Nipple clamps.

Abbie drew a sharp intake of breath as Ms Johnson began to caress and squeeze her nipples, which immediately grew and throbbed under the attention they were receiving. Then she felt the cold steel as the nipple clamps were put into position and the clamps tightened.

She looked down and saw her nipples peeking out from the clamps. A chain connected them, and she began to wonder what she had let herself in for.