String breast implants also go by the name polypropylene implants. This type of breast implant does not have approval by the Food and Drug Administration (FDA) in the United States, but is in use throughout other areas of the world. This type of breast implant was available in the U.S. for a limited time, several years ago. They are no longer available in the United States. This particular type of breast implant has a design to allow for extreme breast sizes. This type of implant continues to grow after they implantation surgically in the tissues. This method allows for abnormally large breasts. A man named Doctor Gerald W. Johnson developed them. The material used is a synthetic material commonly known as polypropylene.