## 5 Coating Materials with Embedded Antibiotics and Other Antibofilm Agents
### Antibiotic Based Antib Biofilm Coatings
They achieved the sustained release of minocycline hydrochloride (MH), a broad-spectrum antibiotic, and anti-inflammatory drug belonging to the tetracycline family, from hydrophilic multilayer coatings prepared by the LbL self-assembly technique. The active antibiotic ingredient MH was loaded into the polyelectrolyte multilayer film as one of the multilayer components. As a tetracycline derivative, MH has a strong tendency to chelate Ca\({}^{2+}\) ions. Chelation sites include the end system in positions 1 and 3 of the ring A, the carboxamide function (Position 2), as well as the \(\beta\)-diketone system present at positions 11 and 12 (Figure 1a) [29]. One molecule of MH can chelate one or two Ca\({}^{2+}\) ions (Figure 1b) [30]. Dextran sulfate (sodium salt) was used as anionic polyelectrolyte (Figure 1c), while the cationic polyelectrolyte was gelatin type A (GA), a biodegradable natural polymer derived from collagen that is positively charged at physiologically relevant pH values. The self-assembled multilayer coatings were deposited on UV-transparent or black 96-well plates and on silicon wafers, the substrates being previously coated with an initiating positively charged base layer of PEI. The deposition was achieved simply by alternately dipping the substrate into solutions of DS, MH, and GA with or without Ca\({}^{2+}\) for 10 min and rinsing with deionized water between dipping steps. The LbL multilayer film growth and incorporation of MH and GA in the film were monitored by UV-vis spectroscopy and fluorescence spectroscopy, respectively. The incorporation of MH in the LbL coating was confirmed or infirmed spectrophotometrically by detecting the characteristic absorbance marker peak at 245 nm (DS and GA have negligible absorbance at this wavelength). Similarly, GA incorporation was evidenced by measuring the fluorescent intensity of the fluorescein isothiocyanate (FITC)-labeled GA. Interestingly, the authors found out that the formation of the LbL assembly and MH incorporation into the multilayered coatings crucially depend on the presence of Ca\({}^{2+}\) ions in the DS solution. This conclusion relies on the following experimental outcomes: (i) Negatively charged DS and positively charged MH-Ca\({}^{2+}\) chelates failed to form bilayer LbL assembly; (ii) on the other hand, when Ca\({}^{2+}\) ions were added to the DS solution, the LbL assembly was formed regardless of the presence or absence of Ca\({}^{2+}\) ions in the MH solution; (iii) although multilayered films of DS/GA could be successfully constructed, MH incorporation into the hydrophilic polyelectrolyte multilayer was not achieved in the absence of Ca\({}^{2+}\) from the DS solution. The authors rationalized these data concluding that MH incorporation is not ruled by electrostatic interactions but rather by a different mechanism mediated by calcium-binding. By chelating two Ca\({}^{2+}\) ions, MH acted as an effective crosslinker to bind Ca\({}^{2+}\) ions in adjacent DS layers, thereby promoting LbL assembly. Moreover, this calcium binding-mediated loading mechanism provided an opportunity to achieve the \"on-demand\" release of the loaded antibiotic The chelating ability is pH-dependent, i.e., the Ca\({}^{2+}\) binding affinity of MH decreases with decreasing pH. Hence, if the surrounding medium becomes more acidic, the chelation between MH and DS-bound Ca\({}^{2+}\) is weakened, facilitating MH release. Because acidic pH predominates at inflammatory loci and infection sites, this polyelectrolyte multilayer nanoplatform provides a pH-triggered smart drug release system for targeted delivery of MH.