To dye for

Engineered microbes, light-reflecting bacteria and nanocellulose gels are among the technologies that could change the way we lend colour to textiles, Vanessa Zainzinger reports

Dyeing textiles is dirty business. Transforming clothes into the colourful pieces we see in the shops requires copious amounts of water, energy and chemicals. In textile manufacturing powerhouses, like Indonesia, Bangladesh and India, whole rivers have turned unnatural shades of red and blue from textile mills dumping dyes and waste into nearby freshwater sources. In China, estimates say more than 60% of groundwater wells are polluted and 72 toxic chemicals in the water supply are from textile dyeing.¹

Jim Ajioka experienced the problem first-hand during his University of Cambridge research into water contamination in Bangladesh. ‘It’s really awful. Textile mills are dumping waste into the water system and people are effectively poisoned to death,’ Ajioka remembers. ‘The industry is using at least 70 highly toxic chemicals, like benzene, in the dyeing process, it creates huge amounts of greenhouse gases too, and it’s responsible for a fifth of total industrial water usage. It’s not sustainable and if we don’t do something now it’s going to be a train-wreck.’

Reports of unacceptable pollution have already inspired government crackdowns and sustainability pledges from international apparel brands. But truly changing this competitive $1000bn industry will require innovation that can be scaled up and adopted without cost or disruption for manufacturers.

Ajioka is chief scientific officer at one of the companies trying to crack the formula for a sustainable and cost-competitive dyeing technology that textile giants will want to adopt. Norwich-based Colorifix bioengineers microbes to reproduce colours that are found in living things.

Via DNA sequencing, Ajioka’s team works out what encodes the instructions to make a pigment in, say, a parrot feather, and translates that message into engineered microorganisms. The bacteria then start making that pigment in the same way the parrot does.

The colour is created at the lab before Colorifix ships a tiny quantity of live microorganisms to local fermentation partners, who then grow the colour, just like they would ferment beer, using by-products of the sugar production industry. The microorganisms can then be transported and used directly in place of dye liquor.

Transferring the microbe colour to fibres, Ajioka explains, is a lot like mould or mildew depositing a stain on the surface it grows. ‘Mildew is actually melanin. We just take this and turn it into an industrial process, allowing the microbe to do its natural job of attaching on textiles and depositing that colour.’

At the same time, the technology takes petrochemical synthesis out of the dyeing process. Colour transfer occurs at body temperature – 37°C – rather than the conventional, energy-intensive 150°C, and it uses one tenth of the water of standard processes.

Bacteria cultivated and fixed/killed after growth on a black T-shirt, above.

Indigo inks

The vast potential of bioengineered organisms as a vehicle for natural dyes is a sign of the advances of synthetic biology, says Michelle Zhu, co-founder and Chief Executive at San Francisco biotech firm Huue. ‘The synthetic biology space is growing and evolving so rapidly that a lot of people compare it with the revolution that happened in the chemistry space with the use of petroleum years ago,’ Zhu says. ‘It has so much potential for being optimised and making so many advancements quickly.’

Huue produces genetically engineered bacteria to mirror the way the Japanese indigo plant, Persicaria tinctoria makes and holds its colour. ‘Indigo and many other colours were plant-based originally, before being replaced by chemical alternatives that were cheaper, high-performing and scalable,’ Zhu says. ‘Now bioengineering has advanced to a point where we can go back to nature and create colours in a more sustainable way.’

The company uses indican, a naturally occurring precursor to indigo. It is largely responsible for the colour in indigo plants and Huue replicates the mechanism in the plant by combining it with the enzyme β-glucosidase to create an indigo solution that can be used to dye textiles.

Unlike Colorifix, which grows colour onto the textile, Huue’s dye can be transferred in exactly the same way as conventional dyeing methods. Zhu hopes this can increase adoption and ensure minimal disruption to the denim supply chain.

