Superior Pre-Osteoblast Cell Response of Etched Ultrafine-Grained Titanium with a Controlled Crystallographic Orientation

Comparative analysis of the physical, chemical, and microbiological properties of Ti-6Al-4V disks produced by different methods and subjected to surface treatments

STATEMENT OF PROBLEM

To improve the osseointegration of dental implants and reduce microbiological growth, different micro- and nanoscale surface topographies can be used.

PURPOSE

The purpose of this in vitro study was to evaluate the influence of Ti-6Al-4V with 4 surfaces, machined (DU), machined+hydroxyapatite (DUHAp), machined+acid-alkali treatment (DUAA), and additive manufacturing (DMA), on the physical, chemical, and microbiological properties.

MATERIAL AND METHODS

The topography of Ti-6Al-4V disks with the 4 surfaces was evaluated by scanning electron microscopy (SEM), the chemical composition by energy dispersive X-ray spectroscopy (EDS), and the crystalline structure by X-ray diffraction (XRD). Physical and chemical properties were analyzed by using wettability and surface free energy, roughness, and microbial adhesion against Staphylococcus aureus by colony forming units (CFU). One-way ANOVA analysis of variance and the Tukey multiple comparisons test were applied to evaluate the data, except CFU, which was submitted to the Kruskal-Wallis nonparametric test (α=.05).

RESULTS

DU photomicrographs showed a topography characteristic of a polished machined surface, DUHAp and DUAA exhibited patterns corresponding to the surface modifications performed, and in DMA the presence of partially fused spherical particles was observed. The EDS identified chemical elements inherent in the Ti-6Al-4V, and the DUHAp and DUAA disks also had the ions from the treatments applied. XRD patterns revealed similarities between DU and DMA, as well as characteristic peaks of hydroxyapatite (HA) in the DUHAp disk and the DUAA. Compared with DU and DMA the DUHAp and DUAA groups showed hydrophilic behavior with smaller contact angles and higher surface free energy (P<.05). DMA showed a higher mean value of roughness, different from the others (P<.05), and a higher CFU for S. aureus (P=.006).

CONCLUSIONS

DUHAp and DUAA showed similar behaviors regarding wettability, surface free energy, and bacterial adhesion. Among the untreated groups, DMA exhibited higher roughness, bacterial adhesion, and lower wettability and surface free energy.

Laser cladding of bioactive glass coating on pure titanium substrate with highly refined grain structure

Free from toxic elements biomaterial potentially applicable for load bearing biomedical implants was obtained for the first time by laser cladding of S520 bioactive glass onto ultrafine-grained commercially pure titanium. The cladding process affected the refined structure of the substrate inducing martensitic transformation near its surface. The α' acicular martensite gradually passes into relatively large grains with increasing distance from the substrate surface, which subsequently are transformed into smaller grains of about 2 μm in diameter. Both the melted zone, where the martensite crystalline structure was found, and the HAZ are characterised by relatively lower hardness in comparison with that of the substrate core indicating increased ductility. Such a combination of zones with different properties may have a synergistic effect and is beneficial for the obtained biomaterial. A characteristic region in the form of about 3 μm width band was formed in the melted zone at about 10 μm below the titanium surface. The results of EDS analysis indicate that several glass elements moved into the region while the titanium content in the same area was decreased. High bioactivity of the coated S520 glass was revealed by in vitro testing with SBF solution and almost complete reduction of P concentration occurred after 14 days.

The Effects of Chemical Etching and Ultra-Fine Grain Structure of Titanium on MG-63 Cells Response

In this work, we study the influence of the surface properties of ultrafine grained (UFG) and coarse grained (CG) titanium on the morphology, viability, proliferation and differentiation of osteoblast-like MG-63 cells. Wet chemical etching in H2SO4/H2O2 and NH4OH/H2O2 solutions was used for producing surfaces with varying morphology, topography, composition and wettability. The topography and morphology have been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition was determined by time of flight mass-spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). The results showed that it is possible to obtain samples with different compositions, hydrophilicity, topography and nanoscale or/and microscale structures by changing the etching time and the type of etching solution. It was found that developed topography and morphology can improve spreading and proliferation rate of MG-63 cells. A significant advantage of the samples of the UFG series in comparison with CG in adhesion, proliferation at later stages of cultivation (7 days), higher alkaline phosphatase (ALP) activity and faster achievement of its maximum values was found. However, there is no clear benefit of the UFG series on osteopontin (OPN) expression. All studied samples showed no cytotoxicity towards MG-63 cells and promoted their osteogenic differentiation.

Biological Applications of Severely Plastically Deformed Nano-Grained Medical Devices: A Review

Metallic materials are widely used for fabricating medical implants due to their high specific strength, biocompatibility, good corrosion properties, and fatigue resistance. Recently, titanium (Ti) and its alloys, as well as stainless steel (SS), have attracted attention from researchers because of their biocompatibility properties within the human body; however, improvements in mechanical properties while keeping other beneficial properties unchanged are still required. Severe plastic deformation (SPD) is a unique process for fabricating an ultra-fine-grained (UFG) metal with micrometer- to nanometer-level grain structures. SPD methods can substantially refine grain size and represent a promising strategy for improving biological functionality and mechanical properties. This present review paper provides an overview of different SPD techniques developed to create nano-/ultra-fine-grain-structured Ti and stainless steel for improved biomedical implant applications. Furthermore, studies will be covered that have used SPD techniques to improve bone cell proliferation and function while decreasing bacterial colonization when cultured on such nano-grained metals (without resorting to antibiotic use).

