Technologies
Medical Urethanes
Beginning with the commercial introduction of Tecoflex® aliphatic TPU in 1983, Thermedics Polymer Products have continued to be supplied to the medical device industry with an outstanding array of thermoplastic polyurethanes (TPUs). Each of our family of products — Tecoflex®, Tecothane®, Carbothane®, Tecoplast® and Tecophilic® TPUs has been specifically formulated to have good biocompatibility, flexural endurance, high strength and processing versatility over a wide range of applications. Lubrizol's family of TPUs are now being used in many medical devices with new applications continually being found by device manufacturers who encounter demanding tissue or blood contact situations.
| Meeting the Challenge | Medical Urethanes Selection Guid |
| Materials of Choice | Summary of Differences |
| The TPU Product Family | Radiopaque and Color Compounding |
| Applications | Custom Tubing Extrusion |
Meeting the Challenge
Body tissue and blood present difficult environments for elastomeric components of indwelling medical devices. Such components must be able to withstand extended exposure to hostile aqueous environments at body temperature and the corrosive biochemical composition of blood and body fluids that can degrade many materials. At the same time, the device must cause as few complications to the patient as possible. Blood clotting, rejection responses, tissue inflammation and leaching of toxic chemicals into the body must all be minimized for a material to meet the safety requirements of an indwelling medical device. Furthermore, the elastomeric component must be strong and easy to manufacture into small, precise shapes and sizes specified by device designers. For example, it is important to keep catheters as small as possible when they are entering the circulatory system — a common application for cardiovascular diagnostic and clinical devices. Resins with biocompatibility and biostability are critical components in medical device manufacturing. Lubrizol offers a wide variety of thermoplastic polyurethanes that are designated as medical grade biomaterials having passed either USP Class VI, MEM Elution or other relevant tests in order to establish their biocompatibility and biostability.
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Materials of Choice
Lubrizol's polyurethanes are reaction products synthesized from diisocyanates, macrodiols and chain extenders. The characteristics of each polyurethane can be attributed to its structure. Polyurethanes are made of hard and soft domains where the diisocyanate and extender make up the hard domains and the macrodiol makes up the soft domain. Varying the ratios of these two domains allow polyurethanes to be formulated with durometers as soft as 72A or as hard as 84D (Shore Hardness).
Polyurethanes are designated aromatic or aliphatic on the basis of the chemical nature of the diisocyanate component in their formulation. Tecoflex®, Tecophilic® and Carbothane® resins are manufactured using the aliphatic compound, hydrogenated methylene diisocyanate (HMDI). Tecothane® and Tecoplast® resins use the aromatic compound methylene diisocyanate (MDI). All the formulations, with the exception of Carbothane®, are formulated using polytetramethylene ether glycol (PTMEG) and 1,4 butanediol chain extender. Carbothane® is specifically formulated with a polycarbonate diol (PCDO). These represent the major chemical composition differences among the various families.
Aromatic and aliphatic polyurethanes share similar properties that make them outstanding materials for use in medical devices. In general, there is not much difference between medical grade aliphatic and aromatic polyurethanes with regard to the following chemical, mechanical and biological properties:
• High tensile strength (4,000 – 10,000 psi)
• High ultimate elongation (250 – 700%)
• Wide range of durometer (72 Shore A to 84 Shore D)
• Good biocompatibility
• High abrasion resistance
• Good hydrolytic stability
• Can be sterilized with ethylene oxide and gamma irradiation
• Retention of elastomeric properties at low temperature
• Good melt processing characteristics for extrusion, injection molding, etc.
With such an impressive array of desirable features, it is no wonder that both aliphatic and aromatic polyurethanes have become increasingly the material of choice in the design of medical grade components. There are, however, distinct differences between these two families of polyurethane that could dictate the selection of one over the other for a particular application:
Yellowing
In their natural states, both aromatic and aliphatic polyurethanes are clear to very light yellow in color. Aromatics, however, can turn dark yellow to amber as a result of melt processing or sterilization, or even with age. Although the primary objection to the discoloration of aromatic clear tubing or injection molded parts is aesthetic, the yellowing, which is caused by the formation of a chromophore in the MDI portion of the polymer, does not appear to affect other physical properties of the material.
Radiopaque grades of Tecothane® TPU also exhibit some discoloration during melt processing or sterilization. However, both standard and custom compounded radiopaque grades of Tecothane® TPU have been specifically formulated to minimize this discoloration.
Solvent Resistance
Aromatic polyurethanes exhibit better resistance to organic solvents and oils than do aliphatics — especially as compared with low durometer (80 – 85 Shore A) aliphatics, where prolonged contact can lead to swelling of the polymer and short-term contact can lead to surface tackiness. While these effects become less noticeable at higher durometers, aromatics exhibit little or no sensitivity upon exposure to the common organic solvents used in the health care industry.
