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.

Comparative in vitro performances of bare Nitinol surfaces.

Comparative fibrinogen adsorption and platelet morphology were evaluated on a wide array of well-characterized Nitinol surfaces (polished, chemically etched, boiled in water, electropolished in different electrolytes and heat treated). XPS, SEM, AFM, atomic adsorption spectroscopy and electrochemistry were employed to acquire information on surface chemistry, topography and Ni release. Obtained surfaces, of various topographies and crystallinity from mostly amorphous to nano-crystalline with Ni concentration from 1 to 8%, induced Ni release into biological medium in a subtoxic range (0-11 ng/ml/cm(2)). Fibrinogen adsorption to Nitinol surfaces ranged from that characteristic to pure Ni (130 ng/cm(2)) to pure Ti (300 ng/cm(2)). It was directly proportional to the Ti surface concentration and correlated with open circuit potential related to surface charge. Human platelet morphology varied from round to fully spread depending on surface treatment. Base layer of fully spread cells detected on all surfaces could be even and smooth with no propensity for thrombosis or sticky causing platelet aggregation and thrombus-like structures. Using appropriate surface treatments thrombogenicity of Nitinol can be manipulated to satisfy both the requirements for stents and defect closures.

Effect of chemical etching and aging in boiling water on the corrosion resistance of Nitinol wires with black oxide resulting from manufacturing process.

The effect of chemical etching in a HF/HNO(3) acid solution and aging in boiling water on the corrosion resistance of Nitinol wires with black oxide has been evaluated with the use of potentiodynamic, modified potentiostatic ASTM F746, and scratch tests. Scanning-electron microscopy, elemental XPS, and Auger analysis were employed to characterize surface alterations induced by surface treatment and corrosion testing. The effect of aging in boiling water on the temperatures of martensitic transformations and shape recovery was evaluated by means of measuring the wire electroresistance. After corrosion tests, as-received wires revealed uniformly cracked surfaces reminiscent of the stress-corrosion-cracking phenomenon. These wires exhibited negative breakdown potentials in potentiostatic tests and variable breakdown potentials in potentiodynamic tests (- 100 mV to + 400 mV versus SCE). Wires with treated surfaces did not reveal cracking or other traces of corrosion attacks in potentiodynamic tests up to + 900-1400-mV potentials and no pitting after stimulation at + 800 mV in potentiostatic tests. They exhibited corrosion behavior satisfactory for medical applications. Significant improvement of corrosion parameters was observed on the reverse scans in potentiodynamic tests after exposure of treated wires to potentials > 1000 mV. In scratch tests, the prepared surfaces repassivated only at low potentials, comparable to that of stainless steel. Tremendous improvement of the corrosion behavior of treated Nitinol wires is associated with the removal of defect surface material and the growth of stable TiO(2) oxide. The role of precipitates in the corrosion resistance of Nitinol-scratch repassivation capacity in particular-is emphasized in the discussion.

Surface conditions of Nitinol wires, tubing, and as-cast alloys. The effect of chemical etching, aging in boiling water, and heat treatment.

The surface conditions of Nitinol wires and tubing were evaluated with the use of X-ray photoelectron spectroscopy, high-resolution Auger spectroscopy, electron backscattering, and scanning-electron microscopy. Samples were studied in the as-received state as well as after chemical etching, aging in boiling water, and heat treatment, and compared to a mechanically polished 600-grit-finish Nitinol surface treated similarly. General regularities in surface behavior induced by the examined surface treatments are similar for wires, tubing, and studied as-cast alloy, though certain differences in surface Ni concentration were observed. Nitinol wires and tubing from various suppliers demonstrated great variability in Ni surface concentration (0.5-15 at.%) and Ti/Ni ratio (0.4-35). The wires in the as-received state, with the exception of those with a black oxide originating in the processing procedure, revealed nickel and titanium on the surface in both elemental and oxidized states, indicating a nonpassive surface. Shape-setting heat treatment at 500 degrees C for 15 min resulted in tremendous increase in the surface Ni concentration and complete Ni oxidation. Preliminary chemical etching and boiling in water successfully prevented surface enrichment in Ni, initially resulting from heat treatment. A stoichiometric uniformly amorphous TiO(2) oxide generated during chemical etching and aging in boiling water was reconstructed at 700 degrees C, revealing rutile structure.

Chemical modification of carbonaceous stationary phases by the reduction of diazonium salts.

