The electropolishing process is almost a 100 year old electrochemical process applied to metals, metals alloys and intermetallic compounds for the purposes of smoothing the surface by minimizing macro and micro roughness, to make the work-piece surface shiny and reflective, to remove the stressed and deformed cracked layer (Beilby layer), to improve corrosion resistance, and in the case of metallic (human body) implants make them more bio- and hemo-compatible. The electropolishing process mainly uses direct current (DC), the exception to this regimen is the platinum metal group, which is electropolished by using alternating current (AC).
The electropolishing and magnetoelectropolishing processes can be performed under two different oxygen regimes, namely; below and under oxygen evolution. The optimum current density for electropolishing as well magnetoelectropolishing processes have to be determined experimentally. Voltage-current curves have to be plotted and the plateau current density established. In most cases the best results are obtained when the potential is adjusted to a value just below the oxygen evolution potential. The best example of this kind of electropolishing is electropolishing of niobium cavities for a superconducting installation where surface smoothness is the main importance. However, many exceptions to the above rule are reported and occasionally the best polishing is obtained outside the current density plateau. For many metals, alloys and intermetallic compounds a second range of potentials and current densities corresponding to good polishing exists at values beyond the plateau, i.e., under the conditions of oxygen evolution.
Anodizing is an electrolytic process that creates a homogeneous anodic oxide layer on the surface of some metals, alloys and intermetallic compounds in order to improve corrosion and wear resistance and to achieve demanded tribological properties, etc. Magnetoanodizing is the anodizing process performed in an externally imposed constant magnetic field. In connection with the present invention, both of these processes are to be carried out under an oxygen evolution regime.
Due to its unique mechanical properties of pseudoelasticity, shape memory and good corrosion resistance Nitinol has found a permanent place as an advanced functional biomedical material. Nitinol is a compound consisting essentially of equal parts of Nickel and Titanium, represented as NiTi. Nitinol is used in the production of implantable medical devices, e.g., stents, heart valve frames, IVC filters, septal occluders, as well as medical and dental instruments, e.g., arthroscopic instruments, blood clot stent retrievers, guide wires, endodontic rotary files, etc. The implantable medical devices, as well as the medical instruments, in many cases undergo very severe bending, twisting, and cyclic stretching-contracting conditions during implantation and continued or continuing use.
Another advantage of Nitinol as an implantable material is its resistance to corrosion due to the presence of titanium oxide, which is the predominant compound residing in the passive film that spontaneously covers Nitinol on exposure to the ambient atmosphere. In order to improve corrosion resistance of the passive film layer, as well as removing other contaminants, smoothing the surface, and improving fatigue resistance, the material is subjected to electropolishing as a finishing step prior to sterilization.
Although titanium oxide is the dominant component in the passive film layer, it is not the only such component. An undesirable component of the passive film layer on substantially all electropolished Nitinol devices is Nickel. The amounts of Nickel vary depending upon electropolishing protocol used in the final step of the production of Nitinol medical devices. The sole presently available electrochemical process for substantially eliminating and removing Nickel in any form from the surface of Nitinol, resulting in the creation of pure titanium oxide on the Nitinol surface, is the magnetoelectropolishing process.
Generally, Nitinol possesses all of the attributes of a very good metallic biomaterial, but it is not totally without drawbacks. The primary drawback is the unavoidable intermetallic inclusions in Nitinol. Carbides [TiC], oxides [Ti4Ni2Ox, TiO2] and intermetallic precipitates [Ni4Ti3, Ni3Ti] are contained within the inclusions. Inclusions are randomly distributed throughout the entire Nitinol alloy material with their concentrations depending upon the melting procedure and purity of each component of the compound during production forming of the alloy material. Consequently, some of the inclusions will reside on the surface of the Nitinol material. Any inclusion formations in Nitinol are unwelcome, but inclusions appearing on Nitinol surfaces are particularly troublesome, not only because they are major crack initiation points along with internal inclusion sites, but also corrosion initiation sites. All bodily fluids, including blood, contain chlorides which are very corrosive to all metallic implantable materials including Nitinol. The corrosion mechanism associated with Nitinol having surface inclusions immersed in bodily fluids depends on the type of inclusion, but is intrinsically connected to an affinity of chloride ions to Nickel.
In the case of Titanium Carbide [TiC] or Titanium Oxide [TiO2], chloride anions will react with the abundant Nickel content of Nitinol created by the drainage of Titanium from the matrix which surrounds the inclusion sites during the process of formation. The Nickel bearing inclusions become the source of free Nickel that will undergo dissolution in the chloride containing bodily fluid. Nickel, which is both an allergen and a carcinogen, will be released into the surrounding environment and into bodily tissues surrounding the Nitinol implantable device creating inflammatory and hypersensitivity reactions. For example, in the case of vascular stents made from Nitinol, the leached Nickel anions could lead to restenosis which diminishes stent patency. Another example of a product made from Nitinol is an implantable permanent birth control or sterilization device for women that is inserted into and across the fallopian tubes; one such device named Essure® is manufactured by Bayer®. The insertable device consists of two metal coils, one of which is made of Nitinol. In many reported cases the Nitinol coil has fractured and become embedded in or perforates the uterus, and/or migrates to other organs. In addition, nickel ions, which are released from the inclusions or from the nickel enriched matrix adjacent to the inclusions, trigger havoc in women's bodies prone to nickel allergies resulting in excessive menstrual bleeding, skin rashes, hair loss, headaches, including migraine headaches, and many more auto-immune disorders. Simultaneously, the dissolving Nickel from around inclusion sites and from the inclusions themselves will weaken the mechanical integrity of the stent or nitinol coil of the permanently implanted birth control device which will consequently lead to ultimate fracture.
