Exciting advancements have been made in the field of flexible electronic devices in the last two decades and will certainly lead to a revolution in peoples’ lives in the future. However, because of ...the poor sustainability of the active materials in complex stress environments, new requirements have been adopted for the construction of flexible devices. Thus, hierarchical architectures in natural materials, which have developed various environment-adapted structures and materials through natural selection, can serve as guides to solve the limitations of materials and engineering techniques. This review covers the smart designs of structural materials inspired by natural materials and their utility in the construction of flexible devices. First, we summarize structural materials that accommodate mechanical deformations, which is the fundamental requirement for flexible devices to work properly in complex environments. Second, we discuss the functionalities of flexible devices induced by nature-inspired structural materials, including mechanical sensing, energy harvesting, physically interacting, and so on. Finally, we provide a perspective on newly developed structural materials and their potential applications in future flexible devices, as well as frontier strategies for biomimetic functions. These analyses and summaries are valuable for a systematic understanding of structural materials in electronic devices and will serve as inspirations for smart designs in flexible electronics.
Compared with traditional stimuli‐responsive devices with simple planar or tubular geometries, 3D printed stimuli‐responsive devices not only intimately meet the requirement of complicated shapes at ...macrolevel but also satisfy various conformation changes triggered by external stimuli at the microscopic scale. However, their development is limited by the lack of 3D printing functional materials. This paper demonstrates the 3D printing of photoresponsive shape memory devices through combining fused deposition modeling printing technology and photoresponsive shape memory composites based on shape memory polymers and carbon black with high photothermal conversion efficiency. External illumination triggers the shape recovery of 3D printed devices from the temporary shape to the original shape. The effect of materials thickness and light density on the shape memory behavior of 3D printed devices is quantified and calculated. Remarkably, sunlight also triggers the shape memory behavior of these 3D printed devices. This facile printing strategy would provide tremendous opportunities for the design and fabrication of biomimetic smart devices and soft robotics.
3D‐printed photoresponsive devices are successfully fabricated through combining fused deposition modeling and photoresponsive shape‐memory composites based on polyurethane and carbon black. This study demonstrates the shape‐memory behavior of these 3D‐printed devices is well triggered by sunshine. This development provides tremendous opportunities for customized stimuli‐responsive devices with complex geometric structure, which have potential in soft robots, selectively pliable tools, and orthopedic surgery.
Compared to transmission systems based on shafts and gears, tendon-driven systems offer a simpler and more dexterous way to transmit actuation force in robotic hands. However, current tendon fibers ...have low toughness and suffer from large friction, limiting the further development of tendon-driven robotic hands. Here, we report a super tough electro-tendon based on spider silk which has a toughness of 420 MJ/m
and conductivity of 1,077 S/cm. The electro-tendon, mechanically toughened by single-wall carbon nanotubes (SWCNTs) and electrically enhanced by PEDOT:PSS, can withstand more than 40,000 bending-stretching cycles without changes in conductivity. Because the electro-tendon can simultaneously transmit signals and force from the sensing and actuating systems, we use it to replace the single functional tendon in humanoid robotic hand to perform grasping functions without additional wiring and circuit components. This material is expected to pave the way for the development of robots and various applications in advanced manufacturing and engineering.
Stretchable strain sensors play a pivotal role in wearable devices, soft robotics, and Internet‐of‐Things, yet these viable applications, which require subtle strain detection under various strain, ...are often limited by low sensitivity. This inadequate sensitivity stems from the Poisson effect in conventional strain sensors, where stretched elastomer substrates expand in the longitudinal direction but compress transversely. In stretchable strain sensors, expansion separates the active materials and contributes to the sensitivity, while Poisson compression squeezes active materials together, and thus intrinsically limits the sensitivity. Alternatively, auxetic mechanical metamaterials undergo 2D expansion in both directions, due to their negative structural Poisson's ratio. Herein, it is demonstrated that such auxetic metamaterials can be incorporated into stretchable strain sensors to significantly enhance the sensitivity. Compared to conventional sensors, the sensitivity is greatly elevated with a 24‐fold improvement. This sensitivity enhancement is due to the synergistic effect of reduced structural Poisson's ratio and strain concentration. Furthermore, microcracks are elongated as an underlying mechanism, verified by both experiments and numerical simulations. This strategy of employing auxetic metamaterials can be further applied to other stretchable strain sensors with different constituent materials. Moreover, it paves the way for utilizing mechanical metamaterials into a broader library of stretchable electronics.
