Burst-Pulse Control of Microstimulator for Bladder Controller

Chen-Yueh Huang, Shuenn-Yuh Lee, Jia-Hua Hong, Ming-Chun Liang, and Cheng-Han Hsieh Department of Electrical Engineering, National Chung-Cheng University, Chia-Yi, Taiwan

Email: ieesyl@ccu.edu.tw

Abstract: This paper presents a method for the control of the variable burst biphasic pulse of a bladder stimulator. The stimulator is used to pass current through the tissue and to generate useful action potentials. The binary-weighted digital-to-analog converter combined with a current mirror has been employed as a microstimulator because of its higher linearity without requiring the decoding of digital inputs. Given that the use of a biphasic pulse could prevent ion-charge accumulation in tissues, two pairs of switches controlled by different clock phases are implemented to provide the biphasic electrical stimulation pulses. According to measurement results, the pulse frequency can be programmed between 1.49 Hz and 47.66 Hz, the burst frequency can be controlled from 190.8 Hz to 763 Hz, and the pulse width can be adjusted between 21 µs and 325 µs. These stimulation parameters are adapted by the clock divider and by the number of controlled bits in the digital circuits.

I. INTRODUCTION Lower urinary tract dysfunction caused by detrusor

sphincter dyssynergia is the most serious problem that arises from spinal cord injury (SCI). Vesicoureteral reflux caused by detrusor sphincter dyssynergia results in upper urinary tract infections and thereby facilitates the deterioration of kidney functions and causes secondary damage in SCI patients. Three treatment approaches are used by doctors to improve the quality of a patient's life: medication, surgical intervention, and functional electrical stimulation (FES).

Medication is the first approach for SCI treatment. However, the effects of medication are often inconspicuous. Moreover, medications also have accompanying side effects. In traditional surgical intervention, bladder augmentation is commonly used for clean intermittent catheterization (CIC). This treatment can increase the time interval of CIC, which can be performed by the patient himself. However, if the procedure is not sanitarily performed, a high risk of infection exists. In FES, Brindley sacral anterior root stimulation (SARS) with dorsal rhizotomy is a proven treatment for voiding dysfunction. However, the major disadvantage of SARS is that the large stimulation current used to induce detrusor muscle contraction will inevitably activate the external urethral sphincter and thus interfere with micturition.

Given that SARS with dorsal rhizotomy is an irreversible procedure aimed at improving bladder emptying, the patient will lose reflex erection, ejaculation, and defecation. Numerous stimulation techniques are currently used to resolve these problems.

Pudendal nerve stimulation (PNS) is another technique under FES. Dorsal rhizotomy is not required with pudendal nerve (PN) afferent stimulation. The onset of hyper-reflexive bladder contractions is also detected from the electrical activity of the PN trunk [1]. In most cases where implanted cuff electrodes are used to stimulate peripheral nerves such as PN, only less than 1 mA of stimulus current is utilized [2]. In neurology and urodynamic research, variable patterned stimulus is considered as a method to improve stimulation. Burst pulse was found to be more effective than continuous single pulse in inducing reflex bladder contractions [3], [4]. Bladder activation by selecting PN afferents with different frequencies can possibly restore bladder function in SCI patients [5].

This study aims to present a method to control variable burst biphasic pulse in a bladder stimulator. The stimulator is used to pass current through the tissue and to generate useful action potentials. The binary-weighted digital-to-analog converter (DAC) combined with a current mirror is used as a microstimulator. The desired stimulus parameters for this study are the high-frequency and the low-frequency stimulus for bladder control. The remainder of this paper is organized as follows: Section II presents the system architecture and circuit structure of the proposed implantable bladder stimulator. Section III describes the measurement result. Section IV concludes the study.

