INTRODUCTION
Urinary bladder functions can be lost after spinal cord injury at the
T12 level or higher. In order to regain partial bladder control, functional electrical
stimulation has been performed at various sites including the spinal cord, the
pelvic nerves, the bladder wall and the sacral roots. It is well established that sacral roots
stimulation shows the most promising results but it induces a simultaneous
contraction of both the detrusor muscles and the external urethral sphincter.1)
Somatic fibers supplying the bladder sphincter are more sensitive to
electrostimulation than autonomic fibers innervating the detrusor muscle. A
stimulus large enough to provoke detrusor contraction will inevitably induce
external urethral sphincter activation and prevent micturition. Recently,
selective electrical stimulation of the detrusor muscles was achieved by using
a signal composed of two distinctive trains of bipolar-current pulses. A
high-amplitude, low-frequency train provokes detrusor contraction while a
low-amplitude, high-frequency train relaxes the external urethral sphincter for
complete voiding of the bladder.
To perform electrostimulation, many devices are now available but they
lack important features such as a wide range for programmable parameters, a
high efficiency in energy transfer and data transmission, a user-friendly
interface, high-frequency stimulus generation and waveform flexibility.2)
Several implantable multichannel stimulators and
their external controllers have already been proposed by members of our team to
fill the gaps and carry out functional electrical sacral roots excitation. We
propose in this paper a new miniaturized integrated implantable stimulator
which is based on a prototype currently used for chronic experiments and
realized with commercially available components. This paper describes methods
an materials, gives simulation and experimental results and main conclusions.
METHODS
In order to perform selective neuromuscular stimulation of the bladder
through sacral roots, a new integrated stimulator was designed to meet the
needed parameters and precision. This device is a component of a complete
system which includes three parts:
-
The external
controller providing the parameters and the energy transcutaneously to the
stimulator;
-
The
implantable stimulator receiving the information and generating the current
pulses necessary for bladder emptying;
-
The
bipolar cuff electrode wrapped around the nerve.
Two complementary models of the external controller have been realized.3)
The first one, attached to a personal computer (PC), offers the generation of a
large sets of parameters. Through dedicated software, each stimulation
parameter can be easily changed. The current set of parameters can also be
saved and retrieved from disk or exported to a binary file for subsequent use
with the portable controller. The second model is a hand-held device dedicated
to moving individuals. This memory based unit allows the generation of eight
preprogrammed sets of parameters. The user-friendly interface enables to view
and choose from the set of parameter's description displayed on a liquid
crystal display (LCD). Both models include an amplitude modulation (AM) emitter
to send the
Figure 1: Block diagram of the implantable
stimulator.
The new
miniaturized stimulator is composed of two main modules (figure 1): 1) RF
electronics part, and 2) a mixed signal (analog – digital) integrated circuit
which is the subject of this paper and identified by the gray box on the block
diagram. The implant receives the data and the energy through an inductive
coupling link. The received 20 MHz AM waveform is rectified to power up the
implant while the 300 kHz clock signal and data are extracted from the
The digital modules of the implant completed with very high speed
integrated circuit hardware description language (VHDL) where synthesized with Synopsys tool. The resulting circuit was simulated and
merged with the full-custom current source under Cadence design environment.
The complete design (control logic as well as the current source) has been
realized with complementary metal-oxide semiconductor (CMOS) 0.35 mm technology on a 1.8 mm x 2.2 mm
silicon die. The integrated digital part of the implant receives the data
serially and detects the header of the bit stream. There are two valid 10-bits
command words to be interpreted by the control logic. The first one forces the
stimulator to test mode in which it generates a bipolar ramp. This mode
facilitates the current source validation and characterization. The second
header shifts the system in stimulation mode (normal mode). Received parameters
are then transferred to the stimulus generation and control modules to produce
the selective stimulation waveform. The generated 5-bits control commands are
interpreted by the bipolar current source to transform the digital signals into
a current waveform.
