Structure and method of manufacture of an implantable microstimulator

An implantable microstimulator has a structure which is manufactured to be substantially encapsulated within a hermetically-sealed housing inert to body fluids, and of a size and shape capable of implantation in a living body, by expulsion through a hypodermic needle. The internal structure of the microstimulator comprises a coil adapted to function as the secondary winding of a transformer and receive power and control information. Circuit means, including control electronics, a capacitor and electrodes are provided. The electrodes, which may be made one of iridium and the other of tantalum and placed on opposite ends of the microstimulator, or alternatively, an iridium electrode at each end of the microstimulator, are at least partially exposed and provide electrical, stimulating pulses to the body.

This invention relates to the structure and method of manufacture of a 
microstimulator for implantation in a living body, in the immediate 
vicinity of tissue, fluids or other body cells which are desired to be 
electrically stimulated. It is of a size and shape capable of implantation 
by expulsion through the lumen of a hypodermic needle. 
The microstimulator is substantially encapsulated within a 
hermetically-sealed housing which is inert to body fluids and provides 
exposed electrodes for electrically stimulating the desired body cells, 
whether muscle, nerve, receptor, gland or other area or organ of the body. 
This application relates to two other patent applications, one filed on 
Dec. 18, 1991, entitled Implantable Microstimulator, Ser. No. 07/812,136 
and having the same inventors as herein and the other filed on Dec. 12, 
1991, entitled Implantable Device Having an Electrolytic Storage 
Electrode, Ser. No. 07/806,584, invented by one of the inventors hereof. 
BACKGROUND 
Neurological disorders are often caused by neural impulses failing to reach 
their natural destination in otherwise functional body systems. Local 
nerves and muscles may function, but, for various reasons, injury, stroke, 
or other cause, the stimulating nerve signals do not reach their natural 
destination. For example, paraplegics and quadraplegics have intact nerves 
and muscles and only lack the brain to nerve link, which stimulates the 
muscles into action. 
Prosthetic devices have been used for some time to provide electrical 
stimulation to excite muscle, nerve or other cells. Such devices have 
ranged in size and complexity from large, bulky systems feeding electrical 
pulses by conductors passing through the skin, to small, implanted 
stimulators which are controlled through telemetry signals, such as are 
discussed in U.S. Pat. No. 4,524,774, Apparatus and Method for the 
Stimulation of a Human Muscle, invented by Jurges Hildebrandt, issued Jun. 
25, 1985. Other devices have comprised a centrally-implanted stimulator 
package sending stimulation signals to a multitude of distant target 
sites. 
Complications, including the possibility of infection, arise in the use of 
stimulators which have conductors extending through the skin. On the other 
hand, in the use of implanted stimulators, difficulties arise in providing 
suitable, operable stimulators which are small in size and have the 
capability to receive and store sufficient energy and control information 
to satisfactorily operate them without direct connection. 
The device of the invention uses a source of electrical energy outside the 
skin, modulated by desired control information, to selectively control and 
drive numerous, small stimulators disposed at various locations within the 
body. Thus, for example, a desired, progressive muscular stimulation may 
be achieved through the successive or simultaneous stimulation of numerous 
stimulators, directed by a single source of information and energy outside 
the body. 
The construction of a microstimulator presents problems of its own, which 
are not encountered in the construction of larger-sized biomedical 
appliances. The extremely small size involves problems and solutions of a 
different nature than are ordinarily involved. The appropriate design of a 
suitable, small stimulator, a microstimulator, which can be easily 
implanted, such as by expulsion through a hypodermic needle, is difficult 
to achieve. Notwithstanding the small size and required shape, the 
microstimulator structure must contain means for receiving and storing 
sufficient energy to provide the desired stimulating pulses, as well as 
electronics which provides control of the characteristics desired of the 
stimulating pulse. 
SUMMARY OF THE INVENTION 
This invention teaches an implantable, microstimulator useful in a wide 
variety of applications. Others have proposed microstimulators and have 
suggested constructing them, but none have taught all the elements set 
forth herein for successful construction and operation of the 
microstimulator. 
The device of the invention is a very small stimulator, a microstimulator, 
which can be easily implanted, such as by expulsion through a hypodermic 
needle and which microstimulator provides electrical stimulation pulses of 
desired characteristics. Stimulation pulses are delivered to the body 
through electrodes exposed on the outer surface of the microstimulator. 
Within the microstimulator, an induction coil receives energy from outside 
the body and a capacitor is used to store electrical energy which is 
controllably released to the microstimulator's exposed electrodes. The 
body fluids and tissue between the exposed electrodes provide the 
electrical path for the stimulating pulse. The capacitor is controllably 
recharged, using the same or different exposed electrodes. In this manner, 
a "charge balancing" is achieved, that is, a balancing of current flow 
through the body tissue in both directions to prevent damage to the tissue 
which results from continued, preponderance of current flow in one 
direction. 
