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Journal of Medical and Biological Engineering, 31(5): 353-357
353
Self-closing Cuff Electrode for Functional Neural Stimulation
and Recording
Naga S. Korivi
Pratul K. Ajmera*
Electronic Material & Device Laboratory, Department of Electrical & Computer Engineering, Louisiana State University, LA 70803, USA
Received 6 Aug 2010; Accepted 6 Dec 2010; doi: 10.5405/jmbe.819
Abstract
This paper reports on a novel cuff electrode design for applications in neural electrical stimulation and recording.
One of the more commonly employed electrode designs for the functional electrical stimulation of nerves and nerve
fibers is the cylindrical cuff electrode, which has a lengthwise opening that allows placement of the target nerve within
the cuff. The cuff opening is subsequently closed to secure the cuff and to decrease electrical noise from the
surrounding ambient. These additional intra-operative steps required for cuff closure are a major limitation of most of
the current cuff electrode designs. Some cuff electrode designs specifically proposed to address this cuff closure issue
suffer from other inherent drawbacks. Therefore, there is a need for an electrode design that can preserve the advantages
of a cuff structure, while overcoming the cuff closure problem. The cuff electrode design proposed here addresses the
problem of securing the cuff opening after placement on the target nerve tissue. The proposed design consists of a
normally closed cuff that has a pinch hinge. By applying a small force on the arms of the pinch hinge, the cuff can be
opened for placement on a nerve. Subsequent removal of the force returns the cuff to its closed state. This self-closing
cuff design is expected to enhance the ease of implanting cuff electrodes on nerves for functional electrical stimulation
and recording applications.
Keywords: Functional electrical stimulation (FES), Nerve stimulation, Cuff electrode, Neural signal recording
1. Introduction
The functional electrical stimulation (FES) of nerves and
nerve fibers is of interest in the control and treatment of
numerous physiological conditions [1]. Cuff electrodes offer
several advantages compared to other commonly used
electrodes, such as hook electrodes or wire-wound electrodes,
for the FES of nerves and nerve fibers. The advantages include
a reduction in the stimulus intensity required for n erve
activation, minimization of the mechanical distortion of the
nerve, reduced probability of lead failure, and selective
stimulation of the target tissue [2,3]. A typical cuff electrode
has a lengthwise opening that requires closing upon placement
on the target nerve to ensure that the electrode stays secured
and in contact with the target nerve. In addition, in applications
involving bio-potential recording, a closed cuff electrode
reduces electrical noise from the surrounding physiological
ambient [4]. During implantation, conventional cuff electrodes
are manually opened, placed on the target nerve, and closed.
The cuff opening is typically secured by epoxy, wax, small
* Corresponding author: Pratul K. Ajmera
Tel: +1-225-5785620; Fax: +1-225-5785200
E-mail: ajmera@lsu.edu
pieces of silicone rubber, or suture thread [5-7], all of which
require intra-operative procedures. These additional
intra-operative procedures may become significantly more
complicated when the cuff electrodes have small dimensions,
such as those required for small-diameter nerves and nerve
fibers. The spiral cuff design addresses the cuff closure
problem, but some spiral cuffs do not reliably interface with
small nerves [8]. Therefore, there is a need for an electrode
design that preserves the advantages of a conventional cuff
structure while overcoming the cuff closure problem.
2. Methods and materials
A self-closing cuff electrode design (Fig. 1) is proposed
here to address the cuff closure problem. The proposed device
incorporates metal wire or foil electrodes in the inner diameter
of a polymeric cuff. The cuff is normally closed. A
pinch-hinge-type structure in the cuff electrode allows the user
to conveniently open and close the cuff to enclose the target
tissue [9]. Once implanted, the cuff stays on the nerve until the
pinch hinge is used to open it.
A flat sheet of acrylic was cleaned with de-ionized (DI)
water and air dried for use as a base substrate. A layer of
polyvinyl alcohol (PVA) was spin-coated onto the substrate to
function as a mold release layer (Fig. 2A). An aqueous solution
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J. Med. Biol. Eng., Vol. 31 No. 5 2011
of PVA (1:8 PVA to DI water ratio, by weight) was used for this
purpose. After spin-coating, the mold release layer was allowed
to solidify at room temperature for 8 h.
release coating on the wire, allowing for the wire to be easily
pulled out from the silicone (Fig. 2F).
