Fig. 4.

Microneedle coating methods that have been shown to coat both the microneedle shafts and the base substrate of the array. (A-i and ii) Fluorescent micrographs of titanium microneedles after immersion in a solution containing fluorescently-labeled ovalbumin (Matriano et al., 2002), (B-i) illustration of a drop placed on a microneedle array to coat the microneedles, (B-ii) top view of a silicon microneedle array after coating with a drop containing fluorescein isothiocyanate, white arrows point to tip of a microneedle (Vrdoljak et al., 2012), (C-i) concept of using a gas jet-assisted coating in which gas is blown at an angle theta (θ) to dry the drop of coating formulation placed on a microneedle array (Chen et al., 2011), (C-ii) fluorescent micrograph of a silicon microneedle array coated with polio virus labeled with red fluorescent DyLight 550, white arrows point to coated microneedles (Muller et al., 2017), (D-i) concept of spray-coating microneedles (McGrath et al., 2011), (D-ii) scanning electron micrograph of a silicon microneedle array spray-coated with a coating formulation containing fluorescein isothiocyanate (Vrdoljak et al., 2012), (D-iii) fluorescent micrograph of a single microneedle spray-coated with a coating formulation containing fluorescein isothiocyanate (Vrdoljak et al., 2012), (E) schematic illustrating the concept of electrohydrodynamic spray coating, (F) confocal micrograph of a microneedle array coated using the layer-by-layer method with alternate layers of diphtheria toxoid (green fluorescently labeled with AlexaFluor 488) and N-trimethyl chitosan, a cationic adjuvant that was red fluorescently labeled with rhodamine B isothiocyanate (Schipper et al., 2017), (G) scanning electron micrographs of poly(lactic-co-glycolic acid) microneedles coated using the layer-by-layer method with alternate layers of a poly(β-amino ester) and cross-linked vesicles (DeMuth et al., 2012). Figures used with permission from publishers.