Recent Advances in Nanowire-Based Wearable Physical Sensors.

Wearable electronics is a technology that closely integrates electronic devices with the human body or clothing, which can realize human-computer interaction, health monitoring, smart medical, and other functions. Wearable physical sensors are an important part of wearable electronics. They can sense various physical signals from the human body or the surrounding environment and convert them into electrical signals for processing and analysis. Nanowires (NW) have unique properties such as a high surface-to-volume ratio, high flexibility, high carrier mobility, a tunable bandgap, a large piezoresistive coefficient, and a strong light-matter interaction. They are one of the ideal candidates for the fabrication of wearable physical sensors with high sensitivity, fast response, and low power consumption. In this review, we summarize recent advances in various types of NW-based wearable physical sensors, specifically including mechanical, photoelectric, temperature, and multifunctional sensors. The discussion revolves around the structural design, sensing mechanisms, manufacture, and practical applications of these sensors, highlighting the positive role that NWs play in the sensing process. Finally, we present the conclusions with perspectives on current challenges and future opportunities in this field.

A soft microrobot with highly deformable 3D actuators for climbing and transitioning complex surfaces.

The climbing microrobots have attracted growing attention due to their promising applications in exploration and monitoring of complex, unstructured environments. Soft climbing microrobots based on muscle-like actuators could offer excellent flexibility, adaptability, and mechanical robustness. Despite the remarkable progress in this area, the development of soft microrobots capable of climbing on flat/curved surfaces and transitioning between two different surfaces remains elusive, especially in open spaces. In this study, we address these challenges by developing voltage-driven soft small-scale actuators with customized 3D configurations and active stiffness adjusting. Combination of programmed strain distributions in liquid crystal elastomers (LCEs) and buckling-driven 3D assembly, guided by mechanics modeling, allows for voltage-driven, complex 3D-to-3D shape morphing (bending angle > 200°) at millimeter scales (from 1 to 10 mm), which is unachievable previously. These soft actuators enable development of morphable electroadhesive footpads that can conform to different curved surfaces and stiffness-variable smart joints that allow different locomotion gaits in a single microrobot. By integrating such morphable footpads and smart joints with a deformable body, we report a multigait, soft microrobot (length from 6 to 90 mm, and mass from 0.2 to 3 g) capable of climbing on surfaces with diverse shapes (e.g., flat plane, cylinder, wavy surface, wedge-shaped groove, and sphere) and transitioning between two distinct surfaces. We demonstrate that the microrobot could navigate from one surface to another, recording two corresponding ceilings when carrying an integrated microcamera. The developed soft microrobot can also flip over a barrier, survive extreme compression, and climb bamboo and leaf.

Recent Advances in Multifunctional Wearable Sensors and Systems: Design, Fabrication, and Applications.

Multifunctional wearable sensors and systems are of growing interest over the past decades because of real-time health monitoring and disease diagnosis capability. Owing to the tremendous efforts of scientists, wearable sensors and systems with attractive advantages such as flexibility, comfort, and long-term stability have been developed, which are widely used in temperature monitoring, pulse wave detection, gait pattern analysis, etc. Due to the complexity of human physiological signals, it is necessary to measure multiple physiological information simultaneously to evaluate human health comprehensively. This review summarizes the recent advances in multifunctional wearable sensors, including single sensors with various functions, planar integrated sensors, three-dimensional assembled sensors, and stacked integrated sensors. The design strategy, manufacturing method, and potential application of each type of sensor are discussed. Finally, we offer an outlook on future developments and provide perspectives on the remaining challenges and opportunities of wearable multifunctional sensing technology.

Assembly of complex 3D structures and electronics on curved surfaces.

Electronic devices with engineered three-dimensional (3D) architectures are indispensable for frictional-force sensing, wide-field optical imaging, and flow velocity measurement. Recent advances in mechanically guided assembly established deterministic routes to 3D structures in high-performance materials, through controlled rolling/folding/buckling deformations. The resulting 3D structures are, however, mostly formed on planar substrates and cannot be transferred directly onto another curved substrate. Here, we introduce an ordered assembly strategy to allow transformation of 2D thin films into sophisticated 3D structures on diverse curved surfaces. The strategy leverages predefined mechanical loadings that deform curved elastomer substrates into flat/cylindrical configurations, followed by an additional uniaxial/biaxial prestretch to drive buckling-guided assembly. Release of predefined loadings results in an ordered assembly that can be accurately captured by mechanics modeling, as illustrated by dozens of complex 3D structures assembled on curved substrates. Demonstrated applications include tunable dipole antennas, flow sensors inside a tube, and integrated electronic systems capable of conformal integration with the heart.

