Hydrogel actuators, comprising gels that convert external stimuli into mechanical motion for actuation, are attracting attention for their promising applications, such as in robotics. The driving force is the absorption or release of water or another solvent, which results in swelling and shrinking motions, leading in turn to more complex functionalities. However, practical hydrogel actuators that can be controlled locally, such as ones that allow local actuation around the joints in rigid‐bodied robots, do not exist. Herein, the driving target of a thermo‐responsive hydrogel, poly(N‐isopropyl acrylamide), is integrated with the stimulation module using a liquid metal. The stimulation module provides heat as an external stimulus to the hydrogel actuator. The motion of the actuator is triggered by the heat supplied by an ultrasoft hydrogel coil, with liquid metal surrounding the driving target. The heat generated by current flowing through the liquid metal changes the temperature only around the desired part of the actuator, which enables the electrical control of an individual part of the hydrogel actuator. The concept of integrating the driving target and stimulator is expected to facilitate functional movement of actuators and expand the range of potential applications of hydrogels.

In recent years, wiring and system construction on ultrasoft materials such as biological tissues and hydrogels have been proposed for advanced wearable devices, implantable devices, and soft robotics. Among the soft conductive materials, Ga-based liquid metals (LMs) are both biocompatible and ultrasoft, making them a good match for electrodes on the ultrasoft substrates. However, gels and tissues are softer and less wettable to the LMs than conventional soft substrates such as Ecoflex and polydimethylsiloxane. In this study, we demonstrated the transfer of LM paste composed of Ga-based LM and Ni nanoparticles onto ultrasoft substrates such as biological tissue and gels using sacrificial polyvinyl alcohol (PVA) films. The LM paste pattern fabricated on the PVA film adhered to the ultrasoft substrate along surface irregularities and was transferred without being destroyed by the PVA film before the PVA's dissolution in water. The minimum line width that could be wired was approximately 165 μm. Three-dimensional wiring, such as the helical structure on the gel fiber surface, is also possible. Application of this transfer method to tissues using LM paste wiring allowed the successful stimulation of the vagus nerve in rats. In addition, we succeeded in transferring a temperature measurement system fabricated on a PVA film onto the gel. The connection between the solid-state electrical element and the LM paste was stable and maintained the functionality of the temperature-sensing system. This fundamental study of wiring fabrication and system integration can contribute to the development of advanced electric devices based on ultrasoft substrates.

In this study, a highly transformable electrocardiograph that can considerably deform the position of stretchable electrodes based on the lead method for diagnosing heart disease was developed; these electrodes exhibited high resistance stability against considerable stretching and multiple stretching. To realize the large deformable functionality of the electrodes of a system, liquid metal electrodes and a heteroconnector composed of a liquid metal paste and carbon-based conductive rubber were employed. The developed device can achieve a 200% strain with only 6% resistance change and a high stability of resistances after the 100-time stretching test. In addition, the study demonstrated electrocardiograms in different lead methods of adult and child using the same device. The proposed combination of large deformable electrodes with high electric stability and a robust heteroconnector is an important technology, and it presents a considerable advancement in the application of stretchable electronic systems.

Hydrogel actuators are ultrasoft and pliable but achieving high driving speeds with large deformation and fine local controllability is difficult because the driving force originates from the external air pressure or heat, and the base material is fragile. Herein, hydrogel actuators that allow high‐speed driving and large deformation with high‐frequency local controllability while maintaining softness are fabricated based on liquid metal gel fibers as electrodes by using microfluidic technology. The Lorentz force produced by an electric current and a magnet is used for actuation control. An ultrafast response of 260.5 mm s−1 with high‐frequency controllability (6 Hz) and a large deformation of 172% with hydrogel actuation are observed. As proof of concept, moving stages, micromixers, and grippers exhibiting high speeds with high mechanical deformability while maintaining the inherent characteristics of hydrogel phases are demonstrated. Different hydrogels can be used with the proposed actuator architecture and fabrication scheme, enabling different functionalities.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.201900794.Many micro total analysis systems (microTAS) and lab-on-achip applications have been developed that provide various functions such as mixing, [1,2] heating, [3,4] separation, [5][6][7] and extraction. [8,9] These applications are crucial to point-of-care testing (POCT), especially in developing countries. [10] These