and regenerative tissue remain a challenge. The present studies focuses on recent advancements in the development of strategies for interface engineering using different biomaterial surface modification techniques. Anyway, an artificial material, small or not, bulk or coated, after insertion into the human body suffers in vivo degradation, leading to the damage of integrity which is in detriment to their performance, which accelerate the device failure, electrical shorting, and following surgical removal. The degradation products can also migrate to the surrounding tissue or other functional organs and change their functionality. The main goal is to minimize the negative effect both on the human body and material and to delay or repeal the rejection process.The article collection on Functional surfaces and interfaces topic contains 5 papers accepted based on their scientific quality, originality, technical merit and timeliness of submission. As the Guest Editors, we would like to express our gratitude and appreciation to all of the reviewers for their competent efforts, and to editors and publishers who have supported this Research Topic.All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.We would like to thank all the authors, reviewers, editors, and publishers who have supported this Research Topic.

Editorial: Functional Surfaces and Interfaces

Tissue engineering aims to bring together biomaterials, cells, and signaling molecules within properly designed microenvironments in order to create viable treatment options for the lost or malfunctioning tissues. Design and production of scaffolds and cell-laden grafts that mimic the complex structural and functional features of tissues are among the most important elements of tissue engineering strategy. Although all tissues have their own complex structure, an even more complex case in terms of engineering a proper carrier material is encountered at the tissue interfaces, where two distinct tissues come together. The interfaces in the body can be examined in four categories; cartilage-bone and ligament-bone interfaces at the knee and the spine, tendon-bone interfaces at the shoulder and the feet, and muscle-tendon interface at the skeletal system. These interfaces are seen mainly at the soft-to-hard tissue transitions and they are especially susceptible to injury and tear due to the biomechanical inconsistency between these tissues where high strain fields are present. Therefore, engineering the musculoskeletal tissue interfaces remain a challenge. This review focuses on recent advancements in strategies for musculoskeletal interface engineering using different biomaterial-based platforms and surface modification techniques.

Tissue engineering aims to bring together biomaterials, cells, and signaling molecules within properly designed microenvironments in order to create viable treatment options for the lost or malfunctioning tissues. Design and production of scaffolds and cell-laden grafts that mimic the complex structural and functional features of tissues are among the most important elements of tissue engineering strategy. Although all tissues have their own complex structure, an even more complex case in terms of engineering a proper carrier material is encountered at the tissue interfaces, where two distinct tissues come together. The interfaces in the body can be examined in four categories; cartilage-bone and ligament-bone interfaces at the knee and the spine, tendon-bone interfaces at the shoulder and the feet, and muscle-tendon interface at the skeletal system. These interfaces are seen mainly at the soft-to-hard tissue transitions and they are especially susceptible to injury and tear due to the biomechanical inconsistency between these tissues where high strain fields are present. Therefore, engineering the musculoskeletal tissue interfaces remain a challenge. This review focuses on recent advancements in strategies for musculoskeletal interface engineering using different biomaterial-based platforms and surface modification techniques.

Engineering Musculoskeletal Tissue Interfaces

Nanocrystalline Zn-substituted hydroxyapatite coatings were deposited by radiofrequency magnetron sputtering on the surface of ultrafine-grained titanium substrates. Cross-section transmission electron microscopy provided information about the morphology and texture of the thin film while in-column energy dispersive X-ray analysis confirmed the presence of Zn in the coating. The Zn-substituted hydroxyapatite coating was formed by an equiaxed polycrystalline grain structure. Effect of substrate crystallinity on the structure of deposited coating is discussed. An amorphous TiO2 sublayer of 8-nm thickness was detected in the interface between the polycrystalline coating and the Ti substrate. Its appearance in the amorphous state is attributed to prior to deposition etching of the substrate and subsequent condensation of oxygen-containing species sputtered from the target. This layer contributes to the high coating-to-substrate adhesion. The major P–O vibrational modes of high intensity were detected by Raman spectroscopy. The Zn-substituted hydroxyapatite could be a material of choice when antibacterial osteoconductive coating with a possibility of withstanding mechanical stress during implantation and service is needed.

