8.2.1　Skin barrier and TDD pathways

The skin barrier is a critical physiological structure that hinders TDD.The stratum corneum，a dense structure consisting of dead cells（mainly containing keratin and lipids），is the main component of the skin barrier，with a thickness of 10-20μm.In the stratum granulosum below，cells are tightly connected to further obstruct the drug penetration（see 1.2.1 Barrier function，2.1.1 Skin barrier and extended reading 1 in this chapter for details）.

There are three main pathways for transdermal absorption of drugs passing through the intact skin barrier（Figure 8-1）.(https://www.daowen.com)