Metalworking: Making the Cuts of Tomorrow, Today

Originally Published MDDI July 2002

COVER STORY

Medical device manufacturers must determine whether their suppliers have the right materials, forming technologies, and finishing methodologies for the job.

Ani Grigorian

In the field of metalworking, change comes in many forms. For companies serving the medical device industry, it is usually fueled by manufacturers who require their suppliers to possess the technological know-how and processing capabilities to produce high-quality metal components or devices with excellent precision and repeatability.

Metalworking companies are further challenged by the need for smaller, more intricate products. While suppliers are expanding their capabilities to be able to produce smaller products, device manufacturers are realizing that the more involved a metal component or product becomes, the more it costs and the longer it takes to manufacture.

Device manufacturers need to determine whether a supplier has the right materials, the right forming technology, and the right finishing methodology for the job at hand, according to Sanjay Shrivastava, principal engineer for Edwards Lifesciences (Irvine, CA) and chair of ASM International's Materials for Medical Devices (MMD) Task Force.

MATERIALS

The range of metals available to medical device manufacturers includes various grades of stainless steel, titanium, nitinol, cobalt-chromium alloys, and tantalum, among others. Many factors come into play when trying to determine which material is best suited for a specific component or device. "When it comes to vascular implants, the durability (fatigue resistance) and biocompatibility of an alloy are key. Manufacturers should also consider whether a certain alloy can be formed into the desired shape at the required size," notes Shrivastava.

For orthopedic implants, Dana Medlin, principal engineer of Zimmer Inc. (Warsaw, IN) and member of the MMD Task Force, says that cobalt-chromium alloys and titanium are popular choices. Cobalt-chromium alloys exhibit corrosion and wear resistance. Titanium, a durable and biocompatible metal, is commonly used in heart valves, pacemakers, artificial hips and joints, dental implants, and surgical equipment. "Cobalt-chromium and titanium perform quite well, so surgeons and manufacturers feel very comfortable using them," Medlin adds.

When titanium is combined with nickel, the result is nitinol, a superelastic alloy that is biocompatible, corrosion resistant, and cytocompatible. The shape-memory effect and elasticity of nitinol make it a popular choice for manufacturers of implantables. Vascular stents made from nitinol can be bent to facilitate their manipulation within the body with an endoscope and then returned to a prescribed shape at a certain temperature.

Stents also can be made using stainless steel, which offers good corrosion resistance, formability, and reasonable fatigue resistance. Stainless-steel tubes are laser cut to form stents. The stents are then electropolished for good surface finishes. "Often, coronary stent manufacturers specify materials that are not available in tube form, or if they're available in tube form, they're just not available in the right size or thickness," says Shrivastava.

MATERIALS DEVELOPMENT

Ideally, the device industry would have a steady stream of innovative new materials to consider for product development. Realistically, however, suppliers and manufacturers have to settle for less, according to Medlin. He notes, "It's very difficult and time-consuming to develop a new material for implantables, for example. It would take a lot of research because there is a tremendous amount of liability when dealing with materials that enter the human body."

Even so, suppliers are spending time on extending the range of metals available to them to come up with innovative products. One example is trabecular metal technology, which has been successfully used for hip and knee replacements by a variety of companies. Trabecular metal is a new porous metal made of elemental tantalum, which offers biocompatibility, durability, and corrosion resistance. The material has pore diameters of 430 µm and a porosity volume of 80%.

Medlin says that tantalum's biocompatibility makes it a popular choice for surgeons. He describes trabecular metal as a material that is very similar to trabecular bone in structure. The interconnecting pores in trabecular metal allow bone to grow into the implanted device. After years of researching this material, Zimmer expects to have acetabular cups that incorporate the trabecular metal technology toward the end of this year.

METAL PROCESSING

"Medical device manufacturers are specifying metals that are usually more difficult to machine than materials being used in other industries," says Kevin Noble, executive vice president of Norman Noble Inc. (Highland Heights, OH). For this reason, determining the appropriate metalworking technology for a specific product can be a complex process. Manufacturers need to ensure that their suppliers not only have the systems in place to form the desired material into the desired shape with excellent repeatability, but that they also have R&D staffs that are capable of determining the right tooling and parameters for a job before production is under way.

