Biomedical Engineering Reference
In-Depth Information
Current Density Normally, an increase in current will increase drug delivery, but once
a certain transport number is achieved, further increase in current does not increase
delivery flux. Actually, the plateau may be achieved at a certain current density,
depending on the formulation used and the physicochemical properties of the drug
molecule. Once the limiting transport number is achieved, further increase in current
does not increase delivery flux. It was also shown that due to electroosmotic flow,
transdermal delivery of a large anion (or negatively charged protein) from the anode
compartment is more effective than delivery from the cathode compartment [43,44] .
Electrode Material Proper selection of electrodes is an aspect that is vital to success-
ful iontophoretic delivery of a drug. The electrode material in an iontophoretic device
is very important as it decides the type of electrochemical reaction taking place at the
electrodes. The possibility of introducing metallic ions into the skin must be care-
fully considered. In the past, stainless steel, nickel, or other iron alloys have been
used, but these are known to produce allergic reaction in the skin and are not being
used nowadays. Electrodes silver-silver chloride are the most preferred as they resist
the changes in pH which are generally seen during the use of platinum or zinc-zinc
chloride electrode. Platinum offers another inert material for electrode construction.
However, platinum causes electrolysis of water, resulting in pH drift. The oxidation
reaction at the anode and reduction at the cathode may be described as follows:
H O
2
H
Ο e
2
(
on the surface of the anode)
(12.2)
2
2
2
H O
2
e
H
2
OH
(
on the surface of the cathode)
(12.3)
2
2
The electron is released in the circuit, and insoluble AgCl precipitates at the anode
surface. In the case of other metals like platinum, the chloride ion at the anode will
be converted to Cl 2 , which will in turn react with water to generate hydronium ions.
As a function of iontophoresis time, the solution under the cathode becomes progres-
sively more basic, and the solution under the anode becomes gradually more acidic.
Because the type and magnitude of charge on peptides and proteins is directly depen-
dent on the background pH or solution, such pH shifts must be prevented. The small
mobile hydrogen and hydroxyl ions produced will carry a significant fraction of the
current. They will thus compete with the peptide for electric current, thereby reducing
the efficiency of iontophoretic transport. These pH changes may be avoided by using
reversible electrodes such as silver-silver chloride electrodes. Reversible electrodes
are consumed by the electrochemistry, and thus they do not force electrolysis of water
to be a fuel for the electrochemistry. The use of a silver wire for the anode and chlori-
dized silver for the cathode in a buffer containing chloride ion provides an ideal elec-
trochemical system [45] . At the anode, the silver will react with chloride ions to form
insoluble silver chloride. Simultaneously, the silver chloride cathode is reduced to sil-
ver metal, and these reactions prevent the electrolysis of water:
Ag
Cl
AgCl
e
(
at anode
)
(12.4)
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