However, the most innovative aspect of the Biocoat programme is the use of biomolecules as an antibacterial agent. Cécile Van de Weerdt: “Our skin is involved in our immune system; it is equipped with a defence mechanism. This is even truer of batrachians, which often live in putrid environments. Their defence system is armed with ancestral molecules against which there is no known resistance. They are small, often positively-charged peptides because bacterial walls are negatively charged.  They work by directly attacking the walls of the bacteria; these aren't targeted attacks unlike our antibiotics, which attack the construction of the wall in a far more targeted manner by attempting to block this mechanism. The enzyme that recognises the antibiotic just has to change slightly for the bacteria to bypass the problem and adapt to its attacker.  This isn’t possible with the batrachians’ peptides. These molecules have yet another advantage. While small doses of these antibacterials are active against bacterial membranes, in larger doses, they are active against cell membranes such as human cells and, more specifically, cancerous cells.  Antimicrobial peptides are therefore of great interest to researchers.  New peptides are being entered into international databases every day.”

Silver nanoparticles

However, before developing a bio-inspired coating – the great innovation in the Biocoat project – it was necessary to master all the techniques. This was easier to achieve with a more classic, better known but non-biological antibacterial: silver nanoparticles. “One of the major functionalities at the core of our project”, Christophe Detrembleur explains, “aimed to provide industrial steel with permanent broad-spectrum antibacterial properties. Numerous methods can be used to provide surfaces with these types of properties but not many of them can be transposed to an industrial scale because it is too complicated and/or they use expensive compounds, or large quantities of toxic organic solvents. Developing a new concept of functional coating seemed important to us here. In concrete terms, the challenge was to develop an aqueous solution of a multifunctional polymer that combined bio-inspired adhesive and broad-spectrum antibacterial properties. In order to reduce the costs of the process, we envisaged applying the coating in thin, highly active layers (several tens of nanometres) in a minimum number of steps.  In 2009, we published a paper on such a highly efficient bio-inspired approach (J. Mater. Chem. 2009, 19, 4117-4125).”

So that’s the end of the research? Not at all. First of all because the method developed required yet more successive applications of the active ingredients. Which, of course, isn’t appealing to manufacturers. “Therefore, we wanted to simplify it”, Christophe Detrembleur explains, “by using the same basic products but by changing the method of applying them. We cleverly pre-assembled the active ingredients (bio-inspired hydrosoluble polymers for the adhesive properties and antibacterial silver nanoparticles for antibacterial properties) in water in order to have a ‘ready-to-use’ solution, i.e. ready to be applied to the substrate by simply dipping or spraying it. The system turned out to be highly effective and a lot faster than the previous one.” This research was published recently (Langmuir 2012, 28, 7233-7241).

Research that didn’t stop there because the antibacterial substance wasn’t organic and had no lasting characteristics: silver is expensive and polluting and as for the nano form, it raises questions if not fears. Not forgetting the real point of the matter: that the antibacterial properties of such assemblies aren’t permanent. The silver nanoparticles actually migrate off the film covering the steel to go and kill the bacteria. Once all the nanoparticles have left the film, it no longer has any antibacterial properties. The following step in the research therefore consisted of using antibacterial peptides that wouldn’t be rejected by the coating so that activity is maintained. “Grafting peptides wasn’t easy”, Christophe Detrembleur remembers. "They are sensitive molecules that mustn’t be heated, and you can’t use organic solvent you want. The goal was to work at room temperature in water." But mission accomplished here too (J. Mater. Chem. 2011, 21, 7901-7904).