Liquid crystal elastomers (LCEs), as an intriguing class of soft active materials, exhibit excellent actuation performances and biocompatible properties, as well as a high degree of design flexibility, which have been of increasing interest in many disciplines. This review summarizes recent developments in this inspiring area, providing an overview of fabrication methods, design schemes, actuation mechanisms, and diverse applications of LCEs. Firstly, two-stage and one-pot synthesis methods, as well as emerging fabrication techniques (e.g., 3D/4D printing and top-down microfabrication techniques) are introduced. Secondly, the design and actuation mechanisms are discussed according to the different types of stimuli (e.g., heat, light, and electric/magnetic fields, among others). Thirdly, the representative applications are summarized, including soft robotics, temperature/strain sensors, biomedical devices, stretchable displays, and smart textiles. Finally, outlooks on the scientific challenges and open opportunities are provided.

Thermal management for wearable devices is evolving to make ubiquitous applications possible based on advanced devices featuring miniaturization, integration, and ultrathin designs. Thermal management and control integrated with wearable devices are highly desirable for various applications for human body monitoring, including external heat exposure and metabolic heat generation, in various activities. Recently, dynamic change materials have been integrated with micro/nano thermal management platforms to address the potential for active thermal management. In this article, recent advances in the architecture of effective thermal management in wearable devices are reviewed, along with the essential mechanisms for managing thermal conditions for users in external/internal thermal environments. Appropriate thermal management approaches are proposed for the design and integration of materials/structures tailored to specific targets in wearable devices. In particular, this review is devoted to materials/structures based on five thermal management strategies: conduction, radiation, evaporation/convection, heat absorption/release, and thermoelectric (TE). Finally, the challenges and prospects for practical applications of thermal management in wearable devices are discussed.

Continuous feedback on a tire is an essential means to ensure tire safety. Smart tires are an important part of the future vehicle control system, which affects the safety and comfort of vehicles by combining sensors with traditional tires to achieve continuous monitoring of real-time dynamic parameters. A stretchable and flexible sensor made of laser-induced graphene (LIG) and PDMS, designed for use in smart tires, is presented in this work. The sensor is known as a LIG-PDMS sensor. Using transfer printing, LIG is formed on a commercial polyimide film under the scribing of a laser beam following the predesigned route before being transferred to a PDMS film. This technology is used to successfully prepare flexible sensors for measuring the tire road interaction at different driving speeds due to its flexibility and shape-following characteristics. The real-time monitoring of the wheel speed and the shape of the tire grounding mark during the driving process is realized by embedding multiple LIG sensors in the tire to monitor the strain information of the tire grounding. Results show that the tire deformation can be accurately feedbacked with the LIG sensors, demonstrating our method's capability for designing and manufacturing intelligent tires.

Recent advancements in soft electronics and robotics have expanded the possibilities beyond the capabilities of traditional rigid devices, indicating promise for a range of applications in electronic skins, wireless biomedical devices, and others. Magnetic materials exploited in these soft systems can further broaden the modalities in sensing and actuation. These magnetic materials, when constructed in the forms of nanoparticles, nanomembranes, or other types of nanostructures, exhibit some unique characteristics, such as the magnetoresistance effect and size-dependent coercivity. Soft electronics and robotics employing such magnetic nanomaterials offer a variety of functions, including the detection of the intensity and direction of magnetic fields, measurement of various types of mechanical deformations, manipulation and transport at small scales, and multimodal complex locomotion in a controllable fashion. Despite recent advancements in soft electronics and robotics, challenges remain in developing advanced materials and manufacturing schemes to improve performance metrics and facilitate integration with other devices. This review article aims to summarize the progress made in soft electronics and robotics based on magnetic nanomaterials, with an emphasis on introducing material and device performance. The discussions focus on soft electronics and robotics based on magnetic nanomembranes/nanostructures and magnetic composites. As a concluding remark, this article summarizes the current status of the field and discusses opportunities that underpin future progress.

Using flexible neuromorphic electronics that emulate biological neuronal systems is an innovative approach for facilitating the implementation of next-generation artificial intelligence devices, including wearable computers, soft robotics devices, and neuroprosthetics. Stretchable synaptic transistors based on field-effect transistors (FETs), which have functions and structures resembling those of biological synapses, are promising technological devices in flexible neuromorphic electronics owing to their high flexibility, excellent biocompatibility, and easy processability. However, obtaining stretchable synaptic FETs with various synaptic characteristics and good stretching stabilities is challenging. Significant efforts to produce stretchable synaptic FETs have been undertaken, and remarkable advances in materials, fabrication processes, and applications have been achieved. From this perspective, we discuss the requirements for neuromorphic devices in flexible neuromorphic electronics and the advantages of stretchable synaptic FETs. Moreover, representative methods used to implement stretchable synaptic transistors, including the structural design and development of intrinsically stretchable devices, are introduced. Additionally, the application of stretchable synaptic transistors in artificial sensory systems such as light, tactile, and multisensory artificial nervous systems is also discussed. Finally, we highlight the possible challenges in implementing and using stretchable synaptic transistors, propose solutions to overcome the current limitations of these devices, and suggest future research directions.

