A newly discovered protein may be the key to the world’s toughest spider silk, researchers reveal in a finding that may soon lead to new kinds of super-tough materials.
Darwin’s bark spider (Caerostris darwini), native to Madagascar, produces the largest orb-shaped webs in the world, with diameters up to 2.8 metres.
The species exploits insect prey that fly above rivers, positioning webs midway between the banks, suspended from silk “bridge-lines” that can be as long as 25 metres.
Gram for gram, spider silk is an extremely tough and flexible material, and is the focus of widespread research because of its potential industrial uses.
This May, for instance, a team led by Kamil Kucharczyk from the Poznan University of Medical Sciences in Poland demonstrated a precision drug delivery device made from bioengineered spider silk and iron oxide nanoparticles.
Only a few months earlier another group had shown its potential incorporation into artificial soft muscles.
Darwin’s bark spider, however, takes tensile strength an entirely new level. Its dragline silk – the type that forms the energy-absorbing primary spokes of the orb-web – is twice as strong as that of any other silk thus far tested, and an astonishing 10 times stronger than Kevlar.
This property was long ago noticed by researchers in the field of biomechanical design, but the exact biological processes that result in the super-strong fibre remained a mystery.
Now, however, scientists led by Jessica Garb from the University of Massachusetts Lowell in the US might have found the answer. In a paper published in the journal Communications Biology, they report that the silk is the result of “a suite of novel traits from the level of genes to spinning physiology to silk biomechanics”.
When Garb and colleagues analysed the structure of bark spider dragline silk they made an unexpected discovery. All orb spiders produce silk containing two distinct sets of repetitive proteins, called spindroins, known as MaSp1 and MaSp2. The number of repeats, and the ratio between the two types, govern the properties of the various types of silk each spider produces.
The dragline silk of Darwin’s bark spider, however, contains a third spindroin, which the researchers dubbed MaSp4a. This protein set lacks some of the components of the other two, but is unusually rich in a type of amino acid called proline, which is associated with elasticity.
This, the researchers suggest, “may in part explain the greater extensibility and toughness” of the fibre.
Dragline silk is produced in all spiders from glands known as major ampullae. The researchers found that these glands in the Darwin’s bark spider are “unusually long” – an evolutionary adaptation, they suggest, that may influence the structure and tensile strength of the silk, although exactly how, they add, “remains an important open question”.
Given the interest in the potential industrial applications of materials that mimic spider silk, the researchers acknowledge that their discovery is unlikely to be of interest only to arachnophiles.
“We anticipate these findings will be leveraged to produce silk-based materials mimicking the extraordinary toughness of C. darwini dragline,” they conclude.
