A three-dimensional electrode device is disclosed. The device is useful as a neuron interface or as a cortical implant. A plurality of spire-shaped electrodes, formed of semiconductor material, are associated with a rigid base. The electrodes are electrically isolated from each other at the base. Multiplexing circuitry allows the electrodes to be addressed individually.

BACKGROUND OF THE INVENTION 
1. Field 
The present invention is directed toward a three-dimensional electrode 
device and a method of manufacturing such a device. The device may be 
particularly useful for neuron interface, and more specifically as a 
cortical implant for vision prosthesis. 
2. State of the Art 
It is well known that messages are transmitted throughout the nervous 
system by means of electrical signals. Electrical signals are generated by 
various parts of the body, such as the sensory organs, and are transmitted 
to the brain. The brain in turn generates electrical signals to control 
muscular and other activity. Certain devices have been developed to 
electrically interface with neural tissue to either receive messages from 
or deliver messages to the neurons. 
For example, various attempts have been made to provide a cortical implant 
to interface with the region of the cortex corresponding to the sense of 
sight. Using such implants, blind persons have been made to perceive 
simple sensations of sight in the form of spots of light, referred to as 
"phosphenes," in simple geometric patterns. Such interface systems 
typically include a two-dimensional array of flat electrodes. 
These attempts have not been completely satisfactory. Two-dimensional 
arrays reside on the surface of the cortex. However, the neurons that 
initiate phosphene perception lie somewhat below the surface. The depth of 
these neurons is believed to be about 1.5 mm. In surface arrays, 
relatively high current, in the neighborhood of 3 mA is required to 
stimulate neurons. Such high currents may pose pathogenic problems. In 
addition, a phenomenon has been experienced in which when two nearby 
electrodes are energized, signals from the electrodes interact to produce 
a phosphene at an anomalous geometric position. Such electrode 
interactions severely limit the number of electrodes that can be used in 
surface arrays. 
To induce in a blind person the perception of sight, it appears necessary 
to produce a large number of contiguous phosphenes, similar to the way a 
cathode ray tube produces a complete image by appropriate illumination of 
a large number of contiguous "pixels" on a television screen. Because of 
their construction and limited electrode spacing, previously known 
implants have not produced the sensation of a sufficient number of 
contiguous phosphenes to produce an acceptable sense of vision. 
Also, some means must be provided to address each of the electrodes 
individually. One approach has simply been to run a wire to each 
electrode. With even a small to moderate number of electrodes, such a 
bundle of wires is cumbersome and disadvantageous, since these wires must 
lead from the blind person's cerebral cortex to some point external to the 
blind person's head. 
Electrode arrays are also used in applications other than as neuron 
interfaces. For example, various arrays of photoreceptors or 
light-emitting diodes are used for image sensing or image producing 
devices. It is often useful to form such arrays of semiconductor material, 
particularly silicon, because of a wide variety of characteristics that 
may be imparted to semiconductors by means of such processes as doping, 
etching, etc. Such arrays are typically formed of "wafers" of 
semiconductor material. The electrodes on these wafers are formed by 
conventional photolithographic techniques. Such wafers may have 
thicknesses of a millimeter or less, with the electrodes formed on such 
wafers being in the range of a few microns in thickness. 
There remains a need for a three-dimensional semiconductor device that has 
the capability of providing a large number of electrodes that may be 
addressed individually for signal transmission and/or reception. Such an 
array would be particularly advantageous as a neuron interface device, 
such as a cortical implant for vision prosthesis. Such a three-dimensional 
array of elongated electrodes may be positioned with the active tips of 
the electrodes at a depth in the cortex where very localized stimulation 
of or recording from neurons may more effectively take place. Such an 
array would preferably be strong and rigid. The array would preferably be 
formed of a semiconductor material, such as silicon, to make use of the 
unique electrical properties of semiconductors. The individual electrodes 
would each be preferably addressable without the need for a large number 
of lead wires, such as by the provision of a multiplexing system. 
SUMMARY OF THE INVENTION 
The present invention provides a three-dimensional electrode device. This 
device may be particularly adapted as a neuron interface device, and more 
specifically, a cortical implant. A base of rigid material is provided. A 
plurality of elongated electrodes are mounted to the base to extend away 
from the base. The electrodes are electrically isolated from each other at 
the base by means of a second material positioned between the electrodes. 
Each of the electrodes has a distal end. Signal connection means is linked 
with the electrodes for providing electrical connection to each of the 
electrodes individually. 
