DOI: 10.1002/admt.201800628 applications (e.g., soft robotics, medical devices).[1–6] Despite state-of-the-art bulkbased planar integrated-circuit devices, their rigid and brittle nature gives rise to the incompatibility with curvilinear and soft human bodies, restricting the development of newborn human-friendly interactive electronics. In contrast, the bendable and flexible wearable electronics could be conformally attached onto human bodies almost without discomfort and succeed in performing a great deal of sensing functionalities. Realization of such promising goals requires the flexible sensor platforms provided with crucial characteristics of light weight, ultrathinness, superior flexibility, stretchability, high sensitivity as well as rapid response.[7–10] Inspired by the perceptive features of human skins, the wearable sensor systems are capable of acquiring abundant information from the external environment with the assistance of sensing modules, such as pressure sensors, strain sensors, temperature sensors, etc.[11] A typical example is their application in prosthetics that could afford the capacity to perceive touch or temperature for the disabled.[12] Additionally, the wearable sensor systems are able to identify physical or chemical signals produced by the human body, providing promising opportunities to evaluate health states.[5,13–15] Conventional skin-like sensor platforms primarily comprise one or two sensing modules, data processing units, and power supplies. Their unitary functionality, however, cannot satisfy the increasing demands of IoTs. Recently, the rapid advances in novel sensing materials, fabrication strategies, and innovative electronic constitution contribute to the development of versatile integration of multimodal sensors, which could synchronously distinguish diverse stimuli from the complex environment and monitor multiple vital signs from the human body.[16,17] In spite of several attempts done in terms of such multimodal sensor systems, one of the cumbersome issues originates from the crosscoupling effect among different categories of signals simultaneously generated by various sensors. Furthermore, the skin-like multiple sensor systems usually suffer from the limited number of repeated use, resulting in their high use-cost. The development of separable versatile devices may address this issue with one layer realized by costeffective materials and fabrication manners for disposable use and the other composed of relatively expensive components for repeatable applications.[18] Additionally, the multiple bending or Skin-inspired wearable devices hold great potentials in the next generation of smart portable electronics owing to their intriguing applications in healthcare monitoring, soft robotics, artificial intelligence, and human–machine interfaces. Despite tremendous research efforts dedicated to judiciously tailoring wearable devices in terms of their thickness, portability, flexibility, bendability as well as stretchability, the emerging Internet of Things demand the skininterfaced flexible systems to be endowed with additional functionalities with the capability of mimicking skin-like perception and beyond. This review covers and highlights the latest advances of burgeoning multifunctional wearable electronics, primarily including versatile multimodal sensor systems, self-healing material-based devices, and self-powered flexible sensors. To render the penetration of human-interactive devices into global markets and households, economical manufacturing techniques are crucial to achieve large-scale flexible systems with high-throughput capability. The booming innovations in this research field will push the scientific community forward and benefit human beings in the near future.

Multifunctional Skin-Inspired Flexible Sensor Systems for Wearable Electronics

Recent advances in the design and implementation of wearable resistive, capacitive, and optical strain sensors are summarized herein. Wearable and stretchable strain sensors have received extensive research interest due to their applications in personalized healthcare, human motion detection, human–machine interfaces, soft robotics, and beyond. The disconnection of overlapped nanomaterials, reversible opening/closing of microcracks in sensing films, and alteration of the tunneling resistance have been successfully adopted to develop high‐performance resistive‐type sensors. On the other hand, the sensing behavior of capacitive‐type and optical strain sensors is largely governed by their geometrical changes under stretching/releasing cycles. The sensor design parameters, including stretchability, sensitivity, linearity, hysteresis, and dynamic durability, are comprehensively discussed. Finally, the promising applications of wearable strain sensors are highlighted in detail. Although considerable progress has been made so far, wearable strain sensors are still in their prototype stage, and several challenges in the manufacturing of integrated and multifunctional strain sensors should be yet tackled.

Wearable and Stretchable Strain Sensors: Materials, Sensing Mechanisms, and Applications

The authors acknowledge generous support of the King Abdullah University of Science and Technology (KAUST). The authors acknowledge Dr. Joanna M. Nassar, Dr. Galo A. Torres, Dr. Mohamed T. Ghoneim, Andres A. Aguirre-Pablo, Davide Priante, Dr. Jhonathan P. Rojas, Sigurdur T. Thoroddsen, and Prof. Boon S. Ooi who contributed to the “pause-embed-resume” data. The authors thank Kelly Rader for proof reading this manuscript.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/aisy.202000128

Soft Actuators for Soft Robotic Applications: A Review

Highly Stretchable and Self-Healable MXene/Polyvinyl Alcohol Hydrogel Electrode for Wearable Capacitive Electronic Skin

Conductive hydrogels can be prepared by incorporating various conductive materials into polymeric network hydrogels. In recent years, conductive hydrogels have been developed and applied in the field of strain sensors owing to their unique properties, such as electrical conductivity, mechanical properties, self-healing, and anti-freezing properties. These remarkable properties allow conductive hydrogel-based strain sensors to show excellent performance for identifying external stimuli and detecting human body movement, even at subzero temperatures. This review summarizes the properties of conductive hydrogels and their application in the fabrication of strain sensors working in different modes. Finally, a brief prospectus for the development of conductive hydrogels in the future is provided.

