Why did Harime Nui's eye not heal? Sometime before episode 12, we learn Harime Nui lost her left eye from Matoi Isshin slashing her with a half-scissor.

Around episode 20, Harime Nui is pierced through the chest by Ryuuko, also by a half-scissor, but that time, she did not experience pain and healed from this more serious wound instantly.

It is also revealed that Harime Nui was fused with Life Fibers from birth having been developed inside a Life Fiber cocoon.

Given these facts, why did


Harime Nui experience pain when slashed by Isshin?
Her eye not heal from the wound?
Based on the Nui Harime wikia page, her eye was destroyed.


  Nui was distracted by the sound of Ryūko calling for her father, allowing Isshin to destroy her left eye with the purple Scissor Blade.


It's also worth noting that the Scissor Blades were able to 


  prevent Nui's arms from regenerating when they were cut off.


The power of the Scissor Blades are known to be the one thing that can cut through Life Fibers, and this is likely what prevents the regeneration of Nui's body parts that were removed by them, as it destroys them.

As an aside, in many anime and shows, when a limb or body part is destroyed, it is not able to be regenerated. The best example I can think of is


  In Claymore, when a claymore loses a limb, they can have it heal by reattaching the severed limb. However, when Clare's arm is destroyed, she has no arm until one is given to her by Irene.