Abstract—This paper describes the process flow and testing of a substrate for a fully implantable neural recording system. Tungsten microwires are hybrid-packaged on a micromachined flexible polymer substrate forming an intracortical microelectrode array for brain machine interfaces. The microelectrode array is characterized on the bench top and tested in vivo. The microelectrode noise floor is less than 2 µV and acute recording results show a signal to noise ratio of 9.9- 17.3 dB. The technique of hybrid fabrication of the electrodes on a flexible substrate provides a general platform for the development of an implantable neural recording system I. INTRODUCTION NTRACORTICAL microelectrodes have been used since the 1970s to invasively record action potentials from neurons in the brain. Some of the first published works [1], [2] developed two trends in microelectrode manufacture that still exist today: arrays made from planar micromachining techniques and arrays made with discrete components (micro-wires). Such recording devices have provided the necessary means to communicate with single neurons for therapeutic technologies—i.e. brain machine interfaces (BMIs)—designed for neurologic diseases and injuries [3], [4]. Since the first publications on microelectrodes 40 years ago, the research community has introduced many silicon- based and polymer-based micromachined electrode arrays [5]-[14]. Recently, signal processing and telemetry electronics have been incorporated with the microelectrode array to achieve fully implantable systems [13], [14]. All current efforts are aimed at increasing the efficacy of these recording electrode arrays. Chronic neuronal recording performance can be improved by floating electrodes that provide strain relief to electrodes and thus minimize tissue damage [15]. Devices best suited to achieve reduced tissue strain use a flexible cable to connect the recording sites to the external connector or circuitry anchored to the skull [16]. However, flexibility comes with trade-offs. Rousche et al. have fabricated flexible polymer substrate microelectrode arrays with thin film recording sites [7]. One drawback to this design is that because of the devices’ dimensions and the flexibility of the polymer, the devices are susceptible to buckling during insertion into the tissue and thus incisions Manuscript received April 7, 2006. This work was supported by a NIH NINDS grant (#NS053561). Erin Patrick, Sheng-Feng Yen, Viswanath Sankar, William Rowe, and Toshikazu Nishida are with the Department of Electrical and Computer Engineering, University of Florida, Gainesville, FL 32611 USA (e-mail: ee1@ufl.edu, sfyen@cnel.ufl.edu, vsanka1@ufl.edu, wrore001@ufl.edu, nishida@ufl.edu). Justin C. Sanchez is with the Department of Pediatrics, Division of Neurology, University of Florida, Gainesville, FL 32610 USA, (e-mail: jcs77@ufl.edu). are necessary for them to be implanted. Cheung et al. report a polymer substrate microelectrode that can be implanted without buckling [10]. However, the extreme flexibility of these and other polymer thin-film arrays requires that they be implanted by hand [6], [17], [18], thereby increasing the labor and reducing implant accuracy. This paper describes a second generation microelectrode based on [19] using insulated tungsten wires as the electrodes. This design furnishes a flexible microelectrode array capable of implantation without buckling that also provides a flat substrate anchored parallel to the skull yielding a platform for future flip-chip bonded electronics. This design is the first step in an effort to develop a fully implantable microelectrode system (Florida Wireless Implantable Recording Electrode (FWIRE)) [20]. The paper will first describe the process flow and then give bench-top impedance, intrinsic noise experimental data, and acute in vivo recording statistics. II. MICROELECTRODE DESIGN The prototype tungsten microelectrode array (Fig. 1) consists of three major components: a polymer substrate with encapsulated wiring, tungsten microwires, and nuts used for anchoring and grounding. Rigid 50 μm diameter tungsten micro-wires are attached to the end of a micromachined flexible cable in a 1-D array, allowing for insertion into the neural tissue without buckling. The micro- wires are spaced 250µm apart as prescribed for decoupled neural recording [21]. Nuts are provided for screws that anchor the device to the skull and supply the reference potential. Incorporating the fasteners yields a secured flat platform for future population of flip-chip bonded electronics. Device dimensions are given in Fig. 1. Figure 1. a. Polymer microelectrode array with Omnetics connector. b. Flexibility of microelectrode is shown with assumed in vivo position. Flexible Polymer Substrate and Tungsten Microelectrode Array for an Implantable Neural Recording System Erin Patrick, Viswanath Sankar, William Rowe, Sheng-Feng Yen, Justin C. Sanchez, and Toshikazu Nishida I 30th Annual International IEEE EMBS Conference Vancouver, British Columbia, Canada, August 20-24, 2008 978-1-4244-1815-2/08/$25.00 ©2008 IEEE. 3158