(Photo: Wei Tchou)
Technically speaking, Ziploc is a brand name. Whatever the brand, the design concept is the same: a bag that separates and reseals with a plastic zipper. Mechanically speaking, it's quite complex--more than you might think for a cheap, disposable product.
(Image: US Patent Office)
Danish inventor Borge Madsen developed the practical resealable bag in 1950. It's a zipper with tiny hooks. Outside runners help guide your fingers so that you align the two sections properly. Bruce Peterson decsribes the history and technology of the resealable bag in Wired:
The zipper features microscopic J-shaped grooves (“hooks”) and arrowhead-like stems that interlock. Look closely at a double zipper bag and you’ll see this line of tiny teeth on the upper row, toward the lips of the bag. Running your thumb and forefinger along the track clasps the hooks around the stems. The zipper clicks at about 50 decibels to let you know you’re doing it right. But misalignment due to your clumsiness can create gaps.
So to solve this, Ziploc added a row of Xs and opposing convex dimples that create a haptic interface to guide your fingers for proper alignment. “I liken it to walking on a sidewalk with holes,” says one Ziploc researcher who can’t give his name because parent company SC Johnson won’t let him. “Suddenly there’s a hole and you step down and then you step back up and then down into another hole. [It] creates the sensation of the teeth—the bomp-bomp-bomp-bomp.”
-via Debby Witt