U.S. patent number 3,826,244 [Application Number 05/381,234] was granted by the patent office on 1974-07-30 for thumbtack microelectrode and method of making same.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the. Invention is credited to Martin J. Bak, Michael Salcman.
United States Patent |
3,826,244 |
Salcman , et al. |
July 30, 1974 |
THUMBTACK MICROELECTRODE AND METHOD OF MAKING SAME
Abstract
A thumbtack microelectrode for making extracellular chronic
recordings from single nerve cells in the cerebral cortex in
unrestrained animals over prolonged periods of time comprises a
rigid electrode shaft, which is microwelded to one side of a tack
head-like disc, and a flexible electrical conductor which is
microwelded to the opposite side of the tack head-like disc. After
a cleaning operation including ultrasonic desiccation the entire
microwelded assembly is electrically insulated. The insulation
covering the recording tip of the electrode shaft which is tapered,
prior to the cleaning and insulating operations, by electrolytic
etching, is then removed so as to expose a small area for use as
the recording surface.
Inventors: |
Salcman; Michael (New York,
NY), Bak; Martin J. (Germantown, MD) |
Assignee: |
The United States of America as
represented by the Secretary of the (Washington, DC)
|
Family
ID: |
23504214 |
Appl.
No.: |
05/381,234 |
Filed: |
July 20, 1973 |
Current U.S.
Class: |
600/377; 29/874;
600/378 |
Current CPC
Class: |
A61L
31/10 (20130101); A61B 5/283 (20210101); A61L
31/10 (20130101); C08L 65/04 (20130101); A61L
31/10 (20130101); C08L 65/04 (20130101); Y10T
29/49204 (20150115) |
Current International
Class: |
A61B
5/0408 (20060101); A61B 5/042 (20060101); A61L
31/10 (20060101); A61L 31/08 (20060101); A61b
005/04 () |
Field of
Search: |
;128/2E,2.6B,2.6E,2.6R,2.1E,2.1R,404,418,DIG.4,2R ;29/63R |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Levick, "Medical & Biological Engineering," Vol. 10, 1972, pp.
510-514. .
Wise et al., "IEEE Transactions on Bio-Medical Engineering," Vol.
BME 17, No. 3, pp. 238-247..
|
Primary Examiner: Kamm; William E.
Claims
What is claimed is:
1. A thumbtack microelectrode for use in the chronic recording of
electrical discharges from single nerve cells in the cortex of the
brain of an animal comprising:
a first electrical conductor means adapted to be imbedded in a
single nerve cell of the cortex of the brain of an animal for
detecting electrical discharges from the single nerve cell, said
first electrical conductor means having a blunt end and a tapered
end;
a second electrical conductor means for anchoring and limiting the
entry of said first electrical conductor means within the cortex,
said second electrical conductor means having an upper and lower
surface, the lower surface being substantially perpendicularly
attached to the blunt end of said first electrical conductor means
and adapted to be cemented to the outer surface of the cortex;
a third electrical conductor means fixedly attached to the upper
surface of said second electrical conductor means for transmitting
the detected electrical discharges from the cortex to the exterior
of the skull of the animal; and
insulation means totally encompassing the outer surface of said
second and third electrical conductor means and all of said first
electrical conductor means except the tapered end thereof for
insulating the covered areas when the microelectrode is inserted in
the cortex.
2. The device of claim 1 wherein said first electrical conductor
means is a rigid elongated iridium shaft.
3. The device of claim 1 wherein said second electrical conductor
means is a rigid platinum disc.
4. The device of claim 1 wherein said third electrical conductor
means is a flexible platinum-iridium wire.
5. The device of claim 1 wherein said insulation means
includes:
a Teflon covering for said third electrical conductor means;
and
a parylene covering for said first and second electrical conductor
means.
6. The device of claim 1 wherein the non-insulated tapered end of
said first electrical conductor means is from 10 to 20 micrometers
in length and from 1 to 3 micrometers in diameter.
