Foam neoprene used for wetsuits is always closed cell, in that the gas bubbles are mostly not connected to each other inside the neoprene. This is necessary to prevent water absorption, and the gas bubbles do most of the insulation. Thick sheets of neoprene are foamed inside a mould, and the surfaces in contact with the mould take on the inverse texture of the mould surfaces. In the early days of wetsuits this was often a diamond pattern or similar, but can also be slick and smooth for low drag and quick drying. The cut surfaces of the foam have a slightly porous mat finish as the cutting process passes through a large number of bubbles, leaving what is called an open cell surface finish, but the bulk of the foam remains closed cell. The open cell finish is the most stretchy and the least tear resistant. It is relatively form fitting and comfortable on the skin, but the porosity encourages bacterial growth if not well washed after use, and the foam surface does not slide freely against skin.