Nitinol wire is an alloy of nickel and titanium known for its superelasticity and shape memory properties. These unique characteristics allow it to return to its original shape after deformation, making it highly resistant to permanent bending or looping. Unlike stainless steel, gold, or TMA wires, nitinol cannot be easily manipulated into loops because it automatically recovers its straight form when the bending stress is removed. This property is crucial in clinical applications where maintaining wire shape and reducing permanent deformation is required. Therefore, nitinol wire is almost impossible to bend into loops due to its intrinsic metal behavior.

Reference: Orthodontics: Current Principles and Techniques, 6th Edition.