In order to make the touch feeling mostly close to real human body, the Breast are always under improving. Now there's three kind of making the breast.
1. Solid.
2. Hollow.
3.Gel Filled.

Solid

Hollow

Gel Filled

Gel FilledHollowSolid
Softness★★★★★★★★☆☆★★☆☆☆
Elasticityy★★★★★★★★☆☆★★☆☆☆
Durability★★★★★★☆☆☆☆★★★☆☆

Please see below video to show you the Gel filled Breast: