Integrated electronics for targeted intraspinal microstimulation

Abstract

Intraspinal microstimulation (ISMS) is an emerging method that is applied to neuroprosthesis aimed at individuals with spinal cord injury. Compared to traditional spinal stimulation or peripheral nerve stimulation methods, ISMS can activate muscle groups in organised synergies and thus can provide finer control of the generated force with reduced muscle fatigue. As the spinal cord is the neural link between the central and peripheral nervous systems, it is convenient to use this in accessing neurons associated with limb movement within a small area. For example, the relevant length of the spinal cord controlling the lower limbs in humans is only 5 cm. However, this means that any implant surgery is limited to some extent, on the other hand, this means ISMS needs to use invasive electric neural stimulation (ENS) with microelectrodes to access the target motor neurons to achieve a higher spatial resolution. Similar to other implantable ENS systems, an ISMS system needs to be compact, safe and energy efficient (in addition to effectively provide the required therapy). Although existing implantable neural stimulators fulfil these basic requirements, there is still much room for improvement. Depending on whether the stimulus is current or voltage controlled, the stimulator can be good for either safety and controllability or energy efficiency. Since the trend in the semiconductor industry is to reduce power consumption in integrated circuits, a current controlled stimulator is usually preferable by experimental neuroscientists. However, there is a new trend to combine these two control modalities, to enjoy the benefits of both.

Following this trend, this thesis starts by focusing on a third modality -- charge controlled stimulation, which delivers the stimulus in charge packets. This eliminates the voltage headroom required for relatively high output resistances in current controlled stimulators whilst preserving the controllability over the total charge delivered. Charge controlled stimulation is thus proposed for having the potential to be as energy efficient as voltage controlled stimulation and as safe as current controlled stimulator. A novel circuit for charge mode stimulation is described based on a charge metering approach that has been adopted from nuclear engineering. Experimental results demonstrate the feasibility of this approach and also identify the key challenges. This is then extended to a novel reconfigurable, multi-modal and multichannel stimulator circuit. This is the first integrated system to implement current, voltage and charge control stimulation within a reconfigurable channel architecture. This has been developed to investigate the effect of dynamic multipolar electrode reconfiguration with the aim of focusing or steering the stimulus. To this end, different stimulus delivery methods can be tested for multipolar spatial control.

The concept of multipolar stimulation is then investigated from a theoretical standpoint. The ability to apply this in improving the spatial resolution in ISMS can be achieved by confining the stimulus spread (thus reducing destructive crosstalk). This method can also be used to shift the stimulus voltage field away from the delivering electrode so as to correct implant placement error during surgery. A theoretical computational model is developed to investigate the effect of dynamic multipolar electrode reconfiguration with the aim of focusing or steering the stimulus. It is intended, together with the developed multichannel stimulator, that this will be used in future to develop advanced multipolar strategies that can achieve spatial hyperacuity for ISMS, and more generally in ENS.