High voltage constant current source for iontophoresis

A regulated current high voltage source for use in driving an iontophoresis probe. The source provides a highly regulated current to the iontophoresis probe up through voltages substantially in excess of a kilovolt in order to maintain the probe ion injection rate under conditions of partial probe blockage. The source employs a DC-to-DC inverter, comprising a blocking oscillator and pulse transformer, with the load provided in the output circuit of the transformer in a feedback arrangement to a regulating amplifier and driver circuit for the DC input to the pulse transformer and blocking oscillator. A separate timing oscillator having a period of several seconds is optionally employed to disable the source and eliminate the high voltage output in order to generate intermittent ion injection.

FIELD OF THE INVENTION 
The present invention relates to regulated current sources and in 
particular to a current source for use in driving an iontophoresis probe. 
BACKGROUND OF THE INVENTION 
In anatomical studies aimed at tracing the path of body sensory signals 
between the location of a stimulus and the brain cortex, it is common to 
use an iontophoresis probe to inject a controlled and constant rate of 
ions into the region of the body under study with a resulting effect upon 
the cortex area responsive to that region of stimulation permitting the 
correlation of source and receptor sights. The iontophoresis probe 
typically comprises a pipette having a very narrow opening, as small as 5 
microns, through which ions generated within the probe are applied to the 
anatomical specimen. The ions are generated by electrolytic reaction 
within the pipette from voltage applied to it from an external source. 
Typical electrolytes include HRP (HORSERADISH, PEROXIDASE) or TPL 
(TRITIATED PROLINE OR LEUCINE). 
In effect, the electrolyte converts the electrical voltage applied to the 
pipette into an ionic current which then flows into the body when the body 
is made a part of the circuit with the current source. It is important to 
maintain a constant flow of ions at a relatively low current level, 
typically measured in small numbers of microamperes, for valid and 
accurate use of the iontophoresis technique. While it is possible to 
maintain well regulated currents of this level under normal conditions, 
the pipettes used in iontophoresis, particularly those with small 
apertures, are subject to clogging through blood clots, air bubbles, or 
other mechanisms. Such clogging drastically increases the impedance of the 
pipette circuit through the electrolyte and body under study from megohm 
ranges up to near open circuit conditions. Such blockages can impede or 
eliminate the flow of ions from the pipette to the body resulting in 
abandonment of the experiment 
BRIEF SUMMARY OF THE INVENTION 
It has been discovered that the effect of clogging of pipettes and other 
phenomenon contributing to a drastic increase in the impedance of the 
pipette and body circuit can be counteracted using a current source which 
has the capability to provide instantaneously, upon demand, extremely high 
voltage outputs to counteract the drastic increase in impedance and in 
effect to flush out the pipette clog. Such high voltages, up to 
substantially over a kilovolt, required to clear a clogged pipette under 
operating conditions without interruption of the iontophoresis effect, is 
achieved in a current source according to the present invention in which a 
DC-to-DC inverter featuring a pulse transformer and blocking oscillator is 
provided to generate a high voltage output in response to a relatively low 
voltage input. The high voltage output is applied to the iontophoresis 
probe and body circuit in a closed loop. The current flowing in this loop 
is sensed at a low voltage level and regulated at low voltage with the 
regulated low voltage signal used to drive the blocking oscillator in the 
primary of the pulse transformer. 
The combination of low voltage regulation permits very accurate output 
current regulation within less than a percent and in combination with the 
DC-to-DC inverter permits the realization of extremely high output 
voltages, typically 1,500 volts, where necessary to overcome the effect of 
pipette clogging as normally encountered in the iontophoresis process. 
An optional oscillator having a long period of several seconds is employed 
to intermittently disable the output of the DC-to-DC inverter so as to 
provide intermittent operation of the iontophoresis probe. This 
intermittent operation is advantageous in circumstances where continuous 
operation might damage the tissue under study. 
The low voltage control and regulation circuitry is provided with an 
adjustment in the low voltage output used to drive the DC-to-DC inverter 
so that the current applied to the iontophoresis probe may be regulated at 
any desired level.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention contemplates a current source particularly adapted 
for use in driving an iontophoresis probe in a manner to provide a high 
voltage output to the probe under conditions of probe clogging or other 
phenomenon resulting in a very high increase in probe circuit impedance 
that would otherwise terminate the flow of a regulated source of ions to 
the body site under study. 
With reference not to FIG. 1, there is shown a generalized block diagram 
illustrative of the present invention in its application as a regulated 
current source for an iontophoresis pipette probe. As shown in FIG. 1 the 
tissue 12 of a body under study has the tip 14 of an iontophoreses pipette 
probe 16 applied therein to a region to be stimulated by the regulated and 
constant flow of a desired ion such as from electrolysis of HRP or TPL. 
The application of such ions is utilized for such purposes as correlating 
stimulus source and receptor regions. 
The probe 16 is energized by current through a lead 18 which may include a 
current limiting resistor 20 of very high, several megohm resistance, to 
prevent excessive current drain in the case of short circuit conditions. 
The probe 16 is typically of pipette construction having within the 
central region 22 an electrolyte of one of the above-identified or other 
materials suitable for use in iontophoresis. The electrolyte is contacted 
by an extension of the lead 18 into the pipette probe 16. 
Current for electrolytic reaction within the probe 16 is supplied by a 
current source 24 in accordance with the present invention to provide 
regulated current of a selected magnitude in combination with the ability 
to achieve high voltage application through the probe 16 to correct or 
compensate for extraordinary high and varying impedances encountered as, 
for example, from clogging of the tip 14. Such an effect is achieved by 
driving current through the lead 18 from a high voltage secondary 26 of a 
pulse transformer 28 with a voltage capability of typically 1,500 volts to 
overcome the high impedances in contrast to normal operating voltages of a 
few hundred volts or less. Transformer 28 is driven at its primary coil 30 
by a transistor blocking oscillator 32 having a feedback coil 34 to 
promote and control oscillation. The pulse transformer 28 typically 
provides a step-up of 3 : 1000 in voltage and generates a squarewave 
output at the secondary 26 which is rectified and filtered in a 
filter-rectifier module 36. 
