Soft, Stretchable, and Conductive Hydrogel Composites

Hydrogels, crosslinked polymeric networks mostly filled with water, have unique mechanical properties, including extreme compliance, stretchability, and conformability. However, their intrinsic ionic conductivity (10⁻⁵ to 10⁻¹ S cm⁻¹) limits their use in soft electronics, wearable technologies, or bioelectronics. One way to improve their electrical properties is to introduce conductive fillers into a soft hydrogel matrix. In turn, the high concentrations of metallic fillers required to achieve adequate conductivity for digital circuit applications degrade hydrogel’s desirable mechanical properties. To overcome this limitation, we need a new material architecture that maintains the soft (< 10² kPa) and stretchable (> 50% strain) properties of hydrogels while also enabling high electrical conductivity (> 100 S cm⁻¹). 

This dissertation introduces various soft conductive composites that achieve low electrical resistance through percolating networks of embedded silver particles. Most of this work focuses on a novel material architecture and a processing method in which silver flakes are suspended with a tough hydrogel. By controlling the water content inside, the composite functions as either a conductor or an insulator. Moreover, this property is reversible so that it can reconfigure the conductive pathways within the composite. Overall, this dissertation contributes to the diversity of hydrogel-based conductors for soft and stretchable electronics.