Poly[3,4-ethylene dioxythiophene (EDOT)-co-1,3,5-tri[2-(3,4-ethylene dioxythienyl)]-benzene (EPh)] copolymers (PEDOT-co-EPh): optical, electrochemical and mechanical properties

Abstract

PEDOT-co-EPh copolymers with systematic variations in composition were prepared by electrochemical polymerization from mixed monomer solutions in acetonitrile. The EPh monomer is a trifunctional crosslinking agent with three EDOTs around a central benzene ring. With increasing EPh content, the color of the copolymers changed from blue to yellow to red due to decreased absorption in the near infrared (IR) spectrum and increased absorption in the visible spectrum. The surface morphology changed from rough and nanofibrillar to more smooth with rounded bumps. The electrical transport properties dramatically decreased with increasing EPh content, resulting in coatings that either substantially lowered the impedance of the electrode (at the lowest EPh content), leave the impedance nearly unchanged (near 1% EPh), or significantly increase the impedance (at 1% and above). The mechanical properties of the films were substantially improved with EPh content, with the 0.5% EPh films showing an estimated 5x improvement in modulus measured by AFM nanoindentation. The PEDOT-co-EPh copolymer films were all shown to be non-cytotoxic toward and promote the neurite outgrowth of PC12 cells. Given these results, we expect that the films of most interest for neural interface applications will be those with improved mechanical properties that maintain the improved charge transport performance (with 1% EPh and below).

POSS-ProDOT crosslinking of PEDOT

“…Hundreds of nanometers of blue shifts in absorption maxima were observed with EPh as crosslinker, leading to dramatic changes in color. 12 The blue shift of absorption was resulted from the decreased conjugated chain length which could potentially lead to an eventual loss in charge transport efficiency, usually larger blue shift could result in more loss of conductivity. However, in this system, much smaller shifts in absorption were observed.…”

“…These shifted oxidation peaks indicated that the crosslinker decreased the conjugation length of PEDOT. 12, 35, 37 …”

“…9-11 These soft materials usually have a typical Young's modulus on the order of ∼1 GPa and thus are expected to help reduce the mechanical properties mismatch between hard metal electrodes and soft living tissue. 12, 13 In addition, they provide high surface areas that can facilitate ion exchange between the microelectrodes and tissue, therefore building more efficient pathways for abiotic-biotic communication. 14 Because of their good electronic and ionic conductivities, the impedance is usually significantly reduced and charge storage capacity is substantially improved.…”

“…17-19 Though conjugated polymers have many advantages over other materials, these materials are often fragile and may delaminate from the surfaces of metal electrodes. 12 Another concern is their electrochemical and mechanical stability for long-term neural recording or stimulation applications. Loss of device performance such as decreased charge carrying capacity and increased impedance of PEDOT coated neural electrodes under chronic neural recording or stimulation conditions has been reported by different groups.…”

Recent advances in neural electrode–tissue interfaces

“…Conducting polymer coatings are known to dramatically decrease electrode impedance and increase charge injection limit [18, 106], and have been used to enable recording from ultrasmall electrode sites [47, 107]. New advances in conducting polymers may be found in novel dopants [61, 89], drug delivery [89], and novel monomers and crosslinkers for ease of functionalization or improved stability [108–111]. These novel polymer coatings may allow for further decreased size of both recording and stimulating electrodes for central and peripheral nervous system applications.…”