Welcome to the first of a series of blog posts intended to familiarize you more with the different types of Elastomeric materials; key components in the design, development, and manufacturing of many medical devices.
Elastomeric or rubber materials are found in various forms of tubing, packaging (e.g., IV solution bags), coatings, soft touch components, etc. Elastomers used in medical devices can be categorized into three broad material categories: Silicone, polyurethane, and other (this “other” category can be further subdivided: polyolefin elastomers, styrenic block copolymers, PVC, and so on; for the purposes of this post, however, these materials can be grouped together for reasons that will be explained, further down).
Let’s introduce you to the first Elastomer category:
Silicone: In medical applications, silicone rubber typically refers to cross-linked polydimethylsiloxane (PDMS). Silicone is the most biocompatible of the three categories. It elicits low levels of platelet activation and is resistant to environmental stress cracking, for example. The stiffness can span a relatively broad range, with the ability to achieve durometers as low as around 10 Shore A and as high as approximately 80 Shore A. Silicone has two primary drawbacks compared with the other two categories: processing and mechanical strength. As a thermoset material, silicone is more difficult to mold or extrude than its thermoplastic counterparts, and it cannot undergo secondary melt processing steps such as RF tipping or be dissolved in a solvent for coatings. Additionally, it is less tough than polyurethane and many other medical elastomers.
The second Elastomer category is:
Polyurethane: The term polyurethane refers to a broad category of materials that can include foams, rigid plastics, and adhesives in addition to elastomers—any polymer that contains –NH–(C=O)–O– urethane linkages. Polyurethane elastomers can be thermosets or (as is the case for most medical applications) thermoplastics. Thermoplastic polyurethane (“TPU”) is generally stronger than silicone and can be easily melt processed via extrusion, molding, and many secondary processes. TPU can range from a relatively soft elastomer around 65 Shore A durometer up to rigid plastics high on the Shore D scale. Though not quite as soft or biocompatible as silicone, TPU is more biocompatible than most other polymers and can, therefore, be used for implantable devices, etc. Depending on the particular chemistry of a given TPU, it can be susceptible to degradation in vivo via environmental stress cracking or metal ion oxidation. See more of our polyurethane products.
Other: This broad category includes many polymers that have been used for decades (plasticized PVC, styrenic block copolymers) as well as newer materials developed more recently (polyolefin elastomers). These materials are generally less expensive than silicone or TPU. They are used in many important biomedical applications but due to their comparatively lower biocompatibility/biostability are not often directly competing with silicone or TPU for many applications such as implantable devices.
The design engineer developing an implantable device is therefore typically left weighing the trade-offs between silicone and TPU in the selection of elastomeric materials. Silicone is often the default choice unless thermoplastic processing and/or increased mechanical strength is required, in which case TPU is selected. What, then, is the designer to do for a long-term implant application requiring the biostability of silicone that also requires the toughness of a polyurethane?
Elast-Eon: The Elast-Eon family of polyurethane siloxane copolymers was designed to eliminate those trade-offs by combining the advantageous properties of both silicone and polyurethane. These materials have typical polyurethane chemistry and structure, except that in contrast with the polyether or polycarbonate soft segment typical of most medical TPU, Elast-Eon has a PDMS soft segment. The overall silicone content of Elast-Eon varies among particular grades of Elast-Eon but is over 50% of the material.
Due to the high silicone content, Elast-Eon exhibits superior biocompatibility and biostability. For example, a three-month study of samples stretched to 150% elongation and implanted in sheep resulted in vastly diminished degradation as observed both by visible cracking in micrographs as indicative of environmental stress cracking and quantification of ether oxygen as indicative of oxidation.1
Elast-Eon also retains many of the advantages of TPU. Its mechanical strength and durability are much greater than silicone (only slightly lower than comparable traditional medical TPUs)2, and Elast-Eon is easily melt processed and is suitable for solution processing (dip-coating or -casting, electrospinning, spray-coating, etc.).
Elast-Eon has found use in a wide range of applications including pacing leads, cardiopulmonary cannulas, stent coatings, synthetic heart valves, and many others.
In our next blog post, we’ll discuss and focus on applications in which Elastomeric polymers can be used in more detail.
- Martin et al., Biomaterials 21 (2000), 1021.
- Gunatillake et al., Appl. Poly. Sci. 76 (2000), 2026.
For more information about this topic or to discuss your elastomeric needs, please contact Nat Fredin, Director, R&D and Materials.