Multifunctional coatings based on tannic acid and pyrogallol compounds resembling the more complex polyphenols found in tea, wine and chocolate, were made by Northwestern University researchers.
Polyphenols are naturally occurring molecules found in plants whose functions include structural support and defense against bacteria and oxidative damage.
Dissolving polyphenol powder in water with a dash of salt produces colorless coatings that have antioxidant properties, are non-toxic and can kill bacteria on contact.
For example, emulating the natural role of plant polyphenols in biological defense, polyphenol coatings derived from tannic acid (TA) and pyrogallol (PG) displayed strong contact-based antibacterial properties against both gram-negative and gram-positive strains.
P. aeruginosa and S. aureus viability was reduced at least 30-fold upon exposure to polycarbonate (PC) modified with TA for three hours.
In a seperate study, researchers created antimicrobial packaging films containing high flavanol cocoa extracts to inhibit the growth of pathogens.
Sticky coating
The coatings can stick to virtually anything, including Teflon and could be used on materials used in food processing, packaging and during preparation, said Phillip B Messersmith.
Messersmith, the Erastus O. Haven Professor of Biomedical Engineering, told us that they exploited the stickiness of polyphenols.
“You can immerse the object into the solution, for food packaging, and for surfaces a spray coating approach might work.
“The time in the solution is about optimization steps depending on the needs of the application, from tens of minutes to several hours to deposit the coating, it depends on how thick you want them. In food packaging thinner coatings would probably be sufficient.
“It could be used in cases where you have transport of processed foods or raw foods or harvested plant tissues as it has a role in preventing spoilage and microbial growth.”
Immersing method
They found that immersing objects into a saline solution of tannic acid or pyrogallol results in spontaneous coating deposition.
Using these inexpensive precursors instead of the extracts improves the speed and lowers the cost of the process, increasing its commercial viability, the researchers said.
The researchers have also studied a coating called polydopamine, also based on phenols, which are found in the sticky glue that marine mussels use to stick to rocks.
Polydopamine has shown promise but the plant polyphenol-based coatings are colorless, so they don’t alter a material’s optical properties, and the compounds used to produce them are roughly 100 times cheaper.
Source: Angewandte Chemie
Online, ahead of print DOI: 10.1002/anie.201304922
“Colorless Multifunctional Coatings Inspired by Polyphenols Found in Tea, Chocolate, and Wine”
Authors: Tadas S. Sileika, Dr Devin G. Barrett, Ran Zhang, Dr King Hang Aaron Lau, Prof Phillip B. Messersmith