Alignment of optical microscope, AFM and HD-MEA. The alignment ensured that the fluorescence, emitted by the sample on the HD-MEA, was collected through a sapphire window in the cantilever holder of the AFM and passed through the optical microscope to a large CMOS sensor camera via a 2.5x f-mounted projector lens. The optical microscope is equipped with a TTL-pulse-controllable monochromatic LED illuminator for filter-cube-free fluorescence imaging. Fluorescence images of the entire chip are used to correlate the locations of the neurons with electrodes of the HD-MEA chip. The HD-MEA is mounted on a custom-made, piezo-controlled x , y stage, while the AFM moves the AFM cantilever in the z direction.
