The catch is any notch you make will weaken the material significantly and you'll have fatigue failures. That's the sneaky part of using a flexible sleeve, you don't introduce any undesired weaknesses.
I did experiment with various ways of allowing light to escape but nothing came close to the properties of a total air gap. You can actually measure (relative) bend angle with it like a protractor since the attenuation is very linear!
There is already existing work that uses colored segments for something similar but those techniques are hard to do outside a well equipped lab.
I didn't include this in my article but I did some experiments early on (for a different idea) with air bubbles in oil inside a Teflon coated tube but that presented a lot of challenges (mainly the bubble breaking up) that made it not ideal for something like this.
This can certainly be miniaturized with the right manufacturing techniques but I left that for the future.
I used it more for future-proofing in case I wanted to do sensor fusion or something like that later on -- currently it's just 1D filtering so I could have used anything. Also I'm just way more familiar with using Kalman filters so it was also a comfort thing!
Since the sleeve is a stretchy rubber as long as the inner diameter of it is a bit smaller than the outer diameter of the fiber it holds just fine. For more dynamic applications, though, a silicone adhesive, or even super glue for more permanent strands, helps!
There are a lot of cool applications indeed! I was able to use it to do gait for a soft robot "leg", but I have to wait for the paper to be published later this year before going into too much detail.
There are a lot of similarities in the approach to the linked paper (which is a very cool concept) and I saw a lot of similar concepts in my lit review. At a high level, my sensor targets bend localization with simple fabrication techniques while the linked paper is doing more general camera-based gesture recognition. I have a more thorough comparison to existing work in my actual dissertation.
