Polypropylene breast implants, also known as string breast implants, are a form of breast implant using polypropylene developed by Dr. Gerald W. Johnson. Due to a number of medical complications, the device has been banned in the European Union and United States.

The implants absorb fluid and expand once inside the body, resulting in “extreme, almost cartoonish breast sizes.”^[1] The result is almost continuous breast growth after surgery. String implants were only available for a very short time before being removed from the market by the FDA around 2001.^[2] Polypropylene implants have created the largest recorded increases in breast size due to surgical augmentation. Polypropylene implants are rarely seen outside the adult entertainment industry.