The development of devices that can be mechanically deformed in geometrical layouts, such as flexible/stretchable devices, is important for various applications. Conventional flexible/stretchable devices have been demonstrated using two-dimensional (2D) geometry, resulting in dimensional constraints on device operations and functionality limitations. Accordingly, expanding the dimensions in which such devices can operate and acquiring unique functionality that is difficult to implement in 2D planar structures remain challenging. As a solution, the development of a flexible/stretchable device embedding a three-dimensional (3D) structure fabricated through the precise control of a 2D structure or direct construction has been attracting significant attention. Because of a significant amount of effort, several 3D material systems with distinctive engineering properties, including electrical, optical, thermal, and mechanical properties, which are difficult to occur in nature or to obtain in usual 2D material systems, have been demonstrated. Furthermore, 3D advanced material systems with flexibility and stretchability can provide additional options for developing devices with various form factors. In this review, novel fabrication methods and unprecedented physical properties of flexible/stretchable 3D material systems are reviewed through multiple application cases. In addition, we summarized the latest advances and trends in innovative applications implemented through the introduction of advanced 3D systems in various fields, including microelectromechanical systems, optoelectronics, energy devices, biomedical devices, sensors, actuators, metamaterials, and microfluidic systems.

Smart wearable electronic devices capable of information exchanging (such as human-machine interfaces) have developed into key carriers for the interconnection, intercommunication, and interaction between humans and machines. Multimodal electronic textiles that incorporate multifunctional sensors into daily clothing are an emerging technology to realize smart wearable electronics. This has greatly advanced human-machine interface technology by bridging the gap between wearing comfort and traditional wearable electronic devices, which will facilitate the rapid development and wide application of natural human-machine interfaces. In this article, we provide a comprehensive summary of the latest research progress on multimodal electronic textiles for intelligent human-machine interfaces. Firstly, we introduce the most representative electronic textile manufacturing strategies in terms of functional fiber preparation and multimodal textile forming. Then, we explore the multifunctional sensing capability of multimodal electronic textiles and emphasize their advanced applications in intelligent human-machine interfaces. Finally, we present new insights on the future research directions and the challenges faced in practical applications of multimodal electronic textiles.

The development of flexible and stretchable materials has led to advances in implantable bio-integrated electronic devices that can sense physiological signals or deliver electrical stimulation to various organs in the human body. Such devices are particularly useful for neural interfacing systems that monitor neurodegenerative diseases such as Parkinson’s disease or epilepsy in real time. However, coupling current brain-interfacing devices with magnetic resonance imaging (MRI) remains a practical challenge due to resonance frequency variations from inorganic metal-based devices. Thus, organic conductive materials, such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), have recently been considered as promising candidates. Nonetheless, their conformability on curvilinear tissues remains questionable. In this study, we developed an injectable conductive hydrogel (ICH) composed of tyramine-conjugated hyaluronic acid (HATYR) and PEDOT:PSS for MRI-compatible brain-interfacing electrodes. Our ICH produced low impedance around 5 kΩ even under 10 Hz, demonstrating high confidence volumetric capacitance. Due to HATYR’s biocompatibility, histological and cytotoxicity assays showed almost no inflammation and toxicity, respectively; in addition, ICH was able to degrade into 40% of its original volume within four weeks in vivo. An electrocorticogram (ECoG) array was also patternable by syringe injections of ICH on a stretchable and flexible elastomeric substrate layer that conformed to curvy brain tissues and successfully recorded ECoG signals under light stimulation. Furthermore, MRI imaging of implanted devices did not show any artifacts, indicating the potential of the MRI-compatible hydrogel electrodes for advanced ECoG arrays. This study provides a promising solution for MRI-compatible neural electrodes, enabling the advancement of chronic neural interfacing systems for monitoring neurodegenerative diseases.

Piezoelectric materials directly convert energy between electrical and mechanical domains, and have been widely employed in electronic devices as sensors and energy harvesters. Recent research endeavors are mainly devoted to dealing with problems such as high stiffness, brittleness, toxicity, poor durability, and low piezoelectric coefficients. Among developed strategies, chemical modification captures much attention. However, the exact physical properties and direct experimental evidence of chemical modification remain elusive or controversial thus far. In this review, we discuss the recently developed piezoelectric modification strategies for piezoelectric composites and assess the effect of different chemical modification approaches on piezoelectric properties. Moreover, we outline existing challenges and new applications of piezoelectric composites.

Modulating the structural order of conjugated polymers has emerged as a significant approach to enhance the organic thermoelectric performance. Among these materials, poly(3,4-ethylenedioxythiophene) is considered a promising candidate due to its high conductivity. However, its low thermopower remains a major obstacle to further improve its performance as an organic thermoelectric material. To address this issue, a series of thiophene derivatives with high rigidity and containing dioxyethylene groups were synthesized, and polymer films were prepared through a simple and mild in-situ polymerization method. The polymer molecule containing a thiophene block, named poly[bi(3,4-ethylenedioxy)-alt-thienyl] , exhibits significant self-rigidification due to non-covalent interactions between oxygen and sulfur atoms, resulting in highly ordered assembly. By adding thiophene and thieno[3,2-b]thiophene structures to the intermediate precursor bi(3,4-ethylenedioxy), the 3,4-ethylenedioxy content in the polymer molecule is altered, leading to an almost four-fold increase in the thermopower of the thin film polymer and achieving a maximum thermopower of around 26 μV·K^-1. Although poly[bi(3,4-ethylenedioxy)-alt-thienyl] shows a significant increase in thermopower compared to poly[bi(3,4-ethylenedioxy)], the thin film conductivity exhibits a nearly imperceptible decreasing trend due to its highly ordered microstructure. This work highlights the potential to control the aggregation state of polymer molecules and achieve an approximate decoupling between the conductivity and thermopower of thermoelectric materials by rationally designing polymer molecules.