Texture-Governed Cell Response to Severely Deformed Titanium

The phenomenon of superior biological behavior observed in titanium processed by an unconventional severe plastic deformation method, that is, hydrostatic extrusion, has been described within the present study. In doing so, specimens varying significantly in the crystallographic orientation of grains, yet exhibiting comparable grain refinement, were meticulously investigated. The aim was to find the clear origin of enhanced biocompatibility of titanium-based materials, having microstructures scaled down to the submicron range. Texture, microstructure, and surface characteristics, that is, wettability, roughness, and chemical composition, were examined as well as protein adsorption tests and cell response studies were carried out. It has been concluded that, irrespective of surface properties and mean grain size, the (101̅0) crystallographic plane favors endothelial cell attachment on the surface of the severely deformed titanium. Interestingly, an enhanced albumin, fibronectin, and serum adsorption as well as clearly directional growth of the cells with preferentially oriented cell nuclei have been observed on the surfaces having (0001) planes exposed predominantly. Overall, the biological response of titanium fabricated by severe plastic deformation techniques is derived from the synergistic effect of surface irregularities, being the effect of refined microstructures, surface chemistry, and crystallographic orientation of grains rather than grain refinement itself.

In vitro and in vivo evaluation of structurally-controlled silk fibroin coatings for orthopedic infection and in-situ osteogenesis

Biomedical device-associated infections (BAI) and osteosynthesis are two main complications following the orthopedic implant surgery, especially while infecting bacteria form a mature biofilm, which can protect the organisms from the host immune system and antibiotic therapy. Comparing with the single antibiotics therapeutic method, the combination of silver nanoparticles (AgNPs) and conventional antibiotics exert a high level of antibacterial activity. Nevertheless, one major issue that extremely restricts the potential application of AgNP/antiviotics is the uncontrolled release. Moreover, the lack of osteogenic ability may cause the osteosynthesis. Thus, herein we fabricated a structure-controlled drug-loaded silk fibroin (SF) coating that can achieve the size and release control of AgNPs and high efficient osteogenesis. Three comparative SF-based coatings were fabricated: α-structured coating (α-helices 32.7%,), m-structured coating (β-sheets 28.3%) and β-structured coating (β-sheets 41%). Owning to the high content of α-helices structure and small AgNPs (20 nm), α-structured coating displayed better protein adsorption and hydrophilicity, as well as pH-dependent and long-lasting antibacterial performance. In vitro studies demonstrated that α coating showed biocompatibility (cellular attachment, spreading and proliferation), high ALP expression, collagen secretion and calcium mineralization. Moreover, after one month subcutaneous implantation in vivo, α-structured coating elicited minimal, comparable inflammatory response. Additionally, in a rabbit femoral defect model, α-structured coating displayed a significant improvement on the generation of new-born bone and bonding between the new bone and the tissue, implying a rapid and durable osteointegration. Expectedly, this optimized structure-controlled SF-based coating can be an alternative and prospective solution for the current challenges in orthopedics. STATEMENT OF SIGNIFICANCE: In this study, an AgNPs/Gentamycin-loaded structured-controlled silk fibroin coatings were constructed on Ti implant's surface to guarantee the success of implantation even in the face of bacterial infection. In comparison, the α-structured coating had the lowest content of β-sheets structure (19.0%) and the smallest particle size of AgNPs (~ 20 nm), and owned pH-responsive characteristic due to reversible α-helices structural. Thanks to pH-responsive release of Ag⁺, the α-structure coating could effectively inhibit adhesive bacteria and kill planktonic bacteria by releasing a large amount of reactive oxygen radicals. Through in vitro biological results (cell proliferation, differentiation and osteogenic gene expression) and in vivo rabbit femur implantation results, the α-structure coating had good biocompatible and osteogenic properties.

Influence of Implant Material and Surface on Mode and Strength of Cell/Matrix Attachment of Human Adipose Derived Stromal Cell

A fundamental step for cell growth and differentiation is the cell adhesion. The purpose of this study was to determine the adhesion of different cell lineages, adipose derived stromal cells, osteoblasts, and gingival fibroblast to titanium and zirconia dental implants with different surface treatments. Primary cells were cultured on smooth/polished surfaces (titanium with a smooth surface texture (Ti-PT) and machined zirconia (ZrO₂-M)) and on rough surfaces (titanium with a rough surface texture (Ti-SLA) and zirconia material (ZrO₂-ZLA)). Alterations in cell morphology (f-actin staining and SEM) and in expression of the focal adhesion marker were analysed after 1, 7, and 14 days. Statistical analysis was performed by one-way ANOVA with a statistical significance at p = 0.05. Cell morphology and cytoskeleton were strongly affected by surface texture. Actin beta and vimentin expressions were higher on rough surfaces (p < 0.01). Vinculin and FAK expressions were significant (p < 0.05) and increased over time. Fibronectin and laminin expressions were significant (p < 0.01) and did not alter over time. Strength of cell/material binding is influenced by surface structure and not by material. Meanwhile, the kind of cell/material binding is regulated by cell type and implant material.

