RHEO is a microprocessor-controlled prosthetic knee system that automatically adapts to an individual’s walking style and environment. In search of a solution to their leaking seal problem, the manufacturer presented a challenge to the leading seal design firms in the U.S. Each company was tasked with designing a custom seal that would meet Federal testing regulations, ensuring it could withstand a minimum of 1,000,000 test cycles without leaking. The company that produced the longest-lasting seal, surpassing the test requirements, would secure the contract.

Upon analyzing the application, we identified that the most difficult challenge would be sealing the viscous fluid, which contained small metallic spheres magnetically programmed to simulate the human leg's movement while walking or running. These spheres were essential to the knee's function, but they presented a significant sealing challenge. Our analysis revealed that conventional seal designs would likely fail, as the spheres could easily slip under the sealing lip. In a typical shaft seal, oil is pumped back and forth beneath the lip to reduce friction, aided by micro asperities in the rubber and varying film thickness. However, the presence of the spheres could cause leaks by disrupting this process.