Here's Yanko Design's O2 Magnetic Dumbbells and as it's name implies, it uses magnets.

It works by splitting into four bracelets, and you wear one above each elbow and another below each one. To get a workout, try pushing the two together and feel the pressure of them repelling one another. Each arm holds two bracelets of repelling electromagnetic force which can be adjusted with a small dial on them.

It's interesting, but it does have to prove safe to use. After all, if these work, you won't need them traditional dumbbells anymore!

Yanko Design, via The Huffington Post