Submillimeter-scale multimaterial terrestrial robots.

Robots with submillimeter dimensions are of interest for applications that range from tools for minimally invasive surgical procedures in clinical medicine to vehicles for manipulating cells/tissues in biology research. The limited classes of structures and materials that can be used in such robots, however, create challenges in achieving desired performance parameters and modes of operation. Here, we introduce approaches in manufacturing and actuation that address these constraints to enable untethered, terrestrial robots with complex, three-dimensional (3D) geometries and heterogeneous material construction. The manufacturing procedure exploits controlled mechanical buckling to create 3D multimaterial structures in layouts that range from arrays of filaments and origami constructs to biomimetic configurations and others. A balance of forces associated with a one-way shape memory alloy and the elastic resilience of an encapsulating shell provides the basis for reversible deformations of these structures. Modes of locomotion and manipulation span from bending, twisting, and expansion upon global heating to linear/curvilinear crawling, walking, turning, and jumping upon laser-induced local thermal actuation. Photonic structures such as retroreflectors and colorimetric sensing materials support simple forms of wireless monitoring and localization. These collective advances in materials, manufacturing, actuation, and sensing add to a growing body of capabilities in this emerging field of technology.

Island Effect in Stretchable Inorganic Electronics.

Island-bridge architectures represent a widely used structural design in stretchable inorganic electronics, where deformable interconnects that form the bridge provide system stretchability, and functional components that reside on the islands undergo negligible deformations. These device systems usually experience a common strain concentration phenomenon, i.e., "island effect", because of the modulus mismatch between the soft elastomer substrate and its on-top rigid components. Such an island effect can significantly raise the surrounding local strain, therefore increasing the risk of material failure for the interconnects in the vicinity of the islands. In this work, a systematic study of such an island effect through combined theoretical analysis, numerical simulations and experimental measurements is presented. To relieve the island effect, a buffer layer strategy is proposed as a generic route to enhanced stretchabilities of deformable interconnects. Both experimental and numerical results illustrate the applicability of this strategy to 2D serpentine and 3D helical interconnects, as evidenced by the increased stretchabilities (e.g., by 1.5 times with a simple buffer layer, and 2 times with a ring buffer layer, both for serpentine interconnects). The application of the patterned buffer layer strategy in a stretchable light emitting diodes system suggests promising potentials for uses in other functional device systems.

Highly-integrated, miniaturized, stretchable electronic systems based on stacked multilayer network materials.

Elastic stretchability and function density represent two key figures of merits for stretchable inorganic electronics. Various design strategies have been reported to provide both high levels of stretchability and function density, but the function densities are mostly below 80%. While the stacked device layout can overcome this limitation, the soft elastomers used in previous studies could highly restrict the deformation of stretchable interconnects. Here, we introduce stacked multilayer network materials as a general platform to incorporate individual components and stretchable interconnects, without posing any essential constraint to their deformations. Quantitative analyses show a substantial enhancement (e.g., by ~7.5 times) of elastic stretchability of serpentine interconnects as compared to that based on stacked soft elastomers. The proposed strategy allows demonstration of a miniaturized electronic system (11 mm by 10 mm), with a moderate elastic stretchability (~20%) and an unprecedented areal coverage (~110%), which can serve as compass display, somatosensory mouse, and physiological-signal monitor.

An Anti-Fatigue Design Strategy for 3D Ribbon-Shaped Flexible Electronics.

Three-dimensional (3D) flexible electronics represent an emerging area of intensive attention in recent years, owing to their broad-ranging applications in wearable electronics, flexible robots, tissue/cell scaffolds, among others. The widely adopted 3D conductive mesostructures in the functional device systems would inevitably undergo repetitive out-of-plane compressions during practical operations, and thus, anti-fatigue design strategies are of great significance to improve the reliability of 3D flexible electronics. Previous studies mainly focused on the fatigue failure behavior of planar ribbon-shaped geometries, while anti-fatigue design strategies and predictive failure criteria addressing 3D ribbon-shaped mesostructures are still lacking. This work demonstrates an anti-fatigue strategy to significantly prolong the fatigue life of 3D ribbon-shaped flexible electronics by switching the metal-dominated failure to desired polymer-dominated failure. Combined in situ measurements and computational studies allow the establishment of a failure criterion capable of accurately predicting fatigue lives under out-of-plane compressions, thereby providing useful guidelines for the design of anti-fatigue mesostructures with diverse 3D geometries. Two mechanically reliable 3D devices, including a resistance-type vibration sensor and a janus sensor capable of decoupled temperature measurements, serve as two demonstrative examples to highlight potential applications in long-term health monitoring and human-like robotic perception, respectively.

