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Multi‐layer electrical interconnects are critical for the development of integrated soft wearable electronic systems, in which functional devices from different layers need to be connected together by vertical interconnects. In this work, electrohydrodynamic (EHD) printing technology is studied to achieve multi‐layer flexible and stretchable electronics by direct printing vertical interconnects as vertical interconnect accesses (VIAs) using a low‐melting‐point metal alloy. The EHD printed metallic vertical interconnection represents a promising way for the direct fabrication of multilayer integrated electronics with metallic conductivity and excellent flexibility and stretchability. By controlling the printing conditions, vertical interconnects that can bridge different heights can be fabricated. To achieve reliable VIA connections under bending and stretching conditions, an epoxy protective structure is printed around the VIA interconnects to form a core‐shell structure. A stable electrical response is achieved under hundreds of bending cycles and during stretching/releasing cycles in a large range of tensile strain (0–40%) for the printed conductors with VIA interconnects. A few multi‐layer devices, including a multiple layer heater, and a pressure‐based touch panel are fabricated to demonstrate the capability of the EHD printing for the direct fabrication of vertical metallic VIA interconnects for flexible and stretchable devices.

 

Nanomaterial‐enabled flexible and stretchable electronics have seen tremendous progress in recent years, evolving from single sensors to integrated sensing systems. Compared with nanomaterial‐enabled sensors with a single function, integration of multiple sensors is conducive to comprehensive monitoring of personal health and environment, intelligent human–machine interfaces, and realistic imitation of human skin in robotics and prosthetics. Integration of sensors with other functional components promotes real‐world applications of the sensing systems. Here, an overview of the design and integration strategies and manufacturing techniques for such sensing systems is given. Then, representative nanomaterial‐enabled flexible and stretchable sensing systems are presented. Following that, representative applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human–machine interactions are provided. To conclude, perspectives on the challenges and opportunities in this burgeoning field are considered.

 

Printed electronics is attracting a great deal of attention in both research and commercialization as it enables fabrication of large‐scale, low‐cost electronic devices on a variety of substrates. Printed electronics plays a critical role in facilitating widespread flexible electronics and more recently stretchable electronics. Conductive nanomaterials, such as metal nanoparticles and nanowires, carbon nanotubes, and graphene, are promising building blocks for printed electronics. Nanomaterial‐based printing technologies, formulation of printable inks, post‐printing treatment, and integration of functional devices have progressed substantially in the recent years. This review summarizes basic principles and recent development of common printing technologies, formulations of printable inks based on conductive nanomaterials, deposition of conductive inks via different printing techniques, and performance enhancement by using various sintering methods. While this review places emphasis on conductive nanomaterials, the printing techniques and ink formulations can be applied to other materials such as semiconducting and insulating nanomaterials. Moreover, some applications of printed flexible and stretchable electronic devices are reviewed to illustrate their potential. Finally, the future challenges and prospects for printing conductive nanomaterials are discussed.

 

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.