Nicola Pugno, at the University of Trento in Italy, has succeeded in making by far the toughest material in the world — by taking a conventional piece of fiber… and tying it in a slip knot. This method seems (and is) so simple that the inventor is calling it the Egg of Columbus, which refers to a discovery that has eluded mankind forever — but seems incredibly obvious and easy after the fact.
If you weren’t a boy/girl scout, a slip knot is essentially a loop of rope/string/thread passed through a bight (a u-shaped section of the same rope/string/thread). For a bit of extra flavor, you can throw in a few coils around the loop (see above). As you pull the slip knot tight, the loop tightens, dissipating the energy through friction, until the loop is tight enough that friction holds the knot in place. In theory, by pulling the loop through the bight (by tugging on the loose end), the knot can be undone — but in the case of Pugno and the world’s toughest material, it’s just the tying of slip knots that we care about.
The toughness of a material is measured by how much energy it can absorb before breaking. Strength, on the other hand, is a measure of how much force a material can support: Glass is strong, but not tough; Rubber is tough, but not strong. Toughness is usually measured as the number of joules (think newtons rather than heat) that a gram or cubic meter of a material can withstand before breaking. Kevlar, for example, can withstand 80 joules per gram before snapping, while spider silk can withstand 170 joules per gram. On the upper end of the scale, in a realm that is mostly theoretical for now, carbon nanotubes and graphene are close to 1,000 joules per gram.
In Pugno’s research paper, he is reporting that he has boosted a standard, commercial Endumax polymer fiber from a toughness of 44 joules per gram up to 1,070 joules per gram — by far the toughest material in the world – just by tying the fiber into a slip knot. As the loop of Endumax slips against the bight, massive, mind-boggling amounts of energy are dissipated through friction. Once the loop has been pulled all the way through — once there’s no more friction to be had — the fiber breaks. The paper suggests that Pugno has tried a wide variety of knots, with the basic overhand knot (with a few added coils) being very effective. Basically, the more friction provided by the knot — the more coils, the longer the loop — the better.