8.4.1.1 The first generation：passive diffusion-based TDD
The first generation of TDD technology selects suitable drugs or simply chemically modifies existing drugs to achieve transdermal effects.The first generation of TDD drugs are generally highly lipophilic，with logP＞4，small molecular weight（MW＜400）so that they spread quickly through the skin barrier.Therefore，the categories of the first generation of TDD drugs are extremely limited.(https://www.daowen.com)
