PVC has long been the material of choice for tubing. Increasingly, however, issues such as those focusing on the phthalate plasticizers used to make PVC flexible are causing some tubing producers or their device manufacturer customers to look for alternative materials. Until now, the proposed replacements have failed to duplicate the advantages that made PVC the preferred material for so many tubing applications over a period of decades.
Tests and full-scale production trials conducted by Teknor Apex Company (Pawtucket, RI) over a three-year period have established the Medalist® MD-500 range of medical-grade thermoplastic elastomers (TPEs) as the first practical alternative to PVC in tubing. The compounds are now in commercial use by medical device manufacturers. This article will present information on how Medalist MD-500 elastomers compare with PVC in every phase of manufacture and end use typical for tubing, from extrusion through everyday handling by healthcare workers.
To demonstrate the viability of these compounds as alternatives to PVC, researchers in the TPE Division of Teknor Apex worked with medical application experts in the Vinyl Division, established tubing manufacturers, and medical device OEMs. The researchers took advantage of the unusual versatility of Teknor Apex as a supplier to medical manufacturers. The company produces materials representing the entire technology spectrum for medical compounds, including DEHP, non-DEHP, and non-phthalate plasticizer, standard PVC compounds, new BioVinyl™ PVC compounds with bio-based plasticizer, and TPEs. Drawing on this broad involvement, the developers of Medalist MD-500 compounds placed great emphasis on meeting “real-world” requirements that are not typically considered in a product data sheet.
Basics: Flexibility, Clarity, Sterilization Resistance
Flexibility is one of three functional properties that are fundamental in medical tubing. In PVC compounds, this property is the result of high levels of plasticizer — as much as one-third of the formulation by weight. The inherent elasticity of TPEs, on the other hand, renders the use of plasticizer unnecessary.
The degree of flexibility varies with the level of hardness as measured on the Shore scale. Because compounds with Shore A hardness levels of 75 to 85 are most common in tubing, Teknor Apex has established a standard portfolio of MD-565, MD-575, and MD-585 grades covering this range. In addition, the company supplies custom formulations of MD-500 compounds that span the range from 45 to 90 Shore A (or wider as needed). Even at the highest end of the hardness spectrum, the Medalist compounds exhibit greater values of elongation at break than comparable PVC materials, indicating their greater inherent flexibility. Medalist MD-500 TPEs are also more flexible than other elastomeric materials that have been proposed as alternatives to PVC.
Another basic functional property is clarity, which is essential for medical staff to assess flow, bubble appearance, and the condition of a fluid as evidenced by its color. Medalist MD-500 compounds provide the same crystal clarity as PVC and in this respect also surpass other alternative materials.
The capacity to withstand even the most aggressive methods of sterilization is a third basic requirement for tubing. In tests for color shift following heat aging of gamma irradiated samples, Medalist compounds proved markedly superior to PVC. Gamma exposure causes degradation of the PVC polymer, resulting in discoloration, embrittlement, and a falloff in mechanical properties. Compounders like Teknor Apex have developed gamma-stable grades of PVC that exhibit reduced levels of such deterioration, but the Medalist compounds surpass even these in gamma resistance, with heat-aged color shift values less than a third of those of the PVC (Table 1).
Medalist MD-500 compounds provide these basic functional properties while exhibiting values comparable to PVC for mechanical properties critical in tubing, such as tensile strength and tensile stress. Their specific gravity is 25 percent lower, indicating that a pound of Medalist compound yields more linear feet of tubing than a pound of PVC.
Everyday Clinical Considerations
Thus far, the greatest failings of PVC alternatives have been specific to tubing as employed daily by healthcare workers. Teknor Apex researchers screened candidate Medalist compounds in terms of their success in delivering such properties in comparison with PVC.
Kink resistance: In many applications, tubing is used for infusion of fluids into the body. If the tube is bent through a radius that is too tight, it collapses, resulting in potentially life-threatening consequences. This kinking occurs when the compressive forces on the tubing surface inside of the bend exceed the tensile forces on the external bend surface. Changes in wall thickness and hardness or modulus will impact the kink resistance.
Teknor Apex used a hand kink test in which the ends of conventionally sized infusion tubing of an appropriate length (7 to 10 cm) were brought together and parallel, shortening the loop formed until kinking occurred, and then measuring the outer diameter of the loop portion of the tubing. A minimum bend diameter of 1.5 cm was achieved with the standard Medalist MD-500 grades (Table 2). This performance was comparable to that of commercially available PVC tubing and was an improvement over traditional TPEs and other alternatives to PVC in tubing.
