Conducting polymers serve as an interface between electrodes and biological organisms in bioelectronic devices. Some examples are poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), polypyrrole (PPy), and polyaniline (PAni) which have favorable electrical and mechanical properties and biocompatibility. However in a wet physiological environment, weak and unstable adhesion occurs leading to interfacial failures. In this article on Science Advances journal, Dr. Xuanhe Zhao and his team spanning from MIT in USA to JSR Corporation in Japan and Jiangxi Science and Technology Normal University in China present a successful method to achieve strong adhesion of various conducting polymers on substrates in wet physiological environments. They introduced a hydrophilic polymer adhesive layer of only a few nanometers thick but strongly bonds wet conducting polymers to various substrates. Furthermore, a variety of fabrication methods can be used to produce this adhesion layer.

The team start with a smooth substrate functionalized with primary amine groups. This provides an enhanced interfacial adhesion between the substrate and adhesive layer via covalent bonds and/or electrostatic interaction. Next, the adhesive layer is applied via spin coating, spray coating or dip coating. A layer of conducting polymer is applied on top of the adhesive layer which then results in a swelling of the adhesive layer due to polymer precursors diffusing into it. A schematic of the process is seen in the image above.

Tests conducted to measure the mechanical and electrical integrity of the adhesive layer on the substrate include lap-shear tests, standard four-point probe, electrochemical impedance spectroscopy analysis and tensile tests. The CellScale UStretch performed tensile tests to determine the effect of the adhesive layer on the conducting polymer under varying thickness of the adhesive layer. Images below show the test set-up and results.

To read the full article, click here: https://doi.org/10.1126/sciadv.aay5394

To read more about Dr. Zhao’s research, click here: http://meche.mit.edu/people/faculty/ZHAOX@MIT.EDU

To read about a lap-shear test done with the UStretch, click here.