Micro-/nano-voids guided two-stage film cracking on bioinspired assemblies for high-performance electronics

Bioinspired 3D flexible devices and functional systems

Flexible devices and functional systems with elaborated three-dimensional (3D) architectures can endow better mechanical/electrical performances, more design freedom, and unique functionalities, when compared to their two-dimensional (2D) counterparts. Such 3D flexible devices/systems are rapidly evolving in three primary directions, including the miniaturization, the increasingly merged physical/artificial intelligence and the enhanced adaptability and capabilities of heterogeneous integration. Intractable challenges exist in this emerging research area, such as relatively poor controllability in the locomotion of soft robotic systems, mismatch of bioelectronic interfaces, and signal coupling in multi-parameter sensing. By virtue of long-time-optimized materials, structures and processes, natural organisms provide rich sources of inspiration to address these challenges, enabling the design and manufacture of many bioinspired 3D flexible devices/systems. In this Review, we focus on bioinspired 3D flexible devices and functional systems, and summarize their representative design concepts, manufacturing methods, principles of structure-function relationship and broad-ranging applications. Discussions on existing challenges, potential solutions and future opportunities are also provided to usher in further research efforts toward realizing bioinspired 3D flexible devices/systems with precisely programmed shapes, enhanced mechanical/electrical performances, and high-level physical/artificial intelligence.

Highly stable and strain-insensitive metal film conductors via manipulating strain distribution

Metal film-based stretchable conductors are essential elements of flexible electronics for wearable, biomedical, and robotic applications, which require strain-insensitive high conductivity over a wide strain range and excellent cyclic stability. However, they suffer from serious electrical failure under monotonic and cyclic tensile loading at a small strain due to the uncontrolled film cracking behavior. Here, we propose a novel in-plane crack control strategy of engineering hierarchical microstructures to achieve outstanding electromechanical performance via harnessing the strain distribution in metal films. The wrinkles delay the crack initiation at undercuts which should be the most vulnerable sites during the stretching process. The surface protrusions/grooves/undercuts inhibit the crack propagation because of the effective strain redistribution. In addition, hierarchical microstructures significantly improve cyclic stability due to the strong interfacial adhesion and stable crack patterns. The metal film-based conductors exhibit ultrahigh strain-insensitive conductivity (1.7 × 107 S m-1), negligible resistance change (ΔR/R0 = 0.007) over an ultra-wide strain range (>200%), and excellent cyclic strain durability (>15 000 cycles at 100% strain). A range of metal films was explored to establish the universality of this strategy, including ductile copper and silver, as well as brittle molybdenum and high entropy alloy. We demonstrate the strain-insensitive electrical functionality of a metal film-based conductor in a flexible light-emitting diode circuit.

Switching ultra-stretchability and sensitivity in metal films for electronic skins: a pufferfish-inspired, interlayer regulation strategy

The booming development of electronic skins necessitates stretchable electrodes and flexible sensors that exhibit distinctly opposite requirements of electromechanical properties, both of which are difficult to be fulfilled on a single material. Here, a pufferfish-inspired, interlayer regulation strategy is proposed that realizes the above opposite properties in simple metal films, exhibiting either ultra-stretchability (295% strain) or sensitivity (maximum GF: ∼5500) on demand. It is revealed that the stretchability of the intrinsically strain-sensitive metal films can be improved by ∼20-fold via regulating the surface morphology of the inserted interlayer, accompanied by an intriguing transition in film cracking behavior from cut-through cracks to network patterns. By featuring these two antithetical but valuable properties, common metal films can be applied as diverse sensors and stretchable electrodes in electronic skins, showing application prospects in healthcare monitoring, human-machine interaction, and engineering services. Our proposed strategy substantially advances the application of metal film conductors in flexible electronics and broadens the horizons for developing more sophisticated electronic skins by interlayer engineering.

