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HighlightsUltrathin, lightweight, and flexible carbon nanotube buckypaper enhanced using MXenes (Ti3C2Tx) for high-performance electromagnetic interference shielding is synthesized through facile electrophoretic deposition.The obtained Ti3C2Tx@CNT hybrid buckypaper demonstrates outstanding EMI shielding effectiveness of 60.5 dB in the X-band at 100 μm and a specific SE value of 5.7 × 104 dB cm2 g−1 at 5 μm.Lightweight, flexibility, and low thickness are urgent requirements for next-generation high-performance electromagnetic interference (EMI) shielding materials for catering to the demand for smart and wearable electronic devices. Although several efforts have focused on constructing porous and flexible conductive films or aerogels, few studies have achieved a balance in terms of density, thickness, flexibility, and EMI shielding effectiveness (SE). Herein, an ultrathin, lightweight, and flexible carbon nanotube (CNT) buckypaper enhanced using MXenes (Ti3C2Tx) for high-performance EMI shielding is synthesized through a facile electrophoretic deposition process. The obtained Ti3C2Tx@CNT hybrid buckypaper exhibits an outstanding EMI SE of 60.5 dB in the X-band at 100 μm. The hybrid buckypaper with an MXene content of 49.4 wt% exhibits an EMI SE of 50.4 dB in the X-band with a thickness of only 15 μm, which is 105% higher than that of pristine CNT buckypaper. Furthermore, an average specific SE value of 5.7 × 104 dB cm2 g−1 is exhibited in the 5-μm hybrid buckypaper. Thus, this assembly process proves promising for the construction of ultrathin, flexible, and high-performance EMI shielding films for application in electronic devices and wireless communications.

Nanomaterials with low-dimensional morphology have been explored for enhancing the performance of strain sensors, but it remains difficult to achieve high stretchability and sensitivity simultaneously. In this work, a composite structure strain sensor based on nanomaterials and conductive liquid is designed, demonstrated, and engineered. The nanowire-microfluidic hybrid (NMH) strain sensor responds to multiscale strains from 4% to over 400%, with a high sensitivity and durability under small strain. Metal nanowires and carbon nanotubes are used to fabricate the NMH strain sensors, which simultaneously exhibit record-high average gauge factors and stretchability, far better than the conventional nanowire devices. Quantitative modeling of the electrical characteristics reveals that the effective conductivity percolation through the hybrid structures is the key to achieving high gauge factors for multiscale sensing. The sensors can operate at low voltages and are capable of responding to various mechanical deformations. When fixed on human skin, the sensors can monitor large-scale deformations (skeleton motion) and small-scale deformations (facial expressions and pulses). The sensors are also employed in multichannel, interactive electronic system for wireless control of robotics. Such demonstrations indicate the potential of the sensors as wearable detectors for human motion or as bionic ligaments in soft robotics. Nanowire-microfluidic strain sensors: a stretchable multi-scale sensing solution By combining metal nanowires and conductive polymers, high-performance stretchable nanowire-microfluidic strain sensors are realized. A team lead by Chuan Liu from the School of Electronics and Information Technology at Sun Yat-Sen University developed a hybrid strain sensor consisting of brittle metal nanowires and conductive polymers. These robust nanowire-microfluidic strain sensors are sensitive to multiscale strains––from 4% to over 400%––and show record-high gauge factor, a figure-of-merit that quantifies the level of sensitivity. The hybrid strain sensor’s high performance is made possible by the electric percolation pathways formed between the parallel nanowire network and the microfluidic channels. The combination of high stretchability and high sensitivity over a large strain range enables the device to be suitable for multiscale sensing. Liu and coworkers demonstrate the applicability of their nanowire-microfluidic strain sensors to human motion detection and human–machine interactive systems.

