Soft Science

Tri-axial tactile sensors that provide real-time information on both normal and shear forces are enabling technologies for tactile perception, which open up new possibilities in robotics, human-machine interfaces, environmental sensing, and health monitoring. Among tri-axial tactile sensors based on different mechanisms, inductive sensors possess good robustness against environmental contamination. Their low sensitivity to normal and shear loads, however, is a critical barrier. This work presents the rational design of soft inductive tri-axial tactile sensors that are capable of distinguishing static or dynamic normal and shear loads, with exceptional tactile sensitivity. Dual mechanisms of Biot-Savart law and Eddy current effect are explored to overcome the long-standing sensitivity issue. In addition, a hybrid coil with non-uniform spacing is designed to generate uniform magnetic fields, addressing the limitations of traditional uniform coils and significantly improving the sensor’s tactile sensitivity. The picosecond pulsed laser scribing technique makes it possible to pattern silver nanowires into inductive coils with high fidelity. A porous compressible layer is adopted to enable adjustable sensitivity and sensing range to meet diverse application demands. Finally, the sensor is integrated between the user’s leg and the orthosis, showcasing the sensor’s capability for real-time monitoring of tri-axial forces and its robustness against environmental objects.

Heterointerface engineering has drawn considerable interest in tuning interfacial polarization and promoting impedance matching. Therefore, it has become a key strategy for optimizing electromagnetic wave (EMW) absorption. This comprehensive review primarily focused on the EMW absorbing strategies of polymer-based materials, emphasizing the critical developments of heterointerface engineering. A possible EMW absorbing mechanism of polymer-based materials was proposed, emphasizing the synergism of multi-components, microstructure design, and heterointerface engineering. Key innovations in structural design such as porous structure, multilayered structure, and segregated structure are explored, highlighting their contributions to enhancing EMW absorption. Also, the review highlights the latest research progress of advanced conductive polymer-based and insulating polymer-based materials with desirable EMW absorption performance; their fabrication methods, structures, properties, and EMW absorption mechanisms were elucidated in detail. Key challenges on polymer-based EMW absorbing materials are presented followed by some future perspectives.

The real-time assessment and personalized monitoring of human bladder status is important for individuals with involuntary voiding, overactive bladder and bladder disorders such as urinary incontinence. To address the shortcomings of traditional urodynamic methods where the equipment is bulky, complex, invasive, expensive and unable to continuously monitor bladder status, and to meet the needs of healthcare professionals and family members to know the patient’s bladder capacity, this paper designs the biocompatible integrated bladder electronics for wireless capacity monitoring assessment. The device employs chitosan, which exhibits favorable biocompatibility, to fabricate patch electrodes, and optimizes their performance through the plasticizing effect of glycerol, with a polarization resistance of 4.8983 kΩ, a maximum tensile force of up to 107.5 kPa, and remains chemically stable for long-term wear. The principle of bioelectrical impedance analysis is employed to integrate a hardware system comprising multiple modules, including a microcontroller, information processing, communication, display and power supply. After the integrated system design is completed with electrodes connected and encapsulated, data on bladder electrical impedance changes is gathered and transmitted wirelessly to the user interface for non-invasive real-time monitoring and intelligent assessment of bladder capacity. The experimental results demonstrate a high correlation between human bladder electrical impedance and bladder volume, with a systematic measurement correlation coefficient reaching 96.7%. The research equipment is portable, simple to operate, and radiation-free to the human body. It has significant potential for real-time monitoring and intelligent alarm of bladder capacity.

Biohybrid robots (bio-bots), made of biocompatible skeletons with living drives (e.g., biological living tissues or cells), represent a new direction of robotics technology due to their attractive advantages of softness, flexibility, adaptability and biocompatibility, accompanied by the remarkable capabilities of self-assembly, self-healing, and self-replication. This paper provides a brief review of recent advances of bio-bots from a functional view, including walking, swimming and non-locomotion bio-bots, by exploring their structure designs along with their operational principles. The performances of these bio-bots are summarized and compared followed by the discussions of challenges and perspectives, which provide valuable insight and guidance for future developments of bio-bots.