Venation-Mimicking, Ultrastretchable, Room-Temperature-Attachable Metal Tapes for Integrated Electronic Skins

Future electronic skin systems require stretchable conductors and low-temperature integration of external components, which remains challenging for traditional metal films. Herein, a bioinspired design concept is reported to endow metal films with 200% stretchability as well as room-temperature integration capability with diverse components. It is revealed that by controllable implantation of defects, distinctive venation-mimicking cracking modes can be induced in strained metal films, leading to profound stretchability regulation. An intriguing exponential-to-linear transition of the film electromechanical performance is observed, which is elucidated by a unified model covering the essence of all modes. Combined with room-temperature integration capability, an integrated electronic skin is constructed with metal films serving as stretchable electrodes, diverse sensors, and "tapes" to attach subcomponents, showing prospects in helping disabled people. This one-step, defect implantation strategy is applicable to common metals without special substrate treatments, which enables fascinating ultrastretchable metal film conductors with low-temperature integration capability to spark more sophisticated flexible electronic systems.

Bioinspired robot skin with mechanically gated electron channels for sliding tactile perception

Human-like tactile perception is critical for promoting robotic intelligence. However, reproducing tangential "sliding" perception of human skin is still struggling. Inspired by the lateral gating mechanosensing mechanism of mechanosensory cells, which perceives mechanical stimuli by lateral tension-induced opening-closing of ion channels, we report a robot skin (R-skin) with mechanically gated electron channels, achieving ultrasensitive and fast-response sliding tactile perception via pyramidal artificial fingerprint-triggered opening-closing of electron gates (E-gates, namely, customized V-shaped cracks within embedded mesh electron channels). By imitating cytomembrane to modulate membrane mechanics, local strain is enhanced at E-gates to effectively regulate electron pathways for high sensitivity while weakened at other positions to suppress random cracks for robust stability. The R-skin can directly recognize ultrafine surface microstructure (5 μm) at a response frequency (485 Hz) outshining humans and achieve human-like sliding perception functions, including dexterously distinguishing texture of complex-shaped objects and providing real-time feedback for grasping.

Topological Gradients for Metal Film-Based Strain Sensors

Metal film-based stretchable strain sensors hold great promise for applications in various domains, which require superior sensitivity-stretchability-cyclic stability synergy. However, the sensitivity-stretchability trade-off has been a long-standing dilemma and the metal film-based strain sensors usually suffer from weak cyclic durability, both of which significantly limit their practical applications. Here, we propose an extremely facile, low-cost and spontaneous strategy that incorporates topological gradients in metal film-based strain sensors, composed of intrinsic (grain size and interface) and extrinsic (film thickness and wrinkle) microstructures. The topological gradient strain sensor exhibits an ultrawide stretchability of 100% while simultaneously maintaining a high sensitivity at an optimal topological gradient of 4.5, due to the topological gradients-induced multistage film cracking. Additionally, it possesses a decent cyclic stability for >10 000 cycles between 0 and 40% strain enabled by the gradient-mixed metal/elastomer interfaces. It can monitor the full-range human activities from subtle pulse signals to vigorous joint movements.

Bioinspired Superspreading Surface: From Essential Mechanism to Application

ConspectusThe dynamic behavior of liquids on surfaces is ubiquitous in nature and has aroused wide attention from researchers. Among them, the superspreading surface has been extensively investigated and applied in areas ranging from film fabrication to antibiofouling, separation, etc. However, the traditional equilibrium contact angle (CA) at the thermodynamic steady-state cannot completely depict the dynamic spreading process of liquids, because the performance of these surfaces is controlled not only by the final steady superhydrophilicity (CA < 5°) but also by the superspreading speed of liquids with a CA of ∼0°. Moreover, as the most basic prerequisite for superspreading, the long-held intrinsic wetting threshold (IWT) of 90°, which divides hydrophobic and hydrophilic surfaces, is also controversial.In this Account, we summarize and condense the commonalities of our related research, further formally propose the concept of "superspreading", and recommend using "superspreading time (ST)" and "curve of superspreading radius versus spreading time (SRST)" to quantify its performance. Learning from nature is the most effective way to artificially fabricate superspreading surfaces. To begin, we first review some typical superspreading surfaces we found in nature and introduce the strategies adopted by the surfaces for surviving or realizing special functions. Then, we systematically review our recent understanding of the essential mechanism of superspreading surfaces across multiple length scales─from the molecular origin of the newly found IWT of ∼65° for water to the macroscopic respective functions of nanostructure and microstructure in superspreading. Armed with the in-depth fundamental mechanism, we propose the designing principle of high-performance superspreading surfaces. Following that, we summarize the commonly utilized methods, including modifying surface composition to give the surface intrinsic hydrophilicity and changing surface structure to improve the superspreading performance. Subsequently, we introduce the recently developed practical applications by virtue of the outstanding property of the superspreading surface, including the fabrication of a self-assembled film on the solid-gas surface and solid-liquid interface, a self-assembled water barrier for antibiofouling and oil repellency, high-efficiency separation and heat dissipation, etc. Finally, we discuss the remaining major challenges and the future development trends in the superspreading field. This Account serves to arouse wide attention and efforts in the superspreading field to strengthen mechanism research and promote practical large-area applications.

