The safest material to use for Butt Enhancement Injections is your own fat. During a Brazilian Butt Lift I remove fat from selected areas of your body, purify the fat, and then reinject selected donor fat cells into your buttocks. The process of fat reinjection involves scores (or hundreds of injections) and the procedure is designed to fill the upper quadrant of the buttocks so that the butt appears lifted and perky.
Butt implants made of silicon could be used but only if you're really tiny and simply have no body fat stored in your abdomen or elsewhere. Hydrogels are, and have been for several years now, impractical and often unsafe. Be sure to consult with a board certified plastic surgeon when discussing your options.