The induction coil which receives energy and control information from a 
modulated, alternating magnetic field acts as a secondary winding of a 
transformer and receives the energy which is rectified and stored on a 
capacitor. The modulation is detected and decoded to provide the desired 
control information. 
Of great importance in the microstimulator are the structures of the 
electrodes. They must meet the requirements of inertness to body fluids, 
be hermetically-sealable to the housing, and formable to the desired size 
and shape. They must meet the operating requirements of the electrical 
circuit and not deteriorate in cathodic and anodic operation, as the 
stimulating pulses are generated and as the recharging of the system 
occurs. 
It is, therefore, an object of this invention to provide a microstimulator 
that is inert and hermetically-sealed, and of a size and shape capable of 
implantation by expulsion through a hypodermic needle. 
Another object of this invention to provide a microstimulator structure 
housing a coil adapted to receive an alternating magnetic field to provide 
a source of power for the microstimulator. 
It is another object of this invention to provide a microstimulator 
comprising capacitor means for storage of electrical energy. 
Still another object of this invention is to provide a microstimulator 
which contains an assembly of induction coil and electronics to receive 
and detect a signal modulating an alternating field and to control a 
stimulating pulse in accordance with such modulation. 
A still further object of this invention is to provide a microstimulator 
having at least partially-exposed electrodes. 
Another object of this invention is to provide a method of manufacture of a 
microstimulator. 
A further object of this invention is to provide a method of manufacture of 
the electrodes of a microstimulator. 
A final object of this invention is to provide a method of sealing said 
microstimulator to its exposed electrodes.

DETAILED DESCRIPTION OF THE DRAWINGS 
The microstimulator of the invention is on the order of 2 mm in diameter 
and 10 mm long. Because of such diminutive character, it is readily 
implanted in a living person or animal through the lumen of a hypodermic 
needle. But, because of its small size, it has been difficult to establish 
the parameters of such a microstimulator in order to obtain the desired 
operating characteristics. The following description sets forth suitable, 
working parameters for constructing such a microstimulator. 
FIG. 1 shows, figuratively, how an external, primary coil 1, which produces 
an alternating magnetic field, at a frequency, say, of 2 mHz, is disposed 
with respect to a number of microstimulators such as 2, 3, and 4, 
implanted, say, in an arm 5. The microstimulators, of course, may be 
planted in or near any part of the body, in the brain, a muscle, nerve, 
organ or other body area. The system operates as an air-gap transformer in 
which coil 1 is the primary winding, exterior to the body, and the 
microstimulators 2, 3 and 4 each have coils within them which act as 
secondary windings of the transformer. 
Coil 1 may, for example, be 12 to 20 turns of #200/38 Litz wire, and wound 
20 cm long and 9 cm in diameter for operation with, for example, 256 
microstimulators implanted in an arm. Each microstimulator has its own 
identifying address and, therefore, is individually addressable. The wire, 
for example, may be 50 or 51 gauge copper wire (diameter of 0.001 inch) 
insulated with less than a 0.4 mil thick polyimide insulation. 
Alternatively, coil 1 may be a pancake type coil or a saddle-type coil and 
disposed on the surface of the skin and not necessarily entirely encompass 
a limb or other body part. It may not, in such case, be as efficient in 
transferring energy to the microstimulators. 
The means of driving coil 1 is, preferably, a class E driver, but may be 
any one of those suitable drivers known to those skilled in the art. Class 
E drivers are well-known in the art and an analysis of them may be found 
in an article entitled, "Exact Analysis of Class E tuned Power Amplifier 
at any Q and Switch Duty Cycle," Kazimierczuk and Puczko, IEEE 
Transactions on Circuits and Systems, Vol. CAS-34, No. 2 February, 1987, 
pp. 149-159. Numerous additional references are therein cited. Inductive 
transdermal links are further disclosed and discussed in U.S. Pat. No. 
4,679,560, for Wide Band Inductive Transdermal Power and Data Link, 
inventor, Douglas C. Galbraith and in "RF Powering of Millimeter- and 
Submillimeter-Sized Neural Prosthetic Implants,", William J. Heetderks, 
IEEE Transactions on Biomedical Engineering, Vol. 35, No. 5, May 1988. 
FIG. 2 is a block diagram illustrating the transcutaneous transmission of 
power and information to implanted microstimulators by a class E driver. 
It shows a modulated, power source on the left, the skin and two implanted 
microstimulators on the right. Coil 1 is driven by a modulated oscillator 
6 which in turn is driven by a stimulation controller 7. Underneath (shown 
to the right of) skin 8 are implanted microstimulators such as 9 and 10. 
Microstimulator 9 is shown in greater detail. Secondary coil 11, within 
microstimulator 9 receives energy and control information from the 
modulated, alternating magnetic field provided by coil 1 and passes such 
energy and information to power supply and data detector 12 which, in 
turn, provides power through an electrode recharge current controller 13 
to stimulating electrodes 14 and 15. 
