March 20, 2017

Improved materials for interfacing neural tissue with electronic biomedical devices

Modern electronic biomedical devices are enabling a wide range of sophisticated health interventions, from seizure detection and Parkinson’s disease therapy to functional artificial limbs, cochlear implants and smart contact lenses.

An effective direct interfacing material is essential to communication between these devices and neural tissue, which includes nerves and the brain.

In recent years, a conjugated polymer known as PEDOT — widely used in applications such as energy conversion and storage, organic light-emitting diodes, electrochemical transistors, and sensing — has been investigated for its potential to serve as this interface.

In some cases, however, the low mechanical stability and relatively limited adhesion of conjugated polymers like PEDOT — short for poly (3,4-ethylene dioxythiophene) — on solid substrates can limit the lifetime and performance of these devices. Mechanical failure might also leave behind undesirable residue in the tissue.

A research team led by the University of Delaware’s David Martin has reported the development of an electrografting approach to significantly enhance PEDOT adhesion on solid substrates.


Schematic representation of the adhesion-promoting layer.

Science Advances - Enhanced PEDOT adhesion on solid substrates with electrografted P(EDOT-NH2)



Compared to other methods, surface modification through electro-grafting takes just minutes. Another advantage is that a variety of materials can be used as the conducting substrate, including gold, platinum, glassy carbon, stainless steel, nickel, silicon and metal oxides.

The actual chemistry usually takes multiple steps, but Martin and his team have developed a simple, two-step approach for creating PEDOT films that strongly bond with metal and metal oxide substrates, yet remain electrically active.

“Our results suggest that this is an effective means to selectively modify microelectrodes with highly adherent and highly conductive polymer coatings as direct neural interfaces,” Martin says.

Abstract

Conjugated polymers, such as poly(3,4-ethylene dioxythiophene) (PEDOT), have emerged as promising materials for interfacing biomedical devices with tissue because of their relatively soft mechanical properties, versatile organic chemistry, and inherent ability to conduct both ions and electrons. However, their limited adhesion to substrates is a concern for in vivo applications. We report an electrografting method to create covalently bonded PEDOT on solid substrates. An amine-functionalized EDOT derivative (2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methanamine (EDOT-NH2), was synthesized and then electrografted onto conducting substrates including platinum, iridium, and indium tin oxide. The electrografting process was performed under slightly basic conditions with an overpotential of ~2 to 3 V. A nonconjugated, cross-linked, and well-adherent P(EDOT-NH2)–based polymer coating was obtained. We found that the P(EDOT-NH2) polymer coating did not block the charge transport through the interface. Subsequent PEDOT electrochemical deposition onto P(EDOT-NH2)–modified electrodes showed comparable electroactivity to pristine PEDOT coating. With P(EDOT-NH2) as an anchoring layer, PEDOT coating showed greatly enhanced adhesion. The modified coating could withstand extensive ultrasonication (1 hour) without significant cracking or delamination, whereas PEDOT typically delaminated after seconds of sonication. Therefore, this is an effective means to selectively modify microelectrodes with highly adherent and highly conductive polymer coatings as direct neural interfaces.

SOURCES- University of Delaware, Science Advances

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