The TPUs are highly resistant to chemical agents, wear-proof and with good elastic comeback.

Mark7 = 70 Shore A, the most flexible, but it requires a perfect calibration of the printer.
Mark8 = 80 Shore A, it is a good mixture of flexibility and stiffness, easier to print too.
Mark9 = 90 Shore A, is the stiffest of all the Flexs, and lets you print as usual.