Ultrathin Gold Microelectrode Array using Polyelectrolyte Multilayers for Flexible and Transparent Electro‐Optical Neural Interfaces

Abstract

Electro‐optical neural interface technologies provide great potential and versatility in neuroscience research. High temporal resolution of electrical neural recording and high spatial resolution of optical neural interfacing such as calcium imaging or optogenetics complimentarily benefit the way information is accessed from neuronal networks. To develop a hybrid neural interface platform, it is necessary to build transparent, soft, flexible microelectrode arrays (MEAs) capable of measuring electrical signals without light‐induced artifacts. In this work, flexible and transparent ultrathin (<10 nm) gold MEAs are developed using a biocompatible polyelectrolyte multilayer (PEM) metallic film nucleation‐inducing seed layer. With the polymer seed layer, the thermally evaporated ultrathin gold film shows good conductivity while providing high optical transmittance and excellent mechanical flexibility. In addition, strong electrostatic interaction via the PEM alters the electrode‐electrolyte interfaces, thereby reducing the electrode impedance and baseline noise level. With a simple modification of the fabrication process of the MEA using biocompatible materials, both excellent transmittance, and electrochemical interface characteristics are achieved, which is promising for efficient electro‐optical neural interfaces.

A mechanically adaptive hydrogel neural interface based on silk fibroin for high-efficiency neural activity recording

“…4 The reliable functions of the electrodes are dependent on the following requirements: (i) mechanical compatibility with target tissues and stable interface contact, (ii) stable and efficient electrochemical properties for collection and amplification of micro neural signals, (iii) biocompatibility to minimize acute and chronic immune responses. Traditional metallic electrodes such as silicon, 5 gold, 6 and platinum (Pt) 7 are widely applied due to their excellent electrical properties. However, the inherent mechanical mismatch with soft tissue prevents tight interface coupling, leading to chronic neuroinflammation and loss of neural signals.…”

Inkjet-Printed Polyelectrolyte Seed Layer-Based, Customizable, Transparent, Ultrathin Gold Electrodes and Facile Implementation of Photothermal Effect

“…Among various NISL materials, we chose a polymer seed layer of polyethylenimine (PEI) because polymer materials are inkjet-printable. Moreover, PEI has a large number of amine side chains, which are known to interact with Au atoms (Figure b). ,, …”

“…Moreover, PEI has a large number of amine side chains, which are known to interact with Au atoms (Figure 1b). 9,26,35 We optimized the formulation of the inkjet printable PEI ink for the NISL. For stable jetting of the PEI solution, we first adjusted the viscosity of the PEI solution.…”

“…In addition to the bioelectrical signal recording capability of such electrodes, optical transparency can enable hybrid approaches such as microscopic imaging of the cells of interest, optical modulations including optogenetics, photo­biomodulation therapy, and photothermal neuromodulation. All of these options are possible using transparent electrodes to implement the additional functionalities simultaneously. − In addition, because the demand for wearable healthcare devices has also increased in recent years, the design of the electrode arrays needs to be easily customizable depending on the shapes of the region of interest in a patient’s body for more efficient biosignal sensing. , Therefore, there is a considerable need for customization-friendly mask-less fabrication processes to provide such transparent microelectrodes for use in wearable biomedical devices. However, conventional mask-based photolithography is limited that it is not only impossible to easily customize the mask design, but also it is cost-inefficient for designing low-density micropatterns over a large area, which is frequently required in wearable biomedical electronics applications.…”

“…As an alternative approach to the fabrication of transparent, flexible, biocompatible electrodes, vacuum-deposited ultrathin gold electrodes (<10 nm) with a polymer nucleation-inducing seed layer (NISL) have recently been developed. These outperform other types of transparent electrode materials regarding their electro-optical characteristics and simplicity of fabrication. , However, because the micropatterning of the ultrathin metal electrodes relies on conventional mask-based photolithography, design customizability and large-area applicability are still severely lacking.…”