FIG. 2 shows secondary coil 11 at or near the surface of the skin. Such is 
for illustration only. The microstimulator may be much deeper, if desired, 
or at any location within the arm, along the length of the transmitting 
coil 1, FIG. 1 and, even, for some distance beyond the ends of the coil 1. 
In one experimental determination, it was found that the microstimulators 
may lie as far as about 5 cm. outside the volume encompassed by coil 1. 
The power supply portion of 12 provides voltage at two levels, for example, 
approximately -7 to -15 volts, for providing stimulating pulse energy 
storage and -2 to -4 volts for power for digital logic 16. Data detector 
12 also provides clock and digital data information to logic 16 which 
decodes the control information contained within the modulated, 
alternating magnet field. Such decoded information is used by the logic 16 
to control switch 17 which controls the charge stored on the capacitor 20, 
between electrodes 14 and 15. Logic 16, which is preferably high speed, 
low current, silicon gate CMOS, also controls switch 18, (which may be a 
transistor), which controls the stimulating pulse current (which is a 
discharge of the stored charge between electrodes 14 and 15) which flows 
between electrodes 14 and 15. Logic 16 also controls current amplitude 
buffer 19. This controls the amount of current allowed to flow in each 
stimulating pulse. 
In FIG. 3, electrode 14, a preferred embodiment, comprises an iridium ball 
having a stem extending into the microstimulator. The iridium ball and 
stem are formed by melting a fine iridium wire such that it forms a ball 
at the end of the wire. A substantial portion of the iridium ball is 
exposed outside the stimulator and is activated, as described hereinafter. 
In FIG. 3, electrode 15, in the preferred embodiment, is placed at the 
opposite end of the microstimulator from electrode 14, and is comprised of 
anodized, sintered tantalum, and has a stem 25 extending into the 
microstimulator. A substantial portion of the tantalum electrode is also 
exposed outside the stimulator. 
Electrode 15 is constructed of powdered tantalum metal molded and sintered 
into a pellet on the end of a 0.25 mm diameter wire, tantalum stem. It is 
then anodized, to form a thin anodized layer 15A, and the tantalum stem is 
threaded through a glass bead of N51A soda-lime glass and the portion of 
the stem protruding inside the glass bead is gold plate (plating not 
shown). The anodization may be left until after the housing is sealed to 
the glass bead because the heat may affect the anodization. The glass bead 
is preferably of the size of the tantalum pellet, approximately 0.060" in 
diameter and 0.042" in width. The entire length of the tantalum electrode 
is approximately 0.110". The porous nature of the pellet allows intimate 
relationship with the body fluids, but is of sufficiently small cellular 
structure that fibrous growth does not occur within the cells. The pellet 
is the outer, exposed portion of the electrode and is formed as a 
cylindrical section approximately less than 2 mm long and 2 mm in 
diameter, (approximately 6 or 7 mm.sup.3). The outer exposed pellet 
comprises, by its porous structure and anodized layer, an electrolytic 
capacitor, shown as capacitor 20, FIG. 2, with resistance 21 illustrating 
the resistance of the path through the body, approximately 300 ohms, 
between the electrodes. the electrolytic capacitance of capacitor 20, 
provided by tantalum electrode 15 and iridium counterelectrode 14, can be 
significant, being on the order of 2 to 30 microfarads. For greater 
capacitance, the outer cylindrical section of tantalum electrode 15 can be 
larger, but it is expected that sufficient capacitance can be achieved by 
a volume of 7 mm.sup.3. It has been found by others that anodized tantalum 
has a very low DC leakage level when biased up to 80% of the anodization 
voltage and tends to self-heal in body fluids. In other embodiments, a 
discrete capacitor within the microstimulator, in series circuit with 
electrode 15 may provide such capacitance, as illustrated by capacitor 50 
in FIG. 9. Such discrete capacitor, constructed in accordance with 
well-known art, would occupy a substantial amount of space within the 
microstimulator in order to achieve the same capacitance as the sintered 
electrode. 
Thus, all of the elements to receive and store modulated, electrical energy 
and to decode and use the modulating information to cause stimulating 
pulses, is provided by the microstimulator. Such elements are all within 
the microstimulator except for the exposed electrodes and, in the 
preferred embodiment, the storage capacitor for storing the energy for the 
stimulating pulses. Such a storage capacitor, which is an electrolytic 
capacitor, is provided by one of the exposed electrodes, porous tantalum 
electrode 15, immersed in body fluids, together with its iridium 
counterelectrode 14, FIG. 2. 
A further embodiment would be one in which one or both of said electrodes 
are formed in the shape of a single or plural nerve cuffs, as taught in 
U.S. Pat. No. 4,934,368, Multi-Electrode Neurological Stimulation 
Apparatus, in order to innervate nerves, as opposed to localized muscles. 
For example, platinum wires extending out of each end of the 
microstimulator could then be connected to the appropriate nerve cuff. The 
stimulating pulse would not need to be as large as those described herein. 
Although the size is miniature, in the nerve cuff application, the device 
would likely be surgically implanted. 
