Design of Iontophoresis Devices

Iontophoretic devices vary in complexity, but the basic design is a unit with a power source (either a battery or an on-line unit with a voltage regulator), a milliampere meter to measure the current, a rheostat to control the amount of current flowing through the system, and two electrodes. Platinum is the material of choice for the electrodes, since it releases almost no ions, undergoes degradation at a slow rate, and is nontoxic.

A variety of iontophoretic apparatuses exist for use in ocular iontophoresis. They mainly consist of either an eyecup or an applicator probe. Figure 1 shows a diagram of ocular iontophoresis of a positively charged drug in a rabbit. The eyecup, with an internal diameter of ~ 1 cm, is placed over the cornea and filled with the drug solution. A metal electrode that is connected to a direct current power supply is submerged in the solution in the eyecup without making contact with the surface of the eye. The ground electrode, connected to the other terminal of the power supply, is attached to the ear of the rabbit via wet (0.9% NaCl) gauze to ensure a good connection. With the hand-held applicator probe, the metal (platinum) electrode extends into the eyecup that is filled with the drug solution. The eyecup is placed against the eye and is held in place throughout the entire iontophoresis procedure. Iontophoresis requires a complete electrical circuit with direct current passing from the anode to the cathode and from the cathode back to the anode. The two electrodes are placed as anatomically close to each other as possible on the body, which is an excellent conductor of electricity, to complete the circuit.

ANODE CATHODE

ANODE CATHODE

Figure 1 Ocular iontophoresis in the rabbit. The drug is placed in a cylindrical eye cup with a central diameter of 9-12 mm; the inner circumference of the eye cup fits within the corneoscleral limbus. The current is controlled by a rheostat on the direct current transformer. In general, the current should not exceed 2.0 mA and the time be no longer than 10 min. In the case illustrated here, the drug molecules (cations) have a positive charge. Therefore, the platinum electrode connected to the anode (the positively charged pole) is placed in contact with the solution. The other electrode (cathode) is connected to the ear or front leg of the rabbit to complete the circuit. The positively charged anode drives the positively charged drug molecules from the solution into the eye at a greater rate than would be observed with simple diffusion.

Figure 1 Ocular iontophoresis in the rabbit. The drug is placed in a cylindrical eye cup with a central diameter of 9-12 mm; the inner circumference of the eye cup fits within the corneoscleral limbus. The current is controlled by a rheostat on the direct current transformer. In general, the current should not exceed 2.0 mA and the time be no longer than 10 min. In the case illustrated here, the drug molecules (cations) have a positive charge. Therefore, the platinum electrode connected to the anode (the positively charged pole) is placed in contact with the solution. The other electrode (cathode) is connected to the ear or front leg of the rabbit to complete the circuit. The positively charged anode drives the positively charged drug molecules from the solution into the eye at a greater rate than would be observed with simple diffusion.

The drug solution or preparation to be iontophoresed should be devoid or have a minimum of extraneous ions. Drugs with one or more pKa values either below pH 6 or above pH 8 are generally excellent candidates for iontophoresis into the eye because these drugs will be in the ionized form at the physiological pH of the eye (12). The salt form of a drug is also preferred for iontophoresis since the dissociated salt is highly soluble. The drug is driven into the ocular tissue with an electrode carrying the same charge as the drug while the ground electrode, which is of the opposite charge, is placed elsewhere on the body (usually the ear) to complete the circuit. The drug or bioactive substance serves as a conductor of the current through the ocular tissue. The transported drugs or bioactive substance either remain in the tissue until they are altered/metabolized or are carried away by the blood vascular network.

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