1. 
Abstract— An instrument designed for the implantation of
neural electrode array devices has been refined in preparation
for use in cortical implantation procedures in non-human
primates. This instrument has undergone extensive testing to
ensure its successful first use in a live surgical setting. This
work describes the modifications made to the instrument and
the testing performed on it during that preparatory period as
well as planned future modifications and augmentations.
Figure 1. Comparison of the instrument presented here (left) with a
previous embodiment (right). Notable changes and commonalities are
highlighted.
I. INTRODUCTION
As reported previously [1], an instrument has been
designed for the purpose of inserting microelectrode array
devices into cortical neural tissue. The goal of this instrument
is to ensure the successful implantation of cortical electrode
devices while minimizing tissue damage in the surrounding
area. Key factors in this goal are control of implantation
velocity, control of stroke depth, and minimizing contact time
with neural tissue.
The instrument presented here has greater control over
velocity and position of implanted devices than previous
instruments, where the electrode array device was projected
*Support: Telemedicine and Advanced Technology Research Center,
U.S. Army Medical Research and Material Command, Contract W81XWH-
12-1-0394; Gifts to IIT; Sigenics, Inc. R&D.
1
Department of Biomedical Engineering, Illinois Institute of Technology,
Chicago, IL USA (phone: 312-567-6902; e-mail: troyk@iit.edu).
using either a spring [2] or pneumatic [3] action. In this
instrument, an electronic drive system is utilized, allowing
fine control over the insertion velocity and depth, as well as
minimizing damage to underlying and surrounding tissue. In
addition, this instrument includes a method for handling
WFMA devices which cannot be handled directly without
damaging their electrodes due to their lack of a wire tether.
This work represents an update of the previously reported
instrument [1]. The instrument described in that work has
been refined in preparation for its inaugural use in vivo for
the implantation of two pairs of floating microelectrode
stimulator devices into primate motor cortex. Extensive
testing has been performed to ensure the instrument’s
readiness for that procedure.
Fig. 1 shows cross-sectional views of the instrument
reported previously (right) and the instrument as reported
here (left). Major alterations include reversing of the motor
orientation and removal of the sensor housing. In addition, a
mounting post was added to facilitate use in the expected
surgical environment and an improved collet and collet
retention system.
Figure 2. Comparison of the motor configuration of this version of the
instrument (left) and the previous version (right).
The motor used was a VM1614-100 voice coil motor
(GeePlus, Inc. Greer, SC). In this version of the instrument,
Preparation of a neural electrode implantation device for in-vivo
surgical use*
Omar Tawakol1
, Samuel D. Bredeson1
, and Philip R. Troyk, Senior Member, IEEE1

2. the drive shaft of the motor was removed and replaced with a
shaft optimized for the reversed orientation of the motor. A
comparison of the motor arrangement of the instrument as
previously reported and with modifications made for this
study is shown in Fig. 2. The motor configuration was
reversed to increase the stroke distance and to maximize
initial acceleration. In the previous version, the motor began
its travel at its weakest position and ended at its strongest
position, resulting in low initial acceleration. In this version,
this behavior has been reversed.
Figure 3. Modifications made to the collet and instrument tip. Previous
version (left) and this version (right) of the instrument are shown.
An important feature of the devices for which this
insertion instrument was designed is their plurality. The
devices will be implanted in large numbers and in close
proximity in the visual cortex of the brain. During the
surgical implantation procedure, visibility of the implant site
and minimization of the gap between adjacent devices are
crucial. The front end of the insertion instrument was reduced
in size to improve both of those factors. Fig. 3 illustrates the
modifications made to the collets and collet retention system.
The components of the tool tip were reduced in thickness,
allowing for a smaller overall diameter and larger viewing
angle. The collet wall was reduced from 1 mm to 0.25 mm at
the front end and the tip of the tool was cut back so that only
that thin section of collet is present at the implant site. This
thin wall was used as a spacer and a guide to accurately
implant the WFMA devices side to side with a minimal gap
between adjacent devices.