II. SYSTEM ARCHITECTURE Figure 1 shows the system architecture of a low-power

microstimulator that is developed for bladder control. An external device encodes the stimulus parameters, which are stored in packets and are modulated by phase shift keying (PSK). The information on the stimulus parameters will then be transmitted to the human body to update the stimulus. Implantable devices receive the information from the radio frequency (RF) front-end, restore the data through the PSK demodulator with clock and data recovery, and then extract data by using digital circuits. In addition, an RF coil can

This research was supported by the Chip Implementation Center (CIC) and National Science Council (NSC) in Taiwan under the grant numbersNSC-101-2628-E-194-002 and NSC-101-2220-E-194-001.

84978-1-4577-1729-1/12/$26.00 ©2012 IEEE.

Fig. 1 Block diagram of the implantable bladder stimulator

energize rechargeable batteries through a rectifier based on charging detector control. A regulator and a supplying detector are utilized to provide stable voltage for all internal circuits. Behind PSK demodulation, a system controller is designed for decoding the information, checking errors, restoring data on the parameters, judging, and handling the required stimulus current. A preamplifier, a band-pass filter, and an analog-to-digital converter (ADC) are combined as a sensing channel for the bio-signal monitor. The sensing channel captures neural signals at the pressure rise and then provides a potential binary value for the controller. Finally, a DAC and a pulse generator provide the correct burst pulse for stimulation according to the command from the system controller. In this study, a required system controller, an 8-bit segmented DAC, and a nerve load for the overall system will demonstrate our proposed system-controlled method. A. System Controller

Figs. 2 and 3 show our custom packet format and the block diagram of the system controller. Each segment length is purposefully arranged. The header and end bits are used for the packet location. The combinational logic circuit with parity check is utilized to enhance reliability during wireless communication. If the data are correct, the system can read or write the digital codes in the register to update the stimulation parameters and the controlled bits. Given that pudendal neural signals are also associated with bladder pressure, the latter can be monitored in real time by the sensing channel. The ADC can also provide an immediate alarm signal to the system controller when the pressure goes beyond the threshold value. The system controller will remind the patient to release urine at an appropriate time.

B. DAC and Output Load

Fig. 4 shows an 8-bit segmented current-mode DAC with a current mirror. The thermometer code scheme is employed in the DAC design because of its small glitch error. The 8-bit

digital code is divided into 4-bit coarse codes and 4-bit fine codes. Two 2-to-4 binary-to-thermometer decoders are used to control the current sources. The cell current of the least significant bits (LSBs) is one-sixteenth of that of the most significant bits (MSB). This current value can provide a precise current for the microstimulator. A cascode current mirror is used to boost the output impedance instead of a simple current mirror or an active-feedback cascode current mirror because high output impedance with low-power consumption is required for the implantable devices.

A stimulator scheme with a set of switches, as shown in

Fig. 5, is proposed to provide the required stimulation current for the PN because the use of a biphasic pulse could prevent ion-charge accumulation in tissues [6], [7]. Two pairs of switches 1S and 2S , which are operated by the system controller, are designed to produce biphasic electrical

Fig. 4 8-bit Segmented DAC structure

Fig. 3 Block diagram of the system controller

Fig. 2 Packet format

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stimulation pulses. The switches 4S and 4S with blocking capacitors are used for charge balance during stimulation. These switches are switched by the opposite phase of a high-frequency clock. When switch 3S is turned on, the current is passed through the nerve load according to switch

4S or 4S , and the blocking capacitor will be either charged or

discharged. Conversely, when switch 3S is turned off ( 3S is turned on), it releases residue from ion-charge accumulation in tissues and blocks the leakage current passage to the nerve load.

To reduce power consumption, the system controller and the 8-bit DAC are operated at 1.8 V supply voltage. Only the output stage is operated at 3.3 V for the requirement of the microstimulator.

C. Variable Stimulation Parameters

Fig. 6 demonstrates the stimulus parameter definitions adopted in [3] as well as the proposed simple method to control the pulse width (PW), pulse period (1/ TRf ), pulse amplitude, number of burst times (#p), and burst period (1/

PUf ).

