## 5 Coating Materials with Embedded Antibiotics and Other Antibofilm Agents
### Biofilm Control by Inhibitors or Modulators of Bacterial Signaling Processes
#### 5.5.2 Antibiofilm Coatings Are Interfering with c-di-GMP Signaling
[70] used branched PEI scaffold with plenty of secondary amino groups to cross-link \(N\)-succinyl chitosan sodium (CPCS) through amide linkage as depicted in Scheme 13. The intermediate CPCS cross-linked to branched PEI are gel materials capable of sustained release and were further treated with NO at a gas pressure of 70 psi for seven days to yield NONOate conjugated polymeric materials with a considerable increase in NO donor groups per mass unit. An overall amount of 2.031 umol NO was released in 5 h per mg material prepared at a molar ratio between CPCS and branched PEI of 1:4. The CPCS-branched PEI-NONOate conjugates showed excellent inhibition of both _S. aureus_ and _E. coli_ as well as low toxicity towards healthy mammalian fibroblasts. **Scheme 13. ** Synthesis of \(N\)-succinyl chitosan sodium (CPCS) cross-linked to branched poly(ethyleneimine (PEI) NONOate NO donor conjugates. The group of [PERSON] [71] incorporated the lipophilic dibutylhexyldiamine diazeniumdiolate (DBHD/N\({}_{2}\)O\({}_{2}\)) (Figure 6) into the hydrophobic, biodegradable, and biocompatible polymer PLGA. To minimize diazeniumdiolate or amine product leaching, the polymer matrix was further coated with a protective top coating of silicone rubber (SR). Since decomposition of the diazeniumdiolate NO donor is a proton-driven reaction, the pH within the inner polymer matrix can modulate the kinetics of NO release from these coatings. From this point of view, the matrix chosen by the authors is especially suited for controlled release applications as polyester PLGA can provide protons through its intrinsic carboxyl end groups and through its hydrolysis products, i.e., lactic acid (LA) and glycolic acid (GA). Hence, nicely exploiting the above property, the authors were able to prepare coatings with tunable NO release kinetics by controlling the rate of PLGA hydrolysis, which in turn is adjustable with the LA/GA ratio in the PLGA polymer. First, the authors used two types of PLGA, both having the same LA/GA ratio (50/50) but differing by the nature of the end groups: Methyl ester terminated PLGA that contained no acid groups (capped polymer) and -COOH terminated PLGA (uncapped polymer). The uncapped 50/50 PLGA showed a much higher NO starting flux compared to the capped material due to the presence of the terminal carboxyl groups that donate protons to promote DBHD/N\({}_{2}\)O\({}_{2}\) decomposition and NO release. Moreover, the water uptake of the bilayer coating caused the hydrolysis of the PLGA layer. While NO was released, the produced DBH diamine remained in the film-forming a quaternary ammonium hydroxide, which in turn may act as a basic catalyst accelerating the rate of PLGA hydrolysis. Consequently, the NO flux decreased rapidly with elapsed time due to the fast depletion of the NO donor. The hydrolysis rate could be slowed down by increasing the LA ratio in the PLGA polymer. Using a capped 75/25 PLGA, the NO release period could be extended up to eight days, while methyl ester terminated poly(lactic acid) (capped poly(lactic acid)) released NO for 10 days at 37 \({}^{\circ}\)C. The antibofilm activity of the prepared NO-releasing coatings was evaluated under dynamic conditions. Various PLGA/SR coatings containing 30% weight DBHD/N\({}_{2}\)O\({}_{2}\) and differing from one another by the LA/GA ratio and controls were incubated in a flow bioreactor at 37 \({}^{\circ}\)C with Gram-positive _S. aureus_ or Gram-negative _E. coli_ cultures for seven days. Two types of controls were used in these experiments: An uncapped 50/50 PLGA/SR bilayer and a SR layer alone. On the eighth day, the films were taken out of the reactor, stained with bacterial live/dead dyes, and biofilm growth was visualized with a fluorescence microscope. Compared to the control SR surface, the capped poly(lactic acid)/SR bilayer coating with 30% weight DBHD/N\({}_{2}\)O\({}_{2}\) produced about 98.4% reduction in biofilm biomass of _S. aureus_ and about 99.9% reduction for _E. coli_[71]. Recently, [PERSON] et al.