Flexible and skin-wearable triboelectric nanogenerators (TENGs) have emerged as promising candida... more Flexible and skin-wearable triboelectric nanogenerators (TENGs) have emerged as promising candidates for selfpowered tactile and pressure sensors and mechanical energy harvesters due to their compatible design and ability to operate at low frequencies. Most research has focused on improving tribo-negative materials for flexible TENGs, given the limited options for tribo-positive materials. Achieving biocompatibility while maintaining the sensitivity and capability of energy harvesting is another critical issue for wearable sensors. Here, we report a TENG-based biocompatible and self-powered pressure sensor by simple fabrication of layer-by-layer deposition methods. The Laminated Flexible-TENG comprises polytetrafluoroethylene (PTFE) and polymethyl methacrylate (PMMA) films embedded within a flexible and biocompatible polydimethylsiloxane (PDMS) matrix. A nanostructured PDMS surface obtained by oxygen plasma facilitated the sputter deposition of a layered indium tin oxide copper electrode and a tribo-positive PMMA thin layer on top. The addition of the indium tin oxide layer to copper significantly improved the quality and performance of the indium tin oxide-copper electrode. Self-powered Laminated Flexible-TENGs demonstrated impressive pressure-sensing capabilities, featuring dual sensitivity of 7.287 V/kPa for low pressure and 0.663 V/kPa for higher pressure. Moreover, the PDMS-encapsulated TENG sensor effectively traced the physiological motions, such as wrist and finger bending, and efficiently harnessed the waste energy from everyday physical activities, such as walking and jogging. The maximum peak-to-peak voltages of 18.3 and 57.4 V were recorded during these motions. Encapsulated TENGs have broad potential in wearable technology, including healthcare, human-machine interfaces, and energizing microelectronics.
Cryosurgery and cryopreservation, as two important categories in cryobiology, have been impeded b... more Cryosurgery and cryopreservation, as two important categories in cryobiology, have been impeded by the poor thermal conductivity of biological tissues or specimens. To improve this, diverse adjuvants, e.g., carbon-based materials, metallic nanoparticles, metallic oxide nanoparticles, etc., have been exploited to improve the heat transfer in heat-targeted regions to increase the tumor elimination efficiency as well as the post-thaw viability of cryopreserved specimens. Nevertheless, these materials suffer poor thermal conductivities, controversial biosafety problems, and high expense. Gallium and its alloys, as a class of room-temperature liquid metals (LMs), have been widely studied in the past decade for their low melting point, minor toxicity, outstanding transformability, and conductivity. Integrated with these superior properties, they have been widely applied in multiple fields, such as thermal management, flexible electronics, and soft robotics. Recently, our laboratory has been devoted to fusing LMs with cryobiology and has made a series of progress. In this article, we will first briefly introduce preparation pathways to LM-based functional nanomaterials and composites. Then, how these materials realize improvement in biological heat transfer will be presented, followed by a discussion about the biosafety of these materials, which is an essential concern for the cryobiological field. Recent studies employing LMs in advanced cryosurgery and cryopreservation will also be highlighted. The present challenges and prospects of LMs towards further development in cryobiology will be put forward to point out the possible research direction.
Simultaneous monitoring of the body's biochemical and biophysical signals via wearable devices ca... more Simultaneous monitoring of the body's biochemical and biophysical signals via wearable devices can provide a comprehensive assessment of an individual's health state. However, current multifunctional sensors for synchronous biochemical and biophysical sensing rely on discrete sensing units, posing a limitation in increased complexity in device assembly, signal processing, and system integration. In this study, we report a dual-mode and self-powered wearable sensor with ion and pressure-sensing capabilities by interfacing a hydrogel film with a solid ion-selective electrode. The hydrogel film can not only collect natural sweat from the skin but also offer a piezoionic response to pressure. We show that wrist pulse-induced pressure response can be incorporated into the noise of the response to sweat sodium ions, allowing for the simultaneous measurement of heart rate and sweat electrolytes. This work provides an example of simplifying the development of wearable multimode sensors through the rational design of classic electrochemical sensors.