Softening at Body Temperature
Both aliphatic and aromatic polyether-based polyurethanes soften considerably within minutes of insertion in the body. Many device manufacturers promote this feature of their urethane products because of patient comfort advantage as well as the reduced risk of vascular trauma. However, this softening effect is less pronounced with aromatic resins than with aliphatic resins.
Carcinogenic By-Products
If aromatic polyurethanes are improperly processed, such as when tubing is extruded from resin with too high a moisture content or the finished components are steam sterilized, it is possible to experience the formation of measurable amounts of methylene dianiline (MDA). MDA is listed as a carcinogen. It is not possible to form MDA with an aliphatic polyurethane. Moreover, the analogous diamine which could be formed from HMDI is not listed as a carcinogen.
Melt Processing Temperatures
The melt temperatures of Tecothane®, Tecoplast® and Carbothane® TPUs are considerably higher than Tecoflex® and Tecophilic® TPUs. Therefore, processing by either extrusion or injection molding puts more heat history into products manufactured from Tecothane®, Tecoplast® and Carbothane® TPUs. For example, Tecoflex® EG-80A and EG-60D resins mold at nozzle temperatures of approximately 310º F and 340º F respectively. Tecothane® and Carbothane® products of equivalent durometers mold at nozzle temperatures in the range of 380º F to 435º F.
The TPU Product Family
In 1983, Tecoflex® TPU, a highly versatile aliphatic polyether polyurethane, entered the medical market. This portfolio has since been expanded to meet the ever changing demands of the medical and industrial markets. This extensive product line also includes Tecothane® (aromatic polyether-based polyurethane), Carbothane® (aliphatic polycarbonate-based polyurethane), Tecophilic® (high moisture absorption aliphatic polyether-based polyurethane) and Tecoplast® (aromatic polyether-based polyurethane).
Whether you are interested in single or multi-lumen, clear, radiopaque, colored or striped tubing, let us help you meet your needs. Our technical support team is ready to work with your product design staff in supplying prototype extrusions of 300 feet or full production runs.
Major Benefits of Tecoflex® Aliphatic TPUs:
Tecoflex (aliphatic, polyether-based TPU), Shore A 75 through Shore D 75
A family of aliphatic, polyether-based TPUs available over a wide range of durometers, colors, and radiopacifiers. These resins are easy to process and do not yellow upon aging. Solution-grade versions are candidates to replace latex. Caution must be observed in evaluating these resins, especially the low durometer grades, in long-term implant applications because of potential for stress cracking.
Major Benefits of Tecothane® TPUs:
Tecothane (aromatic, polyether-based TPU), Shore A 74 through Shore D 75
A family of aromatic, polyether-based TPUs available over a wide range of durometers, colors, and radiopacifiers. You can expect Tecothane® TPU resins to exhibit improved solvent resistance and biostability when compared with Tecoflex® TPU resins of equal durometer. As with any aromatic polyurethane, Tecothane resins tend to yellow upon aging or when subjected to radiation sterilization.
Major Benefits of Carbothane® Aliphatic TPUs:
Carbothane (aliphatic, polycarbonate-based TPU), Shore A 75 through Shore D 72
A family of aliphatic, polycarbonate-based TPUs available over a wide range of durometers, colors, and radiopacifiers. This type of TPU is reported to exhibit excellent hydrolytic stability, a property which may equate to excellent long-term biostability.
Major Benefits of Tecophilic® TPUs:
Tecophilic ((hydrophilic, aliphatic, polyether-based TPU), Shore A 90 through Shore D 60
A family of aliphatic, polyether-based TPUs which have been specially formulated to absorb equilibrium water contents of up to 150% of the weight of dry resin. Extrusion grade formulations are designed to provide maximum physical properties of thermoformed tubing or other components. Solution grade formulations are designed to provide greater solubility in organic solvents to prepare lacquers for coating applications. This technology has been expanded to a Tecophilic® Gel. It is a hydrogel that can be formulated to absorb equilibrium water contents between 500% and 2000% of the weight of dry resin. The materials were designed as a coating cast from an ethanol/water solvent system. Other solvent systems such as THF/water and DMAC can be used. Tecophilic Gel is melt processible using modified injectin molding and extrusion methods.
Major Benefits of Tecoplast® TPUs:
A family of aromatic, polyether-based TPUs formulated to produce rugged injection-molded components exhibiting high durometers and heat deflection temperatures. Tecoplast is intended for use as hubs and fittings manufactured as individual components or insert molded onto tubing. Available as clear pellets as well as in transparent and opaque colors.