High Sensitivity and a Wide Sensing Range Flexible Strain Sensor Based on the V-Groove/Wrinkles Hierarchical Array

Flexible strain sensors occupying a large part of human body detection and wearable electronics, which have a wide sensing range and high sensitivity, are crucial in fully monitoring human motion signals. This study proposed a strategy to construct flexible strain sensors based on the V-groove/wrinkles hierarchical array. The V-groove array was prepared on a polydimethylsiloxane (PDMS) substrate through mold transfer printing. The gold film was sputtered on the prestretching PDMS substrate, and the V-groove/wrinkles hierarchical array was formed after strain release. Compared with the sensors based on single-scale wrinkle structures and a V-groove array, the fabricated strain sensor with the hierarchical array showed high sensitivity (maximum gauge factor up to 2,557.71) and a wide sensing range (up to 45%). In addition, the dynamic characteristics of the sensor were investigated in detail, indicating that the sensor had a fast response (less than 130 ms), a low detection limit (0.1% strain), and good stability (almost no performance loss after 10,000 cycles). In practical applications, the sensor was used to detect sizable physical motion and weak physiological signals, demonstrating great potential application value in human motion detection. This study could provide new ideas for preparing high-performance flexible strain sensors.

Fingerprint-Inspired Strain Sensor with Balanced Sensitivity and Strain Range Using Laser-Induced Graphene

Sensitivity and strain range are two mutually exclusive features of strain sensors, where a significant improvement in flexibility is usually accompanied by a reduction in sensitivity. The skin of a human fingertip, due to its undulating fingerprint pattern, can easily detect environmental signals and enhances sensitivity without losing elasticity. Inspired by this characteristic, laser-induced graphene (LIG) with a fingerprint structure is prepared in one step on a polyimide (PI) film and transferred into an Ecoflex substrate to assemble resistive strain sensors. Experimentally, the fingerprint-inspired strain sensor exhibits a superfast response time (∼70 ms), balanced sensitivity and strain range (a gauge factor of 191.55 in the 42-50% strain range), and good reliability (>1500 cycles). Self-organized microcracks, initiated in weak mechanical areas, cause prominent resistance changes during reconnection/disconnection but irreversibly fail after excessive stretching. The robust function of fingerprint-inspired sensors is further demonstrated by real-time monitoring of tiny pulses, large body movements, gestures, and voice recognition.

Continuous Energy Harvesting from Ubiquitous Humidity Gradients using Liquid-Infused Nanofluidics

Humidity-based power generation that converts internal energy of water molecules into electricity is an emerging approach for harvesting clean energy from nature. Here it is proposed that intrinsic gradient within a humidity field near sweating surfaces, such as rivers, soil, or animal skin, is a promising power resource when integrated with liquid-infused nanofluidics. Specifically, capillary-stabilized ionic liquid (IL, Omim⁺ Cl^- ) film is exposed to the above humidity field to create a sustained transmembrane water-content difference, which enables asymmetric ion-diffusion across the nanoconfined fluidics, facilitating long-term electricity generation with the power density of ≈12.11 µW cm^-2 . This high record is attributed to the nanoconfined IL that integrates van der Waals and electrostatic interactions to block movement of Omim⁺ clusters while allowing for directional diffusion of moisture-liberated Cl⁺ . This humidity gradient triggers large ion-diffusion flux for power generation indicates great potential of sweating surfaces considering that most of the earth is covered by water or soil.

