Spinal cord injury (SCI) leads to the loss of motor and sensory function below lesion, and thus often causes the loss of partial or complete bladder function. The patients after SCI are often faced with the double dysfunction of bladder storage and urination at the same time. If not treated in time, it might eventually lead to kidney failure which is the first cause of death of SCI patients. Patients with bladder dysfunction after SCI are numerous and high incidence. Bladder dysfunction after SCI seriously affects the patient’s quality of life, psychological and social relations, and gives rise to a heavy economic burden to their family and social. Therefore, the restoration and rebuilding of bladder function has great significance for improving the quality of life and reducing the mortality rate of SCI patients.
Gastro-esophageal Reflux Disease (GERD) is defined as “a condition which develops when the reflux of stomach contents causes troublesome symptoms and/or complications”. GERD is one of the most common gastrointestinal diseases around the world, and has a high incidence. According to statistics, there are about 250 million GERD patients around the world. GERD is not only seriously affecting the patient’s quality of life, but also causing the heavy burden of economy and society. Therefore, treatment methods of GERD have become a worldwide concern and research hot spot.
Based on the previous research route of “From peripheral to central nervous” and “From in vitro to in vivo”, the system (“in vitro”) of Electromyographic Bridge (EMGB) has entered the stage of clinical certification. This paper mainly focuses on the reconstruction of bladder dysfunction after SCI and treatment of GERD associated with electronic system design and experimental research for implantable (“in vivo”) neuromuscular electrical stimulation. The specific works of this paper are presented as follows:
1） The experiment research of electrode configuration choice and the stimulating waveform parameters optimization: The suitable electrode configurations and stimulation parameters are explored by experiments, which aims to further improve the performance of the stimulator under the condition of not changing circuit hardware and software design. This paper discussed the effects of the different electrode configuration, the pulse width ratio and interphase gap (IPG) for nerve electrical stimulation.
2） The design of nerve stimulator circuit for controlling the bladder function: The method of integrated circuit design was adopted for the design of stimulator which can generate stimulus pulse for the treatment of bladder dysfunction after SCI. The design of stimulator circuit mainly includes the DAC circuit, current driver circuit and switch network.
3） The design of nerve signal detection front-end circuit for the reconstruction of bladder function: The method of integrated circuit design was adopted for the design of nerve signal detection front-end circuit which was used for sacral nerve signal detection to identify the bladder status information. The design of detection front-end circuit mainly includes the OTAs and pseudo feedback resistance. In addition, the noise of the detection front-end circuit is analyzed theoretically.
4） The passive type of implantable system design and experimental research for esophageal lower sphincter electrical stimulation: Based on RF-wireless across skin transmission technology, the passive type of implantable system was designed for the treatment of GERD. Then, a kind of biocompatible material was used to encapsulate the body part of the system, and the encapsulation system was tested by the signal characteristics and the mismatch position. Finally, the system was used to the LES electrical stimulation experiment and implantation experiment.
The specific novel ideas of this paper are presented as follows:
1） By studying the effects of the different electrode configuration, the pulse width ratio and interphase gap (IPG) for nerve electrical stimulation, we can draw the following conclusion: the longitudinal tripolar electrode configuration with lowest threshold appear to be well suitable for the low power design of implantable stimulator; the transverse configurations with gentle slope and large dynamic range are suitable for the muscle force fine control. The1:6 of pulse width ratio well combines the advantages of the single-phase pulse with low threshold and the biphasic pulse with charge balance. In order to the subsequent anode phase not affect the action potential propagation caused by cathode phase, it needs to add an interphase gap between two phases. The best IPG is 300 μs corresponding to the cathode pulse width of 50 μs, because it can obtain a lower threshold, a larger biggest EMG response and the maximum dynamic range at the same time. These results can provide the experiment instruction for the design of implantable NMES system circuit system.
2） On the one hand, a current type DAC and a current driver were adopted to realize the current mode stimulus, which do not need the voltage-current convert circuit that significantly reduces the power consumption of the stimulator. On the other hand, using the same current source and switch network implement biphasic stimulation current pulses which eliminate to use two independent current sources for generating cathode and anode current that saved the chip area and reduces the control signal interconnect. In addition, using a symmetrical regulated cascode with a current mirror as a current driver circuit, the method combines the advantages of the voltage-controlled resistor technology and current mirror, which can obtain a high voltage tolerance and high output impedance that make sure to send the charge to the tissue effectively.
3） The front-end circuit for nerve signal detection adopts two-stage fully differential amplifiers with a high common mode rejection ratio and a high power supply rejection ratio which can restrain the common-mode noise interference, the power supply ripple and the digital circuit. The two-stage signal amplification is utilized for achieving a high gain and a good linearity. The capacitively coupled amplifiers are used as the neural recording circuit for injecting DC offset introduced by the electrochemical reaction at electrode tissue interface. The first stage adopts telescopic-cascode OTA for reducing the noise of the system, while the second stage adopts folded-cascode OTA for obtaining a high output swing. Compared with previous research, the neural signal detection front-end circuit has the advantages of low noise and low power consumption.
4） A passive type of implantable system is designed for esophageal lower sphincter electrical stimulation. The RF-wireless signal across skin transmission technology was used for the design of this system which has the characteristics of simple circuit structure, good reliability, small volume, long service life and low cost. The stimulating signal from in vitro transmitted to in vivo is coupled by the coils in the transmission circuit. The passive structure in vivo was adopted to solve the problem that the traditional implantable devices need batteries. In addition, according to the situation of patients, doctors can effectively adjust the stimulation parameters in time by radio program control, in order to undertake personalized treatment.