Most people expect to break in a new pair of shoes by wearing them for a few weeks until the material softens and stretches to fit. But the shoe designer Martin Keen has a better idea.

In April, Keen will launch Mion Footwear, a line of mass-produced shoes, designed for sailing and water sports, that promise rapid custom-grade cushioning. Like a memory-foam mattress, Mion insoles compress after about 10 hours of normal wear to assume the unique contours of the owner’s foot – right down to your inward-curling pinkie toe. The transformation is permanent, and in the end, the shoes fit no one else, hence their name, which is pronounced "my own."

The secret lies in a material that Keen calls ergomorphic foam – an off-the-shelf polyethylene that is blasted with nitrogen. For years, medical-supply firms have used this material in shoes intended for diabetics, who must avoid developing blisters on their feet that could, if neglected, turn gangrenous. But Keen, a competitive sailor based in Rhode Island and a co-founder of Keen Footwear, realized that the material might also appeal to kayakers, yachtsmen and other so-called amphibious athletes, by more snugly locking them into their shoes. (Mion’s nonmarking soles also become stickier in water.) "You get increased performance," Keen explains, "because you are in the product, not on the product."

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