Material choice impacts every part of medical-device design, from cosmetic qualities such as weight and feel, to manufacturing, sterilization, and packaging. Even legislation such as RoHS means designers will be looking at more alternatives to metals. And according to Michael Hansen of Mack Molding, “material specifications are shifting more responsibility to the molder who has to work closely with the designers to meet all requirements and figure out how to manufacture the device.” He says it is not uncommon to work with a range of materials and a series of molding sequences to produce a single product. “For example, one recent battery case used a clear polycarbonate window insert molded into a rigid base and overmolded with an elastomeric seal. In another example, a surgical instrument was overmolded with a soft touch handle material - this required a hard base and a thermoplastic polyolefin for softness.” The following examples show some of the latest medical materials on the market.

Biomaterials resist chemicals

Implantable materials must be chemically compatible, withstand temperature variations, be sterilizable, and handle residual stresses and external loads. Solvay Advanced Polymers's family of Solviva biomaterials (www.solvivabiomaterials.com) is said to handle such applications in four grades. Zeniva PEEK (polyetheretherketone) is said to have high chemical resistance, high strength and stiffness along with good toughness and fatigue resistance. Proniva SRP (self-reinforced polyphenylene) is the stiffest and strongest unreinforced thermoplastic in the family, with good chemical resistance and hardness. Veriva PPSU (polyphenylsulfone) has good toughness combined with transparency and excellent chemical resistance. And the company says Eviva PSU (polysulfone) offers toughness in a strong, transparent polymer.

Replace lead shielding with tungsten-filled polymers

RoHS, or Restriction of Hazardous Substances, are directives enacted by the European Union and other countries to restrict the use of six materials classified as potentially hazardous, including lead. In North America, a few states are discussing similar legislation. While these regulations have yet to affect medical devices, manufacturers are looking for lead replacements where possible in advance of plans to strengthen the directive to a complete ban by 2010.

Tungsten-filled polymers, one candidate, can replace lead in radiation shielding applications throughout the healthcare industry. Tungsten is a non-toxic transitional metal with a density 70% higher than that of lead. Testing shows that tungsten-filled polymer products (in varying densities) provide radiation shielding and attenuation up to and including a one-to-one equivalency to lead. Green Shielding Solutions, Avon Lake, Ohio (www.greenshielding.com) forms these materials using conventional methods such as injection molding. The company is a joint venture between Thogus Products and Vulcan GMS that will provide eco-friendly, tungsten-polymer radiation shielding for the healthcare market.

Polymer withstands high temps and wear

The Victrex Max-Series of polymers from Victrex plc, is said to perform in demanding environments, specifically in applications requiring a combination of high-temperature mechanical performance and dimensional stability. Victrex Max-Series polymers are a blend of Victrex PEEK polymer and genuine ExtemUH thermoplastic polyimide (TPI) resin from SABIC Innovative Plastics. The Max series is melt-processable, and an ultra-high temperature performance material (up to 275C) with chemical resistance, high purity, and controlled coefficient of thermal expansion.