Stretchable Microelectrode Arrays for Brain Spheroid Electrophysiology

Human neural spheroids, i.e. 3D cellular aggregates composed of neurons and glial cells, are a compelling biological model to study neuronal communication both in physiological and pathological conditions. We propose the design of a perforated and stretchable microelectrode array (MEA) to monitor the electrophysiology of brain spheroids under static and dynamic mechanical loading. This is a relevant neuroelectronic system to investigate the consequences of traumatic brain injury (TBI).<br/>The MEAs are prepared with thin-film technology using a platinum thin film sandwiched between two 1 μm thick polyimide films. Stretchability is engineered by design introducing patterns of micron scale, Kirigami-like Y-shaped cuts on the whole MEA surface.<br/>Each MEA hosts 8 Kirigami-patterned recording electrodes (diameter = 70 μm), and a macroscopic ground electrode to probe one brain spheroid (spheroid diameter = 500-1000 μm). Electrode contacts are coated with electrodeposited PEDOT. The coating conformably follows the profile of the Kirigami-patterned electrodes independently of electrode size and reduces the electrochemical impedance at 1 kHz by 60 folds (n = 24, diameter = 70 μm).<br/>Next, the permeability of the perforated MEAs is quantified as a function of pattern designs and perforation methods; this is to ensure sufficient nutrient and oxygen exchange at the air-liquid interface of the brain spheroid.<br/>The electromechanical properties of the MEAs are then evaluated when subject to uniaxial elongation at a constant speed (100 μm/s). The strain parameters are aligned with typical TBI loadings (5-35%). Kirigami-patterned PEDOT-coated electrodes sustain strains up to 10% without showing significantly increased electrochemical impedance amplitude, while patterned platinum electrodes sustain strains only up to 5%. Similar results are shown with cyclic stretching. The electromechanical properties of the MEAs are also evaluated when subject to fast mechanical loading comparable with TBI impacts (elongation speed 1-2 m/s).<br/>Next, the ability of the Kirigami-patterned electrodes to record neural activity from brain spheroids is verified. The MEA is mounted on a fluidic platform to allow for cell culture media change and the brain spheroids are integrated into the system. Spontaneous neural activity from the brain spheroids is reliably recorded for up to 5 days.<br/>This work paves the way for studying the effect of mechanical loading on neural activity in 3D <i>in vitro</i> brain models.