On level 3 you have to take a medical-device from the floor.

But there is not written that you have to use that device later on
in the game!!!!!So many people that read that walkthrough use that
device to heal their selves!! So don't!!!
almost in the end of the game you will need it!!!!!!!!!!
Please make a note of that in your walkthrough.

Thanx,    Benito DeJong