Designing new biocompatible glass-forming Ti75-x Zr10 Nbx Si15 (x = 0, 15) alloys: corrosion, passivity, and apatite formation.

Glass-forming Ti-based alloys are considered as potential new materials for implant applications. Ti75 Zr10 Si15 and Ti60 Zr10 Nb15 Si15 alloys (free of cytotoxic elements) can be produced as melt-spun ribbons with glassy matrix and embedded single ß-type nanocrystals. The corrosion and passivation behavior of these alloys in their homogenized melt-spun states have been investigated in Ringer solution at 37°C in comparison to their cast multiphase crystalline counterparts and to cp-Ti and ß-type Ti-40Nb. All tested materials showed very low corrosion rates as expressed in corrosion current densities icorr < 50 nA/cm(2). Electrochemical and surface analytical studies revealed a high stability of the new alloys passive states in a wide potential range. This corresponds to low passive current densities ipass = 2 ± 1 µA/cm(2) based on the growth of oxide films with thickness d <10 nm. A homogeneous constituent distribution in the melt-spun alloys is beneficial for stable surface passivity. The addition of Nb does not only improve the glass-forming ability and the mechanical properties but also supports a high pitting resistance even at extreme anodic polarization up to 4V versus SCE were oxide thickness values of d ∼35 nm are reached. With regard to the corrosion properties, the Nb-containing nearly single-phase glassy alloy can compete with the ß-type Ti-40Nb alloy. SBF tests confirmed the ability for formation of hydroxyapatite on the melt-spun alloy surfaces. All these properties recommend the new glass-forming alloys for application as wear- and corrosion-resistant coating materials for implants.

Elastic softening of ß-type Ti-Nb alloys by indium (In) additions.

Recent developments showed that ß-type Ti-Nb alloys are good candidates for hard tissue replacement and repair. However, their elastic moduli are still to be further reduced to match Young׳s modulus values of human bone, in order to avoid stress shielding. In the present study, the effect of indium (In) additions on the structural characteristics and elastic modulus of Ti-40 Nb was investigated by experimental and theoretical (ab initio) methods. Several ß-type (Ti-40 Nb)-xIn alloys (with x ≤ 5.2 wt%) were produced by cold-crucible casting and subsequent heat treatments (solid solutioning in the ß-field followed by water quenching). All studied alloys completely retain the ß-phase in the quenched condition. Room temperature mechanical tests revealed ultimate compressive strengths exceeding 770 MPa, large plastic strains (>20%) and a remarkable strain hardening. The addition of up to 5.2 wt% indium leads to a noticeable decrease of the elastic modulus from 69 GPa to 49 GPa, which is closer to that of cortical bone (<30 GPa). Young's modulus is closely related to the bcc lattice stability and bonding characteristics. The presence of In atoms softens the parent bcc crystal lattice, as reflected by a lower elastic modulus and reduced yield strength. Ab initio and XRD data agree that upon In substitution the bcc unit cell volume increases almost linearly. The bonding characteristics of In were studied in detail, focusing on the energies that appeared from the EDOSs significant for possible hybridizations. It came out that minor In additions introduce low energy states with s character that present antibonding features with the Ti first neighboring atoms as well as with the Ti-Nb second neighboring atoms thus weakening the chemical bonds and leading to elastic softening. These results could be of use in the design of low rigidity ß-type Ti-alloys with non-toxic additions, suitable for orthopedic applications.

Investigation of early cell-surface interactions of human mesenchymal stem cells on nanopatterned ß-type titanium-niobium alloy surfaces.

