Croptop bikinis are also good for tanning because they require less clothing and thus expose more of the body to the sun. A Croptop Bikini is a bikini top that only covers the chest and exposes the midriff. Typically, they are made of nylon, polyester, or spandex. Croptop bikinis are ideal for women who want to show a little skin while still covering their shoulders. A crop top bikini woman can flaunt her arms without fear of exposing too much skin.