Electrochemical glucose sensors that rely on two-dimensional (2D) oxides have attracted significa... more Electrochemical glucose sensors that rely on two-dimensional (2D) oxides have attracted significant attention owing to the strong sensing activity of 2D oxides, but their practical application is hindered by the complexity and high cost of fabrication of electrodes and integrated devices. Herein, a convenient and effective fabrication route that includes printing a Ga-based liquid metal (LM) as a current collection electrode, followed by growing electrochemically active 2D oxides directly on the surface of Ga-based LMs under mild conditions, is developed for non-enzyme-based electrochemical sensors. Specifically, 2D annealed Cu-Oxide (ACO) is successfully grown on a printed Ga electrode through a galvanic replacement reaction, resulting in the formation of a mechanically and electrically well-matched interface between the active sensing materials and the current collection substrate. Benefitting from the high quantity of 2D ACO and good charge transfer at the interface, the as-prepared ACO electrode exhibits attractive glucose sensing performance, with a wide linear range (1 μM-10 mM) of effective detection, low detection limit down to 1 μM, and high sensitivity of 0.87 μA·mM-1·cm-2. Our study highlights the potential of using LMs in bio-sensing applications and provides a non-enzyme-based electrochemical biosensor platform for effective glucose detection in diets and clinical diagnostic settings.
Sweating is an important physiological reaction and a clinical symptom in a variety of diseases. ... more Sweating is an important physiological reaction and a clinical symptom in a variety of diseases. However, it remains underrated in clinical use. Gold standards to measure the sweat rate are neither continuous nor easily or labindependently applicable. With the emergence of novel wearable devices, using the sweat rate as a digital biomarker shows promise for clinical monitoring and diagnostics. In this Commentary, we discuss the potential and importance of the sweat rate as a digital biomarker in clinical medicine beyond sports science.
Epidermal electrodes can be directly attached to the human skin for high-fidelity electrophysiolo... more Epidermal electrodes can be directly attached to the human skin for high-fidelity electrophysiological monitoring owing to their preponderance in thinness, lightweight, conformability, biocompatibility, self-adhesiveness, mechanical flexibility, gas-permeability, etc. These devices have attracted immense attention due to their emerging applications in personalized health care, human/brain-machine interfaces, and soft robotics. This Perspective focuses on the most recent significant progress in this area, especially materials, properties, and applications. Challenges and prospects are summarized to underscore the unexploited areas and future directions toward digital health and on-skin digitalization.
Nerve stimulation technology utilizing electricity, magnetism, light, and ultrasound has found ex... more Nerve stimulation technology utilizing electricity, magnetism, light, and ultrasound has found extensive applications in biotechnology and medical fields. Neurostimulation devices serve as the crucial interface between biological tissue and the external environment, posing a bottleneck in the advancement of neurostimulation technology. Ensuring safety and stability is essential for their future applications. Traditional rigid devices often elicit significant immune responses due to the mechanical mismatch between their materials and biological tissues. Consequently, there is a growing demand for flexible nerve stimulation devices that offer enhanced treatment efficacy while minimizing irritation to the human body. This review provides a comprehensive summary of the historical development and recent advancements in flexible devices utilizing four neurostimulation techniques: electrical stimulation, magnetic stimulation, optic stimulation, and ultrasonic stimulation. It highlights their potential for high biocompatibility, low power consumption, wireless operation, and superior stability. The aim is to offer valuable insights and guidance for the future development and application of flexible neurostimulation devices.
Inorganic p-type Sb 2 Te 3 flexible thin films (f-TFs) with eco-friendly and high thermoelectric ... more Inorganic p-type Sb 2 Te 3 flexible thin films (f-TFs) with eco-friendly and high thermoelectric performance have attracted wide research interest and potential for commercial applications. This study employs a facile in-situ reaction method to prepare flexible Sb 2 Te 3 thin films by rationally adjusting the synthesized temperature. The prepared thin films show good crystallinity, which enhances the electrical conductivity of ~1,440 S•cm-1 due to the weakened carrier scattering. Simultaneously, the optimized carrier concentration, through adjusting the synthesis temperature, causes the intermediate Seebeck coefficient. Consequently, a high-power factor (16.0 μW•cm-1 •K-2 at 300 K) is achieved for Sb 2 Te 3 f-TFs prepared at 623 K. Besides, the f-TFs also exhibit good flexibility due to the slight change in resistance after bending. This study specifies that the in-situ reaction method is an effective route to prepare Sb 2 Te 3 f-TFs with high thermoelectric performance.
