Presumably, the screen would be made out of a flexible material, allowing it to roll up inside the phone without damaging it, but also sturdy enough to have some rigidity when extended. In my mind, I think of something like those snap bracelets which were a fad about 15-20 years ago. A feature like this will not only allow you to have a larger web browsing area, but also be able to play games that require more viewing area and perhaps even run applications like Photoshop as the specs of phones continue to increase.
Phones, after all, are moving down the track into becoming viable personal computers in themselves, and a concept like this would perfectly blend the portability of a typical phone, the innards of a powerful computer, and the visual area of something like an iPad. You can see that the concept pictures make this out to be truly tiny, able to fit into one's wallet much like a credit card. Impractical at the moment, sure, as I can't see how one would fit the processing power into such a tiny phone along with the screen and a battery capable of powering the phone and the larger screen.
Regardless of practicality, ideas like this will mark the way for future phones and it will be interesting to see how things change in the near future. After all, just picture the phone you had 10 or 15 years ago and compare it to something like an iPhone or Droid. For more incredible phone concepts, check out the Line Phone or the dual-screened AVA Concept Phone.
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