Indian Institute of Science Education and Research, Mohali
Spider silk, produced by spiders for its web, is a wonderful biological material. It is one of the toughest natural fibres, yet elastic and made entirely of proteins. Actually, the spider produces seven different kinds of silk, each for a specific purpose. For example, the silk it uses for catching prey is very elastic so as to absorb the shock of a flying insect, while the first silk that babies get from their mother is very tender. It makes its toughest silk when in danger to escape. Spiders spend their lifetime on these silks and have been surviving on Earth from millions of years ago, much before our ancestors.
There are many exceptional qualities to these silks. One particular type, called dragline silk, which is used at the outer rim of a spider’s web, can be five times stronger than steel. It is only a few microns in diameter, about 20 times thinner than human hair. It is this amazing strength of dragline silk that has got highlighted in the Spider-Man movies.
Spider silk, particularly dragline, has always interested scientists. It has not yet been possible to mimic this material in the laboratory due to its complex hierarchical organisation. Many have explored the possibility of exploiting this inherent strength of dragline silk for some practical use. However, scientists have encountered difficulties in putting these silks to meaningful use. For any material to be put to use, it should be amenable to processing — cutting, patterning, bending and welding with other materials. Also, the processing should not alter its beneficial material property.
The problem is that at the very fine level that it exists, the silk cannot be processed without losing its inherent high strength, the very property that needs to be exploited. And it cannot be integrated with other materials to combine their favourable properties. Scientists have in the past tried chemical techniques, without much success.
We have been working on spider silk for many years. Recently, we were able to show that this silk can indeed be processed with lasers without losing its key properties. It opens up new opportunities in the use of spider silk as a useful biomaterial.
We achieved this by exposing the silk to femtosecond laser pulses. One femtosecond is a millionth of a billionth of a second. The femtosecond lasers fire short, intense pulses of light. Each pulse is composed of zillions of photons, or light particles. Now if a few photons are fired towards the spider silk, nothing happens. This silk is ‘transparent’ to a single photon carrying the energy of infra-red light range. However, if we slowly increase the energy of the laser pulse, so that it becomes possible for silk molecules to absorb two or more photons simultaneously, it leads to interesting changes.
First, the silk is no longer ‘transparent’ to energies when it simultaneously absorbs two or more photons. It produces a ‘local’ rupturing of bonds. It is similar to melting. Because the chances of finding two or more photons simultaneous by a silk molecule are localised in space, the damage too is very localised. We also observed that when the silk absorbs over six photons simultaneously, it causes a nano-scale explosion. The neighbouring areas remain unaffected, because of which the overall property of the material is not changed.
Ideally, we would like to see the rupture of just a few atoms in the material. But that kind of precision is yet to be achieved with lasers. What we could manage was processing of an area of about 100 nanometres. This is similar to making a 100-nanometre cut into the material. This small cut then opens up the possibility of all kinds of patterning of the material.
Our result adds a new dimension to spider silk in that it can be put to a variety of new uses in material science, nano-engineering and biomedical applications. We ourselves were able to demonstrate how spider silk can perhaps be used to weld tissue-like materials. We successfully made tiny sensors from silk that are as sensitive as with the best commercial sensors. As our understanding about amazing biomaterials advances, we anticipate many futuristic applications of our idea.