Another drawback in using Nitinol is fatigue fracture. The fracture of Nitinol is a crack initiation and propagation phenomenon. This means that the existence of a crack is the point of no return giving rise to fatigue fracture. Taking under consideration the small diameters and cross sections of medical devices and tools, the second phase of crack propagation, however interesting from the material behavior point, is totally irrelevant from the practical one. The main source of crack initiation is surface intermetallic inclusions, which accounts for nearly all cases of Nitinol fracture. According to research undertaken by the U.S. Food and Drug Administration, Nitinol fatigue fractures are initiated from surface inclusions in nearly all cases and micro-cracks caused by cold working stresses, heat effected zones created by EMC or laser cutting.
One method which to some degree is capable of removing these imperfections from the surfaces of Nitinol is the electropolishing process, which by dissolution action is able to smooth the surface and by this smoothing eliminates micro-cracks and some of the minute surface intermetallic inclusions. Almost all Nitinol electropolishing processes are proprietary, but it is well-known that all of the processes are performed below the oxygen evolution regime and in the best cases they are able to improve fatigue life of Nitinol within a range of only ¼ to ½ fold. Those proprietary processes performed below the oxygen evolution regime employ three main groups of electrolytes: 1) methyl alcohol—sulfuric acid mixture; 2) perchloric—acetic acid mixture; 3) electrolytes containing citric acid. In order to achieve a higher fatigue life for surface inclusions-free Nitinol medical implantable devices and instruments the electropolishing process, as well as the magnetoelectropolishing process, has to be carried out under an oxygen evolution regime. By applying these processes the fatigue resistance can be elevated five-fold.
During electropolishing and magnetoelectropolishing of Nitinol under an oxygen evolution regime, in addition to electrolytic smoothing and reduction of nickel content in the passive protective layer, the Nitinol surface outermost oxide layer and consecutive deeper under layers are enriched in oxygen without any significant thickness changes. These additional oxygen ions are incorporated into the profile of the passive layer and are responsible for bridging and saturating the oxide lattice defects making the passive film more stoichiometric and homogeneous. The more perfect homogeneous oxide with lower lattice defects consequently improves fatigue resistance of Nitinol medical implantable devices and instruments by improved elasticity of titanium oxide crystals covering the surface, which slows the crack initiation phenomenon. It should be mentioned that electrolytically introduced oxygen into the passive layer does not enrich the metal-oxide interface in metallic nickel and its compound as thermal oxidation processes do and by this process eliminates another possible source of crack initiation.
The passive film formation during electropolishing under an oxygen evolution regime and anodizing can be explained by the Cabrera & Mott theory. According to high field mechanism for oxide film formation and growth theory the main prerequisite is the absorption of oxygen on a metal surface which creates an oxide monolayer. The next step is electron tunneling from the metal to the monolayer of adsorbed oxygen which by adding electrons became an electron trap on the outer surface of the oxide. As the number of electron traps increases the potential drop across the film grows. The drop in potential creates the electric field across the passive film which lowers the activation energy necessary for further transport of ions through the passive film.
The oxide on the Nitinol which is composed predominantly of titanium dioxide [TiO2] is classified as an N-type semiconductor which means that anion transport thought the film is the dominate way of film growth and is due to oxygen ion movement toward the bulk of the Nitinol intermetallic compound. The thickening of the oxide film increases the activation energy necessary for further transport of oxygen ions and limits further passive film formation. The only way for further growth of the passive film at this point is to increase the potential drop across the film which simultaneously increases the electric field.
When the electropolishing and anodizing under an oxygen evolution regime are carried out in a magnetic field, i.e., magnetoelectropolishing and magnetoanodizing, the properties of oxygen and its behavior in the magnetic field are the critical factors. Oxygen is a paramagnetic element with two unpaired electrons that are attracted and aligned by a magnetic field. Due to a magnetic field more oxygen will adsorb on the Nitinol surface and more oxygen ions will be tunneled toward the Nitinol surface through vacant and dislocation sites. It should be mentioned that simultaneously more Nickel ions will be leaving the oxide layer and entering the electrolyte due to its ferromagnetic properties. The oxide layer will become composed almost entirely of titanium dioxide [TiO2]. The highest oxygen concentration will lead to a higher extent of saturation and bridge lattice defects making the passive film more homogeneous and elastic with increased fatigue resistance.