Choosing the optimal material for a medical tube involves understanding the fundamental physical properties of each material under consideration for use. In this example, we are considering two materials: silicone and thermoplastic polyurethane (TPU). Silicone is used for a variety of medical tubing but it is not a thermoplastic. TPU, as its name states, is a thermoplastic and has gained wide acceptance as an alternative to silicone. In designing a medical tube, the engineer should start by considering four basic physical properties, which are critical to function. They are:

1. durometer (the hardness of the resin),
2. flex modulus (the flexibility of the resin),
3. ultimate tensile (the force it takes to break the tube) and
4. elongation (how much the material will stretch before breaking).

Two other important criteria are biocompatibility of the resin and postextrusion processability. Let’s take a general view of the physical properties of both materials and consider how they are related to see how tube design may be affected.

Durometer, flexibility, elongation and tensile strength are all functionally critical in a catheter tube. Typically, low-durometer (soft) polymers or silicones have lower flex modulus (higher flexibility), lower tensile strength, and higher elongation. If kink resistance is important to a tube, then a soft, low-durometer material would be a likely choice. However, what is gained in flexibility is usually lost in strength. If a tube needs high tensile strength but also must be very flexible, then the wall thickness of a tube and its outside diameter must be increased to compensate for the softer material’s lower tensile strength. Conversely, the harder, or higher durometer, materials for both silicones and TPUs have higher flex modulus (stiffness), higher tensile strength and lower elongation.

Both silicone and TPUs meet USP Class VI standards and both perform well inside the body. However, it is my experience that there is more variation with “in the body” performance among the different types of TPUs than there is among the different silicones. In this area, TPU technology has been advancing rapidly and one should stay abreast of the latest customised resins.

In terms of postextrusion processing and chemistry, silicone tubing and TPU tubing differ primarily in that silicone is not a thermoplastic, is inert, and will require special procedures for tipping, bonding and printing. TPUs are thermoplastics and, therefore, conventional heat forming methods can be used for tipping. And because they are not inert, they can be solvent bonded, RF welded and printed on, as the ink carriers readily bond to polymer chains.

In conclusion, material selection for medical tubing starts with consideration of the fundamental physical and chemical characteristics previously mentioned. It is important to understand how the physical characteristics and chemistry are functionally related—staying informed of the latest technology cannot be underestimated.