Bacterial cellulose comes out of the woodwork

Plant-based plastics have become increasingly popular in recent years with growing concerns over plastic pollution. But plant-based ‘bioplastics’ are not new. The first man-made plastic was made in 1862 using cellulose, the main component of plant cell walls, pre-dating the petroleum-based plastics that litter our planet today. Now, scientists are looking back to the birth of plastics and seeking inspiration from nature to find novel raw materials that are environmentally neutral.

The incredible versatility of plastic has made it one of the most widely used materials in the world. However, its strength is also its greatest weakness. Typical petroleum-based plastics, such as the ubiquitous PET (poly(ethylene terephthalate)), can take centuries to decompose.

For Toshifumi Satoh and his team at Hokkaido University’s Polymer Chemistry Lab, tackling plastic pollution by finding all natural, biodegradable alternatives is a priority. And their work goes beyond biodegradable cups and cutlery. “There are so many different types of plastic being produced today,” says Satoh. “Our goal is to find a suitable replacement for every single one.”

Researchers across Hokkaido University have been united by this mission. Takuya Isono, who joined Satoh’s lab in 2014, is an expert in synthetic polymer chemistry. Together they have been finding ways to turn sugar into carbohydrate-based components for bioplastics. Meanwhile, Kenji Tajima, at Hokkaido University’s Biomolecular Chemistry Lab, has been manipulating microbes to produce cellulose nanofibres. These ultrafine threads can be weaved into strong and lightweight materials with many potential uses, from food additives and resin reinforcement materials, to drug carriers.

The challenge is finding eco-friendly ways to bind these natural nanofibres, which is where Satoh and Isono’s work with carbohydrates comes in. “Our projects share a sustainable objective: to reduce the use of petroleum-based plastics,” says Isono, “and now we are combining forces.”

A sweet solution

For the past 10 years, Satoh has been transforming simple sugars into strong and stretchy carbohydrate-based polymers that could replace conventional plastics. This involves extracting starch from plants such as corn and potatoes and breaking it down into simple sugar compounds. These could be anything from glucose molecules to chains of two to ten simple sugars called oligosaccharides.

The next step involves combining these compounds into new plastic-like products such as copolymers, which combine soft segments to provide elasticity with hard segments for strength and rigidity.

In 2020, Satoh’s team produced fully bio-based ‘block’ copolymers — so-called because the molecules are all aligned. They did this by combining chains of various lengths of glucose (malto-oligosaccharides) with polydecanolactone, a soft polyester that can be synthesized from a natural product of the fragrant Cryptocarya massoy tree.

The resulting material was just as bendy as common petroleum-based rubbers and could be stretched to seven times its length before breaking. The team also successfully created a carbohydrate-based compatibilizer — a material that helps bind two substances that would otherwise not mix — thereby combining the strengths of the two substances, and presenting an opportunity to make Tajima’s work with cellulose more eco-friendly.

Bacterial building blocks

Cellulose nanofibres are widely available, strong and recyclable. In general, cellulose nanofibres made from tree pulp are several micrometres-long or less, and therefore have particular uses.

For the past 30 years, Tajima has been harnessing fermentation, during which some bacteria metabolize sugar to produce cellulose nanofibres that are much longer and more useful.

“It’s very easy to find cellulose-producing bacteria,” he explains. “Cellulose is insoluble, so we can see it floating on the surface of fermented liquids. Bacteria produce cellulose as a network of very narrow fibres, about a thousandth of the width of fibre from plant pulp, making it simultaneously strong and malleable,” he adds.

Experimenting with different fermentation methods, Tajima’s team has produced malleable cellulose nanofiberes, known as bacterial cellulose nanofibres, from Gluconacetobacter that are 30 times longer than those extracted from plant pulp, and considerably stronger. “In stress-strain experiments, the mechanical strength of our nanofibres matched that of metals such as gold and silver,” says Tajima.

The downside is that productivity is very low, so for the last 10 years Tajima’s lab has been working with Kusano Sakko — a construction company based in Hokkaido — on a technique to produce bacterial cellulose on an industrial scale.

Last year, Kusano Sakko opened a commercial scale bacterial nanocellulose factory in Ebetsu, Hokkaido, applying Tajima’s innovative fermentation method to mass produce cellulose nanofibres, called Fibnano. By culturing bacteria with molasses (a high-sucrose by-product of sugar refining) and nitrogen sources, they cultivate nanofibres within three days.

“Sugar beet is one of the most important crops in Hokkaido, but its production volume is decreasing,” says Tokuo Matsushima from Kusano Sakko. “This new tech could also help protect an agricultural system that has lasted for 150 years.”

Time for change

Another issue is figuring out how to synthesize many different types of plastic without using fossil fuels. Last year, Tajima’s team, working with Satoh and Isono, found that reinforcing biodegradable resin with bacterial cellulose nanofibres creates a stiffer, stronger plastic.

Kusano Sakko then developed a cellulose acetate resin reinforced with the nanofibres in collaboration with Kenji Takahashi at Kanazawa University. They will start pilot-scale production in 2023. The team expect to commercialize their bioplastics within 10 years.

“We’ll replace everything made from plastic with bioplastic, from computer hardware to homewares and clothing,” says Satoh. Matsushima adds: “We also aim to develop many other plant-based products, from foods to vegetable leather.” Kusano Sakko’s trademarked bacterial cellulose, Fibnano, is already used in medicines, cosmetics, foods and more.

A big concern is then what happens to these bioplastics when they are discarded. “Our aim is to create materials that break down naturally and return to the environment as water and carbon dioxide,” says Isono.

The team are already collaborating with Yutaka Takeuchi at Kanazawa University, which has developed a rapid technique for assessing biodegradability in freshwater that reduces the time needed to test bioplastics from six months to six weeks. A secondary objective for Satoh’s team is to change people’s behaviours so that they choose bioplastics, despite it being more expensive. “We need to convince people to make the right choice, not the cheap one,” he says.

It seems these bioplastics represent a cost-effective and eco-friendly alternatives to fossil fuel-derived plastics. “But we must continue striving for the easiest, most energy efficient way to transform carbohydrates and cellulose into a wide range of materials,” he concludes.