Google Inc. is designing tiny magnetic particles to patrol the human body for signs of cancer and other diseases, in the latest example of the Internet giant’s sweeping ambition.

Google said its nanoparticles, less than one-thousandth the width of a red blood cell, would seek out and attach themselves to cells, proteins or other molecules inside the body. The company also is working on a wearable device with a magnet to attract and count the particles, as a monitoring tool.

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