An Implantable Spinal Cord Stimulator with Adaptive Voltage Compliance for Freely Moving Rats

When the neural pathway within the spinal cord gets damaged, a person’s ability to initiate loco- motion can be affected. Currently, no restorative treatments exist for individuals that suffer from such an injury. However, recent studies have shown epidural spinal cord stimulation (ESCS) to be a potential option for re-establishing some voluntary control. Further research is required to increase our understanding and allow us to investigate the neuronal response to different stimulation parameters. For these purposes, experiments have been conducted in spinal cord injured rats but they suffer from limitations due to the difficult criteria that an electrical stimulation device has to fulfill. In order to investigate the effect of different stimulation parameters, the system needs to offer full programmability to the researcher. Furthermore, since it is desired to investigate the natural behavior patterns of the animal, they should not have to be tethered to a setup during experiments. Ideally such an experimental setup would be fully implantable, which greatly limits the acceptable size of the system. Moreover, an implantable device needs to operate on a minimum amount of power in order to extend the lifetime of the implant. Current controlled stimulation (CCS) devices are favored in electrical stimulation applications due to the accurate control that they offer over the injected charge. Nonetheless, they are known to suffer from poor power efficiency due to the highly variable and dynamic range of electrode-tissue impedance. A high voltage compliance is required to provide stimulation to high impedance loads and most devices include a system block to boost up the internal voltage. Therefore, during operation with lower loads, a lot of power is wasted within the system as the excess voltage is dissipated across its current source. Furthermore, high-voltage compliant devices often exhibit large current spikes at the output when driving lower loads, compromising the overall charge control. The aim of this work was to design and fabricate a fully programmable control unit for epidu- ral spinal cord stimulation in freely moving rodents. The control unit would provide rectangular current pulses to an active electrode array, that was previously designed to be implanted within the spinal canal of the rat. It proposes a novel approach that implements an adaptive voltage compliance in order to reduce the unnecessary power consumption of CCS. Instead of providing a fixed voltage value at its output, the boost converter can be controlled and adjusted to the required value across the load. Moreover, lowering the voltage compliance of the device both enhances the speed of the system and reduces the generated current spikes when operating on lower loads or generating smaller stimulation currents. Thus, it provides safe, charge-balanced stimulation pulses to the tissue. The fabricated version of the system is 13.5 cm3 and therefore fulfills the size restrictions of a system to be implanted in a rat. Because it is battery-operated, animals can move freely during stimulation, which allows the experimenter to investigate the effect of electrical stimulation during natural behavior patterns. Results indicate that a total power efficiency up to 35.5% can be achieved. Compared to a fixed compliance, the adaptive procedure has been shown to save around 60 mW when operating on lower loads. Finally, the overall system was fabricated using discrete components, which results in an inexpensive product that can easily be manufactured and batch produced.