A patent has been filed by inventor Gregory Rondinone of Connecticut for a knife that will shoot a jet of compressed air into whatever it punctures at the press of a trigger. The concentrated burst would be powerful enough to rupture soft tissue in organs, not to mention pump a ton of air (or some other gas) deep into places it shouldn't be. It's been suggested that the knife has valuable applications for underwater use, such as a diver staving off a large animal, both injuring the offending creature and rendering it unnaturally buoyant so that it floats away.

Still, a good ol' knife blade is a sturdy design that is usually only tinkered with in the realms of fiction. A vibroblade, or a knife with a vibrating blade from ultrasonic waves, for instance, has been in the lore of everything from the cult-classic RPG Rifts to video games like Metal Gear Solid. Combining a knife with a chainsaw is also a popular idea, the most famous example being the chainsword from the Warhammer 40,000 universe. There are several real world knife/weapon pairings, like a gun-knife, but they rarely focus on how to make the blade more deadly, as the compressed gas jet would.

Patent, via Wired Danger Room