Titanium and Protein Adsorption: An Overview of Mechanisms and Effects of Surface Features

Titanium and its alloys, specially Ti6Al4V, are among the most employed materials in orthopedic and dental implants. Cells response and osseointegration of implant devices are strongly dependent on the body–biomaterial interface zone. This interface is mainly defined by proteins: They adsorb immediately after implantation from blood and biological fluids, forming a layer on implant surfaces. Therefore, it is of utmost importance to understand which features of biomaterials surfaces influence formation of the protein layer and how to guide it. In this paper, relevant literature of the last 15 years about protein adsorption on titanium-based materials is reviewed. How the surface characteristics affect protein adsorption is investigated, aiming to provide an as comprehensive a picture as possible of adsorption mechanisms and type of chemical bonding with the surface, as well as of the characterization techniques effectively applied to model and real implant surfaces. Surface free energy, charge, microroughness, and hydroxylation degree have been found to be the main surface parameters to affect the amount of adsorbed proteins. On the other hand, the conformation of adsorbed proteins is mainly dictated by the protein structure, surface topography at the nano-scale, and exposed functional groups. Protein adsorption on titanium surfaces still needs further clarification, in particular concerning adsorption from complex protein solutions. In addition, characterization techniques to investigate and compare the different aspects of protein adsorption on different surfaces (in terms of roughness and chemistry) shall be developed

Porous Titanium by Additive Manufacturing: A Focus on Surfaces for Bone Integration

Additive manufacturing (AM) is gaining increasing interest for realization of customized porous titanium constructs for biomedical applications and, in particular, for bone substitution. As first, the present review gives a short introduction on the techniques used for additive manufacturing of Ti/Ti-Alloys (Direct Energy Deposition—DED, Selective Laser Melting—SLM and Electron Beam Melting—EBM) and on the main bulk properties of additively manufactured titanium porous structures. Then, it discusses the main advancements in surface modifications of additively manufactured titanium constructs for bone contact applications. Even if specific surface modifications of constructs from AM are currently not widely explored, it is a critical open issue for application in biomedical implants. Some thermal, chemical, electrochemical, and hydrothermal treatments as well as different coatings are here described. The main aim of these treatments is the development of surface micro/nano textures, specific ion release, and addition of bioactivity to induce bone bonding and antibacterial activity. Physicochemical characterizations, in vitro bioactivity tests, protein absorption, in vitro (cellular/bacterial) and in vivo tests reported in the literature for bare and surface modified AM Ti-based constructs are here reviewed. Future perspectives for development of innovative additively manufactured titanium implants are also discussed

Fatigue resistance of light alloy sheets undergoing eco-friendly chemical milling: metallurgical and chemical aspects

Abstract Component lightening is a key issue for automotive and aerospace industries. Lightening processes on design profile are not always possible by means of traditional machining processes. Then, chemical milling processes are used, often by acid etching. The effect on fatigue behavior is related on many factors, such as chemical surface composition and surface roughness. Material removal through chemical milling is in particular interesting for Additive manufactured components where surface finish still remains a parameter difficult to be controlled and repeated. The environmental aspects related to alkaline and acid processing are still an important issue. In the present paper, an overview on chemical milling for component lightening is presented with focus of the effects on mechanical and chemical resistance of materials. Then, the results of an experimental campaign on an aluminum alloy is presented. In particular, high cycle fatigue tests results are presented on specimens subjected or not subjected to an eco-friendly alkaline chemical milling process (called " Green Etching") and chemical, profilometric and metallographic analyses are presented and related to fatigue resistance results. Wettability and surface charge results will be presented

Tannic Acid Coatings to Control the Degradation of AZ91 Mg Alloy Porous Structures

Porous structures of magnesium alloys are promising bioimplants due to their biocompatibility and biodegradability. However, their degradation is too rapid compared to tissue regeneration and does not allow a progressive metal substitution with the new biological tissue. Moreover, rapid degradation is connected to an accelerated ion release, hydrogen development, and pH increase, which are often causes of tissue inflammation. In the present research, a natural organic coating based on tannic acid was obtained on Mg AZ91 porous structures without toxic reagents. Mg AZ91 porous structures have been prepared by the innovative combination of 3D printing and investment casting, allowing fully customized objects to be produced. Bare and coated samples were characterized using scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), fluorescence microscopy, Fourier transformed infrared spectroscopy (FTIR), tape adhesion test, Folin– Ciocalteu, and degradation tests. Different parameters (solvent, dipping time) were compared to optimize the coating process. The optimized coating was uniform on the outer and inner surfaces of the porous structures and significantly reduced the material degradation rate and pH increase in physiological conditions (phosphate-buffered saline—PBS)

UV-Cured Chitosan-Based Hydrogels Strengthened by Tannic Acid for the Removal of Copper Ions from Water

