>>11247543>https://www.ncbi.nlm.nih.gov/pubmed/7361738>https://www.ncbi.nlm.nih.gov/pubmed/660741/Done on children with micropenis, but still interesting. There's definitely the potential for growing the penis by manipulating it with the right hormones and chemicals, it just isn't clear how to do so in an adult. The last link is interesting also, it suggests that by inducing a controlled priapism you can achieve growth of the penis á la megalophallus (people who have natural priapisms during puberty sometimes develop megalophallus). They also used a mixture of chemicals break down the fibrous tunica that envelop the erectile tissue with the intention that this would help the priapism stretch the tunica out.
Maybe various collagenases (or other chemicals that break down the tunica) coupled with pumping at low intensities could deform the tunica giving additional volume to the penis.
Then there's also various hypoxia inducible factors (HIFs) that have been speculated as the cause for megalophallus. They cause (amongst other things) angiogenesis when the oxygen levels are low in the cell. Priapisms can result in low (dangerously low sometimes) oxygen levels in the penis causing HIFs to work. I recall a study where rats had their penises clamped to induce hypoxia the HIF levels were elevated after 4-5minutes.
