A solid, flexible silicone shell filled with saline. Yes, you read that right; if you thought that a saline implant had no silicone content whatsoever, you'd be partly right. The filler has no silicone, but the shell is a silicone elastomer, the same material which is commonly used in other implant surgeries, such as the type of implant when one wants to improve on a receding chin (ie, chin implant).

These can be factory-sealed (depending on the implant model), but are most commonly filled during surgery. The implants which are filled during surgery have a clever valve design which permits easy filling and fine tuning of the fill volume during surgery, but once the surgeon is satisfied with the fill volume, a simple tug on the fill tube allows the valve in the fill port to close up rather securely.