How do hook-and-loop fasteners stick together?

1. Rrrip! Ava's shoe strap peels open, and with a press it sticks shut again. There is no glue and no button. So what is holding it?
2. Zoom in close. One strip is covered with tiny stiff hooks, like little walking sticks. The other is a tangle of soft thread loops.
3. Press the strips together. The hooks push in among the loops, and many of them catch a loop. Each catch is tiny, but there are hundreds of them.
4. Now pull the strips sideways. Every caught hook pulls on its loop at the same moment. They share the pull, so each one feels only a little. Together they are very strong.
5. Peel one strip up from its end. Now only the row of hooks at the edge feels the pull. They let go, then the next row, then the next. A small pull is enough.
6. That is the trick. When Ava runs and jumps, her foot pulls the strap sideways, and all the hooks hold together. To open it, she peels, and they let go row by row.

This is a strap magnified many times. Tap each part.

• Hooks. The hooks are tiny and stiff, shaped like walking sticks or mushrooms. They are springy enough to bend open and let a loop go when you peel.
• Loops. The loops are soft threads that stand up in little arches. There are far more loops than hooks, so a hook nearly always finds one.
• A hook in a loop. Here a hook has caught a loop. One catch can hold only a tiny weight, about as much as a few paper clips.
• Hook strip. All the hooks are fixed to a tough strip. The strip is sewn or stuck to one side of the thing you want to fasten.
• Loop strip. The loops are part of a soft fabric strip. It goes on the other side of the thing you want to fasten.
• Peel edge. The free end is where you start to peel. Lifting it pulls on just the first row of hooks.

Drag the handle up to peel, or to the side to slide. The spring shows how hard you are pulling. Watch the hooks in the magnifier.

Experiment: Which needs a bigger pull: peeling the strips apart, or sliding them apart sideways?

1. Drag the handle straight up and watch the hooks let go.
2. Let go so the strips stick again.
3. Now drag the handle sideways as far as you can.

What we found: Sliding sideways needs a far bigger pull. Sideways, every caught hook holds on together and shares the pull. Peeling pulls on only the row of hooks at the edge, so they let go one row at a time.

Turn on JavaScript to try it yourself.

Hang a 3 kg bucket with hook-and-loop. Choose where to fix it and how big the patch is. For 3 stars, use the smallest patch that holds.

Hook-and-loop fasteners were invented by an engineer who went for a walk and came home with prickly seed heads, called burrs, stuck to his clothes and his dog. Under a microscope, he saw that each burr was covered in tiny hooks that had caught in the loops of the cloth. He copied the idea with two strips: one of hooks and one of loops.

One tiny hook holding one tiny loop is very weak. It can carry about as much as a few paper clips. But a strap has hundreds of them. When you pull the two strips sideways, every caught hook pulls at the same time. They share the load, like many hands on a rope, so together they can hold a lot.

Peeling is different. When you lift one end, only the hooks right at the edge are pulled. Those few let go easily. Then the next row is at the edge, and it lets go too. You never fight all the hooks at once. That is why you can open a shoe with two fingers.

This is why straps are fitted so that the load pulls along them, not away from them. And it is why an old strap gets weak: fluff fills the hooks and some loops break, so fewer hooks can catch. Cleaning out the fluff helps. The burr comparison is not perfect: a burr's hooks grab anything fuzzy, but a hook strip is made to fit its own loop strip.

Key words

Hook

A tiny stiff, curved piece of plastic that can catch a loop.

Loop

A tiny soft arch of thread that a hook can catch.

Shear

A pull that tries to slide two things past each other.

Peel

Lifting one strip away from the other, starting at an end.

Safety factor

Making something stronger than it needs to be, just in case.

Check your thinking

1. What holds a hook-and-loop strap shut?

   Show the answer

   Hundreds of tiny hooks caught in tiny loops. There is no glue. Each hook catches a loop, and there are hundreds of them.

2. Why is peeling the strap so easy?

   Show the answer

   Only the hooks at the edge are pulled, one row at a time. You only ever fight one row of hooks, never all of them together.

3. You want a strap to hold a heavy bag. Which way should the bag pull on it?

   Show the answer

   Along the strap, so all the hooks share the pull. Pulling along the strap is the strong direction. Pulling away from it peels it off.

4. Why does an old, fluffy strap not hold as well as a new one?

   Show the answer

   Fluff fills the hooks, so fewer of them can catch a loop. Fewer hooks catching means fewer hooks sharing the pull.

5. Why does the peel force not depend on the length of the strip?

   Show the answer

   Only the row of hooks at the peel front carries the load at any moment. However long the strip, the pull acts on one row at a time. A longer strip just takes longer to peel.