Synthesis, copolymerization and peptide-modification of carboxylic acid-functionalized 3,4-ethylenedioxythiophene (EDOTacid) for neural electrode interfaces

Abstract

Background

Conjugated polymers have been developed as effective materials for interfacing prosthetic device electrodes with neural tissue. Recent focus has been on the development of conjugated polymers that contain biological components in order to improve the tissue response upon implantation of these electrodes.

Methods

Carboxylic acid-functionalized 3,4-ethylenedioxythiophene (EDOTacid) monomer was synthesized in order to covalently bind peptides to the surface of conjugated polymer films. EDOTacid was copolymerized with EDOT monomer to form stable, electrically conductive copolymer films referred to as PEDOT-PEDOTacid. The peptide GGGGRGDS was bound to PEDOT-PEDOTacid to create peptide functionalized PEDOT films.

Results

The PEDOT-PEDOTacid-peptide films increased the adhesion of primary rat motor neurons between 3 and 9 times higher than controls, thus demonstrating that the peptide maintained its biological activity.

Conclusions

The EDOT-acid monomer can be used to create functionalized PEDOT-PEDOTacid copolymer films that can have controlled bioactivity.

General Significance

PEDOT-PEDOTacid-peptide films have the potential to control the behavior of neurons and vastly improve the performance of implanted electrodes. This article is part of a Special Issue entitled Organic Bioelectronics—Novel Applications in Biomedicine.

Introduction

Functionalization of materials with biological molecules is often necessary for tissue engineering and long-term implantation of devices. In these applications control of the position, differentiation and chemical signaling of cells is desirable. Previous research has demonstrated the use of peptides and proteins for directing cell behavior on a variety of materials [1]. This type of functionalization may be especially important for stable implantation of chronic neural probes. To work optimally, neural electrodes should be able to record or stimulate neurons in the brain indefinitely. However, when studied in animals, an inflammatory reaction occurs upon implantation of these devices and an insulating glial scar forms around the probes within 4 weeks [2]. Therefore, options need to be explored to functionalize neural probes with biological molecules that can either minimize scar formation or retain neurons close to the electrode surface.

Conjugated polymers, such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), can be electrochemically polymerized on neural probe electrodes to produce semiconducting films with relatively low electrical impedances [3], [4]. These materials can interface electrically with neurons, but lack the chemical functionality to covalently bind biological molecules. To address this problem, Lee et al. synthesized carboxylic acid-functionalized PPy and demonstrated that the carboxylic acid could be used to covalently attach arginine–glycine–aspartic acid (RGD) peptides [5]. RGD is a cell-binding domain found in the extracellular matrix protein fibronectin that increases cell attachment to surfaces through integrin binding [6]. RGD-functionalized PPy increased the adhesion of endothelial cells compared to regular PPy. There is interest in developing a similar system with PEDOT, which is generally more stable and more conductive than PPy. Luo et al. developed two carboxylic acid-functionalized 3,4-ethylenedioxythiophenes (EDOTs) [7]; however, these monomers have relatively long linkers to the acid group, which could decrease their solubility in water, the preferred polymerization solvent. In addition, the ester functionality in one of the monomers may be easily hydrolyzed in a biological environment. Therefore, we developed a simple carboxylic acid-functionalized EDOT with the acid group directly attached to the EDOT unit—EDOTacid. The ability of this monomer to electrochemically polymerize has been investigated, along with the reaction of RGD peptide with the carboxylic acids. Although RGD does not exclusively increase neuron attachment it has been extensively studied and is used to demonstrate the ability of conjugated polymers containing EDOTacid to attach to peptides that remain bioactive.