7. A thumbtack microelectrode for use in the chronic recording of
electrical discharges from single nerve cells in the cortex of the
brain of an animal comprising:
a rigid, elongated, electrically conducting electrode shaft having
a blunt end and a tapered end, the tapered end being the only
portion of said electrode shaft not electrically insulated;
a rigid electrically insulated, electrical conducting disc having
an upper and lower surface, the lower surface being substantially
perpendicularly attached to the blunt end of said electrode shaft;
and
a flexible, electrically insulated, elongated wire fixedly attached
to the upper surface of said disc for transmitting the detected
electrical discharge from the cortex to the exterior of the skull
of the animal.
8. A method of making a thumbtack microelectrode comprising the
steps of:
providing a disc of an upper and lower surface;
microwelding to the upper surface of said disc an elongated,
flexible, insulated electrical conductor;
microwelding to the lower surface of said disc one end of an
elongated, rigid, bare electrical conductor;
bending said bare electrical conductor at substantially a right
angle to the lower surface of said disc;
electrolytically etching the free end of said bare electrical
conductor until the desired tip configuration is achieved;
cleaning the microwelded assembly;
vacuum depositing a layer of insulating material of low biological
toxicity over said disc and said bare electrical conductor; and
removing a small amount of the insulating material from the
electrolytically etched tip of said bare electrical conductor.
9. The method of claim 8 wherein the bare electrical conductor is
electrolytically etched in a bath of supersaturated sodium
cyanide-- 30 percent sodium hydroxide.
10. The method of claim 8 wherein the cleaning step includes:
rinsing the microwelded assembly in hydrochloric acid, then in
distilled water;
placing the microwelded assembly in an acetone bath;
ultrasonicly desiccating the microwelded assembly for 15 seconds in
distilled water, then for another 15 seconds in methanol; and
oven drying the microwelded assembly at 60.degree.C.
11. The method of claim 8 wherein the vacuum deposition step
includes:
vaporizing 2 gm of parylene dimer at 260.degree.C;
pyrolysizing the parylene at 700.degree.C; and
subsequent polymerization of the parylene.
12. The method of claim 8 wherein the insulation removal is
achieved by burning away the insulation.
13. The method of claim 8 wherein the insulation removal is
achieved by melting the insulation with the beam of a ruby
laser.
14. The method of claim 8 wherein the insulation removal is
achieved by abrading off the insulation.
Description
FIELD OF THE INVENTION
The present invention relates to measuring intracortical brain
waves and, more particularly, to microelectrodes for making chronic
recordings of electrical discharges from single cells in the
cerebral cortex of an animal's brain.
BACKGROUND OF THE INVENTION
In defining chronic recording from single cells of the brain, it is
essential to accurately locate the electrode. One technique has
required the attachment of a special chamber to the animal's head
through which new electrodes are inserted on each successive
recording day while the animal's head is rigidly bolted to prevent
any movement. However, such techniques are highly undesirable for a
number of reasons including the fact that they are difficult, are
extremely uncomfortable to the animal, and are less accurate than
desirable.
In all truly chronic recording techniques the following design
conditions are essential:
A. THE ELECTRODES MUST BE IMPLANTED ONCE DURING SURGERY, REMAIN
FIXED IN POSITION, AND THEN MUST NOT BE SUBJECTED TO FURTHER
EXTERNAL MANIPULATION;
B. DURING RECORDING SESSIONS, THE HEAD OF THE ANIMAL MUST BE
UNRESTRAINED EXCEPT FOR THE CONNECTING CABLE WHERE TELEMETRY IS NOT
POSSIBLE;
C. The animal must be awake and unmedicated; and
D. THROUGH THE USE OF SPIKE DISCRIMINATORS, STATISTICAL MEASURES,
CONTINUAL VISUAL MONITORING AND OTHER STRINGENT CRITERIA, THE
IDENTITY OF A SINGLE NERVE CELL MUST BE ESTABLISHED AND THE CELL
SUCCESSFULLY MONITORED FOR SEVERAL HOURS TO MANY DAYS.
