From Semperfli:

Semperfli Nano Silk Pro 20D - Amazing For Trico and Micro Flies Sizes 18 to 32!

Possibly the worlds strongest fly tying silk for its diameter. The Nano Silk Ultra is ideal for flies sizes 18 and smaller. And you can still split it down further or use it to tie dubbing loops. 24/0 (20 denier) use with ceramic bobbins.

At just 20D this has a breaking strain of 750gm or 26oz! This will lay flat and disappear yet still have incredible strength for fly tying micro flies.

100 metre spools of 24/0