Huue, which is currently in the process of creating its first sample denim fabrics, has big plans to bring a broader palette of colours to the textile industry. But indigo is a great springboard for a sustainable colouring business. For one, precursors to indigo can be used to form a number of related colours, such as teal and purple. And the denim dyeing industry is desperately due a sustainability makeover.

While indigo is a natural dye that’s been used for centuries, most blue jeans today are dyed with synthetic indigo powder, that requires a great deal of harmful chemicals. Strong reducing agents (eg sodium hydrosulfite) are needed to reduce the powder into a liquid that has affinity for cotton fibres. And synthetic indigo contains the aquatic toxicant aniline, some 300t/year of which end up in the environment as wastewater discharge.

Larger industry players, too, are tackling indigo. In 2018, specialty chemicals company Archroma became the first to launch a pre-reduced, aniline-free indigo dye. The company was able to eliminate aniline on the back of its alternative indigo manufacturing process that uses hydrogen gas under pressure to pre-reduce the indigo powder by catalytic hydrogenation, which means the mill does not have to use additional chemicals. The pure indigo solution is delivered under a blanket of nitrogen, with zero salts polluting the effluent.

Indigo and many other colours were plant-based originally, before being replaced by chemical alternatives that were cheaper, high-performing and scalable. Now bioengineering has advanced to a point where we can go back to nature and create colours in a more sustainable way.
Michelle Zhu co-founder and CEO at San Francisco biotech firm Huue

The technology is gaining traction, with aniline beginning to feature on industry’s Restricted Substance List (RSL). ‘We are now seeing quite a number of collections that are truly aniline-free,’ says Paul Cowell, who heads the global textile competence centers and brand studio strategy for Archroma. ‘Our primary focus for more sustainable indigo is to convert the powder users to pre-reduced and then take the extra step towards aniline-free.’

Colour me nano

For garments other than classic faded jeans, the vivid hues and extraordinary sustainability potential of structural colour is one to watch. Like Colorifix and Huue’s engineered microbes, structural colour techniques are inspired by nature. But rather than growing colour, they play tricks with light.

Many insects, plants, butterflies, birds and arachnids generate colour through microscopically structured surfaces fine enough to interfere with visible light, sometimes in combination with pigments.

‘The simplest analogy [for structural colour] is that it’s surprisingly close to the feathers of a peacock,’ says Colin Ingham, CEO of the Dutch biotech company, Hoekmine. ‘In the feathers, there are repeating nanostructures that have to be exactly the right spacing to interact with light - at best, a broad-spectrum white light - and then you get positive and negative interfering effects that create the colour.

‘If the geometry changes just a little bit, the wavelength of the light changes as well, and you get a different colour,’ Ingham explains.

Hoekmine organises bacteria into colonies to create structural colour. The company accesses and finetunes the genetics of bacteria, using CRISPR, to grow them into a meticulously organised structure that reflects light as a 2D photonic crystal. The bacteria are killed via vapour treatment, then preserved with epoxy resins. ‘Unless you mess up the order of the bacteria they’re perfectly stable as a colour,’ Ingham says.

Apart from the use of epoxies, which Hoekmine is working to replace, the technique is highly eco-friendly. The structurally coloured particles from the bacteria don’t release any dye and getting new colours is easily done by tweaking the structure. What’s challenging, Ingham says, is to do it reproducibly so the manufacturer’s product is always the exact same colour. He sees this as the reason why structural colour, which has been tried in the paint, cosmetics and textile industries before, hasn’t caught on in the past. ‘It depends on perfectionism and it’s a real manufacturing challenge. That can be expensive,’ he says.

For a method that poses less of a manufacturing challenge, there is the option of using the same, conventional dyes that are already abundant in textiles, but creating a new carrier to transfer them to fibres with fewer chemicals, less wastewater and energy. A team of scientists from the University of Georgia (UGA) in the US is using nanocellulose as a carrier of textile dyes. Their technology converts cellulose, a readily available natural polymer found in the cell wall of green plants, into a colour-carrying hydrogel.