Flexible Light Emitting Diodes are versatile lighting solutions that offer bendable and adaptable... more Flexible Light Emitting Diodes are versatile lighting solutions that offer bendable and adaptable illumination possibilities. A soft, flexible white luminescent film (1 mm) shows promise for foldable electroluminescent devices and applications. This film was fabricated using ZnS:Ag and Mn. Under different excitation wavelengths, the phosphors emit blue light due to Ag + luminescence centers and red light from the d-d transition of Mn 2+. The blue emission is greatly suppressed at high Mn 2+ doping levels, requiring reduced Ag + doping in co-doped ZnS:Ag,Mn compared to solo-doped ZnS:Ag samples. By adjusting Ag + and Mn 2+ concentrations, the ZnS:Ag(1%),Mn(0.2%) phosphors show a proper intensity ratio of blue and red emissions, making them a promising candidate for future white light applications.
In the face of pandemic infectious diseases and increasing aging trends, traditional public healt... more In the face of pandemic infectious diseases and increasing aging trends, traditional public health systems lack the capacity for real-time monitoring, immediate clinical detection, continuous vital sign monitoring, and the implementation of long-cycle treatment protocols, among other deficiencies. On the basis of the rapid development of wearable electronic devices, the Internet of Things, and artificial intelligence, the future healthcare model will transform from a therapeutic, centralized, passive, and even one-size-fits-all treatment to a new paradigm of proactive, preventive, personalized, customized, and intelligent way. The development of wearable electronics has facilitated the evolution of healthcare from healthcare to biological monitoring, enabling continuous monitoring of critical biomarkers for diagnostic treatment, physiological health monitoring, and assessment. Electronic textiles (e-textiles) are among the rapidly developing wearable electronics in recent years. They have revolutionized the functionality of traditional textiles by incorporating smart attributes, enabling unique and multifunctional applications. Significantly, e-textiles have made notable advancements in the field of personalized healthcare. The article introduces several common e-textiles and their applications in personalized medicines, which also gives a forward-looking outlook on their future growth in infectious diseases, real-time health preventive monitoring, auxiliary therapy, and rehabilitation training.
Featuring low cost, low melting points, excellent biocompatibility, outstanding electrical conduc... more Featuring low cost, low melting points, excellent biocompatibility, outstanding electrical conductivity, and mechanical properties, gallium-based liquid metals (LMs) have become a promising class of materials to fabricate flexible healthcare sensors. However, the extremely high surface tension hinders their manipulation and cooperation with substrates. To address this problem, the inspiration of nanomaterials has been adopted to mold LMs into LM nanoparticles (LMNPs) with expanded advantages. The transformability of LMNPs endows them with functionalities for sensors in multiple dimensions, such as intelligent response to specific molecules or strains, various morphologies, integration into high-resolution circuits, and conductive elastomers. This review aims to summarize the superior properties of LMs, transformability of LMNPs, and correlated advantages for sensor performance. Multidimensional functional sensing forms consisting of LMNPs and corresponding applications as healthcare sensors will be presented. In the end, the existing challenges and prospects in the processing and application of LMNPs will also be discussed.
Mechanoluminescence is the phenomenon in which certain materials emit light when subjected to mec... more Mechanoluminescence is the phenomenon in which certain materials emit light when subjected to mechanical stimuli, such as bending, stretching, or compression. Soft devices containing embedded mechanoluminescent materials are capable of responding to mechanical deformation by emitting light, which can be utilized for various applications, including sensing, display, communication, and visual feedback. In this Perspective, we discuss recent advancements and emerging applications of mechanoluminescent materials for soft devices, with a focus on the remaining challenges in mechanoluminescent materials, such as performance, mechanism, synthesis, and device fabrication, that need to be addressed for developing advanced soft devices, and propose the potential solutions.
