There are three main classes of biopolymers, classified according to the monomeric units used and the structure of the biopolymer formed: polynucleotides, polypeptides, and polysaccharides. More specifically, polynucleotides, such as RNA and DNA, are long polymers composed of 13 or more nucleotide monomers. Polyactides are commonly used as surgical implants. Starch based biopolymers can be used for creating conventional plastic by extruding and injection molding. Biopolymers based on synthetic are used to manufacture substrate mats. Cellulose based Biopolymers, such as cellophane, are used as a packaging material.