The human body is soft and complex in shape; consequently, electronic devices attached to the human body should be biocompatible, flexible, stretchable, and recoverable. Recently, ion gels (based on hydrogel and non-hydrogel) have attracted much attention as a material for human-electronics interaction. In particular, hydrogel is a promising material that satisfies the above requirements, and has been employed in various applications such as contact lenses,8 drug delivery, tissue engineering, and human motion. However, the mechanical and electrical properties can change drastically depending on the humidity and temperature because water between the polymer networks of the hydrogel can evaporate or be absorbed over time under dry or wet air conditions.

For small-scale focus-tunable optical applications, the all-solid-state polymer lens technology could allow lens operation without auxiliary actuation elements, and solve issues in established liquid-based lenses or better emulate the human eye. In tunable optical systems, the active polymer lenses are more robust against fluctuations in temperature, pressure, and motion as compared to liquid-based lenses. However, for application in tunable imaging devices, the shape-deformable membrane suffers from severe optical loss due to light absorption by the translucent compliant electrode, and the required stiffness of the electroactive dielectric elastomer restricts the shape deformation. Thus, these lenses have limited optical performance and are incapable of reversible, invertible, and multifocus-tunable operation. Soft smart materials are urgently needed to improve the transmittance and the focus tunability (focus-tunable level) in the optical path of self-contained adaptive lenses based on the electromechanical actuators. 

Electrochemical electronics have found diverse applications including energy storage devices  and batteries, sensors , and displays [for example, electrochemiluminescence (ECL) and electrochromic (EC) devices]. The use of typical liquid or solid electrolytes limits the range of device applications such as wearable electronics. For example, liquid electrolytes easily leak under applied strains. Hence, the realization of flexible/stretchable devices is difficult. In addition, ionic motion is severely hindered in solid electrolytes, leading to extremely low ionic conductivity even at room temperature. Therefore, their application in room-temperature electronics is limited. These issues have been addressed by the development of highly conductive and mechanically robust gel electrolytes, which are referred to as ion gels. Ion gels consisting of copolymers and ionic liquids are nonvolatile even at high temperature or under reduced pressure. Their properties are easily tunable by modifying the chemical structures of the constituents or adjusting the gel composition. Moreover, the introduction of ECL luminophores or EC chromophores into the gels yields functional ion gels, and their utilization extends the application spectrum to flexible/stretchable ECL and EC displays.