The current source uses identical p-channel transistors placed in
parallel or in series to deliver a preprogrammed binary weighted current level.4)
This architecture as a low sensitivity to fabrication process errors. Bipolar
signal generation is achieve through an H output architecture (figure2). The
current mirror is used for both polarity creating well-balanced stimuli and
reducing the possibility of nerve polarization by charge induction. Figure 2
depicts the typical selective stimulation current waveform where all parameters
can be adjusted within the ranges shown in table 1.
Figure 3: Typical selective stimulation
waveform (meaning of abbreviations are given in table 1)
Table 1
Parameters range and resolution (HF: High
frequency, LF: Low frequency)
The last part of the stimulation system is the dedicated cuff electrode
which connects the implant to the sacral root. The electrode consists of two
stainless-steel leads covered with polytetrafluoruethane (Teflon®)
and soldered to 25 mm platinum foil forming
the contact surface to the nerve inside the cuff of the electrode.5)
RESULTS
The external controller have been used twice a day for chronic
experiments stimulation for more than 2 years showing expected performance and
good reliability.3) Following a previous version of the stimulator
based on a FPGA (field programmable gate array) which has been completed to
prototype the stimulator and start validation of the selective stimulation
technique, the central part of the implant, which is the integrated
mixed-signal circuit, has been processed in 0.35 mm CMOS technology. The control part of the
integrated circuit was validated through a series of functional tests. Various
internal signals where externally connected allowing simplified testing by
reading intermediate signal and applying the desired value. The remaining part
which is the current-source block proved good functionality using the ramp
generation built-in test routine.
DISCUSSION
Reducing the dimensions of the whole implant is an important issue to
validate the stimulation technique in small animals like rats. Miniaturizing
the implant is mainly based on the integration of mixed-signal functions on an
application specific integrated circuit (ASIC). A completely integrated
implantable stimulator will also reduce the power consumption allowing to
decrease the emitting signal amplitude, therefore extending battery life in the
portable controller. The design of a custom version of the whole implant (RF
and stimulator parts) that will incorporate all features on the same silicon
die is undertaken in our team to obtain a minimal power consumption, to
decrease the whole implant dimensions and to continue the selective stimulation
validation. The realization of the digital part of the design with a high-level
hardware description language will allow to rapid modification for the
implementation of new functions like telemetry. The full-custom current source
allows a large maximum current output and a precise and well-balanced
stimulation.
ACKNOWLEDGMENTS
Authors acknowledge financial support from The Natural
Sciences and Engineering Research Council of Canada (NSERC) and from the Kidney
Foundation of Canada (KFC). We would also like to thank J.-C. Voghell for his inputs to this project.
REFERENCES
1)
N. J. M. Rijkhoff, H. Wijkstra, P. E. V. van Kerrebroeck
and F. M. J. Debruyne, "Urinary Bladder Control
by Electrical Stimulation: Review of Electrical Stimulation Techniques in
Spinal Cord Injury," Neurourology & Urodynamics, vol. 16, pp. 39-53, 1997.
2) M. Sawan, F. Duval, M. Hassouna, F. S. Li, M. M. Elhilali, "A New Bladder Stimulator – Hand-held Controller and Miniaturized Implant: Preliminary Results on Dogs," Biomed. Instrum. Tech., vol. 27, pp. 143-149, March/April 1993.
3) S. Boyer, M. Sawan, M. Abdel-Gawad, S. Robin, M. M. Elhilali, "Implantable Selective Stimulator to Improve Bladder Voiding: Design and Chronic Experiments in Dogs," accepted for publication in IEEE Transactions on Rehabilitation Engineering.
4) J.-C. Voghell, M. Sawan, S. Bourret, M. Roy, "Programmable Current Source Dedicated to Implantable Microstimulators," 10th International Conference on Microelectronics, Monastir, 1998.
5) M.-A. Crampon, M. Sawan, V. Brailovski, F. Trochu, "New Nerve Cuff Electrode Based on a Shape Memory Alloy Armature," 20th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, 1998.