There are plenty of fancy self-defense tools available on the market these days. You can even buy iPhone cases with a stun gun or pepper spray built-in. The HemiSERE Titanium Lapel Knife is a tiny tool you can wear under your collar or anywhere else on your shirt. It weighs only 4 grams, with a thickness of 1.3mm.
You can stick the knife to the back of your credit card or store it inside your smartphone case.[where to get it]