Inventing is a special skillset that allows the Engineer to create a wide variety of items to assist in and out of combat. While there are dozens of items that can be made, chemicals are arguably the most important for the engineer to master. Chemicals can be combined with Ballistics and Focus to help wear your opponents down.
Chemicals can only be purged from the bloodstream with a new curative known as domecap. This new fungi reacts differently depending on the number of chemicals in the bloodstream. A person with only one chemical in their body will find that it takes a long time before they can use it again. Whereas someone with many chemicals will be find themselves quickly recovered from the effects of the domecap. It is important to note that domecap and toadstools share the same balance.
Be aware that inventing is expensive skill to use. Every item, compound, and chemical has a cost associated with it.