Printed flexible materials have garnered considerable attention as next-generation materials for bioelectronic applications, particularly hydrogels and elastomers, owing to their intrinsic softness, tissue-like mechanical compliance, and electrical conductivity. In contrast to conventional fabrication approaches, printing technologies enable precise spatial control, design versatility, and seamless integration with complex biological interfaces. This review provides a comprehensive overview of the progress in printable soft conductive materials, with a particular emphasis on the composition, processing, and functional roles of conductive hydrogels and elastomers. This review first introduces traditional fabrication methods for conductive materials and explains the motivation for using printing techniques. We then introduce two major classes of soft conductive materials, hydrogels and elastomers, and describe their applications in both in vitro systems, such as biosensors and soft stimulators, and in vivo settings, including neural interfaces and implantable devices. Finally, we discuss current challenges and propose future directions for advancing printed soft bioelectronics toward clinical translation.

A pneumatic soft gripper, composed of multiple elastomeric materials, was designed and manufactured based on gripping simulations. The simulation results demonstrated that the different stiffnesses of the elastomeric materials can influence the internal air pressure required for the proper actuation of the gripper. Considering the substrate properties and the morphological changes of the soft gripper during gripping, a piezoresistive force sensor was developed using elastomers and conductive filaments with the aid of additive manufacturing techniques. We confirmed the reproducibility and stability of the proposed piezoresistive force sensor through evaluations under various fabrication conditions. Results from touch experiments and compressive force measurements indicated that the stiffness of the sensor substrate and the thickness of the conductive part of the sensor affected the sensitivity and reliability of the sensor with respect to different levels of applied forces. Incorporating a rigid panel between the soft gripper and the piezoresistive force sensor diminished the effect of the variable stiffness and curvature of the gripper on the measurement of electrical resistance generated by the piezoresistive force sensor. Our 3D-printed sensor combined with elastomeric materials showed the possibility of differentiating the simple actuation and the gripping demonstration of the soft gripper.

An alternative conductive ink based on carbon nanotubes (CNTs) was developed using a platinum-catalyzed silicone elastomer and isopropyl alcohol (IPA). The inclusion of IPA in the conductive CNT ink facilitated the optimization of its mechanical strength, electrical conductivity, and viscosity. Compared to conventional silicone rubber-based conductive polymers that often solidify in a few hours at room temperature or with heating, this liquid composite of CNT particles and IPA exhibited a prolonged duration of up to several months in a hermetic environment, maintaining chemical stability even with the elastomer and its curing agent. The gradual evaporation of IPA initiates a well-known cross-linking process, leading to stretchability and electrical conductivity derived from the silicone elastomer and CNT particles, respectively. The relationship between the mechanical strength and electrical conductivity of the hardened conductive CNT ink was studied, which helped determine the optimized concentration of CNT particles in the conductive CNT ink. Subsequently, a piezoresistive sensor was designed, fabricated, and evaluated. The conductive CNT ink-based piezoresistive sensor showed high sensitivity and good repeatability with respect to a wide range of external forces. The effect of the concentration of CNT particles on the viscosity of the conductive CNT ink was also investigated, providing a better understanding of the entanglement of CNT particles within the silicone elastomer. A coating test using the conductive CNT ink with a paper cutting machine demonstrated its potential for adaptation to various printing techniques, including screen printing. The proposed conductive CNT ink, characterized by a simple chemical composition, facile fabrication process, use of non-toxic elements, high electrical conductivity, and stretchability, combined with an extended duration, has the potential to be applied for multiple purposes, such as various types of flexible and wearable electronics.