Rapidly deployable and morphable 3D mesostructures with applications in multimodal biomedical devices.

Structures that significantly and rapidly change their shapes and sizes upon external stimuli have widespread applications in a diversity of areas. The ability to miniaturize these deployable and morphable structures is essential for applications in fields that require high-spatial resolution or minimal invasiveness, such as biomechanics sensing, surgery, and biopsy. Despite intensive studies on the actuation mechanisms and material/structure strategies, it remains challenging to realize deployable and morphable structures in high-performance inorganic materials at small scales (e.g., several millimeters, comparable to the feature size of many biological tissues). The difficulty in integrating actuation materials increases as the size scales down, and many types of actuation forces become too small compared to the structure rigidity at millimeter scales. Here, we present schemes of electromagnetic actuation and design strategies to overcome this challenge, by exploiting the mechanics-guided three-dimensional (3D) assembly to enable integration of current-carrying metallic or magnetic films into millimeter-scale structures that generate controlled Lorentz forces or magnetic forces under an external magnetic field. Tailored designs guided by quantitative modeling and developed scaling laws allow formation of low-rigidity 3D architectures that deform significantly, reversibly, and rapidly by remotely controlled electromagnetic actuation. Reconfigurable mesostructures with multiple stable states can be also achieved, in which distinct 3D configurations are maintained after removal of the magnetic field. Demonstration of a functional device that combines the deep and shallow sensing for simultaneous measurements of thermal conductivities in bilayer films suggests the promising potential of the proposed strategy toward multimodal sensing of biomedical signals.

Geometrically reconfigurable 3D mesostructures and electromagnetic devices through a rational bottom-up design strategy.

Microelectronic devices with reconfigurable three-dimensional (3D) microarchitecture that can be repetitively switched among different geometrical and/or working states have promising applications in widespread areas. Traditional approaches usually rely on stimulated deformations of active materials under external electric/magnetic fields, which could potentially introduce parasitic side effects and lower device performances. Development of a rational strategy that allows access to high-performance 3D microdevices with multiple stable geometric configurations remains challenging. We introduce a mechanically guided scheme to build geometrically reconfigurable 3D mesostructures through a bottom-up design strategy based on a class of elementary reconfigurable structures with the simplest ribbon geometries. Quantitative mechanics modeling of the structural reconfigurability allows for the development of phase diagrams and design maps. Demonstrations of ~30 reconfigurable mesostructures with diverse geometric topologies and characteristic dimensions illustrate the versatile applicability. The multimode nature enables customized distinct beamforming and discrete beam scanning using a single antenna capable of on-demand reconfiguration.

Soft three-dimensional network materials with rational bio-mimetic designs.

Many biological tissues offer J-shaped stress-strain responses, since their microstructures exhibit a three-dimensional (3D) network construction of curvy filamentary structures that lead to a bending-to-stretching transition of the deformation mode under an external tension. The development of artificial 3D soft materials and device systems that can reproduce the nonlinear, anisotropic mechanical properties of biological tissues remains challenging. Here we report a class of soft 3D network materials that can offer defect-insensitive, nonlinear mechanical responses closely matched with those of biological tissues. This material system exploits a lattice configuration with different 3D topologies, where 3D helical microstructures that connect the lattice nodes serve as building blocks of the network. By tailoring geometries of helical microstructures or lattice topologies, a wide range of desired anisotropic J-shaped stress-strain curves can be achieved. Demonstrative applications of the developed conducting 3D network materials with bio-mimetic mechanical properties suggest potential uses in flexible bio-integrated devices.

Electro-mechanically controlled assembly of reconfigurable 3D mesostructures and electronic devices based on dielectric elastomer platforms.

The manufacture of 3D mesostructures is receiving rapidly increasing attention, because of the fundamental significance and practical applications across wide-ranging areas. The recently developed approach of buckling-guided assembly allows deterministic formation of complex 3D mesostructures in a broad set of functional materials, with feature sizes spanning nanoscale to centimeter-scale. Previous studies mostly exploited mechanically controlled assembly platforms using elastomer substrates, which limits the capabilities to achieve on-demand local assembly, and to reshape assembled mesostructures into distinct 3D configurations. This work introduces a set of design concepts and assembly strategies to utilize dielectric elastomer actuators as powerful platforms for the electro-mechanically controlled 3D assembly. Capabilities of sequential, local loading with desired strain distributions allow access to precisely tailored 3D mesostructures that can be reshaped into distinct geometries, as demonstrated by experimental and theoretical studies of ∼30 examples. A reconfigurable inductive-capacitive radio-frequency circuit consisting of morphable 3D capacitors serves as an application example.

