from the interweb
Nano Silk: A new hybrid GSP thread that is virtually unbreakable. The strongest and smallest fly tying thread available on the market. Not only for small flies, but for ALL types. Now you can easily tie any fly with no build up. Imagine spinning deer hair using a 12/0 thread.
This thread is a stranded form of GSP which has incredible strength for its size. Advertised as "almost unbreakable", it allows tying techniques never before possible while minimizing thread build-up on small flies. Loosely twisted, it is the only GSP thread that can be split for dubbing loops, although it is fine enough to be doubled for loops without excessive build-up. The white thread takes marker colors easily. This 30 denier thread has a breaking strength of 40 oz. (1.14 kg). 50 yd. per spool. See Other Details for more information on its strength.
i think nano silk is different than pearsalls silk