Stretchable Structures Research Articles

High-energy all-in-one stretchable micro-supercapacitor arrays based on 3D laser-induced graphene foams decorated with mesoporous ZnP nanosheets for self-powered stretchable systems

Micro-supercapacitors are promising energy storage devices that can complement or even replace lithium-ion batteries in wearable and stretchable microelectronics. However, they often possess a relatively low energy density and limited mechanical stretchability. Here, we report an all-in-one planar micro-supercapacitor arrays (MSCAs) based on hybrid electrodes with ultrathin ZnP nanosheets anchored on 3D laser-induced graphene foams (ZnP@LIG) arranged in island-bridge device architecture. The hybrid electrodes with a large specific surface area demonstrate excellent ionic and electrical conductivities, impressive gravimetric (areal) capacitance of 1425 F g −1 (7.125 F cm −2 ) at 1 A g −1 , and long-term stability. In addition to high energy (245 m Wh cm −2 ) and power (12.50 mW kg −1 at 145 m Wh cm −2 ) densities, the MSCAs with excellent cycling stability also showcase adjustable voltage and current outputs through serial and parallel connections of MSC cells in the island-bridge design, which also allows the system to be reversibly stretched up to 100%. Meanwhile, theoretical calculations validated by UV–vis absorption spectra partially suggest that the enhanced capacitance and rate capability may result from the improved electrical conductivity and number of adsorbed charged ions (Na + in Na 2 SO 4 aqueous electrolyte and K + in PVA/KCl gel electrolyte) on the pseudocapacitive non-layered ultrathin ZnP nanosheets. The integration of the all-in-one stretchable MSCAs with a crumpled Au-based triboelectric nanogenerator and stretchable crumpled graphene-based strain sensor demonstrates a self-powered stretchable system. The coupled design principle of electronic materials and device architecture provides a promising method to develop high-performance wearable/stretchable energy storage devices and self-powered stretchable systems for future bio-integrated electronics. • We report hybrid electrodes with ZnP nanosheets anchored on 3D laser-induced graphene foams. • The hybrid electrodes exhibit excellent ionic/electrical conductivities and long-term stability. • The hybrid electrodes with stretchable structures enable stretchable micro-supercapacitor arrays (MSCAs). • The MSCAs with adjustable voltage/current outputs can be reversibly stretched to 100%. • The integration of MSCAs with TENG and sensors demonstrates a self-powered stretchable system.

Structural Innovations in Printed, Flexible, and Stretchable Electronics

AbstractResearch in stretchable, printed electronics combines multidisciplinary, state‐of‐the‐art developments in material science and structural engineering. In addition to major advances based on developing novel materials and fabrication processes, synergistic structural innovations are of equal importance for enabling stretchability in printed devices and should not be overlooked. Planar printing techniques are preferred, compared to microfabrication or 3D printing processes, owing to their low cost, high throughput, and compatibility with a wide range of materials. Various printing strategies for controlling the substrate, bonding, distribution of strain, and buckling can be used to fabricate a variety of devices featuring wrinkled, textile‐embedded, serpentine, island‐bridge, or 2D‐transformed 3D and 4D structures. Such structural innovations allow the use of ordinary printable materials for creating highly stretchable devices with minimal compromise in device performance and mechanical resiliency. This article provides an overview of the structures used in printed devices and summarizes their corresponding fabrication strategies and distinct features. The challenges of advancing the structural designs in printed devices and the prospects of transforming stretchable structures toward smart, responsive devices are also discussed. Future efforts will greatly expand the possibilities of using planar printing processes for fabricating complex structures with new functionalities.

3D Printing of an In Situ Grown MOF Hydrogel with Tunable Mechanical Properties.

