Minimally invasive devices rely upon the unique properties available from a wide of polymers. These properties are particularly critical for thin wall tubing used in endovascular applications. These applications require materials with unique combinations of flexibility and strength properties, such as polyolefins, polyamides, polyesters, ethylene-vinyl acetate (EVA) and thermoplastic elastomers (TPEs).
Polyolefins are used in a wide range of catheter applications, from dilators used in vascular introducer sets, to lumen liners in interventional catheters. This family includes polypropylene (PP), low density polyethylene (LDPE) to high density polyethylene (HDPE). These materials are melt processible on conventional equipment, economically priced, and offer low friction properties approaching that of polytetrafluoroethylene (PTFE).
Polyamides, also known as nylons, and polyamide copolymers (e.g., Arkema’s Pebax® polymer) are used in a wide range of catheter shafts and outer layers. These are materials of choice in balloon and stent delivery catheters. The polyamides that dominate this catheter market are soft grades, such as nylon 11 and nylon 12. Polyamide copolymers offer a wide range of durometer and flexibility so that catheters can be designed with variable flexibility along the shafts.
Thermoplastic elastomers (TPEs) are unique synthetic compounds that combine some of the properties of rubber with the processing advantages of thermoplastics. Like rubbers, the thermoplastic elastomers key properties are softness and flexibility. This polymer category includes thermoplastic elastomers based on polyurethanes (TPU), polyamides (COPA), polyesters (COPE), polyolefin (TPO), styrene (SBC) and vulcanizates (TPV). In general, TPUs and COPAs offer the best biocompatibility and mechanical properties for critical medical device applications such as catheter tubing, catheter balloons for catheters, strain reliefs, and more.
Thermoplastic Polyurethanes (TPU)
Thermoplastic polyurethanes (TPUs) are created from three chemical components: a polyol (long-chain diol), a chain extender (short-chain diol), and a diisocyanate. The soft portion of the molecular chain is created from polyol and isocyanate. This provides elastomeric properties, such as flexibility. The hard segment is created from a chain extender and isocyanate. This provides physical strength properties.
All TPUs generally offer very good abrasion resistance, low temperature flexibility, oil resistance and mechanical strength compared to other TPEs. The various types of TPUs, and unique property characteristics, are achieved through variations in polyols, chain extenders and diisocyanates.
Medical grade TPUs are manufactured from polyester, polyether and polycarbonate polyols. Polyester types offer the best mechanical properties. Polyether types offer better hydrolysis resistance and low temperature flexibility than polyester types, but are more expensive. Polycarbonate types combine excellent strength with good oxidation and hydrolytic stability, and are typically more expensive.
Isocyanates used in the manufacturing of TPU can be aromatic or aliphatic. Aromatics contain benzene rings, and are commonly used since they are tougher, stronger and less expensive than aliphatic types. Aliphatics are made with hydrocarbon backbones and no benzene rings. These isocyanates are strong, and offer good light stability and optical clarity, but lack chemical resistance.
This unique chemistry allows TPUs to be used in a wide variety of medical applications. These include catheter tubing, surgical drains, feeding tubes, dialysis devices, non-allergenic gloves, nonwoven gowns and drapes, hospital bedding, compression stockings, instrument cables, wound dressings, respiration devices, artificial hearts and more.
Thermoplastic Copolyamides (COPA)
This family of thermoplastic elastomers includes polyether block amides (PEBA, more commonly known by the Arkema tradename Pebax®), which are comprised of polyamide 12 and soft polyether groups. These materials offer very low material density when compared to the other thermoplastic elastomers. Modifications in the polyamide and polyether blocks results in a wide range of grades that offer a variety of performance characteristics, such as hardness (i.e., durometer) and flexibility. Higher durometer and stiffness are the result of increased proportions of nylon 12 in the block copolymer.
COPAs are used in a variety of medical applications, including interventional cardiology and neurology catheters, due to compliance with USP class VI, sterilization resistance, lightweight, range of flexibility, chemical resistance and ease of processing.
Thermoplastic Copolyesters (COPE)
The COPE family or thermoplastic polyester elastomers are often recognized by DuPont’s tradename Hytrel®. These thermoplastic elastomers offer an unusual combination of strength, elasticity and dynamic properties. COPEs offer a high degree of heat and chemical resistance as well. Hytrel is used in medical device applications in which superior mechanical properties and flexibility is critical.
Thermoplastic Polyolefins (TPO)
Thermoplastic polyolefin elastomers are comprised of various mechanical blends of polyolefin resins, resulting in relative economical formulations. These polymers are frequently used in high-volume medical applications. In some cases, TPO materials compete with flexible grades of polyvinyl chloride (PVC) used in medical equipment tubing, blood lines and blood bags.
Styrenic Block Copolymers (SBC)
Styrenic thermoplastic elastomers obtain their thermoplastic properties due to a multiphase composition in which the phases are chemically bonded by block polymerization. In all cases at least one phase is a styrenic polymer that is a hard phase at room temperature while the other phase is a rubber-like material that is soft at room temperature. The proportion of the styrenic phase controls the hardness and stiffness properties, whereas the choice of soft segment will control the chemical resistance and thermal properties.
Thermoplastic Vulcanizates (TPV)
TPVs are elastomeric alloys composed of mixtures of a plastic and a rubber in which the rubber phase is cured or cross linked. The plastic phase is commonly a polyolefin, such as polypropylene, although a wide variety of thermoplastics have been used. The rubber phase is commonly an EPR or EPDM rubber. The end product is produced during a dynamic vulcanization mixing process. This process gives a useful elastomeric alloy with properties of a cured rubber but has the processing characteristics of a thermoplastic. ExxonMobil’s Santoprene® polymer is perhaps the most commonly known TPV used in medical applications.
Radiopaque Polymer Additives
A common requirement for minimally invasive medical devices is visibility during X-rays or fluoroscopy. Polymers are inherently transparent to this imaging and require specific additives to achieve visibility. Radiopaque fillers are often blended with polymers for radiopacity. These additives include:
- Barium sulfate – off white powder that compounds easily into most thermoplastics and is easily colored
- Bismuth subcarbonate – pale white power provides excellent X-ray absorption with some color matching limitations
- Bismuth trioxide – yellow powder provides excellent X-ray absorption, limited to low/moderate processing temperature polymers and color matching options
- Bismuth oxychloride – white power with good temperature stability for processing yet susceptible to UV degradation.
- Tungsten – dark grey powder with superior X-ray absorption with color matching limitations.