The electrodes may advantageously be spire shaped, or tapered from the base 
toward the distal ends. The electrodes may also advantageously be formed 
of a semiconductor material, such as silicon. In one embodiment, the 
electrodes are electrically isolated from each other by means of 
semiconductor doped with an impurity. In another embodiment, the 
electrodes are electrically isolated from each other by glass. 
The signal connection means may include an electrical gate associated with 
each of the electrodes. The electrical gates may be arranged in a 
two-dimensional array and multiplexed to be addressable individually. The 
electrodes also preferably include a charge transfer material at the 
distal ends. 
The invention also provides a method of manufacturing an electrode device. 
A three-dimensional chunk of a first material is provided. A first surface 
of the chunk is impinged to a preselected depth with a second material to 
provide isolating regions of the second material and isolated regions of 
the first material between the isolating regions. The second material is 
adapted to provide electrical isolation between the isolated regions. A 
second surface of the chunk is sawed opposite the first surface at a 
preselected depth in criss-crossing channels to provide pillars of the 
first material between the channels. The pillars are electrically isolated 
from each other by means of the isolating material. The pillars are 
tapered to reduce their cross-sectional size toward their distal ends, and 
the distal ends are metallized. 
In one embodiment, the method includes the additional step of positioning 
electrical gates upon the first surface of the chunk to provide electrical 
connection to each of the pillars. 
In a preferred embodiment, the method further comprises the step of coating 
the pillars and the base with an ion impermeable material. The 
semiconductor device may be advantageously used as a neuron interface 
device. This neuron interface device may be a cortical implant for vision 
prosthesis. 
The first material may be a semiconductor material. In one embodiment, the 
second material is a semiconductor material doped to provide pn junctions 
between the first material and the second material. In another embodiment, 
the second material is glass. 
The invention provides an electrode device that has the capability of 
providing a large number of electrodes that may each individually be 
addressed for signal transmission and/or signal reception. This device may 
be particularly advantageous for a neuron interface, and particularly a 
cortical implant for a vision prosthesis. Since the device is three 
dimensional, the active tips of the electrodes may be positioned at a 
depth in the cortex where interface with neurons may more effectively and 
directly take place. The array is strong and rigid. In preferred 
embodiments, the array is multiplexed such that only a small number of 
lead wires need to be attached.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
FIG. 1 is a perspective illustration of a three-dimensional electrode array 
of the invention. This electrode array is specifically adapted to be used 
as a neuron interface device and to be implanted in the cortex of the 
brain. More specifically, the illustrated electrode array is adapted to be 
used as a vision prosthesis for a blind person. A visual image is reduced 
by some means, for example, a video camera, to an electrical signal that 
is then provided to the electrode array of FIG. 1. This electrical signal 
is used to control the amplitude of the signal provided to each electrode 
in the array. An actual image is scanned to produce the series of signals. 
This series of electrical signals is then multiplexed across the electrode 
array to produce phosphenes corresponding to the image. The blind person 
perceives the image in his mind, thus providing a usable sense of sight. 
The electrode array of FIG. 1 includes a base 30 to which are connected a 
plurality of electrodes, of which electrode 32 is typical. As shown, each 
electrode 32 is spire-shaped. In other words, electrodes 32 are relatively 
large in cross-sectional size at their bases 36 where they connect to base 
30 and taper toward distal ends 34. Electrodes 32 are electrically 
isolated from each other at base 30. Base 30 and electrodes 32 are formed 
substantially of semiconductor material, preferably silicon. Electrodes 32 
should be at least about 1000 microns in length and preferably are about 
1500 microns in length, in order to penetrate outer cortical structures to 
stimulate the underlying normal inputs to neural structures, which 
generally lie about 1500 microns below the surface of the cortex. 
FIG. 2 is a schematic representation of the electrode device of FIG. 1 
showing multiplexing circuitry associated with the electrodes 32. A 
plurality of AND gates, of which gate 40 is typical, are associated with 
the back side 42 of base 30. Face 42 is a two-dimensional flat plane. 
Using conventional photolithographic techniques, gates 40 may be formed as 
two-gate field effect transistors (FET) which are formed over and 
electrically connected to the projections of each of electrodes 32. 
Electrodes 32 are electrically isolated one from another by means of 
channels 38 of isolating material. The portions of base 30 connected to 
electrodes 32 are isolated regions of which region 44 is typical. As 
shown, gates 40 are connected to these isolated regions. 
