What kind of drugs can be absorbed through skin? Some drugs such as nicotine can be administered through skin. I thought the layers of skin are designed to prevent in-flow of any chemical/germs. Not all drugs get absorbed in this fashion. So do drugs like nicotine have any specialities(such as a special chemical group) that fool the barriers of skin? A link to an article/site would be very helpful, or even a book recommendation.
The skin is able to prevent some things from passing in and out of a person but it is not a perfect barrier.

The lipophilic things like nicotine, hydrogen flouride, VX, hexane and iodine vapour can pass through undamaged skin. These things are able to dissolve in the fatty parts of skin. Then they diffuse in through the skin into the person. Thus they enter the person.

Some water soluble ionic things can enter / leave a person through the skin, this can occur through the sweat pores.

For example cesium in humans does come out in sweat, in a 1980s case in Brazil where a lot of people were internally contaminated with cesium-137. It was found that cesium kept appearing on the surface of the skin of the contaminated people.