How to upgrade deffenders

[dannyfox]

Hi everyone, im playing this game for 6 years, and i've recently started to play it again, got a new profile in atomic age, but a still dont understand, how does the shield appears on the deffenders?
At first i saw ot on tanks from bunkers, but it spreads also on troopers and main tanks

What should i upgrade to get this option too? Couldn't find the answer in wiki

 

[King Crimson]

This is a research in the new Manufactory building.

 

[dannyfox]

This is a research in the new Manufactory building.

Thanks a lot