Leonardo da Vinci proposed silk for the wings of his ornithopter.
His flying-machine studies borrowed ideas from bird and bat anatomy, especially the relationship between wing movement and body weight. In several designs, he considered lightweight materials suitable for a human-powered machine.
Silk was attractive because it was light and could cover a framework of reeds or other supports. Leonardo also studied bird flight extensively, but his proposed machines lacked an engine capable of producing enough power for sustained human flight.
An ornithopter is a machine intended to fly by flapping wings, not by using fixed wings like an aeroplane. Leonardo's ornithopters remained designs; he did not successfully fly one.