HA, PMMA, etc. are dermal fillers that can be injected between the layers of the penis to make it thicker (it also is soft when flaccid and hard when erect). I've seen way more silicone'd dicks. Fillers don't have nearly the safety risk of silicone when done by a professional.
It's a relatively recent procedure (starting in the mid 2000's), but I'm surprised it isn't as big of a thing? .