Silk with the addition of graphene and nanotubes

Silkworms ate graphene and made a silk thread that conducts electricity

Anatoly Alizar, Geektimes

The silkworm is a relatively large domesticated butterfly that the Chinese have been using for silk production for at least 5,000 years (probably after several centuries or millennia of breeding). For a long time, China was a global monopolist, supplying beautiful fabric to Europe along the trade route, which was called the Silk Road. Then the Byzantines stole silkworm eggs, and then the Europeans got them thanks to the Crusades.

However, this is not about the cunning Chinese with their millennial business plans, but about the unique material, which is silk itself. It is a natural protein, one of the most durable in nature. The fiber consists of 75% fibroin and 25% sericin. Under the microscope, two parallel strands of fibroin with clump-like deposits of sericin are noticeable. In addition to them, waxes and fats are present in silk, as well as minerals. The width of the silk thread is 32 microns, the length can reach 1.5 kilometers. The breaking voltage is about 40 kgf/mm2.

Silk is an outstanding material, and scientists have made repeated attempts to improve silk fiber with various functional components, such as paints, fluorescent proteins, antimicrobials, metal/semiconductor nanoparticles, as well as electrically conductive polymers.

Two main strategies are used to modify silk fiber: modification of the finished thread and enrichment of the material during its production (digestion) inside the silkworm. The first of these technical processes is quite complex, multi-stage and requires the use of toxic reagents. For comparison, the recently invented method of enriching silk inside a silkworm is quite an environmentally friendly and relatively simple process. You just need to put silkworms on a diet.

Scientists from the Faculty of Chemistry and the Center for Nano- and Micromechanics of Tsinghua University (Beijing) have proposed a new way to enrich silk fiber using carbon nanotubes and graphene.

Carbon nanotubes and graphene have excellent mechanical characteristics and are widely used in the production of high-strength materials. There have been several attempts to add carbon nanotubes to silk by modifying the finished thread or adding to the diet of silkworms. Similar experiments were conducted with spiders. In a previous experiment, silkworms were fed multi-walled nanotubes with a diameter of about 30 nm. Now Chinese scientists have logically assumed that single-walled nanotubes with a diameter of about 1-2 nm will be much more acceptable for the digestive system of silkworms and the introduction of fibroin into the structure. Looking ahead, they were not mistaken.

In addition to single-walled nanotubes, scientists decided to feed graphene to silkworms, also a potential hardener. To feed the materials to animals, scientists used a simple method: they sprayed a suspension with single–walled nanotubes and graphene on mulberry leaves, which silkworms feed on - and then collected the product from the cocoon.

The experiment ended with success. A diet of silkworms with additives of single-walled nanotubes and graphene led to the production of silk thread with improved properties. The thread is obtained naturally from a cocoon, just like ordinary silk thread.

Scientists have studied the raman scattering spectra of silk fiber and silkworm excrement – and confirmed in both cases the introduction of carbon nanotubes into silk fiber. They also checked how the properties of the fiber changed after the introduction of carbon nanotubes.

The illustrations show the scheme of the experiment, photographs of cocoons obtained after feeding silkworms with mulberry leaves with a suspended nanotube concentration of 0.2% and 1.0% by weight and a graphene suspension with a concentration of 0.2% and 2.0%. Photographs from a scanning electron microscope for each sample of silk thread and a diagram with the characteristics of the stretchability of the thread are shown

Other mechanical characteristics of improved silk fibers are shown in the table: breaking stress, maximum tensile strength to rupture and modulus of elasticity.

As it is clear from the table, a number of experiments will have to be carried out to find the optimal concentration of carbon nanotubes and graphene in the diet of silkworms so that they get threads of greater strength. We see that a diet with a weaker concentration of SWNT1-S and GR1-S resulted in the production of fiber with much better properties than a diet with a higher concentration of SWNT2-S and GR2-S.

It is not surprising that after the addition of graphene and carbon nanotubes, the silk thread became a conductor of electricity. In the best sample of silk with graphene particles, the electrical conductivity was quite high 120 siemens per centimeter.

Such silk can be used in electronics. It is convenient to power wearable gadgets sewn directly into silk clothes. Actually, it's quite easy to make a luminous fabric.

The photos below from a transmission electron microscope clearly show that silk fibers with foreign nanotubes (in the middle) and graphene (at the bottom) are much better structured than ordinary silk (at the top).

The scientific article was published on September 13, 2016 in the journal Nano Letters (Wang et al., Feeding Single-Walled Carbon Nanotubes or Graphene to Silkworms for Reinforced Silk Fibers).

Portal "Eternal youth" http://vechnayamolodost.ru  18.10.2016