Titanium and Titanium Alloys

Titanium is one of the most important materials in implan-tology. Therefore, many investigations with nanoanalyti-cal tools are performed on titanium or titanium-containing implants.

Single Crystalline Model Surfaces To get insight into the basic material interactions, a defined surface has to be prepared. In the case of titanium and its natural surface oxide, these standards are TiO2 single crystals.

An overview covering atomic resolution investigations with STM and SFM of different TiO2 modifications is given in [120].

Intrinsic defects on single crystals of TiO2 were characterized with STM [121-123]. In these investigations STM was also combined with other techniques like XPS and low-energy electron diffraction (LEED) to investigate the structure after argon sputtering and annealing in oxygen plasma and to detect calcium impurities. These are eliminated after the treatment, which improves the surface morphology of the TiO2 (110) crystal [124].

TiO2 as substrate and the structure of the adhering biofilm were monitored also via SFM [125].

Surface Treated Titanium The high biocompatibility of titanium has its origin in the natural oxide layer on the titanium surface. A further property that influences the ingrowths of titanium implants into bone is their surface roughness. So different surface treatments, for example, chemical, electrochemical, or mechanical techniques, are used to increase the biocompatibility of titanium.

The oxide layer at electropolished and anodically oxidized titanium was investigated in different ways, for example, by SEM, XPS, AES, SIMS [126]. This showed that the oxide formed is mainly TiO2, but the chemical composition can be modified by anion adsorption when H2SO4 or H3PO4 electrolytes are used. The hydrophilicity of TiO2 films obtained by different techniques (radio frequency magnetron sputtering (RFMSD) and plasma enhanced chemical vapor deposition (PECVD)) and the influence of UV irradiation were monitored via SFM. The interaction caused by the capillary water bridge formed at the tip-sample interface was investigated [127]. The adhesive force shows a highly inhomoge-neous surface for RFMSD films and an increase in adhesion force upon UV irradiation. For PECVD a more homogeneous surface was observed. Surface characterization of a titanium oxide layer, grown on commercial Ti substrates by metal organic chemical vapor deposition (MOCVD) technique, was monitored with STM and the chemical composition with XPS [128].

Another widely used surface modification method is sandblasting. This not only increases the surface roughness but also changes the chemical composition because some shot particles remain adhered on the metal. In [129] the percentage of surface area covered by the remaining shot particles was ascertained by SEM and a following digital image analysis.

Titanium surfaces used in oral implantology were submitted to various treatments such as mechanical polishing, acid attack in HCl/H2SO4, after mechanical polishing or sandblasting, and titanium plasma-spray. Polished titanium exhibits a peak-to-valley roughness of 81 nm; the acid-treated surfaces show an increased roughness in the micrometer range (2100 nm for polished and acid attacked; 3600 nm for sandblasted and acid attacked). The chemical composition has been measured by Auger electron spectroscopy. The treatments had no major influence on the surface chemical composition [130].

Titanium implants with different surface properties reached by various surface treatments (mechanically polished, mechanically ground, sandblasted with alumina and etched in HF/HNO3, sandblasted with alumina and etched in HCl/H2SO4, mechanically polished and etched in HCl/H2SO4, and plasma-sprayed) were investigated according to roughness and morphology and the subsequent biofilm formation [131]. The adsorption of human plasma fibrinogen (HPF) was tested but the SFM phase imaging technique could only detect HPF on the first two of the investigated seven samples due to resolution problems. Additionally, the chemical composition was examined by XPS and ToF-SIMS and some impurities used during sample production and treatment were detected.

A further item of investigation are titanium alloys and, in this field, the cell attachment to the implants [132]. As on Ti, there are different ways to prepare the alloy surface, for example, thermal oxidation, machining, or electrochemical polishing. TEM and STEM studies [133] show that the microstructure of the anodic oxide films is rather heterogeneous with areas of different porosity, which can be correlated with the grain structure of the bulk metals. These oxides on Ti alloys are more heterogeneous than that on pure Ti, due to the more complex microstructure of the bulk material. TiOx films deposited on TiAl6V4 by plasma immersion ion implantation (PIII) with different compositions and properties were fabricated as model for artificial heart valve implants. They were evaluated with SFM revealing a dense surface. Microhardness was tested and related to the oxygen pressure upon production. A maximum hardness of 17 GPa is obtained with an oxygen partial pressure of 3 x 10-2 Pa [134]. This is also correlated with a development of Ti3O5 and TiO2 phases as monitored with XPS.

