A note from the manufacturers website:
The bristles are made from a BPA FREE NYLON polymer resistant to microbial growth during normal use, to ensure safety and durability. We have tried to find a biodegradable bristle but as of this time there is nothing available – apart from boars hair (don’t want to go there). No they are not Nylon 4 (as far as we know there are no bristles made from Nylon 4 on the market today).
We have tried to bring you a toothbrush that is better for the environment- over the years everyone had been misled by manufactures about bristles being made of Nylon 4 (this is not true). We still believe that our toothbrush is a better alternative to a full Plastic toothbrush. Better to be 99% Bamboo than 100% PLASTIC