Development of Conductive Hydrogels for Fabricating Flexible Strain Sensors.

Stretchable strain sensors with high sensitivity or gauge factor (GF), large stretchability, and long-term durability are highly demanded in human motion detection, artificial intelligence, and electronic skins. Nevertheless, to develop high-sensitive sensors without sacrificing the stretchability cannot be realized using simple device configurations. In this work, an acid-interface engineering (AIE) method was proposed to develop a stretchable strain sensor with high GF and large stretchability. The AIE generates a layer of SiO x at the interface between the carbon nanotube (CNT) film and Ecoflex, playing a key role in enhancing the sensor's GF. Compared to devices without AIE (GF = 2.4), the ones with AIE are significantly improved. At an AIE time of 10 min, the GF up to 1665.9 is achieved without sacrificing the stretchability (>100%). The AIE-generated cracks are found to modulate the electrical behaviors and enhance the GFs of sensors with AIE through the crack-induced rapid reduction in the electrical conduction pathway, which is manipulated by the CNTs bridging over the cracks. The device with AIE proves its high mechanical durability through a cycling test (>10 000 cycles) at a high strain up to ∼80%, further paving its practical applications in various human motion detections.

Acid-Interface Engineering of Carbon Nanotube/Elastomers with Enhanced Sensitivity for Stretchable Strain Sensors.

In this paper, the selective laser melting (SLM) forming method was combined with the requirement of boiling heat transfer for porous structure to study the forming process and boiling heat transfer characteristics of porous structure of G-surface lattice formed by SLM. 316L stainless steel powder was used to rasterize the G-surface structure. Besides, porosity and surface morphology of G-surface lattice were analyzed. Deionized water was used for boiling heat transfer experiment to study the effect of technological parameters on boiling heat transfer enhancement. On this basis, the bubble escape modes in different stages of boiling heat transfer of G-surface lattice were considered, and the delay of the critical heat flux caused by G-surface lattice was explained. This study provides a useful reference for the design of lattice heat cooling structure.

/pdf/selective-laser-melting-of-g-surface-lattice-forming-process-1v1qeb1zzq.pdf

Selective laser melting of G-surface lattice: forming process and boiling heat transfer characteristics

The additive manufacturing technology of laser-based powder bed fusion (L-PBF), which is used to produce boiling heat transfer structures, offers a high processing flexibility and can provide lattice structures with a high surface-to-volume ratio. As an important part of the phased array radar, the plentiful transmit/receive (T/R) modules can generate considerable heat. Targeting this local overheating problem, this study discusses the pass design, the optimal formation process, and boiling heat transfer performance of microchannel liquid-cooled plates based on L-PBF additive manufacturing technology. The optimum design and process parameters were obtained by performing basic channel experiments. On this basis, the design and formation experiments of the microchannel structure were performed, and then the porosity and pore morphology of microchannel liquid-cooled plate samples were analysed. The boiling heat transfer experiments were conducted with deionised water, and the boiling heat transfer characteristics were compared with the saturated boiling curve of a traditional copper-tube liquid-cooled plate. The average wall temperature of the designed samples decreased by 4% compared with that of the traditional liquid-cooled plate under the same heat flow density the value reduced from 111.9 °C to 108.2 °C. Furthermore, within the same optimal boiling temperature range, the average heat flow densities of all the prepared samples increased by >60% compared with those of the traditional liquid-cooled plate the value increased from minimum 16 W∙cm−2 to maximum 34 W∙cm−2. The self-supporting microchannel structure can considerably improve the heat dissipation effect of T/R modules and solve the local overheating problem.

Self-Supporting Microchannel Liquid-Cooled Plate for T/R Modules Based on Additive Manufacturing: Study on Its Pass Design, Formation Process and Boiling Heat Transfer Performance

/pdf/retraction-note-selective-laser-melting-of-g-surface-lattice-1nu5ga4eau.pdf

Retraction Note: Selective laser melting of G-surface lattice: forming process and boiling heat transfer characteristics (Journal of Nanoparticle Research, (2020), 22, 7, (178), 10.1007/s11051-020-04914-7)

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Acid-Interface Engineering of Carbon Nanotube/Elastomers with Enhanced Sensitivity for Stretchable Strain Sensors.

Selective laser melting of G-surface lattice: forming process and boiling heat transfer characteristics

Self-Supporting Microchannel Liquid-Cooled Plate for T/R Modules Based on Additive Manufacturing: Study on Its Pass Design, Formation Process and Boiling Heat Transfer Performance

Retraction Note: Selective laser melting of G-surface lattice: forming process and boiling heat transfer characteristics (Journal of Nanoparticle Research, (2020), 22, 7, (178), 10.1007/s11051-020-04914-7)