Pulsed electrochemical Machining (PECM) for medical devices

Computer numerical control (CNC) milling, electrical discharge machining (EDM), and metal injection molding (MIM) have long been used in medical device manufacturing, however some innovative designs can be challenging for existing capabilities. Pulsed electrochemical machining (PECM) offers a viable alternative.

Process

PECM removes material and manufactures parts by dissolving metal, atom-by-atom, until it reaches the intended shape. Being a non-contact process, the tool electrode (cathode) used to shape a workpiece (anode) never touches the material. Instead, the tool is formed in the inverse shape of the desired part and placed close to the workpiece, with only a miniscule gap between. An electrical current creates controlled corrosion, leaving only the desired part behind. Simultaneously, an electrolyte solution flushes away removed metal.

PECM provides pristine surfaces and thin walls in hard-to-machine metal alloys, while delivering repeatability and virtually no tool wear. It’s often confused with sinker EDM, and although there are some similarities, PECM is faster, eliminates tool wear, and produces higher quality surfaces. Systems have high initial costs and are not well-suited to prototyping quantities, but they offer great return on investment (ROI) for high-volume and/or high-value manufacturing.

Surgical tools

For surgical tools, progressive stamping and metal injection molding (MIM) are the go-to manufacturing options. In stamping, a flat sheet of metal is inserted into a press, where a die (or multiple dies) cuts and bends the metal into the desired part. MIM uses powdered metal mixed with a binder material, melted and injected into a mold in the shape of the final part. Subsequent steps remove the binder and compact the metal into a solid.

The low-cost, quick production of parts make these processes appealing, but they aren’t compatible with delicate parts. For example, the dies used in stamping deform the workpiece materials, so dies must be made from hard materials that can withstand the pressure and abrasion, and the workpiece materials to be shaped must be thinner, softer, and more ductile. This limits workpiece material choice, and small features can be too fragile to form. Also, dies can become dull or damaged, requiring expensive replacement to maintain part tolerance.

MIM processes have challenges creating thin, high aspect ratio features due to the fluid dynamics of filling the mold. Also, when systems remove the binder and compact the metal via sintering, shrinking can cause the part to crack or fail. So, part sizes and transitions from thick to thin areas of the part are limited. MIM can also necessitate additional processing to remove gate marks or drafts that are a function of the MIM tooling.

While the average PECM part may be more expensive, it can achieve more delicate features and has improved material flexibility. For example, PECM has achieved greater than 20:1 aspect ratios, can be used on parts of virtually any size or weight, and creates blemish-free surfaces.

PECM can create surgical tools – handheld and robot end effectors – with more complex shapes, higher aspect ratios, and from a broader range of metal materials. Given trends in miniaturization and single use devices, the ability to create volumes of intricate parts with PECM can help enable the next generation of affordable surgical tools.

Imaging machines

In many medical imaging machines, the arrangement typically includes an anode device and a filament as a source for X-rays, which reflect off the anode and are directed toward the area of the body to be imaged. Only 1% of the X-ray energy is reflected off the anode, while the remainder is turned into heat at the anode’s surface. This heat requires high temperature refractory metals such as tungsten, molybdenum, and rhenium (or composites thereof) for the rotating anode and its support. Additionally, some companies have added cooling passageways to further regulate the temperature.

Common processes to create these anodes from refractory metals may include grinding for the anode surface and wire EDM for the cooling features on the anode holder. However, refractory metals can be difficult to machine for exactly the reason they are used in this high-stress application: their resistance to heat and wear. In addition, the required aspect ratios of these devices are difficult to achieve using conventional manufacturing processes.

Voxel Innovations, a U.S.-based PECM manufacturer, has recently developed a technique to effectively process refractory metal components, enabling faster manufacturing and lower costs, increasing the accessibility of imaging machines while preserving part performance and longevity.

Orthopedic implants

Structural orthopedic implants can include a variety of materials, shapes, and manufacturing techniques depending on the application requirements.

Joint implants such as knees, hips, and shoulders are traditionally made from metals such as titanium and cobalt chrome, which is cast or forged into a rough shape with additional machining via milling and grinding. Significant innovation has improved patient outcomes by improving the osseointegration of the implant to the bone structure with additive manufacturing (AM) or advanced coating systems. In addition, where one part of the implant touches another, a hard, smooth, bearing-like surface is required.

Hard-to-reach areas such as the femoral implant-to-bone interface on a knee joint can be readily machined with PECM. Even if manufacturers opt for the increasingly popular option of 3D printing, PECM can be a useful tool to correct any surface roughness or deformation that can be generated by down-skin surfaces or support structure remnants.

Trauma fixation devices such as plates and staples can also be improved by manufacturing with PECM. These devices are principally made from titanium or titanium alloys, however, the popularity of super-elastic nitinol has increased in this application, given its biocompatibility and ability for low profile shapes, less invasive surgery, and faster healing. While this material can be difficult to machine with traditional machining processes, PECM is well suited to nitinol because it avoids chip re-cutting, burrs, and recast layers, and enables plates and staples with 3D curvature. Particularly, as nitinol applications rise in volume, PECM can help reduce the per-part price by processing multiple parts simultaneously.

Summary

While PECM doesn’t have the same recognition or supply base scale as conventional manufacturing techniques, it provides important benefits to cost, quality, and material flexibility for multiple areas in medical manufacturing. As the industry continues to look for device innovation, manufacturing processes such as PECM can be a source of inspiration, allowing engineers to create new features that were previously impractical or prohibitively expensive.

Voxel Innovations
https://www.voxelinnovations.com