Development of Micro-Electrocorticography Device Covering Wide Area of Cortex in Mice

The multichannel neural recording can be a breakthrough technology in the development of neuroscience research and new diagnostic/treatment for neurological disorders. Micro-Electrocorticography device (μECoG device) with small electrodes fabricated by semiconductor process technology is expected as a great tool for local field potentials (LFPs), a neural activity in the cerebral cortex, recording with an acceptable minimal invasiveness and covering wide area of cortex. However, especially when applied to small animals such as mice, the μECoG device is limited to be applied for the dorsal parts of cerebral cortex in which the device can be easily installed. In this study, a μECoG device for mice has been developed that has a structure for recording LFPs from a wide range of cortices, including the temporal cortex (the ventral parts of cerebral cortex).While a high bending stiffness is necessary to slide the device into the very narrow space between the temporal cortex and the skull to approach the temporal cortex, oppositely, a softness for the adhesion to the cerebral cortex is important to record the LFPs with high signal-to-noise ratio. Therefore, developments of the structure with different thickness of substrate (parylene C) corresponding to distinct cortical regions is thought as an effective method to meet the requirements. Then, the thickness in substrate required to approach the temporal cortex was considered. A bending stiffness coefficient and an adhesion energy as a function of parylene thickness was calculated. Although these parameters were a trade-off relationship, the device in the temporal cortices was in close contact necessarily because it is placed in a narrow gap between the temporal cortex and the skull. Considering this, the parylene thickness was determined to be 3 µm and 22 µm, respectively. When 22-µm-thickness parylene sheet was challenged to slide the narrow gap, it could be easily placed on the temporal cortex. Prior to the approach to the temporal cortex, the availability of the μECoG device we fabricated was tested. The 32 channel 3-µm-thickness µECoG device with Au electrode of 80×80 µm² was fabricated. The 3-µm parylene sheet was deposited on Si substrate. Then, EB evaporator was used to deposit Ti/Au (50 nm/200 nm) metal wiring layer. Subsequently, the surface was covered with parylene and the electrode area was opened by dry etching. Finally, the μECoG device was bonded to a PCB board by flip chip bonder. The impedances were approximately 0.4 MΩ at 1 kHz. While the μECoG device was placed on the dorsal parts of cerebral cortices, the LFPs corresponding to sensory stimuli was detected from all channels.<br/>On the basis of these basic experiments, the μECoG device with different parylene thickness corresponding to distinct cortical regions has been fabricated. This device was excellent in handleability because the parylene substrate has a thick area. The device was placed on the auditory and the visual cortex. The sensory-evoked LFPs recording was performed by providing auditory and visual stimuli. The recorded signals were analyzed by signal-averaging technique and wavelet transform approach. The LFP signals corresponding to each sensory stimulus were detected only for specific neural electrodes, indicating that this device has a high spatial resolution to identify characteristic response between cortical areas. It would be expected that a characteristic response could be obtained by applying this device to the temporal cortex.