Although prior art devices which meet these design requirements do
exist they suffer from a variety of deficiencies. Most pertinently,
such prior art devices have been successfully employed only in deep
structures, such as the basal nuclei, hypothalamus, brain stem,
etc., because they comprise a comparatively long length of wire,
the electrode, which is cemented directly to the skull. Since the
rotational acceleration of the head results in displacements of the
brain relative to the skull, an electrode rigidly connected to the
skull is certain to move within the substance of the superficial
cortex.
Therefore, prior art microelectrodes for recording from single
cells in the brains of unrestrained animals are not capable of
successfully recording electrical discharges from single cells in
the cerebral cortex over prolonged periods of time. Further, such
devices are of comparatively large diameter (e.g., 80 micrometers)
and do not provide means for limiting the entry of the electrode
into the cortex.
SUMMARY OF THE INVENTION
The shortcomings of the prior art microelectrodes for recording
from single cells in the cerebral cortex of the brain are
satisfactorily overcome by the present invention. It is,
accordingly, an object of the present invention to thus overcome
the defects of the prior art, such as indicated above.
Another object of the present invention is to provide for improved
brain wave recording of single brain cells.
Another object is to provide an improved technique of brain wave
measurement.
Another object of the present invention is to provide a
microelectrode capable of extracellular chronic recording from
single nerve cells in the superficial layers of the cerebral cortex
in unrestrained animals over prolonged periods of time.
Another object is to provide a microelectrode which makes chronic
recordings of electrical discharge from single cells with minimal
tissue reaction.
A further object is to provide a chronic recording microelectrode
having maximal fidelity.
In furtherance of these and other objects, a principal feature of
the present invention is a microelectrode capable of detecting
electrical discharges from a single cell in the cerebral cortex of
the brain over prolonged periods of time due to the flexible
connection and stabilizing influence of the microelectrode (tack)
head on the pial surface of the brain. The tack head of the
microelectrode provides an anchoring surface for a mechanical
inserter or micromanipulator and, therefore, dispenses with the
need for manual insertion. Also, the tack head limits the entry of
the microelectrode into the cortex and provides a good surface for
attachment of pia by the application of isobutyl cyanoacrylate.
Another feature of the invention is that minimum tissue damage is
achieved through the use of a small diameter tip (1-3 micrometers)
and electrode shaft which, depending upon the animal to be studied,
may be as short as one-half mm. Although small, the electrode is
strong enough to pierce the intact pial membrane, thus leaving
superficial circulation undisturbed and decreasing the incidence of
meningocerebral adhesions.
A further optional feature is the incorporation of preamplifiers
into the tack head of the microelectrode, thus reducing cable
problems, such as stray capacitance, pickup, etc., to an absolute
minimum.
According to the preferred manufacturing method, a microelectrode
component is formed by microwelding an electrode shaft of bare
iridium wire, which is approximately 0.001 inch in diameter, to the
intended undersurface of a platinum disc-like tack head 0.005 inch
thick and 1 mm in diameter. After welding, the electrode shaft is
bent at a right angle to the intended undersurface of the tack head
and cut to the appropriate length. A flexible lead which
electrically attaches the tack head to a connector fixedly attached
to the skull is similarly welded to the dorsal or upper surface of
the tack head.
The electrode shaft is then electrolytically etched until the
desired tip configuration is achieved. After cleaning, the
electrode shaft is coated with an insulator (e.g., parylene). The
final step in the fabrication involves the removal of a small
amount of parylene from the very tip of the electrode shaft so as
to expose a small area of iridium for use as the recording
surface.
