After silkworms were fed with graphene or single-walled carbon nanotubes, the silk spewed out twice as much, and the conductivity of carbonized silk was 10 times higher. This "super-strong" silk can be used in durable protective fabrics, biodegradable medical implants, and environmentally friendly wearable electronic devices.

Every child who loves nature may have experienced silkworm rearing. The green mulberry leaves, the white silky baby of silkworms, and the peanut-sized silk cocoons on a broom became a happy piece of childhood memory. Recently, a report from the website of "Scientific American" magazine made the silkworm baby a "protagonist" of the topic.

According to reports, Tsinghua University researchers fed silkworms with graphene or single-walled carbon nanotubes, and their spit silk was stronger and stronger. The silk containing carbon nanomaterials can be used in durable protective fabrics, biodegradable medical implants, and environmentally friendly wearable electronic devices.

The "super-strong" silk spit that silkworms of high-tech "blessing" may have splattered may only be comparable to spider silk. So, curiosity has brought a series of problems: Mulberry leaves with graphene added. Do silkworms eat well? How strong is "super strong" silk? Wearable electronic devices made with it?

Mistakes in the "Super" Silk Discovery Trip

Carbon nanotubes or graphene doesn't sound like serious food. How can you think of feeding silkworm babies with something that Mizuki Hachiko doesn't have? According to Zhang Yingying, an associate professor of chemistry at Tsinghua University, this is not her team’s “first initiative.” To make silk “born” differently, Korean scientists and Chinese scientists have previously conducted similar experiments.

The development of materials for wearable electronic devices is one of Zhang Yingying's main directions. In her view, silk as a skin-friendly natural material is the first choice for wearable electronic devices. As a material of a wearable electronic device, not only strength and toughness are required, but also conductive properties. The silk itself is not electrically conductive, and it needs to be transformed into highly conductive carbonized silk by a simple high-temperature heat treatment.

However, in the process of high-temperature carbonization, the silk becomes hard and brittle, and the flexibility is reduced. Can you feed conductive material to the silkworm and let it spit out its conductive function? With this in mind, Zhang Yingying and colleagues sprayed an aqueous solution containing carbon nanotubes or graphene directly on the mulberry leaves eaten by the silkworm larvae, and then collected the silk after the larvae had been cocooned.

Taking into consideration the taste of silkworms and the adaptability of the stomach, Zhang Yingying team made two kinds of formulas, namely the concentration of 1.0% and 0.2% of the aqueous solution, from the third instar to the fifth instar stage. Just like people's age, the growth of silkworms is also age of silkworms. The first time the molting is called first age, and so on. By the fifth instar, silkworms enter the scab stage.

Concentration 1.0% of the aqueous solution may be too heavy taste, and some of the silkworm babies fed with this concentration solution will have an early misfortune. However, the team members were pleased that the silkworm baby feeding a 0.2% solution was white and fat and could not see any discomfort. After 20 days of careful feeding, it finally gave birth to a round scorpion.

Soon, the time for the "draft" is up. It is frustrating that the collected silk does not have its own conductive function. However, the surprise is behind. The researchers found that its toughness against external forces doubled and the stress was at least 50% higher.

Since it cannot conduct electricity, it must be carbonized. The team heated the wire to 1050 degrees Celsius in an inert atmosphere and studied the conductivity and microstructure of the carbonized silk. This carbonized silk has a conductivity about 10 times higher than that of ordinary carbonized silk, and Raman spectroscopy and electron microscopy imaging show that carbonized silk containing carbon nanomaterials is more highly graphitized.

“In general terms, although the natural silk we obtained could not be electrically conductive, its strength and toughness were significantly improved. The carbonized conductive properties were better than those of ordinary silk after carbonization,” explains Zhang Yingying.

Looking forward to interrogating multi-disciplinary mechanisms

Why do silkworms that eat carbon nanotubes and graphene spit out different kinds of silk? How does the formula solution eaten in the silkworm body absorb and transform? What is the correspondence between the solution concentration and the mechanical properties of the silk?

