Second time I come across this idea...

The first time was in a sci-fi book, I think Snowcrash by Neal Stephenson. That one also had a strong soporific effect, which I believe makes it much more useful as the use of force is the first thing one wants to avoid in case of rape, and more so in the event that one retaliates...
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Femdefence — an idea for a rape-protection device

The project Femdefence is an attempt to contribute to the debate on men’s sexual violence against women in society, using the esthetics of design, and the semiotics of marketing.

Femdefence is an on-going project first presented in 2003. The project includes the creation of an imaginary product, which bears the project’s name. The “product“ is a kind of protection against rape, somewhat similar to a tampon in that it’s user carries it inserted into her vagina. The basic idea is that the woman carries the protective device in her vagina. In it there is a sharp pin which has a penetrating effect on the perpetrator’s penis in the event of a rape. The construction is such that the pin still cannot injure the bearer.