Physico-mechanical design of conducting polymers for neural interface applications

Abstract

A key limitation associated with conducting polymers (CP) for implantable electrode applications is their inferior physico-mechanical properties and the effect of this on biological performance. This research investigates the physico-mechanical cues of conventional conducting polymer coatings and aims to understand their effects on neural adhesion and neurite extension. The underlying hypothesis was that the biological performance of CPs can be effectively controlled by physical cues such as surface topography and mechanical softness. Poly(3,4-ethylenedioxythiophene) (PEDOT) doped with perchlorate, benzenesulfonate, tosylate (pTS), dodecylbenzenesulfonate and polystyrenesulfonate were compared across a range of baseline material properties. Additionally, the deposition charge used to produce PEDOT was varied from 0.05 to 1 C/cm2 to determine an optimal thickness for electrode coatings. To address the need for electroactive biomaterials with improved neural interfacing, nanobrush-CP hybrids were fabricated. Dense poly(2-hydroxyethyl methacrylate) (PHEMA) brushes were grafted via surface-initiated atom transfer radical polymerisation (SI-ATRP). PEDOT/pTS was electrochemically deposited through this nanobrush substrate. The formation of the hybrid was confirmed and characterised across implant performance metrics. The physical, mechanical, electrical and biological performance of PEDOT coatings was used to assess ideal fabrication parameters, optimised for neural cell interactions. Nanoindentation techniques were used to yield the first quantitative values for stiffness moduli of electrodeposited CP coatings on metal substrates. It was found that the nodularity of the CP surface increased with increasing coating thickness and decreasing dopant size. A major finding of this study was that high roughness of conventionally doped PEDOT produced on the micron scale, prevented attachment of neural cells. Consequently, thin PEDOT films doped with the low toxicity anion, pTS, supported the greatest cell attachment and neurite outgrowth. Electrochemical performance was analyzed and supported the finding that thin PEDOT/pTS provides significant biological and electrochemical advantages over platinum electrodes. The nanobrush/CP hybrid further improved the electrochemical properties of conventional CPs and offers a new approach for selective cell attachment via the CP coated region of the brush substrate. This thesis demonstrates that the biological performance of CPs is strongly influenced by the physico-mechanical properties with optimal coatings produced in the sub-micron range using conventional doping ions. A new hybrid nanobrush/CP is presented with fabrication parameters which can be tailored for target material properties. Future work will focus on delineating the interfacial structure of the hybrid to optimise the cushioning effect of the brushes for neural interface applications.