The FFE software is based on the Hansen Solubility Parameters (HSP), which help determine how likely one material will dissolve in another to create a solution. The more similar a material is to another, the more likely that they will dissolve in one another. The three solubility parameters are δD which is the energy from dispersion bonds between molecules, δH which is the energy from hydrogen bonds between molecules, and δP which is the energy from dipolar intermolecular force between molecules.

The molar volume of a substance is the volume occupied by one mole at a constant pressure and temperature. A solid will not diffuse into the skin, which is why it is important to dissolve it in a substance that will deliver it. If an active is not fully dissolved, it has a smaller chance of being effective. Smaller molecules diffuse faster than bigger ones, and diffusion is how actives are carried through multiple layers of the skin.

Initially looking at the molar volume of certain actives can be discouraging because actives greater than 600 cm^3/ mol are unlikely to enter the skin. However, dissolving these actives using the Hansen Solubility Parameters give them a much better chance of entering the SC which then gives them a chance to work.