Bionics Chemical Synapse

Abstract

This thesis presents the very first bionics chemical synapse which has the capability to sense the neurotransmitter (glutamate) and imitates the physiological behaviour of certain chemical synapse receptors (i.e. AMPA, NMDA, GABAA and GABAB). This bionics chemical synapse consists of two main parts: the glutamate ISFETs that act as neurotransmitter sensors and the current-mode CMOS circuits that have been designed to match the physiological behaviour of the chemical synapses. This bionics chemical synapse requires a sub-nano Siemens operational transconductance amplifier (OTA) to develop a low conductance gain for each chemical synapse receptor (0.1nS). A combination of two OTA designs was required to decrease the overall transconductance gain, which were: the bulk driven transistor and the drain current normalisation. To create the bionics chemical synapse, a neurotransmitter sensor is required as the chemical front-end for each receptor circuit. The sensor that was used is an enzyme-modified ISFET with glutamate oxidase immobilisation, to make the ISFET sensitive to glutamate ions. Additionally, a fast chemical perturbation technique called iontophoresis was applied to generate the glutamate stimulus, which represents the neurotransmitter signal. This signal has a one millisecond time duration. Finally, the current-mode CMOS circuits biased in the weak inversion region have been designed to match a biological model of the four mentioned chemical synapse receptors. Circuit techniques, such as the log domain filter and the translinear loop, were applied to realise the complex mathematical functions in the chemical synapse model. The measured response of the fabricated AMPA and NMDA receptors, where the glutamate ISFET was used to sensed the artificial neurotransmitter stimulus, closely matches with the circuit simulation results.