Any guide to designing with advanced polymers could be a book. So this introduction will be limited to tips for finding appropriate polymers and considerations for strength and wear.

Finding the right polymer can be daunting because matching it to a design means examining hundreds of families and grades. Engineers at GE Plastics, Pittsfield, Mass (geplastics.com), suggest considering several important factors, such as the need for visual clarity and resistance to impact, heat, and chemicals. However, beyond that there are three additional points of consideration for whittling down the long list of plastics to find a suitable match. The material must meet requirements for strength and wear resistance as well as those specific to healthcare, including coefficient of friction.

• First, quantify the needed strength. It is important to quantify the anticipated loads or impact the product must sustain. For example, a surgical stapler might be squeezed with 20 lb and dropped from a height of five feet. Each generates stress levels in the polymer.

  “Stress versus strain plots are commonly used design data for characterizing polymers,” says GE Plastics' healthcare marketing manager Clare Frissora. “The curves show several characteristics and their relationships. For instance, the single stress-strain curve shows relationships between applied load, deformation, and stiffness or modulus from simple tension. It also indicates the maximum load a material can sustain without permanent damage.” Other characteristics include the ultimate stretch a plastic can handle before failure, and a value for the plastic's energy absorption comes from summing the area under the curve. The one caveat is that plastics tend to creep more than metals, so values from polymer data sheets are good for one temperature and for short-term loading. GE Plastics provides multiple stress strain curves for a given material to help designers understand plastics' physical property changes at different conditions. Companies such as Frissora's also provide online design tools for further support.

• Secondly, consider healthcare specific requirements. For example, what type of sterilization will the product experience, such as autoclave, gamma, or EtO? If the part will be autoclaved, it is necessary to determine the temperature and time of exposure. And what chemicals will the device be exposed to, such as disinfectants, cleaners, and IV solutions?

  “Exposures may also include body fluids, and in particular, blood,” says Frissora. “The blood-care industry uses a lot of polycarbonate because of its clarity and good mechanical properties.” Hemocompatibility, an additional requirement, refers to how well materials coexist with blood. Certain biomarkers (a blood component) stick to plastic, treating it as foreign material. Surgery sometimes calls for removing blood from the body, warming and oxygenating it, then returning it. But blood components can be lost in the process. So look for materials that preserve the integrity of blood. “A recently introduced polycarbonate, Lexan HPM1944 resin, has additional compatibility to blood contact when compared to standard polycarbonate and based on several tests conducted by GE Plastics. The new material also has low-protein binding qualities,” says Frissora.

• Finally, characterize the required wear resistance by specifying wear factors or the necessary coefficient of friction. Also quantify the expected life of the device.

GE Plastics says resistance to motion — the coefficient of friction — is often not critical as long as it is reasonably low. However, some designs require controlling the level of friction. Wear resistance, a related consideration, is primarily a function of material hardness, contact pressure, and the coefficient of friction between adjacent material surfaces. Contact pressure is easily controlled by changing its dimensions. Such pressures change over long periods due to stress relaxation when stress levels exceed material limits at the given temperature. Contact pressures can also change significantly with temperature. When different materials are used in a mechanism, contact pressure can be severe due to differences in coefficients of thermal expansion when temperatures vary during use. This can be true even for parts made from the same material.

Contact-surface finishes can also influence the coefficient of friction. Mold features that shape contact areas are often highly polished. The coefficient can also change from wear. Semi-crystalline materials (those with molecules and elements in a precise and repeated order) tend to have lower coefficients of friction than amorphous materials (polymers with most macromolecules randomly distributed) when rubbing against metals or a different crystalline material. Lubricants may reduce coefficients in parts that frequently move. “Not all lubricants are chemically compatible with engineering thermoplastics, so test before applying a lubricant to a thermoplastic. Or consider inherently lubricated polymers and compounds, such as those offered in GE Plastics' LNP Lubricomp specialty copolymer materials,” adds Frissora.

“Identifying loads, temperatures, fluid exposures, and other key application requirements helps narrow the decision to a product family such as polycarbonate or polyertherimide rather than an ABS or PPO,” says GE Plastics' industry manager Brian Nourse. “A second level of questioning further refines the search. For example, if the product is to be autoclaved, identifying temperatures and number of cycles it must handle assists in drilling down from a product family to a particular grade.”