No abstract

Glassy carbon (GC) has high potential to serve as a biomaterial in neural applications because it is biocompatible, electrochemically inert and can be incorporated in polyimide-based implantable devices. Miniaturization and applicability of GC is, however, thought to be partially limited by its electrical conductivity. For this study, ultra-conformable polyimide-based electrocorticography (ECoG) devices with different-diameter GC electrodes were fabricated and tested in vitro and in rat models. For achieving conformability to the rat brain, polyimide was patterned in a finger-like shape and its thickness was set to 8 µm. To investigate different electrode sizes, each ECoG device was assigned electrodes with diameters of 50, 100, 200 and 300 µm. They were electrochemically characterized and subjected to 10 million biphasic pulses—for achieving a steady-state—and to X-ray photoelectron spectroscopy, for examining their elemental composition. The electrodes were then implanted epidurally to evaluate the ability of each diameter to detect neural activity. Results show that their performance at low frequencies (up to 300 Hz) depends on the distance from the signal source rather than on the electrode diameter, while at high frequencies (above 200 Hz) small electrodes have higher background noises than large ones, unless they are coated with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS).

monitoring of neural activity during tumor resection neurosurgery, [6][7][8] identification of epileptic foci in chronic implants, [9][10][11] and neuroprosthetics. [12][13][14][15][16][17] In the effort to minimize invasiveness while preserving substantial task-related information, electrocorticographic (ECoG) and micro-electrocorticographic (μECoG) techniques underwent extensive investigation. [18][19][20][21][22] With respect to intracortical microelectrodes, both ECoG and μECoG exhibit some inherent limitation due to increased distance from the signal source. [23] Furthermore, μECoG suffers from noise enhancement due to electrode miniaturization and subsequent increased impedance. [24,25] In this scenario, brain recordings would highly benefit from an in situ first-stage signal amplification strategy. Among various strategies to overcome these limitations, semiconductor technology has been used in neurophysiological applications. Inorganic field-effect transistors were successfully demonstrated as transducers of bioelectrical activity in vitro, [26][27][28] yet their application in vivo is limited by the chemical and mechanical features of inorganic semiconductors, especially when exposed to aqueous environments. [29] This has relegated inorganic transistors to the role of integrated multiplexers for microelectrodes. [30] Successful translation of organic transistors as sensors and transducers to clinical settings is hampered by safety and stability issues. The operation of such devices demands driving voltages across the biotic/abiotic interface, which may result in undesired electrochemical reactions that may harm both the patient and the device. In this study, a novel operational mode is presented for electrolyte-gated organic transistors that avoid these drawbacks: the common-drain/grounded-source configuration. This approach reverts the standard common-source/common-ground configuration and achieves maximum signal amplification while applying null net bias across the electrolyte, with no parasitic currents. The viability of the proposed configuration is demonstrated by recording in vivo the somatosensory evoked activity from the barrel cortex of rats. The main inherent advantage of transistors with respect to passive electrodes is preserved in the proposed scheme: a superior signal-to-noise ratio is achieved which enables the detection of evoked activity at the single-trial level. Then, common-drain/grounded-source organic transistors are proposed as ideal candidate devices for a harmless translational recording platform.