One of the biggest complaints from engineers at metal companies is that designers are stingy with information on their applications. Too little knowledge can lead to a wrong choice. Some designers simply call and ask for a quote for different materials, without discussing the specs of the application. A choice of metal should be driven by more than price. To help figure out which metal is the best fit, we talked to application engineers at metal supply companies about the benefits and limitations of several medical-grade metals.

Stainless steel

Stainless steel is a workhorse medical metal. It's easier to machine than Titanium, corrosion resistant, and strong. This makes stainless good for surgical instruments, bone screws, stents, and other instruments in doctors' offices.

There are over 60 grades of stainless steel. It is essentially a low-carbon material with 10% or more chromium by weight. Chromium makes it corrosion resistant. The 300 series is often used in medical applications, particularly types 304 and 316. Type 316 has more molybdenum which increases its pitting and corrosion resistance, making it slightly more expensive. There are also lower carbon versions, commonly referred to as 304L and 316L. These have carbon contents of 0.03% in comparison to 0.08% in regular 304 and 316, which eliminates harmful carbide precipitation during welding.

Why else choose stainless steel? “When you want a good appearance,” says Bob Bubencik, President and CEO of Eagle Stainless, Franklin, Mass., (eagletube.com). “Stainless steel is a cosmetically and aesthetically better appearing material. It's brighter and you can easily enhance the appearance by electropolishing. This passivates the surface and maintains an enhanced shine. Some people, including doctors, associate things that look good with cleanliness,” he adds. “Stainless is strong, tough, and compatible with bodily fluids, blood, and enzymes, which is why it is FDA approved. It is easily manufactured, and can be bent, formed, cut, machined, and welded.”

One drawback to stainless steel may be its weight. “Titanium has the lightness of aluminum, but the strength and toughness of stainless. If someone is implanting the device, or if a surgeon has to hold the tool for long periods of time, weight may become an issue,” adds Bubencik.

Titanium

Titanium has grown in use over the last 10 years as a strong, lightweight, and bio-compatible metal. It is used for prosthetic implants, spine and trauma systems, instruments, and as dental implants.

There are a few grades of titanium that can be used in medical applications. Alpha types contain no stabilizers and examples include commercially pure 1 and 4 types. The alpha-beta types, contain stabilizers and include Ti-6Al-4V, Ti-6Al-4V ELI, and Ti-6Al-7Nb. There are also a number of beta-type alloys, which also contain stabilizers, and include Ti-15Mo. “A lot of the commercially pure, and some of the beta-type alloys are used where formability is important,” says Jim Ferrero, Metallurgical Services Engineer at Dynamet, a Carpenter Company, Washington, Pa., (www.dynamet.com). “The commercially-pure versions are generally a little lower strength and are chosen for trauma fixtures — devices surgeons have to manipulate that hold healing bones together. Alpha-beta-titanium grades are used for prosthetic implants where strength is important.”

“One benefit of titanium over other prosthetic materials is its lower modulus. Many newer alloys have an even lower modulus, comparable to that of bone. This helps eliminate problems including bone recession and implant loosening.”

Ferrero suggests keeping in mind the following criteria when specifying titanium: First, share as much information as possible. Know the required strength, the final application, environment, and what material titanium might be replacing. With this information, Ferrero says he can better recommend the right titanium alloy.

Nitinol

Nitinol is one of the newer metals used in medical applications and arguably one of the more interesting. Nitinol is a generic trade name for NiTi alloys, which stands for Nickel (Ni), Titanium (Ti), and Naval Ordnance Laboratory (NOL) where the alloy was discovered in the early 1960s.

The superelasticity of Nitinol makes it stand out from stainless steel or titanium. “What that really means is you can put a strain of between 6 and 8% and the material recovers within 0.1 or 0.2% of its original shape,” says Mark Polinsky, Director of Engineering at Memry Corp., Bethel, Conn., (memry.com). “I don't know of another material that can be formed into a shape, deformed significantly, and when heated or cooled, comes back to its shape,” he adds.

This “shape memory” characteristic is popping up in interesting applications. The “superelastic” properties of the material are widely used in instruments including laprascopic and orthopedic tools used in spine surgery. The shape-memory devices recover their shape inside the body at body temperature or at a preset temperature whereas the super elastic devices recover their shape once deployed from the delivery system or catheter. Nitinol tubing is also used in stents and catheters.

“There are multiple grades of Nitinol,” Polinsky continues. “One that's widely used is a binary, which is essentially a nickel and titanium. Within the family, as you change the percentage of nickel you change the material properties including transformation temperature. So designers can tailor the metal to their applications if they want a device to react at body temperature, or if they want the device to be superelastic at body temperature.”

Nitinol is not the right solution for every application. “If you wanted to produce something large out of Nitinol like a plate material, or want to cast it into a shape, it really doesn't lend itself to that,” suggests Polinsky. Most Nitinol parts are sold as tube, wire, or strip, which typically range from about 0.003 to 0.5-in outer diameter.

The two most common finishes on Nitinol are electropolishing and chemical etching. Both electropolishing and chemical etching take material off the surface, making it shiny and aesthetically pleasing. These processes can also be used to add a passive surface for corrosion resistance.

Polinsky's advice for designers considering Nitinol is to describe as much detail as possible to the Nitinol application engineer. “Nitinol knowledge has grown in the industry so a lot of people hear about the superelasticity or that it changes shape with temperature. Designers often don't realize there are limitations in both of those categories.”

Specialty alloys

Cobalt-based alloys are used for medical implants because of their superior corrosion resistance. One example is BioDur CCM, from Carpenter Technology Corp., Reading, Pa., (www.cartech.com), which contains 28 weight percent cobalt, 6 molybdenum, and a small amount of carbon and nitrogen for strength. Cobalt, a dense, hard metal, adds corrosion resistance. BioDur CCM is used for implantable hip and knee components. Other high-cobalt alloys used in implants include L605 and MP35N. These are used for coronary stents, pacemaker leads, and other cardiovascular applications.

“There are many factors to consider when choosing a material for an implantable device”, explains Carpenter's Michael Walter, Staff Specialist, Medical Alloys. “For example, Cobalt-chrome has higher strength and wear resistance than Titanium, while Titanium has lower modulus and is generally thought to be better for bone growth. Most medical OEMs offer both Cobalt based and Titanium-based orthopedic implants to satisfy the preference of the surgeon.”

Another new alloy gaining acceptance in the medical field is Carpenter's BioDur 108. It is an essentially nickel-free alloy with extremely high strength. It is used in medical screws, trauma fixation devices, and hip and knee parts. Tests show BioDur 108 to have higher strengths than other common nickel-containing stainless alloys. The high nitrogen content, which helps maintain the alloy's austenitic structure, contributes to the high strength, ductility, and corrosion resistance.