FIG. 6.

Drug diffusion from the microneedle through the homogenous skin is based on two different models: (a) Limited source diffusion model for coated microneedles, and (b) Constant source diffusion model for dissolving microneedles. (c) Drug diffusion demonstration through a porous skin model with porosity of $\epsilon$. Significant parameters for this model are the hydrodynamic radius of the drug $r_p$ and the hypothesized pore radius created on the skin $r_s$ that can be used to evaluate parameter $\lambda$. Geometric parameter considerations are the length of the microneedles, h, number of microneedles in a MNA, n, and the diffusion path length x. (d) Schematic diagram for a multilayer skin model. The structure consists of corneocytes blocks that are organized in a lipid lamellae medium. Significant geometric parameters in the multilayer skin model for drug diffusion are length, d, and height, h, of the corneocytes, horizontal gap, s, and vertical gap, g, between corneocytes layers, longest overlapping lipid path, $l_L$, shortest overlapping lipid path, $l_S$, and the number of layers, N. Subfigures (a) and (b) were reproduced from Wu et al.,⁹⁰ J. Drug Deliv. Sci. Technol. 61, 102192 (2021); licensed under a Creative Commons Attribution (CC BY) license. Subfigure (c) was reproduced with permission from Zoudani et al.,⁸⁷ Eur. J. Pharm. Sci. 150 (2020). Copyright 2020 Elsevier B.V.