James W. Anderegg

Critical overview of Nitinol surfaces and their modifications for medical applications

Nitinol, a group of nearly equiatomic shape memory and superelastic NiTi alloys, is being extensively explored for medical applications. Release of Ni in the human body, a potential problem with Nitinol implant devices, has stimulated a great deal of research on its surface modifications and coatings. In order to use any of the developed surfaces in implant designs, it is important to understand whether they really have advantages over bare Nitinol. This paper overviews the current situation, discusses the advantages and disadvantages of new surfaces as well as the limitations of the studies performed. It presents a comprehensive analysis of surface topography, chemistry, corrosion behavior, nickel release and biological responses to Nitinol surfaces modified mechanically or using such methods as etching in acids and alkaline solutions, electropolishing, heat and ion beam treatments, boiling in water and autoclaving, conventional and ion plasma implantations, laser melting and bioactive coating deposition. The analysis demonstrates that the presently developed surfaces vary in thickness from a few nanometers to micrometers, and that they can effectively prevent Ni release if the surface integrity is maintained under strain and if no Ni-enriched sub-layers are present. Whether it is appropriate to use various low temperature pre-treatment protocols (< or = 160 degrees C) developed originally for pure titanium for Nitinol surface modifications and coatings is also discussed. The importance of selection of original Nitinol surfaces with regard to the performance of coatings and comparative performance of controls in the studies is emphasized. Considering the obvious advantages of bare Nitinol surfaces for superelastic implants, details of their preparation are also outlined.

Fabrication and electroluminescence of double-layered organic light-emitting diodes with the Al2O3/Al cathode

The effects of a controlled Al2O3 buffer layer on the behavior of highly efficient vacuum evaporated aqua regia-treated indium tin oxide (ITO)/triphenyl diamine (TPD)/8-tris-hydroxyquino-line aluminum Alq3/Al2O3/Al light-emitting diodes are described. It is found that, with a buffer layer of suitable thickness, both current injection and electroluminescence output are significantly enhanced. The enhancement is believed to be due to increased charge carrier density near the TPD/Alq3 interface that results from enhanced electron tunneling, and removal of exciton-quenching gap states that are intrinsic to the Alq3/Al interface.

Surface spectroscopic characterization of TiNi nearly equiatomic shape memory alloys for implants

In spite of the generally good biocompatibility of TiNi revealed in numerous in vivo studies, medical applications of these shape memory alloys as implants are hindered due to the lack of knowledge on the nature of their biocompatibility. Better material characterization is necessary for understanding the chemical and physical properties which determine biocompatibility. The purpose of the present investigation is to use angle‐resolved x‐ray photoelectron spectroscopy to identify the differences in surface chemical state and composition between the layers formed on the surface of TiNi alloys after mechanical polishing, chemical etching, autoclaving in water and steam, and exposure to H2O2 or air. The surface chemistry drastically depends on the preparation method; the Ni surface concentration can be varied in the range 0–30 at. %. In natural conditions (air, water, and steam) TiNi surfaces revealed a tendency to be covered by TixNiyO2x oxides (where x varies in the range of 7–20 and y varies in the range ...

The electrochemical characteristics of native Nitinol surfaces.

The present study explored the avenues for the improvement of native Nitinol surfaces for implantation obtained using traditional procedures such as mechanical polishing, chemical etching, electropolishing and heat treatments for a better understanding of their electrochemical behavior and associated surface stability, conductivity, reactivity and biological responses. The corrosion resistance (cyclic potential polarization, open circuit potential and polarization resistance) of Nitinol disc and wire samples were evaluated for various surface states in strain-free and strained wire conditions. The surface response to tension strain was studied in situ. Surface chemistry and structure were explored using XPS and Auger spectroscopy and photoelectrochemical methods, respectively. It was found that the polarization resistance of the Nitinol surfaces varied in a range from 100 kOmega to 10 MOmega cm(2) and the open circuit potentials from -440 mV to -55 mV. The surfaces prepared in chemical solutions showed consistent corrosion resistance in strain-free and strained states, but mechanically polished and heat treated samples were prone to pitting. Nitinol surface oxides are semiconductors with the band gaps of either 3.0 eV (rutile) or 3.4 eV (amorphous). The conductivity of semiconducting Nitinol surfaces relevant to their biological performances is discussed in terms of oxide stoichiometry and variable Ni content. Such biological characteristics of Nitinol surfaces as Ni release, fibrinogen adsorption and platelets behavior are re-examined based on the analysis of the results of the present study.

The role of copper compounds as fillers in transfer film formation and wear of nylon

Abstract The friction and wear behavior of filled nylon 11 composites was studied. The fillers used were CuS, CuO, CuF 2 and Cu(C 2 H 3 O 2 ) 2 ·H 2 O (copper acetate). A fixed filler proportion of 35% by volume was used. Sliding tests were performed in a pin-on-disk configuration at 1.0 m s −1 speed and 0.65 MPa nominal contact pressure under ambient conditions. The pin was made of composite specimens and the disk of hardened tool steel. It was found that CuS, CuF 2 and CuO as fillers reduced the wear rate of nylon while Cu(C 2 H 3 O 2 ) 2 ·H 2 O increased it. The cause of this behavior was investigated in terms of the ability of these materials to develop a transfer film and its bonding to the counterface. The wear rates of nylon and copper-acetate-filled nylon were high because the ability of nylon to form a transfer film was poor and the composite transfer film had poor adhesion to the counterface. On the other hand, the wear-resistant nylon composites which were filled with CuS, CuO and CuF 2 transferred well to the counterface and their transfer films were thin, uniform and strongly adherent to the counterface. The adhesion of the transfer film to the counterface for CuO-nylon composite was investigated by X-ray photoelectron spectroscopy analysis.