The free-standing cured silicone structure with a hollow
cylinder was manually trimmed at the sides using a blade to the
desired lateral dimensions of the cuff. One side of the structure
was manually cut to remove some silicone material to form a
pinch hinge (Fig. 2G). On the other side of the structure, the
edges were trimmed (Fig. 2H) to reduce the overall size and
mass of the device. The cuff opening was then defined by
manually making a slit on the cured structure (Fig. 2I).
Figure 1. Schematic representation of the proposed cuff electrode with
its normally closed cuff.
Figure 3. Schematic of attaching electrical leads to the cuff body.
Figures are not drawn to scale.
Figure 2. Schematic of the fabrication process for the cuff body. Figures
are not drawn to scale.
A thin layer of medical-grade silicone (Silastic®
MDX4-4210 elastomer kit) was spin-coated onto the solid mold
release layer (Fig. 2B). This layer was cured in a convection
oven at 60°C for 3 h. A metal wire of a known diameter was
then dip-coated with a thin layer of PVA (Fig. 2C). The wet PVA
layer was allowed to solidify at room temperature for 8 h. The
coated metal wire was placed in conformal contact with the
silicone layer on the substrate (Fig. 2D).
A thick layer of uncured silicone liquid was poured over
the wire and cured thermally at 60°C for 3 h (Fig. 2E). As this
thicker layer cured, it bonded permanently with the underlying
thin silicone layer. Subsequently, the cured silicone structure
was released from the substrate by dissolving the mold release
coating on the substrate in an ultrasonic bath of water. Further
ultrasonic treatment in a bath of water dissolved the mold
At this stage of the fabrication process, the main body of
the device is obtained, with the cuff structure in a normally
closed position. Depressing the pinch hinge allows the cuff to
be opened. To complete the cuff electrode device, electrodes
need to be incorporated in the cuff. A segment of multi-strand
stainless steel electrode wire with Teflon insulation was
stripped of its Teflon coating at one end to expose a length of
bare wire. This bare strand of wire, 50 µm in diameter,
functions as an electrode lead inside the cylindrical space of the
cuff. The cuff body (Fig. 3A) can be opened slightly by holding
the arms of the pinch hinge and applying a squeezing force
(Fig. 3B). A 26½ gauge syringe needle was employed to pierce
the cuff body from the pinch hinge side (Fig. 3C). The needle
was allowed to exit from the cuff side, through the open cuff.
The insulated end of the electrode wire was inserted into the
needle from the cuff side (Fig. 3D). When the wire appeared at
the other side of the needle, it was held by tweezers and gently
pulled through until a few cm of the wire projected out
(Fig. 3E). At this juncture, the syringe needle was disengaged
from the cuff body by holding the electrode wire at the cuff
opening side and by gently pulling out the needle with a
twisting motion, leaving the wire inside the cuff body (Fig. 3F).
As the needle was pulled out, the cuff body material collapsed
around the wire, holding it snugly. The wire was then pulled
further from the pinch hinge side until only the bare wire was
Self-closing Cuff Electrode for Nerve Tissue
inside the cylindrical opening (Fig. 3G). The entire process was
repeated to introduce the desired number of electrode leads
(Fig. 3H). For the case considered here, three electrode leads
were inserted into the open space of the cuff cylinder (Fig. 3H).
The cuff was then opened to enclose a pin that was rotated to
bend the electrode leads. This operation shaped the electrode
leads, making them lie against the inner wall of the cuff
(Fig. 3I). Subsequently, the electrode wire-cuff body interface
at the point of exit of each electrode wire was reinforced with
silicone (Fig. 3J).
3. Results and disscussion
The proposed electrode device was fabricated with cuff
inner diameters of approximately 400 µm and 800 µm (Fig. 4A),
respectively, with bare metal wires incorporated inside the cuff
and insulated portions of the wires exiting from the cuff body.