This paper describes a new strategy for the creation of chemically modified carbonaceous stationary phases. The strategy exploits the electroreduction of arenediazonium salts as a means for functionalizing the surface of glassy carbon (GC) and porous graphitic carbon (PGC) stationary phases. The one-electron reduction of these salts forms an arene radical which then couples via a carbon-carbon linkage to the carbon framework at the surface of the stationary phase. Two arenediazonium-based modifiers were used in evaluating the potential utility of this strategy: 4-nitrobenzenediazonium tetrafluoroborate for the GC and PGC phases and 4-hexylbenzenediazonium tetrafluoroborate for only the PGC phases. Modifications were carried out by packing the phases into a column used for electrochemically modulated liquid chromatography. The effectiveness of the modifications was assessed by X-ray photoelectron spectroscopy and by comparing the liquid separation of a series of mixtures before and after coating deposition. For the nitrobenzyl-modified GC phase, the test mixture contained both anisole and fluoranthene. The performance of the nitrobenzyl- and hexylbenzyl-modified PGC stationary phases was characterized by the separations of substituted phenols (i.e., nitrophenol and resorcinol) and a few important pharmaceutical agents (i.e., hexobarbital, oxazepam, and nitrazepam). The potential utility of this modification procedure to form stationary phases that are stable upon extended exposure to aggressive mobile phases is discussed and briefly examined.

Potato starch derivatives with some chemically bound bioelements.

Metal alkoxides [Cr(III), Mg(II), Mo(V), V(III), and W(V)] taken in 1:1 equimolar proportions, calculated per one D-glucose unit, reacted with a pregelatinized starch in microwave assisted processes into the corresponding starch cationic metal derivatives. In each case, the crosslinking occurred as a consequence of the substitution of two alkoxide groups in the metal alkoxide reagents with two D-glucose units. Involvement of the 6-CH2OH moiety of the D-glucose units in formation of the valence bond with the metal cation was the most likely, but the metal cation was additionally coordinated to the other hydroxyl groups of the D-glucose units. Chromium(III) and vanadium(II) complexes hydrolysed in full by their contact with water, whereas the other metal complexes hydrolysed only in part.

Neutralization of poliovirus by a monoclonal antibody: kinetics and stoichiometry.

First-order kinetics of neutralization have usually been interpreted as evidence that a single antibody, binding at a critical site, neutralizes the infectivity of a virus particle. In such a case, if all the binding sites were critical, an average of one antibody bound per virion would be required to reduce the infectivity of a virus sample to 37% (1/e) of its initial infectivity. However, in the work reported here using a monoclonal antibody to poliovirus which inactivated with first-order kinetics, an average of four bound antibodies were required. These results are consistent with two different models: one in which only one-fourth of the antibody binding sites on the virion are critical for neutralization; the other, in which none of the sites is critical, but neutralization takes place instead in a stepwise fashion in which each bound antibody reduces the infectivity by a factor of 3/4. The maximum binding capacity of the virion for this monoclonal antibody was approximately 30 molecules. Since the 60 protein subunits of the poliovirus capsid are related by 30 twofold axes of symmetry, it is proposed that each monoclonal antibody binds bivalently to two protein subunits related by a twofold axis. Such a binding mode would crosslink pentamers, the basic structures in picornaviral assembly and dissociation. It is proposed that pentamer crosslinking is an important element in neutralization by this monoclonal antibody. Another antibody, which may neutralize by a different mechanism, is also discussed briefly.

Rapid gas-liquid chromatographic method for determining residues of ethephon (2-chloroethyl phosphonic acid) in tomatoes.

A rapid method is described for the determination of the growth regulator ethephon (2-chloroethyl phosphonic acid) in tomatoes. Samples are extracted with ethyl acetate. The extract is methylated, decolorized with carbon, and analyzed by gas-liquid chromatography on a column packed with 15% QF-1/10% DC-200 on Chromosorb W (HP). A flame photometric detector is used. Recoveries from tomatoes fortified with 0.1-11.0 ppm ethephon were 78-98%. The sensitivity of this method is about 0.1 ppm.

Comparisons of the low resolution structure of several small RNA-containing viruses.

A comparison is made of the structure of five small RNA-containing viruses and their accompnaying particles. The data obtained by a small-angle X-ray scattering at low resolution indicate that the radial distributions of electron density are quite similar for particles with similar percentage of RNA. Evidence is also presented indicating that the RNA probably penetrates the wall of the protein shell of most if not all of the virus particles.

Structural studies on transfer RNA: preliminary crystallographic analysis.

Single-crystal diffraction patterns from Escherichia coli leucine tRNA and yeast formylmethionine tRNA show a tetragonal lattice for the former, with a = 46 angstroms and c = 137 angstroms, and a hexagonal lattice for the latter, with a = 115 angstroms and c = 137 angstroms. Initial analysis suggests a molecule with a long, double helix parallel to the c-axis for both crystals.