FIG. 3 is a cross-section side view of a micro stimulator. A housing 22 is, 
in a preferred embodiment, glass capillary tubing approximately 10 mm long 
and having an outer diameter of approximately 2 mm. Such glass capillary 
tubing is preferably a biocompatible, lime glass or borosilicate glass and 
is commonly available from or through glass fabrication houses such as 
Kimbel Glass or Corning Glass. Additionally, the housing may be a ceramic, 
cast or molded epoxy or silicon rubber, or other material which is 
hermetically-sealable, inert and suitable for implantation. 
At one end, the glass is hermetically-sealed to an electrode 14, which 
electrode comprises an iridium ball having an iridium stem 23 extending 
into the microstimulator. The iridium ball and stem are formed by melting 
a 0.006" or 0.010" iridium wire such that it forms a ball at the end of 
the wire. The wire may be lowered vertically into the tip of an 
oxy-acetylene flame and the iridium will melt and retract to form a ball 
on the end of the stem. Too large a ball will fall off. Care is taken 
during rapid cooling to center the ball on the stem. The other end of the 
stem may be cold-formed to provide a flat for bonding a wire thereto, to 
make electrical connection to the electrode. 
It is important to select a glass which is stable in body fluids and which 
matches pretty well the coefficient of thermal expansion of the tantalum 
and iridium because of the heating operations involved in fusing the 
electrodes to the glass housing. Glass capillary tubing, N51A, has a 
coefficient of thermal expansion similar to tantalum and iridium and thus 
may be sealed thereto. The tubing is cut off square about 2 mm beyond the 
end of ferrite core 24. 
In operation, the tantalum electrode may be charged to +15 volts, with the 
iridium as the counter electrode. Upon discharge, or partial discharge of 
the charge, due to a stimulating pulse, the tantalum may drop 
substantially in voltage, say, to 8 volts, but the iridium remains at 
approximately -0.4 volts. The combination of tantalum and iridium allow 
the tantalum to be charged to a high voltage, necessary for the 
stimulating pulse. 
At the other end of the microstimulator from iridium electrode 14, is 
tantalum electrode 15, which is comprised of anodized, sintered (porous) 
tantalum and has a stem 25, preferably of solid tantalum wire, extending 
into the microstimulator. Before anodizing, the tantalum electrode, is 
cleaned by stripping the oxide from it by a 20 second bath in 40% HF acid. 
The stem is given a diamond polish, where the glass bead 26 is to be 
sealed, to remove the scratches and nicks. The electrode may then be 
anodized (but anodization is preferably left until after fusing the 
electrode and glass bead to the housing) and the glass bead 26 is threaded 
onto the stem 25 and fused thereto. The stem, inwardly of the glass bead, 
is then gold-plated. Anodization is accomplished by a constant current of 
100 microamperes through the anodizing solution (0.1 vol % H.sub.3 
PO.sub.4 at 84 degrees C.). Anodizing for 1 hour to +10 VDC will give 
approximately 10 microfarads capacitance. Anodizing longer to +20 V DC 
will give approximately 6 microfarads capacitance. Tantalum also has a 
coefficient of thermal expansion compatible with N51A glass, but porous 
tantalum will ignite and burn if placed in a flame, such as when seeking 
to seal stem 25 to the glass housing 22. 
When the glass bead 26 is being fused to the tantalum stem 25 and when the 
glass bead 26 is being fused to the housing 22, a heat sink fixture, which 
may be a nose cone of tantalum, is slipped overtthe porous electrode 15 to 
draw heat away from the electrode and keep the tantalum stem 25 from 
reaching the melting temperature required in order to fuse the glass bead 
to the stem and the glass bead to the housing. Also, a two-piece heat 
shield collar, or collet, is placed between the electrode 15 and the glass 
bead 74, not quite touching the tantalum electrode. Such collar can help 
to maintain a 0.010" spacing between the end of the microstimulator 
housing and the electrode 15. Such gap functions as an anchor, allowing 
tissue to fill in and holds the microstimulator in place. 
Anodization of the tantalum would then be done if it has been left until 
after the fusing of the glass bead to the housing. 
A suitable fixture for accomplishing the fusing of the housing to the 
electrodes is an assembly of three simultaneously-adjustable microtorches 
disposed around a rotatable chuck. Such microtorches may use propane (such 
as from tanks used for household plumbing) mixed with oxygen and fed 
through a 22 gauge (0.016" I.D.) needle. The chuck is tapered to reduce 
its heat sinking capability and allow the glass housing to heat better. 
The glass bead 26 is placed on the stem 25 of the tantalum electrode 15 and 
fused thereto. The chuck for holding the tantalum electrode 15 should also 
be of tantalum to avoid scratching and embedding of foreign metal 
particles in the electrode. When the electrodes are to be fused to the 
housing, after the coil and electronics are inserted therein, the housing 
is inserted in a chuck and is held vertically downward to avoid distortion 
due to gravity. 