The adaptability of natural organisms in altering body shapes in response to the environment has ... more The adaptability of natural organisms in altering body shapes in response to the environment has inspired the development of artificial morphing matter. These materials encode the ability to transform their geometrical configurations in response to specific stimuli and have diverse applications in soft robotics, wearable electronics, and biomedical devices. However, achieving the morphing of intricate three-dimensional shapes from a twodimensional flat state is challenging, as it requires manipulations of surface curvature in a controlled manner. In this review, we first summarize the mechanical principles extensively explored for realizing morphing matter, both at the material and structural levels. We then highlight its applications in the soft robotics field. Moreover, we offer insights into the open challenges and opportunities that this rapidly growing field faces. This review aims to inspire researchers to uncover innovative working principles and create multifunctional morphing matter for various engineering fields.
Stretchable and highly conductive elastomers with intrinsically deformable liquid metal (LM) fill... more Stretchable and highly conductive elastomers with intrinsically deformable liquid metal (LM) fillers exhibit promising potential in soft electronics, wearables, human-machine interfaces, and soft robotics. However, conventional LM-elastomer (LME) conductors require a high loading ratio of LM and the post-sintering to rupture LM particles to achieve electric conductivity, which results in high LM consumption and process complexity. In this work, we presented a straightforward and post-sintering-free method that utilizes magnetic aggregation to fabricate stretchable LME conductors. This was achieved by dispersing LM ferrofluid into the elastomer precursor, followed by applying the magnetic field to induce the aggregation and interconnection of the LM ferrofluid particles to form conductive pathways. This method not only simplifies the preparation of initially conductive LME but also reduces the LM loading ratio. The resulting conductive LME composites show high stretchability (up to 650% strain), high conductance stability, and magnetic responsiveness. The stretchable LME conductors were demonstrated in various applications, including the creation of flexible microcircuits, a magnetically controlled soft switch, and a soft hydrogel actuator for grasping tasks. We believe the stretchable LME conductors may find wide applications in electronic skins, soft sensors, and soft machines.
Wearable biosensors have demonstrated enormous potential in revolutionizing healthcare by providi... more Wearable biosensors have demonstrated enormous potential in revolutionizing healthcare by providing real-time fitness tracking, enabling remote patient monitoring, and facilitating early detection of health issues. To better sense vital life signals, researchers are increasingly favoring wearable biosensors with flexible properties that can be seamlessly integrated with human tissues, achieved through the utilization of soft materials. Gallium (Ga)based liquid metals (LMs) possess desirable properties, such as fluidity, high conductivity, and negligible toxicity, which make them inherently soft and well-suited for the fabrication of flexible and wearable biosensors. In this article, we present a comprehensive overview of the recent advancements in the nascent realm of flexible and wearable biosensors employing LMs as key components. This paper provides a detailed exposition of the unique characteristics of Ga-based LM materials, which set them apart from traditional materials. Moreover, the state-ofthe-art applications of Ga-based LMs in flexible and wearable biosensors that expounded from six aspects are reviewed, including wearable interconnects, pressure sensors, strain sensors, temperature sensors, and implantable bioelectrodes. Furthermore, perspectives on the key challenges and future developing directions of
Soft and stretchable strain sensors have aroused great interest in research and engineering field... more Soft and stretchable strain sensors have aroused great interest in research and engineering fields due to their promising application potential in many areas, including human-machine interface and healthcare monitoring. However, developing stable, strain-sensitive, and fatigue-resistant wearable strain sensors remains challenging. Herein, we report a low-cost strain-sensing glove based on a commercial nitrile glove and liquid metal as both sensing units and circuit/interconnects, with excellent response to strains and great stability in long-term use. The liquid metal sensing circuit is prepared by scraping the liquid metal slurry in situ on glove fingers, followed by soft silicone encapsulation. The whole process does not involve toxic chemicals, so no strict requirements on the operating environment are necessary. The strain-sensing glove is capable of real-time monitoring of finger gestures in a very sensitive and accurate way, which exhibits great application potential as a soft controller in manipulating the machine hand to achieve related human-machine interaction.