Due to their precisely modifiable microporosity and chemical functionality, Metal-Organic Frameworks (MOFs) have revolutionized catalysis, separations, gas storage, drug delivery, and sensors. However, because of their rigid and brittle powder morphology, it is challenging to build customizable MOF shapes with tunable mechanical properties. Here, we describe a new three-dimensional (3D) printing approach to create stretchable and tough MOF hydrogel structures with tunable mechanical properties. We formulate a printable ink by combining prepolymers of a versatile double network (DN) hydrogel of acrylamide and alginate, a shear-thinning agent, and MOF ligands. Importantly, by simultaneous cross-linking of alginate and in situ growth of the HKUST-1 using copper ions, we are able to create composites with high MOF dispersity in the DN hydrogel matrix with high pore accessibility. We extensively characterize the inks and uncover parameters to tune modulus, strength, and toughness of the 3D prints. We also demonstrate the excellent performance of the MOF hydrogels for dye absorption. Our approach incorporates all of the advantageous attributes of 3D printing while offering a rational approach to merge stretchable hydrogels and MOFs, and our findings are of broad relevance to wearables, implantable and flexible sensors, chemical separations, and soft robotics.

Multi-scale pattern with surface quasi crystal for wettability tuning

Transparent and stretchable structures with multi-scale patterns on PDMS surfaces for a tunable superhydrophobicity are presented. The samples result in the transfer of photoinduced quasi-crystal structures of azopolymer-based thin films onto an elastomer. The quasi-crystal surface patterns have different complexities generated with superimposed multiple exposures. The obtained samples can be stretched or compressed. Periodic circular nanocavities as small as 500 nm could be obtained with their shapes changed by stretching. The surface stretching or compressing induced on the elastomer surface allows the size manipulation of nanocavities. An application of these surfaces is presented for a large control of the super-hydrophobicity. The wetting of these patterns is compared to the same surface pattern modified with metal nanoparticles obtained by a metal dewetting process where the roughness is increased. We show that in this case the hydrophobicity is higher, and the droplet is sticky on these surfaces leading to a rose-petal effect.

Multi-axial electro-mechanical testing methodology for highly stretchable freestanding micron-sized structures

Recent advances in MEMS technology have brought forward a new class of high-density stretchable/flexible electronics as well as large displacement MEMS devices. The in-situ electro-mechanical characterization of such devices is challenging since it requires: (i) highly delicate sample handling, (ii) controlled application of large (hundreds of µm) multi-axial displacements to mimic service conditions, (iii) integrated electrical testing and (iv) fast actuation for cyclic testing. Techniques already developed for small-scale testing in literature fall short to meet the combined set of requirements. To this end, a characterization methodology that fulfills all these requirements is developed and presented here. The technique is based on a piezo-driven micro-tensile stage, which provides large multi-axial displacements with high resolution and fast actuation (4000 µm/s). This is combined with a method for sample microfabrication on a test-chip to warrant delicate sample handling. Proof-of-principle experiments are shown for multi-axial mechanical characterization, electrical characterization and high cycle fatigue testing of micron-sized highly stretchable interconnects. Experiments are conducted under in-situ microscopic observation using optical microscopy, scanning electron microscopy, and high-resolution profilometry. The generic platform proposed here can be used for other problems where similar requirements are faced, e.g. other miniaturized, large displacement electro-mechanical applications that are currently being developed.

Highly Stretchable Bilayer Lattice Structures That Elongate via In‐Plane Deformation

AbstractMany emerging technologies such as wearable batteries and electronics require stretchable functional structures made from intrinsically less deformable materials. The stretch capability of most demonstrated stretchable structures often relies on either initially out‐of‐plane configurations or the out‐of‐plane deflection of planar patterns. Such nonplanar features may dramatically increase the surface roughness, cause poor adhesion and adverse effects on subsequent multilayer processing, thereby posing a great challenge for flexible devices that require smooth surfaces (e.g., transparent electrodes in which flat‐surface‐enabled high optical transmittance is preferred). Inspired by the lamellar layouts of collagenous tissues, this work demonstrates a planar bilayer lattice structure, which can elongate substantially via only in‐plane motion and thus maintain a smooth surfaces. The constructed bilayer lattice exhibits a large stretchability up to 360%, far beyond the inherent deformability of the brittle constituent material and comparable to that of state‐of‐the‐art stretchable structures for flexible electronics. A stretchable conductor employing the bilayer lattice designs can remain electrically conductive at a strain of 300%, demonstrating the functionality and potential applications of the bilayer lattice structure. This design opens a new avenue for the development of stretchable structures that demand smooth surfaces.

