Hi Fancy Folks! Welcome to another episode of LuxeSci, a podcast that explores the intersection of science and luxury. We’ve opened our season on the science of fashion with a look at fashion trends and a dive into the science behind fabric. This week we’re looking into one particular fabric, silk.
Background
Approximately 90,000 metric tonnes of silk were produced in 2022, which puts it at a billion dollar industry
Less than 0.2% of the global textile market
The top producers are China and India
This makes sense as the production of silk is thought to have originated in China during the Neolithic period (about 12000 years ago)
Earliest evidence are silk protein found in soil from tombs dating back to 8500 years ago
In 2007, archeologists found woven and dyed silk fabrics in a tomb in Jiangxi province dating back to approximately 2500 years ago.
Silk production also has a long history in India - with evidence of it found at sites dating between 2450 and 2000 BCE
The process spread with the advent of the Silk Road (or Silk Routes), which was active from the second century BCE to the mid-15th century and brought silk to places such as Greece, Rome and Egypt. The trade route was ended when the Ottoman empire severed trade between East and West around 1453.Â
There were other trade routes - such as the maritime spice trade with India and Arabia and the Jade Road (southern routes from Xinjian to Eastern China used as far back as 5000 BCE and still used today)
Silk Production
So how is silk made?
There is a large variety of insects that can produce silk. For commercial silk production, the most commonly used species is the larvae of the Bombyx mori (domestic silk moth).
Sericulture is the process of breeding silk worms or the process of gathering the silk worms and harvesting the cocoons
Bombyx mandarina (wild silk worm)
Order Lepidoptera (butterflies and moths)
Can produce large amounts of cocoon protein using dietary nitrogen
Get most of the nitrogen from mulberry leaves (they prefer white mulberry leaves but will eat other types)
Produces silk from a pair of curved glands found on the ventral side of the digestive tube
These glands account for 25% of the weight of the larva
Female silk worms can lay a large number of eggs. (Hundreds at a time). The eggs are incubated in a controlled environment until they hatch into larvae
The larvae feed on the mulberry leaves for about 6 weeks, when they are full, they will start spinning the cocoon
The cocoon spinning involves rotating its body in a figure 8 motion for approximately 300,000 times. (I would be so dizzy)
It takes anywhere from 3-8 days to complete a cocoon and it is formed by a single strand of silk held together by a natural gum called sericin.
Cocoons are placed in boiling water to dissolve the sericin and the silk fiber is slowly reeled from the cocoon in individual long threads
It takes about 2500 silkworms to produce one pound of raw silk
Now I know it can be upsetting to think about the silkworm larvae being boiled alive essentially. And because of that, there are silks that are harvested in a more humane way. One type is Ahimsa silk, which usually uses wild silk moths and waits until the moths hatch from the cocoons to harvest them.
Science
What is is silk actually? An extremely helpful review article from Internal Journal of Molecular Science published in 2016 by Marlene Andersson, Jan Johansson and Anna Rising outlines the process and properties of silk in silkworms and spiders
 Silk fibers - one of nature’s strongest fibers with the equivalent of the strength of steel
For silkworms - the fibers are 10-16 um wide and consist of 2 fibroin monofilaments that originate from the two separate silk glands and are coated in sericin.
Fibroin - insoluble protein and is consider a B-keratin (related to hair, skin and nails)
In silk it has 3 chains, light, heavy and the glycoprotein P25. The light and heavy chains are linked by disulphide bonds (S-S bond) and the P25 links to both by noncovalent interactions (doesn’t involve the sharing of elections
The building blocks of proteins are amino acids and the fibroin in silk has a high glycine content, which allows the sheets of the heavy chain of fibroin to be packed tightly and this leads to the high tensile strength and rigid structure of the silk
Sericin - polymer (remember this word, it keeps popping up), globular protein (water-soluble and shaped like a sphere or globe upon folding)
Consists of 18 amino acids
Very hydrophobic - except in hot water where it becomes soluble
Lower temperatures - becomes a gel
Makes up 25-30% of the weight of cocoon
Proteins can have side chains of amino acids and other chemical groups that give that protein different properties
Sericin has hydrophilic (water-loving) side groups, such as serine. These have a good ability to absorb water, which has been noticed by the cosmetics industry.