Abrasion tolerant, non-stretchable and super-water-repellent conductive & ultrasensitive pattern for identifying slow, fast, weak and strong human motions under diverse conditions

The conversion of mechanical deformation into electrical signals is a widely used principle for various relevant applications. Facile & scalable fabrication, ultrahigh-sensitivity, low-response time and uninterrupted performance under severe conditions are hallmarks of an efficient strain-sensor that would be suitable for realistic application. In the past, various approaches were introduced to achieve high gauge factor-mainly associated with a large tensile deformation. But, in reality, a flexible strain sensor that displays a high gauge factor at low applied strain and remains efficient under practically relevant diverse and challenging conditions would be more appropriate for unambiguous and effective monitoring of human motions and other relevant applications. But, a low-strain sensor with ultrahigh sensitivity and durability is yet to be introduced in the literature. Here, a metal-free, chemically reactive and conductive ink is unprecedentedly introduced following a 1,4-conjugate addition reaction. Furthermore, a strategic integration of a chemically reactive porous paper with the prepared conductive ink allowed the development of a chemically reactive and conductive interface that allowed desired post covalent modification with selected alkylamines under ambient conditions. Taking advantage of the spatially selective deposition of the prepared ink on chemically recative paper and the ability of post covalent modification of the prepared ink, an abrasion tolerant superhydrophobic & conductive patterned interface was developed for achieving a low-strain (below 0.2%) based flexible strain sensor with an ultrahigh sensitivity (gauge factor ∼18 300) and low response time (8 ms). The external low-strain induced cracks on the flexible & durable superhydrophobic and conductive patterned interface provided a facile basis for real-time & wireless monitoring of slow, fast, weak and strong human motions & expressions-under diverse conditions, including continuous aqueous exposures, physical abrasions etc.

Robust Protection of III-V Nanowires in Water Splitting by a Thin Compact TiO2 Layer

Narrow-band-gap III-V semiconductor nanowires (NWs) with a suitable band structure and strong light-trapping ability are ideal for high-efficiency low-cost solar water-splitting systems. However, due to their nanoscale dimension, they suffer more severe corrosion by the electrolyte solution than the thin-film counterparts. Thus, short-term durability is the major obstacle for using these NWs for practical water-splitting applications. Here, we demonstrated for the first time that a thin layer (∼7 nm thick) of compact TiO₂ deposited by atomic layer deposition can provide robust protection to III-V NWs. The protected GaAs NWs maintain 91.4% of its photoluminescence intensity after 14 months of storage in ambient atmosphere, which suggests the TiO₂ layer is pinhole-free. Working as a photocathode for water splitting, they exhibited a 45% larger photocurrent density compared with unprotected counterparts and a high Faraday efficiency of 91% and can also maintain a record-long highly stable performance among narrow-band-gap III-V NW photoelectrodes; after 67 h photoelectrochemical stability test reaction in a strong acid electrolyte solution (pH = 1), they show no apparent indication of corrosion, which is in stark contrast to the unprotected NWs that fully failed after 35 h. These findings provide an effective way to enhance both stability and performance of III-V NW-based photoelectrodes, which are highly important for practical applications in solar-energy-based water-splitting systems.