HighlightsThe common performance optimization strategies of smart composite hydrogel are summarized.The recent advanced progress of smart composite hydrogel-based wearable sensors is systematically discussed from the aspect of health monitoring.The current challenges and future prospects of smart composite hydrogel-based wearable sensors are presented.Growing health awareness triggers the public’s concern about health problems. People want a timely and comprehensive picture of their condition without frequent trips to the hospital for costly and cumbersome general check-ups. The wearable technique provides a continuous measurement method for health monitoring by tracking a person’s physiological data and analyzing it locally or remotely. During the health monitoring process, different kinds of sensors convert physiological signals into electrical or optical signals that can be recorded and transmitted, consequently playing a crucial role in wearable techniques. Wearable application scenarios usually require sensors to possess excellent flexibility and stretchability. Thus, designing flexible and stretchable sensors with reliable performance is the key to wearable technology. Smart composite hydrogels, which have tunable electrical properties, mechanical properties, biocompatibility, and multi-stimulus sensitivity, are one of the best sensitive materials for wearable health monitoring. This review summarizes the common synthetic and performance optimization strategies of smart composite hydrogels and focuses on the current application of smart composite hydrogels in the field of wearable health monitoring.

HighlightsA general strategy of fabricating thickness-controllable hydrogel films with diverse polymer networks for humidity sensing is proposed. The hydrogel film humidity sensor displays an unprecedented sensitivity up to 13,462.1%/%RH.The stretchable humidity sensor can work normally under large tensile strains with even enhanced sensitivity, which makes the humidity responding property of hydrogel mechanically programmable.A wireless respiratory interruption detection mask with Bluetooth transmission function is developed based on the sensor, enabling the real-time, wireless and accurate acquisition of user's breathing state, such as the interval and breathing frequency and apnea.Respiratory monitoring plays a pivotal role in health assessment and provides an important application prospect for flexible humidity sensors. However, traditional humidity sensors suffer from a trade-off between deformability, sensitivity, and transparency, and thus the development of high-performance, stretchable, and low-cost humidity sensors is urgently needed as wearable electronics. Here, ultrasensitive, highly deformable, and transparent humidity sensors are fabricated based on cost-effective polyacrylamide-based double network hydrogels. Concomitantly, a general method for preparing hydrogel films with controllable thickness is proposed to boost the sensitivity of hydrogel-based sensors due to the extensively increased specific surface area, which can be applied to different polymer networks and facilitate the development of flexible integrated electronics. In addition, sustainable tapioca rich in hydrophilic polar groups is introduced for the first time as a second cross-linked network, exhibiting excellent water adsorption capacity. Through the synergistic optimization of structure and composition, the obtained hydrogel film exhibits an ultrahigh sensitivity of 13,462.1%/%RH, which is unprecedented. Moreover, the hydrogel film-based sensor exhibits excellent repeatability and the ability to work normally under stretching with even enhanced sensitivity. As a proof of concept, we integrate the stretchable sensor with a specially designed wireless circuit and mask to fabricate a wireless respiratory interruption detection system with Bluetooth transmission, enabling real-time monitoring of human health status. This work provides a general strategy to construct high-performance, stretchable, and miniaturized hydrogel-based sensors as next-generation wearable devices for real-time monitoring of various physiological signals.

3D printing has attracted a lot of attention in recent years. Over the past three decades, various 3D printing technologies have been developed including photopolymerization-based, materials extrusion-based, sheet lamination-based, binder jetting-based, power bed fusion-based and direct energy deposition-based processes. 3D printing offers unparalleled flexibility and simplicity in the fabrication of highly complex 3D objects. Tactile sensors that emulate human tactile perceptions are used to translate mechanical signals such as force, pressure, strain, shear, torsion, bend, vibration, etc. into electrical signals and play a crucial role toward the realization of wearable electronics and electronic skin. To date, many types of 3D printing technologies have been applied in the manufacturing of various types of tactile sensors including piezoresistive, capacitive and piezoelectric sensors. This review attempts to summarize the current state-of-the-art 3D printing technologies and their applications in tactile sensors for wearable electronics and electronic skin. The applications are categorized into five aspects: 3D-printed molds for microstructuring substrate, electrodes and sensing element; 3D-printed flexible sensor substrate and sensor body for tactile sensors; 3D-printed sensing element; 3D-printed flexible and stretchable electrodes for tactile sensors; and fully 3D-printed tactile sensors. Latest advances in the fabrication of tactile sensors by 3D printing are reviewed and the advantages and limitations of various 3D printing technologies and printable materials are discussed. Finally, future development of 3D-printed tactile sensors is discussed.

