What I've seen is literally the coloring of the small balls inside the e-ink display to get wiped off.
See here a photo I made years ago (when I had access to tools that could do this)
https://en.wikipedia.org/wiki/E_Ink#/me ... psules.png
Inside the display there are balls with 2 colored sides (and some have a gradient, like the Kindle has)
These balls rotate when changing their color.
As you can see, the neighboring pixels also slightly rotate due to the field applied to the pixels.
This is why you may want to update the entire screen every now and then or else there might be some residual image left of the previous image.
This update is then often first writing the image inverted and then flipping all pixels 180 degree.
Now back to the wear-and-tear.
If you only update local to where you want to make changes, the pixels will wear out unevenly.
Similar to a TV or monitor showing burn-in.
So you will see some kind of ghost-image of what was displayed more often.
On top of that the ink may fade due to sunlight.
So if you have some areas which show black in full sunlight, then those pixels may also show a burn-in like effect.
I have seen an e-ink display be worn-out in roughly a weekend (may have been a 'long weekend') when it got updated every second due to a programming error.
Let's assume it was 4 days, then 86400 x 4 = 345600 updates.
No idea if it was already hard to read after 1 day or 3 days.
It probably also makes a difference if the pixels get some time to 'rest'.
So I think a safe amount of updates during its lifetime is about 100'000 updates.
This is an update-count per "pixel" or "area", meaning a local update on one corner and another update on another corner without overlap in pixels should count as 1 update.
Also a local update seems to be more "aggressive" compared to showing an inverted screen and then flipping all pixels 180 degrees. This is because local updates need to be faster, and neighboring pixels may not move and thus may cause more friction.
When aiming for < 100k updates during a display's lifetime then an update every 5 minutes is a life time of 1 year.
I do have one such display on my desk which is about 8 years old and is updated every 15 minutes (full screen update).
It is showing some wear on some pixels, but still very usable, so this 100k updates is quite conservative.
. Great work!
