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How do henna 'tattoos' work?

June 8, 2009
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Q. How do henna "tattoos" work? They're not permanent like an ink tattoo, but they also don't wash off like a pen mark.

A. Henna tattoos are made by making a paste out of leaves of the henna plant and something acidic like lemon juice. The leaves alone won't stain, but acids release a dye called lawsone, which is what actually does the job.

Lawsone molecules form very strong bonds with protein molecules and also penetrate the skin quite well. If you put some lawsone on your skin, which has a lot of protein, the lawsone will seep in deeply and stick to the proteins.

If you apply a paste with lawsone on it and leave it on for a while (traditionally many hours) you will have stained all the way through your outer skin layer. Note that this is more than just the outer layer of dead skin cells - this dye goes well into the living skin.

If you just draw on your hand with a pen, it's very unlikely that the ink will penetrate far, so it usually washes off, perhaps with a little of the upper layer of dead skin cells, with little difficulty.

For a properly done henna stain to disappear, you have to wait for the outer layer of skin to slough off, which it does naturally over a period of days.

A real ink tattoo involves injecting ink below the epidermis and into the dermis, where it sits pretty much forever.

The epidermis is continually being shed and grown back, but the layer of skin below, where the ink is put, is not shed, so the tattoo is permanent.

Dr. Knowledge is written by physicist John Swain of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.