Thermoplastic polyurethane (TPU) is well known and specified in the medical industry for advanced medical and healthcare products, due to its high performance characteristics. Because of its excellent mechanical properties, durability, and resistance against oils and chemicals, TPU is very desirable for medical applications. Since TPU does not contain plasticizers, it also offers the medical industry an environmentally friendly replacement to PVC without sacrificing flexibility. Ether TPU types also meet the requirements of National Sanitation Foundation (NSF) Standard 61, U.S. Food and Drug Administration (FDA) 21 CFR for certain food contact applications, and USP Class VI classification.

Fig. 1 – Distinction of TPE.

Examples of medical TPU applications include devices used for diagnostic, anesthesia and artificial respiration, as well as healthcare mattresses, dental materials, compression stockings, medical instrument cables, gel shoe orthotics, and wound dressings.

TPU belongs to the family of materials known as thermoplastic elastomers (TPE). It attains virtually the same level of elasticity as cross-linked elastomers (rubber) while simultaneously offering the advantages that it can be processed like a thermoplastic and is recyclable. (See Figure 1)

Chemistry

TPU is made up of block copolymer molecules with alternating rigid and flexible segments. It is this combination of flexible, elastic segments—with a high extensibility and low glass transition temperature, with rigid crystallizing segments, with a high melting point, that gives this material its elastomeric nature. (See Figure 2)

By altering the ratio of the rigid phase, it is possible to vary properties such as hardness, strength, rigidity, extensibility, and low-temperature flexibility over a broad range. A distinction is drawn between polyether and polyester polyurethane as a function of the polyols employed. The cable industry uses predominantly polyether TPU on account of its good resistance against microbes and considerably better resistance against hydrolysis.

Properties

Fig. 2 – Block copolymer character of TPU.

Mechanical properties: Thermoplastic polyurethane is supplied with a surface hardness of 35 Shore A (with plasticizer), 70 Shore A (without plasticizer), up to 74 Shore D. Further distinction is made between grades by using flameretardant additives, matting agents (for low-adhesion surfaces), or plasticizer (for even higher flexibility, especially decreased bending modulus and partly flame retardance). However, most of these grades containing additives are not suitable for healthcare applications. (See Table 1)

Flexibility at low temperatures: Dynamic Mechanical Analysis (DMA) is a method used to study and characterize materials. It is used for studying the viscoelastic behavior of polymers. A sinusoidal stress is applied and the strain in the material is measured, allowing to determine the storage modulus G′ and to locate the glass transition temperature TG.

The glass transition temperature TG for the unfilled Elastollan® 1185A based on DMA-analysis is -27.7°C (-17°F). However, feedback from the market shows that, under real environmental conditions, the 1185A performs in flexible installations at even lower temperatures.

Abrasion: While the abrasion values of TPU as per ISO 4649-A are hardly dependent on the hardness, harder versions withstand the typical abrasion test for insulation for an extremely long period of time. In ABS brake cable applications for example, car manufacturers frequently specify at least 150 or 200 double strokes until insulation wears through.

Thermal properties/Long term heat aging: Long-term ther mal behavior is determined according to ISO 2578. The test sample is permanently conditioned at a selected temperature until a defined criterion is no longer achieved. The achieved temperature resistance is therefore dependent from the end criterion and the test time.

altFor TPU the end criterion is usually an elongation at break of 300 percent, which would be 50 percent of initial values, but it is also usual to test to an elongation at break of 50 percent. The electrical engineering sector frequently specifies a 20,000 hour test (2.3 years) for estimating the service life time. TPU can be aged at 105°C (221°F) over this period of time still having an elongation at break of 300 percent. If an elongation at break of 50 percent is sufficient the temperature resistance is increased to 110°C (230°F) over a period of 20,000 hours.

Resistance against hydrolysis: For outdoor applications, the resistance against hydrolysis is absolutely necessary. In general, the rate of hydrolytic degradation of TPU increases with increased temperatures. A standard polyetherpolyol based TPU has very good hydrolytic resistance. Polyester-polyol based TPU in comparison have lower hydrolytic resistance.

