TPU (thermoplastic polyurethane):

It is a flexible, abrasion resistant, rubbery thermoplastic that is used by traditional manufacturing methods for making consumer (protective cases) and industrial (transmission belts) products. Depending on the blend used, it can be very soft (elastic). 

Within the 3D printing community it is used to make flexible, durable parts that can withstand temperature up to 80 °C in air. 

3D printing with TPU:

Unlike the stiffer thermoplastics, due to its intrinsic flexible and conformal nature, TPU will have excellent interlayer adhesion and no curling or del-lamination when 3D printed, even without an enclosure. 

TPU consists of a blend of polymers resulting in various properties, most notable the hardness can vary substantially between different blends. Thermoplastic polyurethane will extrude between 225 °C to 250 °C. A reliable 3D printing head (or extruder) needs to withstand these temperatures for an indefinitely long time. 3FXtrud printers use a whole metal hot end design which works reliably and provides consistent 3D printing of TPU materials. 

TPU 3D printing requirements:

An extruder system able to handle compressible and very flexible materials with a hot end able to withstand ~ 240 °C continuously.

TPU is available in 1.75 mm and 3 mm. Pyra 2 uses 3 mm filament, while the newer 3FXtrud 20 Uno, 3FXtrud 25 Uno, 3FXtrud 25 Duo, and 3FXtrud 30 Duo use 1.75 mm.