Electrochemically deposited conducting polymers for reliable biomedical interfacing materials: formulation, mechanical characterization, and failure analysis
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Conjugated polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) are
of interest for a variety of applications including interfaces between electronic
biomedical devices and living tissue. These polymers provide an improved interface
compared to metal and semiconducting electrodes because of their ionic conductivity,
relatively lower stiffness, and ability to incorporate biological molecules. Even though
the signal transfer and biocompatibility of conjugated polymers are superior as the
biointerfacing materials, the durability has been the weakest part for the long-term
applications. Some efforts have been made to improve the durability of conjugated
polymers, however, little quantitative information of the improved cohesion, adhesion
and durability has been reported. ☐ In this thesis, the methods of improving the durability of conjugated polymer
films, especially PEDOT, were investigated, including alternating the processing
methods and components in synthesis. The 7-month in vivo testing showed a superior
neural signal recording performance, however the durability of PEDOT films still
needed to be improved. ☐ As a coating for biosignal transfer, the cohesion, adhesion and electrochemical
stability of PEDOT are vital to determine the long-term performance. By far, not
much information of cohesion and adhesion of conjugated polymer coatings has been
dug out. In this thesis, a thin film cracking method was developed to measure the
stiffness, strength and the interfacial shear strength (adhesion) of electrochemically
deposited PEDOT. The estimated Young’s modulus of the PEDOT films was 2.6 ± 1.4
GPa, and the strain to failure was around 2%. The tensile strength was measured to be
56 ± 27 MPa. The effectiveness of crosslinker and adhesion promoter was
demonstrated by this method. It was shown that 5 mole% addition of a tri-functional
EDOT crosslinker increased the tensile strength of the films to 283 ± 67 MPa, while
the strain to failure remained about the same (2%). With the modification of EDOTacid
to the surface of stainless steel substrate, the interfacial shear strength was
improved from 11.8 MPa to 32.5 MPa. ☐ To correlate the adhesion with the durability of PEDOT coatings, a tribology
test was introduced. It was found that the durability of PEDOT on Au electrode was
exceptionally good, and even better than the adhesion promoted coatings with EDOTacid
on stainless steel and ITO substrates. The characterization method developed in
this thesis made a critical difference in systematically comparing different materials,
and provided valuable information for materials development and selection.