Carbon Nanotube Fabric-Based Composites for Development of Multifunctional Structures

ABSTRACTCarbon nanotubes (CNTs) possess impressive properties along with low density. Integration of CNTs in the form of fabrics or other preformed assemblies simplifies their handling and allows for the higher CNT content needed to better leverage their properties in multifunctional structures. Here we describe production of non-woven CNT-polyurethane fabrics made from industrial-grade CNTs via a one-step filtration method. Individual sheets were scaled to 30 cm x 30 cm size and subsequently used to fabricate thicker composites, including via lamination with itself to produce simple panels and with other materials to further tailor the nanocomposite properties and address several example applications including electrical heating, fire resistance, electromagnetic shielding, and a skin for stretchable morphing structures.

Structural Anisotropy in Stretchable Silicon

AbstractPatterned planar silicon (Si) semiconductor structures able to conform around 3D surfaces are promising candidates for wearable devices from solar cells to displays. Despite the known anisotropic material properties of crystalline semiconductors, prior evaluations have assumed isotropic stretchable mechanical behavior. The effect of structural anisotropy on the mechanical stretching behavior of {100} single crystalline Si planes is demonstrated through models and experiments. 3D finite element analysis results show serpentines fabricated and strained loaded on the dense {111} family of planes will have maximum von Mises peak stresses that are 20% higher than those fabricated in the <100> direction on the {100} family of planes. A fabrication process is presented to release in‐plane Si serpentine test structures from (100) silicon‐on‐insulator wafers aligned parallel to the <110> and <100> crystallographic orientations. The released test structures can accommodate strains to 84%. Raman spectroscopy is used to characterize anisotropic effects on the internal stresses of Si serpentines. Raman measurements confirm two different maximum stress locations on <110> and <100> Si serpentines, resulting in two different break locations. All results show anisotropy plays a critical role in the mechanical behavior of stretchable Si serpentines and must be considered during design of stretchable semiconductor structures.

Ultrafast volume holography for stretchable photonic structures.

Stretchability and flexibility are two key requirements for manipulating the propagation of light in compact and high-performance lab-on-a-chip systems. These requirements are best met by embedding stretchable and flexible tuning elements such as volume phase gratings (VPGs) in polydimethylsiloxane (PDMS), making them attractive alternatives to conventional rigid optical elements. However, fabrication of these PDMS VPGs is a challenge, requiring extensive modifications to PDMS or complex multi-step processes that require long processing times. In this context, we propose the concept of "ultrafast volume holography" for the fabrication of stretchable photonic structures such as tunable VPGs directly in unmodified PDMS. Our concept translates insights in heat regulation via fs repetition rate control into volumetric patterning, forming periodic refractive index modulation of 1.95 × 10-4 in the PDMS without post-processing. VPGs formed are further demonstrated as active beam steering units and tunable spectroscopic optical elements.

Experimentally and Numerically Validated Analytical Solutions to Nonbuckling Piezoelectric Serpentine Ribbons

Emerging stretchable piezoelectric devices have added exciting sensing and energy harvesting capabilities to wearable and implantable soft electronics. As conventional piezoelectric materials are intrinsically stiff and some are even brittle, out-of-plane wrinkled or buckled structures and in-plane serpentine ribbons have been introduced to enhance their compliance and stretchability. Among those stretchable structures, in-plane piezoelectric serpentine ribbons (PSRs) are preferred on account of their manufacturability and low profiles. To elucidate the trade-off between compliance and sensitivity of PSRs of various shapes, we herein report a theoretical framework by combining the piezoelectric plate theory with our previously developed elasticity solutions for passive serpentine ribbons without piezoelectric property. The electric displacement field and the output voltage of a freestanding but nonbuckling PSR under uniaxial stretch can be analytically solved under linear assumptions. Our analytical solutions were validated by finite element modeling (FEM) and experiments using polyvinylidene fluoride (PVDF)-based PSR. In addition to freestanding PSRs, PSRs sandwiched by polymer layers were also investigated by FEM and experiments. We found that thicker and stiffer polymers reduce the stretchability but enhance the voltage output of PSRs. When the matrix is much softer than the piezoelectric material, our analytical solutions to a freestanding PSR are also applicable to the sandwiched ones.

