EFFICIENT MONITORING OF ELECTRODES-NERVE CONTACTS DURING FNS OF THE BLADDER

C. Donfack , M. Sawan, Y. Savaria.

Department of Electrical and Computer Engineering, École Polytechnique de Montréal,

P.O.Box 6079, Station “Centre-ville” , Montréal, Québec, Canada, H3C 3A7

Abstract

Electrodes-nerve behaviour during functional neurostimulation (FNS) is a major concern in implantable stimulators. In this paper a fully integrated electrodes-tissue contact monitoring system is proposed. This system generates a test current. The voltage across the load which includes two electrode-tissue contacts is first converted into frequency, this frequency is then converted into 8-bit data which is serially transmitted to the external controller through an inductive link. Thus, valuable information pertaining to the state of electrodes and tissue can be estimated. Voltage Controlled Oscillator(VCO) and Frequency Estimator are in idle mode during stimulation ensuring low energy consumption. Each block of circuit have been made fully testable. The proposed monitoring device has been realised in a 3.3V, 0.35 micron CMOS process and has been successfully tested. Its entire size including the stimulation current source is 0.1mm².

Keywords: bladder stimulators, electrode-nerve contact, impedance measurement.

I. Introduction

Sphincters and bladder contraction and inhibition are controlled by complex mechanisms involving both central nervous system and peripheral nervous system ¹⁾. Electrical stimulation of the lumbosacral spinal cord, bladder wall, pelvic nerve and sacral root induce muscular contractions of the detrusor and aid in initiation of voiding in paraplegic ^{2) 3)}. However, electrodes malfunction often reduce the safety and the reliability of the stimulation.

Impedance measurement of the electrode-nerve contact and interelectrode tissues is an efficient mean for the verification of the state of electrodes and stimulated tissues owing to his sensitivity to most commonly encountered anomalies. Impedance measurement is generally associated with device such as computer, scope or frequency analyser. Using those device to test implanted electrodes and its contact with the nerve require percutaneous wire which are not always recommended for implantable stimulators.

Recently, we proposed an FPGA based first version of a circuit using impedance measurement technique which is externally controlled through an inductive link ⁴⁾. In this paper, a fully integrated version of the circuit is proposed. The circuit is suitable for long-term requirements and is able to provides at any time information on the electrodes-nerve contact.

Section II concerning materials and method presents the electrodes-nerve contact model adopted to test the device, and the design description which includes impedance to frequency converter and frequency to 8 bits converter. Section III summarizes the results, and conclusion is the subject of section IV.

II. MATERIALS AND METHOD

A. Electrode-nerve contact modelling

The contact between electrodes and nerve behaves in a very complex manner. Viewed as electrode-electrolyte interface, it has been studied by several authors. After a review of existing electrical models of the electrode-tissue contacts, the simplest one has been chosen in order to validate our electrode testing approach. A parallel resistance–capacitance model has been chosen, since with current stimuli, it adequately represents the voltage induced across real electrode-tissue contacts.

After implantation, electrodes-nerve contact impedance can fall in one of three main ranges: nominal, high and low impedance values. For commonly used electrodes, the nominal impedance value varies from 300W to 3KW. An important point is that impedance variation occurring during first weeks after implantation can be evaluated in order to adjust stimulation parameters.

Monitored impedance over 10kW is an indication of electrodes or lead wire breakage or simply a disconnection between electrodes and the nerve due to mechanical problems. However, when electrodes-nerve contacts exhibit an impedance under 100 Ohms, this can be interpreted as a short circuit of the electrodes. This short circuit is due either to blood or saline in the pelvic cavity surrounding the bladder ⁵⁾ either due to failure in electrodes and leads insulation or to metal dissolution due to corrosion.

A test current provided by the stimulator is injected into two electrode-nerve contacts and the nerve. The voltage induced across electrodes is converted into proportional frequency. This frequency is then converted into an 8-bit data and serially transmitted to the external controller through a FSK modulator and an inductive link already presented in another paper ⁴⁾.

B. Impedance to frequency conversion

As shown in Fig. 1, transistors driven by signals PS1, PS2, NS1 and NS2 are active during the stimulation mode to transform a continuous current into a balanced biphasic current. Two resistive transistors (driven by signals PSR1 and PSR2) are activated during the impedance measurement mode in order to reduce the voltage drop contribution of lower switching transistors on the signal at point X.

