Nanoscale pores stop bacteria sticking to surfaces
The technology, created by researchers from Cornell University and Rensselaer Polytechnic Institute, uses an electrochemical process called anodization to create nanoscale pores that change the electrical charge and surface energy of a metal surface.
This exerts a repulsive force on bacterial cells and prevents attachment and biofilm formation.
Surface-criteria interaction calculated with the extended Derjaguin Landau Verwey-Overbeek (XDLVO) theory indicated less attachment and biofilm formation is due to a synergy between electrostatic repulsion and surface effective free energy, said the researchers.
Applying the process
When the anodization process was applied to aluminum, it created a nanoporous surface called alumina, which prevented E.coli ATCC 25922 and Listeria innocua from attaching.
The team told FoodQualityNews they are hoping to find industrial partners to test and validate the technology in actual processing settings.
Carmen Moraru, associate professor of food science at Cornell University, said the principle could be applied to a variety of surfaces, including walls, floors and conveyor belts.
“The surfaces will have an electrical charge that enables them to electrostatically repel bacteria, but there is no actual electricity going through them. Most surfaces (natural or synthetic) do have electrostatic charges,” said Moraru, who is also the paper’s senior author.
Anodized metals could be used to prevent build-ups of biofilms, which are hard to remove pathogens that get stuck on machinery and other surfaces in food manufacturing plants.
They form a tough surface skin that resist conventional commercial washing and sanitizing methods, resulting in lowered shelf-life of products and potential consumer illness.
Forming of nanoscale pores
Dr Diana Borca-Tasciuc, associate professor at Rensselaer Polytechnic Institute, said the nanoscale pores form by self-structuring of the alumina (aluminium oxide) layer created on the surface of aluminum during the anodization process.
“Anodization is an electrochemical process, in which the aluminium part is immersed in an acidic bath together with an electrode,” said Borca-Tasciuc, who was the anodization expert of the team.
“When current is applied between the part and the electrode, oxygen ions from the solution start reacting with the aluminium surface, forming an oxide layer,” she said.
“The diameter and depth of these pores depend on anodization conditions, such as chemical composition of the bath or applied voltage.”
The pores can be as small as 15 nanometers; a sheet of paper is about 100,000 nanometers thick.
Surfaces with 15 and 25nm pore diameters significantly repressed attachment and biofilm formation, found the study.
Anodized aluminum is already used in commercial applications, including cookware and is known to have anti-stick properties.
Alumina (aluminum oxide) has Generally Recognized as Safe (GRAS) status with FDA, which was one of the incentives to investigate the material, said Borca-Tasciuc.
“Anodic alumina is known to have good mechanical and corrosion resistance. In fact in many applications is used as coating because it is long-lasting. However, it may be affected by strong alkaline cleaners and thus may require special cleaning solutions.”
Moraru said they demonstrated the increase in surface area due to the cylindrical nanopores increases the repulsive forces exerted on the bacterial cells.
“We do expect that these surfaces also trigger a physiological response in the cells, and we are planning work to investigate these aspects.
“We will explore the use of anodization on other materials that are typically used in food processing or biomedical applications for the prevention of bacterial attachment.
“The RPI team will also focus on perfecting the anodization process to reduce the diameter of the pores, which according to our numerical calculations would further improve the bacteria repelling effect.”
Source: Biofouling: The Journal of Bioadhesion and Biofilm Research
Online ahead of print, DOI: 10.1080/08927014.2014.976561
“Alumina surfaces with nanoscale topography reduce attachment and biofilm formation by Escherichia coli and Listeria spp.”
Authors: Guoping Feng, Yifan Cheng, Shu-Yi Wang, Lillian C. Hsu, Yazmin Feliz, Diana A. Borca-Tasciuc, Randy W. Worobo and Carmen I. Moraru
