Wool’s complex chemistry unlocks sustainable applications beyond textiles

Wool is one of the most important materials in human history. The fibres taken from the hair of sheep, goats and other ungulates have been used to produce textiles for over 3500 years. The trade and manufacture of wool products, particularly during the industrial revolution in the UK, was both a huge source of wealth and a key driver of scientific innovation and social change. Woollen clothing was ubiquitous across all tiers of society and variation in wool grades and finishing processes enabled manufacturers to produce a huge range of other textile products including carpets, furniture and industrial felt.

This versatility is embedded within the chemical structure of the fibres: a surprisingly complex network of interlocking protein sheaths and greasy organic molecules. But while the broad fibre structure remains consistent between breeds, variation in the arrangement and composition of these individual constituents give different types of wool an impressive portfolio of diverse properties, explains Louisa Knapp, a marketing executive at British Wool.

At the very centre of each strand lies the helical coil, a tight alpha-helix of keratin which contributes to the overall elasticity and distributes tension across the fibre to prevent breakage. This core structure is surrounded by a complex matrix of keratin-associated proteins, small sulfur-rich units which crosslink between adjacent keratin filaments to provide additional strength. ‘The sulfur atoms give it a natural aptitude for absorbing water molecules without feeling wet,’ Knapp explains. ‘Wool can absorb up to 30% of its own weight before you as a wearer would notice that it’s feeling wet. This also helps it resist static as well as making it fire retardant.’ Sulfur’s excellent chelation properties also help wool absorb and retain dyes during processing and enable the fabric to wick sweat and trap odours when worn, reducing the frequency with which it needs to be washed.

Zooming out slightly further, the cortex layer is formed of a mixture of orthocortical and paracortical cells which produce most of the fibre’s mechanical properties. These twisted chains of tightly packed alpha-keratin give the fibre strength and elasticity, while the air-filled spaces between the different proteins trap air, making wool a good insulator. Orthocortical and paracortical cells differ slightly in their chemical structure – the lower-sulfur ortho structures being softer and more flexible than their paracortical analogues – and this contrast creates an internal wave within the fibre known as crimp. Crucially, the distribution and proportion of cortical cells varies between breeds, with high-crimp breeds like merino producing soft and elastic wool, ideal for next-to-skin clothing, and the gentler waves of lincoln longwool forming a stronger, coarser wool suitable for rugs and textured items.

But it’s the outermost layer, the cuticle, which differs most substantially between breeds. The cuticle anchors the fibre to the sheep and is formed of overlapping cells called scales. Composed principally of keratin beta-sheets, it acts as a barrier to protect the more vulnerable protein structures inside and contributes to the longevity and hardwearing nature of many wool products. Pores in this scale structure allow water vapour to pass in and out of the fibre and a waxy lipid coating called lanolin repels excess moisture. ‘This links to the comfort and breathability of wool garments, and also their ability to clean easily and naturally repel dirt,’ says Knapp.

However, the precise scale structure is the single most important feature which determines the properties, and ultimate use of, the wool, she adds. Warm-weather breeds like merino produce fine fibres with smaller individual scales and have a correspondingly soft texture, perfect for thermal clothing. Meanwhile, sheep reared in colder climates, such as Scottish blackface, have much thicker fibres with fewer, larger scales, resulting in a warm, heavy-duty but relatively coarse fleece, better suited to tough fabrics like carpets.

Enabling wool blend recycling with enzymes

But despite these versatile properties, wool struggled to compete with the advent of cheap and easy-care synthetic fibres like polyester and nylon, and since the 1950s, demand for this textile has plummeted. Today, growing concerns around the colossal impact of fast fashion mean many consumers are returning to natural fabrics and manufacturers are beginning to come up with creative solutions to some of wool’s limitations.

Blended fabrics are perhaps the most obvious solution: combining wool with other fibres to enhance the overall properties of the final material. However, this also creates serious sustainability issues at the product end-of-life. ‘Because fibres are so intimately twisted together, there is currently no technology that can effectively separate different fibre components, and one perishes in the recovery process most of the time,’ explains textile specialist-turned-biotechnologist Chetna Prajapati at Loughborough University in the UK. As a result, the bulk of textile waste is instead degraded mechanically, with less than 1% making it back into clothing.

However, targeting each polymer separately could close this loop and Prajapati is investigating enzymatic methods to recover the individual components of wool blends. ‘Protease hydrolyses the polypeptides that you find in wool fibres and breaks them down into very small polypeptides of varying size and degrees of polymerisation,’ she explains. ‘This leaves the other fibre behind as a piece of fabric which we can process and spin back into a yarn.’ The team have worked with a variety of different blends including polyester, polypropylene, hemp and flax, in each case recovering the intact polymer for reuse elsewhere.

The resulting enzymatic liquor contains a thick dispersion of mixed wool polypeptides, still holding the dye chromophore. ‘We can migrate dye molecules from the broken, fragmented wool to a virgin fibre which is undyed, recovering the dye as well as the wool,’ says Prajapati. Once free of dye, the team can separate the wool polypeptides by particle size and these filtered fragments are then applied directly to raw wool as part of a treatment to sustainably enhance the fibre’s more problematic properties.

Chief among these is wool’s propensity to shrink when washed. ‘Wool shrinks because of the scales,’ explains Prajapati. ‘When we wash it, we’re adding agitation and water, and this causes a differential frictional effect where the scales interact and become interlocked.’ Chemical treatments such as the Hercosett process – which uses chlorine to erode the scale structure and then coats the fibres with a cationic polymer – have been employed in an attempt to make wool machine-washable but can result in an unpleasant synthetic texture.

