Elastomeric polymers offer flexibility to navigate complex anatomical pathways via catheters. Modulus, a measure of strength relative to elongation, is the material property that provides a relative comparison of flexibility for these materials. A low modulus implies greater flexibility and is often correlated with lower hardness (Shore scale A and/or D).
However, many structural medical devices require less flexible polymers. For these applications, rigid thermoplastics with higher modulus values are commonly used. Although rigidity and strength properties of the amorphous and crystalline thermoplastic options are often similar, they are used for different applications.
Crystalline polymers have regions where the molecular chains are orderly, like crystals. This generally results in improved chemical resistance and toughness, yet reduced dimensional stability (i.e. part warpage) and an opaque color. Since this molecular structure can be further oriented to improve strength through post processing, crystalline polymers are commonly used in fiber and film applications.
The molecular structure of amorphous polymers is random, resulting in low shrinkage and excellent dimensional stability for molded components. Amorphous polymers are also inherently transparent and commonly used for optical applications.
Polysulfones v. Amorphous Polymers
Although sulfones are inherently transparent, they often have a slight amber hue. For purely optical applications, polycarbonates or acrylics have greater clarity and are often preferred.
Polysulfones, polycarbonates and acrylics offer similar strength and rigidity properties. Acrylic is the least expensive and used if rigidity is important but toughness is not critical. Polycarbonate has exceptional toughness and is preferred for high impact applications. Polysulfones are more expensive and used when other factors prohibit the use of acrylics or polycarbonates.
Chemical resistance and temperature are the two most common factors that determine the use of polysulfones over other transparent, amorphous polymers. Medical devices are commonly exposed to lipids (a.k.a. fats), plasticizers (used to make PVC flexible), isopropyl alcohol (used to clean devices), various drug related chemicals, and water.
Acrylic is generally resistant to lipids and plasticizers, but less resistant to IPA. Polycarbonates are generally less resistant to lipids and plasticizers, and more resistant to IPA. Due to economics, one of these materials is often specified for disposable devices that require rigidity, transparency and moderate chemical resistance.
However, acrylics or polycarbonates do not offer comprehensive chemical resistance, nor are they resistant to hydrolysis (chemical attack by water). This is particularly true at elevated temperatures, which accelerates chemical degradation of polymers.
Steam sterilization of reusable medical devices requires materials that are resistant to degradation from water at temperatures ranging from 121C (250F) to 134C (273F) for 15 to 30 minutes. In general, acrylics do not have sufficient temperature resistance for steam sterilization. Polycarbonate has a glass-transition and heat-deflection temperature high enough to withstand normal steam sterilization; however, it quickly loses ductility and impact properties following repeated sterilization cycles. Polysulfones and polyetherimide (PEI) are the only amorphous thermoplastics that can withstand repeated steam sterilization without substantial degradation of properties.
Comparing Polysulfones and Polyetherimide
Polysulfones typically compete with PEI (tradename Ultem® by Sabic) for rigid, transparent components that are exposed to frequent sterilization. PEI is considerably stronger and more rigid than polysulfones. PEI also has nearly twice the dielectric strength, a measure of insulation and beneficial for electrical applications.
However, a major weakness of PEI is impact strength. PSU offers better impact resistance and is a more economical alternative to PEI. PPSU is generally priced similar to PEI but offers substantially better impact resistance.
Polysulfone Applications in Healthcare
- Polysulfone is often used for the transparent portion of anesthesia masks. This is combined with a flexible silicone that conforms to the patient anatomy. These masks are reusable and require repeated steam sterilization without degradation of material properties or appearance.
- Polysulfone is commonly used for pharmaceutical filter housings since it is durable, non-toxic, highly break-resistant and transparent. These housings exhibit low protein-binding, produce low levels of trace metals or organic leachables, and are easy to clean.
- Heart valve sizers are used to estimate the size of the heart valve when it is replaced. Molded polysulfone sizers are dimensionally accurate, transparent for maximum visibility, and resistant to sterilizations for repeated use.
- Dialysis filtration cartridges are used for continuous renal replacement therapy. Small, hollow polysulfone fibers offer ideal porosity and filtration characteristics for the membranes that remove toxins. Polysulfone is also used for filter housings and caps due to high physical strength and clarity. These applications require materials that must withstand steam sterilization.
- Polysulfone has been used for the outer shell of implantable catheter ports used for venous access in oncology. Compared to fully metal constructions, the polysulfone portion is transparent during CT scans. Polysulfone is dimensionally stable and able to maintain tight tolerances. It is resistant to a variety of drugs and can be sterilized without degradation.
- Surgical instrument trays often use polysulfone for the transparent covers. Injection molded covers are often made with no sharp edges for easy cleaning. The material is high impact and can be repeatedly steam sterilized without degradation in clarity or performance.
- Many components used in dental instruments are manufactured from polyphenylsulfone due to high strength, rigidity and toughness. These devices must be able to withstand steam, ethylene oxide, hydrogen peroxide or gamma radiation sterilization methods. Polyphenylsulfone is unique among amorphous thermoplastics in its ability to withstand more than 1,000 cycles of steam sterilization without any significant loss of properties.
- Surgical instrument cases and trays made from polyphenylsulfone offer extremely high impact strength and have been proven to withstand repetitive sterilization cycles. Polyphenylsulfone cases can be manufactured from a variety of translucent and opaque colors for aesthetics and differentiation.
Selecting the Right Polysulfone
PSU (tradename Udel® by Sovay) is the most economical of the sulfone polymers, and substantially less expensive than PEI. It has less impact resistance than polycarbonate yet 3 times the thermal performance, 2 ½ times the hydrolytic stability and 2 times the chemical resistance to organic solvents. For many medical device applications that require good dimensional stability, rigidity and repeated sterilization Udel PSU is the material of choice.
PPSU (tradename Radel® by Solvay) offers substantially better impact resistance and chemical resistance than polysulfone (PSU) and polyetherimide (PEI). PPSU is a material of choice for dimensionally stable, rigid parts that demand exceptional toughness and are exposed to repeated sterilization.