The signal produced by the imaging device, such as a video camera, is fed 
into input 46. The signal is provided to all of the gates 40 at the same 
time. Whether or not a particular gate is activated to provide this signal 
to its associated electrode 32 depends on the multiplexing circuitry. 
Gates 40 allow for bidirectional flow of current through each electrode 
32. A clock provides an oscillating series of logic "1" and logic "0" 
signals at a selected frequency to input 48. 
Gates 40 are connected to an X-axis shift register 54 and a Y-axis shift 
register 56. Shift registers 56 and 58 are linked by a carry 60. The gate 
40 that has a "1" logic level on both its respective row and column is 
energized. In FIG. 2, gate 58 is the energized gate. Gate 58 would 
therefore provide the electrical signal (which is supplied to all gates 
40) to be transmitted only to its associated electrode 32. Thus, shift 
registers 54 and 56 allow the entire array to be "scanned," one electrode 
and a time. The implant needs only five wires attached to it: two power 
supply lines (not shown), a ground (not shown), a clock, and the signal 
line. 
The electrode array is three dimensional. The electrodes are intended to 
penetrate the cortical tissue and to position the electrode tips 34 near 
the neurons responsible for phosphene perception. Because the active 
current-passing tips 34 are in close proximity to these neurons, a 
relatively lower current can be used for the signal supplied to the array. 
Such currents can be in the neighborhood of 1 to 100 .mu.A. The close 
proximity of tips 34 to the neurons and the lower currents also reduces 
electrode interaction, thus reducing anomalous phosphene effects and 
allowing for a higher number of contiguous phosphenes. 
The isolation regions 38 are formed in any appropriate manner which 
provides a rigid and strong base 30. At present, two methods are disclosed 
for providing such a base having isolation regions. One is 
thermomigration, and the other is a method that places channels of glass 
in between the isolated regions 44. The array described includes 100 
electrodes. However, a much larger or smaller number of electrodes may be 
fashioned if it is deemed useful in the particular application involved. 
The block or "chunk" from which the electrode arrays are fabricated is a 
1.7 millimeter thick mono-crystalline n-doped silicon, with a resistivity 
of 1-10 ohm-cm. 
Thermomigration is described generally in U.S. Pat. No. 3,898,106 issued to 
Cline et al., entitled "High Velocity Thermomigration Method of Making 
Deep Diodes; High Temperature Melting," the disclosure of which is 
incorporated herein by reference. The method used in the present invention 
to achieve thermomigration is similar to that described in U.S. Pat. No. 
4,001,047 issued to John K. Boah, entitled "Temperature Gradient Zone 
Melting Utilizing Infrared Radiation," the disclosure of which is also 
incorporated herein by this reference. 
A description of the thermomigration technique is described in reference to 
FIGS. 3-5. A block 70 of 1.7 mm thick monocrystalline n-doped silicon is 
provided having a resistivity of 1-10 ohm-cm. A 6 micron thick layer 72 of 
aluminum is deposited on back surface 74 of block 70. Back surface 74 will 
become the surface 42 (FIG. 2) of the array. Referring to FIG. 4, 
conventional photolithographic/etching techniques are used to form one 
hundred 300 micron by 300 micron rectangular pads 78. These pads are then 
annealed to the silicon block 70. 
The silicon block 70, now coated with the 100 aluminum pads, is then placed 
in a thermomigration oven with a positive temperature gradient in the 
direction of arrow 80. Each pad 78 becomes a localized aluminum-silicon 
"melt" that propagates in the direction of the temperature gradient 
through the silicon block 70 in relatively straight columns. The aluminum 
passing through the silicon leaves behind trace aluminum to form the 
columns 82 (FIG. 5) of aluminum-doped silicon. After thermomigration is 
complete, the front surface 84 of the block and the back surface 74 are 
polished. The aluminum-doped silicon is revealed by a staining procedure. 
Each p+ doped column 82 forms a diode in block 70 between column 82 and 
the surrounding n-type block. Therefore, a pair of back-to-back diodes is 
created between each adjacent pair of columns 82, which become electrodes 
32 after the shaping process is complete. Electrical isolation of 
electrodes 32 is thereby achieved and current leakage between electrodes 
is limited to back-biased leakage across these diodes. 
Isolation regions 38 may also be formed by a glass melt process discussed 
in reference to FIGS. 6 and 7. A diamond dicing saw equipped with a 0.125 
mm thick blade, is used to make a series of 0.25 mm deep orthogonal cuts 
90 into block 70. Eleven cuts are made with even spacing along one 
direction. The silicon is rotated 90.degree. and 11 additional cuts are 
made with the same spacing orthogonal to the first set. This cutting 
process leaves 100 silicon stubs 92, each 0.25 mm high. 