Figure 4. Setup used in velocity measurements. A low-friction
potentiometer was used to translate linear motion into voltage, which was
then recorded on an oscilloscope.
II. PROCEDURE
Several tests were performed to quantify the
specifications of the motor within the modified device
assembly and to identify the input parameters required to
operate the instrument. Motor velocity was measured with
different input voltage values, physical orientations, and
forward pulse widths.
A. Setup for measuring velocity
A test setup (shown in Fig. 4) was constructed to observe
the velocity of the motor under different input parameters
such as voltage and pulse width. A potentiometer was
mechanically attached to the tip of the drive shaft and
electrically connected to a differential amplifier for
monitoring. The potentiometer setup was calibrated by
applying known linear displacements and observing the
resulting change in voltage. The data recorded for this
calibration are shown in Fig. 5. The measured values were fit
to a linear equation (1), in which x is the measured
differential voltage (mV) and y is linear displacement (mm).
This equation was used in the rest of the experiments to
convert measured changes in potential to linear displacement.
 y = 0.0266 * x 
Figure 5. Data used to calibrate the potentiometer/linear travel transducer.
A. Velocity vs. orientation
To determine the effects of gravitational acceleration on
motor velocity, the instrument was held in three different
orientations and its velocity was observed while changing
forward drive voltage. The instrument was held vertically
with its tip pointing downward, horizontally, and vertically
with its tip pointing upward. This approach was taken to
observe how much the mass of the drive shaft and the testing
apparatus itself would affect the measured motor velocity.
B. Velocity vs. pulse width
Next, the motor velocity was measured while varying the
width of the forward driving pulse. This was done to
determine the effects of pulse width on motor velocity and
motor position at the end of the stroke. The motor’s final
position was determined by observing the point on the
position-time graph at which the motor was farthest from its
origin (see Fig. 6 for a representative graph).
C. Motor drive control
The waveforms shown in Fig. 6 are a representative
sample of the forward velocity measurements that were
y = 0.0266x
R² = 0.9993
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5
6
0 50 100 150 200
Displacment(mm)
Differential voltage (mv)
Motor calibration

3. collected. The voltage across the potentiometer is represented
by channel 1 (blue). This change in voltage was used to
measure the velocity based on the calibration method
described above and by using equation (1).
Figure 6. Representative waveform captured during voltage
measurements. [Ch. 1]: linear position of the motor. [Ch. 2]: logic toggle
between full-power (low) and low-power (high) mode of the motor. [Ch. 3]:
logic for driving the motor in reverse (low) and forward (high) directions.
Channel 2 (teal) represents the logic signal used to set the
motor in either full-powered forward/reverse mode or a
lower-power holding mode. Full-power mode is used when
the instrument is activated to drive the motor and driveshaft
forward at the desired velocity and then to reverse that
motion to limit contact time with neural tissue. Low-power
mode is used to hold the instrument in the fully reversed
position while the array to be inserted is loaded and the
instrument is positioned above the target implantation site.
This logic signal is used for both modes, so that when full-
power mode is active, low-power mode is inactive, and vice
versa.
Figure 7. Logic control of the motor. Based on two logic signals, the
motor can be set into one of three drive modes. The logic controls here
correspond to channels 2 and 3 in Fig. 6.
Channel 3 (pink) represents the reverse voltage and
forward voltage logic signal. This signal is set to high
(forward) when the motor is triggered and held low (reverse)
at all other times.
Before implantation, the logic signals are set to “low-
power” and “reverse.” When the instrument is activated for
an insertion procedure (‘a’ in Fig. 6), the logic is switched to
“high-power” and “forward.” After the forward pulse has
been completed, the second logic control switches back to
“reverse” (‘b’) and after the reverse pulse has been
completed, the first logic control switches back to “low-
power” (‘c’). Both of these pulse widths can be controlled by
altering the control circuit.
C. Motor behavior
Immediately after activation, the motor undergoes a rapid
acceleration until the drive shaft reaches its terminal velocity
(shown by the linear region in channel 1 between ‘a’ and ‘b’).
