Flexible Thin‐Film Neural Electrodes with Improved Conformability for ECoG Measurements and Electrical Stimulation

Abstract

Neural electrodes for electrocorticography (ECoG) recording and electrical stimulation are used for diagnosis and treatment of epilepsy, which are temporarily attached to the surface of cerebral cortex within the subdural space. However, conventional electrodes have a mechanical mismatch with the surface curvature of the brain cortex. Thus, it is necessary to develop a thin and flexible ECoG electrode to overcome the mechanical mismatch with cortex tissue and achieve long‐term ECoG measurement. Herein, a flexible thin‐film electrode is reported with a thickness of ≈8 µm by exploiting printed electronics. A simple fabrication method of conductive wiring is proposed by inkjet printing Au nanoink on a flexible elastomeric thin film consisting of polystyrene‐block‐polybutadiene‐block‐polystyrene (SBS). The conductive wiring printed on the SBS thin film is encapsulated by another SBS thin film as an insulation layer, with laser‐processed micropatterns. The flexible thin‐film electrode is implanted on the cortical surface of rats and used for in vivo ECoG recording and electrical stimulation without the occurrence of severe inflammatory reactions after 6 weeks of implantation. The flexible electrode with 35 ECoG channels visualized the epileptiform activity in a drug‐induced epilepsy rat model, which demonstrates the potential application for the management of intractable epilepsy.

Printable devices for neurotechnology

“…Carefully selecting the reaction conditions (temperature, pH, sequence, and rate of reagent addition), as well as the right functional additives can tune these characteristics. Due to their high conductivity (σ = 4.42 × 10 7 S.m −1 for gold, and σ = 6.3 × 10 7 S.m −1 for silver; Pajor-Świerzy et al, 2022 ) and low cost, metallic nanoparticles, in particular silver nanoparticles ( Kimtan et al, 2014 ; Kim et al, 2022 ), and gold nanoparticles ( Bachmann et al, 2017 ; Imai et al, 2023 ), are frequently employed in conductive inks for printed electronics ( Jensen et al, 2011 ; Khan et al, 2016 ; Kim et al, 2017 ; Liu et al, 2019 ; Almasri et al, 2020 ), as well as platinum-based inks ( Borda et al, 2020 ). The high surface area of these materials, however, can cause aggregation and poor ink stability, and they can be prone to oxidation and other stability issues ( Kamyshny and Magdassi, 2014 ).…”

“…Moreover, optimizing the printed materials' conductivity, resistance, and other electrical properties is also required (Borda et al, 2020). Electrode impedance, charge storage capacity, and charge injection capacity measurements were performed on printed electrodes in various studies to assess the overall electrical performance of the devices with in some cases, successful proofs of concept of EcoG measurements and in vivo stimulation (Duquesnoy et al, 2017;Shur et al, 2020;Kim et al, 2022;Imai et al, 2023). 4 Post-printing conditions: Post-printing curing conditions are considered important factors for the overall performance of the device.…”

Lifetime engineering of bioelectronic implants with mechanically reliable thin film encapsulations

“…Styrene-ethylene-butylene-styrene (SEBS) (figure 1(J)) is potentially the next most common choice, with PDMS being the more widely used of the two [7,118,121,130,131]. Less common but also used are Ecoflex (a non-FDA approved platinum catalyzed silicone elastomer which can be made extremely soft) (figure 1(K)) [132][133][134], polybutylene adipate terephthalate, which is biodegradable [135], perfluoropolyether, which is a an elastic fluorinated photoresist patternable down to micron scale [136], and SEBS relative polystyrene-block-polybutadiene-block-polystyrene (SBS) [137]. Elastomeric substrates are highly deformable and are therefore often chosen for applications requiring conformability.…”