Weathering and ozone resistance: Weathering performance can be measured according to ISO 4892-2, part A, using injection molded test plates with a thickness of 2 mm. Due to the chemical structure, TPU in general shows very good resistance against ozone. Selected grades have been stored for 72 hours at 40°C (104°F) with an ozone concentration of 200 pphm and a strain of 20 percent. All grades remained free of cracks.

Regulations for Medical Applications

In the following chapters, information is given concerning basic regulations regarding materials intended for use in contact with foodstuffs or as medical devices. It is the responsibility of manufacturers and traders to ensure that intermediate and finished articles are in compliance with all relevant legal requirements and standards for the particular application area.

Approvals for medical goods both in Europe and the USA cover exclusively finished products. In the EU in addition to Directive 93/42/EEC there are some national regulations (e.g., in Germany the “Medizinproduktegesetz”) for the respective member country which must be followed.

Fig. 3 – TPU for nonwoven fabrics for gowns, drapery, and tubing.

For a new application in the EU, it is a precondition that the manufacturer of the product obtains a CE authorization involving biocompatibility testing according to EN ISO 10993 “Biological assessment of Medical Products”. This standard covers a number of test procedures related to the specific application and body contact time. According to experience from tests on injection molded plates, it is most likely that finished products made from Elastollan 1100 types without flame retardant additives or plasticizer will fulfill the requirements for irritation, sensitization, and cytotoxicity, assuming that the materials are correctly processed, and providing no unacceptable additives are included. This must, however, be confirmed by an authorized laboratory.

TPU is not suitable for articles for implantation, blood-bags, containers for liquid medicines or other liquids that are intended for transfer to the human body, nor for articles that could come into contact with saliva for longer time. A formulation of a transparent, plasticizer- free 70 Shore A material is available under the name Elastollan 1170A. The Cytotoxicity Assay in vitro (ISO 10993) is officially confirmed, as well as for the 80 Shore A material Elastollan 1180A. An USP class VI-classification is available for Elastollan 1185A and WY1140 (polyether with UV). For these reasons, more and more applications with TPU in medical segments are provided.

Examples of medical TPU applications include devices used for diagnostic, anesthesia and artificial respiration, as well as healthcare mattresses, dental materials, compression stockings, medical instrument cables, gel shoe orthotics, and wound dressings. (See Figure 3)

Regulations in USA: The additives and raw materials (excluding stabilizers) that are used to manufacture above mentioned Elastollan grades are listed in the Code of Federal Regulations by FDA, Title 21 §177.2600 “Rubber Articles Intended for Repeated Use” in the latest version dated January 4, 2011. The antioxidant stabilizers which are included are listed in § 178.2010 “Antioxidants and Stabilizers for Polymers”.

Regulations in Germany and European Community (EC): The mono meric raw materials and additives which are used for manufacturing the above products are listed in the “Bedarfsgegenstän - deverordnung” dated December 23, 1997, and last updated February 7, 2011. The monomeric raw materials are also listed in Chapter 2 of Recommendation XXXIX of BfR, last updated January 1, 2010, which covers articles manufactured from polyure thane. The used monomeric raw materials and additives are listed in Annex I of Regulation (EU) 10/2011.

Requirements for Food Contact Applications: Based on experience with food contact articles made of above described materials, it can be expected that articles will be in accordance with § 2 chapter 6 (1.) of “Lebensmittel-, Bedarfsgegenstände- und Futtergesetzbuch” for contact with dry foodstuff ( excluding packaging materials) and for contact with wet foodstuff for a maximum period of 10 minutes. Generally application simulating testing, in accordance with regulations in EU (e.g., Regulation (EU) 10/2011) has to demonstrate that global migration, specific migration, and other restrictions for substances does not exceed allowed quantities. Among others it has to be proven that primary amines are not detectable (BfR-Recommendation XXXIX, Nr. 3.1; Regulation (EU) 10/2011, Annex II). It is also a requirement that commodities shall not influence appearance, taste, or odor of the foodstuff. The compliance with legal requirements and generally accepted standards for consumer protection has to be proven for the specific application using the final article.

This article was written by Sarah E. Westerdale, Market Manager - Wire & Cable, Distribution, BASF Corporation, Wyandotte, MI. For more information, Click Here 

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Medical Design Briefs Magazine

This article first appeared in the June, 2013 issue of Medical Design Briefs Magazine.

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