Objective. A number of tissue penetrating opto-electrodes to simultaneously record and optogenetically influence brain activity have been developed. For experiments at the surface of the brain, such as electrocorticogram (ECoG) recordings and surface optogenetics, fewer devices have been described and no device has found widespread adoption for neuroscientific experiments. One issue slowing adoption is the complexity and fragility of existing devices, typically based on transparent electrode materials like graphene and indium-tin oxide (ITO). We focused here on improving existing processes based on metal traces and polyimide (PI), which produce more robust and cost-effective devices, to develop a multi-electrode array for optophysiology. Approach. The most widely used substrate material for surface electrodes, PI, has seen little use for optophysiologicalμECoG/ECoG arrays. This is due to its lack of transparency at optogenetically relevant short wavelengths. Here we use very thin layers of PI in combination with chrome-gold-platinum electrodes to achieve the necessary substrate transparency and high mechanical flexibility in a device that still rejects light artifacts well. Main results. The manufactured surface arrays have a thickness of only 6.5 µm, resulting in 80% transparency for blue light. We demonstrate immunity against opto-electric artifacts, long term stability and biocompatibility as well as suitability for optical voltage imaging. The biocompatible arrays are capable of recording stable ECoGs over months without any measurable degradation and can be used to map the tonotopic organization of the curved rodent auditory cortex. Significance. Our novel probes combine proven materials and processing steps to create optically near-transparent electrode arrays with superior longevity. In contrast to previous opto-electrodes, our probes are simple to manufacture, robust, offer long-term stability, and are a practical engineering solution for optophysiological experiments not requiring transparency of the electrode sites themselves.

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Recording from single neurons in the brain for long periods of time has been a central goal in both basic neuroscience and translational neurology, in order to understand mechanisms underlying brain processes such as learning and to understand the pathogenesis of neurodynamic disease states 1 . Recent advances in materials engineering, digital signal acquisition, and analysis algorithms have brought us closer to achieving this goal, and the possibility has gathered much public attention 2,3 . However, it remains a challenge to record from the same units for weeks to months. Here, we record many high-quality tetrode neuronal signals reliably over long periods of time in both deep and superficial areas of the brain. We achieve this by combining electrochemical roughening and carbon nanotube coating of a flexible platinum/iridium substrate, with materials, packaging, and insertion optimized to minimize tip movement with brain pulsation. This "Magdeburger" probe enables recordings with long-term signal stability and high signal-to-noise ratio at a reasonable cost in both rodent brains and in substantially larger primate brains. Robust tetrode tracking of identified neurons over longer time periods, in multiple independently targeted areas of the brain, will allow fundamental advances in the study of cognitive learning, aging, and pathogenesis, and opens new possibilities for brain interfaces in humans.Currently, four main classes of electrodes are standard for chronic in vivo recordings of neural activity: microwire arrays, Utah arrays, silicon probes, and flexible thin polyimide-based electrodes 4 . These electrodes were designed to record from as many units (neurons) as possible-however, long-term stable recordings from tetrode-identified single units and juxtacellular recordings are rarely reported.Microwire arrays are made of insulated sharpened metals, packaged in brush-or comb-like arrays 5,6,7 . In selected cases, these electrodes demonstrate stable single unit recordings over months 6,7 . Recently, solidstate-based versions of such a thin-wire brush approach have been proposed 8,9,10 . The small diameter of

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