Metamorphic Stretchable Touchpad

AbstractConventional touchpads are an example of a common human–machine interface. They are rigid devices with a limited usability without any form of adaptability and conformity to different morphologies necessary for gaming and virtual reality applications. A metamorphic touchpad is an envisioned conceptual approach of shape changing electronics. To demonstrate this, a multipurpose touchpad able to serve as a flexible, 2D stretchable, and 3D metamorphic device is designed and fabricated. The approach replaces the rigid carrier and rigid interconnects by mechanically stretchable structures. The metamorphic touchpad can be wrapped around 3D shapes or undergo reversible topological changes from a conventional planar to a hemispheric shape.

In-plane deformation mechanics of highly stretchable Archimedean interconnects

Stretchable inorganic electronics are very attractive for many applications, which require large deformation during stretching. Archimedean–inspired interconnect designs can offer and achieve high level of stretchability under extreme deformations. Here, we systematically investigate the relationship between stretchability and the geometrical parameters under in-plane deformation. The stretchable structures are made of amorphous silicon (a-Si), which cracked at very small strain 1.6%. Finite element method (FEM) was carried out to simulate the maximum strain/stress of interconnects. The results show that high stress appears at the base and the half-circle of the Archimedean interconnects. Experimental results agree well with the numerical modeling, both showing that the stretchability more than double when the straight line at the base is replaced by two lines in series. Our results demonstrate a stretchability up to 1020% and 605%, respectively for two types of Archimedean interconnect. The results indicate that the narrower width, the larger gap separated the straight lines (higher radius), and the longer straight lines will achieve lower stress and high stretchability. Further, a numerical study is conducted to explore the mechanical performance of Poly-crystalline silicon based structures where the maximum bending strain should be up to 1%.

Flexible and stretchable metal oxide gas sensors for healthcare

Capable of measuring volatile biomarker produced by the metabolism from several secretion pathways, flexible and stretchable metal oxide gas sensors have received increasing attention and their development for healthcare starts to gain momentum. Integration of semiconducting metal oxide on a soft, thin, flexible substrate is the key to enable the flexible property to the gas sensor and such integration typically involves either a direct growth or post transfer of the metal oxide on or to the flexible substrate. In addition to the planar plastic substrate, textile represents another important class of flexible substrates due to its ease of integration with clothing. Moreover, the integration of metal oxide on a single fiber provides a great versatility for different applications. Though flexible sensors can easily conform to the developable surface (e.g., cylinder or cone) from a bending deformation, the conformal contact between the sensor and the tissue surface that is often non-developable requires the sensor to be capable of stretching. Due to the intrinsically brittle nature of the semiconducting metal oxide, several stretchable structures have been explored. Despite the great strides made to the burgeoning area of flexible and stretchable metal oxide gas sensors, grand challenges still need to be overcome before the technology can be applied for the practical application. The selected challenges discussed in this mini-review also represent a fraction of possibilities and opportunities for the research community in the future.

Recent Development of Flexible and Stretchable Antennas for Bio-Integrated Electronics.

Wireless technology plays an important role in data communication and power transmission, which has greatly boosted the development of flexible and stretchable electronics for biomedical applications and beyond. As a key component in wireless technology, flexible and stretchable antennas need to be flexible and stretchable, enabled by the efforts with new materials or novel integration approaches with structural designs. Besides replacing the conventional rigid substrates with textile or elastomeric ones, flexible and stretchable conductive materials also need to be used for the radiation parts, including conductive textiles, liquid metals, elastomeric composites embedding conductive fillers, and stretchable structures from conventional metals. As the microwave performance of the antenna (e.g., resonance frequency, radiation pattern, and radiation efficiency) strongly depend on the mechanical deformations, the new materials and novel structures need to be carefully designed. Despite the rapid progress in the burgeoning field of flexible and stretchable antennas, plenty of challenges, as well as opportunities, still exist to achieve miniaturized antennas with a stable or tunable performance at a low cost for bio-integrated electronics.