Nanocrystalline Zn-substituted hydroxyapatite coatings were deposited by radiofrequency magnetron sputtering on the surface of ultrafine-grained titanium substrates. Cross-section transmission electron microscopy provided information about the morphology and texture of the thin film while in-column energy dispersive X-ray analysis confirmed the presence of Zn in the coating. The Zn-substituted hydroxyapatite coating was formed by an equiaxed polycrystalline grain structure. Effect of substrate crystallinity on the structure of deposited coating is discussed. An amorphous TiO2 sublayer of 8-nm thickness was detected in the interface between the polycrystalline coating and the Ti substrate. Its appearance in the amorphous state is attributed to prior to deposition etching of the substrate and subsequent condensation of oxygen-containing species sputtered from the target. This layer contributes to the high coating-to-substrate adhesion. The major P–O vibrational modes of high intensity were detected by Raman spectroscopy. The Zn-substituted hydroxyapatite could be a material of choice when antibacterial osteoconductive coating with a possibility of withstanding mechanical stress during implantation and service is needed.

Thin Bioactive Zn Substituted Hydroxyapatite Coating Deposited on Ultrafine-Grained Titanium Substrate: Structure Analysis

We present a generalization of the three-layer model to calculate the surface second-harmonic generation (SSGH) yield, which includes the depth dependence of the surface non-linear second-order susceptibility tensor χ(−2ω; ω, ω). This model considers that the surface is represented by three regions or layers. The first layer is a semi-infinite vacuum region with a dielectric function ϵv(ω) = 1, from where the fundamental electric field impinges on the material. The second layer is a thin layer (ℓ) of thickness d characterized by a dielectric function ϵℓ(ω), and it is in this layer where the SSHG takes place. We consider the position of χ(−2ω; ω, ω) within this surface layer. The third layer is the bulk region denoted by b and characterized by ϵb(ω). We include the effects caused by the multiple reflections of both the fundamental and the second-harmonic (SH) fields that take place within the thin layer ℓ. As a test case, we calculate χ(−2ω; ω, ω) for the Si(111)(1 × 1):H surface and present a layer-by-layer study of the susceptibility to elucidate the depth dependence of the SHG spectrum. We then use the depth-dependent three-layer model to calculate the SSHG yield and contrast the calculated spectra with experimental data. We produce improved results over previous published work, as this treatment can reproduce key spectral features, is computationally viable for many systems, and most importantly remains completely ab initio.

We present a generalization of the three-layer model to calculate the surface second-harmonic generation (SSGH) yield, which includes the depth dependence of the surface non-linear second-order susceptibility tensor χ(−2ω; ω, ω). This model considers that the surface is represented by three regions or layers. The first layer is a semi-infinite vacuum region with a dielectric function ϵv(ω) = 1, from where the fundamental electric field impinges on the material. The second layer is a thin layer (ℓ) of thickness d characterized by a dielectric function ϵℓ(ω), and it is in this layer where the SSHG takes place. We consider the position of χ(−2ω; ω, ω) within this surface layer. The third layer is the bulk region denoted by b and characterized by ϵb(ω). We include the effects caused by the multiple reflections of both the fundamental and the second-harmonic (SH) fields that take place within the thin layer ℓ. As a test case, we calculate χ(−2ω; ω, ω) for the Si(111)(1 × 1):H surface and present a layer-by-layer study of the susceptibility to elucidate the depth dependence of the SHG spectrum. We then use the depth-dependent three-layer model to calculate the SSHG yield and contrast the calculated spectra with experimental data. We produce improved results over previous published work, as this treatment can reproduce key spectral features, is computationally viable for many systems, and most importantly remains completely ab initio.

