Natural polymers can be classified as polysaccharides, polyamides, and polynucleotides (Figure ). They can also be listed as plant-based polysaccharides (e.g., cellulose, alginate, or starch) and animal-derived polymers (e.g., collagen, silk fibroin, or chitosan). They exhibit advantages, such as high biocompatibility, low toxicity, low cost, good mechanical properties, and high biodegradability. Several linkages ensure high biodegradability (Figure ).