Peptides can generally be divided into two major categories: animal-based peptides and plant-based peptides. The small-molecule peptides that are commonly advertised on the market are mostly plant-based peptides, which have a single-chain planar structure and are composed of seventeen to eighteen types of amino acids, such as soy isoflavones in food and kinetin in cosmetics.
Animal-based peptides, which have a structure similar to that of the skin, are composed of twenty types of amino acids and have a triple-chain three-dimensional structure. Because of their larger molecular weight (generally above 3,000 Dalton), they are not easily absorbed directly through the skin, which is currently the main challenge that needs to be overcome.