Medicae has 2 special uses: First Aid and Extended Care.
First aid description states:
Special use: First aid
The Medicae Skill performs first aid for the injured, removing a small amount of damage by suturing lacerations, bandaging abrasions, and plugging punctures. A successful Medicae Test removes Damage equal to the Explorer’s Intelligence Bonus on lightly damaged characters or 1 damage point from heavily or critically damaged characters.
Skill Use: Full Action.
The fact that says that it's a full action means that it can be used in combat. How many times can you use this skill on a single target while in combat?
Besides, can you use this skill out of combat? If so, the characters could simply spend some time to top off their wounds and remove all their critical damage?