Hyaluronic acid (Hyaluronan, hyaluronic acid, also known as glyoxylic acid, hyaluronic acid, lucuronic acid) is a glycosaminoglycan that is composed of the basic structure of disaccharides (D-glucuronide and N-acetylglucosamine). The molecular weight of hyaluronic acid in the body ranges from 5,000 to 20 million daltons. Hyaluronic acid is widely found in connective tissue, epithelial tissue and neural tissue. With its unique molecular structure and physicochemical properties, hyaluronic acid displays a variety of important physiological functions in the body, such as lubricating joints, regulating the permeability of blood vessel walls, regulating proteins, water-electrolyte diffusion and functioning, and promoting wound healing. Especially important is that hyaluronic acid has special water retention effect and is the best moisturizing substance found in nature, which is known as the ideal natural moisturizing factor, for example, 2% pure hyaluronic acid aqueous solution can firmly retain 98% water. Hyaluronic acid has special biological activity, and has non-toxic, low immune response, high biocompatibility, and biodegradable and human absorbable properties, which can be used in eye surgery, intra-articular injection, wound healing, and surgical anti-adhesive. Hyaluronic acid is used as a soft tissue filler to fill wrinkles and depressions and to adjust the shape of facial tissues, and its use has become increasingly popular in recent years. Because of its special high water retention, it can be injected into the dermis to improve the water content of the skin, making the skin smoother and more hydrated, which is the origin of the term “hydrafacial”.