NEW EASY TO INSTALL NERVE
CUFF ELECTRODE USING SMA ARMATURE
M-A. Crampon*, M. Sawan*, V.Brailovski**, F.Trochu**
*Department of Electrical & Computer
Engineering
**Department of Mechanical Engineering
SUMMARY
This paper presents an easy to
install nerve cuff electrode dedicated to functional electrical stimulation. In
this new device, a shape memory alloy (SMA) armature is used to perform the
closing of the electrode. This technique makes the electrode installation
around the nerve much easier, quicker and safer. Both remarkable mechanical
properties of SMA materials namely shape memory effect and superelasticity, can
be used to obtain the desired mode of electrode closing. The fabrication
procedure of the new electrode is described. It does not require any expensive
or complex techniques. Bipolar and tripolar electrodes have been manufactured
with an inner diameter of 1.6 mm and a cuff wall thickness of 0.8 mm. These
electrodes are to be used for functional electrical stimulation of the bladder
in spinal cord injured patients. Acute studies in dogs are being carried out to
validate the device and the implantation procedure.
STATE OF ART
Nerve cuff electrodes are
widely used for functional electrical stimulation (
MATERIALS AND METHODS
General description and materials.
The new nerve cuff electrode with SMA armature is presented on figure 1.
Its design is based on classic split-cylinder cuff electrodes but a SMA
armature has been added inside the cuff wall. This SMA structure enables the
electrode to close by itself around the nerve and to be maintained in place
without requiring any external fixation means such as sutures.
The electrode is exclusively made of biocompatible materials. The
electrode cuff is molded in Silastic® and electrode contacts are made of a
0.025 mm thick platinum foil. The leads are multi-strands stainless steel wires
coated with Teflon® (e.g. Cooner Wire, AS634). For
the SMA armature, medical grade NiTi wires of 0.1 mm of diameter are used
(Shape Memory Applications, inc.). This shape memory alloy (50.7% Nickel, 49.3%
Titanium) is considered as biocompatible and has already been used in different
biomedical applications such as cardiovascular stents. Nevertheless, it is
still undergoing acute long term biocompatibility testing. For this reason and
also because the armature needs to be electrically isolated,
Teflon coated lead wires Contacts
made of platinum foil SMA armature
the electrode SMA structure is completely
embedded in Silastic®.
Fig.1: The nerve cuff electrode with SMA
armature.
Mechanical properties of shape
memory alloys and electrode closing mode.
SMA materials are well known and widely described in literature /6/. In
order to understand the closing mode of the electrode, the two remarkable
properties of SMA materials – the shape memory effect and the superelasticity –
are shortly described here. The shape memory effect is the capacity of a SMA to
recover a memorized shape when deformed at a certain temperature and then
heated to a higher temperature. The material can memorize any desired shape by
undergoing a specific thermal treatment. Its recovery shape temperature can be
fixed at a desired value. Above this recovery shape temperature, the alloy
becomes superelastic. This means that, over a certain amount of mechanical
stress, the material can be easily and reversibly strained up to 8% or more,
rather like a rubber band than like classic metallic materials.
As describes on Figures 2 and 3, the way
of installing the electrode around the nerve depends on the chosen method
(shape memory effect armature or superelastic armature). If a shape memory
effect armature is to be used, then the alloy recovery shape temperature will
be fixed slightly under 37ºC. At room temperature, the electrode is initially
closed (fig.2a). By cooling the electrode around 10ºC, the surgeon can easily
open the cuff and the electrode will remain in open position even if it is
taken back to room temperature (fig.2b). In this open configuration, the
electrode can be easily placed under the nerve (fig.2c). In the biological
environment, the SMA armature warms up to its recovery shape temperature and
then recovers its initial shape, activating the electrode closing (fig.2d).
When closed onto the nerve, the SMA armature makes the cuff rigid enough to insure the
cuff mechanical stability . If a superelastic
|
a) b) c) d)
a) b) c) |
Fig.3: Installation procedure of a cuff electrode with a superelastic
armature. a) Closed electrode at room temperature, b) Electrode opened by the
surgeon at room temperature, c) Elastic closing of the electrode around the
nerve. |
armature is to be used, the recovery shape temperature is fixed below
room temperature. At room temperature, the electrode is initially closed
(fig.3a). The physician pulls apart the two edges of the electrode cuff,
strongly enough to reach the SMA material superelastic state. Then, the
electrode cuff opens easily (fig.3b). Keeping the cuff opened, the surgeon can
place the electrode near the nerve. When he slackens the cuff, the electrode
comes to close elastically around the nerve (fig.3c). The installed electrode
is once again rigid enough to keep stable on the nerve.
Fabrication procedure.
Silastic® foils
for easy manipulation Armature elements Stainless steel
cylinder Platinum contacts Silastic® strip Thin
Silastic® layer
As shown on figure 1, the armature is
composed of several sets of split rings of NiTi wire. In order to give the NiTi
wire its final shape, it is wound into a spring on a metallic rod of 1.9mm of
diameter. It is then thermally treated at 470ºC during one to three hours depending
on the type of mechanical behavior –shape memory effect or superelasticity– we
want to obtain. The treated SMA spring is covered by a first thin layer of
Silastic® in order to create an additional cohesion between the different
spring curls. Then, we longitudinally cut the spring to obtain the armature
elements presented on Figure 4.