Thermoelectric (TE) conversion technology can directly exploit the temperature difference of seve... more Thermoelectric (TE) conversion technology can directly exploit the temperature difference of several Kelvin between the human body and the environment to generate electricity, which provides a self-powered solution for wearable electronics. Flexible TE materials are increasingly being developed through various methods, among which the vacuum filtration method stands out for its unique advantages, attracting the favor of researchers. It has been proven to construct flexible TE thin films with excellent performance effectively. This paper presents a comprehensive overview and survey of the advances of the vacuum filtration method in producing flexible TE thin films. The materials covered in this study include conducting polymer-based materials, carbon nanoparticle-based materials, inorganic materials, two-dimensional materials, and ternary composites. Finally, we explore potential research outlooks and the significance of flexible films, which are at the forefront of research in TE materials science.
Endowing robots with multi-directional tactile sensing capabilities has long been a challenging t... more Endowing robots with multi-directional tactile sensing capabilities has long been a challenging task in the field of flexible electronics and intelligent robots. This paper reports a highly sensitive, flexible tactile sensor with an embedded-hair-in-elastomer structure, which is capable of decoupling normal stress and shear stress. The flexible tactile sensor is fabricated on a thin polyimide substrate and consists of four self-bending piezoresistive cantilevers in a cross-shaped configuration, which are embedded in an elastomer. The sensor can decouple the tactile information into a normal stress and a shear stress with simple summation and differencing algorithms, and the measurement error is kept within 3%. Moreover, the sensitivity and detection threshold of the sensor can be adjusted by simply changing the elastic material. As a demonstration, the flexible tactile sensor is integrated into a robotic manipulator to precisely estimate the weight of the grasped objects, which shows great potential for application in robotic systems.
Flexible pressure sensors with high stretchability, sensitivity, and stability are undoubtedly ur... more Flexible pressure sensors with high stretchability, sensitivity, and stability are undoubtedly urgently required for potential applications in intelligent soft robots, human-machine interaction, health monitoring, and other fields. However, most current flexible pressure sensors are unable to endure large deformation and are prone to performance degradation or even failure during frequent operation due to their multilayered structures. Here, we propose a stretchable all-nanofiber iontronic pressure sensor that is composed of ionic nanofiber membranes used as dielectric layers and liquid metal used as electrodes. This sensor exhibits a high sensitivity of 1.08 kPa-1 over a wide range of 0-300 kPa, with a fast response-relaxation time of about 18/22 ms and excellent stability. The high sensitivity comes from the electric double layer formed at the ionic film/electrode interface, while high stretchability and stability are enabled by in-situ encapsulated all-nanofiber structures. As a proof of concept, a prototype sensor array is integrated into a soft pneumatic gripper, demonstrating its capability of pressure perception and object recognition during the grasping process. Thus, the scheme provides another excellent strategy to fabricate stretchable pressure sensors with superb performance in terms of high stretchability, sensitivity, and stability.
Recent advances in soft sensor technology have pushed digital healthcare toward life-changing sol... more Recent advances in soft sensor technology have pushed digital healthcare toward life-changing solutions. Data reliability and robustness can be realised by building sensor arrays that collect comprehensive biological parameter data from several points on the underlying organs simultaneously, a principle that is inspired by bioreceptors. The rapid growth of soft lithography and printing, three-dimensional (3D) printing, and weaving/knitting technologies has facilitated the low-cost development of soft sensors in the array format. Advances in data acquisition, processing, and visualisation techniques have helped with the collection of meaningful data using arrays and their presentation to users on personal devices through wireless communication interfaces. Local-or cloud-based data storage helps with the collection of adequate data from sensor arrays over time to facilitate reliable prognoses based on historical data. Emerging energy harvesting technologies have led to the development of techniques to power sensor arrays sustainably. This review presents developmental building blocks in wearable and artificial organ-based soft sensor arrays, including bioreceptor-inspired sensing mechanisms, fabrication methods, digital data-acquisition techniques, methods to present the results to users, power systems, and target diseases/conditions for treatment or monitoring. Finally, we summarise the challenges associated with the development of single and multimodal array sensors for advanced digital healthcare and suggest possible solutions to overcome them.
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