Highly Stretchable Micro/Nano Wrinkle Structures for Infrared Stealth Application

We demonstrate a novel infrared stealth structure consisting of SiO2/TiO2 film, which was manufactured as the highly stretchable triangular wrinkle structures. The triangular wrinkle structures have firstly been transferred to the flexible substrate from the surface of Si-substrate, which was manufactured by the MEMS technology. Then, the infrared reflective film have been manufactured to be the triangular wrinkle structures by depositing the materials (noble metal (Ag or Au) or multilayer oxide (SiO2/TiO2)) on the surface of flexible substrate. Due to the lower reflection effect of curved surface, the infrared reflectivity of these structures has been tuned down to 5%. And, compared to the flat surface, the reflection-to-diffuse ratios improved approximately one order of magnitude. These structures can adapt to the environment by changing the reflectivity of triangular wrinkle structures under stretching. Finally, an Au-modified infrared stealth structure has been fabricated as the array structures, which disappeared and then display by stretching the triangular wrinkle structures at room temperature. It features high reflection-to-diffuse ratios, stable repeatability, low-cost, and easy to manufacture. It may open opportunities for infrared camouflage for military security and surveillance field application.

Multifunctionality and Mechanical Actuation of 2D Materials for Skin-Mimicking Capabilities.

Human skin serves as a multifunctional organ with remarkable properties, such as sensation, protection, regulation, and mechanical stretchability. The mimicry of skin's multifunctionalities via various nanomaterials has become an emerging topic. 2D materials have attracted much interest in the field of skin mimicry due to unique physiochemical properties. Herein, recent developments of using various 2D materials to mimic skin's sensing, protecting, and regulating capabilities are summarized. Next, to endow high stretchability to 2D materials, the approaches for fabrication of stretchable bilayer structures by integrating higher dimensional 2D materials onto soft elastomeric substrates are introduced. Accordion-like 2D material structures can elongate with elastomers and undergo programmed folding/unfolding processes to mimic skin's stretchability. That stretchable 2D material devices can achieve effective tactile sensing and protecting capabilities under large deformation is then highlighted. Finally, multiple key directions and existing challenges for future development are discussed.

Softening and Shape Morphing of Stiff Tough Hydrogels by Localized Unlocking of the Trivalent Ionically Cross-Linked Centers.

The mechanical properties (e.g., stiffness, stretchability) of prefabricated hydrogels are of pivotal importance for diverse applications in tissue engineering, soft robotics, and medicine. This study reports a feasible method to fabricate ultrasoft and highly stretchable structures from stiff and tough hydrogels of low stretchability and the application of these switchable hydrogels in programmable shape-morphing systems. Stiff and tough hydrogel structures are first fabricated by the mechanical strengthening of Ca2+ -alginate/polyacrylamide tough hydrogels by addition of Fe3+ ions, which introduces Fe3+ ionically cross-linked centers into the Ca2+ divalent cross-linked hydrogel, forming an additional and much less flexible trivalent ionically cross-linked network. The resulting stiff and tough hydrogels are exposed to an L-ascorbic acid (vitamin C, VC) solution to rapidly reduce Fe3+ to Fe2+ . As a result, flexible divalent ionically cross-linked networks are formed, leading to swift softening of the stiff and tough hydrogels. Moreover, localized stiffness variation of the tough hydrogels can be realized by precise patterning of the VC solution. To validate this concept, sequential steps of VC patterning are carried out for local tuning of the stiffness of the hydrogels. With this strategy, localized softening, unfolding, and sequential folding of the tough hydrogels into complex 3D structures is demonstrated.