The exit point of the wires was reinforced by applying liquid
silicone pre-polymer and curing it thermally (Fig. 4B).
Figure 4. Microscope images of self-closing cuff electrode. The scale
ruler has 0.397 mm spacing markers.
The same type of silicone material was used for the device
body and for the wire reinforcement to form a robust interface
between the cuff body and electrical wiring. Although three
electrode wires were incorporated into the fabricated devices, a
larger number of electrodes can be installed if desired. The
metal wire electrodes of the self-closing cuff are not fixed inside
the device. They lie flush against the inner walls of the cuff.
Other studies that employed this metal wire arrangement inside
a cuff did not report interference with the insertion of a nerve
[10]. The metal wires can be fixed during the fabrication process
if desired. The total weight of the developed cuff device
including the electrode wires (Fig. 4) is in the range of 0.2~0.3 g
with total outer surface area in the range of 45~60 mm2. The
cuff inner diameter can be made to be over 2.5 mm.
The developed electrodes were subjected to electrical and
mechanical tests. The electrical impedance between the leads
was measured in 2 M potassium chloride (KCl) solution at room
temperature with an impedance meter (Tenma® dual-display
LCR meter, model 72-960). The electrical impedance between
the electrode leads was found to be in a range of 300 Ω to 3 kΩ.
This is close to the range of values reported in the literature for
other cuff devices [11]. The impedance values indicate that the
electrode leads inside the cuff function properly.
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In another test, electrical signals in the micro-volt range at
a given frequency were fed to the electrodes of the self-closing
cuff (Fig. 5A). Resistor R2 was connected between electrodes
E1 and E2 to simulate an enclosed nerve. The electrical
resistance of a nerve trunk with a diameter of 1.06 mm is
approximately 28.75 kΩ/cm [12]. Due to a spacing of 2 mm
between the electrodes, the value of R2 was chosen to be 5.6 kΩ
to simulate a 2-mm-long nerve trunk enclosed between the
electrodes. The signals read by the electrodes were recorded
after being amplified. A recorded sinusoidal wave voltage signal
at 1 kHz with an amplitude of approximately 150 µV, amplified
by a gain of 91, is shown in Fig. 5. Signals with amplitudes of as
low as 75 µV in a frequency range of 100 Hz~5 kHz were
successfully read by the proposed self-closing cuff. These
signals are in the range of typical electroneurogram (ENG)
signals in terms of magnitude and frequency.
Figure 5. (A) Schematic of in vitro testing of the self-closing electrode to
study its applicability in stimulation and recording. Digital
oscilloscope data of real-time recording of (B) an artificially
generated voltage signal Vin input to the electrodes of the
self-closing cuff through a R2/(R1+R2) voltage divider and (C)
an amplified signal read by the electrodes of the cuff. The
signal inversion is due to the use of an amplifier in the final
stage of the test set-up. No filtering component or equipment
was employed for removing electrical noise.
The self-closing cuff electrodes were evaluated for their
applicability in bio-potential recording. The cuff device was
immersed in stationary KCl solution and the electrical
impedance between its electrode leads was measured (Table 1).
The KCl solution was then agitated by a magnetic stir bar to
simulate the effects of a physiological ambient in motion. The
impedance readings showed considerable, random fluctuations
ranging between 2.5 kΩ~14 kΩ for a 120-Hz measurement, as
shown in Table 1. The two circular openings at the lateral ends
of the self-closing cuff were closed by attaching pieces of
silicone. The cuff electrode was then reintroduced into the KCl
solution. The KCl solution entered the cuff gradually and filled
the cuff interior. The electrical impedance readings were
obtained and found to be in the range of those measured in
stationary KCl solution, with the cuff ends open. The solution
was then agitated by magnetic stirring. The impedance readings
did not show appreciable changes due to the movement of the
solution, ranging between 2.2 kΩ~3 kΩ for a 120-Hz
measurement. Subsequently, the cuff endings were opened by
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removing the silicone pieces and the electrical impedance was
measured in agitated KCl solution. The impedance values were
found to fluctuate considerably. These results are attributed to
the more or less fixed enclosed volume inside the cuff when it
is in the closed position, a situation analogous to when the cuff
encloses a nerve. Therefore, the proposed device can be used
for recording electrical signals with a high signal to noise ratio
from a nerve tissue. For conventional cuff electrodes, a similar
signal to noise ratio may be obtained if the cuff is closed by
epoxy, wax, or silicone pieces. However, these sealing
procedures are intra-operative. If a suture thread is used to
close a conventional cuff, the electrical noise from the
surrounding ambient may not be reduced to the extent possible
by the self-closing cuff.