Flexible display technology is actively explored as a cornerstone of the next generation of wearables and soft electronics, set to revolutionize devices with its potential for lightweight, thin, and mechanically flexible features. Flexible thin-film transistors (TFTs) utilizing promising materials such as amorphous silicon (a-Si), low-temperature polysilicon (LTPS), metal oxides (MOs), and organic semiconductors are essential to enable flexible platforms. Among these, MO semiconductors stand out for flexible displays due to their high carrier mobility, low processing temperature requirements, excellent electrical uniformity, transparency to visible light, and cost-effectiveness. Furthermore, the maturity of MO TFT technology in the existing display industry and its compatibility with complementary-metal-oxide-semiconductor (CMOS) processes are driving active research toward integrated circuits for wearable electronics beyond display applications. Specifically, achieving both high mechanical flexibility and electrical performance in MO TFTs is crucial for implementing complex integrated circuits such as microprocessors and backplanes for ultra-high resolution augmented reality (AR)/virtual reality (VR) displays. Therefore, this review provides recent advances in high-mobility flexible MO TFTs, focusing on materials, fabrication processes, and device architecture engineering methods for implementing MO TFTs on flexible substrates, as well as strategies to reduce the impact of mechanical stress on MO TFTs. Next, MO TFT-based display and integrated circuit applications for next-generation flexible and stretchable electronics are introduced and discussed. Finally, the review concludes with an outlook on the potential achievements and prospects of MO TFTs in the development of next-generation flexible display technologies.

Low-grade heat (below 373 Kelvins) is abundant and ubiquitous, yet the lack of cost-effective recovery technologies frequently impedes its effective utilization. The advent of thermogalvanic hydrogel thermocells has garnered significant attention due to their high thermopower, inherent flexibility, low cost, and scalability. Thermogalvanic hydrogels have significantly enhanced their thermoelectric performance, resulting in the development of functional materials that exhibit flexibility, stretchability, self-healing, and frost resistance. However, there are substantial challenges in developing multifunctional thermogalvanic hydrogels that combine high power density and efficiency with practical applicability. This review discusses the synthesis of the novel redox couple, improving the performance of electrolytes to increase thermopower, creating electrodes with extensive surface areas for better current density and flexibility, and optimizing thermocell structure design to improve performance further. This comprehensive review aims to propel progress toward higher performance levels and broader applications of thermogalvanic hydrogel thermocells.

Microwaves are currently used in many fields, including the military, medical, and communication. However, the ensuing electromagnetic radiation has seriously threatened human life. Therefore, the design of high-performance microwave absorbing materials (MAMs) has become an important development direction. Metal-organic frameworks (MOFs) are regarded as a bright new star among MAMs with broad application prospects due to their advantages of tunable structure, large specific surface area, high porosity, etc . This paper reviews the research progress of MAMs derived from MOFs in recent years, including preparation methods, properties and microwave absorption mechanisms. Finally, the problems and research prospects of MOF-derived MAMs are discussed.

Dual magnetic particles-modified carbon materials have great potential in terms of ultrathin thickness (≤ 2 mm) and super electromagnetic wave (EMW) absorption (≤ -70 dB). Herein, using CoFe-metal-organic framework (MOF)-derived CoFe/Co@NC heterostructures composed of hollow CoFe nanospheres, solid Co nanospheres, and nitrogen-doped carbon (NC) nanosheets, we demonstrate how the dual magnetic particles regulate the electromagnetic response behavior of the heterostructure and thus steer the efficient EMW absorption performance. That is, CoFe/Co@NC heterostructure inherits an ultra-strong reflection loss (R L) of -73.8 dB at 1.78 mm. The effective absorption bandwidth (EAB) value is also available up to 5.4 GHz. Moreover, computer simulation technology (CST) simulations reveal the good radar stealth effect of heterostructures. Experimentally, the outstanding EMW absorption of CoFe/Co@NC heterostructure is due to a large number of heterointerfaces, good conductive networks and dual magnetic nanoparticles, which bring considerable interface polarization, conduction loss, and magnetic loss characteristics. These findings underscore the importance of electromagnetic synergy induced by dual magnetic particles for steering the electromagnetic response of EMW absorbers.

