Owing to its biocompatibility, proven history, and cost-effectiveness, stainless steel has long been a dominant force in the biomaterials market. And nickel-free grades currently entering the market will likely further secure stainless steel's longevity in the industry. While stainless steel is in no danger of becoming an obsolete biomaterial, it is increasingly being passed over in favor of alternative materials. The rising demand for more-complex applications and nonmetal options has manufacturers turning to younger, more inexperienced biomaterials such as nitinol and PEEK for their applications, leaving the veteran medical metal sidelined.

Nitinol Gives Medical Metals a Makeover

"Since the 1960s, nitinol has been looking for a home," observes Audrey Fasching, senior metallurgist for Memry Corp. (Bethel, CT; www.memry.com), a supplier of nitinol and associated engineered components.

In the 1990s, it finally found one in the medical device industry. Named for its nickel-titanium composition and the Naval Ordnance Laboratory where it was discovered, nitinol has steadily emerged in the past decade as a viable alternative to stainless steel for many biomedical applications.

Prized for its shape-memory effect, the alloy is also superelastic, flexible, kink and corrosion resistant, and, of course, biocompatible. This combination of desirable material characteristics has enabled nitinol to carve out a niche in the minimally invasive market, most notably in cardiovascular and peripheral vascular stents.

"Nitinol, because of its elastic nature, is always applying a constant force against the vessel wall," says Dave Niedermaier, director of sales and marketing for Nitinol Devices and Components (NDC; Fremont, CA; www.nitinol.info), a material fabricator that supplies nitinol wire, strip, sheet, and tubes. "And if that force is disturbed, if you have motion crushing, twisting, or bending, the material gives but springs back, so it's a very well-adapted material for the body," he continues.

Despite success in stents and several other minimally invasive devices, nitinol has experienced slow adoption in the medical market. Dennis Norwich, process engineering manager for Memry, attributes reticence toward replacing stainless steel with nitinol to a lack of awareness about the relatively new material.

"Unlike steel--which has been around for hundreds of years with volumes of information on it--nitinol is new and there are no true textbooks to describe it," Norwich says. "But with more exposure, the customer base is becoming more knowledgeable and sophisticated. Awareness is starting to grow."

Jim Hartle, Memry director of marketing, concurs. He states that, over the past eight years, questions from medical device OEMs have evolved from the basics (What is nitinol?) to more-complex application-specific queries (Can nitinol do this for my device?) Memry feels that as information becomes more accessible and success stories are relayed, OEMs will be more receptive to designing with nitinol in lieu of stainless steel.

Cost has also been a deterrent. But Norwich says the bottom line is that OEMs are paying slightly more for functionality with which stainless steel simply can't compare. "If you want to get to the next tier of product capabilities, there's really no other choice," he says.

Although it can replace stainless steel, nitinol is a completely separate material with its own unique properties and processing needs. Niedermaier stresses that OEMs need to understand the effects of heat treatment on the material as well as how to yield a smooth, uniform oxide surface to ensure biocompatibility and corrosion resistance. Nitinol can also be difficult to machine and cut, and can wear out tooling quickly, adds Norwich. Memry, which offers proprietary processes for nitinol, recommends that OEMs let specialty companies handle the material.

As awareness and comfort level continue to grow, so may the potential uses of nitinol as a biomaterial. Niedermaier points to orthopedics as an untapped market; nitinol could be employed in such applications as anchors and fixation devices. Norwich predicts that nitinol will continue to flourish in the minimally invasive sector.

For now, Niedermaier says much of the nitinol R&D is focused on taking baby steps forward in such areas as improving component processing. NDC is preparing to offer the ability to roll nitinol in thin, wide constructions on a continuous coil for laser cutters that want to work with long lengths, and for those who may want to use nitinol for stamping applications. The company's aim is that this offering will enable better tolerance control than is currently available.

"Nitinol is the answer in search of a question," says Hartle. "[It is the answer] any time someone comes up with something that needs a dynamic material that can undergo deformation and return to its original construct."

PEEK-ing into the Future of Biomaterials

Like nitinol, polyetheretherketone (PEEK) is a newer biomaterial that is edging out stainless steel, titanium, and other traditional metals in a number of biomedical applications. Characterized by its stiffness, strength, design flexibility, biocompatibility, strength-to-weight ratio, and chemical resistance, PEEK has become increasingly prominent during the past decade for its use in fusion and stabilization devices, as well as in numerous other implantable devices.

Its future appears promising as well. Studies have planted the seed of doubt regarding the use of metal in implantable devices. As a result, some OEMs are looking for an alternative material that offers the attractive properties of metal without the potential risk. "There are issues with stress shielding," says Shawn Shorrock, global market manager, healthcare, Solvay Advanced Polymers (Alpharetta, GA; www.solvayadvancedpolymers.com). "So they're looking for ways to make implants lighter, specifically speaking to things like orthopedics, but then the [materials] also need to be biocompatible and able to handle the loads that orthopedics [require]."

Moreover, studies have suggested that there could be a correlation between metal-on-metal hip replacements and elevated metal ion concentrations in the body, according to Michael Callahan, president of Invibio (West Conshohocken, PA; www.invibio.com), supplier of PEEK-Optima.

Potential red flags associated with metal, coupled with a swelling market base, have primed the biomaterials industry for an expansion of PEEK. "It's the right time for the market with the aging population and the medical device technology really outpacing the materials that are available," says Shorrock. "It's a good time to come in."

Observing the receptive state of the marketplace, Solvay was attracted to the notion of adding biomaterials to its polymer-based product portfolio. In October, after two years of R&D, the company unveiled its first family of long-term biocompatible materials, Solviva, which includes Zeniva PEEK. It is chemical resistant and features strength, stiffness, toughness, and fatigue resistance comparable with the leading PEEK products, according to the company.

A veteran provider of the polymer, Invibio trumpets the advantages of its PEEK-Optima over metals. "It is less stiff than titanium alloys, which may help prevent stress-related resorption of surrounding bone and the loosening of implants," Callahan says.

The product has also demonstrated good wear resistance almost on par with metal-on-metal orthopedic implants, according to Callahan. As a polymer-based product, the company's PEEK-Optima is not in danger of releasing metallic ions, experiencing corrosion, or generating image artifacts, either. However, Invibio recently introduced PEEK-Optima image-contrast compounds that contain a radiopaque additive enabling visibility of the implant and potentially eliminating the need for tantalum markers. The company also launched carbon-reinforced PEEK for enhanced stiffness.

PEEK and nitinol are still relatively young. However, their influence has the potential to substantially grow as applications require more flexibility or shy away from metal. And, as these biomaterials improve and more applications are determined, stainless steel may just find its grasp on the biomaterials market weakening.