An ancient fabric with, suddenly, a new future: Silk

It is interesting to think that one of the earliest uses of silk was in medicine. The Ancient Greeks and Romans used raw spider silk (essentially, bundled-up spiders’ webs) to treat wounds, in the 2nd century CE. Though catgut (the twisted, dried-out intestines of herbivorous animals) was the primary material for sutures, and remains in use today, spider silk was considered a viable alternative because of its antibiotic, antiseptic properties.

Most of the innovation around this material today is linked to its component proteins: fibroin, which provides molecular structure, and sericin, which acts as the binder. Silk fibroin forms the basis of most new applications. Once extracted, it can be reconstituted into gels, films and sponges. Carry out a few chemical modifications and it turns malleable, almost plastic-like.

“Silk protein is biocompatible (it doesn’t react with and is not rejected by the body) , biodegradable and is therefore suitable for various biomedical applications including drug delivery and tissue-engineering applications,” says T Govindaraju, a professor with the Bioorganic Chemistry Laboratory at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) in Bengaluru.

Scaffolding for damaged tissue

Among the most dramatic experiments underway, is one in which a silk fibroin-based hydrogel controls and delays delivery of drugs within the body.

In a 2020 study by Govindaraju and his team, published in the journal ACS Applied Bio Materials, insulin was encapsulated in a fibroin-based hydrogel formulation derived from the cocoons of the Bombyx mori (which produces mulberry silk, the most widely available variety in the world).

When injected under the skin of diabetic rats, it succeeded in forming a depot that breaks down over a predictable and controllable period of time, releasing the insulin slowly.

For diabetes patients, such a solution could eventually mean one shot every few days, instead of multiple ones a day. In a way, it could also make insulin easier to store.

“Insulin is an unstable substance that has to be refrigerated, is sensitive to environmental conditions, and can’t be taken orally because the gut will break it down,” says Govindaraju. “In our experiments, we found that the insulin showed a stability and viability of four days, inside its hydrogel depot in the body.”

The hydrogels could eventually also help deliver drugs to specific tissues in the body, limiting the damage caused by highly toxic ones such as those used in cancer treatments, Govindaraju says.

In another study, the team combined fibroin with skin pigment cells or melanin, to create nano-fibres that acted as scaffolding for damaged tissue. In lab mice, this scaffolding helped slow-healing cells repair, their study published in ACS Applied Materials and Interfaces stated.

“These techniques have immense potential to replace metallic and polymer implants,” says Govindaraju. “The silk-based scaffolds could potentially help treat injuries to the spinal cord, whose regeneration is extremely poor. Such formulations could offer support in cases of injury to bones, skin, cartilage and neuronal tissue. Essentially, you would be able to regrow injured or diminished cells and tissues, while the silk proteins dissolved away.”

Inks that change colour, a coating for apples and pears

At the Silklab at Tufts University, Massachusetts, researchers are freeze-drying silk fibroin, then heating it to make it malleable, moulding and fabricating it, to make bone screws that aren’t just non-disruptive; they can be loaded with bioactive molecules such as antibiotics and enzymes that speed up healing.

Unlike the metal alloys currently in use, “silk screws” are bioresorbable (or biodegradable), degrading as they are replaced by bone tissue, researchers said, in a paper published in Nature Materials in 2019.

They are now tinkering, to turn silk fibroin into a type of coating for vegetables, fruit and meats. Such wrapping would serve as a barrier to microbes and oxygen, extending the shelf-life of produce. It would be invisible, edible and tasteless.

The highly malleable silk fibroin is also being combined with a dopamine polymer and iron to make a powerful, non-toxic super-sticky glue — in a formula inspired by the mechanisms used by barnacles and mussels, researchers said, in a study published in Advanced Science in 2021.

A year later, the lab reported that it had also used fibroin to make biologically activated inks that change colour in response to chemicals. These could potentially end up in uniforms, sports clothing and other wearables, where they could help map parameters such as glucose levels, the presence of pathogens, even skin composition and hydration.

They could be used in tattoos, to deposit the sensors under the skin that would glow brighter or dimmer in response to changed blood oxygen levels, according to a study published in the journal Advanced Functional Materials in 2022.

Further modification could enable such inks to detect the presence of antibodies, the reported stated, as well as changes in heart and lung function.