"Swiss machining is one of the big components of medical device manufacturing," states Noble. Swiss automatic machining, a process that involves turning, milling, drilling, and tapping functions, is used to make such products as dental tools and bone screws from stainless steel and titanium, among other materials. The multiple functions performed by Swiss automatic machines help increase production speeds for machining parts with midsized diameters.

An alternative to conventional Swiss turning machines is the coil-fed Swiss machine, which rotates tools around a stationary length of continuously fed metal. Noble adds that coil-fed machines are particularly useful for high-volume production runs that require good repeatability. The Escomatic coil-fed Swiss machine used by Norman Noble minimizes vibration, which increases precision. The system's CNC programmability ensures tolerances to ±0.0002 in.

Combining Swiss automatic machining with wire EDM (electrical-discharge machining) or laser machining will allow the production of small, tight-tolerance parts. "In stent manufacturing, laser cutting has been one of the most popular ways to achieve the desired shape," Shrivastava says. Metals are processed using CO₂ lasers, Nd:YAG lasers, and Nd:glass lasers. Metalworkers agree that CO₂ lasers cut faster and produce deeper weld penetration than Nd:YAG lasers. However, Nd:YAG lasers offer higher-pulse energy, which is favorable for drilling holes and cutting metals at certain thicknesses and angles.

Cutting metals at certain thicknesses and angles to produce medical devices such as needles and electrodes becomes a challenge when one considers how much smaller and more intricate devices are becoming. "Up until now, spiral electrodes have been mechanically cut off at the tip. We now take that tip and put a really sharp conical point on it, which reduces tissue trauma and increases ease of entry," explains John O'Brien of Point Technologies Inc. (Boulder, CO).

A manufacturer of probe needles for the semiconductor industry, Point Technologies has found a way to use an electrochemical pointing or etching process to remove small amounts of metal from the surfaces of medical devices made from tungsten—a pure metal that is the main ingredient in its probe needles. Additional metals used in probe needles include tungsten rhenium, beryllium-copper, and a palladium-platinum-gold alloy. "We have introduced some of these metals to medical contractors and device manufacturers for applications like electrosurgery electrodes, electrolysis needles, spiral electrodes, and other products that need points on them," says O'Brien.

"Tungsten is not an alloy, it's a pure metal—and it's four times harder than stainless steel and a lot more corrosion resistant. It can be used in virtually any application where stainless steel can be used," says O'Brien. Because tungsten is much harder than stainless steel, it is also more difficult to machine mechanically. The electrochemical etching process utilizes a cathode, an anode, and an electrolyte to produce ultrasharp tips on wire as small as 0.001 in. diam and submicron sharp edges on tubing as small as 36 GA. Submicron edges are set into stainless-steel hypo tubes (used in such microbiopsy punches as microtrephines). The process used by the company is computer controlled, which enables the manipulation of the number of times a wire or tube is dipped into a solution, the way in which it goes in or comes out, the temperature, and the shape of the tip being cut.

Unlike electrochemical etching, abrasive water-jet cutting systems are intended for larger pieces of metal destined for precision micromachining. Using this process, cuts can be made in materials up to 8 in. thick, such as stainless steel and titanium. Water pressurized up to 60,000 psi cuts the material by way of supersonic erosion.

METAL FINISHING

Forming a metal part may be just the beginning. "The parts may have to be subjected to finishing processes, and it is important that a supplier have the capability to do that," says Jack Fulton, sales manager of Specialized Medical Devices Inc. (Lancaster, PA). Choosing a finishing process is highly dependent on the nature of the component's end use.

For stainless-steel guidewires, "the parameters during the processes of drawing stainless-steel wire, heat-treating, and annealing affect the torque performance. So, we must carefully design and optimize those parameters," says Yoshi Terai, marketing manager of Asahi Intecc Company, Ltd. (Newport Beach, CA). One way the company improves the performance of its guidewires is by applying PTFE or hydrophilic coatings to their surfaces to increase lubricity. "Biocompatibility and lubricity are very important surface characteristics. Coating the guidewires enables physicians to insert them into the patients' vessels smoothly," says Terai.

