Recently there has been an increase in demand for soft and biocompatible electronic devices capable of withstanding large stretch. Conductive hydrogels present a promising class of soft materials for the emerging applications due to their ability to realize charge transport across wet and swollen polymer networks, while preserving the desired mechanical and chemical features. As opposed to electron transfer in traditional electrical conductors, the charge transport across a polymer swollen with an electrolyte solution is achieved through ion migration – such materials are therefore known as ionotronic. The applications of ionically conductive hydrogels are vast, from healthcare and soft robotics to sensors. Moreover, the ability to successfully predict ion migration in soft and wet materials opens the door for implementation of biocompatible hydrogels as an interface between living tissue and external devices.Hydration level and water diffusion through the polymer network have a significant impact on both the conductive and mechanical behavior of hydrogels. In addition to water molecules and mobile ionic species, fixed charge groups might be tethered onto the polymeric backbone. As a consequence of variation in pH, the ions might interact with the fixed groups through association/dissociation chemical reactions, thus changing the overall electrolyte strength. This significant environmental responsiveness of hydrogels identifies them as active materials with highly tunable properties, and therefore front-runners for implementation in soft electric circuits and devices.Ionotronic materials and systems are characterized primarily through DC measurements, electrochemical impedance spectroscopy (EIS), and thermo-mechanical testing. EIS consists of applying a time-varying voltage across the material while measuring the corresponding current, and provides insights into the underlying electrochemical mechanisms driving the ion transport. Despite the rapid increase in the number of potential applications and experimental procedures, there is a lack of overarching design tools, hindering the further advance of ionotronic technology. The main challenge in developing reliable simulation capabilities is the complex and coupled electro-chemo-mechanical behavior of ionically conductive hydrogels.In this work, we propose a multiphysics framework involving the coupled effects of ion transport, electric fields, network hydration, and large deformation. In addition, the developed constitutive model takes into account the difference in conductivity at various hydration levels, along with association/dissociation chemical reactions triggered by pH variations. The model also provides capabilities for computing current generated by the time-varying voltage, and prediction of EIS results based on material microstructure and environmental conditions.We numerically solve this multiphysics system by employing the finite element method. More specifically, we implement the multiphysics framework as a&nbsp;user element (UEL) subroutine within the commercial software Abaqus/Standard. Such an approach will enable quantitative evaluation of material design and ionotronic devices. The accuracy of the developed model and numerical method is ensured through validation against the EIS data. The usefulness of the developed multiphysics model is showcased by simulating the representative ion transport problems and the operation of a soft ionotronic device.

Multiphysics Modeling of Ionically Conductive Hydrogels

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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