It's still up for review by Guinness World Records, but it's a pretty safe bet that this Gillette ad, written on a space less than 100 microns short across a human hair, is the smallest yet.
First step: French ad agency CLM BBDO got someone to shave, collecting the resulting hair samples in a bowl. The samples were then washed and all traces of soap and water were filtered out, and one lucky hair was plucked up and placed under an electron microscope.
The area for the ad is less than 100 microns across, etched by scientists at the University of Nottingham in the U.K. To get an idea of how small that is, 250 micros would be about a quarter of a millimeter thick. So, tiny. Then, using some big, mysterious machine, the crew used ions to etch the ad.
This is what you're left with (click to see it larger — as large as a 100-micron-across ad gets):
Pfft, a big, ion-shooting machine? They should have just used the world's smallest 3D printer. See a video about the creation of the ad down below.