Micro-cylindrical electronic devices represent a rapidly emerging class of electronics distinguished by their unique geometries and superior mechanical properties. These features enable a broad range of applications across fields such as wearable fibric devices, surgical robotics, and implantable medical devices. The choice of micro-cylindrical substrate materials is crucial in determining device performance, as their high curvature and excellent flexibility offer an ideal foundation for functional integration. This paper systematically reviews a wide array of substrate materials suitable for micro-cylindrical electronic devices, analyzing their differences and application potential in terms of mechanical stability, biocompatibility, and processability. The unique requirements of micro-cylindrical devices, specifically their flexibility, integrative capabilities, and lightweight nature, challenge conventional planar fabrication processes, which often fall short of meeting these demands. Thus, we further examine custom fabrication techniques tailored for micro-cylindrical electronics, assessing advantages, limitations, and specific applications of each approach. Additionally, we analyze the current application requirements and developmental progress of these devices across multiple fields. This review also outlines future directions in this field, focusing on enhancing fabrication precision, improving material compatibility and biocompatibility, and advancing integration and intelligent functionalities. With a comprehensive overview, this review aims to provide a valuable reference for the research and development of micro-cylindrical electronic devices, promoting technological advancements and innovation in emerging applications.

The emergence of wearable electronics, along with an increased emphasis on personal health, has catalyzed a transformation in conventional health monitoring methods. Textile electronics are attracting significant attention due to their good flexibility, breathability, biocompatibility, portability and wearability, positioning them as a promising platform for human health monitoring. Consequently, substantial efforts are being dedicated to developing multifunctional, integrated, and reliable health monitoring systems based on textile electronics. This review summarizes recent advancements in textile electronics, focusing on materials, preparation techniques for functional fibers and fabrics, design strategies for textile-based health monitoring systems, and applications in ubiquitous health monitoring. Additionally, some emerging strategies for integration are presented. Moreover, the challenges and future outlook of textile electronics, along with potential solutions are discussed.

With the growing potential of the Internet of Things, displays are being utilized to provide various types of information in every aspect of daily life, leading to the expansion of form-factor-free displays. Stretchable displays are considered the ultimate goal in form factor innovation, and they are not limited to rectangular shapes with deformation characteristics suited to target applications. Because reliable stretchable displays should be robust under uniaxial and biaxial strain, there have been efforts to tailor mechanical stress with promising strategies from structural and material perspectives. This review focuses on strain-engineering stretchable substrates for free-form display applications. First, we introduce deformable substrates with structural stretchability, achieved by incorporating buckling and Kirigami structures into plastic films, and we systematically analyze the tensile deformation characteristics based on design elements. In addition, we examined intrinsically stretchable elastomeric substrates, which have gained considerable attention due to recent advances in material and processing technologies. Their spatial modulus patterning is studied by applying optimized design principles, achieved through network alignment and crosslinking control in homogeneous elastomers, as well as by incorporating heterogeneous structures within the elastomer materials. Finally, we discussed state-of-the-art stretchable display applications employing strain-engineered stretchable substrates, focusing on advantageous materials and structures based on the display components, processes, and target deformation characteristics. Building on this foundation, we discuss the development of next-generation free-form displays and aim to contribute to their application in various static and dynamic deformation environments.

Metal-organic frameworks (MOFs) have recently garnered attention due to their intriguing physiochemical properties; however, their instability and unsatisfactory processability have limited them from achieving a wide range of applications. Therefore, structural new MOFs and MOF-based hydrogels have been developed to address associated drawbacks (such as inherent brittleness and instability in the crystalline state). The development of MOF-based hydrogels has been the focus of some recent attempts. Compared to the original MOFs, they have several better properties (such as improved mechanical strengths). This review will provide the latest overviews of current research developments on MOF-based hydrogels. We primarily focus on the classification of these hydrogels and their associated synergistic effects. We also emphasize microscale composite design, macroscale performance, and extended applications in catalysis, water treatment and biomedicine. Further, we anticipate that this review will be valuable for individuals seeking insights into the applications of MOFs-based hydrogels.

