AN1209

AN1209
Iontophoresis Implementation Using a Low-Cost Microcontroller
Authors:
David Martin
Jonathan Dillon
Joel Mach
Microchip Technology Inc.
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
 2010 Microchip Technology Inc.
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.
DS01209B-page 1
AN1209
The software tests the comparator output to determine
the current level. If the current level exceeds the
required level, then the microcontroller does not switch
the MOSFET, otherwise the MOSFET is switched to
boost the voltage, driving more current through the
skin.
The output current is limited to the power available at
the input, times the efficiency of the converter, divided
by the voltage needed at the output (as shown in
Equation 1).
EQUATION 1:
THE OUTPUT CURRENT
I OUT = Pin *  / V OUT
  0.85 (measured on the demo board)
The duration of the infusion is controlled using the
built-in 16-bit hardware timer plus a 16-bit software
timer. When the desired dose is reached, the microcontroller stops switching the MOSFET and goes to Sleep
to await a button press.
For added patient comfort, the ramp rate of the voltage
output during the power-up sequence can be adjusted.
The software for the microcontroller is available from
the Microchip web site (www.microchip.com).
DS01209B-page 2
 2010 Microchip Technology Inc.
AN1209
FIGURE 1:
SOFTWARE FLOWCHART
Start
Hardware Initialization
A/D Conversion ISR
Start A/D Conversion
Start
Test the
Voltage
Output
Get A/D Result &
Start Next Conversion
Dose
Complete Flag
Set?
Y
Normal Shutdown
END ISR
N
Output Over
Voltage?
Y
Skip Pulse
Timer ISR
N
Start
Output Over
Current?
Y
N
Pulse MOSFET
4 Times
Output Stable
Flag Set?
N
Abnormal Shutdown
Y
Shutdown
Increment Software
Counter
Time the
Dose
Set Output Stable Flag
Has Dose
Been
Reached?
N
Y
Shutdown
Set Dose Complete
Flag
END ISR
 2010 Microchip Technology Inc.
DS01209B-page 3
AN1209
THE CIRCUIT SCHEMATIC
3
2
GP0/AN0/C1N+/ICSPDAT
GP4/AN3/T1G/OSC2
GP1/AN1/C1N-/ICSPCLK
GP2/AN2/C1OUT/CCP1
C6
Electrode
A
Gnd4
8
B
C2
2
R6
GP5/T1CKl/OSC1
GP3/MCLR/VPP
C4
Gnd5
VSS
7
6
5
U4
VDD1
1
U3
1
R2
R1
2
VDD
R3
Gnd3
In the circuit schematic (Figure 2), Q1 is the main
switching transistor. The MOSFET VDS breakdown and
the breakdown voltage of D1 should be greater than
the maximum desired voltage output of the circuit.
When the microcontroller detects that the output
current has dropped below the required level, it pulses
the MOSFET four times in rapid succession to boost
the voltage output. Four pulses are used to generate
more current flow and to speed up the rise time under
load. Alternately, the PWM can be used to drive the
MOSFET which allows higher output from the boost
circuit. R6/C6 is the current sense network.
4
3
C5
Gnd2
U7
2
1
B
A
1
Gnd
1
2
A
K 1
4
A
R5
1
1
2
Vsup
2
1
B
A
PIC12F683
1
1
1
Gnd1
1
R8
A
D2
K
VDD
C1
C1
2
1
B
S
R7
2
2
B
Q1
G
R4
C2
A
D
K
2 1
D1
A
C3
B
A
L1
A
B
FIGURE 2:
The design also includes two LEDs for the user
interface. There is a start button, which is connected to
the Reset of the part.
The PIC12F683 was selected as the microcontroller for
the device because of its small size, low-cost, internal
analog-to-digital converter, fixed voltage reference,
integrated comparator, PWM, hardware timer, and
internal EEPROM. The fixed voltage reference
eliminates the need for a regulator or an external
reference, and keeps the design to an 8-pin device to
lower cost.
DS01209B-page 4
 2010 Microchip Technology Inc.
AN1209
TESTING
Test Results
During testing, the following traces were taken:
FIGURE 3:
TURN-ON WITH 10K LOAD
Typical start-up condition.
The voltage rises over approximately 0.45 mS until the current set point, then remains at a steady level.
The current loop is set to approximately 1 mA.
 2010 Microchip Technology Inc.
DS01209B-page 5
AN1209
FIGURE 4:
TURN-ON WITH 20K LOAD
Typical start-up condition.
The output voltage is dependant on the current set point.
For the current set-point of 1 mA, the output voltage should be approximately 20V and has stabilized in 2.6 mS.
Using a 1µF ceramic capacitor as the output capacitor,
the voltage ripple is shown in Figure 5.
FIGURE 5:
DS01209B-page 6
REGULATED OUTPUT AT 20K LOAD (AC COUPLED TO SEE THE RIPPLE)
 2010 Microchip Technology Inc.
AN1209
Conclusions
The electronics required for Iontophoresis can be
implemented using a small, low-cost microcontroller to
control a DC/DC boost converter to drive a controlled
current through the skin. The software-based control
can be easily modified for additional features and for
changes in the dose and duration without requiring
hardware changes.
REFERENCES
AN1114, Switch
Topologies.
Mode
Power
 2010 Microchip Technology Inc.
Supply
(SMPS)
DS01209B-page 7
AN1209
APPENDIX A: COMPONENTS LIST
TABLE 1:
CIRCUIT COMPONENTS LIST
Component
Value
R1
1 kΩ
R2
1 kΩ
R3
1 kΩ
R4
1 MΩ
R5
36 kΩ
R6
500Ω
R7
0.5Ω
R8
10Ω
C1
10 nF
C2
1 µF
C3
10 nF
C4
10 nF
C5
10 nF
C6
10 nF
L1
10 µH
D1
1N914
D2
1N914
U3
LED
U4
LED
Vsup
3V
DS01209B-page 8
 2010 Microchip Technology Inc.
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DS01209B-page 9
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DS01209B-page 10
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