9.4.1.2 Hyaluronic acid
Hyaluronic acid（HA）is a non-sulfated glycosaminoglycan polymer with a linear conformation，which is the main component of extracellular matrix（ECM）.The total amount of HA in the skin is more than 50％.Due to its strong water holding capacity，HA can increase the moisture content of the skin，which is inherently helpful to increase the skin luster visually.The content of HA in epidermis is decreased with the increase of age.UV also causes the loss of HA and water，resulting in wrinkles.
Evidence-based analysis revealed that the anti-wrinkle efficacy of HA is molecular weight dependent which is expected to be due to differences in percutaneous absorption of different molecular weights of HA across the stratum corneum.In a clinical trial involving 76 female aged between 30 and 60 having periocular wrinkles，these patients were applied with 0.1％cream formulation containing different molecular weights of HA（50，130，300，800，and 2000）twice daily for a period of 60 days.The greater improvements were observed in the skin hydration level，skin elasticity，and reduction in peri-ocular wrinkles in women applied with cream formulation containing low molecular weight HA.In view of molecular weight dependent percutaneous absorption，the swift anti-wrinkle and skin rejuvenating effects of nano-HA containing topical formulations were expected to be due to superior percutaneous absorption of ultra-small HA molecules.(https://www.daowen.com)
HA can also be used for soft tissue filling.After 2007，HA soft tissue filling has become one of the popular non-surgical cosmetic methods，second only to Botox skin filling treatment.The mechanism of dermal filling is shown in Figure 9-3.HA filling in dermis is rarely allergic，relatively painless and has little side effects；Yet，it can not maintain the effect for a long time，with the need of repeat filling.HA soft tissue filling may cause inflammation，swelling，ecchymosis and asymmetry，which may depend on the injection technology.It is usually necessary to inject different doses of HA into different parts of the face and massage to prevent facial unevenness.HA filling has become the main method for the treatment of wrinkles.After filling，collagen synthesis can be optimized for several months or even for a long time.