The manufacturing process for this novel colour carrier follows a ‘microfibrillation’ process invented in the late 1970s: cellulose powder, cut from wood pulp, passes through a homogeniser at high temperature and pressure. The process rips the larger wood fibres into nanofibres and produces a hydrogel. The gel can be dyed instead of cotton fabrics and deposited on the surface of textiles using conventional methods. Because of the high surface-to-volume ratio of the hydrogel, it can take on more dye and the process reduces usage of water, salt and alkali six-fold.

‘It’s just a change of technology but the chemistry remains the same,’ says Sergiy Minko, UGA professor of fiber and polymer science, who developed the technology with UGA professor Suraj Sharma and postdoc Yunsang Kim. ‘We simply shift the process to make it more efficient and we can substantially decrease the amount of chemicals and water we use.’

Nanocellulose has potential as a carrier not just for all kinds of colours, but also different functionalities. Minko’s colleague Smriti Dilliwar, a graduate teaching assistant on the UGA team, is working on introducing biocidal properties, with the aim of creating a hydrogel that dyes textile and introduces antibacterial properties at the same time. And PhD candidate Anuradhi Liyanapathiranage is working on a post-treatment for the coloured textiles that reduces the amount of dye discharged in the wash by 60%.

The technology is at its early stages. Raha Saremi, a materials researcher and laboratory manager at the lab, is leading the commercial efforts to bring it to the market under the wings of UGA startup, EcoaTEX. The next step, she says, is to move its application from the lab to a pilot scale and make prototypes for further tests, before moving on to larger scale production and partnerships with textile manufacturers.

Cracking work

Saremi believes it’s time for the textile industry to adopt technologies that champion saving water and removing harsh chemicals from their production process, as consumers become more environmentally conscious and sustainability is a branding buzzword. But she is conscious of the challenges that stand in the way of taking a new technology to the larger scale. ‘Manufacturers and brands know they cannot continue with the same old methods of production forever and they are willing to make changes,’ she says. ‘But they still want to spend as little money as possible and are very cautious when it comes to investing.’

Archroma’s Paul Cowell has come across this roadblock many times. ‘When I visit sustainability teams about a new dyeing process that reduces water usage we’re high-fiving in the room, saying it’s a win-win; but then the buyer looks at sourcing that T-shirt for a pound less a year,’ he says. ‘The main powerhouse is the cost.’

Cowell adds, however, that not all clothing brands put cost-competitiveness above sustainability. And collaborative industry initiatives, such as the Zero Discharge of Hazardous Chemicals (ZDHC) programme (see Box), have accelerated efforts to change the textile supply chain. The ZDHC, Cowell says, opened up new communication channels between chemical companies, like Archroma, and the fashion brands, speeding up development and uptake of new technologies.

Archroma’s portfolio now includes formaldehyde-free resins and binders for pigment printing, petroleum-free sulfur dyes, metal-free acid dyes for nylon, and an activated enzyme treatment for bleaching cotton that saves on using sodium hydroxide; all about ‘being safe at the core, and making an article with less water, energy and time,’ Cowell says.

Established players like Archroma have an easier time taking innovations to the textiles industry than small start-ups and research groups. Innovators from the UGA to Hoekmine are challenged to break through to a massive, conservative industry. Their biggest hurdle is scalability, with production capacity needed to bring costs down and investment needed to bring production up.

But despite the challenges involved, they are positive that fast fashion, and the cheap dyeing that comes with it, are on their way out.
‘People are becoming really aware of how bad the textile dyeing industry really is. Even in places like China and India, which are usually loose with regulations, governments are trying to make an effort,’ says Colorifix’s Ajioka.

‘I think the fashion industry is due for a complete revolution,’ says Huue’s Zhu. ‘There is an opportunity here to reinvent the colouring space, and it has real momentum behind it.’

References
1 http://documents1.worldbank.org/curated/en/614901468768707543/pdf/922610WP0P11950DEL0FOR0GREEN0GROWTH.pdf