|An SEM image shows a cobalt-chrome surface after high-voltage electropolishing.
Ultrapolishing provides a clean surface for implants, instruments, and invasive medical devices. The process enables the manufacturing of metal medical devices under the same high-quality standards that previously were not possible for machined or chemically etched titanium alloys or high-chromium steels. This article discusses the existing methods of final finishing for medical devices and the problems associated with these methods. It explores how ultrapolishing can be used as a standard final finishing process to produce medical parts that have a superior surface quality over mechanical polishing, chemical etching, and electropolishing.
Manufacturing medical implants, surgical instruments, and other devices that come into contact with bodily tissue creates specific surface-quality challenges. The ASTM standards for medical implants, instruments, and invasive devices (ASTM F86-04 and ASTM B600-91) recommend electropolishing as the preferred method of surface treatment, but it is not required.
It is a well-known fact that electropolishing provides better corrosion protection than the traditional process of mechanical finishing followed by passivation. Mechanical grinding and buffing can produce mirror-like results on surfaces, but the processes leave the surface layer distorted, stressed, and contaminated with grinding media.
The 300-series stainless-steel medical parts must be electropolished. However, the same requirements are rarely applied to other metal alloys, such as titanium alloys and 400-series stainless steels, for two main reasons. Electropolishing methods are highly hazardous and unreliable, and there are no existing methods of electropolishing that can be implemented in production without expensive specialized equipment.
For example, the current method of titanium implant production involves multistep mechanical manual abrasive polishing, which is prone to causing errors and can be labor-intensive. After manual mechanical polishing, the parts must be etched and passivated. A mechanically polished surface will appear bright and clean, but examination under a microscope will reveal a surface that is smudged and covered with titanium particles. Bacteria can multiply under the titanium. In addition, the etching process can cover pockets with a thick layer of titanium particles, making them difficult to remove.
|This sample has been manufactured with an ultrapolished titanium 6Al-4Va alloy and has been powder-baked.
Manufacturing surgical instruments that are used under electrical current can be particularly challenging. For example, when current passes through titanium bipolar forceps that have been mechanically polished, the forceps will stick to tissue, because they have carbonized organic matter on their conducting surfaces. It is highly recommended that forceps be electropolished.
Mechanical polishing is labor-intensive, and parts can be easily damaged by incorrect steps, miscalculations, or simple human errors. A company that wanted to avoid the drawbacks of this process thought that the plasma effects in electrolytes would be useful when cleaning and fine polishing metal alloys. The process would be especially useful when working with hard-to-polish alloys due to their metal grain structure. In less than 30 seconds, the technique burned all hard-to-polish carbides from these surfaces. The treatment removed all organic contamination, revealing a clean implant surface. Most importantly, this method uses ecofriendly electrolytes and is acid free. This polishing process was licensed to an Australian knee-implant manufacturer that wanted to apply it to cobalt chrome alloys. Another company from Italy has also implemented the process with titanium.
The main drawback of this electropolishing method is that it requires special equipment and a high-voltage power supply, thus restricting mass production and the number of parts that can be polished at the same time.
Due to high demand, further research concentrated on safe and ecofriendly methods of electropolishing nitinol wire and stents. Conventional methods of electropolishing nitinol use methanol, which poses a danger to workers. The company’s research team developed a new, patent-pending method of acid-free electropolishing that is safe for the environment and workers. This acid-free electropolishing technology has been implemented at one of the largest continuous-wire producing plants in the United States.
Downsides to electropolishing include low penetration of the electrical power into the narrow areas between the knitted wires. Using the process on laser-cut stents can take 10–12 minutes, depending on the initial roughness of the surface.
After continuous investigation, the company’s engineers discovered ultrapolishing, a new process that achieves the same quality results as high-voltage electropolishing the plasma effects in electrolytes without using special equipment and high voltages. The process enables the mass production of fine-finished implants and instruments. The company licensed the ultrapolishing process for titanium alloy to a U.S. medical device manufacturer in 2009.
Although there are conventional methods of electropolishing 300- and 400-series stainless steel, those methods have the following drawbacks:
- Sulfuric acid emits corrosive fumes that can damage the surroundings of a material.
- Operators need to acquire special skills before they can electropolish complicated shapes and cavities.
- Mainly used in medical instruments, 400-series stainless steel is difficult to electropolish in mass production. As a result, it is common to mechanically deburr, clean, and passivate such surgical instruments. However, without properly leveling and deburring the metal surface, passivating surgical parts will not prevent corrosion; the corrosion will occur under the smudged areas.
