Perfect tools: Why the scorpion's sting is so hard

Whether teeth or skeleton, in the realm of biology it is often a composite material made of organic and mineral components that ensures particular hardness and resilience.

In our teeth, for example, the mineral content consists mainly of calcium phosphate in the form of hydroxyapatite.

In contrast, the shell of a beach crab contains calcium carbonate, more precisely in all parts of the so-called cuticle, which serve as an exoskeleton.

Limpets and beetle snails also build their protective shield from a composite material with calcium carbonate.

For their rasping tongue, however, they use harder iron minerals: some strengthen the teeth they use to grate algae from rocks with goethite (also known as needle iron ore), others with magnetite.

Insects and other arthropods in particular often use a completely different type of building material when particular hardness is required: they do not reinforce their teeth and claws with mineral ingredients, but rather distribute atoms of heavy metals such as zinc, manganese, iron or copper evenly in an organic matrix .

Scientists led by Robert Schofield from the University of Oregon in Eugene recently discovered this.

Neither with the electron microscope nor with special methods of X-ray absorption spectroscopy could they detect mineral components in the mouthparts or the spines of ants, spiders, scorpions and annelid worms - even though the heavy metal content there can be more than twenty percent.

In order to be able to study the distribution of heavy metal atoms in detail using the example of zinc, Schofield and colleagues used a miniature probe adapted to organic material.

The teeth on the jaws of the leafcutter

, the venomous claws of the garden spider

and the mouthparts of the many-bristly

.

Even with a spatial resolution of a few nanometers, the corresponding material samples presented themselves as completely homogeneous, without any granulation.

It also turned out that most of the zinc atoms are bound to nitrogen atoms, presumably to those of the amino acid histidine.

In this way, proteins could be crosslinked with each other with the help of zinc, the researchers write in the "Scientific Reports".

Durable tools in miniature format

The remaining zinc atoms are probably distributed as amorphous zinc hydroxide in the organic matrix of chitin fibers and proteins.

The researchers suspect that when they bind water and fill gaps between the macromolecules, they reduce their mobility and thereby increase the hardness of the material.

In addition, excess zinc atoms could step in when cross-links between the proteins break under heavy stress, and thus quickly repair the damage.

But what are the characteristics of a cuticle whose dry weight consists of up to 24 percent zinc?

To answer this, the physicists examined their hardness, elasticity, breaking strength and other mechanical parameters.

The biomaterial with homogeneously distributed zinc turned out to be as hard and break-resistant as crab claws and salmon teeth, which essentially consist of calcium carbonate and calcium phosphate.

On the other hand, almost all tested samples with heavy metal atoms were significantly more resistant to abrasion than the biomaterials with mineral components.

This also applied to the comparison with Plexiglas and polycarbonates.

In terms of shock resistance, biomaterials with heavy metal atoms were even able to absorb up to seven times more energy than polycarbonates without being damaged,

According to Schofield and his colleagues, biomaterials with homogeneously distributed heavy metal atoms have a major advantage over composite materials with a clear grain: they are suitable for miniature "tools", precisely shaped and often very sharp, but still durable.

Examples are the examined jaws of the leafcutter ant, which resemble serrated knives, or the stinger of scorpions, which ends in a kind of cannula, as well as the venomous claws of spiders.

Some research objects even equip their organic matrix of chitin fibers and proteins with different heavy metals.

The scorpion

for example, has equipped its stinger with manganese - but with zinc at its very tip, which is the first to pierce the victim's body.

In all studied animals using both heavy metals, the zinc-containing biomaterial was significantly harder than the manganese-containing one, which in turn was significantly harder compared to areas of the cuticle without heavy metal atoms.

Manganese is therefore perfect for a gradual transition between very different degrees of hardness.

After all, maximum hardness is not optimal everywhere, because this often goes hand in hand with increasing mechanical stress and decreasing breaking strength.

Basically, a cuticle with homogeneously distributed heavy metal atoms prevents sharp edges, slender tips and other filigree structures from wearing out quickly, deforming or becoming blunt with frequent use.

As model calculations have shown, the owners of such high-quality tools can therefore save a lot of strength, muscle mass and metabolic energy.

Such properties also sound tempting for human applications: perhaps the tried and tested biomaterial that various crawling animals have developed in the course of their evolution can inspire materials scientists to create new materials, for example for medical applications.