8.1　Introduction of Transdermal Drug Delivery

Transdermal drug delivery（TDD）refers to a skin-mediated approach to administration that allows drugs to be delivered to target tissues through the skin or further into the systemic circulation and corresponding lymph nodes through capillaries and lymphatic vessels，resultantly exerting a local or systematic therapeutic effect.Compared with traditional administration approaches（e.g.，oral administration，subcutaneous injection，intradermal injection，and intravenous injection），TDD has its distinctive advantages：①TDD can avoid the gastrointestinal adverse reactions and the first-pass effect of liver caused by oral administration.②In contrast with traditional injections，TDD is mostly noninvasive or minimally invasive，with little pain，no needles left，easy operation and selfoperation，which may effectively improve patients'medication compliance.Finally，TDD is highly controllable because the dosage and time of administration can be flexibly adjusted to ensure the safety of medication.So far，the US Food and Drug Administration has approved more than 20 drugs for TDD，bringing a global TDD market worth up to $32 billion annually.(https://www.daowen.com)