by Paul Polak The single biggest reason that the appropriate technology movement died and most technologies for developing countries never reach scale is that nobody seems to know how to design for the market. Over the past 30 years, I’ve looked at hundreds of technologies for developing countries. Some provided elegant solutions for challenging technical problems. Some were big and clumsy. Some  were far too expensive. Some of were beautifully simple and radically affordable. But only a handful were capable of reaching a million or more customers who live on less than two dollars a day. If you succeed, against all odds, in designing a…