Flexible wearable devices designed to evaluate the biomechanical properties of deep tissues not only facilitate continuous and effective monitoring in basic performance but also exhibit significant potential in broader disease assessments. Recent advancements are highlighted in the structural and principled design of platforms capable of capturing various biomechanical signals. These advancements have led to enhanced testing capabilities concerning spatial scales and resolution modes at different depths. This review discusses the engineering of soft wearable devices for the biomechanical evaluation of deep tissue signals. It encompasses different measurement modes, device design and fabrication methods, integrated circuit (IC) integration schemes, and the characteristics of measurement depth and accuracy. The core discussion focuses on platform development, targeting different monitoring sites and platform structure design, ranging from linear strain gauges and conformal stretchable sensors to complex three-dimensional (3D) circuit-integrated stretchable arrays. We further explore various technologies associated with different measurement mechanisms and engineering designs, as well as the penetration depth and spatial resolution of these wearable sensors. The practical applications of these technologies are evident in the monitoring of deep tissue signals and changes in tissue characteristics. The results suggest that wearable biomechanical sensing systems hold substantial promise for applications in healthcare and research.

Stimuli-responsive hydrogels are innovative soft materials that have garnered significant attention in recent years. These hydrogels can undergo phase transitions or structural changes in response to external stimuli, offering considerable potential for use as actuators. They can effectively be used for drug delivery and disease treatment by responding flexibly to a variety of stimuli. This paper first categorizes the types of external stimuli that hydrogel actuators respond to and outlines their applications in skin therapeutics. It then reviews therapeutic potentials of hydrogel actuators beyond the skin, and discusses the challenges and future prospects for the development of stimuli-responsive hydrogel actuators.

Recently, mechanically deformable displays, such as flexible, foldable, rollable, and stretchable displays, have received considerable attention due to their broad range of applications across various electronic systems. Among the various types of deformable displays, stretchable displays represent the most advanced form factor. The stretchable displays require a sophisticated integration of components, including stretchable conducting, insulating, and semiconducting materials, intricate geometrical patterns, and multiple electronic elements. This comprehensive review explores the recent progress in stretchable displays, emphasizing the critical developments in materials, device architectures, and practical applications. Key innovations in stretchable electrodes and interconnections, light-emitting materials, transistors, circuitry, and deformable substrates are explored, highlighting their contributions to enhancing durability and stretchability. Also, the review highlights the latest research on achieving stretchability using intrinsically elastic materials or through structural engineering with rigid materials. Additionally, we introduce innovative applications of stretchable displays in various emerging electronic systems.

Recent advances in pressure sensors have garnered significant interest due to their promising applications in healthcare, robotics and wearable technology. In these fields, there is an ever-increasing demand for soft sensors that can conform to complex surfaces, such as the human body. However, current sensors often face limitations in measurable pressure ranges and customization involves complex manufacturing processes. In this study, we introduce an innovative solution for producing soft pressure sensors with varying maximum detection pressures. By utilizing a magnetic transduction mechanism and different hyperelastic materials, we have developed sensors that can adapt to irregular surfaces. These sensors measure a wide range of pressures, from ultra-low to medium, and offer variable stiffness, sensitivity, and measurement ranges. The sensors we manufactured exhibit a detection range from 6.8 to 77.4 kPa, a sensitivity range between -5.1 × 10-2 and -0.4 × 10-2 kPa-1, a short recovery time of 0.4 s, low hysteresis values during repeated loading/unloading cycles, and stable response over thousands of pressure cycle. Proof-of-concept experiments validated the sensors’ suitability for breathing monitoring and finger tap detection, highlighting their potential in medical and robotic applications. The results demonstrate a robust strategy for controlling the performance of soft pressure sensors, positioning them as promising candidates for diverse pressure sensing applications.

This review investigates the transformative potential of neuromorphic computing in advancing biointegrated electronics, with a particular emphasis on applications in medical sensing, diagnostics, and therapeutic interventions. By examining the convergence of edge computing and neuromorphic principles, we explore how emulating the operational principles of the human brain can enhance the energy efficiency and functionality of biointegrated electronics. The review begins with an introduction to recent breakthroughs in materials and circuit designs that aim to mimic various aspects of the biological nervous system. Subsequent sections synthesize demonstrations of neuromorphic systems designed to augment the functionality of healthcare-related electronic systems, including those capable of direct signal communication with biological tissues. The neuromorphic biointegrated devices remain in a nascent stage, with a relatively limited number of publications available. The current review aims to meticulously summarize these pioneering studies to evaluate the current state and propose future directions to advance the interdisciplinary field.