There are various methods of assembly. In the preferred method, the entire 
internal assembly is put together and inserted into the housing from one 
end of the housing or the other. Prior to inserting the assembly, the 
glass bead 26 is threaded onto the anodized tantalum stem 25 and fused 
thereto. The inner end of the tantalum stem is gold-plated. A metallized 
film 29 is deposited on the bottom ferrite shelf. The IC electronics chip 
28 is adhered to the metallic film 29 on ferrite shelf 27 by silver epoxy, 
silver solder, an indium-based solder, or other suitable conductive 
adhesive. Two metallized pads 32 and 33 are created on the top half of the 
ferrite core. A polyimide, solder resist line, or barrier 38, is added. 
The tantalum stem 25 is resistance welded or soldered to the weld shim 30. 
Another method of connection of the tantalum stem 25 to electronic chip 28, 
rather than by means of metallized film 29 and the substrate of the 
electronic chip 28, is to replace the shim 30 and the metallized film 29, 
with a small metallized pad disposed on the ferrite shelf 27 to which both 
stem 25 and electronic chip 28 are connected by flying wire bonds, or 
other means. 
One side of the iridium ball is remelted, prior to its assembly, to present 
a very smooth surface for fusing to the housing 22. The iridium electrode 
is cleaned by reduction in a saline solution with 3 to 6 volts applied. 
The iridium stem is inserted through the ferrite channel of the bottom 
half of the ferrite core and by electrical conductor 31, is connected 
electrically to the electronic chip 28. A silver epoxy may be used to 
reinforce such electrical connection. Additional silver epoxy may be used 
to fill up the channel and provide a heat sink for the iridium electrode. 
The top half of the ferrite core is placed over the bottom half of the 
ferrite core and one end of the coil wire is soldered to a metallized pad, 
32, for example, and the coil is wound on the ferrite and the other end is 
soldered to the other metallized pad 33. Gold wires are bonded between the 
metallized pads 32 and 33 and the electronic chip pads 36 and 37. A 
junction coat is applied. 
The iridium ball and its stem, along with the entire inner assembly, are 
inserted into one end of the glass capillary tube. The glass is sealed to 
the smooth surface of the iridium ball, by bringing the glass and iridium 
ball into an oxy-acetylene flame and rotating them within the flame. Ball 
diameters were tested from 0.038" (0.97 mm) to 0.050" (1.25 mm), just 
slightly smaller than the inner diameter of the 2 mm D glass tubing, and 
it was found that the larger ball sealed quicker and were easier to keep 
centered and gave a longer seal path. The preferred diameter is 0.060". 
The larger ball also provides a larger exposed electrode surface, outside 
the glass housing. The sealing operation may be viewed under a microscope. 
It will be noted that the glass will shrink backward during melting, but 
the portion of exposed ball (1/3, 1/2 or 2/3) before sealing is 
proportional to the exposed portion after sealing. In FIG. 3 approximately 
60% of the iridium ball is shown exposed outside the microstimulator. 
The glass bead 26 on the tantalum stem 25 on the other end of the 
microstimulator is flame-sealed, or fused, to the glass tubing, housing 
22. 
The outwardly exposed portion of the tantalum electrode 15 may then be 
anodized, as explained previously. 
The exposed portion of the iridium electrode 14 is activated after the 
sealing of the iridium electrode 14 to the glass tubing 22. This is done 
by immersing the exposed portion of the iridium electrode 14 in a 
phosphate-buffered saline solution and touching its outside surface with a 
whisker probe (0.003") of iridium wire, cycling for 10 to 30 minutes at 
0.5 volts per second between plus and minus 0.8 volts (relative to the 
standard Calomel electrode), just below the voltage at which electrolysis 
occurs. The cyclic voltammetry builds up an electrically conductive layer 
of iridium hydrous oxide, layer 14A, that is capable of being cycled 
reversibly between the +3 and +4 valence states, thereby transforming 
electron motion in the underlying metal into ion fluxes in the surrounding 
solution without inducing irreversible electrolysis of the water or metal. 
The interfacial impedance tends to be very low, also, the voltage which is 
necessary to obtain stimulation is reduced. 
Activation creates a hydroxide or oxide layer on the surface of the 
iridium. Such activation layer, in iridium, provides a substantial amount 
of capacitance. 
It has been determined that the metallized pads 32 and 33 on the top half 
of the ferrite core 24 may be made of indium solder and no barrier 38 is 
then required. Metallized pads 32 and 33 may be created of palladium, 
silver, indium, or solder, or mixtures thereof. The flying, gold, bond 
wires may be attached directly from such pads to the electronic chip 28. 
Other means may used to make the electrical connections within the 
microstimulator. For example, a polyimide cap (Kapton) may be used. It is 
placed on the electronic chip 28 and provides solder pads on the top for 
connecting to the electrodes and the coil and on the bottom for connecting 
to the electronics chip 29. Thru-hole connections, in the cap itself, make 
connections between the top and bottom of the cap. 
