Each style has 2 tread material UAP:Polyurethane(PU) XSP:Polyurethane(PU) electric conductive Steel frame, yellow zinc plated, precision ball bearing in swivel head, covered swivel head, swivel head welded to the steel frame, housing with synthetic cover, wheel center made of Polyamide, tread made of PolyurethaneAvailable for electrically conductive and watertightness casters. For hospital bed ,hospital equipment or other applications.