Dicing saws are used in the fabrication of silicon computer chips. Such 
saws are used to saw apart large numbers of identical chips from a block 
on which they have formed. In the present application, however, the dicing 
saw is used to form a three-dimensional structure in the semiconductor 
block. 
Referring to FIG. 7, after stubs 92 are formed, a slurry is prepared of 
glass frit (Corning Glass Works, 7070), and methyl alcohol. This slurry is 
dripped onto front surface 74. The alcohol in the slurry wets the silicon 
surface and carries the glass powder down into the saw kerfs 90. The 
methanol evaporates quickly. This dripping process is continued until 
there is a layer of glass powder approximately 0.25 to 0.5 mm above the 
top surface of stubs 92. 
The coated silicon chip or block 70 is then placed in a computer controlled 
oven. The oven is evacuated and the temperature quickly increased to a 
temperature sufficient to melt the glass frit, allowing it to flow while 
not melting the silicon. The temperature is held at this level for 60 
minutes, after which the oven is allowed to cool to room temperature. The 
vacuum in the oven is released and the silicon/glass chip is removed. Any 
large bubbles in the glass surface are then broken with a sharp probe. 
The block 70 is placed in the oven again and the oven is evacuated and the 
temperature again increased to the same temperature previously used to 
melt the glass. The temperature is held at this level for 50 minutes, at 
which time the vacuum is released. Releasing the vacuum forces any bubbles 
in the glass to shrink in size. The temperature is maintained for an 
additional 10 minutes, after which the temperature is slowly decreased 
until the glass is cooled. This melting process removes any large bubbles 
from the glass and minimizes internal stresses in the glass while assuring 
good adhesion at the glass/silicon interface. The glass remaining above 
the level of stubs 92 is then ground off, and the stubs 92 and the glass 
is then polished to provide a smooth surface as shown in FIG. 7 with 
regions of glass 94 between stubs 92. 
A description of the formation of electrodes 32 is made in reference to 
FIGS. 8-14. One starts with a block 70 that has had either the 
thermomigration process performed as discussed in reference to FIGS. 3-5 
or the glass isolation region process discussed in reference to FIGS. 6 
and 7. The computer-controlled dicing saw is used to produce a series of 
orthogonal cuts in the silicon block 70. Eleven cuts are made along one 
axis. The block is then rotated by 90.degree. and 11 more cuts are made 
orthogonal to the first set of cuts. Each cut is 1.5 mm deep and has a 
kerf of 270 microns. In thermomigrated blocks, the cutting is aligned 
between the aluminum-doped columns 82, producing electrodes 32, which are 
p+ doped. If the glass melt procedure has been used, the cuts are made as 
shown in FIG. 8 such that the glass regions 94 isolate electrodes from 
each other. 
The dicing process to produce columns 32 is disclosed in terms of 
orthogonal cuts to produce pillars having square cross-sections. However, 
pillars having other types of cross-sections may be useful and 
advantageous. For example, another scheme involves the use of 
criss-crossing cuts at 60.degree. angular separation to produce a 
hexagonal array of hexagonally-shaped pillars. Pillars of hexagonal 
cross-sections may provide electrodes with denser packing than with square 
pillars. 
FIG. 9 illustrates the substrate or block after it has been cut with the 
dicing saw to produce 100 columns 98 by forming a first set of cuts 100 
and a second set of cuts 102 orthogonal to first set 100. As also shown, 
this cutting process leaves 40 rectangular fins 104 and four corner posts 
106. Fins 104 and corner posts 106 are left in place temporarily to 
facilitate subsequent processing. Cuts 100 and 102 are made from the front 
side 84 of the block so that the thermomigration or glass melting 
procedures provides that the electrodes 32 be isolated from each other at 
base 30. FIG. 17 is a scanning electron micrograph of the array after 
columns 98 have been formed by the dicing process. 
After the columns 98 which form the basis for electrodes 32 have been 
formed, these columns 98 must be etched to produce pyramid or spire-shaped 
electrodes 32 as shown in FIG. 1. This etching process is described in 
reference to FIGS. 10-14. A holder is formed of a cylindrical piece 110 of 
Teflon that has a diameter of 0.435 inches. A square hole 112 that is 0.25 
inches on each side 0.080 inch deep is machined into piece 110. Hole 112 
receives block 70 and securely holds it during the etching process. The 
rectangular block 70 is placed in hole 112 such that block 70 registers in 
a press- or friction-fit within holder 110. Block 70 is placed such that 
columns 98 extend toward the opening of hole 112. When block 70 is placed 
within hole 112, the tops of columns 98 are approximately flush with the 
upper surface 116 of holder 110. 