BRIEF DESCRIPTION OF THE DRAWING
For a better understanding of the invention a possible embodiment
thereof will now be described with reference to the attached
drawing, it being understood that the embodiment is to be intended
as merely exemplary and in no way limitative.
FIG. 1 is an elevational view of an embodiment of a completed
thumbtack microelectrode.
FIG. 2 is a schematic diagram of the electrical components of the
thumbtack microelectrode of FIG. 1.
DETAILED DESCRIPTION
Referring to FIG. 1, there is shown a preferred embodiment of the
thumbtack microelectrode, here provided in the form of a disc-like
tack head 10, a flexible lead wire 11, and an electrode shaft 12.
The head 10 is, preferably, a platinum disc 0.005 inch thick and 1
mm in diameter.
One end of the flexible lead 11 is microwelded to the approximate
center of the intended dorsal or upper surface of tack head 10 as
indicated at 13. The other end of the flexible lead 11 is attached
to a connector (not shown) which is fixedly attached to the surface
of the skull. The connector, in turn, is attached by a cable to an
amplifier system, thereby permitting daily recordings of single
nerve cells to be made. The flexible lead 11 is, preferably, a
Teflon-coated platinum-iridium (90/10) wire.
The electrode shaft 12 is microwelded to the approximate center of
the opposite or lower surface of the tack head 10 as indicated at
14 and subsequently bent perpendicularly to that surface. The very
tip 15 of the end of the electrode shaft 12 furthest from the tack
head 10 is electrolytically etched to a fine point. The entire
shaft 12 except for the tip 15 is coated with an insulator such as
parylene or Teflon.
Referring now to FIG. 2, there is shown a schematic diagram of the
electrical components of the preferred embodiment of the thumbtack
microelectrode shown in FIG. 1. The electrical discharge of the
single nerve cell is designated generally by 16 and corresponds to
the biological source voltage. In series with the biological source
voltage 16 are the resistance and capacitance 17 and 18,
respectively, of the electrode recording tip which are in parallel
with one another. Numeral 19 designates the grounded shunt
capacitance of the tack head which is in parallel with the
equivalent electrical components (16, 17 and 18) which are, also,
grounded by the body of the animal.
The resistance and capacitance 17 and 18, respectively, of the
electrode recording tip correspond to the impedance of the
interface between the intestitial fluid and the recording surface
of the electrode shaft. Since the shunt capacitance of the
thumbtack microelectrode which corresponds to the total capacitance
associated with all of the metallic components and the dielectric
outer insulation is considerable, the amplifier circuit must have
the capability of cancelling out the distortion introduced in this
manner.
The fabrication of the preferred embodiment of the thumbtack
electrode shown in FIG. 1 includes the following steps. The head 10
of the microelectrode is a platinum disk 0.005 inch thick and 1 mm
in diameter which may be obtained in any convenient manner, e.g.,
punched from a sheet of platinum with a steel die against a wood
block.
The flexible lead 11 is a 4 inch length of platinum-iridium (90/10)
Teflon-coated wire, 0.001 inch in diameter. It is microwelded to
the approximate center of the intended dorsal or upper surface of
the tack head 10 by any suitable precision welding instrument, such
as a Weltek Model 410E, under direct visual control through a
dissecting microscope. The microwelding operation is performed
right through the Teflon insulation.
The electrode shaft 12 is a length of bare iridium (e.g. 100
percent) wire which is 0.001 inch in diameter. It is microwelded to
the approximate center of the opposite or lower surface of the tack
head 10, also, under direct microscopic control. After
microwelding, the shaft is cut to the appropriate length (for cat
visual cortex-- about 2 mm) and bent at substantially a right angle
to the lower surface of the tack head by a pair of fine
watchmaker's forceps.
