Electropolishing Can Improve Medical Part Performance

A decades-old technology has been updated to achieve new and profound results for the orthopedic industry.

David Z. Pokvitis

The term electropolishing still conjures up the idea of bright and shiny. From a strictly technical perspective, the removal of surface metal using chemicals and electricity has long been used to brighten many metals, especially stainless steel.  Even sectors such as the food processing industry have long used electropolishing to make food contact products look and behave better.

The photos at top, before electropolishing, and below, after electropolishing, show the tip of a drill used in orthopedic surgery. Manufactured from 440A stainless and heat treated, this expensive and critical part is machined to tight tolerances.

In the past decade, advancements in process controls have allowed for significant breakthroughs in the medical arena. Today, literally millions of bone screws, plates, cutting instruments and other implant items routinely are electropolished as part of the manufacturing process every year. Many engineers specify electropolishing these days because of precedent, but few fully understand the how’s and why’s of this proven process.

Electropolishing is a controlled metal-removal process similar in theory to electroplating. Parts are immersed into a precisely controlled chemical bath, and when electrical current is applied to the metal parts, metal ions are dissolved from the surface. Electroplating uses similar techniques to apply metal ions to parts.

Fabrication processes such as stamping, grinding, machining and heat treating all are employed to transform a metal bar, casting or sheet into a finished part. As metal is bent, ground, heated and altered, the metal surfaces alter significantly. These alterations take form in burrs, contamination, scale and tooling marks, most of which need to be removed before use in surgery. In many cases, these surface imperfections can be a focal point for infection or metal contamination for the patient. As a result, fabricators, engineers and regulators expend much energy and time restoring these metal surfaces to an inert condition.

Under higher magnification (top photo) the opposing machining marks are clearly visible on the various surfaces, moving metal to a fine and fragile burr on the lead surface. After electropolishing (bottom photo), enough metal was removed to remove most machining marks together with the burr and yet preserve the highly engineered drill surfaces. This was accomplished without change to the hardness of the drill.

Orthopedic product engineers often look to coating processes to hide or mask a contaminated surface. The chief weakness of coatings is delamination.  Depending on the application (implant, tissue insertion, multiple-use or single-use surgical tools), chipping, peeling or delaminating coatings pose unique and potentially dangerous conditions for the patient.  While the coating itself may not be dangerous, exposing the poor metal surface underneath can be. Exposed burrs could break off in surgery, microscopic surface irregularities could harbor dangerous bacteria and imbedded contamination can cause rust or corrosion under repeated tough chemical sterilization procedures.

There are six key reasons electropolishing is specified on orthopedic products:


1. Deburring. Electropolishing removes surface metal and works especially well on burrs left from stamping, grinding or machining. Since the process is non-mechanical, it dissolves burrs from all surfaces simultaneously.  The more complex or multifaceted a part, the more economical electropolishing is. Bone screws, cutting blades and drills are prime candidates.

2. Microfinishing. As surface metal is removed, grinding, tooling and other marks are reduced, leaving the part much smoother both visually and measurably by profilometer. This drive for smoother parts can result in tools that operate with less applied force, smoother insertion for cannulas or biopsy needles, more consistent drug delivery devices and surgical tools that operate in ever-smaller environments. In general, Ra surfaces are improved by 50%. For example, a 16Ra finish is improved to a finer 8Ra, again on all surfaces simultaneously. Nearly all orthopedic products, especially implantable parts, benefit.

3. Corrosion resistance. Stainless will corrode, especially once contaminated by fabrication. Electropolishing removes surface metal and, with it, imbedded contamination that encourages corrosion. Decades of corrosion testing have proven that electropolished surfaces will resist corrosion 30 times more than commonly specified chemical passivation.  All implantable parts and multiuse surgical tools are excellent examples, especially those that call for passivation in an attempt to prevent damaging corrosion. With increasingly stringent sterilization techniques subjecting parts to corrosion, many engineers are looking for treatments that will exceed the limited benefits of passivation.

4. Fatigue improvement. When surface metal is removed, surface cracks and other “stress risers” are greatly reduced, allowing high-stress parts to last longer without fracture or total failure. Heat-treated or solution-treated metals to very hard condition often are subject to high-speed or high-stress use, and an unexpected metal failure of a cutting tool during surgery can be devastating. The spring industry has used electropolishing for many decades to improve the fatigue life of high-stress parts.  Electropolishing can be applied after the hardening process and, in most cases, at very minimal dimensional change.

5. Decontamination. Most metalworking processes leave a metal surface porous and spongy, a condition called an “amorphous” or “white layer” of smeared metal. Surfaces that are machined, ground or lapped often leave an amorphous layer that can be observed under 100X-300X magnification. This imbedded contamination also has significant air space that is perfect for trapping bacteria or water/chemicals. Research

This surgical cutting blade (Top photo) is fabricated from 420 stainless steel and heat treated. The middle photo (400X magnification) shows how it looked before electropolishing. The cutting teeth were ground, and the cutting surfaces held residual burrs and other fine material. The objective was to remove enough metal to remove loose metal fragments but not to reduce cutting effectiveness. After electropolishing (bottom photo, 400X magnification), the surfaces were cleaned of all fragments. All teeth were done on all angles from root to tooth in a single process step.

in other fields has proven that electropolished surfaces are highly resistant to biofilm production. This is the largest potential application for the process, as concerns grow over resistant bacteria in the surgical arena.

6. EDM/Laser Recast Removal. EDM and laser fabrication have made great strides over the past few years. An increasing number of complex, fragile and tiny parts is cut and shaped by these methods. EDM especially is a preferred fabrication method for products in design for FDA approval, as slight design changes can be accommodated with minimal tooling charges. Unfortunate byproducts of these methods are re-cast or molten metal and contaminates that adhere to the part. As the wire (EDM) or laser beam cuts metal, some of it is vaporized, while a fair portion re-attaches to the metal part. In most cases, this recast metal is extremely brittle and difficult to remove by conventional mechanical finishing operations. Electropolishing dissolves recast on nearly any metal alloy, removing the molten metal, carbon and other contamination. As described earlier, complex curves, parallel and certain interior surfaces are treated simultaneously without damage to the base part.

A Beneficial Process

Thanks to process improvements and exhaustive surface analysis, more people are beginning to see the full range of benefits from controlled metal removal via electropolishing. Restoration of a metal surface almost always is more beneficial than coating a defect, especially when the process is non-mechanical and applicable on fragile and complex parts. In volumes large and small, the process has proven itself as a valuable part of orthopedic product manufacturing.

David Pokvitis is co-owner of Able Electropolishing, headquartered in Chicago with new production facilities opening in Reno, NV. David has more than 25 years of experience conducting seminars on
electropolishing, and has extensive hands-on experience in troubleshooting metal
surface problems. He can be contacted at dpokvitis@earthlink.net