Medical Urethanes Selection Guide
TECOFLEX®
TPU (polyether-based) |
TECOTHANE®
TPU (polyether-based) |
CARBOTHANE®
TPU (polycarbonate-based) |
TECOPHILIC®
TPU (polyether-based |
TECOPLAST®
TPU (polyether-based) |
|
Aliphatic
or Aromatic |
Aliphatic |
Aromatic |
Alphatic |
Aliphatic |
Aromatic |
Durometer Range |
72A - 83D |
75A - 77D |
73A - 75D |
83A - 72D |
82D - 84D |
Radiopacifiers |
Barium Sulfate Bismuth Salts Tungsten |
Barium Sulfate Tungsten |
Barium Sulfate Tungsten |
Barium Sulfate Bismuth Salts Tungsten |
N / A |
Custom
Colors |
Yes |
Yes |
Yes |
No |
Yes |
Solution
Grades |
Yes |
No* |
No* |
Yes |
No* |
Extrusion
and/or Injection Molding |
Both |
Both |
Both |
Both |
Injection Molding (primarily) |
Melt
Processing Temperature Range |
150°C - 190°C 305°F - 370°F |
200°C - 220°C 390°F - 430°F |
190°C - 220°C 370°F - 430°F |
145°C - 190°C 290°F - 370°F |
215°C - 230°C 420°F - 445°F |
Relative
Degree of Biostability |
Good |
Better |
Best |
Not Determined |
Not Determined |
Summary of Differences
Property |
Tecothane®
TPU |
Tecoflex®
TPU |
Carbothane®
TPU |
Techophilic®
TPU |
Yellowing |
Significant concern especially
for aesthetic appearance of clear components |
Minor concern |
Minor concern |
Minor concern |
Solvent
resistance |
Excellent |
Low durometers may exhibit
considerable swelling and tackless upon prolonged exposure
to polar organic solvents |
Similar to Tecoflex®
TPU |
Similar to Tecoflex®
TPU |
Softening
at body temperature |
Yes |
Yes, to an even greater
degree |
Yes |
Yes |
Formation
of MDA (Methylene Dianiline) |
Possible if resin or product
is imporperly processed |
Not possible |
Not possible |
Not possible |
Melt
processing temperature |
High temperatures 195°C to 230°C 380°F to 450°F |
Lower temperatures 155°C to 190°C 310°F to 375°F |
High temperatures 185°C to 215°C 365°F to 420°F |
Lower temperatures 155°C to 190°C 310°F to 375°F |
Radiopaque and Color Compounding
Lubrizol’s polyurethanes can be loaded with radiopaque materials for detection by Xray or fluoroscopy and colored for product identification or coding. All radiopaque or color additives are introduced and dispersed at the time of polymerization, creating extremely consistent mixtures and superior smoothness of the final polymer. All additives are thoroughly screened and carefully tested for chemical stability, biocompatibilty and performance in the resin before use as a radiopaque or coloring agent.
Natural grades of each family are clear (transparent) in color. The polyurethane can be made radiopaque by adding barium sulfate. Tungsten powder has also been used as an effective radiopacifier with many of our grades of resin. Bismuth subcarbonate has been used very successfully in conjunction with the Tecoflex® TPU family of products. Lubrizol has many stock grades containing 20% and 40% barium sulfate and can custom formulate higher loadings upon request.
Transparent colors of our products can be produced using reactive dyes that chemically combine into the urethane chain, creating an unleachable covalent bond for color permanence and non-cytotoxicty. Opaque colors are formed with high-density pigment powders that are thoroughly dispersed for color uniformity and smooth consistency. Opaque colors may be chosen using a color matching chart or by matching existing components.
Custom Tubing Extrusion
Lubrizol operates a complete tubing production facility specially designed to extrude our various polyurethane products. All Lubrizol tubing passes strict quality control criteria at each stage of production and conforms to specifications of Good Manufacturing Practices. All tubing at Lubrizol is produced by custom order to ensure exact dimensions and configuration.
Lubrizol’s families of polyurethane have excellent working characteristics that allow extremely small diameters and very complex lumen configurations to be extruded. Specialty operations, such as radiopaque stripe coextrusion, are done on a regular basis. Tubing requiring specific radiopaque loadings or exact color matching is also done on a regular basis allowing manufacturers to order tubing that meets their desired specifications. Lubrizol’s expert extrusion engineers have developed capabilities to extrude tubing with lumen diameters as small as .005 inch and lumen quantities up to 9 lumens. Orders of 3 to 6 lumens are not uncommon. The technical staff at Lubrizol works with clients to assure all tubing meets the tolerances and characteristics necessary to ensure top performance in its function within the medical device.
Tubing Extrusion Capabilities: Cross sections below show a variety of lumen configurations available.
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Lubrizol Advanced Materials, Inc. disclaims any warranty of its products (Tecoflex®, Tecothane®, Carbothane®, Tecoplast®, and Tecophilic® TPUs) for merchantability or fitness for any particular application. Any person who intends to use these resins in the manufacture of implantable or any other medical device must independently determine the suitability of these resins for such applications. Each person is responsible for obtaining all necessary FDA and other approvals for the use of these resins in such an application and for complying with all applicable laws relating to the manufacture and sale of medical devices.