The effect of sterilization with different techniques such as dry heat, steam autoclaving, ethylene oxide, peracetic acid, and plasma-based sterilization techniques on the surface properties of NiTi (Nitinol) was investigated with AES and SFM. Dry heated, steam autoclaved, and ethylene oxide treated surfaces present thicker oxide layers where the first two treatments yield a threefold increase in surface roughness. Plasma-based sterilization has a lesser effect on the oxide layer thickness, roughness, and morphology [135].

The influence of functionalization of titanium surfaces was monitored in [136]. No change in surface structure was observed upon coating with silanes.

The corrosion of titanium surfaces was monitored in-situ with electrochemical SFM by changing the applied voltage on the titanium. The growth of oxide domes could be observed [137]. Further STM experiments are presented in [120]. Titanium surfaces exposed to conditions of corrosion in a biological environment were examined with STM ex-situ and nanoscale topographical changes could be determined [138].

Structured Titanium Surfaces Nanostructured Ti films were characterized with respect to interaction with proteins like F-actin via SFM. An influence of the structure height between 1 and 4 nm on the alignment and adsorption amount (scan ranges: 0.158, 6, and 17.8 /m) [24] could be determined. Using a photolithographic technique, the flanks of Ti-implant screws were patterned with round etched regions of 10 / m diameter, and placed in a square matrix with a pitch of 20 /m [139]. This surface morphology was investigated by SEM after implantation, but no statistically significant differences were seen in fixation, with respect to bone-to-implant contact, between the patterned and control implants. In another study 1-, 2-, 5-, and 10-/m-wide Ti gratings were produced by microtechnology and plasma etching [140]. The biocompatibility was tested via incubation of rat dermal fibroblasts (RDFs) on the surfaces for 3 days. Observation by SEM, TEM, and confocal laser scanning microscopy showed that the RDFs as a whole and their stress fibers oriented strictly parallel to the surface pattern on the 1- and 2-/m surface, while on the 5- and 10-/m surfaces this orientation was not observed.

Coated Titanium Surfaces A further approach to increase the biocompatibility is to form a new surface consisting of more tissue-like material; especially phosphate (Ca-P) and hydroxyapatite (HA) deposited by different techniques are used to reach this [120, 136, 141-148].

One of these techniques is the ion-beam assisted sputter deposition (IBASD). Ca-P coatings yield columns of 40 to 80 nm on the flat surface as monitored with SFM. The quality of the coatings was monitored with XPS, AES, and FTIR studies [141]. IBASD was also used for the production of TiO2, Al2O3, and HA films. These were investigated concerning the roughness via SFM yielding a higher roughness of TiO2 surfaces [142].

Another deposition technique for Ca-P coatings was developed in [149]. This study showed proliferation of the osteoblast-like cells which was significantly higher on non-coated than on Ca-P-coated samples. On the other hand, more mineralized extracellular matrix (ECM) was formed on the coated surfaces. TEM confirmed that the cells on the coated substrates were surrounded by ECM.

Plasma-spray, sol-gel, and sputtering techniques were compared in [144] by SEM, XPS and X-ray diffraction (XRD). All coatings exhibited a rough morphology suitable for implant applications. The sputtered coatings were found to have a composition most similar to HA; the solgel deposits also showed a high concentration of hydroxyl ions. Plasma-spraying also results in a porous titanium coating in which Ti3O5 was formed in the outermost surface due to oxidation [150]. The macropores in the outermost layer reached a diameter and even surpassed 150 /m, which could be beneficial for tissue growth into the coating. Also plasma-sprayed coatings were investigated in [147] by TEM. Six months after insertion, an occasional lack of HA coating and phagocytosis of HA particles were noted. The implant was surrounded by well-mineralized bone investing the smallest cavities of the plasma-sprayed layer. Plasma-sprayed hydroxy-apatite was also used to increase the biocompatibil-ity of these materials. But there was no obvious difference in the in vitro and in vivo reactions of pure and coated Ti alloys [151].