Effect of surface grain boundary density on preosteoblast proliferation on titanium

Studies since 2004 have shown that the cytocompatibility of ultrafine grain (UG) commercial purity (CP) titanium exceeds that of coarse grain (CG) CP titanium (Ti) by 30% to 20-fold. To isolate the factors affecting this large reported variability of CP titanium's cytocompatibility, discs of UG and CG titanium were fabricated with controlled texture and roughness. The discs were seeded with MC3T3-E1 pre-osteoblastic cells and cultured for 72 h. The proliferation of cells on polished UG-Ti exceeded unpolished CG-Ti 3.04-fold. Cell proliferation was found to correlate with a new biophysical parameter, the average grain boundary length per surface-attached cell.

Enhanced interfacial adhesion and osseointegration of anodic TiO2 nanotube arrays on ultra-fine-grained titanium and underlying mechanisms

The poor adhesion of anodic TiO₂ nanotubes (TNTs) arrays on titanium (Ti) substrates adversely affects applications in many fields especially biomedical engineering. Herein, an efficient strategy is described to improve the adhesion strength of TNTs by performing grain refinement in the underlying Ti substrate via high-pressure torsion processing, as a larger number of grain boundaries can provide more interfacial mechanical anchorage. This process also improves the biocompatibility and osseointegration of TNTs by increasing the surface elastic modulus. The TNTs in length of 0.4 µm have significantly larger adhesion strength than the 2.0 µm long ones because the shorter TNTs experience less interfacial internal stress. However, post-anodization annealing reduces the fluorine concentration in TNTs and adhesion strength due to the formation of interfacial cavities during crystallization. The interfacial structure of TNTs/Ti system and the mechanism of adhesion failures are further investigated and discussed. STATEMENT OF SIGNIFICANCE: Self-assembled TiO₂ nanotubes (TNTs) prepared by electrochemical anodization have a distinct morphology and superior properties, which are commonly used in photocatalytic systems, electronic devices, solar cells, sensors, as well as biomedical implants. However, the poor adhesion between the TNTs and Ti substrate has hampered wider applications. Here in this study, we describe an efficient strategy to improve the adhesion strength of TNTs by performing grain refinement in the underlying Ti substrate via high-pressure torsion (HPT) processing. The interfacial structure of TNTs/Ti system and the mechanism of adhesion failure are systematically studied and discussed. Our findings not only develop the knowledge of TNTs/Ti system, but also provide new insights into the design of Ti-based implants for orthopedic applications.

Molybdenum Disulfide Surface Modification of Ultrafine-Grained Titanium for Enhanced Cellular Growth and Antibacterial Effect

The commercially pure Ti (CP Ti) and equal-channel angular pressing (ECAP) processed Ti can contribute to the downsizing of medical devices with their superior mechanical properties and negligible toxicity. However, the ECAP-processed pure Ti has the risk of bacterial infection. Here, the coarse- and ultrafine-grained Ti substrates were surface-modified with molybdenum disulfide (MoS2) to improve the cell proliferation and growth with antibacterial effect for further dental applications. According to in vitro tests using the pre-osteoblast of MC3T3-E1 cell and a bacterial model of Escherichia coli (E. coli), MoS2 nanoflakes coated and ECAP-processed Ti substrates showed a significant increase in surface energy and singlet oxygen generation resulting in improved cell attachment and antibacterial effect. In addition, we confirmed the stability of the surface modified Ti substrates in a physiological solution and an artificial bone. Taken together, MoS2 modified and ECAP-processed Ti substrates might be successfully harnessed for various dental applications.

Immunomodulatory Effects of Calcium and Strontium Co-Doped Titanium Oxides on Osteogenesis

The effects of calcium (Ca) or strontium (Sr) on host osteogenesis and immune responses have been investigated separately. In clinical practice, these two elements may both be present around an orthopedic device, but their potential synergistic effects on osteogenesis and the immune response have not been explored to date. In this work, we investigated the immunomodulatory effects of Ca and Sr co-doped titanium oxides on osteogenesis in vitro using the mouse macrophage cell line RAW 264.7 alone and in co-culture with mouse bone mesenchymal stem cells (BMSCs), and in vivo using a mouse air-pouch model. Coatings containing Ca and Sr at different concentration ratios were fabricated on titanium substrates using micro-arc oxidation and electrochemical treatment. The in vitro and in vivo results demonstrated that the Ca and Sr concentration ratio has a marked influence on macrophage polarization. The coating with a Ca/Sr ratio of 2:1 was superior to those with other Ca and/or Sr concentrations in terms of modulating M2 polarization, which enhanced osteogenic differentiation of mouse BMSCs in co-culture. These findings suggest that the osteoimmunomodulatory effect of a titanium-oxide coating can be enhanced by modulating the concentration ratio of its components.