The overall size of the self-closing cuffs can be reduced
without any change in the fabrication process. Experiments
show that the cuff size can be decreased to reduce the total
surface area by more than 30% of the value on devices reported
here. A further reduction in size is possible. The control of
pressure on the nerve is important for the safety of nerve tissue.
Previous experimental studies on the circular compression of
rabbit sciatic nerves determined that blood perfusion of the
nerves started decreasing at a mean pressure of 4066.3 Pa [14].
Our theoretical modeling indicates that thinner cuff walls can
accommodate the expansion of an enclosed nerve with minimal
or no compression. Therefore, future work will focus on
reducing the overall size of the cuff, including the cuff wall
thickness.
Table 1. Electrical impedance readings for proposed self-closing cuff
electrode measured between its electrode leads in stationary and
moving KCl ambient, with the cuff’s lateral ends closed and
open, respectively.
4. Conclusion
Cuff condition
Lateral ends open
Lateral ends open
Lateral ends closed
KCl ambient
condition
Stationary
Moving
Moving
Impedance reading at
120 Hz
2.3 kΩ~3.2 kΩ
2.5 kΩ~14 kΩ
2.2 kΩ~3 kΩ
A major goal of the present research is to secure a cuff
electrode to a target tissue without requiring additional steps
during surgery. Force tests performed on the developed cuff
device indicate the efficacy of this normally closed cuff design.
A metal pin was enclosed in the cuff and the cuff was returned
to its closed state. The enclosed metal pin was clamped at both
its ends. The electrical wires exiting the cuff were attached to
the hook of a force gauge (Chatillon, NY, USA, Gauge-R,
Model 516-1000) and a pulling force was applied to the wires.
During the course of testing, the enclosed metal pin did not
bend from the force applied. The force was progressively
increased until the cuff was removed from the enclosed wire.
The force-test results indicate that the developed self-closing
cuff device can withstand a force of 1.9 N before the enclosed
pin is removed. This compares favorably to a force of 0.2 N
measured for a conventional cylindrical cuff device fabricated
using the processes typically used for such devices [10]. For
both types of device tested, the outer surface areas and total
weights of the devices were comparable.
For the self-closing cuff device developed in this work
with a total outer surface area of 45~60 mm2 and a weight of
0.2~0.3 g, an average force of 0.8 ± 5% N is required to open
the cuff to its maximum extent. This relatively low required
opening force indicates that the self-closing cuff design is an
alternative to designs that require the application of a force for
cuff closure. For example, Durand et al. reported a cuff design
with male-female interlocking structures that requires the
application of a 2-N force to achieve cuff closure [13]. This
magnitude of force can harm the nerve if the cuff is improperly
closed. Durability testing of the pinch hinge of the developed
cuff electrode indicates that the cuff can withstand more than
50 open-close operations, which is significantly more than
needed during a typical lifetime of such a device.
A pinch hinge was incorporated in a self-closing cuff
electrode to allow the user to easily open the cuff and place it
on the target nerve tissue for stimulation or recording purposes.
When the pinch hinge is released by the user, the cuff assumes
its normally closed position. The pinch hinge negates the need
to close the cuff electrode by suture threads or some other
intra-operative procedure. The proposed self-closing cuff
electrode offers researchers an easy means to stimulate or
record electrical signals from nerves and nerve fibers with
effective diameters ranging from the micro-scale to the
millimeter-scale.
Acknowledgement
The authors acknowledge the assistance of Mr. Golden
Hwaung in the laboratory.
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