title: Novel flexible multielectrode arrays for neuronal stimulation and recording
Creators: name: Adams, Christopher
Publisher: name: University of Glasgow
Year: 2007
Subjects: subject: Biomedical engineering, artificial vision, neurostimulation.

description: This thesis will focus on developments in coupling the multidisciplinary research interests of Physics, Micro-engineering and neurobiology towards the development of a proof of concept retinal prosthetic device. With recent developments in low-power electronics and semiconductor fabrication techniques many applications in the life sciences have emerged. One such application is in the development of a retinal prosthetic device which relies on the ability to record information from and feed information directly to small retinal neuronal cells which are approximately 5mum diameter. Where successful, we achieve the possibility of restoring sight to people affected by degenerative visual diseases such as Age Related Macular Degeneration and Retinitis Pigmentosa. Both these conditions affect the photosensitive elements of the eye yet leave the remaining pathways to the visual cortex, the area of the brain responsible for our visual precept, intact. High-density electrode arrays axe becoming well established as tools for the measurement of neuronal signals. The fabrication of arrays on flexible materials allows for 2D position sensitive recording of cellular activity in vivo and for the possibility of direct in vivo stimulus. Using flexible polymer materials (Pryalin PI2545), compliant with semiconductor fabrication techniques, a process allowing the fabrication of flexible multi-site microelectrode neuronal recording and stimulating arrays is presented. The development of both 8 and 74 electrode arrays on polyimide substrates with 50mum and 5mum minimum linewidths respectively is described. Implementing low noise amplification, 8muV rms (bandpass typically 80-2000 Hz), the polyimide 8-electrode arrays have been used to stimulate and record electroretinogram and ganglion cell action potentials in vivo from the frog retina (Rana lemporaria). Such arrays coupled to our application specific pixellated CMOS sensors, the IPIX, incorporating an ability to apply neural network algorithms should allow for the recovery of basic functionality in the human retina. The IPIX detector is an Active Pixel Sensor which responds to incident light in the visible region. It responds to the varying intensity of light over 3 log units and outputs varying frequency voltage pulses of similar form to that of a healthy retina. Stimulation studies for electro-deposited platinum electrodes of 4 nA/mum2 indicate upper breakdown limits for charge density approaching 100 muCm-2. Investigations of lifetime stimulation of a 50 mum diameter electrode, of typical impedance less than 20 kO at 1 kHz, suggest operational limits over lifetime in the order of 10 muCm-2. These charge densities are adequate for neuronal cell stimulation. It is believed that the system described in this thesis can form the basis on which to deploy a retinal prosthetic device. Moreover, in the short term, the information provided by this system will allow for investigations into deciphering the 'wiring diagram' of the retina.