By Cat DiStasio

In the grand scheme of things, mobile phones haven't been around that long. Less than 20 years ago, hardly anyone had one, and devices of that era certainly weren't capable of doing what today's smartphones can do. Cell phone technology progresses so quickly, in fact, that most users are prone to replacing their phones with new ones every few years -- even if the old one still works -- just to have something with longer battery life and a better camera. This behavior is inspiring designers to create more sustainable handsets: some with easily upgradable modules, some made with recycled materials and some that charge themselves with little or no interaction from the user. Although a few of these technologies are still in the conceptual phase, it's easy to imagine that they could be available within the next few years -- right about when you'll be thinking about replacing the phone you have now.