Durability of Poly(3,4-ethylenedioxythiophene) (PEDOT) films on metallic substrates for bioelectronics and the dominant role of relative shear strength.

Abstract

Despite growing interest in the use of conducting polymer coatings such as poly(3,4-ethylenedioxythiophene) (PEDOT) in bioelectronics, their relatively poor mechanical durability on inorganic substrates has limited long-term and clinical applications. Efforts to enhance durability have been limited by the lack of quantifiable metrics that can be used to evaluate the polymer film integrity and associated device failure. Here we examine the hypothesis that film failure under the tribological and cyclic electrical stressing becomes substantially less likely when the interfacial shear strength (τ_i) exceeds the shear strength of the film (τ_f). In this paper, we: (1) develop a simple yet robust method to quantify the relative shear strength (τ_i/τ_f); (2) quantify the effect of substrate and surface treatment on the relative shear strength of PEDOT; (3) relate changes in relative shear strength to resistance to interface failure under cyclic electrical and tribological testing. Treating a stainless-steel substrate with an adhesion promoter increased τ_i/τ_f from 0.18 to 0.69 compared to untreated controls. On untreated gold, the τ_i/τ_f of PEDOT increased to 1.46. Whereas both cyclic electrical and tribological testing quickly and severely damaged the interface of PEDOT when τ_i/τ_f < 1, neither stimulus had any quantifiable effect on delamination when τ_i/τ_f > 1.