The mechanisms governing the onset and eventual progression of several neurodegenerative disorders remain poorly understood or even undiscovered. This lack of pathophysiological insight can be partly attributed to reliance on inaccurate in vitro models. Notwithstanding research efforts towards recapitulating brain functions on flat devices, mimicking the brain's three-dimensional (3D) architecture in vitro remains a prime target, as 3D models more closely resemble the functional behavior and dynamic responses of in vivo organs. In this work, we present a novel, wafer-scale approach for microfabrication of soft and transparent 3D microelectrode arrays (MEAs) for in vitro electrical recording and optical inspection of electrogenic cell cultures. The proposed 3D MEAs entail 90μ m -high polydimethylsiloxane-based micro-pyramids featuring multiple, electrically-distinct and vertically-stacked titanium nitride electrodes on their slanted facets. Our innovative 3D MEAs will facilitate the development of physiologically-accurate brain-on-a-chip models capable of monitoring 3D electrical communication in neuronal networks while allowing their simultaneous optical characterization. ... Read more