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Cover Story

Unique Properties Make Polymers Suitable to Medical Tubing Applications


Mr. Dan Lazas, Senior Director,
Sales Optimization Medical Components, Tekni-Plex

Extruded polymer tubes are used prolifically throughout the medical device industry from intravenous (IV) lines to neurovascular catheters. Advancements in polymers, additives and processing technologies have resulted in medical tubing precisely tailored to specific applications. This includes functional attributes spanning mechanical, thermal, electrical and chemical performance.

Often, polymers and processing technologies selected for devices are based on successful application in similar devices. This is rational logic for a device designer intent on reducing time-to-market by avoiding reinvention. The regulatory filing process also encourages use of proven technology. For example, the intent of a United States Food & Drug Administration 510(k) regulatory filing is to prove a device is as safe and effective as a device that is currently marketed, known as a predicate


Unfortunately, the repeated application of polymer technologies based on predicate devices can desensitize engineers to the extraordinary benefits of these materials for healthcare applications. Revisiting unique polymer attributes that precisely benefit certain medical devices can lead to innovative applications in future devices. Some of these unique material benefits stem from properties that are not generally desirable in broad context.



Blood Management


Polyvinyl chloride (PVC) is a material of choice for tubing used throughout the medical device industry. Applications include blood management, cardiovascular pumps, dialysis, and respiratory, suction and drainage tubes. In many instances PVC has revolutionized patient care, reduced infections and improved safety. For example, a blood transfusion system in the middle of the 20th century was a reusable device, which was large, heavy, and diffult to sterilize. Modern transfusion sets, substantially comprised of PVC tubing, are single-use devices that are economical, light weight and easy to use.


Nevertheless, PVC has become controversial in recent years. One reason is a common plasticizer, di-ehtylhexyl phthalate (DEHP), used to convert the inherently rigid polymer into a flexible product suitable for tubes, bags and films. The FDA issued an advisory risk in 2002 pertaining to reproductive and developmental concerns associated with patient exposure to high levels of DEHP.


While there are other plasticizers available for PVC, DEHP is still commonly used for blood management applications. In addition to the economic benefits of using DEHP, studies have shown the plasticizer to be advantageous in preserving blood cells. For example, a 1998 study1 found that DEHP inhibits the deterioration of red blood cells during storage in PVC containers that use this plasticizer. Hemolysis and microvesicle formation were also shown to decrease with the presence of DEHP in the same study.


Therapeutic Delivery


Due to versatility in properties, thermoplastic polyurethanes (TPUs) are used across a wide range of medical devices, including wound care, vascular access devices and IV therapy applications. TPUs are a class of copolymers, created from three chemical components: polyol, chain extender and diisocyanate. The elastomeric segment of the polymer chain is created from polyol and isocyanate, whereas the strength segment is created from the chain extender and isocyanate. Polyester, polyether and polycarbonate polyols can be used in the formulation of TPUs. Likewise, aromatic or aliphatic isocyanates can be used. Variation in the type and amount of each component can change elasticity and a variety of other material properties.


A particularly unique characteristic of TPUs is their ability to soften with relatively small changes in temperature. 2 As such, these materials tend to soften when placed into the body. The degree of softening is dependent on the initial hardness and the chemical make-up of the material. Generally, low durometer polyurethanes soften up to 50% when placed in 98.6o F (37o C) water. Hard polyurethanes generally soften to a greater extent. For central catheters, this property is particularly useful. Peripherally inserted central catheters (i.e. PICC lines) are indwelling devices that can remain in a patients arm vein for weeks or longer to allow for frequent drug delivery. The softening characteristics of TPU are ideally suited for maximizing comfort in patients receiving treatment with central catheters.


Cardiac Intervention


The ability to tailor TPU properties from flexible to rigid using variations in chemical constituents and amounts would be seemingly ideal for the flexible catheter shafts used in percutaneous transluminal coronary angioplasty (PTCA) procedures. Commonly 2 mm (0.079 inches) in diameter and 100 cm (39 inches) long, these catheters require a perfect balance between strength and flexibility to be inserted in the upper leg (i.e. femoral artery) and reach the aortic sinus. However, changing properties due to material softening, as is characteristic of TPUs, would be a disadvantage in these devices.


Like TPUs, polyether block amides (PEBAs) are thermoplastic elastomers created through copolymerization of soft and hard block molecular chains. Variation in the percentage of these components results in physical properties that range from highly flexible to moderately rigid. For PEBAs the hard block is a polyamide, and the resulting polymer is resistant to softening when exposed to human body temperature and fluids. As such, it retains physical performance characteristics throughout the procedure. Also, these copolymers are slightly more rigid than TPUs yet more flexible than nylon 11 or 12. This allows for better “pushability” of these catheters by clinicians while retaining flexibility for vascular navigation.


Since all PEBAs contain common polyamide constituents, they can be thermally bonded to each other regardless of flexibility characteristics. This is particularly useful in the development of cardiovascular guide catheters that can require more rigidity at the proximal end, where the device is controlled by the physician, and more flexibility at the distal end, for atraumatic navigation of vascular pathways. Shafts of this nature can be created by thermally bonding multiple segments of PEBA tubes, with variation in flexibilities.


Data Sheets : Only Part of the Story


Polymer data sheets contain a host of material properties that allow medical device engineers and designers to compare properties of the polymers. For medical polymers used in device tubing applications, these properties include density (i.e. specific gravity), strength, elongation, modulus (i.e. rigidity), hardness (i.e. durometer) and much more.


However, these properties only begin to tell the complete story of these polymers. Often, the distinct attributes of polymers that are not found on data sheets is the determining factor for why specific medical polymers are consistently used in distinct medical tubing applications.


AuBuchon, JP et. el. The Effect of Plasticizer Di-2-Ethylhexyl Phthalate on Survival of Stored RBCs. Blood, Vol 71, No 2 (February), 1988: pp 448-452.


Walder, A. Kulkarni, P. Thermoplastic Polyurethanes as Medical Grade Thermoplastic Elastomer. Lubrizol Advanced Materials, Wilmington, MA..

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