The skin is essentially a membrane barrier to foreign

The skin is essentially a membrane barrier to foreign pathogens such as bacteria, viruses or exogenous allergens. Generally only very few non-polar small molecules (molecular weight < 500 Da) can passively cross the barrier by the skin. Therefore, the delivery of various drugs with different physicochemical properties or active substances, especially biopharmaceutical macromolecular drugs (polysaccharides, peptides, proteins, [0004] enzymes, nucleic acids, etc. with extremely rare molecular weights of ~104Da, ~105Da, or even ~106Da) to the deeper lesion areas of the skin is very challenging, and is also a research challenge and focus in the field of transdermal drug delivery.