The results of transdermal diffusion experiments are influenced by various factors and can be mainly classified into the following four categories:

1、 Drug properties

• Molecular size: Small molecules with a molecular weight less than 500 daltons (such as menthol and niacinamide) are more easily transdermal, while large molecules (such as collagen and sodium hyaluronate) face greater challenges.

• Lipid water partition coefficient: The components need to have a certain degree of lipophilicity (to penetrate the stratum corneum) and hydrophilicity (to enter the dermis layer), and the balance point is crucial.

• Dissociation degree and stability: The chemical stability of components in the testing environment and whether they exist in a molecular state directly affect their transdermal behavior.

2、 Skin condition

• Skin integrity: Weakened skin barrier function in damaged or inflamed areas may increase the speed and degree of drug absorption.

• Skin thickness: There are differences in the thickness of the stratum corneum in different parts of the skin. For example, in the scrotum, the stratum corneum is thin and has many appendages, resulting in fast and large drug absorption, while in the palms and soles, the stratum corneum is thick, making drug absorption relatively difficult.

• Skin temperature and humidity: An increase in temperature can cause blood vessels to dilate and blood flow to accelerate in the skin, while also increasing the moisture content of the stratum corneum, which facilitates drug penetration; When the humidity is high, the hydration of the stratum corneum is enhanced, which can also promote drug absorption.

3、 Experimental conditions

• Temperature: As the temperature increases, the diffusion coefficient and solubility of the drug in the skin rapidly increase, and the transdermal rate increases. When the skin temperature rises by 10 ℃, the permeability coefficient can increase by 3-10 times.

• Humidity: Adequate humidity helps keep the skin surface moist and facilitates the penetration of certain types of drugs.

• Pressure: When the skin is compressed, it may alter the blood circulation and structure of the stratum corneum, thereby affecting the diffusion of drugs.

• Application area: The larger the application area, the more the total amount of drug absorption may be, as more skin surface area is involved in the drug absorption process.

4、 Other factors

• Dosage form and excipients: The form of drug preparations (such as patches, creams, etc.) and the additional ingredients they contain can also affect transdermal absorption. Certain excipients can serve as penetration enhancers to enhance the drug's penetration ability; And other substances may have a protective effect, reducing drug loss or rapid release.

• Individual differences: Physiological characteristics such as age, gender, and race can also affect the effectiveness of transdermal absorption. For example, children and the elderly have weaker skin barrier function and are more likely to absorb external substances.