DI resin is a water filter made from tiny beads that is used to remove impurities from water and chemically change the water. DI resin is used in the last stage of the RO/DI system to reduce Total Dissolved Solids (TDS) to zero. The resin is made from small polymer beads that are charged with either negative or positive charges that make up cation-anion mixed bed resin. DI resin is used to create highly purified water applicable for medical, bio, or laboratory use.