Organic electrochemical transistors for neurotechnology
Creators: name: Tyrrell, James Edward
Publisher: name: Imperial College London
Year: 2022

description: The organic electrochemical transistor is a device that has proven to provide a comprehensive insight into physiological activity. The burden of brain-related illnesses and the potential societal impact of enhancing human interaction and communication with computers motivates the advancement of technologies to provide greater insights into neural activity. This research focuses on the intersection of these fields through developing organic electrochemical transistors for neurotechnology. As such, this research reports the fabrication, characterisation, and operation of the devices to demonstrate their applicability in providing an accurate description of neural activity. In turn, pathways are offered on the optimisation and enhancement of transistor performance, informing the field on methods to maximise the efficacy of biosensing devices. This thesis opens with the motivations to develop neurotechnology in society by considering the burden of neurological disorders and the enhancement of human-machine communication. The roles that bioelectronics and organic electrochemical transistors could take in achieving such goals are then described. The literature on organic electrochemical transistors and their applications in neurotechnology is then examined. Foundational to this research was the design and fabrication of novel transistor architectures. As such, fabrication methods are then described which provide micrometre-scale resolutions, facilitating the production of biocompatible, high-performance devices. The thesis then describes an investigation with the aim to optimise the performance of organic electrochemical transistors for the maximisation of the range of electrophysiological signals that the devices can resolve and amplify. Devices are demonstrated with high amplification factors and bandwidths sufficient for transducing the full bioelectric spectrum. The devices efficaciously resolve electrophysiological inputs with a wide range of voltage and frequency characteristics, demonstrated through measuring the device response to pre-recorded signals and supported through noise quantification. Research is then presented with aims to employ organic electrochemical transistors in a linear amplifier topology that provides precise amplification control. Circuit parameters are identified for stable amplifier operation, and the transistors are physically implemented into amplifier circuits. An equation is then formulated which models the transistor current in the linear, non-linear and saturation regimes, and this equation is used to simulate the amplifier response. Through experimentation and simulation, the amplifier is extensively characterised, providing quantitative measures on amplifier gain, linearity, bandwidth, and noise. The amplifier is then inputted with pre-recorded bioelectric signals and the linear gain of the amplifier is experimentally demonstrated. The thesis then presents an investigation with the aim to determine the effect of electrode architecture on organic electrochemical transistor characteristics and bioelectric performance. The impacts of varying the interconnect resistance on the steady-state and transient characteristics are examined. Then, arrays of devices are fabricated, and the effects of varying the interconnect resistance and the interdigitated electrode geometry on bioelectric performance are quantified. The electrode architecture at the measurement site is then varied for a fixed site area to determine an optimal configuration for resolving bioelectric inputs. The investigation presents a pathway for the optimisation of electrode architecture for a given application. Finally, the thesis describes research that aims to determine the relationships between the morphological properties of the organic layer and the electrical characteristics relevant to biosensing performance. Electrical characterisations and resonant Raman spectroscopy are carried out on organic electrochemical transistors that are immersed in an aqueous solution over a period of 50 days, and the temporal changes in the derived parameters are presented. Then, the effects of varying the applied voltages on the active layer morphology and the device noise are investigated, providing an insight into how the structural changes of organic polymers correlate to variations in noise.