Mechanically-Guided Structural Designs in Stretchable Inorganic Electronics.

Over the past decade, the area of stretchable inorganic electronics has evolved very rapidly, in part because the results have opened up a series of unprecedented applications with broad interest and potential for impact, especially in bio-integrated systems. Low modulus mechanics and the ability to accommodate extreme mechanical deformations, especially high levels of stretching, represent key defining characteristics. Most existing studies exploit structural material designs to achieve these properties, through the integration of hard inorganic electronic components configured into strategic 2D/3D geometries onto patterned soft substrates. The diverse structural geometries developed for stretchable inorganic electronics are summarized, covering the designs of functional devices and soft substrates, with a focus on fundamental principles, design approaches, and system demonstrations. Strategies that allow spatial integration of 3D stretchable device layouts are also highlighted. Finally, perspectives on the remaining challenges and open opportunities are provided.

Harnessing the interface mechanics of hard films and soft substrates for 3D assembly by controlled buckling.

Techniques for forming sophisticated, 3D mesostructures in advanced, functional materials are of rapidly growing interest, owing to their potential uses across a broad range of fundamental and applied areas of application. Recently developed approaches to 3D assembly that rely on controlled buckling mechanics serve as versatile routes to 3D mesostructures in a diverse range of high-quality materials and length scales of relevance for 3D microsystems with unusual function and/or enhanced performance. Nonlinear buckling and delamination behaviors in materials that combine both weak and strong interfaces are foundational to the assembly process, but they can be difficult to control, especially for complex geometries. This paper presents theoretical and experimental studies of the fundamental aspects of adhesion and delamination in this context. By quantifying the effects of various essential parameters on these processes, we establish general design diagrams for different material systems, taking into account 4 dominant delamination states (wrinkling, partial delamination of the weak interface, full delamination of the weak interface, and partial delamination of the strong interface). These diagrams provide guidelines for the selection of engineering parameters that avoid interface-related failure, as demonstrated by a series of examples in 3D helical mesostructures and mesostructures that are reconfigurable based on the control of loading-path trajectories. Three-dimensional micromechanical resonators with frequencies that can be selected between 2 distinct values serve as demonstrative examples.

Deterministic Line-Shape Programming of Silicon Nanowires for Extremely Stretchable Springs and Electronics.

Line-shape engineering is a key strategy to endow extra stretchability to 1D silicon nanowires (SiNWs) grown with self-assembly processes. We here demonstrate a deterministic line-shape programming of in-plane SiNWs into extremely stretchable springs or arbitrary 2D patterns with the aid of indium droplets that absorb amorphous Si precursor thin film to produce ultralong c-Si NWs along programmed step edges. A reliable and faithful single run growth of c-SiNWs over turning tracks with different local curvatures has been established, while high resolution transmission electron microscopy analysis reveals a high quality monolike crystallinity in the line-shaped engineered SiNW springs. Excitingly, in situ scanning electron microscopy stretching and current-voltage characterizations also demonstrate a superelastic and robust electric transport carried by the SiNW springs even under large stretching of more than 200%. We suggest that this highly reliable line-shape programming approach holds a strong promise to extend the mature c-Si technology into the development of a new generation of high performance biofriendly and stretchable electronics.

High performance transparent in-plane silicon nanowire Fin-TFTs via a robust nano-droplet-scanning crystallization dynamics.

High mobility, scalable and even transparent thin-film transistors (TFTs) are always being pursued in the field of large area electronics. While excimer laser-beam-scanning can crystallize amorphous Si (a-Si) into high mobility poly-Si, it is limited to small areas. We here demonstrate a robust nano-droplet-scanning strategy that converts an a-Si:H thin film directly into periodic poly-Si nano-channels, with the aid of well-coordinated indium droplets. This enables the robust batch-fabrication of high performance Fin-TFTs with a high hole mobility of >100 cm2 V-1 s-1 and an excellent subthreshold swing of only 163 mV dec-1, via a low temperature <350 °C thin film process. More importantly, precise integration of tiny poly-Si channels, measuring only 60 nm in diameter and 2 µm apart on glass substrates, provides an unprecedented transparent Si-based TFT technology to visible light, which is widely sought for the next generation of high aperture displays and fully transparent electronics. The successful implementation of such a reliable nano-droplet-scanning strategy, rooted in the strength of nanoscale growth dynamics, will enable eventually the batch-manufacturing and upgrade of high performance large area electronics in general, and high definition and scalable flat-panel displays in particular.

