The cell membrane has selective permeability. This means that only certain substances can come in and go out of the cell.
Transportation across the cell membrane can occur in many different ways, depending on the size and characteristics of the substance.
Two methods of transportation are diffusion and osmosis
Osmosis is the movement of a water down its concentration gradient. This is often because the solute molecules are too large to pass through the pores of a membrane. The force with which this occurs is referred to as osmatic pressure -water is pulled from the dilute solution to the stronger solution across a membrane.
Water/hydration found in skincare products can be drawn into the skin using this method.
Diffusion is the movement of a chemical substance from an area of high concentration to an area of low concentration so that the solution is of equal concentration. It can occur in gases, liquids and solutions.
Some examples include nicotine replacement patches and HRT patches applied to the skin to deliver medication.