The Electrolizing Corp. of Ohio (Cleveland), a partner company of Norman Noble, offers its own proprietary nonmagnetic chromium coating, which can be applied to all grades of stainless steel, most ferrous metals, and some nonferrous metals, such as copper and aluminum. Medcoat 2000 is a hard and dense low-friction coating that increases the lubricity of a cutting tool, explains Noble. Following machining, brazing, welding, heat-treating, and stress relieving of a metal component, the coating is applied to the base metal without the need for an intermediate coating.

The surface characteristics of parts machined from nitinol are also very important. Nitinol parts can exhibit heat-affected zones after thermocutting, crevices between grains, and sharp edges, which is why electropolishing nitinol parts is a necessary step. The process involves passing an electrical current through an electrochemical solution, leaving a smooth surface on the part. Noble says, "Electropolishing nitinol is a very limited field. It is something that a lot of companies want to be able to do because a lot of engineers are specifying it, but there are only a handful of companies that have figured out the right formula of chemicals required to make it a successful process."

COST AND TIME

Working with metals can be a time-consuming and expensive process. From a device manufacturer's point of view, "the cost of an alloy is usually not a concern. I wouldn't mind paying a bit more to get it sooner. For finished components, costs do tend to go up, and after you have your initial prototype, you should look for ways to manufacture the device in a cost-effective manner," says Shrivastava.

James Meier, marketing manager at Remmele Engineering, Micromachining Div. (Big Lake, MN), agrees that the cost of a material is almost insignificant. One of the more important challenges with micromachining metal components or devices is size. He asks, "How can you make a miniature part and then find it, measure it, account for it, and clean it? There's a point where the smaller it gets, the more it costs to handle it."

Another issue to consider is volume. "The quantities involved in the manufacture of metal components are so small that big metal producers tend to put the job on the back burner, which increases lead times," adds Shrivastava. This may push a manufacturer to turn to multiple suppliers to get the job done in a more time-efficient manner. For the most part, multiple suppliers translate into multiple costs, simply because of the amount of capital required to manage various supplier sites, says Fulton.

To keep device manufacturers' business, providers of metalworking technologies are being pushed to expand their capabilities. A company's capabilities (or lack thereof) affect the costs and time required for manufacturing metal components. "Some suppliers don't have advanced CAD capabilities, and if time to market is critical, the manufacturer needs to choose a supplier based on the availability of delivery systems, average lead times, and the ability to produce the part within a certain time frame. They should also look into getting prototypes made before ordering huge quantities," says Shrivastava.

THE NEXT STEP

"The medical world is constantly next-generation, new and improved," says Fulton. In the effort to keep pace with the demands of its customers, Specialized Medical Devices Inc. has added laser marking, 10-axis screw machining, RAM optical inspection equipment, and CMM inspection equipment to its capabilities. "If you want to continue to grow, you must maintain the highest level of sophistication possible. For example, the more complex a part becomes, the more complex the inspection equipment needs to become," says Fulton.

Suppliers are beginning to feel the pressure from medical device manufacturers, and reacting to it, by expanding their capabilities and updating their equipment. It is likely, however, that soon even those efforts will not suffice in an industry that strives to become more competitive than it already is. Shrivastava says, "I'd like to see some of the technologies that exist out there for making bigger products for the automotive and aerospace industries used for producing small metal components and devices. The device industry needs more suppliers doing more-efficient, more technologically advanced work to make it a more competitive market."

Because the demand is there, metalworking technologies can be expected to press forward, but this industry may need even more. Medlin shares a final thought on the future of metalworking: "It would be nice if there were some collaborative forum for device manufacturers and suppliers to get together, but there's so much money and proprietary information involved that this becomes difficult." For an industry that never gives up when faced with challenges, this objective might not be as unattainable as it sounds.

Ani Grigorian is a freelance writer and former MD&DI editor.

Copyright ©2002 Medical Device & Diagnostic Industry