In this regard, Zhang Yingying said that their studies have shown that lower concentrations of carbon nanotubes or graphene can prevent the transformation of random coil structures in the silk fibroin molecules to the β-sheet structure during silk formation. The obtained silk has higher ductility and toughness. However, if the content of carbon nanomaterials is too high, defects such as agglomeration may be caused, and the mechanical properties of the silk may be reduced. The mechanical properties of the silk obtained with a concentration of 1.0% are lower than those of ordinary silk.

“The transfer of carbon nanotubes and graphene in the silkworm is a very complex physiological process. The specific mechanisms and influencing factors are not yet clear. We hope that the team with the biological background can participate and help unlock the mystery. Made of natural silk with excellent mechanical and electrical properties,” said Zhang Yingying.

In fact, limited by experimental conditions, the number of silkworms fed by Zhang Yingying's team is limited, and there are not many samples for more detailed experiments. On the one hand, sericulture is a relatively tedious and time consuming process. In addition, the temperature in the north is relatively low, and it is not suitable for the survival of silkworms for most of the year.

"In the first round of experiments, we only conducted feeding experiments with two concentrations of solution. In the future, we need to make more attempts to find better concentrations to obtain silk with better mechanical properties." said Zhang Yingying. .

Beyond the concentration of the solution, the start time of feeding "special recipes" is another issue worth exploring. From the 3rd instar to the 5th instar, the silkworm has the best feeding effect at any stage of the 20-day period. It also needs further experiments to verify. "We understand that the development of the silk glands of silkworms started from the 5th instar, so it is possible that the feeding of the 3rd to 4th instars will have no effect on the final production of silk. If this is the case, we can delay the use of silkworms. 'Special recipe' time." Zhang Yingying added.

It is reported that silk companies that are interested in this technology have expressed their willingness to provide factory buildings, large quantities of silkworm babies, and technicians to cooperate in carrying out more in-depth and large-scale experiments.

Wearables that are the closest to market

Although it is still in the laboratory stage to feed silkworms with specific concentrations of graphene or carbon nanotubes to obtain "super-strong" silk, Zhang Yingying thinks that if it successfully obtains conductive natural silk in the future, it will have a series of important application values. She said that other scientists at home and abroad have also conducted similar experiments. For example, Professor Shen Qing of Shanghai Donghua University had fed silkworm multi-walled carbon nanotubes with a diameter of about 30 nanometers to produce new silk fibers in 2014. Korean scientists fed quantum dots to silkworms and obtained colored ones. Silkworm cocoons and so on. Experts from Donghua University believe that the single-walled carbon nanotubes with diameters of 1-2 nanometers used by the team of Zhang Yingying are "more suitable for integration into the crystal structure of silk proteins."

In addition, in addition to the above work, Zhang Yingying's team also developed a high-temperature carbonization technology using common silk fabrics on the market as a raw material, and can produce flexible wearable device sensors with high sensitivity and wide strain detection range. This kind of sensor can be wearable to detect the full-scale movement of the human body by directly affixing to human skin or loading on clothing and accessories. Zhang Yingying is very optimistic about the market prospect of this technology, and applied for related patents. She said that the flexible sensors they prepared have significant advantages in terms of performance index, stability, cost and manufacturing technology, and are one of the most wearable electronic devices that have recently become market-oriented.

“This sensor can detect large-scale movements such as running and jumping, and small-scale movements such as pulse, micro-expression, breathing, and vocal cord vocalization. It can also be integrated with signal processing and command generation systems to capture human motion. With the reconstruction, this technology has great application potential in human physiological signal detection, motion tracking and limb rehabilitation, virtual reality, human-computer interaction, etc.” Zhang Yingying further cited.

Does using silk as raw material increase the cost of wearable electronic devices? Zhang Yingying stated that China is the country with the largest silk production. The cost of silk fabrics is only 30 to 50 yuan per square meter, and a large number of flexible sensors can be made per square meter of silk fabric. In the future, if silkworms are fed to obtain silk, the experiment is successful. Even if we count the cost of feeding silkworms with graphene or carbon nanotubes, the cost is acceptable.

The wearable electronic device made of silk sounds cool. In the near future, you may be able to try it yourself.

The picture shows the experimental process diagram. Researchers sprayed an aqueous solution containing carbon nanotubes or graphene on mulberry leaves to feed silkworms and finally obtain cocoons mixed with carbon nanotubes or graphene.

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