Pulsed Electrochemical Machining: Achieved Through Chemistry

Pulsed electrochemical machining (PECM) is a non-conventional and relatively unknown machining process. It could prove to be a viable solution for machining orthopedic devices and help grow medical device manufacturing to keep up with rapidly advancing technology. This article is intended to explain how it works, its capabilities, and how it can affect the future of orthopedic device manufacturing, citing current and hypothetical examples.

What Is PECM?
PECM is a unique machining process that utilizes electrochemistry to dissolve metal, rather than a contact-based process such as CNC machining or thermal-based processes such as electrical discharge machining (EDM).

PECM’s growing reputation has industry leaders currently focused on its applications within aerospace manufacturing—as the aerospace industry has acute specifications that only PECM can fulfill—in projects dealing with high temperatures and unique geometries. One such example is superalloy-based heat exchangers in airplane engines. However, this article’s focus will be on how PECM can be particularly advantageous for medical device manufacturers.

PECM relies on several distinct parts that allow the electrochemical reaction to take place.

The tool: Also known as the cathode, this custom-machined tool is designed as the inverse of the desired workpiece pattern. The cathode design is a complex process requiring an understanding of current density distributions, materials, dielectric coatings, and managing flow of the electrolyte.

The workpiece: Also known as the anode, the material being machined can be any conductive metal. Common materials for medical devices include nitinol, stainless steel, titanium, and cobalt chromium. For non-medical purposes, PECM can also machine metal matrix composites, nickel superalloys, and recently, some refractory metals, with unmatched precision.

The electrolytic fluid: Between the cathode and anode, also known as the interelectrode gap (IED), an electrolytic fluid is continually pumped between the two materials. This serves two important purposes. First, it acts as the conductor for the electrochemical reaction via a “pulsed” electrical charge. Second, it acts as a flushing agent that removes the dissolved metal and any excess heat from the IED. This is the primary factor that leads to PECM’s unique surface finishing qualities, without requiring a secondary finishing process.

Due to the unique qualities of PECM, it is best utilized in specific machining situations that will be later outlined in this article. As important as the use cases are, however, following are examples where PECM may not be the most applicable technology.

• PECM is generally not suited for prototyping projects. PECM becomes more cost-efficient with scalable, high-volume manufacturing projects, but machining a unique cathode for very few prototyping purposes is not cost-efficient.
• PECM will not work for any project where the workpiece is not a conductive metal, such as polymers or plastics. The electrochemical reaction cannot take place here.
• PECM may not be the most efficient machining solution for a project with simple geometries. Large flat or cylindrical surfaces are easily produced with grinding or milling while rough machining of large areas can be more cost efficient with conventional machining. Similarly, industries where tight tolerances are not quite as important, such as certain construction applications, are a poor use of the PECM capability.

Pulsed electrochemical machining has a variety of properties that, under the correct circumstances, offers a cost-efficient, intricate machining process. These benefits may include:

Non-contact machining: The inter-electrode gap (distance between the electrode and workpiece) can be 10 to 100 μm, meaning the cathode and anode never physically touch. This is particularly useful in machining applications of tough alloys that may damage the tool over time. PECM can theoretically utilize the same tool thousands of times without replacement, as there is no wear or erosion on the electrode.

This non-contact approach also means there is no friction—therefore, no heat-affected zones. This is particularly important with thin-walled features of parts, where heat generated can distort the features of the part.

Surface finishing: The electrolytic fluid helps the resulting surface achieve a unique surface finish, included without additional steps. PECM can also localize the process to specific surfaces, which may prove useful in projects such as femoral stems, where a ball joint should have a smooth finish, and the intraskeletal part remain rough. In orthopedic devices and especially surgical tools, these smooth features provide a more sanitary environment, free of re-cast layer or microcracks that may promote bacterial growth.

No regard for material hardness: Conductivity is the only parameter PECM is concerned with; therefore, it can machine a nickel superalloy at around the same rate as copper. Within orthopedic device manufacturing, many tough-to-machine alloys are used such as nitinol and cobalt chrome, and a machining process that can address a range of material types can be useful.

Precision machining: PECM can achieve intricate features and create micrometers-wide wall structures that can be utilized for surgical tools and razor-thin features on other devices.

Scalable manufacturing: For applications where annual volumes may be in the thousands to hundreds of thousands, PECM can be a cost-efficient solution. Since the cathode doesn’t wear, and the process remains the same, PECM can easily repeat its intricate machining features to parts over and over.

As previously noted, these unique advantages make PECM applicable in many other industries aside from medical device manufacturing, such as aerospace applications, additive manufacturing, and the automotive industry.

Current Medical Device Applications
Currently, PECM is already being utilized in an array of applications across medical device manufacturing.

Surgical stapler anvils often must be composed of tough materials, making them an ideal candidate to be fabricated with PECM. PECM is the ideal machining process to create the small, patterned grooves in these materials. A tough, smooth surface ensures the overall part’s safety and sanitation—vital for surgical staplers.

