Programmable active pixel sensor to investigate neural interactions within the retina

Detection of the visual scene by the eye and the resultant neural interactions of the retina-brain system give us our perception of sight. We have developed an Active Pixel Sensor (APS) to be used as a tool for both furthering understanding of these interactions via experimentation with the retina and to make developments towards a realisable retinal prosthesis. The sensor consists of 469 pixels in a hexagonal array. The pixels are interconnected by a programmable neural network to mimic lateral interactions between retinal cells. Outputs from the sensor are in the form of biphasic current pulse trains suitable to stimulate retinal cells via a biocompatible array. The APS will be described with initial characterisation and test results

Development of flexible arrays for in vivo neuronal recording and stimulation

Recent developments in low-power electronics and semiconductor fabrication techniques have found many applications in the life sciences. High-density electrode arrays are becoming well established as tools for the measurement of neuronal signals. The fabrication of arrays on flexible materials allows for 2Dposition sensitive recording of cellular activity in vivo and for the possibility of direct in vivo stimulus. Using flexible polymer materials, compliant with semiconductor fabrication techniques, we demonstrate a process allowing the fabrication of flexible multi-site microelectrode neuronal recording and stimulating arrays. We describe the development of both 8 and 61 electrode arrays on polyimide substrates with 50 and 5 mm minimum linewidths respectively. Further studies have realised 8-electrode arrays using gold on Polydimethylsiloxane (PDMS), an alternative biocompatible material, with linewidths of 14 mm: Implementing low noise amplification, 2.6 mV rms (bandpass typically 80–2000 Hz), the polyimide 8-electrode arrays have been used to record electroretinogram and ganglion cell action potentials in situ from the frog retina (Rana temporaria). Such arrays coupled to pixellated CMOS sensors, incorporating on-board neural networking should allow for the recovery of basic functionality in the human retina. More specifically, where retinal degeneration has affected only the photosensitive elements of the eye we can utilise the remaining neuronal pathways. Initial stimulation studies for electro-deposited platinum electrodes of 4 nA/mm2 indicate upper breakdown limits for charge density approaching 40 mC m2: Investigations of lifetime stimulation of a 50 mm diameter electrode, of typical impedance less than 20 kO at 1 kHz, suggest operational limits over lifetime in the order of 10 mC m2: These charge densities are adequate for neuronal cell stimulation

A CMOS active pixel sensor for retinal stimulation

Degenerative photoreceptor diseases, such as age-related macular degeneration and retinitis pigmentosa, are the most common causes of blindness in the western world. A potential cure is to use a microelectronic retinal prosthesis to provide electrical stimulation to the remaining healthy retinal cells. We describe a prototype CMOS Active Pixel Sensor capable of detecting a visual scene and translating it into a train of electrical pulses for stimulation of the retina. The sensor consists of a 10 x 10 array of 100 micron square pixels fabricated on a 0.35 micron CMOS process. Light incident upon each pixel is converted into output current pulse trains with a frequency related to the light intensity. These outputs are connected to a biocompatible microelectrode array for contact to the retinal cells. The flexible design allows experimentation with signal amplitudes and frequencies in order to determine the most appropriate stimulus for the retina. Neural processing in the retina can be studied by using the sensor in conjunction with a Field Programmable Gate Array (FPGA) programmed to behave as a neural network. The sensor has been integrated into a test system designed for studying retinal response. We present the most recent results obtained from this sensor

Simulated and experimental results from a room temperature silicon X-ray pixel detector

Simulated and experimental results are presented from a silicon X-ray pixel detector which is bump bonded to a PAC5 pixel array of read-out electronics. When coupled to a matching, fully depleted silicon detector the pre-amplifier is observed to have a linear response up to 80 keV, and a pulse height resolution of around 1 keV FWHM over the range 13–60 keV. The Monte-Carlo N-Particle code has been used to simulate the detector response under illumination from a variety of energies. The excellent agreement observed between simulation and experiment illustrates the predictive abilities of such packages