In a second method, the assembly (without the tantalum electrode, including 
the wire) is inserted in the housing. The iridium stem 23 has already been 
inserted into the core and electrically connected to the electronic 
control circuitry 28. Weld shim 30, FIGS. 3 and 4, has solder, conductive 
epoxy or other flux disposed on it. The bottom end of the housing is 
melted back until it is curved inwardly to form a small opening. The stem 
25 of the tantalum electrode has been fed through the glass bead 26 and 
fused thereto. The tantalum electrode 15 and the fused glass bead 26 are 
inserted into the bottom end of the housing for fusing of the glass bead 
to the housing, using the microtorches. The heat of fusing the glass bead 
to the housing melts the flux on the weld shim 30 and tantalum stem 25 
becomes electrically fused to the weld shim. In order to construct the 
embodiment shown in FIG. 5, in which there is no weld shim, the tantalum 
electrode stem 25 is finally connected directly to the electronic chip 28 
by means of conductive epoxy 39. If desired, it may be caused to fuse to 
the tantalum stem 25 and the electronic chip 28 by the heat fusing glass 
bead 26 to housing 22. Iridium electrode 14 may be hermetically sealed to 
the housing 22 before or after the glass bead 26 is hermetically sealed to 
the housing 22. 
In a third method of assembly, the assembly of the tantalum electrode 15, 
the electronic chip 28, the coil 11 and the ferrite core 24, is inserted 
into the housing. Then the iridium stem 22 is inserted into the housing 
and through the channel in the ferrite core, to a conductive epoxy or 
other conductive material which may serve to connect it to the electronic 
chip 28 or to a conductor connecting thereto. The ends of the housing are 
hermetically sealed to the electrodes 14 and 15 as previously described. 
The epoxy disposed within the channel, around iridium stem 23, effectively 
serves to provide a heat sink for the iridium electrode into the ferrite 
core. 
A bottom chuck holds heat sink means which can be used to shield the outer 
end portion of the tantalum electrode from the direct flame. 
The tantalum and iridium wire, used in the microstimulator, will be found 
to have irregularities comprising voids, linear scratches and furrows, 
from the drawing process. Such irregularities may result in leaks when 
fused to the glass. Cleaning, polishing, annealing or other processes for 
smoothing such wires will improve the success rate of hermetic sealing. 
The problem of irregularities as to the iridium electrode 14 was overcome 
by reheating the part of the ball to be fused, cleaning the ball as 
explained previously, and fusing the housing to the back of the iridium 
ball, which had been formed of the iridium wire. 
To achieve a satisfactory success rate for bonding tantalum to glass a 
cleaning process was used for the tantalum wire which comprised abrading 
by hand with 600 grit (30 micrometer) SiO paper, then with 6 micrometer 
diamond paste using the buffing wheel of a Dremel tool, followed by an 
ultrasonic cleaning procedure of 1 minute each in trichloroethylene, 
methanol, detergent and distilled water, rinsed in distilled water, 
methanol, and freon TF. 
Testing for hermeticity may be done by helium, but the helium may leak out 
before the test can be made. Finished devices are best tested by soaking 
in a dye solution and rejecting those parts that exhibit streams of 
bubbles or internal dye droplets. 
The embodiment shown in FIGS. 3 and 4 illustrates the coil 11 which, 
depending on the particular application, has approximately 200 turns or 
more, to provide the necessary induction for the secondary of the 
transformer. In one preferred embodiment, approximately 250 turns of 51 
AWG, (0.00102" diameter or smaller) insulated, copper wire is wound in two 
layers on the ferrite core 24, having a diameter of approximately 0.050". 
Such construction will by its own distributed capacitance be resonant at 
approximately 2 mHz. Although an air core and other cores of 
high-permeability and low losses could be made to work, a ferrite core is 
preferred. A particular core material which was used and found 
satisfactory was a low conductivity, high permeability, nickel-zinc 
ferrite stock, having a permeability of 800. It was formed by cutting and 
grinding two half cylinders, to the shape shown in FIGS. 3 and 4, with a 
groove in each, to form a hollow core when the two half cylinders were 
placed together. 
The bottom half of ferrite core 24 is longer than the top half and provides 
a shelf 27 on which to mount the electronic chip 28 (a custom, integrated, 
microcircuit chip) for the microstimulator. It is noted that the top half 
of ferrite core 24 extends over electronic chip 28 and on the top surface 
of ferrite core 24 are disposed the metallic pads 32 and 33, FIG. 4 to 
which are connected the ends of the coil. In a preferred embodiment, 
electronic chip 28 is a double-poly P-well CMOS (3 micron) process so that 
the substrate is at the V+ supply rail to which the tantalum stem of 
electrode 15 is connected. It is noted that both ends of the coil 11 are 
electrically connected to provide input to the electronic chip 28, 
providing the energy (for powering the microstimulator) and the modulation 
(control) information to such electronic chip 28. The chip dimensions may 
be approximately 0.050" with aluminum pads approximately 0.006" square for 
conventional gold-wire ball-bonding. 