Various methods of improving the hazardous sulfuric acid-based solution that is used in the conventional method of electropolishing stainless steel have been tested. The high-voltage, acid-free electropolishing of stainless steels is sometimes used for specific applications. However, the high-voltage process is difficult to apply on an appropriate scale for mass production because when the production volume is high, the amount of power consumed is economically unjustifiable. One approach was to modify the ultrapolishing method to process 400-series stainless steel. A second approach modifed conventional electrolytes for stainless steel in such a way that they produce the same ultrapolishing results but without a plasma effect on the surface. Some unpublished corrosion tests have revealed high anticorrosion protection of ultrapolishing on stainless steel.
Ready for Mass Production
Ultrapolishing has been developed for mass production and is an environmentally friendly way to electropolish many types of stainless steel alloys. Ultrapolishing various stainless steel alloys has been found to yield high levels of anticorrosion protection in comparison to other electropolishing methods. Other benefits include the following:
- The surface quality after ultrapolishing is a mirror finish with even material removal along the large areas.
- The short duration of the electropolishing process and high through power of the electrolytes provide an economically effective mass production process.
- The ecofriendly electrolyte process involves a simple disposal procedure and produces fewer fumes, ensuring a safe working environment.
Ultrapolishing produces the desired surface quality and reduces the rejection rate of many metal alloys used in the manufacture of medical devices. By implementing ultrapolishing technology, which uses off-the-shelf equipment and mild electrolytes, medical device manufacturers can create products with high-quality surfaces and eliminate outsourcing costs. Based on the production experience of companies that have used ultrapolishing processes, the quality of the surface finishing contributes to low rejection rates of implants and other devices or instruments.
Ultrapolishing can also be used in the mass production of disposable instruments or parts, which often face low-cost pressures. Stamping or powder-baking finishing methods can create a surface quality that isn’t suitable for medical applications. To improve surface quality, highly hazardous etching processes or mechanical abrasion processes are applied. However, these processes produce surfaces with multiple defects. Because ultrapolishing is a reliable and repeatable process that produces a fine-quality surface, electropolishing disposable parts during mass production can reduce high rejection rates during quality assurance testing.
This method can also be applied to recycling used instruments. It restores the fine quality of a metal surface and burns all organic contamination on the surfaces of medical instruments and parts.
Researchers continue to test the method on other metal alloys in the medical device industry, in the hope that one day the process will become a standard final finishing method.
|Potential applications for electropolishing include laser-cut stents and wire-knitted stents, which cannot be polished using conventional methods. This image shows a nitinol surface after electropolishing.
Advantages of Ultrapolishing
Because ultrapolishing does not require special equipment or skills, device manufacturers can install these finishing processes in-house. Depending on the production volume, the process can be manual or fully automated. It can also solve technological problems not addressed by other finishing methods.
For example, large and flat surfaces have been nearly impossible to electropolish evenly (either for further adhesion plating purposes or for mirror reflection radiation equipment). Because the ultrapolishing process has high electrical throwing power, such tasks are achieveable, even in mass production. Ultrapolishing electrolytes are also less hazardous than standard etching solutions; their chemical compositions substitute some hazardous ingredients with nontoxic organics.
To retain a competitive edge, few metal finishing companies advertize the fact that they electropolish alloys such as titanium, nitinol, or hard-to-polish stainless steels. Existing electropolishing processes for nitinol require highly sophisticated equipment with strict controls of each chemical ingredient and temperature maintenance. Conventional nitinol electropolishing is a hazardous process that uses methanol as the main ingredient. It is dangerous to use and requires skilled personnel to operate. In many cases, small companies cannot afford the cost of maintaining such processes.
Ultrapolishing technology can produce products of high quality at low costs. Companies that employ such methods can reduce outsourcing, lower rejection rates during quality assurance testing, and lower hazardous waste costs, especially as compared with the etching-deburring method that uses strong acids. Manufacturers also do not need to buy expensive equipment or hire specialists to conduct the metal finishing.
With fewer hazardous electrolytes, easy-to-follow instructions, and a low use cost, small- and medium-sized medical device companies should consider establishing advanced ultrapolishing processes in-house—especially if they’re seeking results that are superior to those produced by conventional technologies.
Anna Berkovich is president at Russamer Lab LLC (Pittsburgh).