Microelectrodes are the gold standard for measuring the activity of individual neurons at high temporal resolution in any nervous system region and central to defining the role of neural circuits in controlling behavior.Microelectrode technologies such as the Utah or Michigan arrays, have allowed tracking of distributed neural activity with millisecond precision. However, their large footprint and rigidity lead to tissue damage and inflammation that hamper long-term recordings. State of the art Neuropixel and carbon fiber probes have improved on these previous devices by increasing electrode density and reducing probe dimensions and rigidity.Although these probes have advanced the field of recordings, next-generation devices should enable targeted stimulation in addition to colocalized electrical recordings. Optogenetic techniques enable high-speed modulation of cellular activity through targeted expression and activation of light-sensitive opsins. However, given the strong light scattering and high absorption properties of neural tissue optogenetic interfacing with deep neural circuits typically requires the implantation of large-diameter rigid fibers, which can make this approach more invasive than its electrical counterpart.Approaches to integrating optical and electrical modalities have ranged from adding fiber optics to existing Utah arrays to the Optetrode or other integrated electro-optical coaxial structures. These technologies have shown great promise for simultaneous electrical recordings and optical stimulation in vivo. However, the need to reduce the device footprint to minimize immune responses for long-term recordings is still present.