About five years ago, Harvard University chemist George Whitesides had a brainstorm. Because polymers can be constructed with chemical arms able to selectively grab onto other molecules, why not design an indigestible version that can rid the stomach of unwanted substances? Like fat, for instance. The idea spawned GelTex Pharmaceuticals Inc., which by year end hopes to win Food and Drug Administration approval for its first product: a polymer that grabs onto phosphate in the gut, then is excreted. That's vital for patients whose kidneys can no longer remove phosphate from the blood.