Up to 50%. Injured body parts won't be treated.
Next wave, if injured (2-use). Doctor's treatment does not interfere with the item.
"Erm, I've got a Band-Aid left if anyone needs it."
First Aid is an unlockable Equipment which can be used in one of three equipment slots available per Trooper. It allows the dummy to regain up to 50% of his initial Life Points at the end of the each wave where he sustains Damage. Actual injuries are required to trigger use of the item. It is worth to note that Doctors' manual treatment does not interfere with this piece of equipment, meaning that it still can be used even if the owner has been healed previously.
It synergizes well with Survivor, letting a dead trooper revive once with 50%+1 life point (this is in question, please update with evidence for or against), as well as any LP-enhancing skills which increase the chances to sustain serious enough injuries to trigger the ability without the trooper dying from them, as well as increasing the LP recovered.
When used the trooper appears to be wrapped in bandages, like the Doctors' manual treatment.
Can be used twice per battle.