Transdermal transport passive diffusion

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Iontophoresis by definition is the process of transport of ions into or through a tissue by the use of an applied potential difference across the tissue [52], Depending on the physicochemical characteristics of a molecular species, electrorepulsion is usually the primary mechanism of transdermal transport for ions, whereas electroosmosis and increased passive diffusion (as a result of the reduced barrier properties) are more prominent for neutral species [53]. In contrast, enhancement in flux for neutral or weakly charged species during electroporation arises predominantly from the reduced barrier properties of the membrane, whereas direct electrorepulsion is usually of secondary importance [25],... 

Since diffusion of permeants through a disordered bilayer phase can be significantly higher than that through a normal bilayer, transdermal transport in the presence of ultrasound is expected to be higher than passive transport. 

This hypothesis also explains why low-frequency ultrasound can induce transdermal transport of drugs which exhibit very low passive transport. Drugs possessing low passive permeabilities are either i) hydrophilic, which makes their partitioning into the SC bilayers difficult or ii) large in molecular size (for example, proteins), which reduces their diffusion coefficients in the SC. Low-frequency ultrasound may overcome both of these limitations by providing aqueous transport channels across the skin. Since these channels are filled with saline, hydrophilic drugs can easily partition into the SC. In addition, diffusion of... 

---