Using sericin in items such as creams and shampoos could lead to an increase in hydration, elasticity, and antiaging and antiwrinkle effects
Alternative uses for sericin would be a good idea, since currently it is discarded as a waste product from silk productionÂ
Around 1800 MJ/kilogram of fiber is the energy required to produce silk (more than any other fiber)
Over half of this is the process of degumming (getting rid of the sericin)
50,000 tonnes of sericin are dumped into groundwater (along with the chemicals required in the process) for every 400,000 tonnes of silk produced
One group out of a university in Columbia found that autoclaving the silk cocoons led not only to less chemical waste but to better quality silk by-products that could then be up-cycled into other uses
So those are the components of silk, but are there other uses of this beautiful and highly functional thread besides making gorgeous clothes?
Because of silk’s amazing strength and relatively inert status (doesn’t react with much), silk sutures have been used in medicine for quite some time.
There is also a product of silk powder that is uses for topical wound care and dermatological conditions
A wonderful review article published in Advanced Healthcare Materials in 2018, authored by Chris Holland et al about the biomedical uses of silk
The authors cite current uses of silk including silk fibers made into surgical mesh, the sutures and the use of silk garments to treat dermatological conditions. These have been used widely already but what considerations would be in place for new medical indications of silk?
For example, the SERI Surgical Scaffold used in breast reconstruction showed mild inflammatory responses in 59 of 69 patients with some collagen deposition and 1 patient who had to have the mesh removed
biocompatibility (the ability of a material to perform with an appropriate host response in a specific application, essentially, does the material do what it’s supposed to do and not irritate or exacerbate something in the host)
An example would be the consideration of silk nanoparticles for the delivery of cancer chemotherapeutics. This would require hemocompatibility assessments (will the silk interfere with something in the blood)?
What is some new and exciting ways silk is being researched for biomedical uses?
Silk solutions - this has been investigated as a treatment for corneal injury and dry eyes. In mice, the silk treatment inhibited detachment of corneal epithelial cells and increased conjuctival goblet cells leading to the recovery of the tear film and mucus layer of the eye, improved corneal health and reduced dry eye symptoms. All this without some of the side effects of current treatmentssuch as pain and irritation
Silk films - investigated for drug delivery, wound healing, corneal replacement, among others
One potential use that was interesting to me personally was using silk to make microneedle patches for vaccine delivery. These are minimally invasive and painless way to delivery vaccines. Silk would make a good substrate to build a patch out of since it can potentially keep the vaccine at a stable temperature and can ensure controlled release of the vaccine
Silk fibroin tip with the vaccine and a polymer pedestal, also containing the vaccine.
When applied - the polymer would dissolve rapidly releasing a bulk of the vaccine
The microneedle tips would remain inserted into the skin, releasing the vaccine slowly over a few days.Â
One study found a 10X increase in antigen-specifc T cell and humoral responses when compared to traditional immunization approaches
Silk nanoparticles - you all know i had to put this one in. if you’ve listened to Season 1 of the podcast, you know how much we like a good nanoparticle
As a reminder - nanoparticles are simply small particles that range between 1-100 nanometers in size
Using nanoparticles for chemotherapy drug delivery could allow for less side effects and/or increased penetration of the drug, especially into solid tumors
Silk nanoparticles that were researched showed low coagulation in plasma and low inflammation in a simulated venous blood flow
One study showed improved anticancer drug delivery into drug resistant breast cancer cells
While all of these indications are very exciting, none are ready for primetime yet
Though there is something exciting about the potential of using this natural fiber in medicine in new ways so we’ll definitely be keeping our eyes on this one and hopefully will update you if anything new comes to market.
Glossary
Fibroin - insoluble protein that is the core of a silk thread
Sericin - globular protein that is a gum holding the silk thread together
Glycoprotein - carbohydrate plus a protein
Amino acid - organic compounds that contain amino and carboxylic acid groups, molecules that build proteins
Biocompatibility - ability to perform a function and not irritate the host
Nanoparticle - particle 1-100 nm in size
Additional glossary time - how to cells make proteins? DNA is read into mRNA — mRNA transcript is translated into chains of amino acids — folded into proteins
Cocktail Party Facts
Approximately 90,000 metric tonnes of silk were produced in 2022.
Silk worm larvae spin their cocoon by spinning around themselves about 300,000 times
Silk has many biomedical uses such as sutures and mesh
Thank you for listening to this episode of LuxeSci. I hope it’s given your a renewed appreciation for the artistry and the power of silk. Personally, I will be trying to buy ethically made silk going forward and am excited to see what this amazing material can bring to biomedical science.
LuxeSci is produced by me, Dr. Lex and the audio engineering is by Dr. Dimos. Our theme music is Harlequin Mood by Burdy. Follow us all over social media at LuxeSci pod, check our blogs about sophisticated science over at Erevna Media. And drop us a line to say hi, we love hearing from you.
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