Surface oxidation of an AlPdMn quasicrystal, characterized by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) is used to determine the extent of oxidation of each of the three metals which comprise a quasicrystalline alloy. A single-grain sample, oriented with the fivefold axis perpendicular to the surface plane, and with nominal bulk composition Al 70 Pd 21 Mn 9 is used. The oxide which results from exposure to ambient gas at room temperature is compared with that which results from exposure to pure oxygen in ultrahigh vacuum at temperatures up to 870 K. XPS probes the near-surface region (ca. top 100 A), and shows that only the Al can be oxidized. The depth of the oxide layer depends systematically upon the conditions of treatment, but is always very thin — in the range of about 5–30 A. Taken together, the data suggest that the surface forms a thin, passivating, surface layer of aluminum oxide.

The investigation of the action of fillers by XPS studies of the transfer films of PEEK and its composites containing CuS and CuF2

Abstract The wear behavior of polyetheretherketone (PEEK) and CuS-PEEK composite rubbing at 1 m s− 1 speed and 19.6 N load against both the steel and glass disks is examined. While CuS filler decreased considerably the wear rate of PEEK in rubbing against the steel disk, the wear rate of both the filled and unfilled materials in rubbing against the glass disk was almost the same. The wear rate of CuF2-PEEK composite sliding against a steel surface was also considerably lower than that of PEEK. The transfer films of the polymer and its composites containing CuS and CuF2 formed against the steel disk surfaces were studied by X-ray photoelectron spectroscopy (XPS). The analysis of the XPS spectra revealed that both CuS and CuF2 decomposed under the rubbing conditions, because reduced Cu, S, FeF2 and FeSO4 were found in the transfer film. The analysis at two different depths in the transfer film of CuF2-PEEK revealed that the concentrations of Cu and FeF2 were greater close to the transfer film-counterface interface. However, there was no chemical change detected when unfilled PEEK rubbed against the steel disk surface. These results have been used to hypothesize the mechanism for the action of fillers in reducing wear.

Investigation of the influence of CaS, CaO and CaF2 fillers on the transfer and wear of nylon by microscopy and XPS analysis

Abstract The effect of CaS, CaO and CaF 2 fillers on the transfer, friction and wear behaviors of nylon was studied. Sliding was performed in ambient atmosphere at a speed of 1.0 m s −1 and a load of 19.6 N, and a pin-on-disk configuration was used. The filler content in nylon was fixed at 35% by volume. The counterface disk was made of hardened tool steel. The experimental results showed that CaO and CaS were effective in reducing the wear of nylon, but CaF 2 increased it greatly. The tribochemical studies by XPS revealed that some CaS decomposed during sliding so that FeS and FeSO 4 were produced. No such decomposition was found in the case of CaF 2 . The comparison of the bonding strengths of these fillers revealed that the bonding strengths of the compounds that decomposed were lower than that of CaF 2 which did not decompose. It was concluded that FeS and FeSO 4 increased adhesion between the composite transfer film and the steel substrate. Furthermore, the ability of the fillers to help in the formation of a uniform and continuous composite transfer film and to promote adhesion between the transfer film and its substrate contributed to increased wear resistance.

Surface oxidation of Al-Cr-Fe alloys characterized by X-ray photoelectron spectroscopy

Abstract We present X-ray photoelectron spectroscopy (XPS) measurements of several Al–Cr–Fe samples which are mixtures of approximants of the decagonal phase. Some samples also contain a hexagonal γ-brass phase. Our purpose is to evaluate the effect of chemical composition, particularly Cr content, on the response of the surface to oxidation. Under mild conditions only aluminium oxidizes, but under extreme conditions (water immersion at room temperature, or oxygen exposure at high temperatures), chromium oxidizes as well. XPS data also provide a measure of the oxide thickness. Cr has no discernible effect on oxide thickness when the oxidizing environment is the gas phase, but provides significant protection against water immersion, where high concentrations of Cr reduce the thickness by as much as 40%. These results for the Al–Cr–Fe samples are compared with results for approximants and quasicrystals in other systems.

The effect of zinc and copper oxides and other zinc compounds as fillers on the tribological behavior of thermosetting polyester

Abstract The tribological behavior of thermosetting polyester filled with CuO, ZnO, ZnF 2 , and Zn(C 18 H 35 O 2 ) 2 fillers was studied. Sliding was performed in ambient atmosphere at 1.0 m s −1 between a polyester pin and a hardened tool steel disk in a pin-on-disk machine. The worn and the unworn specimen pin surfaces, zinc stearate powder, and the transfer films formed during sliding were studied by X-ray photoelectron spectroscopy (XPS). It was found that the coefficients of friction of all the filled composites were higher than that of the unfilled polyester. As for wear, the wear rate of polyester filled with ZnO and ZnF 2 increased while that of polyester filled with CuO and Zn (C 18 H 35 O 2 ) 2 decreased. The reasons for these kinds of behavior were investigated. XPS results revealed that the Zn(C 18 H 35 O 2 ) 2 filler decomposed during the sliding process while ZnO did not. It has been hypothesized that decomposition of the filler helps in bonding of the transfer film to the counterface. The observed behavior is explained in terms of the topographical features, morphology and bonding of the transfer film to its counterface.