Heteroepitaxial Writing of Silicon-on-Sapphire Nanowires.

The heteroepitaxial growth of crystal silicon thin films on sapphire, usually referred to as SoS, has been a key technology for high-speed mixed-signal integrated circuits and processors. Here, we report a novel nanoscale SoS heteroepitaxial growth that resembles the in-plane writing of self-aligned silicon nanowires (SiNWs) on R-plane sapphire. During a low-temperature growth at <350 °C, compared to that required for conventional SoS fabrication at >900 °C, the bottom heterointerface cultivates crystalline Si pyramid seeds within the catalyst droplet, while the vertical SiNW/catalyst interface subsequently threads the seeds into continuous nanowires, producing self-oriented in-plane SiNWs that follow a set of crystallographic directions of the sapphire substrate. Despite the low-temperature fabrication process, the field effect transistors built on the SoS-SiNWs demonstrate a high on/off ratio of >5 × 104 and a peak hole mobility of >50 cm2/V·s. These results indicate the novel potential of deploying in-plane SoS nanowire channels in places that require high-performance nanoelectronics and optoelectronics with a drastically reduced thermal budget and a simplified manufacturing procedure.

Engineering island-chain silicon nanowires via a droplet mediated Plateau-Rayleigh transformation.

The ability to program highly modulated morphology upon silicon nanowires (SiNWs) has been fundamental to explore new phononic and electronic functionalities. We here exploit a nanoscale locomotion of metal droplets to demonstrate a large and readily controllable morphology engineering of crystalline SiNWs, from straight ones into continuous or discrete island-chains, at temperature <350 °C. This has been accomplished via a tin (Sn) droplet mediated in-plane growth where amorphous Si thin film is consumed as precursor to produce crystalline SiNWs. Thanks to a significant interface-stretching effect, a periodic Plateau-Rayleigh instability oscillation can be stimulated in the liquid Sn droplet, and the temporal oscillation of the Sn droplets is translated faithfully, via the deformable liquid/solid deposition interface, into regular spatial modulation upon the SiNWs. Combined with a unique self-alignment and positioning capability, this new strategy could enable a rational design and single-run fabrication of a wide variety of nanowire-based optoelectronic devices.

Operating principles of in-plane silicon nanowires at simple step-edges.

Growing silicon nanowires (SiNWs) into precise locations is a key enabling technology for SiNW-based device applications. This can be achieved via in-plane growth of SiNWs along a simple step-edge, where metal catalyst droplets absorb an amorphous Si matrix to produce c-SiNWs. However, a comprehensive understanding of this phenomenon is still lacking. We here establish an analytical model to address the driving force that dictates the growth dynamics under various droplet-step contact configurations, and to identify the key control parameters for effective guided growth. These new principles were verified against a series of experimental observations and proved to be powerful in designing a stable guided growth configuration. Furthermore, we propose and demonstrate a unique ability to achieve in situ capturing, guiding and release of the in-plane SiNWs along curved step-edges. We suggest that such a new understanding and results provide a fundamental basis and a practical guide for positioning and integrating self-assembled nanowires in a wide variety of material systems.

In-plane epitaxial growth of silicon nanowires and junction formation on Si(100) substrates.

Growing self-assembled silicon nanowires (SiNWs) into precise locations represents a critical capability to scale up SiNW-based functionalities. We here report a novel epitaxy growth phenomenon and strategy to fabricate orderly arrays of self-aligned in-plane SiNWs on Si(100) substrates following exactly the underlying crystallographic orientations. We observe also a rich set of distinctive growth dynamics/modes that lead to remarkably different morphologies of epitaxially grown SiNWs/or grains under variant growth balance conditions. High-resolution transmission electron microscopy cross-section analysis confirms a coherent epitaxy (or partial epitaxy) interface between the in-plane SiNWs and the Si(100) substrate, while conductive atomic force microscopy characterization reveals that electrically rectifying p-n junctions are formed between the p-type doped in-plane SiNWs and the n-type c-Si(100) substrate. This in-plane epitaxy growth could provide an effective means to define nanoscale junction and doping profiles, providing a basis for exploring novel nanoelectronics.