Giagka, Vasiliki; (2015) Flexible Active Electrode Arrays For Epidural Spinal Cord Stimulation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In spinal cord injured (SCI) individuals the neural pathway between the brain and the extremities is damaged. However, there is still the capacity to elicit muscle activation despite the absence of any supraspinal input. Recent studies have proposed epidural spinal cord electrical stimulation (ESCS) as a means to facilitate locomotor recovery in SCI humans. In epidural stimulation a number of electrodes are placed in the spine, outside the dura, and stimulus current pulses are used to ‘tune’ the spinal circuitries. Some rat studies have supported this concept, but further testing is required to increase our understanding and optimise the stimulation parameters. Testing protocols are currently limited by the available technology. More specifically, the number of electrodes one can use seriously limits the paradigms that could be investigated. For this reason, electrode arrays, as opposed to the conventional pairs of electrodes, can be used to investigate the effect of ESCS at different sites. The development of epidural electrode arrays for chronic testing in rats is a challenging task due to their small size. The difficulties increase radically when a large number of electrodes need to be independently controlled. It has been well documented in the literature that a large number of connections (wires) is highly undesirable because it either makes the implantation procedure more challenging, or, if the device is successfully placed in the body, it could imperil perfusion, result in infections, tissue damage, or simply cause the device to fail. The development of a flexible epidural electrode array suitable for chronic implantation in rats was the main goal of this work. For the first generation of the system, flexible passive 12-electrode arrays, using silicone rubber and annealed platinum foil, were designed and fabricated—suitable for use with an external stimulator. In vivo evaluation of these devices showed that they failed quickly, 87.5% of the connections after a week inside the spine of a rat. The failure analysis performed highlighted the need to reduce the number of connections to avoid inflammation and improve the mechanical stability of the implants. To overcome the connections ‘bottleneck’ without compromising the number of electrodes (which was necessary for the planned paradigm), our approach was to develop application-specific integrated circuits (ASICs) to be embedded on the arrays, acting as electrode drivers. The ASICs reduce the number of connections to 3, feature a very small silicon footprint (less than 0.36 mm2 core area), consume very low power (up to 114 μW during a full stimulation cycle), and allow for the necessary versatility for the testing with a real-time control system, developed by our collaborators (in the FP7 NEUWalk project). A custom designed ‘hub’, designed by Dr. Clemens Eder, is used to electronically – rather than manually – manage the stimulus parameters and the operation of the ASICs. It can be programmed via a graphical user interface (GUI) or the real-time controller. Moving to the second generation of the system, active (with embedded ASICs) epidural arrays were designed, developed and evaluated. For this version, platinum iridium foil, was preferred, due to its superior mechanical strength. The next part of the work focused on the the different aspects of the fabrication and assembly processes. More specifically, size restrictions related to the implantation site dictated the need to use thinned ASICs. To post-process the already fabricated chips, a method for purely mechanical silicon thinning at individual die level was developed and characterised. For the integration of the ASICs on the arrays an evaluation study was conducted to examine the mechanical reliability of the bonds produced by electrical rivet bonding. Combining all the above, a new fabrication process was developed for the active arrays. Despite the fact that, so far, chronic in vivo testing has not been yet implemented, the produced prototypes were electrically and mechanically evaluated in vitro, and results are satisfactory, as no failed tracks were observed during the chronic tests in the lab. The current setup allows power and data to be transferred to the implant real-time through a connector fixed on the rat’s head, while the animal is on a treadmill or on a runway. This implies that there is no need for a wireless system at this stage. However, more complex experiments where the rats would be able to move freely and interact with other rats unrestricted, developing a behaviour that is closer to their natural, could provide significant new knowledge in the future. Although there are still many things to understand regarding epidural stimulation and its effect before planning an experiment like this, this was kept in mind throughout the whole design and development phases of this system. On this basis, the developed subcomponents are compatible with a system level design of a fully implantable platform to be used in freely moving rats, stimulated for 3 – 4 hours per day. This system comprises the active electrode array, which is the focus of this thesis, together with a miniaturised, battery-powered implantable version of the previously mentioned hub (which is on-going work, and is not presented here).

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