Clamp resilience: To temporarily halt the flow of liquid, medical tubing is often clamped using either roller or hemostat clamps. Both compress the tube to block flow, and after release of pressure, the tube must recover and allow flow to resume in a short space of time. Teknor Apex developed its own in-house method using commercially available clamps to assess the recovery of tubing. The Medalist MD- 500 samples proved to be as resilient as, or better than, those produced from PVC.
Necking: This is a phenomenon in which the diameter of a tube is permanently reduced when exposed to a longitudinal force. While traditional TPEs are prone to necking, PVC shows very little. Test samples were subjected to a manual and visual assessment and found levels of resistance to necking equivalent to those of commercially available PVC tubing.
Haptics: Most tubing sets are used manually, with the familiar feel of PVC being accepted as the norm in the healthcare industry. To maximize the acceptance of Medalist compounds by the users of these products, the haptics of the finished tube must be as close to the incumbent material as possible. Initial work utilized the company’s extensive internal knowledge of these materials, but once the compounds were close to expectations, researchers sought the opinions of experienced individuals in the industry. This was done in the form of a blind test with one PVC tube mixed within a grouping of tubes produced from the new compounds. The products made from Medalist MD-500 TPEs proved to be indistinguishable from the PVC tubing.
A summary of how the tubing characteristics of Medalist elastomers compare with those of PVC and traditional PVC alternatives appears in Table 3.
Processing: Extrusion and Downstream
Tubing manufacture involves extrusion and a variety of post-extrusion or “downstream” processes, often carried out in-line with extrusion. Teknor Apex worked collaboratively with companies having expertise in medical tubing manufacture, including HTP Meds, LLC (Ashaway, RI), to evaluate the processability of Medalist MD-500 compounds during their development and to maximize the efficiency and accuracy of production lines in which they are employed.
In a demonstration run on a fully equipped, commercial-scale tubing extrusion line, equipment manufacturer American Kuhne, Inc. (Ashaway, RI) achieved high-speed, tight-tolerance production of tubing from MD-585, an 85 Shore A Medalist compound. The company ran tubing with a 0.105 in. (2.67 mm) OD and 0.020 in. (0.508 mm) wall thickness at speeds up to 830 ft/min. (253 m/min.). At a 600 ft/min. (183 m/min.) line speed, tolerances of ±0.0004 in. on the OD and ±0.0001 in. on the wall thickness were held.
To demonstrate the capabilities of Medalist MD-500-based tubing in downstream processes, Teknor Apex worked with prominent medical device manufacturers Dunn Industries, Inc. (Manchester, NH), which specializes in medical tubing, and Pelham Plastics, Inc. (Pelham, NH), with expertise in assembly techniques. The companies achieved good results in all of the following.
In high-speed extrusion, the compound exhibited a wide processing window and maintained close tolerances, and the tubing was easily cut to length in-line with extrusion.
Hole punching posed no problems, even allowing for tube rotation producing a serpentine pattern; typically, this process is difficult for elastic materials because of stretching or tearing.
Forming a tapered closure at the end of a tube was carried out without need for pre-treatment or release agents, which would require additional regulatory approvals. Cycles were fast, there was no sticking, tapers were consistent, and the aesthetic quality of the finished product was excellent.
Printing was carried out successfully using standard automated corona surface treatment and traditional pad printing with conventional inks. Teknor Apex suggested inks that could be used without the need for pre-treatment.
Insert molding, in which luers were applied to the ends of tubing, involved short cycle times, no tube distortion, and excellent bonding of the tubing to pre-colored luers made either of polypropylene or Medalist elastomers.
Bonding to Standard Connectors
Commercially available adhesive and solvent systems used with PVC tubing either do not enable most TPE tubing to achieve the required bonding strength or do not permit sufficient work time for ease of assembly. Teknor Apex has overcome these limitations for TPE tubing in the 65 to 85 Shore A range with development of three patent-pending bonding systems:
Room temperature-cured adhesive: This system is designed to set slowly in order to allow precise control of assembly. Bonds exhibit cohesive rather than adhesive failure, an indication of the high level of bond strength achieved.
Light-cured adhesive: This system exhibits a higher retention force than is achieved with any commercially available light-cured adhesive.
Solvent bonding: This system uses commonly available solvents and may eliminate the need for multilayer extrusion.
These systems enable common-size TPE infusion tubing to achieve bonds exhibiting a retention force significantly greater than the minimum required by device manufacturers, with 99.7 percent confidence that failures will not occur below the threshold of 35.6 Newton force (8.0 lbf). Globally, there are even lower threshold specifications of 15 and 25 Newtons.
In addition, Teknor scientists have demonstrated equally acceptable retention values using tapered barbs, thereby eliminating the need for solvents or adhesives. The elastic performance of the MD-500 compounds allows for a tight fit onto the male connector barbs.