A British YouTuber, who goes by the name Colin Furze, created retractable Wolverine claws that are sharp enough to slice a watermelon with one swift movement. He created the pair just in-time for the upcoming release of X-Men: Days of Future Past.
Despite the fact that these claws don't gruesomely emerge from under your hand's skin, they are probably the most realistic-looking claws that have hit the Internet or costume market to-date. The claws are 12-inches in length and are fully automatic, thanks to the portable air tank that is connected to them. Thus, they can "deploy" with the simple grip of a button.
Furze teaches his subscribers to make them with stainless steel, but if you want to be more cautious about the safety of others, maybe try out a less dense and less sharp material.