Though the simple mechanics of the modern bicycle seem to leave little room for improvement, new materials and technologies continue to refine this human-powered machine. Our suggestions for your next zero-gallons-per-mile vehicle are functional, socially responsible and sustainable.

Bamboo bikes

Elite bike builder Craig Calfee first developed a bamboo bike prototype in Santa Cruz, where its functionality and earthy appeal made it an instant success. After a trip to Africa, he realized that locals could manufacture bamboo bikes to provide a source of income through exports as well as affordable and non-polluting transportation to the communities. With help from Zambikes, the Peace Core, and Cyclists for Cultural Exchange, "Bambooseros" shops have opened in Ghana, Zambia, Mexico and Uganda and there are plans for more in the Philippines, Cambodia, and El Salvador. Not only does buying a bamboo bike support the green economy in poor countries, but the bikes themselves are also remarkably robust: bamboo is shock-absorbent, as strong as carbon due to its hollow interior and fracture-resistant nodes, doesn't rust, and costs a whole lot less. Ironman triatheletes even testify to riding faster on bamboo bikes than carbon frame ones.

The Contortionist Folding Bike

Lack of space to lock or store bikes in cities is one of the biggest barriers to would-be bike commuters. One solution: a design that allows you to fold up your bike and take it with you wherever you go. While folding bikes have been on the market for years, their small wheels, bulky profile, and greasy chains are not an ideal solution. But the Contortionist Folding Bike seems to have solved these problems. A full-sized bike that can be manufactured in existing bike plants, with hydraulics instead of a chain to power the back wheel, and a sleek form that can unhinge and twist to the size of a single wheel, this bike could be the one that makes folding bikes mainstream. Although it is still in prototype phase, and its long-term durability is still questionable, manufacturers and bikers alike are lining up.

Electric bikes

Electric bikes haven't been so popular in the past because of design flaws: they were often heavy, sometimes unreliable, and they looked clunky. But slimmer, lighter and faster models are popping up, making this mode of transportation a true solution for long, hilly urban treks. The Schwinn electric bikes are particularly pretty, with 18 mph speed and a battery that lasts three years (most electric bike batteries need to be replaced annually). It is, however, a little more costly. At a lower price point -- albeit with some extra weight and a battery that needs replacing more often -- is the EZip Electric from Walmart. For only $350, it's certainly cheaper and healthier than a gas guzzler, and it can travel up to 25 mph for 25 miles on one charge if you use the motor only for hills (10 miles if you use it liberally).