Audrey Moores, an associate professor in McGill University’s Department of Chemistry, never forgot the students who tried to save some money while working in her lab a few summers ago. Michael Malca and Pierre-Olivier Ferko were using polyethylene glycol (PEG) to stabilize nanoparticles for a biomedical application and Moores told them to buy PEG amines for the purpose.
“They said that was too expensive and they wanted to try making it instead,” she recalls. “At the time I couldn’t believe it, because most students don’t seem to pay much attention to how they’re spending the group’s money.”
The students turned to mechanochemistry, running the PEG through a ball miller. Ordinarily that action would break down all of the polymer chains in a sample, but by taking a “soft” approach — placing only a single ball in the mill — the chains instead became functionalized. This technique yielded not just the desired amine, but also a variety of other products, including PEG bromide and PEG sulphate.
Research Assistant Thomas Di Nardo, who worked with Audrey Moores on a new technique for refining chitosan from chitin. Photo credit: Jessica Goodsell, McGill University
Another student, Thomas Di Nardo, subsequently applied this idea to transforming the polymer chitin — a long chain polysaccharide found in the exoskeletons of shellfish —into the valuable source material chitosan, whose linear polymers have a wide range of commercial uses. Specifically he combined soft milling with reactive aging or RAging, a technique where the mixture is simply allowed to mature for several days. As a bio-based feedstock, chitosan has become increasingly attractive for agricultural pesticides, polyurethane paint coatings, and antibacterial bandaging.
“It’s a pretty amazing material,” says Moores, who has been approached by a wide range of prospective industrial partners who regard chitosan as a desirable, bio-based replacement for petroleum feedstocks in their supply chains.
That goal is nothing new and some firms had previously set up plants to generate chitosan from large amounts of shellfish processing waste. However, the standard approach called for large amounts of base, specifically sodium hydroxide (NaOH), more familiar as the caustic soda traditionally used in soap-making. When deployed in solution, NaOH takes a viscous, concentrated form at temperatures around 130C, creating hazardous working conditions that generate a considerable amount of liquid waste.
In contrast, RAging employs NaOH in powder form, which never reaches more than 50C and poses far less risk to workers or the environment. Above all, this method retains molecular features that could add even more value to the resulting chitosan.
“In the solution-based method, you depolymerize your chitin as you produce chitosan,” she explains. “Because the conditions are mild, we can control the chemistry to do de-acetylation but far less de-polymerization. We remain with a system that has long polymeric chains, which means that it no longer dissolves in water.”
While soluble chitosan is desirable for some applications, such as biomedical hydrogels, an insoluble version would lend itself to processes such as trapping metals found in water. Rather than trying to find a way of attaching some hydrophobic material to the chitosan for this purpose, this kind of chitosan can be used by itself.
Moores has placed these initial findings on the preprint server ChemRxiv and expects journal publications to follow soon. In the meantime, she is weighing the many different ways of refining chitosan for commercial purposes.
“The challenge right now is that there’s so much that we can do with it,” she says. “We need to narrow down our focus to the most realistic avenues so we push this to applications.”