Referring to FIG. 13, with block 70 in holder 110, as described, holder 110 
is mounted by means of stock 114 to a clock motor 115, and block 70 is 
immersed in an acid bath of five percent hydrofluoric acid and 95 percent 
nitric acid etchant 120. Block 70 is positioned approximately 0.125 inches 
above a magnetic stir bar 122 held within jar 124. Jar 124 has an 
approximately 1.9 inch diameter. Jar 124 resides on the surface of a 
magnetic stir plate 126 that has a magnetic mechanism to cause stir bar 
122 to rotate at a selected rotational speed. 
FIG. 12 illustrates a top view of this swirling etch method. Magnetic stir 
bar 122 is caused to rotate in the direction of arrow 130 at a preferred 
speed of approximately 375 revolutions per minute. Holder 110 is caused to 
rotate in the direction of arrow 132 at a rotational speed of 
approximately 4 revolutions per minute. Block 70 is swirl etched in this 
manner for approximately three minutes to produce thin columns with 
polished sides as shown in FIGS. 18 and 19. 
The final structure of the electrodes is produced by a wet static chemical 
etching as illustrated in FIG. 14. In this arrangement, holder 110 is 
placed in an "L"-shaped handle 140 and immersed in the same solution 120 
for a period of approximately two to three minutes. The result of this 
procedure is to sharpen the electrodes to the spire shape shown in FIG. 
20. 
As can be seen in FIG. 20, the spires 98 have relatively large 
cross-sectional sizes at their bases and taper toward their tips. The 
bases of the columns are roughly square in cross-section because at this 
point the etchant has had the least effect. These spire-shaped or tapered 
columns provide for effective insertion of the array into cortical tissue 
without doing undue harm to the tissue. The columns are pointed at their 
tips to pierce the tissue and increase gradually in cross-section toward 
their bases. Thus, after the tips of the electrodes are inserted, the 
tissue is gradually spread apart until the array is fully inserted. The 
relatively large cross-sectional size at the base of the electrode 
provides increased strength for each electrode. 
Once the columns 98 have been formed to provide the basis for the 
electrodes, ohmic contact must be provided to the back of each electrode. 
A grid of 200 micron by 200 micron square aluminum pads is deposited over 
the back surface of each column 98. This process is identical to the 
initial steps of the thermomigration process. These aluminum pads are used 
whether the thermomigration process or the melted glass process is used to 
produce the electrodes. The aluminum pads are annealed to the back surface 
of their respective columns, one pad to each column. This process forms a 
low resistance ohmic contact to each column 95. At this point fins 104 and 
corner posts 106 are removed with the dicing saw. 
Referring to FIG. 15, the tips 34 must be then formed to provide active 
current passing electrodes. In other words, some means must be provided to 
transduce electric charge to ionic charge in the neural tissue. In the 
disclosed array, metal is used as such a charge transfer medium. However, 
other materials may be used. For example, iridium oxide may be used as an 
effective charge transfer material. Tips 34 are driven through a metallic 
foil such that the top 1000 microns of each spire emerge from the surface 
of the foil. The array, with foil is then transferred to an electron beam 
evaporator and a few microns of platinum or iridium (which are metallic 
substances) are deposited on the tips of the electrode. The metal foil is 
removed, and the array, with its metal coated tips 34 is annealed at low 
temperature. This annealing drives a small amount of the metal into the 
silicon and forms a thin layer of metal silicide at the metal/silicon 
interface. The metal silicide provides an ohmic contact between the metal 
on the tips 34 and the silicon spires 98. The metal surface forms an 
active current passing electrode between the silicon and the neurons in 
the brain. 
Referring to FIG. 16, lead wires 152 are wire bonded to the aluminum pads 
150 on the back surface 74 of block 70. This bonding process is a 
standardized process using ultrasonic bonding of insulated 25 micron gold 
wires to the aluminum pads 150. The bonded wires are secured to the back 
side of the array with a layer of polyimide, which is subsequently cured 
in an oven. 
The entire structure (with the exception of the tips 34 of each electrode) 
must then be insulated or passivated. Passivation means to coat the array 
with a substance to prevent ion transfer between neural tissue and the 
array. In one passivation method, polyimide, coupled to aluminum chelate 
primer (Hitachi PIQ coupler) is used. The entire array is immersed in 
aluminum chelate primer and the excess aluminum chelate primer is drained. 
