Dong-Wook Park^1,*, Amelia A. Schendel^2,*, Solomon Mikael^1,*, Sarah K. Brodnick³, Thomas J. Richner³, Jared P. Ness³, Mohammed R. Hayat³, Farid Atry⁴, Seth T. Frye⁴, Ramin Pashaie⁴, Sanitta Thongpang⁵, Zhenqiang Ma^1,2 & Justin C. Williams^2,3

¹ Department of Electrical and Computer Engineering, University of Wisconsin.Madison, Madison, Wisconsin 53706, USA. ² Materials Science Program, University of Wisconsin.Madison, Madison, Wisconsin 53706, USA. ³ Department of Biomedical Engineering, University of Wisconsin.Madison, Madison, Wisconsin 53706, USA. ⁴ Department of Electrical Engineering and Computer Science, University of Wisconsin.Milwaukee, Milwaukee, Wisconsin 53211, USA. ⁵ Department of Biomedical Engineering, Mahidol University, Bangkok 73170, Thailand.
* These authors contributed equally to this work.

Correspondence to: Zhenqiang Ma or Justin C. Williams

Abstract
Neural micro-electrode arrays that are transparent over a broad wavelength spectrum from ultraviolet to infrared could allow for simultaneous electrophysiology and optical imaging, as well as optogenetic modulation of the underlying brain tissue. The long-term biocompatibility and reliability of neural micro-electrodes also require their mechanical flexibility and compliance with soft tissues. Here we present a graphene-based, carbon-layered electrode array (CLEAR) device, which can be implanted on the brain surface in rodents for high-resolution neurophysiological recording. We characterize optical transparency of the device at >90% transmission over the ultraviolet to infrared spectrum and demonstrate its utility through optical interface experiments that use this broad spectrum transparency. These include optogenetic activation of focal cortical areas directly beneath electrodes, in vivo imaging of the cortical vasculature via fluorescence microscopy and 3D optical coherence tomography. This study demonstrates an array of interfacing abilities of the CLEAR device and its utility for neural applications.