Flexible and stretchable electrochromic displays (ECDs) perform a crucial function in Internet of Things (IoT) systems, as they have shown superior eye-friendly, energy-saving, mechanical (flexibility and stretchability) properties. They can be integrated with IoT devices and successfully applied as wearable and intelligent electronics. Flexible and stretchable ECD technology has shown promising potential but is still in the early stage of development. A systematic overview from comprehensive perspectives of materials selection and modification, structure design, and advanced fabrication methods of this technology is necessary. In this review, we concentrate on the strategies in substrates, active layers, and electrolyte aspects to fabricate high-performance flexible and stretchable ECDs. We have systematically summarized the materials selection/modification and device structure design in these strategies. We also outline recent advances in flexible and stretchable ECDs based on processing methods for electrode patterns, active layer patterns, electrolyte patterns, and ECD pixels. Moreover, the interactive visual displays integrating ECD pixels with different sensors have been elaborated. Finally, we outline the future directions for developing flexible and stretchable ECDs, focusing on materials, methods, and applications. These prospects aim to overcome the limitations in pattern resolution, electrolyte uniformity, and pixel size/number and realize the manufacturable, commercialized, scalable, and robust flexible ECDs. This review can further promote the basic research and advanced fabrication of flexible ECDs and facilitate the advancement of multifunctional displays to satisfy the increasing demand for next-generation flexible electronics.

As device form factors evolve towards increased complexity and flexibility, the role of adhesives within the display module stack becomes increasingly crucial. These adhesives are essential for bonding functional layers with minimal thickness while mitigating stress during the dynamic behavior of flexible devices. This paper offers a comprehensive overview of the essential properties of adhesives - such as adhesion, viscoelasticity, optical characteristics, and environmental reliability - necessary for the stable operation of flexible display devices across diverse form factors and environments. In particular, it provides an in-depth look at ongoing research in simulation, material selection, polymer network control, and the integration of new functionalities to achieve optimal performance. Furthermore, this paper discusses extensive research outcomes addressing the growing demand for sustainable solutions. Building on this knowledge, we highlight the future direction of adhesives for flexible displays.

Environment-benign indium phosphide (InP) quantum dots (QDs) show great promise as visible emitters for next-generation display applications, where bright and narrow emissivity of QDs should be required toward high-efficiency, high-color reproducibility. The photoluminescence (PL) performance of InP QDs has been consistently, markedly improved, particularly owing to the exquisite synthetic control over core size homogeneity and core/shell heterostructural variation. To date, synthesis of most high-quality InP QDs has been implemented by using zinc (Zn) carboxylate as a shell precursor that unavoidably entails the formation of surface oxide on InP core. Herein, we demonstrate synthesis of superbly bright, color-pure green InP/ZnSe/ZnS QDs by exploring an innovative hybrid Zn shelling approach, where Zn halide (ZnX2, X = Cl, Br, I) and Zn oleate are co-used as shell precursors. In the hybrid Zn shelling process, the type of ZnX2 is found to affect the growth outcomes of ZnSe inner shell and consequent optical properties of the resulting heterostructured InP QDs. Enabled by not only the near-complete removal of the oxide layer on InP core surface through the hybrid Zn shelling process but the controlled growth rate of ZnSe inner shell, green InP/ZnSe/ZnS QDs achieve a record quantum yield (QY) up to unity along with a highly sharp linewidth of 32 nm upon growth of an optimal ZnSe shell thickness. This work affords an effective means to synthesize high-quality heterostructured InP QDs with superb emissive properties.

Skin is a rich source of invaluable information for healthcare management and disease diagnostics. The integration of soft skin electronics enables precise and timely capture of these cues at the skin interface. Leveraging attributes such as lightweight design, compact size, high integration, biocompatibility, and enhanced comfort, these technologies hold significant promise for advancing various applications. However, the fabrication process for most existing soft skin electronics typically requires expensive platforms and clean-room environments, potentially inflating production costs. In recent years, the emergence of laser-induced-graphene (LIG) has presented a practical solution for developing soft skin electronics that are both cost-effective and high-performing. This advancement paves the way for the widespread adoption of intelligent healthcare technologies. Here, we comprehensively review recent studies focusing on LIG-based soft skin electronics (LIGS²E) for intelligent healthcare applications. We first outline the preparation methodologies, fundamental properties of LIG, and standard regulation strategies employed in developing soft skin electronics. Subsequently, we present an overview of various LIGS²E designs and their diverse applications in intelligent healthcare. These applications encompass biophysical and biochemical sensors, bio-actuators, and power supply systems. Finally, we deliberate on the potential challenges associated with the practical implementation of LIGS²E in healthcare settings and offer insights into future directions for research and development. By elucidating the capabilities and limitations of LIGS²E, this review aims to contribute to advancing intelligent healthcare technologies.

