Applications of Diffusion

Being such a broad topic, it's rather hard to approach the subject of applications of diffusion; as diffusion is not often used as a tool, but rather it is a phenomenon which has to be dealt with. A few of the topics listed below deal directly with materials processing applications, and a few may be surprising.

Diffusion Bonding
Diffusion in Geology
Diffusion in Vegetables
Heat treatment of metals
Helium Diffusion in Stars
The Human Body