Advanced MEMS Microprobes for Neural Stimulation and Recording
Akhavan Fomani, Arash
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The in-vivo observation of the neural activities generated by a large number of closely located neurons is believed to be crucial for understanding the nervous system. Moreover, the functional electrical stimulation of the central nervous system is an effective method to restore physiological functions such as limb control, sound sensation, and light perception. The Deep Brain Stimulation (DBS) is being successfully used in the treatment of tremor and rigidity associated with advanced Parkinson's disease. Cochlear implants have also been employed as an effective treatment for sensorineural deafness by means of delivering the electrical stimulation directly to the auditory nerve. The most significant contribution of this PhD study is the development of next-generation microprobes for the simultaneous stimulation and recording of the cortex and deep brain structures. For intracortical applications, millimetre length multisite microprobes that are rigid enough to penetrate into the cortex while integrated with flexible interconnection cables are demanded. In chronic applications, the flexibility of the cable minimizes the tissue damage caused by the relative micro-motion between the brain and the microprobe. Although hybrid approaches have been reported to construct such neural microprobes, these devices are brittle and may impose severe complications if they break inside the tissue. In this project, MEMS fabrication processes were employed to produce non-breakable intracortical microprobes with an improved structural design. These 32 channel devices are integrated with flexible interconnection cables and provide enough mechanical strength for penetration into the tissue. Polyimide-based flexible implants were successfully fabricated and locally reinforced at the tip with embedded 15 µm-thick gold micro-needles. In DBS applications, centimetre long microprobes capable of stimulating and recording the neural activity are required. The currently available DBS probes, manufactured by Medtronic, provide only four cylindrical shaped electrode sites, each 1.5 mm in height and 1.27 mm in diameter. Although suitable for the stimulation of a large brain volume, to measure the activity of a single neuron but to avoid measuring the average response of adjacent cells, recording sites with dimensions in the range of 10 - 20 µm are required. In this work, novel Three Dimensional (3D) multi channel microprobes were fabricated offering 32 independent stimulation and recording electrodes around the shaft of the implant. These microprobes can control the spatial distribution of the charge injected into the tissue to enhance the efficacy and minimize the adverse effects of the DBS treatment. Furthermore, the device volume has been reduced to one third the volume of a conventional Medtronic DBS lead to significantly decrease the tissue damage induced by implantation of the microprobe. For both DBS and intracortical microprobes, the impedance characteristics of the electrodes were studied in acidic and saline solutions. To reduce the channel impedance and enhance the signal to noise ratio, iridium (Ir) was electroplated on gold electrode sites. Stable electrical characteristics were demonstrated for the Ir and gold electrodes over the course of a prolonged pulse stress test for 100 million cycles. The functionality and application potential of the fabricated microprobes were confirmed by the in-vitro measurements of the neural activity in the mouse hippocampus. In order to reduce the number of channels and simplify the signal processing circuitry, multiport electrostatic-actuated switch matrices were successfully developed, fabricated, and characterized for possible integration with neural microprobes to construct a site selection matrix. Magnetic-actuated switches have been also investigated to improve the operation reliability of the MEMS switching devices.