Bioinspired design and assembly of a multilayer cage-shaped sensor capable of multistage load bearing and collapse prevention

Flexible bioinspired mesostructures and electronic devices have recently attracted intense attention because of their widespread application in microelectromechanical systems (MEMS), reconfigurable electronics, health-monitoring systems, etc. Among various geometric constructions, 3D flexible bioinspired architectures are of particular interest, since they can provide new functions and capabilities, compared to their 2D counterparts. However, 3D electronic device systems usually undergo complicated mechanical loading in practical operation, resulting in complex deformation modes and elusive failure mechanisms. The development of mechanically robust flexible 3D electronics that can undergo extreme compression without irreversible collapse or fracture remains a challenge. Here, inspired by the multilayer mesostructure of Enhydra lutris fur, we introduce the design and assembly of multilayer cage architectures capable of multistage load bearing and collapse prevention under large out-of-plane compression. Combined in situ experiments and mechanical modeling show that the multistage mechanical responses of the developed bionic architectures can be fine-tuned by tailoring the microstructural geometries. The integration of functional layers of gold and piezoelectric polymer allows the development of a flexible multifunctional sensor that can simultaneously achieve the dynamic sensing of compressive forces and temperatures. The demonstrated capabilities and performances of fast response speed, tunable measurement range, excellent flexibility, and reliability suggest potential uses in MEMS, robotics and biointegrated electronics.

Helically Intersected Conductive Network Design for Wearable Electronic Devices: From Theory to Application

Flexible and stretchable strain sensors are crucial components for wearable electronics that can detect and quantify the stimuli from the environment and thus realize the rapid feedback and control of smart devices. However, reconciliation of the conflict between resourceful design of conductive networks and large-scale production in the industry still faces a huge challenge. Herein, we present a new flow-manipulated strategy to prepare a wearable strain sensor featuring a helically intersected conductive network, which exhibited easy integration, multidimensional sensibility, and robust mechanical properties. From visualization of simulation and verification of experimental results, the helically intersected conductive network formed in an elastomer ring can simultaneously reflect the static and dynamic mechanical responses with a tunable gauge factor (10.41-31.12), wide linear region (0-40^o), mechanical robustness (σ_s = ∼7 MPa, ε = ∼1400%), and rapid response time (∼300 ms). We further constructed a control system based on smart rings and demonstrated its application in controlling industrial robotic arms and remote-controlled cars. Looking ahead, this kind of a smart ring will be more widely used in space and underwater exploration, intelligent robotics, and human-machine interface technologies.

Hollow-porous fibers for intrinsically thermally insulating textiles and wearable electronics with ultrahigh working sensitivity

Wearable smart devices should be flexible and functional to imitate the warmth and sensing functions of human skin or animal fur. Despite the recent great progress in wearable smart devices, it is still challenging to achieve the required multi-functionality. Here, stretchable hollow-porous fibers with self-warming ability are designed, and the properties of electrical heating, strain sensing, temperature sensing and pressure sensing are achieved. The hollow-porous TPU fiber possesses an ultra-high stretchability (1468%), and the textiles woven from the fibers present a splendid thermal insulation property (the absolute value difference in temperature |ΔT| = 68.5 and 44 °C at extreme temperatures of 115 and -40.0 °C). Importantly, after conductive filler decoration, the fiber-based strain sensor exhibits one of the highest reported gauge factor (2.3 × 10⁶) towards 100% strain in 7200 working stretch-release cycles. A low detection limit of 0.5% strain is also achieved. Besides, the fibers can be heated to 40 °C in 18 s at a small voltage of 2 V as an electrical heater. The assembled thermal sensors can monitor the temperature from 30 to 90 °C in real time, and the fiber-based capacitive type pressure sensor exhibits good sensing performance under force from 1 to 25 N. The hollow-porous fiber based all-in-one integrated wearable systems illustrate promising prospects for next generation electronic skins to detect human motions and body temperature with thermal therapy and inherent self-warming ability.

M13 Bacteriophage-Assisted Morphological Engineering of Crack-Based Sensors for Highly Sensitive and Wide Linear Range Strain Sensing

Despite their extraordinary mechanosensitivities, most channel-like crack-based strain sensors are limited by their poor levels of stretchability and linearity. This work presents a simple yet efficient way of modulating the cracking structure of thin metal films on elastomers to facilitate the development of high-performance wearable strain sensors. A net-shaped crack structure based on a thin platinum (Pt) film can be produced by coating an elastomer surface with M13 bacteriophages (phages) and consequently engineering the surface strain upon stretching. This process produces a Pt-on-phage (PoP) strain sensor that simultaneously exhibits high levels of stretchability (24%), sensitivity (maximum gauge factor ≈ 845.6 for 20-24%), and linearity (R² ≈ 0.988 up to 20%). In addition, the sensor performance can be further modulated by either changing the phage coating volume or adding a silver nanowire coating to the PoP sensor film. The balanced strain-sensing performance, combined with fast response times and high levels of mechanical flexibility and operational stability, enables the devices to detect a wide range of human motions in real time after being attached to various body parts. Furthermore, PoP-based strain sensors can be usefully extended to detect more complex multidimensional strains through further strain engineering on a cross-patterned PoP film.

