In vivo recordings of brain activity using organic transistors

Abstract

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.

Figure 1. Structure of the ECoG probe.

(a) Optical micrograph of the probe conforming onto a curvilinear surface. Scale bar, 1 mm. The inset shows an image of the whole probe, in which the transistor/electrode arrays are on the right-hand side, whereas the external connections, onto which a zero insertion force (ZIF) connector is attached, are on the left-hand side. (b) Optical micrograph of the channel of a transistor and a surface electrode, in which the Au films that act as source (S), drain (D) and electrode pad (E) are identified. Scale bar, 10 μm. (c,d) Layouts of the surface electrode and of the transistor channel, respectively (not to scale).

Figure 2. In vitro characterization of the transistor.

(a) Output characteristics showing the drain current, I_D as a function of drain voltage, V_D for a gate voltage, V_G varying from 0 V to 0.5 V (with a step of 0.1 V) of a PEDOT:PSS transistor in Ringer’s solution and with a stainless-steel gate electrode. (b) Transfer curve and resulting transconductance at V_D=−0.4 V.

Figure 3. Validation in Wistar rats.

(a) Optical micrograph of the ECoG probe placed over the somatosensory cortex, with the craniotomy surrounded by dashed lines. Scale bar, 1 mm. (b) Wiring layout of the transistor, with the blue box indicating the brain of the animal. (c) Recording of a bicuculline-induced epileptiform spike from a transistor (pink), a PEDOT:PSS surface electrode (blue) and 12 of the 16 Ir-penetrating electrodes (black). The transistor was biased with V_D=−0.4 V and V_G=0.3 V, and the scale of 10 mV is for both surface and penetrating electrodes. (d) Current source density map of a bicuculline-induced epileptiform spike (44 events averaged shown as an overlay) showing a strong sink and source around the reversal of the event, in the deeper layers of the somatosensory cortex.

Figure 4. Validation in GAERS rats.

(a) Recordings from an OECT (pink), a PEDOT:PSS surface electrode (blue) and an Ir-penetrating electrode (black). The transistor was biased with V_D=−0.4 V and V_G=0.3 V, and the scale of 10 mV is for both surface and penetrating electrodes. Note the superior SNR of the OECT as compared with the surface electrode. (b) Time–frequency analysis of epileptiform activity during a short period, recorded by an OECT (top), a PEDOT:PSS surface electrode (middle) and an Ir-penetrating electrode (bottom).

Figure 5. Recordings of low-amplitude oscillations.

Recordings from an OECT (top), a PEDOT:PSS surface electrode (middle) and an Ir-penetrating electrode (bottom), shown together with their corresponding time–frequency analysis plots (normalized TF energy), scaled to their minimum/maximum values. These oscillations, which have a lower amplitude than SWD, are picked up equally well by the OECT and the Ir-penetrating electrode, but are not well-resolved by the PEDOT:PSS surface electrode.