Humanity’s reliance on plastic is a significant problem. This material is derived from petroleum and generally ends its life as landfill and or in an incinerator. Either way, that’s unsustainable. So why not develop biocomposites that are more environmentally friendly?
That’s not as simple as it sounds. Most biodegradable plastics rely on a matrix structure derived from petroleum. That’s because biological matrixes generally lack the strength for most engineering and structural applications.
Then there is natural wood, which can be processed to give it properties that rival steel and ceramics. But this processing requires harsh chemical treatments that are not environmentally friendly.
So there is intense interest in finding ways to turn ordinary plants into biocomposites that are sustainable and comparable in mechanical performance to processed wood and to conventional plastics.
Enter Eleftheria Roumeli and colleagues at the California Institute of Technology. This team has found a way to turn cells from tobacco plants into a hugely strong material with wood-like mechanical properties. “We have developed a new method to create natural biocomposite materials based on plant cells,” they say. “[The materials’] stiffness and strength surpass that of commercial plastics of similar density, like polystyrene, and low-density polyethylene, while being entirely biodegradable.”
The manufacturing method is straightforward. The team start with cells from the herbaceous plant Nicotiana tabacum, which they culture in liquid suspension in the lab. This widely grown plant produces leaves that are processed into tobacco.
These cells are well-studied and easily available to researchers. Some cell lines, such as the BY-2 line, can multiply 100-fold within a week when grown in suspension. Roumeli and co do not say what kind of cell they use, although BY-2 cells seem a reasonable choice, given the paper’s references.
Each cell has a cell wall strengthened by microfibrils made of proteins and cellulose, which effectively knot the wall together. The cell wall encloses the cell nucleus, various kinds of biomolecular machinery for processing energy and so on, and the cytoplasm, much of which is water. (BY-2 cell lines do not photosynthesize and so do not contain chlorophyll).
Having cultured the cells, the team harvest and compress them in a mold. The mold is permeable to allow water to escape. “During compression, water diffuses through the plant cell wall and the cell volume is gradually reduced,” they say.
Indeed, the cells lose 98% of their weight during this process. Most of this is due to water evaporation, but there are other processes at work, such as the degradation of complex biomolecules including pectins, hemicellulose, and phenolic compounds.
The team then heat the dehydrated material. This causes the microfibrils to undergo a phase transitions and form crystalline structures. “The obtained material is a biocomposite, comprised of a heterogeneous mixture of naturally synthesized biopolymers,” say Roumeli and co.
And it is remarkably tough. The team measured its mechanical properties and compared it to softwoods such as pine; hardwoods like poplar, oak, and walnut; and commercial plywood and MDF. They also compared it to synthetic plastics of similar density, such as polystyrene, polypropylene, and low-density polyethylene.
The results reveal how good this material is. “The mechanical performance of our biocomposites is comparable to that of commercial engineered woods and plastics,” say Roumeli and co. “They surpass all literature-reported values for materials composed of plant cells, mycelium, or yeast matrixes.
The team go on to make the material even stronger by adding carbon fiber. Indeed, they can further fine-tune the properties of the biocomposite with additives that make it conductive or magnetic.
An important question for sustainability is how this material degrades at the end of its life. The fear is that this kind of processing produces biopolymers that are so strong they do not break down easily.
To find out, Roumeli and co buried their samples in agricultural soil along with some ordinary wood and watched what happened. Both samples initially gained weight by absorbing water from the soil. But then both broke down naturally.
“The detectable mass loss due to biodegradation of the biocomposites begins 3 weeks after incubation, while for natural wood it begins about 7 weeks later,” say the team. “We observe an almost complete biodegradation of the biocomposite 14 weeks after initial incubation.”
That’s interesting work that deserves further attention. Biodegradable laptops, anyone?
Ref: arxiv.org/abs/1909.01926 : Plant Cells-Based Biological Matrix Composites