Why 65-Micrometre Paper Causes the Worst Paper Cuts

It has happened to nearly everyone. You are quietly sealing envelopes or loading a ream into the printer when-tsssss, ouch!-a brand-new paper cut appears, spilling ruby-red human juice across an otherwise spotless white surface.

Despite their everyday nature, the physics behind paper cuts remains surprisingly poorly understood. Somehow, the edge of a material that can be crumpled and torn with ease can slice into skin deeply enough to draw blood.

Why 65-micrometre paper cuts are the worst

Physicists at the Technical University of Denmark have now identified a defining trait of the most troublesome paper cuts: sheets roughly 65 micrometres, or microns, thick.

That category includes dot-matrix printer paper, newspapers, and the paper used for certain magazines and scientific journals.

It may seem logical that a finer edge would be more dangerous. Yet extremely thin papers, including tissues, are so weak that their edges buckle as soon as they meet skin. At the other extreme, thicker sheets are simply too blunt. At 65 micrometres, paper occupies the Goldilocks zone for paper cuts: sturdy enough to retain its shape, while still thin enough to form a cutting edge.

"These results allow us to assess the relative safety of various product categories broadly," physicists Sif Fink Arnbjerg-Nielsen, Matthew Biviano, and Kaare Jensen write in their paper.

"While tissues, books, and photos are generally safe, we cannot rule out certain risks of using office paper or magazines. In the future, paper manufacturers, printers, and publishing companies may wish to consider this during the product design process."

Testing paper thickness against gelatin

Paper cuts are not generally regarded as serious injuries-and in most cases, they are not. Usually, they cause brief pain and irritation before we forget about them and carry on.

Mostly.

For some patients, however, they can be genuinely hazardous. In any event, nobody likes getting a paper cut. The underlying physics also presented an intriguing puzzle, prompting Arnbjerg-Nielsen and her colleagues to investigate.

In their experiments, the team used paper sheets of varying thicknesses and attempted to slice them through freshly prepared blocks of gelatin designed to mimic the consistency of flesh. A machine moved each sheet over the gelatin at precisely controlled speeds and at several angles, while a video camera captured the outcome.

The tests showed that thin paper merely buckles under pressure and fails to make a cut, whereas thick paper leaves only an indentation in the gelatin.

The researchers then created a formula to establish the paper thickness that cuts most efficiently through human flesh. They found that paper between 50 and 100 micrometres poses the greatest potential danger, with 65 micrometres as the ideal point.

Papermachete and safer paper handling

As the researchers note, the results could support the creation of paper products less likely to bite back. But the findings can also be applied in the opposite direction. Using their research, the team made a low-cost paper scalpel called the Papermachete, which cut through apple, chicken, capsicum, and banana peel.

Papermachete is not about to replace metal knives, but it can be useful in a pinch, which is rather neat. The 3D-print files for its handle are available free here.

Further research is required to clarify how the cutting angle affects the ease with which paper can slice humans. Until then, greater care when handling paper may help you take control of your own fate.

"It is worth pointing out," the researchers write, "that the habits and dexterity of the user also play a role. In particular, most paper cuts can be avoided by adhering to a strict near normal-contact regimen, which minimizes the cutting likelihood."

We think they have just called us clumsy. Rude.

The research will be published in Physical Review E.