## 2 Results and Discussion
### Effect of Acrylate and Ammonium Oligomer Molar Mass  
The first biological studies were carried out on surfaces obtained from the acrylate oligomers **(1)** (Figure 1). The parameters used to perform the polymerization were optimized and described in a previous publication [17], therefore they will not be discussed in detail here. The oligomer **(1)** possesses two acrylate groups; therefore cross-linked films were formed by the photopolymerization of the liquid viscous mixture of acrylates and (5% weight) of photoinitiator, deposited on the flat bottom of a well of a 24-well cell culture plate (50-70 mg in each). This system proved to be the most suitable to insert the suspension of bacteria in contact with the polymers, having several replicates each time and a control polystyrene surface on the same plate. The \"acrylate\" thick films were yellowish but transparent, non-significantly swelling and flexible. The thermogravimetric analysis (TGA, the thermogram is shown in Figure S1) showed that they start to decompose after heating at 180 \({}^{\circ}\)C, which means they could be sterilized in an autoclave at 100 \({}^{\circ}\)C if needed for a specific application. A procedure was optimized to wash the films after their synthesis and make sure that in the conditions of the biological assays, no leaching was occurring from the polymers. Plates were washed three times in sterile water and sonicated for eight seconds. No leaching being indeed a prerequisite for the possible development of environmentally-friendly polymers, an experiment to assess the toxicity of a medium having been in contact with the NR coatings was carried out. Surfaces obtained from the acrylate oligomers **(1)** (1700 g\(\cdot\)mol\({}^{-1}\)) were incubated with nutrient broth for 3 h and 24 h, at 37 \({}^{\circ}\)C. Then the supernatant media were removed to a fresh plate not containing the polymers and two different bacterial strains, _S. aureus_ (SA) and _Escherichia coli_ (EC), were added and normal 3 h growth assays were carried out (Figures S2). No difference in the growth curves was observed between bacteria grown in a fresh medium and bacteria grown in supernatant media that had been in contact with the coatings for 3 h or 24 h. This indicated that no leaching of toxic molecules had occurred in the conditions adopted for the biological tests. To demonstrate that the test was sensitive enough to evidence possible leaching, we ran the same experiment using a surface made with ammonium oligomers **(5)** (1700 g\(\cdot\)mol\({}^{-1}\)) but not completely cured, thus forming an incomplete three dimensional network from which small oligomers, unreacted photoinitiator, or impurities could be released. The same experiment described above was performed, with nutrient broth in contact with the polymer for 3 h or 24 h, and supernatants used as growth medium (3 h) for SA and EC. At the end of the 3 h growth period, the bacterial number decreased significantly, demonstrating that they had been negatively affected. To conclude, this means that if the curing process is effective and the film is completely formed, the resulting materials are non-leaching. In a first series of experiments, two different batches of acrylate oligomers were synthesized; the first one with molecular weight (MW) of 1700 g\(\cdot\)mol\({}^{-1}\) and the second one with MW of 4000 g\(\cdot\)mol\({}^{-1}\), to see if the chain length had any effect on the antibacterial properties. The influence of the oligomer molecular weight on the photopolymerization process was reported in a previous paper [17], however the surfaces were not submitted to biological tests. It was shown that conversion was lower for longer oligomers (98% for MW of 2000 g\(\cdot\)mol\({}^{-1}\)_versus_ 80% for MW 4000 g\(\cdot\)mol\({}^{-1}\)) because their superior viscosity reduced the chains mobility and limited the diffusion of active centers, leaving statistically unreacted chain ends.