Additionally, the negatively charged PSS outer layer promotes the electrostatic adsorption of the positively charged DOX. Then, the adjustment of pH at 8.0 causes the shrinkage of the PEM, and the drug molecule is trapped
inside Stem Cells inhibitor the film. The subsequent washing will remove any nontrapped DOX molecule. Figure 4A was collected exposing the micropillar arrays to a laser excitation of 488 nm and using a 590 ± 30-nm bandpass emission filter (red channel). Bright red dots appear in correspondence with the micropillar pattern, which confirms the pH-controlled adsorption of DOX in the PAH/PSS multilayer. In addition, PEM-coated and DOX-loaded micropillars were detached from the silicon substrate in order to analyse the conformation of the polyelectrolyte multilayer and, subsequently, the DOX adsorption. Figure 4B shows a number of micropillars with uniform size and shape, exhibiting bright red fluorescence originated from the loaded DOX. This observation indicates a successful deposition of the polyelectrolyte multilayer on the micropillar sidewalls, in which no pore blockage occurred during the LbL self-assembly. The use of a multivalent salt such as CaCl2 assists the formation of the polyelectrolyte layer inside the selleck micropillars owing to a stronger polymer-chain contraction . Figure 4C shows a closer detail of a single hollow micropillar with a
homogeneous distribution of the DOX all along their wall, confirming the conformational PEM deposition along the micropillar walls. Figure 4 Fluorescence confocal images of PEM-coated and DOX-loaded micropillars. Fluorescence confocal micrograph of the micropillar arrays in top view after PEM coating (eight bilayers) and DOX loading (A); detached hollow micropillars with uniform size distribution (B); and single detached micropillar with PEM and DOX all along the walls (C). After the DOX loading, the micropillars were exposed to two different pH media to assess the pH responsiveness. Once in contact with the aqueous medium, the PEM film swells to a certain extent, increasing its permeability and allowing the diffusion of the drug. After the DOX releasing from the PEM film, the molecule
still remains inside the micropillar until it finally diffuses into the release medium through the micropillar opened-end. Figure 5A compares the Sclareol release profile of DOX from the PEM-coated micropillars at pH 5.2 and 7.4 over a period of 24 h. The data indicates that the release at pH 5.2 is higher than that at pH 7.4 (4.8 and 3.2 μg cm−2 after 24 h, respectively). This demonstrates the release rate is pH-dependent and increases with the decrease of pH. The swelling mechanism of PAH/PSS films is mostly related to the variation in charge density of polyelectrolyte chains induced by a change in the media pH. PAH is a weak polyelectrolyte whose amino groups become charged when the pH decreases, causing an increase in the osmotic pressure.