Transparent, conformable, active multielectrode array using organic electrochemical transistors

Abstract

Mechanically flexible active multielectrode arrays (MEA) have been developed for local signal amplification and high spatial resolution. However, their opaqueness limited optical observation and light stimulation during use. Here, we show a transparent, ultraflexible, and active MEA, which consists of transparent organic electrochemical transistors (OECTs) and transparent Au grid wirings. The transparent OECT is made of Au grid electrodes and has shown comparable performance with OECTs with nontransparent electrodes/wirings. The transparent active MEA realizes the spatial mapping of electrocorticogram electrical signals from an optogenetic rat with 1-mm spacing and shows lower light artifacts than noise level. Our active MEA would open up the possibility of precise investigation of a neural network system with direct light stimulation.

Keywords: multielectrode array; optogenetic; organic electrochemical transistors; thin films; transparent electrode.

Fig. 1.

Transparent electrophysiology OECTs array. (A) Image of the transparent electrophysiology OECT array on a parylene substrate. The passivation layer was patterned by SU-8. (B) Magnified image of a transparent OECT single cell. For transparency, the source and drain of OECT were made by a metal grid of Au. (C) A cross-section of the transparent OECT array. (D) Schematic diagram of the OECT array. A certain voltage was applied to the drain line to be measured. The other lines were all connected to the ground of the circuit for preventing cross-talk. The source lines were connected to each current meter. (E) The 3 × 5 transparent electrophysiology array (black dashed square) on the cortical surface of the optogenetic rat. (Scale bar: 1 mm.)

Fig. 2.

Mechanical stability and transparency of Au grid. (A) Microscope image of metal grid on 1.2-μm parylene substrate. (Scale bar: 100 μm.) (B) Image of a metal grid and square grid unit cell parameters. (C) Transmittance spectrum of the metal grid with various calculated transmittance (T_c). (D) A 3D microscope image of a metal grid under (Upper) 0% compression strain and (Lower) 30% compression strain. (E) Resistance change of ITO (70 nm) and Au grid (70 nm) with PEDOT:PSS (150 nm) when compression strain was changed (the number of samples was three). (F) Resistance ratio after cycling test of ITO and Au grid with PEDOT:PSS when the compression strain applied to the film was 50%.

Fig. 3.

Electrical characteristic of transparent OECT and OECT array. (A) OECT during electrical measurement. The electrolyte was placed on the channel of OECTs, and V_g was applied using Ag/AgCl from the top of the electrolyte. (B) Transfer characteristics of the transparent and nontransparent OECTs. The channel width and length were 90 and 20 μm, respectively. The maximum transconductance values were 2.2 and 2.3 mS at a −0.7-V source drain and a 0-V gate voltage, respectively. (C) The response time of transparent and nontransparent OECT. The response times were 110 and 120 µs, respectively. (D) The microscope image of a 3 × 5 transparent OECT array and magnified view of the channel (Inset). The channel width and length were 70 and 20 μm, respectively. (Scale bar: D, 500 μm; D, Inset, 30 μm.) (E) The distribution of transconductance in a 3 × 5 transparent OECT array. (F) The distribution of response times in a 3 × 5 transparent OECT array.

Fig. 4.

In vivo evaluation of transparent OECTs with light stimulation using optogenetic rat. (A) Photograph of the transparent OECT on a neuron-concentrated area of the cortical surface of optogenetic mice with blue laser continuous stimulation through optical fiber (500-µm diameter). (Scale bar: 500 μm.) (B) The recorded signal by transparent OECT with light stimulation of a 475-nm wavelength. The rms was 38 µV. (C) The recorded signal by nontransparent OECT with light stimulation of a 475-nm wavelength. The rms was 60 µV. (D) Evoked ECoG potentials in response to pairs of 2-ms light pulses at 60 mW. The rms was 0.03 µA. (E) Photograph of the transparent electrophysiology array in a neuron-concentrated area on the cortical surface of optogenetic mice with blue laser continuous stimulation through optical fiber (500-µm diameter). (Scale bar: 1 mm.) (F) The spatial distribution of the brain signal intensity measured by a 3 × 5 transparent electrophysiology array. The electric potential was calculated using each OECT’s transconductance, which is measured by a V_g sinusoidal signal and I_d.