NiTi wire rings
|
Fig.4: SMA armature element composed by
several split rings of NiTi wire maintained by a thin layer of Silastic®. |
Fig.5: Bipolar electrode under fabrication,
before deep coating in Silastic®. The platinum contacts are maintained onto
the steel cylinder by silicone rubber elastic bands. The armature elements
are fixed onto Silastic® strips. |
The electrode is assembled on a stainless steel cylinder according to
the Haugland’s method /2/. The armature elements are
placed in between the electrode platinum contacts. They are isolated from the
internal side of the cuff by a Silastic® strip. The exceeding Silastic® strip
lengths of the different armature elements are linked together by an extra
Silastic® sheet (Figure 5) in order to facilitate the electrode manipulation.
When all the armature elements and contacts are mounted on the mandrel, they
are deep coated in a fluid Silastic® and heptane solution in order to obtain a
cuff of minimal thickness. The
overlapping Silastic® parts are cut by the physician after installation of the
electrode. In acute experiments, they can be left on the cuff so that the
electrode can be easily removed and left intact.
RESULTS
Different types of electrodes have been fabricated: bipolar electrodes for FES of the bladder and tripolar electrodes for electroneurograms (ENG) recording. For both types, electrodes with memory effect armature and others with superelastic armature have been completed. The shape recovery temperature of the SMA material is fixed at 35ºC in the first case and at 10ºC in the second case. The electrode cuffs are 1.6 mm of diameter and about 0.8 mm of thickness. Their lengths are 12 mm for bipolar electrodes and 15 mm for tripolar ones. These dimensions are adequate for implantation on S2 sacral root of dogs and respect the AAMI recommendation for safe cuff electrode use /7/.
As expected, the electrodes with superelastic armature are closed at
ambient temperature and can be easily manipulated and opened by pulling the
Silastic® edges of the cuff apart. And as soon as the stress is released, the
cuff returns to close again. The behavior of the electrodes with SMA is
slightly different of the one predicted (Figure 2). They almost behave as the
electrodes with superelastic armature because of the high elasticity of the
silicone cuff. It is difficult to keep the electrode opened even at low
temperature because the silicone cuff spring back force is high compared to the
armature rigidity. The rigidity can be improved by increasing the number of SMA
rings in the different sets of the armature. Acute experimentation on dogs has
started recently and our results will be reported soon.
DISCUSSION
The armature rigidity has to be carefully chosen. Undertaken experiments
show that a low rigidity shape memory armature does not lead to a correct
behavior of the whole electrode. Inversely, a too rigid armature could tear the
silicone cuff during closing. The armature rigidity is evaluated by mechanical
testing and by simulating the SMA material mechanical behavior.
CONCLUSION
We have described a new type of nerve cuff electrode activated by a SMA
armature. It is easier to install on the nerve than other available electrodes.
It is also fabricated at low cost without requiring any complex technique. The
superelastic armature design seems to be more promising than the one with a
shape memory armature. Acute and chronic studies are undertaken in animals
(dogs) to evaluate the electrode mechanical behavior and biocompatibility. In
the future, we will consider the feasibility of electrode cuffs of smaller
diameter that could extend in case of nerve diameter increase.
ACKNOWLEDGMENTS
Authors would like to acknowledge the financial support from the Natural
Sciences and Engineering Research Council of Canada (NSERC) and the
International Council for Canadian Studies.
REFERENCES
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²Stimulator Design and Subsequent Stimulation Parameter Optimization for
Controlling Micturition and Reducing Urethral
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vol. 4, No. 1, March, pp. 39-46, 1996.
/2/ M. Haugland, "A Flexible Method for Fabrication of Nerve
Cuff Electrodes", IEEE-EMBS Proceedings, Amsterdam, 1996.
/3/ G.G. Naples, J.T. Mortimer, A. Scheiner,
J.D. Sweeney, "A Spiral Nerve Cuff Electrode for Peripheral Nerve
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/4/ I. Yu. Khmelevskaya et al.,
"Application of Ni-Ti SME Alloys to X-Ray Endslenting
and Other Medical Fields", Proceedings of the First Int. Conf. on Shape
Memory and Superelastic Technologies, Asilomar , CA.,
USA, 1994.
/5/ J.B.Niemi, J.D.Harry, "Stabilization and Insertion of Peripheral
nerve Electrodes Using a Ni-Ti Cuff", Proceedings of the First Int. Conf.
on Shape Memory and Superelastic Technologies, Asilomar
, CA., USA, 1994.
/6/ H. Funakudo, “Shape Memory Alloys”,
Gordon & Breach, Amsterdam 1987.
/7/ Association for the Advancement of Medical Instrumentation, "American National Standard for Implantable Peripheral Nerve Stimulators", 1984.
AUTHOR’S ADRESS
Marie-Agathe Crampon
Department of Electrical & Computer Engineering, Ecole
polytechnique de Montreal
P.O.Box 6079, Station Centre-Ville, Montreal, Qc, Canada H3C 3A7
Tel: (1)514 340 4711 (ext.4190), fax: (1)514 340 4147,
e-mail: crampon@vlsi.polymtl.ca