Auxetic mechanical metamaterials are employed to significantly enhance the sensitivity of stretchable strain sensors, by regulating the transverse Poisson effect due to auxetic expansion. High sensitivity with almost 24‐fold improvement is achieved, together with high maximum stretchability and cyclic durability. Additionally, the underlying mechanism, elongated microcracks, is proven by both experiments and numerical simulations.
Soft and stretchable electronic devices are important in wearable and implantable applications because of the high skin conformability. Due to the natural biocompatibility and biodegradability, silk ...protein is one of the ideal platforms for wearable electronic devices. However, the realization of skin‐conformable electronic devices based on silk has been limited by the mechanical mismatch with skin, and the difficulty in integrating stretchable electronics. Here, silk protein is used as the substrate for soft and stretchable on‐skin electronics. The original high Young's modulus (5–12 GPa) and low stretchability (<20%) are tuned into 0.1–2 MPa and > 400%, respectively. This plasticization is realized by the addition of CaCl2 and ambient hydration, whose mechanism is further investigated by molecular dynamics simulations. Moreover, highly stretchable (>100%) electrodes are obtained by the thin‐film metallization and the formation of wrinkled structures after ambient hydration. Finally, the plasticized silk electrodes, with the high electrical performance and skin conformability, achieve on‐skin electrophysiological recording comparable to that by commercial gel electrodes. The proposed skin‐conformable electronics based on biomaterials will pave the way for the harmonized integration of electronics into human.
Silk protein is plasticized and demonstrated as a soft and stretchable substrate for skin‐conformable stretchable electronics. The plasticization is enabled by the addition of CaCl2 and ambient hydration, and the mechanism is investigated using molecular dynamics simulation. Moreover, highly stretchable (>100%) silk electrodes are fabricated for on‐skin electrophysiological measurements. This progress will enable more biomaterial‐based wearable electronics.
The first mechanically and electrically self‐healing supercapacitor has been successfully fabricated. It exhibits excellent self‐healing performance with the restoration of the specific capacitance ...up to 85.7% of its original value even after the 5th mechanical cutting. This achievement may provide a way to expand the lifetime of future energy storage devices and endow them with desirable economic and human safety attributes, as well as promote the development of next‐generation self‐healing electronics.
Flexible electronic devices are necessary for applications involving unconventional interfaces, such as soft and curved biological systems, in which traditional silicon‐based electronics would ...confront a mechanical mismatch. Biological polymers offer new opportunities for flexible electronic devices by virtue of their biocompatibility, environmental benignity, and sustainability, as well as low cost. As an intriguing and abundant biomaterial, silk offers exquisite mechanical, optical, and electrical properties that are advantageous toward the development of next‐generation biocompatible electronic devices. The utilization of silk fibroin is emphasized as both passive and active components in flexible electronic devices. The employment of biocompatible and biosustainable silk materials revolutionizes state‐of‐the‐art electronic devices and systems that currently rely on conventional semiconductor technologies. Advances in silk‐based electronic devices would open new avenues for employing biomaterials in the design and integration of high‐performance biointegrated electronics for future applications in consumer electronics, computing technologies, and biomedical diagnosis, as well as human–machine interfaces.
Silk fibroin is an ancient biomaterial with exquisite mechanical, optical, and electrical properties. Its intriguing properties and environmental benignity render silk fibroin compelling for the advancement of next‐generation biocompatible and biodegradable flexible electronic devices.
A highly sensitive tactile sensor is devised by applying microstructured graphene arrays as sensitive layers. The combination of graphene and anisotropic microstructures endows this sensor with an ...ultra‐high sensitivity of –5.53 kPa−1, an ultra‐fast response time of only 0.2 ms, as well as good reliability, rendering it promising for the application of tactile sensing in artificial skin and human–machine interface.
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