In this work, a new environmentally friendly material for the removal of heavy metal ions was developed to enhance the adsorption efficiency of photocurable chitosan-based hydrogels (CHg). The acknowledged affinity of tannic acid (TA) to metal ions was investigated to improve the properties of hydrogels obtained from natural and renewable sources (CHg-TA). The hydrogel preparation was performed via a simple two-step method consisting of the photocrosslinking of methacrylated chitosan and its subsequent swelling in the TA solution. The samples were characterized using ATR-FTIR, SEM, and Folin–Ciocalteu (F&C) assay. Moreover, the mechanical properties and the ζ potential of CHg and CHg-TA were tested. The copper ion was selected as a pollutant model. The adsorption capacity (Qe) of CHg and CHg-TA was assessed as a function of pH. Under acidic conditions, CHg-TA shows a higher Qe than CHg through the coordination of copper ions by TA. At an alkaline pH, the phenols convert into a quinone form, decreasing the Qe of CHg-TA, and the performance of CHg was found to be improved. A partial TA release can occur in the copper solution due to its high hydrophilicity and strong acidic pH conditions. Additionally, the reusability of hydrogels was assessed, and the high number of recycling cycles of CHg-TA was related to its high mechanical performance (compression tests). These findings suggest CHg-TA as a promising green candidate for heavy metal ion removal from acidic wastewater

Natural Polyphenols and the Corrosion Protection of Steel: Recent Advances and Future Perspectives for Green and Promising Strategies

Corrosion is recognized as an unavoidable phenomenon and steel, particularly carbon steel, is strongly susceptible to corrosion. Corrosion damages cause serious material, energy, and economic losses as well as negative impacts on the environment. As a result, research interest has been focused on the development of effective corrosion prevention strategies. However, some of the most commonly used corrosion inhibitors, such as chromates and pyridines, are harmful to human and environmental health. Polyphenols are natural, non-toxic, and biodegradable compounds from plant sources or agricultural by-products. Polyphenols’ chelating capacity has been acknowledged since the 1990s, and tannins, in particular, have been widely exploited as green rust converters in phosphoric acid-based formulations to recover rusty steel. Polyphenolic compounds have recently been investigated as a method of corrosion prevention. This review overviews not only the polyphenolic rust converters, but also the application of green anticorrosive coatings containing polyphenols. Moreover, polyphenols were discussed as an active component in corrosion-inhibiting primers to also promote strong adhesion between the steel surface and the topcoat layer. Finally, an overview of the use of polyphenolic additives in coatings as sustainable systems to improve corrosion resistance is provided

Exploitation of tannic acid as additive for the adhesion enhancement of UV-curable bio-based coating

The interest in environmentally friendly coatings is rising to substitute for the oil-derived materials in the coating industry. In the present study, natural tannic acid (TA) is investigated as an additive to an epoxidized soybean oil-based (ESO) coating. TA solutions in propylene carbonate at two different concentrations were prepared and added to an ESO matrix with different weight ratios. The UV-curing process of the coatings was deeply assessed through real-time Fourier Transform Infrared (FTIR) spectroscopy and Differential Scanning photo Calorimetry (photo-DCS). A significant increase in high epoxy group conversion, around 90 %, was achieved thanks to the activated monomer mechanism, which involves the TA polyphenols. This mechanism accelerated the photocrosslinking process, but reduced the coatings' crosslinking density, as demonstrated by the dynamic thermal mechanical analysis. The hardness of coatings containing the TA additive decreased, while the hydrophobicity of the surface coatings remained unchanged after the TA incorporation. Lastly, the adhesion of the UV-cured coating was evaluated on low-carbon steel substrates. An outstanding enhancement in adhesion property was provided by the TA additive, whose phenols not only participate in the photocrosslinking reaction but also coordinate iron on the steel surface. Moreover, the influence of two different steel surface pre-treatments, the pickling and plasma processes, on the coatings' adhesion strength was studied

Nano-topography and functionalization with the synthetic peptoid GN2-Npm9 as a strategy for antibacterial and biocompatible titanium implants

In recent years, antimicrobial peptides (AMPs) have attracted great interest in scientific research, especially for biomedical applications such as drug delivery and orthopedic applications. Since they are readily degradable in the physiological environment, scientific research has recently been trying to make AMPs more stable. Peptoids are synthetic N-substituted glycine oligomers that mimic the structure of peptides. They have a structure that does not allow proteolytic degradation, which makes them more stable while maintaining microbial activity. This structure also brings many advantages to the molecule, such as greater diversity and specificity, making it more suitable for biological applications. For the first time, a synthesized peptoid (GN2-Npm9) was used to functionalize a nanometric chemically pre-treated (CT) titanium surface for bone-contact implant applications. A preliminary characterization of the functionalized surfaces was performed using the contact angle measurements and zeta potential titration curves. These preliminary analyses confirmed the presence of the peptoid and its adsorption on CT. The functionalized surface had a hydrophilic behaviour (contact angle = 30°) but the hydrophobic tryptophan-like residues were also exposed. An electrostatic interaction between the lysine residue of GN2-Npm9 and the surface allowed a chemisorption mechanism. The biological characterization of the CT_GN2-Nmp9 surfaces demonstrated the ability to prevent surface colonization and biofilm formation by the pathogens Escherichia coli and Staphylococcus epidermidis thus showing a broad-range activity. The cytocompatibility was confirmed by human mesenchymal stem cells. Finally, a bacteria-cells co-culture model was applied to demonstrate the selective bioactivity of the CT_GN2-Nmp9 surface that was able to preserve colonizing cells adhered to the device surface from bacterial infection