Instead, Prajapati is exploring methods to crosslink the retrieved wool fragments back on to the scale surface of virgin wool products. This natural coating effectively blocks the differential frictional effect to prevent the fabric from shrinking, without creating an artificial texture, and has the added benefit of softening the coarse handle of rougher wool grades. ‘We’re hoping that this will keep wool recyclable,’ says Prajapati. ‘We’ve conducted some early trials and we’re scaling up this technology. I’d like to think this opens up new avenues for wool to be used and recovered.’

Wool as a water filter

Despite the new opportunities created by biotechnology, there are still many farmers for whom wool is a burden rather than a commercial opportunity. Particularly in the UK, sheep are generally reared for their meat rather than their fleece, with the income from selling wool scarcely covering the cost of shearing.

There’s a clear need for applications outside of the traditional textile industry, says Mária Porubská, an analytical chemist at Constantine the Philosopher University in Nitra, Slovakia, and the superb properties of the fibre, notably the chelating ability of keratin, make it ideal for development into a molecular binder. ‘The diversity of composition is the basis of keratin’s ability to bind substances through several types of interactions: hydrogen bonds, electrostatic interactions and coordination bonds,’ she says. ‘Targeted modification of keratin can add or increase affinity for a certain type of substance that we want to isolate from an environment.’

The bulk of Porubská’s work focuses on metal cations, a common contaminant in wastewater. However, rather than using a chemical approach (which only effects changes to surface) to treat the wool, her team specialise in electron beam irradiation that can penetrate through the entire structure to modify the coordination environment of the sulfur groups.

We have verified this in the possibility of cleaning percolating water from a toxic waste landfill 

‘The energy absorbed by the wool first converts the alpha-helix structure to beta-sheets,’ she explains. Raising the energy further dissociates the weaker bonds within the structure, in particular the disulfide bridges which hold the double helices of keratin together. These cleaved bonds form sulfide radicals which rapidly react with atmospheric oxygen to form various S-oxidised species, including cysteine monoxide (–S–SO–) and S-sulfonate (R−S−SO₃). With sufficient radiation, this sequence yields the terminal oxidation product, cysteic acid (R–SO₃ H), which contains hexavalent sulfur, capable of coordinating metal ions in an aqueous environment.

However, the outcomes of these treatment processes are complex. The team evaluated the capacity of wool subjected to different doses of radiation to adsorb and distinguish between copper(II) and chromium(III) ions in a binary solution. Both species readily form complex salts with keratin but modifying the radiation dose changed the selectivity, increasing the absorption of one ion while simultaneously decreasing the other. ‘We found that for lower doses of absorbed energy, copper is preferentially absorbed, and at higher doses, chromium adsorption is preferred,’ Porubská explains. ‘This is related to the higher concentration of cysteic acid generated from a higher amount of cleaved disulfide bonds, and the space required for the chromium cysteinate complex.’

Overall, the preliminary study enabled the team to selectively sequester a single ion and they believe that more targeted modifications could tailor the sorption to other metals, or even organic contaminants such as nitrates. ‘We have verified this in the possibility of cleaning percolating water from a toxic waste landfill which also contains organic substances, including banned pesticides and herbicides,’ says Porubská. In the short term, the team are developing a simple pretreatment procedure to clean the wool for maximum absorption efficiency and ultimately hope to scale the process into a commercial water treatment.

Wool keratin as scaffold for tooth repair

While sorbent applications typically use the entire fibre, the constituent parts of wool also find surprising niches outside of the textile industry. The innate biocompatibility of keratin, combined with its structural rigidity and diverse chemical handles, makes it an excellent material for biomimetic applications in medicine, says Sherif Elsharkawy, a prosthodonthics specialist at King’s College London, UK. The complex protein scaffold can effectively act as a template for tissue regeneration, particularly with tough or slow growing structures such as bones and teeth.

Tooth enamel is a prime example, with an estimated two-thirds of UK adults experiencing decay or damage to one or more teeth. ‘Enamel is the only tissue in our body that cannot regenerate itself. Once we’ve developed the tooth, the ameloblast cells responsible for building the enamel die completely,’ says Elsharkawy. When the tooth first forms, these cells produce a complex protein matrix called amulogenin which provides a scaffold around which the mineral components of enamel assemble and crystallise.

Inspired by this biological templating approach, Elsharkawy’s team developed a biocompatible mimic of this essential scaffold, using keratin extracted directly from wool fibres. The complex wool structure was first broken down under reducing conditions and filtered to give a refined selection of shorter keratin fragments. The team next solubilised these simpler protein units in ultrapure water, manipulating the secondary structure via the formation of selective crosslinks to create a responsive keratin film which attracts both calcium and phosphate ions. ‘The negative amino acids present in the keratin proteins attract the calcium first via electrostatic interactions. These positive ions then attract the phosphorus and you have an early nucleation point which starts to grow the crystals,’ explains Elsharkawy. ‘Once the calcium has bound, the protein changes its conformation into an alpha-helix structure that is capable of orienting the resulting hydroxyapatite crystals for hierarchical mineralisation.’

The entire crystallisation process occurs within seconds and the enamel becomes a fully integrated part of the native tissue, recovering around 70% of the original mechanical properties. The preparation is inherently scalable, says Elsharkawy, and the team are currently working on developing the technology into two streams of products: an at-home cosmetic and a professional dental treatment.

Wool is perhaps no longer the industrial titan and symbol of wealth it once was, but despite its unglamorous reputation in recent years, this natural fibre is undoubtedly still a valuable commodity, with chemistry and biotechnology helping to unlock that potential.

Victoria Atkinson is a science writer based in Saltaire, UK