Soft ionic materials combine charged mobile species and tailored polymer structures in a manner that enables a wide array of functional devices. Traditional metal and silicon electronics are limited to two charge carriers: electrons and holes. Ionic devices hold the promise of using the wide range of chemical and molecular properties of mobile ions and polymer functional groups to enable flexible conductors, chemically specific sensors, bio-compatible interfaces, and deformable digital or analog signal processors. Stand alone ionic devices would need to have five key capabilities: signal transmission, energy conversion/harvesting, sensing, actuation, and signal processing. With the great promise of ionically-conducting materials and ionic devices, there are several fields working independently on pieces of the puzzle. These fields range from waste-water treatment research to soft robotics and bio-interface research. In this review, we first present the underlying physical principles that govern the behavior of soft ionic materials and devices. We then discuss the progress that has been made on each of the potential device components, bringing together findings from a range of research fields, and conclude with discussion of opportunities for future research.

Ionically conductive polymers are commonly made of monomers containing high polarity moieties to promote high ion dissociation, like poly(ethylene oxide) (PEO), polyvinylidene difluoride (PVDF), and poly(vinyl alcohol) (PVA). However, the glass transition temperatures (T g ) of these polymers are relatively high, and therefore, these polymers are in a glassy state at room temperature, which limits the mechanical flexibility of the material. Although polydimethylsiloxane (PDMS) has many attractive physical and chemical properties, including a low glass transition temperature, mechanical flexibility, and good biocompatibility, its low dielectric constant suppresses ion dissociation. In this Letter, we overcome this shortcoming by functionalizing the PDMS with ligands that can form labile coordination with metal ions, which greatly promotes ion dissociation and improves the ionic conductivity by orders of magnitude. By combining an experimental study with a fully atomistic molecular dynamics simulation, we systematically investigated the ion transport mechanisms in this low T g material.

Ionically conductive polymers are commonly made of monomers containing high polarity moieties to promote high ion dissociation, like poly(ethylene oxide) (PEO), polyvinylidene difluoride (PVDF), poly(vinyl alcohol) (PVA). However, the glass transition temperature ($T_g$) of these polymers are relatively high, and therefore yields a glassy state at room temperature and limits the mechanical flexibility of the material. Although polydimethylsiloxane (PDMS) has many attractive physical and chemical properties, including low glass transition temperature, mechanical flexibility, and good biocompatibility, its low dielectric constant suppresses ion dissociation. In this paper, we overcome this shortage by functionalizing the PDMS with ligands that can form labile coordination with metal ions, which greatly promotes the ion dissociation and improves the ionic conductivity by orders of magnitude. By combining an experimental study with a fully atomistic molecular dynamics simulation, we systematically investigated the ion transport mechanisms in this low $T_g$, low intrinsic conductivity material.