1.2.2 Absorption function
The skin has an absorption function which is the basis of external dermatological medicine treatment.The stratum corneum is the main route of transdermal absorption，followed by hair follicles，sebaceous glands，and sweat glands.The absorption function of the skin is mainly affected by the following factors.
1.The condition of the whole body and skin
（1）Age and gender：Some people think that the skin of babies and the elderly is easier to absorb than other age groups.However，most studies have shown that the skin of newborns and infants has reduced or normal transdermal absorption capacity.There is no difference between genders.
（2）Part and structure：The absorption capacity of the skin is related to the thickness，integrity and permeability of the stratum corneum.The absorption capacity decreases along with the thickness of the stratum corneum：scrotum＞forehead＞flexed thigh＞flexed upper arm＞forearm＞palmoplantar area.
（3）Period：Primarily，the difference in the function of the stratum corneum in different periods of growth and shedding can lead to the changes in its absorption capacity.Secondarily，changes in temperature and humidity can also affect its absorption capacity.If the temperature rises from 26℃ to 35℃，the water dispersion of the epidermis can double.
（4）The degree of hydration of the skin：The higher the degree of hydration of the stratum corneum，the stronger the absorption capacity of the skin.If the moisture content of the stratum corneum is less than 10％，the stratum corneum becomes brittle and easily cracked，and the absorption capacity is reduced.
2.Physical and chemical properties of the absorbed substance
（1）Molecular weight and molecular structure：Ammonia with a small molecular weight can easily penetrate into the skin，and substances with a large molecular weight such as mercury ointment and dextran molecules can also penetrate into the skin.This situation may be related to the structures，shapes and solubilities of the molecules.
（2）Concentration：It is generally believed that the higher the concentration of the penetrating substance，the more the skin absorbs.However，there are also a few substances with high concentration have a coagulation effect on keratin，which affects the permeability of the skin and causes malabsorption，such as carbolic acid.(https://www.daowen.com)
（3）Electrolysis：Generally，substances that can be dissociated are easier to penetrate into the skin than those cannot.For example，the skin absorbs sodium salicylate better than salicylic acid.
3.External factors
（1）The increase in ambient temperature can cause skin blood vessels to dilate，increase blood flow speed，and accelerate the diffusion of substances that have penetrated into the tissues，thereby improving skin absorption capacity.
（2）When the environmental humidity increases，the stratum corneum hydrates，and the skin absorption capacity increases.
（3）External dosage form：The dosage form has an obvious influence on the absorption of the substance.The same drug has different skin absorption conditions due to different dosage forms.Powders，aqueous solutions，etc.，are difficult to be absorbed.The medicine in the cream can be absorbed in small amounts.Ointment and plaster can promote the absorption of drugs.Organic solvents（e.g.，dimethyl sulfoxide，azone）can increase the absorption of fat-soluble and water-soluble substances.
4.Pathological conditions
It is easy to penetrate for substances when skin is congested with faster blood flow or is physically or chemically damaged with increased skin permeability.Skin diseases that affect the stratum corneum can affect its barrier function and affect skin absorption capacity.Diseases of hypokeratosis such as psoriasis and eczema will weaken the barrier function and increase the absorption capacity.