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Magnetic Fields and Electropolished Metallic Implants
With increased reports of blood-clotting problems among patients implanted with drug-coated stents, magnetoelectropolishing may provide a solution.
Originally Published MDDI March 2006
|Table I. Roughness parameters after differing processes, from atomic force microscopy.
Blood-contacting implants, such as stents, filters, and sutures, are part of a very special category of metallic devices. These devices have many surface features that must be corrosion resistant, as well as to bio- and hemocompatible. In many cases, such devices are processed using a technique called electropolishing. But in an attempt to refine and improve these critical features, a new process has been developed: magnetoelectropolishing.
Researchers have discovered that an externally applied magnetic field may either enhance or retard the dissolution rate during the electropolishing process. The main cause of this change is the oxygen evolution regime, which influences the properties of the finished surface. Researchers do not fully understand why an externally applied magnetic field causes this change, and it requires a good deal of further research and clarification. However, this article presents one a possible explanation. It also includes the results of a study that examined the new properties of magnetoelectropolished 316L-stainless-steel surfaces. It found that the magnetoelectropolishing process can add many useful properties, including lubricity and antimicrobial peculiarity, to metals used in blood-contacting devices. Antimicrobial peculiarity refers to the fact that magnetoelectropolished 316L stainless steel is less prone to bacteria attachment and biofilm formation than standard electropolished 316L steel.
The electropolishing process modifies surface properties of metals and alloys without affecting their bulk properties. For example, the most commonly electropolished alloy, austenitic 316L stainless steel, becomes more corrosion resistant, smoother, and cleanable after electropolishing. The process dissolves martensitic particles created by mechanical processes like grinding and rolling. It also enriches the chromium oxide in the passive layer.1
It is often assumed that a voltage versus current (V–I) curve must be plotted and that current plateau densities must be established to reach the best conditions for electropolishing. The current plateau usually exists just below the oxygen evolution regime. However, for many materials, the best electropolishing results occur beyond this plateau.
The best example is a process used for electropolishing austenitic stainless steels that is carried out within the oxygen evolution regime. The experimental data for electropolishing the 316L stainless steel with two differing oxygen regimes (within and below oxygen evolution) do not show many differences in appearance and in atomic force microscopy (AFM) roughness data (see Table I). The only significant difference between these two processes is the time required to achieve their results. In the case of electropolishing below oxygen evolution, the time is considerably prolonged.
The electropolishing process is governed by many different mechanisms and depends on many parameters. Given its variability, no single theory for attaining the best electropolish covers all the complexities. Three theories are widely recognized. There is the viscous layer theory by Jacquet, but it is still somewhat obscure.2 There is also the diffusion theory by Elmore.3,4 Finally, the compact solid film theory by Hoar explains many, but not all, electropolishing systems.5–7
|Table II. The influence of magnetic field strength on mass loss for 316L-stainless-steel samples electropolished in fixed-potential mode
The electropolishing process becomes even more complicated when another parameter is added—in this case, an externally applied magnetic field. It is commonly understood that a magnetic field can influence the electrolysis process. Although some research has been done on the influence of an externally applied magnetic field on electrodeposition, little has been done on electropolishing.8
This article posits that a magnetic field can influence the electropolishing process in two distinct ways. This theory differs from existing electropolishing theories and thus will require them to be modified.
In the electropolishing process, an externally applied magnetic field works in two ways: either enhancing or retarding the rate of dissolution. The change in dissolution rate does not depend on the magnetic properties of the dissolved material or the composition of the electrolyte. Instead, it depends on the strength of the externally applied magnetic field (see Table II).
The main factor for the effect is created by the oxygen evolution regime. This parameter previously had not been thought responsible for the influence of a magnetic field. The electropolishing of stainless steel occurs in the transpassive region. That region is characterized by dissolution either by diffusion through the oxide layer or by randomly localized oxidation through the vacancy sites in that layer.
When magnetoelectropolishing is performed in fixed-potential mode, the process should follow normal diffusion principles. The magnetic field creates a Lorentz force, which is a cross product of a magnetic field and a current. The mechanical effect of this force causes the electrolyte to rotate around the axis parallel to direction of the magnetic field. The rotating movement reduces the thickness of the diffusion layer, or viscous layer. Theoretically, thickness reduction should speed up the dissolution process by increasing the rate of mass transport. This effect also should be manifested by increased current densities.
|Table III. (Click to See Full Table) Comparison of mass loss using standard and magnetoelectropolishing processes. Also shown is the influence on the processes of using differing oxygen regimes.
It is important to note, however, that this effect holds only when the electropolishing process is performed below the oxygen evolution regime (see Table III). In the case of fixed-potential electropolishing performed within the oxygen evolution regime, the experimental data in Table III show the process to be diametrically opposed to the one described above.
The influence of the Lorentz force seems to argue against the diffusion theory of the electropolishing process. The thinning—or complete elimination—of the viscous layer by the Lorentz force reduces the dissolution rate. That in turn results in decreased current densities, but still gives a good finish that can be confirmed by visual inspection and AFM data (see Table IV).
The magnetoelectropolishing process carried out within oxygen evolution contradicts, to some degree, all three of the established electropolishing theories. Each theory assumes that a viscous layer is needed to obtain good results. (However, it should be noted that Hryniewicz expressed doubts that a viscous layer was indispensable to high-speed rotation electropolishing.9
|Table IV. Roughness parameters after differing processes, from atomic force microscopy.
