Standardizing Electrochemical Characterizations of Conductive Hydrogels

Conductive hydrogels, composite materials of polymer networks and conductive fillers, are an exciting area of growth in the field of biomaterials. Because conductive hydrogels exhibit mixed ionic and electronic conductivity, they have the potential to interface seamlessly between tissue signaling and traditional electronics, adding to their value as bioactive materials. As the area continues to grow, attention must be drawn to how we benchmark new materials and assess potential performance – specifically their conductivity. The soft, hydrated nature of conductive hydrogels creates confounding variables that pose challenges for reproducible conductivity measurements such as strain induced morphological changes during measurement, the impact of electrolyte environments, and composite ionic and electronic components.
We present a novel method for collecting and analyzing electrochemical impedance spectroscopy on conductive hydrogels that produces repeatable results with low errors. We propose the use of a two-electrode system in a water-tight Swagelok cell which can be fit to a Debye circuit to fully characterize the conductive hydrogel system. We demonstrate the feasibility of our method using several different hydrogel systems exhibiting different degrees of ionic and electronic conductivity and show that our data collection cell and simplified equivalent circuit produce reliable and interpretable figures of merit –ionic conductivity, electronic conductivity, and double layer capacitance. To encourage adoption of this method, we provide a list of materials/suppliers, standard-operating-procedures, and open-source code for fitting the acquired data. To emphasize the benefits of this method of analysis, we explore some unexpected trends in conductivity revealed by our method that are unable to be seen with other common methods of conductivity measurement, namely 4-point probe. Lastly, we explore several confounding factors to hydrogel conductivity measurements and show how our system can deconvolute effects on conductivity due to ionic strength, storage conditions, and mechanical compression. This project proposes a simple-to-execute yet in-depth method of probing conductive hydrogel systems that can not only allow for consistent comparison across different hydrogel systems but can also help parse which elements of hydrogel conductivity are more significant in different applications. We believe such a system would be invaluable to the growing field of conductive hydrogels.