Thermoplastic polyurethanes (TPUs) have historically been used as thermoplastic elastomers (TPEs) for medical applications.1 The well-known biocompatibility, biostability, abrasion resistance, low temperature elasticity, and processing flexibility characteristics of TPUs provide the foundation for the selection of the material as a building block for medical devices.
Until now, the fabrication of softer grades of TPU (Shore hardness ratings <70A) has been accomplished through the use of plasticizers and other additives. These additives may extract or migrate out of the polymer device, resulting in an increase in potential toxicity from the fabricated components.
This article introduces the development of soft plasticizer-free grades of hydrophobic TPU material with Shore hardness ratings as low as 62A. This 100% polymer technology is highly elastic and extremely hydrophobic; has a low specific gravity, high memory, and low hysteresis; and exhibits optimal processing and adhesion properties.
Silicone elastomer and thermoplastic elastomer (TPE) materials are routinely used in medical device applications that require softer components. For example, tubing, medical bags, catheters, and feeding systems all benefit from the properties of these softer materials. While effective for some applications, silicone chemistries are difficult to process and exhibit poor adhesion characteristics for bonding and over-molding components. In the case of TPEs, plasticizers are currently necessary to achieve soft elastomeric properties, which may result in possible plasticizer migration and potential toxicity concerns. In addition, typical TPEs require the use of large amounts of lubricant to reduce tackiness. Excessive amounts of lubricant obstruct the adhesion properties of the material and the lubricant can migrate out of a component much like a plasticizer. Lubrizol’s softgrade hydrophobic TPU technology was designed specifically to overcome these limitations.
Thermoplastic polyurethanes (TPUs) are a class of thermoplastic elastomers (TPEs)2 that are named for the chemical composition of their carbamate or urethane functional group. Typically, TPU polymers are made from combining diisocyanates and diols. The diols can either be of high molecular weight polyols or of low molecular weight chain extenders.
Thermoplastic polyurethanes that exhibit elastomeric characteristics are derived from a mix of both high and low molecular weight diols. As a result, TPUs are segmented two-phase polymers. The hard segment is a urethane linkage of diisocyanate and chain extenders. The soft segment is predominantly comprised of higher molecular weight polyol. The hard segment gives the polymer its strength while the soft segment imparts material flexibility.
Thermoplastic urethanes are available in a variety of chemistries. Hydrophobic TPUs are part of a relatively new class of material that has been developed by Lubrizol specifically for the medical industry. These materials have been produced in the Shore A durometer range of 62A to 85A and are greater than 50% bio-sourced. Hydrophobic TPUs exhibit the same desired processing, strength, elasticity, and tear resistance characteristics that are offered by traditional TPUs. However, benefits offered by the hydrophobic class of TPUs that are not found in typical TPUs include unique solvent solubility, improved adhesion characteristics, and excellent electrical insulation properties.
The ultimate performance of a thermoplastic urethane is based on its chemistry and a series of optimized processing parameters. Selecting the right balance of chemistry and processing parameters becomes more critical as the desired softness of the material increases.
Lubrizol’s hydrophobic TPU technology achieves this balance and eliminates the need for plasticizers to produce softer grades of material. In addition, these TPUs exhibit relatively low tack with use of a significantly reduced amount of lubricant.
Combining a hydrocarbon polyol with a low molecular weight diol facilitates the production of TPUs with the desired range of soft attributes. The table in this article includes a list of properties for this new hydrophobic class of TPUs. Materials produced using this technology exhibit good tensile strength, even at a low durometer rating of 62A. As the durometer rating increases, so does the tensile and tear strength of the material. The introduction of higher levels of hydrocarbon polyol lowers the durometer rating and imparts attributes such as increased resilience and lower specific gravity.
Typical aromatic TPUs have a specific gravity of roughly 1.15 to 1.22, while aliphatic versions are in the range of 1.05 to 1.12. Some grades of the newly developed hydrophobic class of soft TPUs can float in water with specific gravities as low as 0.994. This overall reduction in specific gravity of the hydrophobic class of TPUs relative to typical TPUs can result in a decrease in material amount per component.
The hydrophobic characteristic of the material results in an improvement in its electrical insulating properties. The dielectric permittivity is significantly lower and more stable over a range of frequencies than that of standard TPUs. This performance feature is beneficial for fabricating device components such as guide wires or pacemaker leads.
Processing and Performance
The new class of hydrophobic TPUs can be processed using standard melt-processing equipment for thermoplastic urethanes. These materials have low water absorption characteristics; however, drying is necessary prior to melt-processing. Typical drying conditions are 4 hours at 65 °C.
Many traditional TPUs are dissolved in polar solvents prior to casting device components or forming thin films. For hydrophobic TPUs, solvent systems that are more aromatic in nature are recommended.
Unique solvent solubility properties increase the compatibility of hydrophobic TPUs with common medical disinfectants. Isopropyl alcohol, a typical disinfectant used in hospitals, does not cause hydrophobic TPUs to swell to the same extent as typical TPUs. Traditional TPUs can swell from 25 to 100 percent of their original size, whereas hydrophobic TPUs will only swell 10 percent.
The properties of these new hydrophobic TPUs facilitate use in applications that are not typically considered for traditional thermoplastic urethanes. For example, this technology may be suitable for use in soft peristaltic pump tubing, ultra-small diameter tubing and catheters, and neo-natal feeding systems.
During processing, many pharmaceutical formulations are transferred through peristaltic pump tubing as ethanol or other alcohol-based slurries. The durability and alcohol-resistant nature of hydrophobic TPUs make them an ideal fit for this type of application.
Clinical applications often involve the transportation of biological or medicinal fluids. Many of these applications require the clinician to observe the fluid in the tube. TPUs based on hydrocarbons, such as hydrophobic TPUs, are generally opaque in comparison to traditional TPUs. Lubrizol’s hydrophobic TPU technology exhibits translucent to clear properties. Such transparency allows fluids to be easily observed in the tubing during clinical use.
This article was written by Anthony Walder, Global Technology Manager, Medical and HealthCare for Lubrizol Advanced Materials, Wilmington, MA; Donald Meltzer, Technical Fellow, and Paul Dulick, Product Manager, Pharmaceuticals for Lubrizol Advanced Materials, Brecksville, OH. For more information, Click Here .
- M.D. Lelah, S.L. Cooper, Polyurethanes in Medicine, CRC Press Boca Raton, (1986).
- A. Walder and D. Meltzer; Thermoplastic Polyurethanes: The Other TPE, Antec 2008, pp 773-774.