This is an interesting subject...I don't think that any body part that contains some form of erectile tissue can be permanently increased in size or made to stay erect. Yes I find erect nipples a total turn-on and enjoy some nipple stimulation myself. I have found that a nipple bulb, which is not really, a permanent nipple enhancer can be a lot of fun to play with, it is a suction device that gently sucks the nipple. I have worn them for about an hour or so at a time and my nips (I am a guy) get pretty huge considering their natural state. As a matter of fact it was a bit disconcerting the first time I used them, as I did not want a permanent change like that. The up side was just how sensitive they became and the act of wearing a shirt for the next day or so was well quite frankly arousing.
You might want to try some of these babies out for kicks but unlike the Marines “the change is not permanent”. The other thing is to wear some sweater bumpers that basically clamp around an erect nipple and keep the area engorged with blood…much in the same manor that a Cock Ring works for guys…. Have fun, Play safe and Take pictures!

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Sex is a tricky thing best left to professionals.