Treating leather with chitosan could help create odour-fighting shoes

Portuguese researchers are working to make anti-microbial shoes by treating leather with a modified chemical from crab shells. Chitosan is a linear polysaccharide made from the deacylation of chitin, the structural component that makes up the shells of various crustaceans. In fact, chitin is the second most common dietary fibre after cellulose; however, it’s chitosan’s anti-microbial properties that are being exploited by Joana Amaral and colleagues at the Polytechnic Institute of Bragança.

Amaral, presenting her work at the 4th EuCheMS Chemistry Congress in Prague this week, described how leather shoes and boots provide exactly the ‘wet and warm’ conditions that fungi and bacteria thrive in. This can cause problems not just with odour and infections, but also with deterioration of the leather. Your smelly shoes can fall apart thanks to the action of the bacteria and fungi.

In acidic conditions, chitosan is protonated and becomes positively charged, allowing it to bind to cell walls. Amaral showed that a chitosan solution could be introduced at the leather dyeing stage, which uses formic acid, impregnating the leather with the polysaccaride and making the leather resistant to bacterial degradation. When patches of the leather impregnated with chitosan were placed in Petri dishes smeared with three different bacteria (Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus) there was an exclusion zone around the patches where no bacteria would grow.

With further optimisation, the group found that the best antimicrobial treatment was a 1% solution of chitosan that also contained chitosan microparticles impregnated with essential oils. This boosted the anti-microbial power of the treatment and gives the leather ‘a pleasant, long-lasting lemon odour’.

Although the research presented by Amar was performed using pilot drums – small examples of the equipment used in the tanning process – she tells Chemistry World that they’ve now extended the work to industrial-sized drums. Unfortunately, she says, this has reduced the anti-microbial effect slightly, which they think is because some of the chitosan solution is being absorbed by wood present in the machinery. They are now working to further optimise the process.

Friction and wear may also reduce the effectiveness of the treatment, as anyone who’s ever transferred their shoe colour onto their socks might suspect. Amar admits they are also worried about this, and while they haven’t yet tested how their treated leather stands up to wear, they are also looking to develop their technology into a spray that can top up the protection, or treat shoes that haven’t been made from treated leather.

On 13/02/14 the institute of Joana Amaral was corrected to the Polytechnic Institute of Bragança