Silkworms that eat carbon nanotubes and graphene spin tougher silk | Chemical & Engineering News
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Web Date: October 5, 2016

Silkworms that eat carbon nanotubes and graphene spin tougher silk

Strong, conductive carbon-reinforced silk could be suitable for wearable electronics, biodegradable sensors, and medical implants
Department: Science & Technology
News Channels: Biological SCENE, Materials SCENE, Nano SCENE
Keywords: biobased materials, silk, biodegradable electronics, silkworm, medical implants, carbon nanotubes, graphene
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Feeding silkworms mulberry leaves sprayed with carbon nanomaterials yields strong silk strands that conduct electricity.
Credit: SarDiuS/Shutterstock
Photo of silkworms eating mulberry leaves
 
Feeding silkworms mulberry leaves sprayed with carbon nanomaterials yields strong silk strands that conduct electricity.
Credit: SarDiuS/Shutterstock

Silk—the stuff of lustrous, glamorous clothing—is very strong. Researchers now report a clever way to make the gossamer threads even stronger and tougher: by feeding silkworms graphene or single-walled carbon nanotubes (Nano Lett. 2016, DOI: 10.1021/acs.nanolett.6b03597). The reinforced silk produced by the silkworms could be used in applications such as durable protective fabrics, biodegradable medical implants, and ecofriendly wearable electronics, they say.

Researchers have previously added dyes, antimicrobial agents, conductive polymers, and nanoparticles to silk—either by treating spun silk with the additives or, in some cases, by directly feeding the additives to silkworms. Silkworms, the larvae of mulberry-eating silk moths, spin their threads from a solution of silk protein produced in their salivary glands.

To make carbon-reinforced silk, Yingying Zhang and her colleagues at Tsinghua University fed the worms mulberry leaves sprayed with aqueous solutions containing 0.2% by weight of either carbon nanotubes or graphene and then collected the silk after the worms spun their cocoons, as is done in standard silk production. Treating already spun silk would require dissolving the nanomaterials in toxic chemical solvents and applying those to the silk, so the feeding method is simpler and more environmentally friendly.

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High-resolution transmission electron microscopy images of silk fibers after carbonization show that unmodified fibers have an amorphous graphite structure (top), whereas the fibers modified with single-walled carbon nanotubes (middle) and graphene (bottom) have more ordered structures.
Credit: Nano Lett.
Transmission electron microscopy images of silk fibers after carbonization
 
High-resolution transmission electron microscopy images of silk fibers after carbonization show that unmodified fibers have an amorphous graphite structure (top), whereas the fibers modified with single-walled carbon nanotubes (middle) and graphene (bottom) have more ordered structures.
Credit: Nano Lett.

In contrast to regular silk, the carbon-enhanced silks are twice as tough and can withstand at least 50% higher stress before breaking. The team heated the silk fibers at 1,050 °C to carbonize the silk protein and then studied their conductivity and structure. The modified silks conduct electricity, unlike regular silk. Raman spectroscopy and electron microscopy imaging showed that the carbon-enhanced silk fibers had a more ordered crystal structure due to the incorporated nanomaterials.

Some questions remain. One is exactly how the silkworms incorporate the nanomaterials in their silk. Another is what percentage of the nanomaterials eaten by the worms make it into the silk instead of being excreted or otherwise metabolized. The carbon materials are not visible in the cross sections of the silk threads, perhaps because the nanoparticle content is low, Zhang says. Answering these questions might be a task for biologists, she adds.

Polymer chemist Qing Shen at Donghua University reported similar work in 2014 using 30-nm-wide multiwalled carbon nanotubes, which also increased the silk fibers’ strength and toughness (Mater. Sci. Eng., C 2014, DOI: 10.1016/j.msec.2013.09.041). Zhang says that the smaller, 1- to 2-nm-wide single-walled nanotubes her team uses “are more suitable for incorporation into the crystalline structures of silk protein.”