Although the origin of the dual influence is not fully understood, one possible explanation of this influence exists: When the electropolishing process is carried out within the oxygen evolution regime, the properties of oxygen and its behavior in a magnetic field are the critical factors. Oxygen is a paramagnetic element with two unpaired electrons that are attracted and aligned by a magnetic field. Some oxygen molecules, which are released during decomposition of the oxide layer, escape to the electrolyte. Others are attracted by the nonzero magnetic field. Those attracted oxygen molecules likely migrate toward the metal surface through the cyclically oxidized surface, or through vacancy sites, and dissociatively adsorb. This makes the oxide-hydroxide layer more compact and homogenous and thus more difficult to dissolve. The dissociatively adsorbed oxygen must be responsible for the decrease in current density and, consequently, for the rate of dissolution of electropolished material.
Magnetoelectropolished Stainless Steel: Hydrophilicity
One big advantage of magnetoelectropolished surfaces of 316L-stainless-steel blood-contacting implants is their improved hydrophilic character. The improvement can be seen in the water contact angle (see Table V).
|Table V. The water contact angle, measured in degrees, after both standard and magnetoelectropolishing.
It is well documented that hydrophilic metallic surfaces are hemocompatible and bacteria resistant. Examples include the results of applying hydrophilic coatings on guidewires, catheters, and similar devices.10,11
The experiment reported here evaluated the thromboresistant properties of the magnetoelectropolished surface of 316L stainless steel. The properties were then compared with standard electropolished properties using a clotting experiment with whole fresh human blood. The 0.1 ml of fresh whole human blood was immediately deposited on two 316L-stainless-steel samples. One sample had gone through a standard electropolishing process, and the other had been magnetoelectropolished in a 500-mT magnetic field.
After 30 minutes of exposure, the samples were transferred to separate glass beakers containing 20 ml of distilled water each and incubated for 10 minutes. The released hemoglobin—dispersed in water from clots—that was not contained by thrombi was measured calorimetrically by monitoring the absorbance at 520 nm using a spectrophotometer. The absorbance of water that contains hemoglobin from the magnetoelectropolished sample was 27% higher than for the standard electropolished sample. The higher absorbance indicates better thromboresistance.
The two main factors that influence metal surface hydrophilicity are roughness and chemistry (functional groups). Functional groups are a combination of atoms that undergo characteristic chemical reactions and influence the property of metal surfaces. In the case presented here, the roughness factor can be set aside because the differences in the electropolished and magnetoelectropolished samples are marginal (see Table IV). The remaining factor, which in this case influenced the hydrophilicity of the 316L-stainless-steel surface, is chemistry—namely, hydroxyl functional groups. The hemocompatibility of hydrophilic metal surfaces is caused by electrostatic and hydrophobic interactions between blood and hydroxyl groups. The hydroxyl groups created by the magnetoelectropolishing process on 316L-stainless-steel surfaces are responsible for their increased hydrophilicity. The hydroxyl groups bind with water from blood and stabilize the dense hydration layer around metallic implants. The water, which is negatively polarized as most blood proteins are, will repel them from the surface, which explains their very low adsorption. This action is essential in minimizing blood interaction with implant surfaces, including clotting, thrombosis, inflammatory activation, and platelet binding and activation.
|Table VI. The difference in atomic titanium concentration (atomic percent) in the passive layer of nitinol following standard and magnetoelectropolishing.
One reason manufacturers electropolish metallic implants is to take advantage of the selective dissolution of elements constituting a particular metallic material.12 Selective dissolution enriches the passive layer in an element in which oxide is the most corrosion resistant and hemocompatible. The best example is 316L austenitic stainless steel. Electrochemical corrosion tests performed on standard electropolished and magnetoelectropolished 316L-stainless-steel samples in a high-chloride solution showed remarkable improvement in overall corrosion resistance. These results include a decrease in nickel ions leaching. The same applies to cobalt-chromium alloys, where magnetoelectropolishing elevates chromium content in the passive layer when compared with a standard electropolished sample. Titanium alloys, such as nitinol (an intermetallic compound of titanium and nickel), owe their corrosion resistance and biocompatibility to titanium oxides. These oxides are created spontaneously on the metal surfaces when exposed to moisture or an ambient atmosphere.13 The combination of nitinol’s rare mechanical (superelasticity, superplasticity) and chemical (biocompatibility, hemocompatibility) properties make it widely used as an implantable material. Electropolishing is one method commonly used to modify the surface of nitinol to make it more bio- and hemocompatible. Although the electropolishing process enriches the passive layer in titanium, magnetoelectropolishing within the oxygen evolution regime further enhances the concentration of titanium in the passive layer of the nitinol (see Table VI).14
The experimental results show that an externally applied magnetic field influences the electropolishing process in two distinctive ways—by enhancing or retarding the dissolution rate of electropolished materials. This dual influence depends on the oxygen regime used during the electropolishing process.
The results also show that an externally applied magnetic field makes electropolished surfaces more hydrophilic than standard electropolished surfaces. This enhances an alloy or intermetallic compound’s passive layer in a particular element, making it more hemocompatible and corrosion and bacterial resistant.
An electropolishing process carried out in an externally applied magnetic field within the oxygen evolution regime can be used for electrochemical surface processing and can, to some extent, be tailored to achieve desirable metallic surface properties.
It is important to remember that during the course of this work, researchers discovered another preliminary, but potentially significant, benefit of magnetoelectropolishing within the oxygen evolution regime: improved fatigue resistance in processed materials. If the results are borne out, this could lead to being able to use thinner struts, which would allow miniaturization of the stents that are used in smaller vessels. Further experiments need to be undertaken to determine the magnitude and mechanism of this phenomenon.
The author would like to thank Z. Rogulski, PhD, from the chemistry department at Warsaw University (Poland) for EDAX analysis and helpful discussion.
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