Thermoplastic urethanes (TPUs) are a subset of thermoplastic elastomers (TPEs). TPUs are copolymers of hard and soft segmented molecular chains that exhibit both thermoplastic and elastomeric properties. There are many types of medical TPUs, with distinctly different 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, providing elastomeric properties such as flexibility. The hard segment is created from a chain extender and isocyanate for physical strength properties.
All TPUs generally offer very good abrasion resistance, low temperature flexibility, oil resistance and mechanical strength compared to other TPEs. The various types of TPUs, and unique property characteristics, are achieved through variations in polyols, chain extenders and diisocyanates.
Medical grade TPUs are manufactured from polyester, polyether and polycarbonate polyols. Polyester constructs 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 the most expensive of the three.
Isocyanates used for manufacturing of TPUs can be aromatic or aliphatic. Aromatics contain benzene rings, and are commonly used since they are tougher, stronger and less expensive than aliphatic types. Aliphatics are made with hydrocarbon backbones and no benzene rings. These isocyanates are strong, and offer good light stability and optical clarity, but lack chemical resistance.
This unique chemistry allows TPUs to be used in a wide variety of medical applications. These include small-diameter 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.