I don’t mean to brag, but I have a really great mom. Sadly though, last week she twisted her ankle quite badly. According to her physical therapist, the injury caused damage to one of the nerves in her leg. So, in addition to the more traditional PT remedies (cold and hot packs, stretching exercises, etc), he prescribed a disposable iontophoresis patch, pictured here, for the administration of the anti-inflammatory drug dexamethasone. And the day after she was done with it, my mother, knowing of my fascination with electronics, mailed it to me for disassembly.
So just what is iontophoresis? (And why isn’t it spelled ionophoresis? That letter “t” really feels out of place to me…) Well, it is a method for drug delivery which utilizes direct electrical current to “push” charged ions through a patient’s skin – no needle required. It works based on the simple principle that like charges repel and opposite charges attract. So, if we have a drug which can be ionized – either positively or negatively charged – we can apply a like charge to the delivery electrode, and an opposing charge to the skin itself. This difference in electrical potential (aka voltage) will cause the charged drug ions to flow into the skin. Cool, right?
In practice, drug delivery using iontophoresis is quite simple. First, a solution of the drug to be dispensed is applied to the appropriate electrode (either positive or negative, depending on the charge of the ionized drug). Then, both the delivery electrode and a second, oppositely-charged electrode (necessary to complete the electrical circuit) are applied to the patient’s skin. Finally, a small electrical current, usually no more than a few milliamps, is applied between the two electrodes for a set amount of time. Dosages are specified in mA-min: the number of milliamps applied, multiplied by the treatment duration in minutes. The battery-powered patch shown above is rated for 80 mA-min when used for 3 hours. Thus, it delivers an average current of 80/(3*60) = 0.444mA.
Alright, let’s get started with the teardown. First off, I removed the labeled cover from the top electrode, which was held on by only a fairly mild adhesive:
As you might have guessed, I found a whole slew of 1.5V alkaline coin-cell batteries, linked together in series to produce 10.5VDC. Towards the bottom-middle of the battery compartment, you’ll notice a small bracket-shaped device. This is actually a spring-loaded switch which is held open by a removable tab (no longer present). This switch is necessary to prevent the batteries from discharging until the patch is applied and the tab removed. The underside of the battery compartment doubles as the positive electrode, and is attached to the conductive gelatinous pad shown in a previous image.
Before going any further, I was curious to see if I could bring the discharged device back to life once more. So, I punched a couple of small holes in the battery compartment and inserted my own wires. Those wires were connected back to a DC supply set to 10.5V:
Indeed, the LED (what Empi’s marketing department calls the “Smart Light”) illuminated once more! Now, I’d already suspected this to be a fairly simple system – basically a set of batteries in series with a resistor – but I decided to take some current measurements between the two electrodes to confirm this. To do so, I grabbed a bit of aluminum foil, some wires, a section of plastic wrap, and assembled it like so:
Not the prettiest thing, granted, but it did the job. The adhesive backing on the patch held the plastic wrap tightly in place, pressing the aluminum foil snugly against the two electrodes. I first inserted a 50k resistor in series with the electrodes, and measured a current of 143uA. I then lowered the resistance to 25k, and recorded a current of 251uA. Finally, I shorted out the two electrodes and saw a current of 1240uA. Clearly, there was nothing here performing current control. The amount of current this device delivers depends solely on the resistance of the patent’s skin. But, that’s probably fine. I mean, does skin conductance really vary much from patient to patient?
With that test complete, I removed my wires and proceeded to extract the opposite end of the circuitry from the drug delivery electrode (it was again held in by mild adhesive):
Nothing terribly exciting here, honestly – a flexible circuit board with five components, seven coin cell batteries, and a switch. The whole thing was quite easily traced:
Now I’m guessing, but I’ll bet that extra diode D1 is there to cause the LED to shut off sooner than it would with just a series resistance. The forward voltage across D1, about 0.7V, requires that much more voltage from the batteries for the LED to be illuminated. The purpose of R3, I’m guessing, is to prevent too much current from being delivered to the patient. And the function of R1 can only be to discharge the batteries more rapidly, which I’ll wager is done to guard against an excessively long dosing time.
So the question is, does iontophoresis actually work? Well I’m afraid the verdict’s still out on this one. One study indicates a measurable difference in the delivery of the drug dexamethasone, but does not demonstrate a tangible benefit to the patient. Overall, the results are mixed, and iontophoresis is still considered experimental by insurance companies (who won’t typically pay for it). Take a look at this great article for more info.
Oh and as for my mom, she’s doing just fine, although she isn’t quite sure whether or not to attribute that to iontophoresis. With so many other treatments being employed in parallel, it’s hard to tell what worked and what didn’t. But at a cost of $12, the patch she used was probably worth the try. I certainly got a kick of out it anyway.