Interpenetrating Conducting Hydrogel Materials for Neural Interfacing Electrodes

Abstract

Conducting hydrogels (CHs) are an emerging technology in the field of medical electrodes and brain-machine interfaces. The greatest challenge to the fabrication of CH electrodes is the hybridization of dissimilar polymers (conductive polymer and hydrogel) to ensure the formation of interpenetrating polymer networks (IPN) required to achieve both soft and electroactive materials. A new hydrogel system is developed that enables tailored placement of covalently immobilized dopant groups within the hydrogel matrix. The role of immobilized dopant in the formation of CH is investigated through covalent linking of sulfonate doping groups to poly(vinyl alcohol) (PVA) macromers. These groups control the electrochemical growth of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and subsequent material properties. The effect of dopant density and interdopant spacing on the physical, electrochemical, and mechanical properties of the resultant CHs is examined. Cytocompatible PVA hydrogels with PEDOT penetration throughout the depth of the electrode are produced. Interdopant spacing is found to be the key factor in the formation of IPNs, with smaller interdopant spacing producing CH electrodes with greater charge storage capacity and lower impedance due to increased PEDOT growth throughout the network. This approach facilitates tailorable, high-performance CH electrodes for next generation, low impedance neuroprosthetic devices.

A living electrode construct for incorporation of cells into bionic devices

“…The electrochemical properties for the CH coatings are in line with literature, demonstrating a substantial drop in impedance and increase in both CSC and injection limit. [16][17][18]28] In prior studies, the lamination of the CH with the additional PVA layer to create a living electrode was found to double the CSC (from 25 mC/cm 2 for the CH to 50 mC/cm 2 for the living electrode), but had minimal impact on charge injection limit. In these studies, only a small increase in CSC was observed (27%) and a greater increase (50%) was found for charge injection limit.…”

“…In the current study, a recently designed CH, with PEDOT grown through PVA-Tau, was employed. [18] This new CH has tailored placement of sulfonate doping groups (i.e., taurines) along the PVA backbone, which enables more controlled electrodeposition of PEDOT. It is expected that this CH system improves the stability and reproducibility of the CH component and subsequently the living electrode.…”

“…Deposition was carried out at 1 mA/ cm 2 for 1 min. The non-degradable hydrogel was then formed from a macromer solution of 10 wt% methacrylate and taurinemodified PVA (PVA-Tau), using methods established by Goding et al [18] The hydrogel film was cross-linked to form a thin layer across the electrode surface with ultra-violet light (70 mW/cm 2 , 365 nm) for 180 s. PEDOT was deposited through this gel from an aqueous solution of 0.03 M EDOT at 0.5 mA/cm 2 for 30 min. The thickness of the CH layer was ∼10 µm, in line with prior studies using this technique and similar fabrication parameters.…”

“…The thickness of the CH layer was ∼10 µm, in line with prior studies using this technique and similar fabrication parameters. [16][17][18] The final layer was produced from a macromer solution of 8 wt% phenol-modified PVA, 1 wt% gelatin, and 1 wt% sericin (S5201 Sigma Bombyx mori) using the methods developed by Lim et al [20] Neuroprogenitor cells (described below) were added to the macromer solution at 5 × 10 6 cells/mL. The initiators ruthenium (2 mM) and sodium persulfate (20 mM) were included.…”

“…[15] Conversely, conductive hydrogels (CHs), being interpenetrating hybrids of CPs and hydrogels, [16] have been shown to be stable under implant conditions, [17] and swelling can be controlled through design of the hydrogel and CP components. [18] By encapsulating cells in an overlying hydrogel layer above a CH coating, the cells can be protected from the solvents and high DC voltages required for CH (or CP) fabrication. Since the biologic environment exclusively passes charge in ions, this design and specifically the CH layer will also protect encapsulated cells from high-voltage capacitive charging that occurs on metallic electrodes.…”