Particulate-Aggregated Adhesives with Exudate-Sensitive Properties and Sustained Bacteria Disinfection to Facilitate Wound Healing

Wound-associated infections create additional suffering and come at a high cost for patients and their families, which urgently require wound disinfection biomaterials with improved healing efficacy. Here, we report an adhesive with sustained bacteria disinfection ability, which is aggregated from hydrogen-bonded polymer particulates. The particulate-aggregated adhesive shows strong binding ability on different surfaces from rigid substrates to soft skins. Moreover, water-sensitive mechanical properties are shown in wound exudates, resulting from the dissociation of hydrogen bonds under the competition of water and thus the sustained release of particulates. Synergizing with the strong binding ability, exudate-sensitive behaviors, and sustained release of antibacterial particulates, the adhesive achieves sustained bactericidal activity and can facilitate the healing process in bacteria-infected skin wounds.

Recent progress in controlled nano/micro cracking as an alternative nano-patterning method for functional applications

Generally, cracking occurs for many reasons connected to uncertainties and to the non-uniformity resulting from intrinsic deficiencies in materials or the non-linearity of applied external (thermal, mechanical, etc.) stresses. However, recently, an increased level of effort has gone into analyzing the phenomenon of cracking and also into methods for controlling it. Sophisticated manipulation of cracking has yielded various cutting-edge technologies such as transparent conductors, mechanical sensors, microfluidics, and energy devices. In this paper, we present some of the recent progress that has been made in controlling cracking by giving an overview of the fabrication methods and working mechanisms used for various mediums. In addition, we discuss recent progress in the various applications of methods that use controlled cracking as an alternative to patterning tools.

Magnetic Actuation Multifunctional Platform Combining Microdroplets Delivery and Stirring

Multifunctional droplets manipulation devices are in urgent need for various laboratory operations such as chemical reaction and biological analysis. However, most current techniques that achieved a controllable droplet transport system mainly rely on passive diffusion for mixing, limiting their practical applications. Here, we develop a magnetic controlled dimple on slippery surface (MCDSS) that enables arbitrary direction or even uphill droplet transport through the synergy between gravitational force and asymmetrical droplet deformation. Further experiments demonstrate that our system could also be used for stirring microdroplets and accelerating the mixing speed by more than one hundred times. In addition, the microstir strategy could help to avoid locally uneven production of precipitation or gas in heterogeneous reactions. This combination of droplet delivery and agitation may have a promising future for application in various fields, for example, laboratory-on-a-chip platforms and microengines.

Dual-Cross-Linked Supramolecular Polysiloxanes for Mechanically Tunable, Damage-Healable and Oil-Repellent Polymeric Coatings

Polymeric coatings that show tunable mechanical strength, healing ability of mechanical damage, and proper liquid repellency will be promising in various areas across life and industry. However, the exploitation of such coating materials is largely limited by their molecular design. In this work, polymeric coatings with ion-controlled mechanics and coloration and damage-healing and oil-sliding properties have been demonstrated based on a supramolecular design of dual-cross-linked polysiloxanes. The coating color and mechanical properties can be adjusted by coordinative metal ions with various metal-ligand binding abilities. Dense and dynamic hydrogen bonds and coordination bonds lead to the ready healing ability and high durability of the coating. The extreme smoothness of the flat silicone coating facilitates not only the sliding of impinging oil but also the restoration of topological integrity from mechanical damage. The coating can be selectively patterned and applied to large-scale substrates by diverse coating operations, making it feasible for versatile applications.