HighlightsA transparent, conductive, and flexible metal mesh film has been developed by a low-cost, uniform self-forming crackle template and electroplating strategy.The Cu mesh films show an ultra-low sheet resistance (0.18 Ω □−1), high transmittance (85.8%@550 nm), high figure of merit (> 13,000), excellent stretchability and mechanical stability.The metal mesh film can be used as a flexible heater and electromagnetic interference shielding film (40.4 dB at 2.5 μm).Despite the growing demand for transparent conductive films in smart and wearable electronics for electromagnetic interference (EMI) shielding, achieving a flexible EMI shielding film, while maintaining a high transmittance remains a significant challenge. Herein, a flexible, transparent, and conductive copper (Cu) metal mesh film for EMI shielding is fabricated by self-forming crackle template method and electroplating technique. The Cu mesh film shows an ultra-low sheet resistance (0.18 Ω □−1), high transmittance (85.8%@550 nm), and ultra-high figure of merit (> 13,000). It also has satisfactory stretchability and mechanical stability, with a resistance increases of only 1.3% after 1,000 bending cycles. As a stretchable heater (ε > 30%), the saturation temperature of the film can reach over 110 °C within 60 s at 1.00 V applied voltage. Moreover, the metal mesh film exhibits outstanding average EMI shielding effectiveness of 40.4 dB in the X-band at the thickness of 2.5 μm. As a demonstration, it is used as a transparent window for shielding the wireless communication electromagnetic waves. Therefore, the flexible and transparent conductive Cu mesh film proposed in this work provides a promising candidate for the next-generation EMI shielding applications.

Stretchable, self‐healing, and breathable skin‐biomimetic‐sensing iontronics play an important role in human physiological signal monitoring and human–computer interaction. However, previous studies have focused on the mimicking of skin tactile sensing (pressure, strain, and temperature), and the development of more functionalities is necessary. To this end, a superior humidity‐sensitive ionic skin is developed based on a self‐healing, stretchable, breathable, and biocompatible polyvinyl alcohol–cellulose nanofibers organohydrogel film, showing a pronounced thickness‐dependent humidity‐sensing performance. The as‐prepared 62.47‐μm‐thick organohydrogel film exhibits a high response (25,000%) to 98% RH, excellent repeatability, and long‐term stability (120 days). Moreover, this ionic skin has excellent resistance to large mechanical deformation and damage, and the worn‐out material can still retain its humidity‐sensing capabilities after self‐repair. Humidity‐sensing mechanism studies show that the induced response is mainly related to the increase of proton mobility and interfacial charge transport efficiency after water adsorption. The superior humidity responsiveness is attributed to the reduced thickness and the increased specific surface area of the organohydrogel film, allowing real‐time recording of physiological signals. Notably, by combining with a self‐designed printed circuit board, a continuous and wireless respiration monitoring system is developed, presenting its great potential in wearable and biomedical electronics. Stretchable, self‐healable, and breathable biomimetic ionic skin with superior humidity‐sensing performance is developed based on fully physical cross‐linked organohydrogel film, which features a superior responsiveness, excellent repeatability, and long‐term stability. Particularly, it can be used for real‐time and remote respiration monitoring and to implement a breathing interruption alarm system for sleep apnea syndrome patients after combining with a specially designed circuit and mask, showing its great potential in wearable and biomedical electronics.

