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.

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 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.

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.

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.

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 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-of-the-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 LM-enabled wearable and flexible biosensors are also discussed.

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 two-dimensional 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.

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.

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.