WASP Injector Knife
No matter how far personal weaponry advances, the last line of defence has remained the same. If a threat gets really up close and personal, divers, soldiers, campers and hunters all have to rely on their trusty knife when all else has failed. The trouble is, if your shark, elephant or enemy combatant is close enough for you to stab them, they're well within range to lay some serious hurt on you if your knife aim isn't true - and there are plenty of angry animals out there capable of disposing of you even with a blade hanging out of them. That's why WASP have created the Injector Knife, which forces a massive charge of freezing compressed air into the stab wound. WASP claims the shock and tissue freezing can stop the largest of land predators in its tracks, and it's even more effective on underwater predators.
The WASP knife is fairly simple in principle; the removable handle contains a bulb of compressed gas, and there's a thumb button which releases that gas almost instantly through a thin tube that exits near the point of the blade. The gas is expelled at around 850psi of pressure, and expands to around the size of a basketball inside whatever the knife has been poked into. The handle unscrews to fit new gas bulbs, and it's easy to carry more than one handle if there's the chance that the knife's lethal charge could be needed more than once in relatively quick succession.
The initial gas release is extremely cold, and can snap-freeze all tissue and organs in the area surrounding the blast, which has a devastating and instant effect on the hapless victim. The effect is compounded if used on an underwater predator - not only is there the instant shock and freezing effect, but the large injection of air causes the recipient to float upwards, and as it rises and the atmospheric pressure decreases, the ball of air continues to expand with catastrophic consequences.
The WASP Injector Knife is also virtually silent when discharged inside an enemy's flesh - not that the enemy can be expected to remain silent through such internal trauma - but it effectively maintains the knife's potential as a stealth killing option while massively multiplying its lethality.
The WASP Injector Knife system is available through the WASP website for US$380. It's still a last resort for when all else has failed and a big, nasty threat is virtually upon you - and you'll still need to be smuggling coconuts in your shorts to have the nerve to stab and hit that button at the right moment - but if I was standing there with a knife in my hand and an angry brown bear advancing on me, I'd want that knife to be one of these.
Here's a video of the knife in action.