Analogous to the heat conduction through multiple layers, derive the molecular mass diffusion of a drug,

A through two layers $($skin and muscle$)$ of thicknesses x$1$ and x$2,$ respectively, and diffusivity of the drug

being DA$1$ and DA$2$ respectively. If the concentration of the patch drug on this slab geometry was CA$0$ at

the surface of the skin, and it is CA$1$ at the end of the muscle layer, derive the concentration profile

$($particular solution$)$ of the drug across both the layers, under steady$-$state conditions.