Chronically stable stretchable thin film electrodes for in vivo and ex vivo tissue stimulation

Poppy Oldroyd

Thin film electrodes are becoming increasingly common for interfacing with tissue. However, their long-term stability has yet to be proven in modulation applications where electrical stimulation over months to years is desired. To evaluate the stability of different thin film electrodes, we developed an ISO-informed accelerated aging protocol for stimulation implants. These include electrodes made of Au, electrodes coated with the conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and electrodes made of PEDOT:PSS (metal free). Conducting polymer coatings are known to offer mixed ionic-electronic conductivity, which improves performance in neural recording and stimulation applications. We show that pristine PEDOT:PSS electrodes outperform all other configurations, exhibiting little degradation over an extrapolated aging time exceeding two years.1 However, over time, polymer swelling, and subsequent delamination of insulation and substrate layers was identified as a mode of failure of the thin film devices. Currently, thin-film devices are fabricated using Parylene C or Polyimide, stiff polymers, with little ability to accommodate the conductive polymer swelling. In this work, we present a thin film fabrication method using PDMS and PEDOT:PSS to develop devices which can stretch with, and accommodate, the PEDOT:PSS swelling. Due to the hydrophobicity of PDMS, we add a fluorosurfactant and a hydrophilic polymer to the PEDOT:PSS solution. The resultant solution is spin-coat processible and offers enhanced adhesion to the PDMS substrate and insulation layers. The PDMS/PEDOT:PSS/PDMS devices demonstrate excellent mechanical and electrical stability under cyclic tensile testing both in air and in liquid. Under accelerated aging, they show reduced delamination while maintaining conductivity and mechanical properties. The application of these devices is vast; the electrodes can be fabricated out of the cleanroom for macro-scale applications or using conventional lithography for micro-scale applications. Here we present two macroscale ex-vivo applications; monitoring the contractility of the gut and expansion of the heart. The electrodes offer simultaneous monitoring of impedance changes and electrophysiology. Due to the functionalization capacity of PDMS, established adhesion methods can be readily integrated, increasing contact with the tissue improving signal to noise ratio.