Despite the lower efficiency for thermoelectric cooling technology compared to conventional mechanical cooling technology, it finds application in commercial portable cooling due to its compactness, simple device design, and low noise. The rapid progress in flexible and wearable electronics opens the need for flexible cooling technology for local thermal regulation where thermoelectric cooling technology offers niche advantages suitable for flexible cooling such as light weight and no moving parts. Organic thermoelectrics hold promise for flexible and wearable cooling applications due to their intrinsic mechanical flexibility, low thermal conductivity, and ease of processing. However, research on organic Peltier cooling devices remains limited, and more work is required to exploit their potential for flexible cooling applications. This review discussed the state-of-the-art organic Peltier cooling devices and the materials and device design considerations required for advancing organic Peltier device technology toward practical applications.

The lack of infrastructure and accessibility in medical treatments has been considered as a global chronic issue since the concept of treatment and prevention was presented. After the COVID-19 pandemic, the medical reaction capability for epidemic outbreak/spread has been spotlighted as a critical issue to the fore worldwide. To reduce the burden on the medical system from the simultaneous disease emergence, the personalized wearable electronic systems have arisen as the next-generation biomedical monitoring/treating equipment for infectious diseases at the initial stage. In particular, electronic skin (e-skin) with its potential for multifunctional extendibility has been enabled to be applied to next-generation long-term healthcare devices with real-time biosignal sensing. Here, we introduce the recent enhancements of various e-skin systems for healthcare applications in terms of material types and device structures, including sensor components, biological signal sensing mechanisms, applicable technological advancements, and medical utilization.

Liquid metal (LM), an emerging functional material, plays increasing roles in biomedical and healthcare areas. It has particular values in neural interfaces as it combines high conductivity, flowability, and biocompatibility properties. Neuro-electrical interfaces (NEIs) are effective tools to provide a bridge between the nervous system and the outside world. The main target of developing neural interfaces is to help disabled people repair damaged nerves and enhance human capacity above normal ability. This article systematically summarizes LM-based neural interface technologies, including neural electrodes for electrical signal acquisition and administration of electrical stimulation and nerve guidance conduits for neural connectivity and functional reconstruction. The discussion begins with an overview of the fundamental properties associated with LM materials involved in the field of neural interface applications. The fabrication methods of LM-based neuro-electrodes and conduits are then introduced, and the current development status of LM-based neuro-electrodes and conduits is elaborated. Finally, the prospects and possible challenges of LM-based neural interfaces are outlined.

Soft electronics have achieved significant development, attracting substantial interest due to their promising potential as a dominant form of future electronics. In this rapidly evolving field, the fully soft Schottky diode plays a critical role as a fundamental building block for electronic circuitry systems. These systems, constructed entirely from soft materials, can tolerate various mechanical deformations when interfaced with human skin, making them ideal for use in health monitoring systems and interactive human-machine interfaces. In this study, we introduce a Schottky diode fabricated entirely from soft materials using a facile solution process, further enabling all-printing fabrication systems. Utilizing the mechanical softness of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-based soft electrode, poly(3-hexylthiophene) nanofibril composite soft semiconductor, and liquid metal, we successfully fabricated a fully soft Schottky diode. This diode exhibits exceptional electrical characteristics even under various mechanical deformations, showcasing the high durability of the device. We have further developed fully soft rectifiers and logic gates, highlighting the versatility of our study. By incorporating these devices with a piezoelectric nanogenerator in a skin-interfaced energy harvesting system, they exhibit sufficient capability for rectification, ensuring a stable power supply as part of a power supply management system. This approach offers substantial potential for future skin-interfaced electronics, paving the way for advanced wearable technology.