I want to help prevent self harm. I need to know how to detect it before it happens. I was thinking of having a sensor in a knife that would detect when your about to cut and stop you?

Most knives are used for other things then self harm, so that won't work. Also, not that I am an expert on the subject, but from the few people I have had interactions with that did self harm, it seemed they used razor blades. They are easier to conceal and generally make cleaner cuts.