Depth-Dependent Three-Layer Model for the Surface Second-Harmonic Generation Yield

In this paper, Ni–TiO2 nanocomposite coatings with different sizes of TiO2 nanoparticles were successfully prepared by electrodeposition process from a nickel electrolyte in which the TiO2 nanoparticles were suspended. The influence of relevant deposition parameters on the nanocomposite coating characteristics was discussed. X-ray diffractometer has been applied in order to investigate the phase structure of the nanocomposite coatings. The surface morphology of nanocomposite coatings was characterized by a scanning electron microscopy equipped with an energy dispersive spectroscopy. The electrodeposited nanocomposite coatings obtained at different deposition parameters were evaluated for their mechanical and corrosive properties. Obtained results show that the size of TiO2 nanoparticles and applied current density during deposition process has a direct effect on mechanical and corrosive properties of nanocomposite coatings. Increasing current density and smaller nanoparticle size has affirmative effect on mechanical properties, whereas corrosion resistance of nanocomposite coatings deposited at 3 A dm−2 current density are higher than the coatings prepared at higher current density values.

In this paper, Ni–TiO2 nanocomposite coatings with different sizes of TiO2 nanoparticles were successfully prepared by electrodeposition process from a nickel electrolyte in which the TiO2 nanoparticles were suspended. The influence of relevant deposition parameters on the nanocomposite coating characteristics was discussed. X-ray diffractometer has been applied in order to investigate the phase structure of the nanocomposite coatings. The surface morphology of nanocomposite coatings was characterized by a scanning electron microscopy equipped with an energy dispersive spectroscopy. The electrodeposited nanocomposite coatings obtained at different deposition parameters were evaluated for their mechanical and corrosive properties. Obtained results show that the size of TiO2 nanoparticles and applied current density during deposition process has a direct effect on mechanical and corrosive properties of nanocomposite coatings. Increasing current density and smaller nanoparticle size has affirmative effect on mechanical properties, whereas corrosion resistance of nanocomposite coatings deposited at 3 A dm−2 current density are higher than the coatings prepared at higher current density values.

Preparation and Characterization of Ni–TiO2 Nanocomposite Coatings Produced by Electrodeposition Technique

With regard to biocompatibility, the cardinal requirement for dental implants and other medical devices that are in long-term contact with tissue is that the material does not cause any adverse effect to the patient. To warrant stability and function of the implant, proper osseointegration is a further prerequisite. Cells interact with the implant surface as the interface between bulk material and biological tissue. Whereas structuring, deposition of a thin film, or other modifications of the surface are crucial parameters in determining favorable adhesion of cells, corrosion of metal surfaces and release of ions can affect cell viability. Both parameters are usually tested using in vitro cytotoxicity and adhesion assays with bone or fibroblasts cells. For bioactive surface modifications, further tests should be considered for biocompatibility evaluation. Depending on the type of modification, this may include analysis of specific cell functions or the determination of antimicrobial activities. The latter is of special importance as bacteria and yeast present in the oral cavity can be introduced during the implantation process and this may lead to chronic infections and implant failure. An antimicrobial coating of the implant is a way to avoid that. This review describes the essential biocompatibility assays for evaluation of new implant materials required by International Organization for Standardization 10993 and also provides an overview of recent test methods for specific coatings of dental implants.

With regard to biocompatibility, the cardinal requirement for dental implants and other medical devices that are in long-term contact with tissue is that the material does not cause any adverse effect to the patient. To warrant stability and function of the implant, proper osseointegration is a further prerequisite. Cells interact with the implant surface as the interface between bulk material and biological tissue. Whereas structuring, deposition of a thin film, or other modifications of the surface are crucial parameters in determining favorable adhesion of cells, corrosion of metal surfaces and release of ions can affect cell viability. Both parameters are usually tested using in vitro cytotoxicity and adhesion assays with bone or fibroblasts cells. For bioactive surface modifications, further tests should be considered for biocompatibility evaluation. Depending on the type of modification, this may include analysis of specific cell functions or the determination of antimicrobial activities. The latter is of special importance as bacteria and yeast present in the oral cavity can be introduced during the implantation process and this may lead to chronic infections and implant failure. An antimicrobial coating of the implant is a way to avoid that. This review describes the essential biocompatibility assays for evaluation of new implant materials required by International Organization for Standardization 10993 and also provides an overview of recent test methods for specific coatings of dental implants.