One output of electronic chip 28 is to the tantalum electrode 15, and is 
shown in FIG. 3 as being through the substrate (base) of the electronic 
chip 28 to a thick, conductive film 29 which may be plated or adhered to 
ferrite shelf. Electrical connection between the electronic chip 28 and 
the film 29 may be accomplished by a silver-filled conductive epoxy or 
other electrically conductive means. The preferred thick, conductive film 
is screenable or imageable (by screen printing or photolithography or 
other imaging method). An alloy of the platinum group and gold 
(specifically, Pt/Pd/Au) in a fritted paste is preferred. Such is then 
fired for about 60 minutes, being raised evenly, 50 degrees Centigrade per 
minute, to approximately 850 to 1050 degrees Centigrade in 30 minutes and 
cooling at the same rate. Such film shows good weldability and good 
solderability. 
Weld shim 30, which is conductive, is also bonded or otherwise adhered to 
conductive film 29 and is resistance welded, or otherwise electrically 
connected to tantalum stem 25. Stem 25 comprises part of tantalum 
electrode 15 and extends through glass bead 26 to the exposed pellet of 
tantalum electrode 15. 
Electrical connection between the tantalum stem 25 and the electronic chip 
28 may also be made by a wire-bond between them or by a flying wire bond 
from each of them to a small metal pad (not shown) on ferrite shelf 24. 
Such connection may also be made as described in connection with FIG. 5, 
hereafter. 
The other output of electronic chip 28 is connected to iridium stem 23 by 
means of wire 31. 
The electric stimulation occurs through discharge of electrolytic capacitor 
20, FIG. 2 (provided by the porous, exposed end of electrode 15) upon 
connecting electrodes 14 and 15 together inside the control circuit. The 
stimulation pulse, of course, passes through the body between electrodes 
14 and 15. 
FIG. 4 is a top view of a microstimulator with the housing in 
cross-section. The ends of coil 11 are connected to two metallized pads 32 
and 33 (of palladium-silver, for example) plated on ferrite 24, by means 
of conductive epoxy or soldering. Such pads 32 and 33 are connected by 
flying, gold wires 34 and 35 which are bonded to aluminized pads 36 and 37 
on electronic chip 28 and protected with a junction coat. Barrier 38, 
which may, for example, be polyimide isolates the conductive epoxy and 
solder from flowing to undesired areas. 
The microstimulator may be filled with a harmless, inert gas which is also 
compatible with the internal structure of the microstimulator. Prior to 
fusing of the second electrode to the housing, the inert gas may be 
introduced into the microstimulator, or the microstimulator may be 
assembled in an inert gas atmosphere. The inert gas may be 10% helium and 
90% argon or krypton or other commonly-used, suitable, 
biologically-compatible gas. Assembly in a dry, relatively clean 
atmosphere has been found suitable. All epoxy inside the microstimulator 
must be allowed to fully cure before sealing, otherwise undesired 
by-products are generated within the microstimulator as the epoxy cures. 
FIG. 5 shows an alternate means of connecting the tantalum stem of 
electrode 15 to electronic chip 28. Conductive epoxy 39 is used to provide 
an electrical connection between the stem and a metallic pad on the 
electronic chip 28. This construction would be most adapted to the second 
method of assembly, mentioned previously, in which the heat of fusing the 
electrode 15 to the housing 22 provides heat to a flux (the conductive 
epoxy) to connect stem 25 to the electronic chip 28. 
FIG. 6 shows a microstimulator having an anchor skirt 40 fitted around the 
waist of microstimulator 2, to provide a means for anchoring the 
microstimulator within the body. As the microstimulator is released from 
the hypodermic needle, the arms 41 and 42 (and any other arms) resiliently 
spring away from the body of the microstimulator to an extended position 
where they hold the microstimulator against movement within the body. The 
microstimulator may also be anchored by the spacing of the enlarged end of 
the electrode 15, a small distance, say, 0.010", away from the end of the 
microstimulator, as explained previously in connection with the discussion 
concerning the tantalum electrode. 
It is possible to construct the electrodes 14 and 15 in other shapes, sizes 
and disposition. They may be constructed of platinum or other suitable 
metal. For example, platinum wire may be used to make electrical 
connection through the ends of the microstimulator housing to electrodes 
plated or otherwise attached to the housing, or to electrodes removed from 
the housing. 
FIG. 7 shows a microstimulator 2 having an extended electrode 43 having a 
barb at its end, to hold the electrode and the microstimulator in place. 
At the other end of microstimulator 2 is shown a short electrode 44 having 
two barbs at its outer end. It is to be appreciated that two or more 
electrodes, such as 43 may extend from one or both ends of the 
microstimulator. 