HighlightsA stretchable, transparent, and ultra-broadband (0.1–10 THz) terahertz shielding MXene (Ti3C2Tx) film has been fabricated by a structure engineering strategy.Theoretical calculations indicate that the wrinkled structure enhances the film's conductivity and surface plasmon resonances, resulting in an improved THz wave absorption.The wrinkled MXene films exhibit superb conformability to surfaces with random curvatures, and can be used as a shielding film for THz imaging.With the increasing demand for terahertz (THz) technology in security inspection, medical imaging, and flexible electronics, there is a significant need for stretchable and transparent THz electromagnetic interference (EMI) shielding materials. Existing EMI shielding materials, like opaque metals and carbon-based films, face challenges in achieving both high transparency and high shielding efficiency (SE). Here, a wrinkled structure strategy was proposed to construct ultra-thin, stretchable, and transparent terahertz shielding MXene films, which possesses both isotropous wrinkles (height about 50 nm) and periodic wrinkles (height about 500 nm). Compared to flat film, the wrinkled MXene film (8 nm) demonstrates a remarkable 36.5% increase in SE within the THz band. The wrinkled MXene film exhibits an EMI SE of 21.1 dB at the thickness of 100 nm, and an average EMI SE/t of 700 dB μm−1 over the 0.1–10 THz. Theoretical calculations suggest that the wrinkled structure enhances the film's conductivity and surface plasmon resonances, resulting in an improved THz wave absorption. Additionally, the wrinkled structure enhances the MXene films' stretchability and stability. After bending and stretching (at 30% strain) cycles, the average THz transmittance of the wrinkled film is only 0.5% and 2.4%, respectively. The outstanding performances of the wrinkled MXene film make it a promising THz electromagnetic shielding materials for future smart windows and wearable electronics.

Flexible and wearable sensors have broad application prospects in health monitoring and artificial intelligence. Many different single-functional sensing devices have been developed in recent years, such as pressure sensors and temperature sensors. However, it is still a great challenge to design and fabricate tactile sensors with multiple sensing functions. Herein, we propose a simple direct stamping method for the fabrication of multifunctional tactile sensors. It can detect pressure and temperature stimuli signals simultaneously. This pressure/temperature sensor possesses high sensitivity (0.67 kPa −1 ), large linear range (0.75–5 kPa), and fast response speed (15.6 ms) in pressure sensing. It also has a high temperature sensitivity (1.41%/°C) and great linearity (0.99) for temperature sensing in the range of −30 to 30 °C. All these excellent performances indicate that this pressure/temperature sensor has great potential in applications for artificial intelligence and health monitoring.

Highlights A highly aligned and compact MXene film for ultra‑broadband and ultra-high electromagnetic interference shielding was fabricated by continuous centrifugal spraying strategy. The film exhibits an exceptional electromagnetic interference shielding effectiveness (EMI SE) of 45 dB, specific shielding effectiveness of 1.545 × 10 6  dB cm 2  g⁻ 1 in gigahertz band (8.2–40 GHz), and EMI SE of 59 dB in terahertz band (0.2–1.6 THz). The film exhibits an ultra-low infrared emissivity of 0.1 in the infrared band (2.5–16.0 μm) and an efficient electrothermal performance. With the rapid development of electronic detective techniques, there is an urgent need for broadband (from microwave to infrared) stealth of aerospace equipment. However, achieving effective broadband stealth primarily relies on the composite of multi-layer coatings of different materials, while realizing broadband stealth with a single material remains a significant challenge. Herein, we reported a highly compact MXene film with aligned nanosheets through a continuous centrifugal spraying strategy. The film exhibits an exceptional electromagnetic interference shielding effectiveness of 45 dB in gigahertz band (8.2–40 GHz) and 59 dB in terahertz band (0.2–1.6 THz) at a thickness of 2.25 μm, owing to the high conductivity (1.03 × 10 6 S m −1 ). Moreover, exceptionally high specific shielding effectiveness of 1.545 × 10 6  dB cm 2  g⁻ 1 has been demonstrated by the film, which is the highest value reported for shielding films. Additionally, the film exhibits an ultra-low infrared emissivity of 0.1 in the wide-range infrared band (2.5–16.0 μm), indicating its excellent infrared stealth performance for day-/nighttime outdoor environments. Moreover, the film demonstrates efficient electrothermal performance, including a high saturated temperature (over 120 °C at 1.0 V), a high heating rate (4.4 °C s −1 at 1.0 V), and a stable and uniform heating distribution. Therefore, this work provides a promising strategy for protecting equipment from multispectral electromagnetic interference and inhibiting infrared detection.