In [145] different surface treatments were investigated by SEM. Here the HA coating on grid blasted Ti surfaces significantly improved fixation to the bone, while HA onto porous coating did not compared to pure Ti surfaces. Further SEM studies were performed on plasma sputtered HA thin coatings on Ti alloys [146]. For deposition temperatures as low as 67 °C, the crystalline phase of the HA coating is detectable and an underlying (TiAlV)N coating increases the crystallinity and thermostability of the HA coating before and after heat treatment. In a combined TEM, SEM, XRD, and FTIR study [148] it was shown that the morphology and composition of a plasma assisted elec-trophoretic deposited HA layer were significantly influenced by the solution pH values. SEM images out of this investigation are given in Figure 9. The electrophoretical method performed with ultrahigh surface area HA powder produces dense coatings when sintered at 875-1000 °C. SEM inspection of these layers showed that the use of dual coatings solves the problem of cracking HA layers during sintering

[152]. Sol-gel derived hydroxyapatite coating morphology was also investigated with SFM and SEM in [143], revealing good homogeneity and a high surface roughness.

Other investigations on HA are presented in [120]. In

[153] bioactive calcium phosphate invert glass-ceramic coatings on substrates such as pure titanium, conventional Ti-6Al-4V, or Ti-29Nb-13Ta-4.6Zr were analyzed to determine the optimum condition for preparation of fine joining. Cross-sectional SEM photographs and EDX measurements around the joining interface between substrate and coating show gradient zones of 3-4 /m in thickness between substrate and ceramic layers. Ti concentrations in the zones decrease with decreasing distance from the glass-ceramic layers.

Another approach to increase the biocompatibility of Ti alloys is a laminin-5-coating which enhances cell attachment, spreading, and hemidesmosome assembly [154]. This may reflect better integration between epithelial cells and titanium alloy. TEM analysis of such a compound showed that cells formed significantly more hemidesmosomes on the surface of laminin-5 coated and passivated than on the surface of laminin-5 coated, unpassivated Ti alloys.

In some cases inert areas and areas of high biocompat-ibility are needed on the same substrate. Teflon (PTFE)

Figure 9. SEM micrographs of plasma-assisted electrophoretic deposited coatings [148]. (a) Calcium-phosphate coating, cathodic deposition by electrochemical reaction in a HA solution of pH 4-5. (b) HA coating anodic deposited at pH 10-11.

coatings on titanium substrates are an approach to build up an inert layer on a biocompatible titanium substrate. In Figures 10 and 11, SFM pictures of a Teflon layer on commercially pure titanium are shown [155]. In Figure 10 the Teflon is deposited by a dip coating technique. The structure of the underlying polished titanium surface is not visible. It is possible to compensate substrate roughness by this method. In Figure 11 an additional surface structure is caused by powder coating a Teflon layer, which increases the surface roughness.

Implanted Materials Implants can also be investigated after implantation.

SEM investigations showed that failed oral titanium implants [156] contained varying amounts of tissue residues while two control samples were essentially free from macroscopic contamination. The efficacy of cell adhesion on Ti, Ti-alloy, dental gold, and ceramic implants were also investigated by SEM [132]. By this investigation the better adhesion and spreading of the cells on metallic surfaces could be shown. One result of TEM is that the attachment zone between bone and implanted biomaterials consists of a mineralized collagen fiber matrix associated with an inorganic (hydroxyapatite) matrix [157, 158].

An important field of investigation is the drift of titanium far away from the dental implant into the surrounding bone tissue [159]. The same happened at metal-metal interfaces, for example, in hip implants, as was shown by TEM [160].

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