FIG. 8 shows a microstimulator having electrodes on the surface of the 
housing of the stimulator. Electrode 45, for example, may be an adhered 
film or plated conductor, attached, of course, to a conductor penetrating 
into the housing of microstimulator 2, as shown by tantalum wire 25 in 
FIGS. 3 and 4, for example. Iridium, platinum or other metal may also 
provide such conductor. Metallic band 46 may be electrically connected to 
electrode 45, or isolated therefrom, having its own connection to the 
microstimulator. The external electrodes may also take the form shown by 
comb electrode 47 and serpentine electrode 48. Such structures would serve 
to reduce heating of the external electrodes in the alternating magnetic 
field. The comb, serpentine and similar electrodes, on a much larger 
scale, are disclosed in U.S. Pat. No. 4,006,748, Implantable Unipolar 
Pacemaker with Improved Outer Electrode Plate, invented by Joseph H. 
Schulman. Such electrodes may be suitably made in the instant invention by 
microphotolithographic techniques. 
In one embodiment, an iridium ball electrode may be utilized at both ends 
of the microstimulator. In that event, the body fluids capacitor 20, FIG. 
2, would be replaced by a capacitor internal to the microstimulator. This 
embodiment is more fully discussed in connection with FIG. 10. 
Also, in the event the electrolytic tantalum electrode 15 is not used, or, 
if it is desired to supplement the capacitance provided by such electrode, 
an additional internal capacitance may be provided. In FIGS. 3 and 4, the 
weld shim 30 could be replaced by a tantalum sintered slug capacitor which 
may be purchased. One side of the internal capacitor, as with the weld 
shim, is connected to the tantalum stem 25 and the other side of the 
internal capacitor is connected to the metallized film 29, which is 
further connected to electronic circuitry 29. 
The construction details of a successful microstimulator are vital because 
of the extremely small size. Space is at a premium. In a preferred 
embodiment, the core 24 provides a coil form, a mount for the electronics 
chip 28, a mount for the metallized connection pads 32 and 33 and a 
channel through which one of the electrodes extends. 
FIG. 9 illustrates another embodiment of the microstimulator in which 
multiple electrodes 15B, 15C, and 15D are disposed in one end and an 
electrode, such as iridium electrode 14 is disposed in the other end. In 
this embodiment, neither of the electrodes are electrolytic and the 
storage capacitor means is provided inside the microstimulator. In FIG. 9, 
electrodes 15B, 15C and 15D pass through individual holes in glass bead 
26, are fused thereto, and the glass bead is fused to the housing 22. 
Individual storage capacitors, such as axial capacitor 50 are connected to 
each electrode and provide the electrical storage capacitance therefor. 
Such capacitors, on their inward end, are connected to connection pads on 
electronic circuitry chip 28. Such capacitors may be electrolytic, axial, 
tantalum capacitors or other suitable, miniature capacitors which are 
readily commercially available. 
A single capacitor may be used internally, and all electrodes 15B, 15C and 
15D may be mutually connected to one side and the other side of such 
capacitor, such as capacitor 50, may be connected to a pad on the 
electronic circuitry chip 28, to be controlled thereby. One embodiment 
uses an iridium electrode at each end of the microstimulator. One of the 
iridium electrodes is connected to a single, internal capacitor, or 
multiple capacitors in parallel, such as capacitor 50, whose other end or 
ends would be connected to the electronic circuitry chip 28. 
FIG. 10 illustrates a preferred embodiment in which iridium electrodes 14 
and 51 are disposed at opposing ends of the microstimulator. Iridium stem 
23 extends through glass bead 52 and then through the center of ferrite 
24. Diode 53 may be mounted as shown or be constructed within electronic 
chip 28. It has been found that ferrite shelf 27 is prone to break and 
therefore, if shortened, a miniature metal plate 55 may be adhered to it 
and provide adequate space for mounting electronic chip 28, diode 53 and 
shim 56. Electrolytic capacitor 58 is connected to shim 56 through 
electrode 57. The other electrode of capacitor 58 is its external surface, 
or case, which is connected to metal plate, or metallization 59 which, in 
turn, is connected by a wire 60 to iridium electrode stem 61 which extends 
through glass bead 62. 
Hypodermic syringe 63 may be utilized to fill up the housing 22 with epoxy, 
silicon rubber or other suitable inert material which is impervious to 
water. Such material may be added while the internal assembly is being 
moved into the housing 22 or after the internal assembly is in place and 
iridium electrodes 14 and 51 are disposed to extend out each end of the 
microstimulator. The material itself, or other sealant, may seal up the 
hole or aperture through which the syringe enters the housing. 
Inasmuch as the electronic chip 28 is light sensitive, a light barrier must 
be provided. Such light barrier may be a film or mask placed on the chip, 
an opaque or colored material used to fill the microstimulator, or a 
housing which is opaque or colored so as to prevent undesired light from 
reaching the chip 28. 
Although specific embodiments and certain structural arrangements have been 
illustrated and described herein, it will be clear to those skilled in the 
art that various other modifications and embodiments may be made 
incorporating the spirit and scope of the underlying inventive concepts 
and that the same are not limited to the particular forms herein shown and 
described except insofar as determined by the scope of the appended claims 
.