(Invited) A Stretchable Leaf Sensor for Plant Monitoring

Ecological deterioration, increasing population, and nature resource shortage post huge pressure on the conventional agriculture mode. More advanced modes based on precision agriculture apply geospatial techniques and sensors (e.g., geographic information systems, remote sensing, GPS) to obtain quantified field and environmental data, and hold the promise to improve efficiency of resource distribution and production yield. Typical approaches in precision agriculture rely on aerial sensing information or non-contact measurement. However, such measurement techniques are difficult to provide continuous information, making prompt response to disastrous environmental conditions and precision guidance of agricultural activities extremely difficult. Stretchable and flexible electronic sensors that have been applied to human skin for monitoring the health conditions of the wearers can be potentially applied to the surface of plants, and grow with the plants over their entire life spans. Here, we present a highly stretchable, ultrathin sensor that can conformably attached on leaf and monitor both the physiological and environmental conditions of plants (Figure 1). The sensor possesses excellent stretchability that is compatible to the growth of the leaf with minimized influence to plant biology. The sensor contains stacked layers of polymeric and metallic meshes and four sensing elements to measure hydration, stain, temperature, and light intensity on leaf, respectively (Figure 2). The serpentine interconnects within the sensor is made of two levels of stretchable structures, allowing extremely large stretchability to adapt to the growth of leaf. The hydration sensing element contains a pair of electrodes in the form of an inner circular disk (200 μm in radius) and an outer circular annulus (400 μm in inner radius and 600 mm in outer radius). The strain sensing element consists of two perpendicular SWCNTs strips (500×30 μm2) in response to strain changes both in vertical and horizontal directions. The temperature sensor exploits ultrathin copper film (thickness ~50nm) that designed to serpentine shape, in order to increase the resistance compared to the serpentine interconnections. In addition, a phototransistor is used to measure the ambient light intensity. The sensor can be connected with a wireless measurement circuits to conduct remote data transmission. The circuit, which is 3×3×2 cm2 in its dimension, consists of 4 stacked PCB layers, and uses Zigbee protocol to enable longer data transmission range (Figure 3). The power of the circuit is both supplied by a polymer rechargeable battery and solar cells to accommodate the continuous requirement of data monitoring and transmission on field. We have conducted preliminary experiments of this sensor under controlled environment. The impedance measurement results obtained by injecting a piece of filter paper with different amount of water demonstrate that the sensor can response to a hydration changes from wet to dry, with more than 11% changes in its impedance at a frequency of 70 kHz (Figure 4). In addition, the temperature sensing element has been calibrated on a hot plate with varied temperature from 40 to 100 °C. The sensing element exhibits linear resistance changes from 4100 to 4250 Ω (Figure 5). Furthermore, the phototransistor can response to light intensity changes from 0 to 100000 lux (figure 6), which is equivalent to the light intensity during the night and the day. More experiments related to other sensing elements as well as the wireless circuit will be reported during the meeting. In conclusion, the stretchable leaf sensor offers high stretchability to match the mechanical properties of leaf. The leaf sensor can be conformably attached on the surface of leaf and accommodate to the growth of leaf. The experimental results have demonstrated that the sensor can be used to monitor the biophysiological and environmental conditions on leaf to facilitate precision agriculture. Figure 1

One‐Step Selective Adhesive Transfer Printing for Scalable Fabrication of Stretchable Electronics

AbstractTransferring patterned thin foil structures onto an elastic substrate is one important process for realizing heterogeneous integration and high‐performance stretchable electronics. Usually, specific equipment and long learning curve are necessary for transferring fragile micro/nanostructures. In this work, a one‐step selective transfer printing technique can selectively transfer fine metal structure from a donor substrate to a sticky polydimethylsiloxane receiver by tuning its adhesion force and hence corresponded energy release rate (G). The method greatly simplifies the transferring process compared with traditional ones. The selective transferring lies in the fact that a wider line requires a higher energy release rate, based on experimental and numerical analysis. With the selective adhesive transferring technique, a few simple stretchable devices and structures are fabricated, where a device does not exhibit any fatigue after stretching for 10 000 cycles under a 40% strain. Further, this method does not use any new specific equipment and is believed to be a reliable and cost‐effective technique for potential mass production of stretchable electronics.

Mechanisms and Materials of Flexible and Stretchable Skin Sensors

Wearable technology has attracted significant public attention and has generated huge societal and economic impact, leading to changes of both personal lifestyles and formats of healthcare. An important type of devices in wearable technology is flexible and stretchable skin sensors used primarily for biophysiological signal sensing and biomolecule analysis on skin. These sensors offer mechanical compatibility to human skin and maximum compliance to skin morphology and motion, demonstrating great potential as promising alternatives to current wearable electronic devices based on rigid substrates and packages. The mechanisms behind the design and applications of these sensors are numerous, involving profound knowledge about the physical and chemical properties of the sensors and the skin. The corresponding materials are diverse, featuring thin elastic films and unique stretchable structures based on traditional hard or ductile materials. In addition, the fabrication techniques that range from complementary metal-oxide semiconductor (CMOS) fabrication to innovative additive manufacturing have led to various sensor formats. This paper reviews mechanisms, materials, fabrication techniques, and representative applications of flexible and stretchable skin sensors, and provides perspective of future trends of the sensors in improving biomedical sensing, human machine interfacing, and quality of life.