At the end of the forward signal, the motor coasts briefly
(shown by the linear region immediately following ’b’) as
current in the motor coil reverses direction, then rapidly
decelerates, stops, and reverses direction (‘d’) as the reverse
current takes over. The motor accelerates in the negative
direction until the reverse current is discontinued (‘c’). It
continues at a constant velocity until the motor is fully
retracted. Finally, the drive shaft is held in place by the low-
power reverse mode.
III. RESULTS
The effects of pulse width, applied voltage, and the
instrument’s orientation were examined. The velocity of the
motor was measured along its full travel length, rather than
only the last 1 mm of travel as previously reported, while
varying those input parameters.
Velocity tended to linearly increase with increasing input
voltage. A slight decrease in this trend was observed when
the motor had to push the mass of the shaft and the testing
device against the force of gravity.
A. Velocity vs. orientation
Fig. 8 shows the forward velocity of the motor measured
while the instrument was held in three orientations:
horizontal, vertical with tip pointed down, and vertical with
tip pointed up. As expected, the velocity was greatest with
the motor held in the vertical position with its tip pointing
down, as the force of gravity aids the drive force of the
motor. The lowest velocity values were obtained when the
orientation is vertical with the instrument tip pointing
upward, as it must act against the force of gravity. The
horizontal orientation lies in between, as the force according
to gravity is neutral.
B. Velocity vs. pulse width
Velocity increased with increasing the forward pulse
width while the tool was placed in the horizontal position. By
increasing the pulse width, the force delivered by the motor
acts on the drive shaft for a longer time interval, which
imparts a greater overall increase in average velocity due to
the acceleration caused by motor force. This phenomenon
should have a practical limit due to the limited travel distance
of the motor; once the motor reaches its stopping point,

4. further pulse width will not cause any change in motor
velocity. These behaviors are shown in Fig. 9.
Figure 8. Motor velocity as a function of applied forward drive voltage in
three orientations. As expected, the motor velocity increased when aligned
with gravity and decreased when opposing gravity.
IV. DISCUSSION
Several improvements are being planned for a future
embodiment of this instrument. A three-axis manipulator will
be designed to facilitate positioning of the instrument during
surgical procedures. This manipulator will allow the
instrument’s position to be precisely controlled while still
allowing for quick and easy removal of the instrument from
the surgical area for loading and removal of the device
collets.
This instrument was used to successfully implant four
wireless floating microelectrode array devices into the motor
cortices of two non-human primates (two devices per animal)
[4]. As evidenced by visual inspection of the cortex and no
signs of bleeding, there was minimal damage to the
surrounding tissue and all four devices are functional in
preliminary testing. The instrument was also used to implant
nine WFMA devices into cortical tissue of a human cadaver
as a test of surgical procedure.
Figure 9. Motor velocity as a function of applied forward pulse width.
Velocity increases approximately linearly with pulse width, but will reach a
plateau as the pulse width is longer than the full stroke length of the motor.
This trend can be seen beginning at approximately 10ms and above.
ACKNOWLEDGMENT
The authors would like to thank Craig Johnson,
supervisor of the Illinois Institute of Technology machine
shop.
REFERENCES
[1] Bredeson, SD; PR Troyk, “Device for the implantation of neural
electrode,” 36th Annual International Conference of the IEEE
EMBS, pp. 434-437, Aug 2014.
[2] McCreery, DB; LA Bullara, SH Waldron, “Electrode insertion tool.”
U.S. Patent 6304785, Oct 16 2001.
[3] Normann, RA; EM Maynard, PJ Rousche, DJ Warren, ”A neural
interface for a cortical vision prosthesis.” Vision research, vol. 39, no.
15, pp. 2577-87, Jul 1999.
[4] Troyk, PR; D Frim, B Roitberg, VL Towle, K Takahashi, S Suh, M
Bak, SD Bredeson, Z Hu, “Implantation and testing of WFMA
stimulators in macaque,” 38th Annual International Conference of the
IEEE EMBS, Aug 2016 (forthcoming).
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Forward velocity vs. voltage
Horizontal Vertical (down) Vertical (up)
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