The contemporary tampon was put into mass production during the mid-19th century and approved by the American Medical Association in 1945. All tampons have a cord for removal. Some have an additional outer cover to aid insertion and withdrawal (the applicator). Production materials are mainly rayon or rayon/cotton blend, with an applicator in plastic or cardboard with a design similar to a syringe.  The outward appearance of tampons is similar between brands with absorbency ratings ranging from 6 to 18 grams. The main difference between brands lies in either axial or radial direction of expansion. Tampons are sold individually-packed for hygienic reasons and should be changed every 4-6 hours once inserted.