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2010 Microchip Technology Inc. DS01209B-page 1
AN1209
ABSTRACT
Iontophoresis is a process used to deliver drugs
through the skin into the body. A transdermal drug is a
charged compound driven through the skin by the flow
of electrical current. To deliver the correct dosage the
current flow through the skin must be actively
controlled. This can be performed by means of an
automated system.
INTRODUCTION
Iontophoresis is the method of using an electrical
current to assist the infusion of a drug through the skin.
The advantages to this approach are many. First, the
medicine can be dosed at very high levels locally,
rather than a lower dose distributed throughout the
body. Second, there are far fewer side-effects
associated with localized application of the medicine.
At very high levels, the efficacy of the medication can
be greatly improved. In order to accomplish this, a
specially formulated medicine is prepared, which
bonds to the electrons and is moved by current through
the skin. Historically, this has required significant
electronics and a trained operator to monitor the
current and the necessary safety features to protect the
patient. However, with recent advances in technology,
switched mode power supply design, and
cost-effective, high-performance microcontrollers, the
production of low-cost or single-use dispensers for
these drugs has become possible. This proof of
concept design uses a low-cost, 8-bit PIC12F683
microcontroller with mixed signal features and some
off-the-shelf components.
IMPLEMENTATION
To infuse the drug through the skin, the device must
produce sufficient voltages to drive the current level
needed for the specific infusion dose rate for the
required duration period. The goal is to control the
current flow through the skin, but, for safety reasons,
the device should ensure that it does not generate
excessive voltage. Otherwise, should the device
become detached from the patient, breaking the
current path, the control electronics would attempt to
increase the voltage to maintain the current flow, which
could cause discomfort on reattachment.
A boost regulator is used to step up the voltage from a
low-voltage battery to sufficient levels to pass the
required current through the skin. A discontinuous
boost regulator topology was selected as it does not
require the processor to provide a pulse at a specific
time, allowing the current through the inductor to fall to
zero. This simplifies the software development.
The microcontroller is configured with an external
asynchronous Reset pin (Master Clear/MCLR
).
Bringing this pin low will reset and wake the
microcontroller from a low-power shutdown state
(Sleep mode). The software currently goes to Sleep
once it completes administering an infusion, and the
button connected to the MCLR
pin pulls the line low,
triggering a wake-up from Sleep mode. When the
device is woken from Sleep, it begins executing code
from the Reset vector (0x0000 in program memory),
which is the same for any other Reset including
power-up. The number of infusions that have been
administered is stored in the internal EEPROM, which
may be important depending on the implementation
and the medication being administered. The circuit
uses two AA alkaline cell batteries to provide power to
the microcontroller and to the switching regulator.
The software monitors the voltage supplied to the skin
using the microcontroller’s built-in A/D converter, and
compares it against a set threshold. If the voltage
exceeds the predefined limit, the microcontroller will
stop switching the MOSFET, preventing the voltage
from being boosted higher. This feature limits the
output voltage to a safe level, should the device
become detached from the skin. The predefined limit is
set in the software, however there is some scaling of
this value as the voltage applied to the skin is greater
than what can be applied to an input pin of the
microcontroller or can be converted by its A/D
converter. The applied voltage is scaled by resistors R1
and R2 as shown in Figure 2 (The Circuit Schematic) to
within the supply rails of the microcontroller, 0 and 3V.
The current used in Iontophoresis varies with the
medication and, in general, needs to be validated with
the particular formulary. The current is controlled by an
external resistor, R3, and the internal comparator of the
PIC12F683. The comparator threshold is set in the code
by defining the desired current level, 0.5 mA-4 mA.
Authors: David Martin
Jonathan Dillon
Joel Mach
Microchip Technology Inc.
Iontophoresis Implementation Using a Low-Cost Microcontroller
