Figure 6

Proposed model of the interactions between PLL and alginate in function of genipin crosslinking for the studied microspheres. The first PLL coating is well integrated in the alginate matrix by means of attractive electrostatic interactions. The second PLL coating interacts in a different way depending on whether or not genipin is included. (a) In the APP microspheres, the PLL chains of the second coating find less available negative charges in the alginate matrix to interact with. In addition, the positive charges of the first coating produce repulsion forces that hamper the integration of the second coating. This results in a loosely attached second PLL layer. The second PLL layer is therefore bulkier and generates high peaks on the surface that are detected as soft by AFM. (b) In GDP microspheres, genipin creates covalent bondings that pin both PLL coatings strongly to the alginate matrix, creating more densely packed membranes. Genipin crosslinking is enough to overcome the repulsion forces between the PLL chains, making possible for the second coating to reach the alginate matrix and increase the PLL-alginate interactions. These features can be readily probed with AFM, resulting in the disappearance of the above-mentioned soft peaks.