Anything placed at the bottom of your feet would be undetectable.

Did the government put these machines through a complete test to determine if they were effective against an enemy that is "clever and ever evolving"?

We don't even know for sure if it would have caught the underwear bomber.

I believe that there are many ways to defeat the machine. It is too late now for the government to back down and admit error. So in true government fashion, charge forward and worry about things later.