This work provides an “easy way to produce high-strength silk fibers on a large scale,” says materials scientist Yaopeng Zhang of Donghua University, who has fed titanium dioxide nanoparticles to silkworms to create superstrong silk resistant to ultraviolet degradation. The electrical conductivity of the carbon-reinforced silk might make it suitable for sensors embedded in smart textiles and to read nerve signals, he says.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Comments
blam (Wed Oct 05 17:34:43 EDT 2016)
Next up, they will put graphene in viagra.
lo (Sat Oct 08 11:24:48 EDT 2016)
ha ha ha !
bowlsheet (Thu Oct 20 10:24:34 EDT 2016)
and nanotubes in Cialis
Sumanta (Fri Oct 07 01:26:07 EDT 2016)
Definitely it will give insight for advance clothing technology. How about silkworm production efficiency after many time feeding ?
Prof.Kamani.K.K (Sun Oct 09 23:53:59 EDT 2016)
Glad to know and very interesting...After this what will be the future applications...??? Bio sensors I think best applications .
David Eckhous (Mon Oct 10 17:51:06 EDT 2016)
Add some thermistors, a photovoltaic panel and a thermostat to create self heating clothing, greenhouse heaters or car hood blankets for engines.
Rodrigo Albuquerque  (Wed Oct 12 16:14:53 EDT 2016)
One important point to be considered is the final destination of other excreted substances by the same worm, since this can go to the environment and will very likely contain carbon nanotubes or graphene.
Dr Richard Ray Shreve, ACS Emeritus (Wed Oct 12 17:54:42 EDT 2016)
Why stop with silkworms? How about sheep, llamas or even crops like cotton and flax? The benefits of Chemistry are indeed endless provided there is creativity and money.
Wolfgang K.-D. Brill (Thu Oct 13 03:30:24 EDT 2016)
I would like to congratulate the researchers to this finding.
One of the obvious ways to enhance silk may be BN-nanotubes (BNNT) or flakes (BN-powder). BN has a great affinity to proteins. Silk could be an ideal template for BN. The resulting fibers could be extremely temperature resistant. However I don't think that sikworms could digest BN-fullerenes (WO 2008102357 A2), which react well with nucleophiles and my kill them.
Dr David Thomas (Thu Oct 13 06:45:21 EDT 2016)
"The carbon materials are not visible in the cross sections of the silk threads, perhaps because the nanoparticle content is low". Yet "silks are twice as tough and can withstand at least 50% higher stress". Really? The silk [with this apparently low level of incorporation] conducts electricity. Really? Publishing this sort of nonsense will bring C&EN a poor reputation.

Comment to editor: I've a doctorate in peptide and protein synthesis from Oxford so I'm offering a reasonably informed opinion.
KM (Thu Oct 13 07:51:29 EDT 2016)
David Thomas - I agree with you. No quantification of the amount of material incorporated into the silk threads, no discussion of percolation threshold. Also look at the plot in Figure 1. If I am reading that correctly the "effect" was observed when the leaves the worms eat was sprayed with low concentrations of grapheme/nanotubes, but disappears at higher concentrations.
KM (Thu Oct 13 18:20:32 EDT 2016)
David Thomas - I agree with you. There is no discussion or acknowledgement of the percolation threshold in that paper, nor do the authors report to amount of the nanomaterials in the silk threads. Moreover, if I read their Figure 1 correctly, the effects (if real) are observed only at the lower feeding rate and disappears to equal the control at higher rates.
Cruz-Silva Rodolfo (Wed Oct 19 03:42:15 EDT 2016)
I am also skeptical about this paper and I agree with Dr. Thomas. Nanoletters should review this paper. The main question in this work, which actually was not addressed, is how the nanotube or graphene was loaded into the silk fibers. Everybody in the media has assumed that somehow the nanotubes were metabolized by the worm, based on evidence from previous studies. But it is not the same to metabolize a small organic molecule (a dye for example) than a carbon nanostructure of several tens of nanometers. They should have dissected the silk glands, not just analyze the silk. By reading the methodology is clear that CNT or graphene could have easily contaminated the silk by contact, explaining the small loading (if any). The worms were grazing over leaves sprayed with nanotubes and graphene, so it is expected that some of the nanotubes attached to the silkworm skin. If you see a timelapse video of a silkworm building a cocoon, you will see that they literally rub against the silk fibers, and if there are some nanomaterials in the silkworm skin, they will inevitably land in the fibers. I am also a researcher in carbon nanotecnology.
Olivia (Thu Oct 13 16:26:31 EDT 2016)
This is fascinating! I named my pet silkworm Carbon Nanotube after this article!
JustConcerned (Thu Oct 20 09:23:59 EDT 2016)
It is perhaps important to comment on "and then collected the silk after the worms spun their cocoons, as is done in standard silk production". I wonder if it is widely known that the pupa is boiled in water to collect the threads. Another way human can exploit a creature with no choice. Science is great and fun I agree, but it does not always come at no cost to the environment and its creatures.
JP Djukic (Thu Oct 20 10:36:03 EDT 2016)
This is what one would call a "butterfly effect" applied to natural silk production.

Knowing the nasty effect of carbon nanotubes on cellular membrane I would have expected a much worse fate for the worm.

I am pretty sure that Spiderman will quickly fetch some nanos and graphies on ebay and try that ASAP. Or ?

:))
Paul H Dear (Tue Nov 01 15:25:15 EDT 2016)
To quote the well-known biomechanician Prof. V. Meldrew, "I don't believe it!".

It's highly unlikely that carbon nanotubes or graphene would be absorbed through the silkworm's gut, much less find their way into the silk glands and be incorporated into the silk. And, to have any measurable effect on the tensile strength, the carbon structures would have to be more or less contiguous along the silk, _and_ make up a measurable percentage of its cross-section.

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