In vivo electrophysiological recordings of neuronal circuits are necessary for diagnostic purposes and for brain-machine interfaces. Organic electronic devices constitute a promising candidate because of their mechanical flexibility and biocompatibility. Here we demonstrate the engineering of an organic electrochemical transistor embedded in an ultrathin organic film designed to record electrophysiological signals on the surface of the brain. The device, tested in vivo on epileptiform discharges, displayed superior signal-to-noise ratio due to local amplification compared with surface electrodes. The organic transistor was able to record on the surface low-amplitude brain activities, which were poorly resolved with surface electrodes. This study introduces a new class of biocompatible, highly flexible devices for recording brain activity with superior signal-to-noise ratio that hold great promise for medical applications.

The development of transistors with high gain is essential for applications ranging from switching elements and drivers to transducers for chemical and biological sensing. Organic transistors have become well-established based on their distinct advantages, including ease of fabrication, synthetic freedom for chemical functionalization, and the ability to take on unique form factors. These devices, however, are largely viewed as belonging to the low-end of the performance spectrum. Here we present organic electrochemical transistors with a transconductance in the mS range, outperforming transistors from both traditional and emerging semiconductors. The transconductance of these devices remains fairly constant from DC up to a frequency of the order of 1 kHz, a value determined by the process of ion transport between the electrolyte and the channel. These devices, which continue to work even after being crumpled, are predicted to be highly relevant as transducers in biosensing applications.

A photolithographic process was used to integrate the conducting polymer poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with parylene C, yielding highly conformable electrode arrays that were only four micrometers thick (shown here to conform to the midrib of a small leaf). The arrays were sufficiently self‐supporting to allow in vivo evaluation in rats, yielding high‐quality electrocorticography recordings.

The bulk of the currently available biosensing techniques often require complex liquid handling, and thus suffer from problems associated with leakage and contamination. We demonstrate the use of an organic electrochemical transistor for detection of lactate (an essential analyte in physiological measurements of athlete performance) by integration of a room temperature ionic liquid in a gelformat, as a solid-state electrolyte.

The integration of an organic electrochemical transistor with human barrier tissue cells provides a novel method for assessing toxicology of compounds in vitro. Minute variations in paracellular ionic flux induced by toxic compounds are measured in real time, with unprecedented temporal resolution and extreme sensitivity.

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A generic lithographic process is presented that allows the fabrication of high density organic electrochemical transistor arrays meant to interface with aqueous electrolytes. The channels of the transistors, which were 6 μm long, were made of the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) and were in direct contact with phosphate buffered saline. Source and drain electrodes and interconnects were insulated by parylene C, a biocompatible material. The transistors operated at low voltages and showed a response time of the order of 100 μs.

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