Stents are inserted into the urethra to aid in the flow of urine, or in the prostatic urethra to combat particular conditions in men. The materials used in stenting have evolved over time and differ based on whether placement is permanent or temporary. Temporary stents may be biodegradable or non-biodegradable. All stents must meet certain criteria: Ease of insertion and removal, biocompatibility, rigid enough to avoid obstruction, resistant to infection, resistant to migration, and radio-opaque for confirmation of placement via radiography. Metal alloys and polymers are both used. Prior to the introduction of biodegradable stents, polyurethane and other plastics were primarily used, but both plastics and metal have disadvantages compared to newer materials in regard to migration and biocompatibility. Read more about the materials used in urethral and prostatic stents.