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This work reports an approach to print complex patterns of metal nanowires on curvilinear substrates with high conductivity. 

Abstract Soft electrothermal actuators have drawn extensive attention in recent years for their promising applications in biomimetic and biomedical areas. Most soft electrothermal actuators reported so far demonstrated uniform bending deformation, due to the deposition based fabrication of the conductive heater layer from nanomaterial-based solutions, which generally provides uniform heating capacity and uniform bending deformation. In this paper, a soft electrothermal actuator that can provide twisting deformation was designed and fabricated. A metallic microfilament heater of the soft twisting actuator was directly printed using electrohydrodynamic (EHD) printing, and embedded between two structural layers, a polyimide film and a polydimethylsiloxane layer, with distinct thermal expansion properties. Assisted by the direct patterning capabilities of EHD printing, a skewed heater pattern was designed and printed. This skewed heater pattern not only produces a skewed parallelogram-shaped temperature field, but also changes the stiffness anisotropy of the actuator, leading to twisting deformation with coupled bending. A theoretical kinematic model was built for the twisting actuator to describe its twisting deformation under different actuation effects. Based on that model, influence of design parameters on the twisting angle and motion trajectory of the twisting actuator were studied and validated by experiments. Finite element analysis was utilized for the thermal and deformation analysis of the actuator. The fabricated twisting actuator was characterized on its heating and twisting performance at different supply voltages. Using three twisting actuators, a soft gripper was designed and fabricated to implement pick-and-place operations of delicate objects. 

In recent years, there has been an increasing interest in the research in soft actuators that exhibit complex programmable deformations. Soft electrothermal actuators use electricity as the stimulus to generate heat, and the mismatch between the thermal expansions of the two structural layers causes the actuator to bend. Complex programmable deformations of soft electrothermal actuators are difficult due to the limitations of the conventional fabrication methods. In this article, we report a new approach to fabricate soft electrothermal actuators, in which the resistive heater of the electrothermal actuator is directly printed using electrohydrodynamic (EHD) printing. The direct patterning capabilities of EHD printing allow the free-form design of the heater. By changing the design of the heating pattern on the actuator, different heat distributions can be achieved and utilized to realize complex programmable deformations, including uniform bending, customized bending, folding, and twisting. Finite element analysis (FEA) was used to validate the thermal distribution and deformation for different actuator designs. Lastly, several integrated demonstrations are presented, including complex structures obtained from folding, a two-degree-of-freedom soft robotic arm, and soft walkers.