Low voltage of a regulated and preselected magnitude is, on the other hand, 
applied to the blocking oscillator 32 at a level which is more readily 
dealt with using highly accurate transistor and integrated circuit control 
circuitry. The low voltage signal applied to the blocking oscillator 32 on 
a lead 38 is buffered by a driving circuit 40 to provide sufficient 
current for driving the blocking oscillator 32 and pulse transformer 28. 
The driver 40 receives a regulated DC signal from a regulator 42 having a 
control 44 to adjust the magnitude to which the voltage applied to the 
driver, and ultimately, transformer 28 is regulated. 
To provide current regulation, negative feedback is received from the body 
tissue 12 through a connection 46 in series with a meter 48, useful in 
adjusting the control 44, to the regulator 42. The current flowing through 
the lead 46, the same current which is applied through the probe 16 to the 
tissue 12, is converted to voltage through a resistor 50 for application 
to the regulator 42. The regulator 42 operates to maintain a set 
relationship between the voltage signal applied to regulator 42 by action 
of resistor 50 and a reference signal supplied by the setting of the 
control 44. Elements 40, 32, 28 and 36 are viewed as a DC-to-DC inverter 
in the example of the detailed description. In this manner, the control 
circuitry and in particular the regulator 42, driver 40, and blocking 
oscillator 32 are all operated at low voltage permitting accurate 
regulation using transistor circuitry. High voltage is generated through 
the pulse transformer 28 and filter circuit 36 permitting and making 
available high voltage signals upon demand to respond to abnormal, high 
impedance conditions of the probe 16. 
As an optional feature of the current source of the present invention, a 
periodic disable generator 54 is provided to disable the regulator 42 and 
shunt its control function so as to remove the output voltage applied 
through the driver 40 to the blocking oscillator 32 thereby removing the 
output voltage from the secondary winding 26 and interrupting the flow of 
current to the probe 16. The generator 54 is typically operated at a 
period of several seconds per cycle in an automatic mode to interrupt the 
operation of the probe 16 and the supply of ions through the tip 14 in 
order to permit the tissue 12 to recover periodically and prevent its 
saturation and injury in the operation. 
With reference now to FIG. 2, the complete circuitry in accordance with the 
preferred embodiment for the present invention is shown. It is to be 
understood that deviations from the specifically indicated circuitry are 
contemplated within the scope of the invention. In particular, the 
transformer 28 is shown in FIG. 2 to have secondary winding 26 in series 
with a rectification diode 60 and filter capacitor 62 with an output taken 
across the capacitor 62 between circuit common and the junction with the 
diode 60. The primary coil 30 is connected on one end to the collector of 
a common emitter transistor 64 used as a blocking oscillator. The other 
end of primary coil 30 receives on a terminal 66 the regulating low 
voltage DC for the blocking oscillator. A zener diode 68 in series with a 
resistor 70 connects the collector of transistor 64 to ground as a 
protection against excessive voltages at the output of the transformer 28 
under conditions of open circuit load. The feedback coil 34 in the primary 
of pulse transformer 28 is connected between the base of the transistor 64 
and the terminal 66 through a current limiting resistor 72. Stabilization 
capacitors 76, 78 and 80 are provided respectively to ground from coil 34 
and terminal 66 and between the base and collector of transistor 64. 
The driver circuit 40 includes a series of buffer transistors 82, 84 and 86 
with the transistors 82 and 86 emitter-follower coupled and the collector 
transistor 84 coupled into the base of the transistor 82. The three 
transistors 82, 84 and 86 buffer the output of an amplifier 88 having 
differential inputs 90 and 92. The output of the amplifier 88 is applied 
through a voltage divider composed of resistors 93 and 95 to the base of 
transistor 86. 
The amplifier 88 is preferably a high gain FET input transistorized or 
monolithic amplifier with a response time of at least a few microseconds. 
The inverting input 90 receives feedback current from the output of the 
secondary 26 as applied through a limiting resistor 20 and the load 94 of 
pipette and body tissue through a double pole, double throw, center 
off-switch 96, which provides polarity reversal. Current meter 48 is also 
provided in the feedback circuit. 
The noninverting input 92 of amplifier 88 is supplied with a variable 
reference voltage through a resistor 98 from a potentiometer 100 adjusted 
in potential between a positive voltage 102 and ground. 
The periodic disable generator 54 includes a multivibrator 104 operating in 
an astable mode to provide a periodic squarewave or rectangular wave 
output through a resistor 106 to the base of transistor 108 operative to 
periodically ground the noninverting input 92 forcing the output of the 
amplifier 88 to assume a value which eliminates the drive voltage applied 
to the blocking oscillator transistor 64. The same output from the 
multivibrator 104 is also applied through an input resistor 109 to the 
base of a transistor 110. Transistor 110 drives a relay 112 in its 
collector circuit. The relay 112 when energized closes a set of contacts 
114 and 116 to connect the load 94 to ground during the period of 
nonenergization to prevent continued energization from the stored charge 
in the capacitor 62. A Switch 118 is provided to permit deactivation of 
the periodic disable generator 54 as by maintaining an on bias at its 
output as may be desired or not by the particular conditions of operation. 
The above-described specific implementation for the preferred embodiment of 
the present invention is to be considered as exemplary only with 
alterations and improvements intended to fall within the scope of the 
invention as limited only in accordance with the following claims.