Shields meant to be used as protection for circuits and often used in bunkers. SHLD does not conduct heat. SPRK makes it grow, and it is destroyed by excessive pressure. 
When SHLD is sparked, it will spread to all empty spaces surrounding that spark. It will also increase in level (SHLD->SHD2->SHD3->SHD4)
All types except SHLD will fill surrounding empty spaces with the level below. For example, SHD3 will surround itself with SHD2.

SHLD (Element ID: 119, level 1) destroying at 7 pressure
SHD2 (Element ID: 120, level 2) destroying at 15 pressure
SHD3 (Element ID: 121, level 3) destroying at 25 pressure
SHD4 (Element ID: 122, level 4) destroying at 40 pressure