Thermoplastic bearings used to create prosthetic foot and ankle

In the development of prosthetics, there can be few more important considerations than light weight and toughness. Since these factors ensure the usability and durability of the end product, they make the difference between a prototype and a hard-wearing and effective aid to the end user. These strictures apply to all components within the prosthetic and not least to the bearings.

Fitbionic of Boulder, Colorado is a developer and manufacturer of next-generation prosthetic technology and has recently launched its first product, an innovative prosthetic foot that features high load-carrying bearings made of Torlon polyamide-imide (PAI) from Solvay Specialty Polymers. The ultra-high performance thermoplastic delivers long-term strength and stiffness along with exceptional wear and fatigue resistance in this highly demanding load-bearing application. The Fitbionic prosthetic foot, developed specifically for lower-activity and diabetic amputees who comprise more than 80% of the U.S. amputee population, utilises 'Active Stability' technology to help wearers feel more stable and potentially prevent injuries while walking. Effortless ground conformance and reduced pressures on the wearer's tissues combine with propulsive energy to deliver the most human-like walking foot commercially available, according to Jerome Rifkin, chief technology officer and founder of Fitbionic. The prosthetic foot assembly includes a lubricious, nickel-plated aluminium axle that rides on and between two bearings made of Torlon PAI, thus providing multi-axial motion. The 19 x 13 x 13 mm bearings are machined from Torlon PAI plate stock. Fitbionic may potentially switch to injection moulding Torlon PAI bearings to handle larger production volumes since the material offers the flexibility to convert the application. "Patients put these components through demanding situations and varying loads for years of hard wear," says Rifkin. "Torlon PAI is strong and tough enough to withstand those forces reliably and consistently." The thermoplastic exhibits metal-like performance and is routinely specified for precision components used in repetitive-use, load-bearing operations. The material's exceptional wear resistance withstands the forces of walking that are projected on the bearing's 13mm wide surface without any material loss or non-uniform wear. As a thermoplastic, Torlon PAI provides high strength and stiffness up to 275°C (525°F). It has outstanding resistance to wear, creep, and chemicals and is ideally suited for severe service environments. Rifkin says: "Torlon PAI exhibits exceptional bearing properties (fatigue resistance and compressive strength), resulting in long-lasting performance. It also yields a stable tribological interface by virtue of its strong mating and compatibility to a receptacle made of PEEK." Fitbionic offers the new prosthetic foot for body weights up to 113kg. The company currently sells the prosthetic device to prosthetists throughout the U.S.