Cosmetics and skin care

Sericin, the other silk protein, is not as malleable or versatile as fibroin, but is a bio-polymer full of antioxidants that has found uses in high-end cosmetics, as an additive that helps the skin and hair retain moisture. Sericin is also used as a layer of protection on the skin, where its antioxidants repel harmful ultraviolet radiation.

Incidentally, silk fibroin makes up 75% to 83% of most cocoon threads. Fibroin is a long, repetitive protein chain that gives silk its ability to bend and stretch without breaking, allowing it to be woven in extremely high thread counts that give the eventual fabrics its softness and smoothness. Sericin, which makes up 17% to 25% of cocoon threads, is the cement that holds fibroin strands together. It is removed from raw silk during the boiling process.

New spools from India

All this evolving research could act as an added boost for sericulture in India, a country already actively seeking to expand the industry across new states.

Currently, at least 70% of the silk produced in India comes from the Bombyx mori moth. Karnataka is the biggest producer of this mulberry silk. (It is also home to the Central Silk Board and several sericulture research institutions.) Tamil Nadu, Andhra Pradesh and Telangana are major producers too.

There are three types of non-mulberry silkworm silks, all of which are also indigenous to India. These include tussar, made from the cocoon of “tussar moths”, of the genus Antheraea, and produced in Chhattisgarh, Jharkhand, Odisha and West Bengal; muga, made by the Antheraea assamensis moth, a mainstay of Assam; and eri silk, made by the Samia cynthia moth, and grown and processed in Meghalaya and Nagaland.

“The sericulture industry has the potential to boom in India if we diversify the applications of silk beyond textiles and fashion,” says Govindaraju of JNCASR.

A key concern, globally, is the cruelty involved in silk production. The silk moth cocoon is typically boiled to extract the silk in a single, unbroken thread.

Eri is an exception. It has traditionally been a cruelty-free process in which farmers wait for the moth to leave the cocoon, before harvesting it. In the early 2000s, the late textile entrepreneur Kusuma Rajaiah patented the ahimsa method, which does the same with mulberry silk.

India’s great potential lies in its wild moths, says Aarathi Prasad, an honorary research fellow at University College London’s Department of Genetics, Evolution and Environment, and author of Silk: A History in Three Metamorphoses (2023). “Historically, China has always led the world in silk production. We should be looking at how we can do things differently, sustainably, and with the wild moths.”

Of course, no one thing can flourish in isolation. “Like the Bombyx mori and its mulberry bush, each species of moth needs a specific plant,” Prasad says. “In order to raise each type, we would need to protect and preserve the wild habitats on which they depend.”

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DOWN THE SILK ROAD: A LOOK BACK

• Silk was likely invented in Neolithic China, at least 5,000 years ago. Today, China is the world’s largest producer of raw silk, producing more than 50,000 metric tonnes a year.

• In India, the earliest traces of silk have been found at Indus Valley sites dating to more than 4,000 years ago. They are the earliest examples of silk threads found outside China. India is currently the world’s second-largest producer, generating about 36,000 metric tonnes of raw silk a year.

• What’s interesting about the Indus artefacts (which include jewellery and cutlery) is that some of the silk used as embellishment in them came not from the Bombyx mori — the most commonly used moth species, and the one first domesticated in China — but from wild moth species now found in Assam.

• Over 95% of the silk produced in the world today is mulberry silk, made by breeding the Bombyx mori moth in its cocoon stage, and then boiling it. The domestication has altered these moths and the silk they produce, says Aarathi Prasad, an Honorary Research Fellow at University College London’s Department of Genetics, Evolution and Environment, and author of Silk: A History in Three Metamorphoses (2023).

• “The females have become larger, make more silk and produce more eggs. But domestication has also severely compromised them as organisms,” Prasad says. “They can’t fly, have no camouflage, are born blind, and are completely dependent on humans for everything from feeding to propagating.”

• All silkworm silk is primarily made up of two proteins: fibroin, which gives it its structure, and sericin, which binds the fibroin fibres together. When silk is processed, the sericin is separated via boiling. What is left is mainly fibroin, a long, repetitive protein chain that gives silk its ability to bend and stretch without breaking, allowing it to be woven in extremely high thread counts that give the eventual fabrics their softness and smoothness.

• Silk, incidentally, gets its sheen from the triangular prism-like structure of this protein. The prisms refract light, giving the fabric its characteristic glow.