Multi-potent adult mesenchymal stem cells (MSCs) derived from bone marrow have therapeutic potential for bone diseases and regenerative medicine. However, an intrinsic heterogeneity in their phenotype, which in turn results in various differentiation potentials, makes it difficult to predict the response of these cells. The aim of this study is to investigate initial cell-surface interactions of human MSCs on modified titanium alloys. Gold nanoparticles deposited on ß-type Ti-40Nb alloys by block copolymer micelle nanolithography served as nanotopographical cues as well as specific binding sites for the immobilization of thiolated peptides present in several extracellular matrix proteins. MSC heterogeneity persists on polished and nanopatterned Ti-40Nb samples. However, cell heterogeneity and donor variability decreased upon functionalization of the gold nanoparticles with cyclic RGD peptides. In particular, the number of large cells significantly decreased after 24 h owing to the arrangement of cell anchorage sites, rather than peptide specificity. However, the size and number of integrin-mediated adhesion clusters increased in the presence of the integrin-binding peptide (cRGDfK) compared with the control peptide (cRADfK). These results suggest that the use of integrin ligands in defined patterns could improve MSC-material interactions, not only by regulating cell adhesion locally, but also by reducing population heterogeneity.

Surface treatment, corrosion behavior, and apatite-forming ability of Ti-45Nb implant alloy.

The low modulus ß-type Ti-45Nb alloy is a promising new implant alloy due to its excellent mechanical biocompatibility and composition of non-toxic elements. The effect of surface treatments on the evolution of controlled topography and roughness was investigated by means of scanning electron microscopy and optical profilometry. Severe mechanical treatments, for example sand-blasting, or etching treatments in strongly oxidizing acidic solutions, like HF:HNO(3) (4:1) or H(2)SO(4):H(2)O(2) (1:1) piranha solution were found to be very effective. In particular, the latter generates a nanopatterned surface topography which is expected to be promising for the stimulation of bone tissue growth. Compared to Ti and Ti-6Al-4V, the ß-type Ti-45Nb alloy requires significantly longer etching durations due to the high chemical stability of Nb. Severe surface treatments alter the passive film properties, but do not deteriorate the outstanding corrosion resistance of the Ti-45Nb alloy in synthetic body fluid environments. The Ti-45Nb appears to have a lower apatite-formation ability compared to Ti. Etching with H(2)SO(4):H(2)O(2) (1:1) piranha solution inhibits apatite formation on Ti, but not on Ti-45Nb.

Production of Porous ß-Type Ti-40Nb Alloy for Biomedical Applications: Comparison of Selective Laser Melting and Hot Pressing.

We used selective laser melting (SLM) and hot pressing of mechanically-alloyed ß-type Ti-40Nb powder to fabricate macroporous bulk specimens (solid cylinders). The total porosity, compressive strength, and compressive elastic modulus of the SLM-fabricated material were determined as 17% ± 1%, 968 ± 8 MPa, and 33 ± 2 GPa, respectively. The alloy's elastic modulus is comparable to that of healthy cancellous bone. The comparable results for the hot-pressed material were 3% ± 2%, 1400 ± 19 MPa, and 77 ± 3 GPa. This difference in mechanical properties results from different porosity and phase composition of the two alloys. Both SLM-fabricated and hot-pressed cylinders demonstrated good in vitro biocompatibility. The presented results suggest that the SLM-fabricated alloy may be preferable to the hot-pressed alloy for biomedical applications, such as the manufacture of load-bearing metallic components for total joint replacements.

Controlled surface modification of Ti-40Nb implant alloy by electrochemically assisted inductively coupled RF plasma oxidation.

Low temperature metal oxidation induced by plasma in the absence of liquid electrolytes can be useful for the surface preparation of orthopedic devices since residues from these may be harmful and need to be removed before implantation. In this study the oxidation of Ti-40Nb for biomedical application was achieved by employing an inductively coupled radio frequency oxygen plasma. The correlation between the growth mode of the surface oxide and the electric conductivity ratio of the plasma and the oxide phase were studied by varying the sample temperature, oxygen gas pressure and additional bias potential. The plasma treated samples were characterised by confocal laser microscopy, SEM, EBSD, XPS, TEM and ToF-SIMS. The surface energy was determined by contact angle measurements using the Owens-Wendt-Rabel-Kaelble method. Well adhering oxide layers consisting of TiO2 and Nb2O5 with thicknesses between 50 and 150 nm were obtained. Surface roughness values and microstructure indicate that the growth mode of the oxide can be well controlled by the sample temperature and oxygen gas pressure. At temperatures above 450°C a migration of Ti ions towards the surface controls the growth process. A bias potential higher than +50 V causes rough and defective surfaces with high surface energies.