Biomed Microdevices (2006) 8: 81–85 DOI 10.1007/s10544-006-6386-6 BIOMEMS MATERIALS AND FABRICATION TECHNOLOGY Fabrication and testing of microelectrodes for small-field cortical surface recordings Joseph Kitzmiller · David Beversdorf · Derek Hansford C  Springer Science + Business Media, Inc. 2006 Abstract A microfabrication approach to produce a micro- electrode array that is suitable for use with human patients has been developed. The device is comprised of materials that are consistent with those of clinically used macroelectrodes (platinum electrode contacts suspended within a biomedi- cal grade polydimethylsiloxane, PDMS). Photolithography, metal deposition, wire bonding, and PDMS encapsulation were used to fabricate the device. Cytotoxicity testing with both mammalian and human cortical cells suggests that the device is suitable for use with human patients and implemen- tation of the device in animal studies revealed that reliable evoked potentials could be acquired with the designed spatial resolution. Keywords Surface microelectrodes . Cortical column potential measurement . Platinum lift-off 1. Introduction Both penetrating and non-penetrating electrode arrays have been developed and utilized by scientists to investigate the electrical activity of the cortex. Penetrating microelectrode arrays have been used extensively in animal studies but their use with humans has been limited (Burmeister, 2002; Hep- pelmann, 2001; Waren, 2001). As a result of the associated potential risks, studies utilizing penetrating microelectrode arrays with humans have only involved areas of cortex that are destined for surgical resection (Bechtereva , 2000; Oya, 2002; Schwartz, 2000; Williamson, 1993). J. Kitzmiller · D. Beversdorf · D. Hansford () The Ohio State University, Columbus, Ohio, USA e-mail: hansford.4@osu.edu Clinically, non-penetrating surface macroelectrode arrays are used subduraly to help localize a seizure focus in prepara- tion for epilepsy surgery (e.g. Radionics Cortical Grid Elec- trodes distributed by Integra Neuroscience, Plainboro, NJ). However, no studies investigating the use of non-penetrating subdural surface microelectrode arrays with human patients have been reported. Such studies have presumably not been performed due to a lack of biocompatible microelectrodes for human use. The aim of our study is to develop a non- penetrating surface microelectrode array potentially suitable for use with human patients, which will allow investigators to study areas of cortex that penetrating electrodes cannot study. Investigators would be able to examine all portions of exposed cortex, even those not destined for surgical re- section. This paper reports the development of an electrode array and testing for this eventual purpose. 2. Surface microelectrode array design and fabrication The design of our microelectrode array was restricted to ma- terials acceptable for human use. Therefore, we chose to fab- ricate our device using the same materials that comprise com- mercially available surface macroelectrodes (Cortical strip electrode, Cat. Nr. 16166-2, Integra NeuroSciences, Plains- boro NJ). Platinum electrode contacts suspended within a flexible biomedical-grade polydimethylsiloxane (PDMS) were selected. Due to limitations of designs that result from stresses from the addition of metals to PDMS microstruc- tures (e.g. Maghribi, 2002), a process was desired in which the metal processing was performed prior to incorporation of the PDMS matrix into the process. The dimensions and spacing of the microelectrode contacts were chosen as 200- micron squares with bi-directional pitch of 400 microns to Springer