The entire array with the lead wires is then oven cured. 
In an identical fashion, the array is immersed in polyimide and the excess 
polyimide drained. The entire array with lead wires is then oven cured. 
The tips of the array are then pushed through a thin foil, and the 
polyimide and the aluminum chelate primer are etched from the exposed tips 
of the spires in an oxygen plasma. The 25 micron lead wires is then 
soldered to a percutaneous connector. The back of the connector is then 
filled with epoxy. 
Another passivation technique involves the use of silicon nitride and 
silicon dioxide, instead of the polyimide. Silicon nitride, when properly 
deposited on a silicon dioxide coating, can provide very long-term 
passivation. 
FIG. 21 is a side view of a pneumatic impact inserter of the invention. The 
inserter of FIG. 21 is used to implant the electrode array of FIG. 1 into 
the cortex of the brain. The cortex of the brain has a consistency 
somewhat like gelatin. The electrode array of FIG. 1 is pressed into the 
cortex with tips 34 being inserted first until base 30 comes into contact 
with the cortical tissue. If the electrodes 32 are slowly pushed into the 
cortical tissue, the tissue tends to deform and dimple under the array. 
However, if the array is rapidly inserted by a sharp impulse force, the 
electrodes 32 penetrate the cortical tissue without substantial 
deformation of the tissue. 
The impact inserter of FIG. 21 includes a delivery tube 160, a piston 162, 
an insertion mass 164, and an end spring 166. Delivery tube 160 is a 
cylindrical tube, which may be formed, for example, of aluminum. Piston 
162 is a cylindrical mass sized to slidingly fit within tube 160. In a 
working model of the inserter of FIG. 21, tube 160 has an inside diameter 
of 0.477 cm and a length of 17.4 cm. Piston 162 has an outside diameter of 
0.468 cm. and a mass of 1.76 grams. In the illustrated embodiment, piston 
162 is formed of stainless steel. The electrode array is placed on face 
168 of insertion mass 164 with electrodes 32 pointing away from face 168. 
A pressure tube 170 connects to tube 160 at cap 172. Before the array is 
inserted into the cortex, piston 162 is held in the position shown in FIG. 
21 by means of a vacuum being presented in tube 170 behind piston 162. 
When it is desired to insert the array into the cortex, an air pressure 
pulse is supplied to tube 170 to force piston 162 to slide toward 
insertion mass 164. The small cylindrical extension 174 on piston 162 
enters a cylindrical channel 176, formed in tube 160 and strikes the rear 
face 178 of insertion mass 164. Insertion mass 164 has a mass of 0.9 
grams. Insertion mass 164 accepts momentum transfer from piston 162 to 
achieve a high velocity impact insertion of the array into the cortex. 
After the array has been inserted into the cortex, spring 166 returns 
insertion mass 164 to its original position. Spring 166 also controls the 
distance of travel of insertion mass 164 after it has been struck by 
piston 162. Useful values for the pressure applied at tube 170 for 
insertion is a value of 12 pounds per square inch of pneumatic pressure 
applied for a period of 0.13 seconds. 
FIG. 22 illustrates an alternative impact inserter for electrode arrays of 
the invention. The impact inserter of FIG. 22 uses a mechanical spring 
rather than pneumatic pressure to achieve insertion. The impact inserter 
includes a cylinder 190, a sliding plunger 192, a trigger 194, a spring 
196, an insertion mass 198, and a spring 200. The array is placed on face 
202 of the mass 198 with the electrodes pointing away from face 202. The 
user grasps cylinder 190 and pulls back trigger 194, which is connected to 
plunger 192, to thereby withdraw plunger 192 to a position where trigger 
194 may be rotated to move into a notch 204. In notch 204, trigger 194 is 
held in place against face 206 by spring 196. To insert the array, the 
user rotates trigger 194 out of notch 204. Spring 196 then urges plunger 
192 rapidly toward insertion mass 198 to strike mass 198 on face 208, to 
deliver momentum to mass 198 and to thereby quickly insert the array into 
the cortex. 
While both the inserters of FIGS. 21 and 22 are useful, it is currently 
believed that the pneumatic inserter of FIG. 21 is preferable, as 
providing greater control over the parameters of insertion and providing 
for more consistent insertion results. 
Reference herein to details of the illustrated embodiment is not intended 
to limit the scope of the appended claims, which themselves recite those 
features regarded important to the invention.