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Bioelectronic devices employ, usually biocompatible, functional electronic components that interact with biological elements. The keys to the development of such devices are suitable functional materials and effective fabrication methods. Conducting polymers (CPs) have emerged as one of the promising candidate materials for bioelectronics, owing to their remarkable properties, including electrical conductivity, biocompatibility, mechanical flexibility and the possibility of added functionality by chemical synthesis. Therefore, CPs have been widely used as electroactive bio-interfaces in applications such as biosensors, electrically stimulated tissue engineering substrates and in stretchable and conformable organic bioelectronics. However, the majority of these applications are based on the electropolymerised or solution deposited two-dimensional (2D) films of CPs, and as such cannot fully probe the actual three-dimensional (3D) cell environment within tissues and organs. The fabrication methodology developed in this thesis is based on the precise manufacture of individually addressable, high aspect ratio, 3D CP pillar microelectrode arrays by mean of ‘micro-extrusion printing’ using our in-house constructed system. The printing principle is simple ‘extrusion’ of CP ‘ink’ from a micro-pipette into ‘pillars’, accompanied by the evaporation of the solvent (water) during the process. CP pillars of different aspect ratios (from 2 to 7000) were successfully fabricated and extensively characterised. The pillars showed excellent stability and mechanical and electrochemical properties. The gold micro-electrodes in an array format were fabricated by the photolithography technique, followed by electropolymerisation of a thin poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer to reduce the contact resistance, and then the 3D writing of PEDOT pillars. A comparison of the electrochemical properties between three types of electrodes (bare Au, Au + PEDOT film and Au + PEDOT film + pillars) indicated much improved performance of the 3D pillar electrodes compared with the other two. The 3D CP pillar microelectrode arrays were demonstrated to be useful in a variety of bioelectronics applications, from biological sensing to electrical stimulation of cells and tissues, with the design of the arrays being easily adjustable to a specific application. In particular, the use of 3D CP pillars in electrical stimulation of human neural stem cells (NSCs) that were encapsulated within a conductive biogel was demonstrated. The stimulated NSC in the biogel showed widespread tracts of high-density mature neurons and enhanced maturation of functional neural networks in comparison to nonstimulated cells. This 3D CP microarray platform was also used for exosome capture, electrochemical sensing and electrochemical release studies, where the CP pillars could be easily functionalised with biological recognition probes (aptamers) for selective capture of exsomes. The capture and release were further confirmed by confocal fluorescent microscopy imaging. Overall, the 3D CP microelectrode arrays provide a very versatile substrate and soft organic probes that can be customized and functionalised to interface biological elements, and can be usefully employed in both bioelectronics research and in various biomedical applications. |
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