leading edge?

I think the leading edge of a razor is too sharp to touch. But what is the leading edge here?

Context:


Stretching a moistened material contained in a strip on the leading edge of disposable razors demonstrates a long-mysterious phenomenon that causes some fluids to form beads while others do not. Whereas beads form when a bit of saliva is stretched, they do not form when the material on the razor strip is stretched. Researchers have learned why the beading occurs, which could help improve industrial processes and for administering drugs in "personalized medicine." (Credit: Gareth H. McKinley/MIT)