To facilitate the handling of the microelectrode, the flexible lead
is loaded into a blank micropipette previously drawn on a vertical
micropipette puller with the fine end broken off. Again under
direct microscopic control, the iridium electrode shaft is
electrolytically etched in a bath of supersaturated sodium
cyanide-- 30 percent sodium hydroxide until the desired tip
configuration is achieved. In the case of the preferred embodiment
of FIG. 1 the diameter of tip 15 is approximately 1-3 micrometers
and increases slowly to 10 micrometers at a distance of 100
micrometers from the tip.
After etching, the microelectrode is rinsed in hydrochloric acid
and then distilled water. After being placed in an acetone bath,
the microelectrode is subjected to approximately 15 seconds of
ultrasonic desiccation in distilled water and then approximately 15
seconds of ultrasonic desiccation in methanol in a device similar
to that manufactured by Ultrasonics Inc. Following the above
cleaning steps the microelectrode is oven dried at 60.degree.C.
Once the microelectrode has been sufficiently dried, it is placed
in a low vacuum (about 1 torr), room temperature chamber and coated
with a 3 micrometer layer of parylene. The coating process is
carried out by vaporizing 2 gm of the parylene dimer at
260.degree.C and subsequent polymerization in the chamber mentioned
above. The microelectrode emerges from the chamber with a uniform
coating of 3 micrometers of parylene along the entire length of the
etched electrode shaft and over both surfaces of the tack head.
Parylene is used because it is a superior insulator with negligble
water uptake and excellent conformability. Also, its biological
toxicity is almost nil. It should be noted that Teflon or (glass)
could be used in place of the parylene.
The final step in the fabrication process involves the removal of a
small amount of parylene from the very tip of the electrode shaft
so as to expose a small area of iridium for use as the recording
surface. This step may be carried out by several methods. The first
method involves burning away the parylene by bringing the 0.003
inch heating element of a specially designed microforge (See: Dolde
and Burke, EEG Clin. Neurophysiol. 1972) in close proximity to the
tip of the electrode shaft. Secondly, the parylene may be melted
with the beam of a ruby laser. The final method involves abrading
the parylene off with oxygen or argonions through the use of a
micromilling device. The first and second methods may be repeated
until a desired tip impedance is achieved, about 1 megohm in the
case of the preferred embodiment of FIG. 1.
In operation, the microelectrode is attached to a connector which
is then cemented to the surface of the skull. The microelectrode is
then attached to the nose of a vacuum electrode inserter and is
driven into the cortex by positive pressure acting on the piston of
the electrode inserter. The piston rides on sapphire jewel bearings
and a vacuum is delivered to the head of the thumbtack through the
centerbore of the piston. After insertion, the microelectrode is
cemented to the pial surface of the cortex by isobutyl
cyanoacrylate.
A chamber is built up around the bony incision in the skull and
after all of the air is evacuated, the chamber is filled with a
saline solution, thereby maintaining a constant pressure in the
skull cavity and preventing leakage of fluid therefrom.
The connector which is located remote from the incision and,
therefore, not enclosed by the chamber is attached by a cable to
the amplifier system and daily recordings are made of the same
single nerve cell for many hours. As mentioned above, the shunt
capacitance of the tack head is relatively large and, therefore,
the amplitude circuit must have the capability of cancelling out
the distortion introduced in this manner. Displacement of the brain
relative to the skull during movement of the animal are absorbed by
the flexibility of the microelectrode lead, without movement of the
microelectrode tip which stays in contact with the selected brain
cell.
The foregoing description of the specific embodiment will so fully
reveal the general nature of the invention that others can, by
applying current knowledge, readily modify such specific embodiment
and/or adapt it for various applications without departing from the
generic concept, and, therefore, such adaptations and modifications
should and are intended to be comprehended within the meaning and
range of equivalents of the disclosed embodiment.
It should be noted that the instant device is not necessarily
limited to use in the cortex of an animal. Also, other suitable
materials, dimensions, or processes besides those recited herein
may be adapted without departing from the inventive concept of the
instant invention.
It is to be understood that the phraseology or terminology employed
herein is for the purposes of description and not of
limitation.
* * * * *