As you work your way through the hospital you will get specific items you are going to need to get your hospital scrub sets.
These items are only found at the Hospital currently. Once you have Empty Syringes you can start to extract blood from the zombies. You can extract blood from the same zombie multiple times before it no longer allows you to do so. As long as you have Empty syringes in your inventory you will not be able to scavenge the zombies for their brains. If all of your syringes are full off infected blood you can scavenge their brains. I recommend gathering up brains before you even head to the hospital to work on your scrub sets. Or set up a shack etc... close to the hospital where u can farm them at your leisure.
Once you have the 4 parts you need you can discover the recipe for the "Treated Brain" you don't actually need to know the recipe because you place these items in a medical station and it turns them into the treated brains on it's own.
1 Syringe of H1Z1 Infected Blood 1 Vial of H1Z1 Reducer 1 Empty Specimen Bag 1 Infected Brain
This is the Medical Station you will place the ingredients in. There are a few scattered throughout the hospital's Second floor.
Once you have created 10 Treated Brains you can go to a Specimen Box and click on it. It will give you One part of a coded message. Place the Coded Message in your discovery to learn the recipe for the Airdrop Code.
You will need 4 parts of the coded message to make one air drop code.
It takes 40 treated brains to make one air drop code. One Airdrop will give you one piece of the learnable hospital scrub sets, It's random what you will get. There are 4 different sets all of them are 3 pieces each.
That's a LOT of math ;)
Once you have the Airdrop Code go to one of the 4 Longwave Radio locations, use the airdrop code, and, collect your scrubs. When you have all 3 pieces of a set put them in your discovery and you will learn the permanent skin, these are account bound skins and can not be traded or sold on the market. They don't even appear in your steam inventory.