[FenrisFrost]

We have a character in our group who is a min-maxed ubertwink troll, and we often like to wax poetic on the numerous ways we would all attempt to defeat him. One of them was to just spam tranquilizer patches on him.

He quickly countered that, because he wears full body armor and a helmet and mask, that the patch would harmlessly touch his armor, and not him.

I found a way around this by secretly putting ten wireless tranquilizer patches inside his armor, since he lives in my character's apartment anyway. But thoughts / relevant rules as to why this is or isn't true would be appreciated.

[ZeConster]

I think he's correct:

Slap patches are adhesive dermal drug dispensers that allow continual, safe administration of necessary chemicals. They are applied directly to the patient’s skin. Applying a slap patch to an unwilling patient requires a successful melee attack (which deals no damage), which may be tricky if your target doesn’t have much exposed skin (Called Shots, p. 195).

Of course, I don't see a "bypass armor" option under Called Shots, so I'm not sure why they mention that section, but the rules definitely say you need to touch the skin.

[Xenon]

No exposed skin = immunity to hostile slap patch.
Only little exposed skin = unarmed combat with -4 dice.

[FenrisFrost]

I'm glad I planned ahead and placed them all inside his armor while he was asleep then. Heh.

[ZeConster]

4e Arsenal had the possibility of giving grenades Pilot programs and movement capabilities - although very expensive, if you use it on a bunch of minigrenades, you could have them sneak into his bedroom and all blow up at the same time. Who knows? Maybe 5e's version of Arsenal will have something like that too.

[Insaniac99]

I had the impression that Full Body Armor had exposed skin unless they had sealed system and it was activated.  I always imagined the helmet closing down or something.

One nice thing about slap patches is you get +2 on your to hit so when you have to make a called shot you are only at -2