Advanced Sealing Systems Improve Prosthetics’ Performance

It is a very unfortunate trend that the incidences of limb loss are increasing. According to Forbes, there are between 10 and 15 million amputees in the world. According to the Amputee Coalition, nearly two million of those are in the United States with approximately 185,000 amputations occurring each year. The main causes are vascular disease (54 percent)—including diabetes and peripheral arterial disease—trauma (45 percent), and cancer (less than 2 percent). 
 
Prosthetic manufacturers concentrate on the development of devices that provide the best quality of life possible for amputees. The needs of users vary depending on lifestyle. Older people may be satisfied with a simpler device, while other people may require a complex prosthetic that allows them to fully participate in even the most extreme of sporting activities and fully replicates a natural gait.
 
Prosthetic performance, especially for complex devices, is improving at a rapid rate and most prosthetics now incorporate a hydraulic system whereby the user can vary the damping force to help cushion shock loads. More advanced systems use a microprocessor with some element of intelligent “learning” to make adjustments when a prosthetic user is standing, walking, running, or climbing stairs.
 
Advanced Prosthetic Knee and Ankle Devices
One of the main challenges with advanced knee and ankle devices is the development of a robust sealing system for their hydraulic cylinders. The sealing systems for the piston and rod in ankle devices are generally similar to those used in knees. However, performance is more difficult to achieve in the ankle than the knee because ankles are more compact, have a much shorter stroke, and must allow a degree of rotation. This makes side-loading more problematic.
 
The cylinder needs a sealing system for both the piston and rod. The piston must be sealed on the outside diameter to maximize the effectiveness of electronic valves, forcing the fluid through the piston valve in a controlled manner. Seal materials must be compatible with the lubricating fluid to ensure long life of the prosthetic in typical operating temperatures range from zero to 190 degrees Fahrenheit. Nitriles, Urethanes, Fluoroelastomers (FKMs), Polytetrafluoroethylenes (PTFEs), Hytrel, and Ultra-High-Molecular-Weight Polyethylene (UHMWPE) materials are generally suitable.
 
For most devices, engineers strive for a zero leakage system, which can be difficult with peak pressures near 2,000 psi. In addition, bleed down can occur during prolonged inactivity (such as when the prosthetic is sitting idle overnight). In this case, oil stuck on the cylinder rod slides down, creating a pool of oil on top of the outer seal. In addition, a balance of seal and lip load must be struck to seal in the hydraulic fluid without creating too much rod friction.
 
To avoid stick-lip, which can cause an impact shock to the prosthetic user, low-friction materials should be used. A double-acting PTFE seal with an O-Ring energizer is ideal for this application. Adding a wear ring or two on the piston keeps the piston aligned properly during side loading. The seal and bearing materials must be selected to be compatible with the cylinder-bore material, surface finish, and coatings to maximize cycle life.
 
The rod sealing system serves two distinct functions—it prevents external contamination from entering the system and keeps hydraulic fluids within the cylinder. To achieve this, a complex configuration of multiple seals is required, with each seal performing a separate function and, in combination, give the optimum performance.
 
A wiper or scraper seal with sealing lips designed to scrape the rod, pushing debris away from the cylinder, will prevent ingress of media such as dirt, dust, and sand. The rod seal keeps lubricating fluid in the cylinder during high-pressure actuations and spikes, while minimizing friction and stick-slip. A spring-energized PTFE or Hytrel seal generally provides the best performance in this application. The seal material should be selected for compatibility with the rod material, hardness, surface finish, and coatings. Finally, to keep the rod aligned and prevent metal-to-metal contact, a rod bearing should be included in the system. DU bushings and thermoplastic bearings provide long life and help absorb shocks and side loads.
 
Bearings are also used outside the cylinder as rod ends. They minimize binding and the effects of side loading on the cylinder. Metal rod end spherical bearings or composite bearings made of resins impregnated with fine weaves of fabric and lubricants can be used in these applications. Being plastic, they also provide some vibration damping into the system.
 
Prosthetics of the Future
What’s the next step for prosthetics? Every year, there seems to be no limit to the technological advancements when it comes to new prosthetic designs and the sealing solutions behind them. Futuristic versions of advance prosthetics have been seen in some of the most popular movies over the past decades. These systems are already a reality. Electronic actuators as part of exoskeletons are helping to rehabilitate patients with limited mobility, due to a stroke, for example, enabling them to move when they cannot. Progress is also being made with bionic systems where limbs are connected to the body’s nervous system, allowing a user to control prosthetic movement just as they would their own limbs.

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Jerome Zawada is life sciences segment manager for Trelleborg Sealing Solutions, where he has worked for over 14 years. Previously, Zawada was VP for a start-up life sciences company. He has over 18 years of experience designing hydraulic components and valves and holds 3 U.S. patients.
 
Luis Tissone is the director of life sciences at Trelleborg. He has worked as a regional sales manager for Helix Medical and prior to that, in positions ranging from supply chain operations to marketing and sales. Tissone has a Bachelor of Science degree in Industrial Engineering from Argentinean Catholic University and a Business Administration and Management degree from Harvard University.