Caffeine is not absorbed by the body the same way Nicotine is. Nicotine can be absorbed by the skin, aerosolized, liquid, solid, and is absorbed at 99% rate, regardless of method. Heat increases the absorption rate to increase perceived effects... Caffeine is best absorbed in liquid form, ingested into the stomach, and processed into the blood-stream by the kidneys/intestines. Ingestion has an absorption rate of 99%, but takes up to 45-min to feel the effects. Caffeine gum and sprays are absorbed into the bloodstream through the mouth and throat, but only at the level of 50-75% the rate of an equal volume ingested into the stomach. Caffeine "could" be added to e-liquids, but to get the equivalent of a cup of coffee, you'd have to vape 300mg (or a liter of 33% solution), and it would take 45-min for the effects to be felt. Basically, you'd have to drown in the stuff, and wouldn't get much benefit for the effort