I heard about aquafilling "hydrophilic gel", is it safe for breast augmentation?

In no way would I suggest you fill your breasts with anything besides FDA approved implants and potentially some fat. Its just not safe. A small implant under the gland in a body builder is a great and reliable operation

I personally do not recommend its use.  Aquafilling is made of a polyamide macromolecule. Polyamides occur naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Nylon is an example of a man-made polyamide molecule. It is NOT FDA approved in the USA and I would hesitate to recommend this treatment without seeing large number scientific studies. Breast Augmentation can be safely done in weight lifters.

This is not recommended.  There are many horror stories of women injecting things into their breasts.  The results can be disastrous with a significant risk of VERy serious infections and even death.  DO NOT DO IT!!!