Breast shells are hollow plastic isks worn inside the brassiere to protect the nipple from becoming flattened. The disk has a hole in the middle worn toward the nipple side. It is slightly concave to conform to the shape of the breast, but can sometimes still be slightly visible under tight clothing. Shells come apart for washing. This should be done frequently, as the shell also tends to make the mother's breast sweat, which can increase bacteria growth and cause irritation.