Thermoplastic urethanes (TPUs) fall within a class of materials known as thermoplastic elastomers (TPEs). Yet, TPUs comprise a class of materials unto themselves due to versatility in the polymer chemistry. As such, there are many types of medical TPUs, with distinctly different properties.
TPUs are copolymers of hard and soft segmented molecular chains that exhibit both thermoplastic and elastomeric properties. The molecular structure of TPU is created from three chemical components: a polyol (long-chain diol), a chain extender (short-chain diol), and a diisocyanate. The soft portion of the molecular chain is created from polyol and isocyanate. This provides the flexibility and elastomeric properties. The hard segment, created from a chain extender and isocyanate, provides toughness and physical strength properties.
All TPUs generally offer very good abrasion resistance, low temperature flexibility, oil resistance and mechanical strength compared to other elastomers. Mixing and matching various types of polyols, chain extenders and diisocyanate allows for optimization of certain key properties.
Medical grade TPUs are manufactured from polyester, polyether and polycarbonate polyols. Polyester types offer the best mechanical properties. Polyether types offer better hydrolysis resistance and low temperature flexibility than polyester types, but are more expensive. Polycarbonate types combine excellent strength with good oxidation and hydrolytic stability, and are typically more expensive.
Isocyanates used in the manufacturing of TPU can be aromatic or aliphatic. Aromatics, which contain benzene rings, are commonly used since they are tougher, stronger and less expensive than aliphatic types. Aliphatics, made with hydrocarbon backbones and no benzene rings, are strong but lack chemical resistance. However, aliphatic offer good light stability and optical clarity.
Because of this unique versatility in chemistry, TPUs are used in a wide variety of medical applications. These include catheter tubing, surgical drains, feeding tubes, dialysis devices, non-allergenic gloves, nonwoven gowns and drapes, hospital bedding, compression stockings, instrument cables, wound dressings, respiration devices, artificial hearts and more.