Multifunctional soft machines based on stimuli-responsive hydrogels: from freestanding hydrogels to smart integrated systems

Hydrogels possess exceptional physical and chemical properties that render them appealing components for soft actuators, wearable technologies, healthcare devices, and human interactive robots. Especially, the stimuli-responsive hydrogels can sense and perform smart functions in the presence of various stimuli that collectively contribute to the intelligence of the soft machine systems. Furthermore, facile modification of hydrogels with other functional groups/additives/nanofillers substantially expands their functionalities and further broadens the scope of their application. Designing suitable hydrogels with adequate capabilities and engineering effective configurations are of supreme importance for the development of advanced hydrogel-based soft machines. Herein, this review summarizes recent advances of stimuli-responsive hydrogels in multifunctional soft machines, such as robotics, actuators, and sensors. Functions including multistimuli responsiveness, self-healing, and high biocompatibility can be endowed to the soft machines through designing advanced hydrogel materials, which would not be possible with an approach based on conventional elastic materials (e.g. rubbers, elastomers). To close, future opportunities and challenges this field faces are emphasized and discussed for the development of exciting new hydrogel-based devices in real-world conditions.