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on the scale of a world war, prosthetics are still needed to replace limbs lost or amputated through accident and disease.The U.S. popu- lation includes more than one million amputees, with an estimated 100,000 lower-limb amputees added yearly.And in some parts of the world, there is a residue of war that maims thousands of people a yearthe unexploded land mines strewn around many countries, from Afghanistan to Mozambique. Land mines are cheap and effective weapons, and estimates range up to 100 million of them buried in 62 countries, with Cambodia having one of the densest concentrations. The result is that the business of making prostheses, along with the allied industry of orthotics (limb braces), is estimated to be a $2 bil- lion undertaking worldwide.

This industry has seen significant technological progress.Where metals are used, they are the lightest available, including titanium, but increasingly they are replaced by new materials such as graphite com- posites like those used in tennis rackets, and plastics, which can be formed into natural-appearing limbs. The mechanical systems that articulate the limbs have also been improved, using pneumatic or hy- draulic fittings to provide smooth motion. Some artificial legs are good enough that their wearers can enter athletic events with satisfy- ing performances, such as a time of 12.4 seconds for the 100-meter dash turned in by one runner equipped with a prosthetic leg.

The power sources that move artificial limbs have become more sophisticated as well. Energy-storing artificial feet, designed in the 1980s, incorporate a spring that compresses as the foot strikes the ground, and then extends to release the stored energy and help propel the leg into the next stride forward. Extremely small electric motors have also been developed. Some of them are tiny enough to fit into artificial fingers and hands and powerful enough, for instance, to pro- vide a grasping function, while drawing so little electrical energy that battery operation seems feasible.

However, no matter how effective the engineering and aesthetic design of an artificial arm or leg, it still lacks an important capability. An artificial leg has no sensors to test the nature of the walking sur- face in order to adjust pace and maintain balance, nor does it receive

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