The voltage V_Re is applied to a VCO. The linear zone of the VCO corresponds to V_Re range during the measurement mode. Also, due to the reactive component of electrodes-tissue interface, V_Re can present some variations. This effect is reduced by an averaging mechanism of the frequency estimator. Table I shows the main frequency ranges expected at the output of the VCO and their typical meaning.

On chip calibration technique makes it possible to limit process and temperature variation problems. As shown in Fig.1, a replica of the stimulation switching circuit is included. However, the calibration current is 8 times lower than the test current in order to reduce circuit size. The transistors driven by signals T1 and T2 are designed to operate in the triode region.

They simulate two resistances (R and R/2) which allow to drive the VCO with known voltages R.I_test and (R.I_test)/2, thus calibrating its response. To insure both fidelity of the replica switching circuit and a wide range for stimulation current, a current source based on a 5 bit-DAC and a wide swing current mirror has been used. Also, transistors matching of the two switching circuits has been done.

The versatility of the realised chip is due in part to the performance of its logic controller. This controller can stop the whole system at any moment, control the current direction, choose the impedance measurement mode, or the stimulation mode, and determine the calibration transistors logic state.

B. Frequency to 8-bit data conversion

The frequency estimator followed by a parallel to serial converter is presented in Fig.2. It is composed of two main blocks : an 8-bit counter which counts, in a fixed duration, the number of clock cycles produced by the VCO and a finite state machine which determines the fixed counting period.

The parallel to serial converter is based on the classic scan path technique.

III. Test results

Physical test of the fabricated chip were conducted with loads based on the electrodes-nerve electrical model discussed early. Table II summarizes different employed values of R and C, and VCO output frequency for low impedance measurement results are shown in Fig.3.

Fig.3 Test results for low impedance measurement

Fig.4 Tested output data transmission: in top side, Test _CLK is shown, and the bottom side is the serially transmitted data.

The frequency to data conversion shows a good accuracy. For input frequency 100 kHz, measurement gives the number 85 (01010110) at its output as shown in Figure 4 which was very close to 84 expected theoretical value. Experimental frequency-impedance curve of the fabricated chip is presented in Fig.5.

When the signal Test_R (Fig.1) is not activated, the VCO output frequency is in mHz range as observed during experimentions ensuring energy saving.

The 5-bit DAC associated to a wide swing cascode current mirror was shown to deliver its maximal current (1.5mA) to resistive loads with values up to 1.5 kW while maintaining acceptable linearity.

IV. Conclusion

We have designed, fabricated and tested the proposed circuit which allows efficient monitoring of electrodes-nerve contact

during FNS of the bladder. Based on an impedance to frequency converter, the circuit is compatible with the stimulator, harmless, linear and accurate. Also, it occupies a small area of 0.1mm² and can be set in the idle mode during the normal stimulation. This monitoring module is quite efficient and can be implemented in any other stimulator.

ACKNOWLEDGMENTS

The authors would like to thank the Canadian Microelectronics Corporation (CMC), the Canadian International Development Agency (CIDA) and the National Science and Engineering Research Council of Canada (NSERC) for their support.

References

[1] Sawan, M. “Conception, réalisation et tests in vivo des stimulateurs neuro-musculaires destinés aux patients souffrant de dysfonctions urinaires”, Ph. D. Thesis, Université de Sherbrooke, July 1990.

[2] H. Friedman; B. S. Nashold; P. Senechal “Spinal cord stimulation and bladder function in normal and paraplegic animals”, J. Neurosurg., Vol. 36, pp 430-437, April 1972.

[3] N.J.M. Rijkhoff; H. Wijkstra; P.E.V. van Kerrebroeck and F.M.J. Dubruyne “Urinary bladder control by electrical stimulation: Review of electrical stimulation techniques in spinal cord injury”, Neurology and Urodynamics, Vol. 16, pp 39-53, 1997.

[4] Sawan, M.; Donfack, C.; Schneider, E.; Boyer S.; Roy M Externally-powered implantable device dedicated to monitor functional electrical events and parameters”, Proceedings of the 4^th International Congress of the International Neuromodulation Society. Lucerne Switzerland, sept.1998.

[5] W.H. Boyce; J.E. Lathem and L.D. Hunt “Research related to development of an artificial electrical stimulation for the paralysed human bladder: a review”, The Journal of Urology, Vol. 91, No 1, pp 41-51, 1964.