Polymers for Implantable Devices

Due to the growing demand for implantable devices, there is also a growing demand for approved materials for use in manufacturing these devices. Polymers are an increasingly important material for use in medical implantable devices, with UHMW PE and PEEK being the most popular.

The Implantable Device Market

According to the FDA, implants are defined as devices intended to stay within the body for more than 30 days. Implantable devices are an ever growing market that was already worth $96.6 billion back in 2018 and expected to reach $143.3 Billion by 2024. These implantable devices include joint replacements, deep brain neurostimulators, pacemakers, insulin pumps, gastric stimulators, and more. 

Requirements for Implantable Device Materials

When it comes to materials, the implantable device market is divided into metals, ceramics, polymers, and biologics. For implant materials to be considered safe, they must remain stable in the presence of body fluids, not elicit an adverse response from the host, and, if applicable, meet requirements related to strength, stiffness, and performance. Long-term long-term biostability and biocompatibility are also key, as well as compatibility with medical imaging methods. For load bearing implants such as those used in hip replacements, how a polymer responds to wear can also be critical.

Implantable Polymers

The two most widely used implantable polymers are FDA approved grades of UHMW PE (Ultra-High Molecular Weight Polyethylene) and PEEK (Polyether ether ketone), although there are other polymers in use.

UHMW PE for Implantables

UHMW PE has been in use for orthopedic applications for over 30 years. UHMW PE can be found in joint replacements and spinal disc implants, where properties such as a low coefficient of friction, excellent fatigue resistance, and good impact strength are vital. Its primary drawback is a lack of strength, which is why it is not used in applications that involve significant load bearing. Note that UHMW PE can be machined or molded, but cannot be injection molded.

PEEK for Implantables

Implantable PEEK offers some properties similar to UHMW PE, including good impact strength and a low coefficient of friction. It is also self-lubricating, possesses a high compression strength, and offers good wear resistance. As a matrix material combined with carbon and graphite fibers, it can be customized to offer improved properties related to strength, stiffness, and dimensional stability. Unlike UHMW PE, PEEK is more versatile when it comes to manufacturing: it can be machined, molded, extruded, and even powder coated. One of PEEK’s more recent applications is implantable spinal cages, which requires significant load bearing capacity.

Other Implantable Polymers

Other polymers for implantables include PSU (Polysulfone), which is known for its toughness, dimensional stability, and moldability. PSU can be found in implantable devices such as shunts and access ports. Another polymer is PPSU (Polyphenylsulfone), which combines toughness with transparency and is often used as a coating for wires and leads. Both of these polymers also provide excellent tensile strength and biocompatibility.

Conclusion

Materials for use in implantable devices must be stringent requirements, and for good reason. There are a host of issues, such as biocompatibility, biostability, wear properties, friction, stiffness, and strength, that all depend heavy on the application. However, there are polymers such as UHMW PE and PEEK whose performance and safety have been well established.

Spring-Energized Seals for Cryogenic Environments

Cryogenic environments deal with temperatures that range from below freezing all the way down to absolute zero, which is -460° F (or -273° C). Trying to seal fluids and gases at those types of extreme temperatures, but one of the most effective sealing solutions for cryogenic environments is the use of a spring-energized seal. 

Spring-Energized Seals

Energized seals are often used in situations where standard seal designs simply cannot provide the performance needed. And when cryogenic temperatures are involved, finding a reliable seal design is extremely challenging — and failure to do so can be fatal. 

How Spring-Energized Seals Work

A spring-energized seal includes a metallic spring (usually in the form of a coil) that applies a near constant load throughout the circumference of the seal. That load enables the lip to remain in contact with the mating surface, even during pressure and temperature variations as well as dimensional changes. In addition, the spring helps the seal compensate for eccentricity, misalignment, and wear. More importantly in cryogenic environments, energized seals can compensate for dimensional changes related to temperature variations.

The result of using a spring energizer is a highly reliable, almost leak-proof seal. This can be especially important when there are safety and environmental regulations are involved, as is often the case when cryogenic materials are in use. In addition, spring-energized seals can be used with both static and dynamic applications, with dynamic including rotary, linear, and oscillating motion (as well as any combination of these).

There are several different types of spring-energizers, from simple elastomeric o-rings and familiar coil springs to V springs, helical flat springs, and cantilevered finger springs. For cryogenic applications, V ribbon springs are typically used. They are found in the harshest of applications, including vacuum pressures and/or cryogenic temperatures.

For cryogenic applications, the spring lip is often made of PTFE and the energizing spring is made of metal. Typical spring metals include 17-7 precipitation hardening stainless steel or 301/304 stainless steel, Elgiloy, Hastelloy, Inconel, or 316 stainless steel.

Cryogenic Applications for Spring-Energized Seals 

Spring-energized seals are successfully being used for a wide in a variety of cryogenic applications:

• Magnetic resonance imaging (MRI)
• Biosystems
• LNG fueling systems and compressors
• Pharmaceutical and medical research
• Rocket propulsion filling systems
• Scientific instrumentation
• Specialty gas manufacturing
• Infrared telescopes
• Radio astronomy
• Satellite tracking systems

Typical fluids and gases involved include LOX (Liquid Oxygen), liquid Helium, liquid Hydrogen, liquid Xenon, refrigerants, coolants, liquid methane, LNG (Liquid Natural Gas), and LPG (Liquid Petroleum Gas). More specific examples include LAR (Liquid Argon) used in surgical environments, LN2 (Liquid N2) used in environmental test chambers, LCO2 (Liquid CO2) for cleaning and deburring. 

Conclusion

While finding a solution for a cryogenic sealing application can be challenging, it is certainly far from impossible. Spring-energized seals can provide durable, reliable sealing power in even the harshest conditions — including extremely low temperatures. By combining the right type of coil, seal geometry, and materials, you can engineer the right spring for applications ranging from rocket development to bioengineering.