Iontophoresis

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Iontophoresis is a non-invasive method of propelling high concentrations of a charged substance, normally medication or bioactive agents, transdermally by repulsive electromotive force using a small electrical charge applied to an iontophoretic chamber containing a similarly charged active agent and its vehicle. To clarify, one or two chambers are filled with a solution containing an active ingredient and its solvent, termed the vehicle. The positively charged chamber, termed the anode will repel a positively charged chemical, while the negatively charged chamber, termed the cathode, will repel a negatively charged chemical into the skin.

Iontophoresis is well classified for use in transdermal drug delivery. Unlike transdermal patches, this method relies on active transportation within an electric field. In the presence of an electric field electromigration and electroosmosis are the dominant forces in mass transport. These movements are measured in units of chemical flux.

[edit] Mechanism

There are a number of factors that influence iontophoretic transport including skin pH, drug concentration and characteristics, ionic competition, molecular size, current, voltage, time applied and skin resistance. The current density of the treatment electrode is perhaps the most important variable relative to the degree of ion transfer. Studies suggest that comparable iontophoretic doses delivered at low currents over longer periods are more effective than those delivered by high currents over a short periods (Anderson et al, 2003).

The isoelectric point of the skin is ~4; therefore, under physiological conditions, with the surface of the skin also buffered at or near 7.4, the membrane has a net negative charge and electroosmotic flow is from cathode (-) to anode (+). The phenomenon of electroosmosis has been used as a means to augment the anodic delivery of (in particular) large, positively charged drugs, the transport numbers of which are often extremely small (and whose iontophoretic enhancement therefore depends heavily upon electroosmosis) and to promote the transdermal migration of uncharged, yet polar, molecules, the passive permeation of which is typically very small.

The application of a charge to the skin alters the skin’s permeability increasing migration of the active ingredient into the epidermis. There are a number of pathways that the ingredients could take, but research suggests that the majority of drugs permeate the skin via appendageal pores, including hair follicles and sweat glands, although some delivery is via the paracellular channels and minimal quantities are transcellular.

Transport of lipophilic drug molecules is believed to be facilitated by its dissolution into the lipid matrix of the stratum corneum however hydrophilic drugs which are thought to permeate through the open pores or cutaneous appendages (hair follicle and sebaceous glands) only accounts for 0.1% of the total skin surface area.^[1]

[edit] Uses

Reverse iontophoresis is the term used to describe the process whereby molecules are removed from within the body for detection. In reverse iontophoresis the negative charge of the skin at buffered pH causes it to be permselective to cations causing solvent flow towards the anode. This flow is the dominant force allowing movement of neutral molecules, including glucose, across the skin. This technology is currently being used in such devices as the GlucoWatch which allows for blood glucose detection across skin layers using reverse iontophoresis.

Iontophoresis is commonly used by physical therapists for the application of anti-inflammatory medications. Common diagnoses treated with Iontophoresis include plantar fasciitis, bursitis and some types of hyperhidrosis ^[2]. There are around ten current iontophoresis machines to treat hyperhidrosis^[3]. In this specific application, the solution chosen is usually tap water but better results can be obtained using glycopyrronium bromide, a cholinergic inhibitor^[4]. Iontophoresis of Acetylcholine is used in research as a way to test the health of the endothelium by stimulating endothelium dependent generation of nitric oxide and subsequent microvascular vasodilation. Acetylcholine is positively charged and therefore placed in the anode.

Use with Diagnosis and Monitoring of Cystic Fibrosis (CF): The most commonly-used form of testing for CF is the sweat test. Sweat-testing involves application of a medication that stimulates sweating (pilocarpine) to one electrode of an apparatus and running electric current to a separate electrode on the skin. This process, called iontophoresis, causes sweating; the sweat is then collected on filter paper or in a capillary tube and analyzed for abnormal amounts of sodium and chloride. People with CF have increased amounts of sodium and chloride in their sweat.

[edit] References

1. ^ Katrin Moser et al, Passive skin penetration enhancement and its quantification in vitro. 2001. J Pharm.
2. ^ Reisfeld, Rafael. "Iontophoresis and Hyperhidrosis". http://www.sweaty-palms.com/iontophoresis.html. Retrieved on 2007-11-23.
3. ^ "Iontophoresis experiences using various machines". http://www.no-ets.com/forums.
4. ^ "Iontophoresis". http://www.stopthesweat.info/iontophoresis/.