A 50 MHz clock generator is included in the system to create a safe stimulation environment by using on-chip blocking capacitors. Moreover, the clock output is divided to provide low-frequency clocks with 97.08, 1.526, and 95.37 Hz, respectively, to produce the required burst pulse for the microstimulator.

Fig. 7 shows that the pulse period counter is operated at 95.37 Hz. The output of the pulse period counter is initiated at 0 and then further calculated to reach the required value, which is defined by the stimulus parameters. Moreover, 6 bits are used for the counter. Therefore, the stimulation can be programmed at 1.29 Hz to 47.685 Hz. Once the output of pulse period counter reaches 1, which appears briefly at 10.5 ms, this value will trigger the burst period counter. The burst period counter is operated at 1.526 kHz clock and used to control the burst period and burst times. The product of the burst period and the burst times cannot be more than 10.5 ms. Otherwise, the last burst pulse would not appear. The parameter segment length is relative to the operated clock. In our work, 3 bits for the burst period and 2 bits for the burst times are defined in the stimulus parameter. When burst period output is at 1, the pulse width counter will be triggered at 97.08 kHz to provide the stimulus pulse width. After stimulation, all counters will be reset to 0. The microstimulator can change stimulated direction and activate the pulse width counter again before stopping. This process will form the biphasic pulse flowing through the nerve.

According to the above operation, the high-frequency components are started after the low-frequency components are triggered. However, the former is accomplished before the low-frequency components’ statement is changed. To avoid the clock synchronization problem and to reduce power consumption, the counter is often idle and is operated by the system controller.

Fig. 7 Control method for burst-pulse generation

Fig. 6 Pattern of the stimulus parameters

Fig. 5 Output stage with blocking capacitor for biphasic pulse

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(a) (b) (c)

(d) (e) (f)

Fig. 8 Measured results: (a) (b) (c) and (d) (e) (f) forms two different burst pulses.

III. MEASUREMENT RESULTS The system controller is verified by a synthesis tool with a

Verilog code. The DE0-Nano (Altera Corporation) is used to implement the required system controller, because it can provide 4.76 V supply voltage and 50 MHz clock frequency. The output of system controller is connected to a DAC and controlled switches for the microstimulator verification. The proposed system controller hardware comprises an AD7524 and a CD 4066 bilateral switch for the 8-bit current output DAC and the output stage, respectively. These components are embedded on a PCB board with a 3 kΩ load to measure the stimulation signals.

In this study, the pulse period controlled by 6 bits can provide from 1.49 Hz to 47.685 Hz of continuous stimulation. The bust period and the burst time set by 3 bits and 2 bits can produce relative frequencies of 190.75 Hz to 763 Hz and 305.184 Hz to 763 Hz, respectively. Moreover, the pulse width controlled by 5 bits can be adjusted between 20 µs to 320 µs.

Fig. 8 shows the measured result on field-programmable gate array implementation. The pulse frequency is from 1.49 Hz to 47.66 Hz [Figs. 8(a) and (d)], burst frequency is from 190.8 Hz to 763 Hz [Figs. 8 (b) and (e)], and pulse width is from 21 µs to 325 µs [Figs. 8 (c) and (f)]. Fig. 8 shows the ripples that are caused by a disturbed charge from the blocking capacitors. These ripples can be reduced by replacing the larger capacitors or by increasing the clock frequency. These ripples can be overcome in future implementations by using integrated circuits.

IV. CONCULSION In this study, a system-controlled method for a bladder

controller is proposed and implemented. The controlled bits

can be programmed by surgeons from an external device to provide the required burst pulse. A total of 52 bits are defined in the communication protocol, and internal codes are used to enhance reliability and to control the stimulated parameters. The measured result shows that the function is efficient. The stimulation results can be a basis for the future fabrication of an implantable stimulator. In the future, we will integrate the circuits in a single chip for animal study.

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