This is hard to believe -- guess I'd have to actually see and wear one to get over my skepticism:
Kranium is a bike helmet that is made from the same cardboard used for the boxes you find at the supermarket. This material, along with some clever construction, turns out to be a lot better at absorbing impact than the more usual polystyrene-filled lids.
Anirudha Surabhi’s design absorbs four times more impact energy that the polystyrene equivalent, and — unlike regular helmets which break on impact — it survives longer. One Kranium was smashed five times in a row and still passed the British Standard (EN 1078) test.
What about rains and sweat? No problem. Surabhi’s helmets mix the cardboard with a “waterproof acrylic compound” which makes them just as rain-resistant as the helmet you have now.
The Kranium is better than regular helmets in another way too. Because they are easy to build to order by scanning the buyer’s head and cutting the cardboard to order, it fits perfectly. This means that it will stay in place if you crash, offering much better protection.
Cardboard Bike Helmet Better than Plastic | Gadget Lab | Wired.com
Here's a video about the introduction of the design at a cycle show:
Cardboard Bicycle Helmet » Cyclelicious