Various surgical tools can be electrochemically machined. Machining razor-thin features on stainless steel is not a concern with PECM. Since PECM is a non-heat conductive process, it can machine extremely thin walls without any worry of thermal distortion. This machines an extremely sharp surgical tool that is not only more durable, but safer for both the patient and medical staff. By machining smooth surface finishing, PECM can also promote a more sanitary surgical tool. Rough-edged tools contain microcracks that can promote harmful bacterial growth. A smoother, microscopic surface is a harsher environment for bacteria and viruses to grow.

In various joint implants, such as knee or hip components, PECM plays an important role in machining these parts for tribological purposes. Often, orthopedic joint implants are hardened for joint longevity and dramatically different surface quality requirements, creating manufacturing challenges. Within a ball and socket or knee joint, for example, the surface must be particularly smooth to reduce any friction or wear. Contrast that with bone facing surfaces which require an intentionally rough or porous finish for optimal bone integration. PECM, using a shaped electrode, can selectively machine or polish some features while leaving others unaffected.

Potential Medical Device Applications
Medical device manufacturers should consider PECM as a cost-effective process for their critical, high-volume projects. As the medical device manufacturing industry often requires a rapid build-up of inventory pre-launch, PECM may be a viable solution for pre-launch production, as well as post-launch production spikes. While PECM requires some up-front development and design costs, part production can quickly be enabled to meet demands frequent of the medical device industry, such as high-volume orders and quick turnaround due to demand fluctuation.

Stents pose unique manufacturing challenges that are representative of many of the medical device industry’s overall challenges. First, stents must be made of a material that is biocompatible. In May 2021, the FDA published a discussion paper on this issue to facilitate a conversation with manufacturers, highlighting the importance of using materials in medical devices that won’t cause harm and promoting discussions with all parties regarding the safety of biocompatible materials in these devices. Stents are a primary example, since if a stent is made of an incompatible material that can be easily worn, harmful materials may eventually be released into the bloodstream.

Only a few biocompatible materials are known to be used for stents that bypass these issues, such as titanium, nitinol, and cobalt-chromium. PECM excels at machining certain biocompatible materials often used in these devices and, under the right circumstances, may reduce manufacturing costs by eliminating electropolishing—a common secondary process. Another reason electrochemical machining is ideal for stents is its surface finishing qualities; excess materials and microcracks are already disposed of in the PECM process, ensuring the completed part will be fully biocompatible. PECM can potentially speed up the manufacturing process of stents by machining the entire surface simultaneously without requiring secondary machining to remove any burrs.

Drug-eluting stent technology is rapidly developing. While the details of these advancements are currently being researched, it is reasonable to infer that drug-eluting stents will have thinner features, higher radial strength, and be more likely to benefit from a machining technology with the ability to shape these alloys into complex geometries. PECM can create thin-walled structures down to the micrometer length, but it also excels at patterned surfaces, which is a likely aspect of drug-eluting stents made of conductive metals.

On a slightly larger scale, these biocompatible alloys are also utilized in bone fixation devices. Nitinol and titanium are especially useful in osteotomy procedures, fractures, and other applications requiring a device to fixture bones together. The fixture device should be small and minimally invasive while maintaining durability and flexibility in challenging areas such as ankles. The comfort and size of the device plays an important role in patient comfort with smaller, smoother, and more conformal or organic shapes preferred in these areas. In this application, PECM is particularly valuable, as it can machine materials such as nitinol and titanium with high precision, small features, and 3D contours without leaving any burrs or heat-affected zones that may compromise the part’s effectiveness.

Pacemakers, neurostimulators, or other implantable electronics packages are another type of implant that may directly benefit from the advantages of PECM. Over the past 20 years, pacemakers have rapidly shrunk in size and increased in performance. Miniaturization, however, causes new issues in manufacturing, requiring smaller features, thinner walls, and tighter component packaging. Only a select few alloys can accomplish this task, with the added obstacle of shielding the electronics from any electromagnetic interference. Titanium and certain refractory metals are among the ideal materials that comprise pacemakers that can perform all the necessary requirements to be a safe, durable pacemaker that will last years and shield the device from electrical interference. Titanium and niobium can be machined with high precision if pulsed electrochemical machining is utilized.

Other medical technology outside of implants may benefit from PECM as well. For example, medical imaging machines often use refractory metals such as tungsten, rhenium, and molybdenum. X-ray devices generate enormous heat and electrical energy, often requiring materials composed of high-temperature resistant materials, notably refractory metals. Recent PECM advances enable machining of these refractory metals as well as the cooling structures necessary to expel this excess heat.

Conclusion
PECM has unique attributes that may be valuable for a variety of medical parts. From surgical tools to nitinol bone plates, pulsed electrochemical machining can deliver high quality parts with complex features for volume production applications involving challenging materials. 

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Kirk Gino Abolafia is the technical sales and marketing manager of Voxel Innovations. Visit Voxel’s education portal for more information on PECM.