Stretchable Material Research Articles

Silver and epoxy binder-based printed electrodes and the effect of silver nanoparticles on stretchability

In recent years, there has been a lot of research on printed paste and ink composite electrode material made with metallic-based materials such as silver nanowires, silver nanoparticles, and flakes with epoxy-based binders. These stretchable electrode materials must exhibit metal-level conductivity and high interfacial adhesion that can withstand necking and deformation stress. To overcome several issues including the low adhesion properties found in silicone-based binders and the low heat resistance found in urethane-based binders, we synthesized a binder material with over 300% stretchability and high adhesive strength. This was done by modifying an epoxy to produce a binder that can be easily applied to various materials without special surface treatment technologies. In addition, we developed a printable, stretchable electrode material with high adhesive strength, high conductivity, and high stretchability by mixing three types of silver powders (flake, spherical micro-particle, and nanoparticle) with the modified epoxy binder. Results from electrode conductivity, stretchability, repetitive fatigue property, and residual strain analyses demonstrate that these properties are highly dependent on nanoparticle content and distribution. The electrode resistivities ranged from 8.5 × 10−5 to 2.5 × 10−5 Ω cm after heat curing at 170 °C and showed excellent stability at strain of over 140%.

Graphene Nanoscrolls with Confined Silicon Nanoparticles as a Durable Anode for Lithium‐Ion Batteries

AbstractSilicon anodes have been given great focus due to the unparalleled theoretical specific capacity. However, destructive volume expansion during lithiation/delithiation processes and intrinsic inferior electrical conductivity cause drastic capacity decay. Forming composites with conductive and stretchable material is an effective solution to address these problems. Here, a three‐dimensional continuous conductive network consisting of one‐dimensional flexible graphene nanoscroll wrapped silicon nanoparticles (Si@GNS) is realized by a simple cold quenching method followed by a facile annealing process. GNS acted as conductive framework and elastic buffer layer, improving electrochemical reaction kinetics and suppressing Si volume expansion, respectively. Hence, Si@GNS shows excellent rate capability of 760 mAh g−1 even at 2 A g−1 and long‐term cycling stability of 81.5% capacity retention after 1000 cycles.

Fabrication and characterization of a polymeric curved compound eye

Curved compound eyes, i.e. microlens arrays on curved surfaces, are highly desirable for their unique optical properties including wide field of view, tracking of fast moving objects and so on. However, it is technically challenging to fabricate biomimetic gapless microlens arrays. In this work, we report a simple method for fabricating close-packed microlenses in a kind of stretchable polymeric material PDMS on curved surfaces inspired by the vertebrate eyes. The successfully fabricated polymeric curved compound eye consists of more than 6000 close-packed ommatidia with an average diameter of 600 µm for each ommatidium. The ommatidia are located on a convex surface with a diameter of 40 mm and thus a total field of view of about 180° has been obtained. The optical test on ommatidia shows that the NA for each ommatidium is about 0.21 and the imaging result of the whole compound eye is also given. Furthermore, an optical relay system is introduced to integrate with the compound eye to form a biomimetic compound eye camera. The formed camera is shown to have a great potential for a broad range of optical imaging applications, such as surveillance imaging, target detection and tracking, surveying and mapping, collision-free navigation of terrestrial and aerospace vehicles.

Wearable and Stretchable Triboelectric Nanogenerator Based on Crumpled Nanofibrous Membranes.

Triboelectric nanogenerator could efficaciously harvest the mechanical energy that come from the ambient environment, which has become a research hotspot in the sphere of wearable electronic technology. Here, a self-powered and highly stretchable single-electrode triboelectric nanogenerator with an undulating three-dimensional surface crumpled structure is reported. The triboelectric nanogenerator has a multilayer structure with a crumpled nanofiber membrane as the triboelectric material. Due to the materials and structural innovations, the triboelectric nanogenerator possesses outstanding electric output stability and stretchability. It could subtly transform the unstretched flexible nanofiber membrane into a stretchable material, while overcoming the deficiency that the nanofiber membrane has a tendency to be delaminated from the electrode layer during long-term operation. Utilizing the triboelectric nanogenerator directly attached to human skin could efficaciously harvest the ignored mechanical energy that come from our daily activities.

Latitudinally deforming rotating sphere

In this study, we consider a sphere with a surface that is fully covered by a stretchable elastic material. The radius of the sphere is fixed and it is also rotating about its radial axis. We investigate how the axisymmetric motion of a triggered fluid flow around the sphere is affected by the presence of both sphere rotation and latitudinal stretching. Considering that the deformation over the sphere commences at the pole, the problem is formulated such that the fluid flow near the pole is similar to the induced flow due to a linearly stretchable rotating disk, which has been described well in previous studies. When the rotation is omitted, the flow develops two-dimensionally under the action of pure stretching; otherwise, a three-dimensional axisymmetric fluid flow occurs, which is computed at each latitudinal angle both numerically and using a perturbation approach. The solution with wall deformation is different from the traditional character of the solution due to a solely rotating sphere. This solution is then used to compute the surface shears due to the physical drag and torque acting over the sphere. The contribution of wall stretching reduces the drag, whereas high rotation suppresses the effects of stretching to enhance the drag. More torque is required to rotate the sphere when both stretching and rotation mechanisms are in action.

Distortion energy-based topology optimization design of hyperelastic materials

Highly stretchable material is widely used in the engineering field ranging from soft robots to stretchable electronics. Some highly stretchable 3D-architected mechanical metamaterials have been developed recently. However, failure of material is still the most critical design constraint when stretchability of the structure is considered, and existing topology optimization methods based on, e.g., von Mises stress failure criterion, are not accurate when applied to design hyperelastic materials under large deformation. To address this issue, this paper presents a topology optimization method based on distortion energy for designing nonlinear hyperelastic material against failure. For this purpose, a new objective function based on distortion energy for hyperelastic materials is proposed in the p-norm form. The adjoint method is applied to obtain the sensitivities, and the corresponding optimization problem is solved by the method of moving asymptotes. Excessive mesh distortion in low-density area is addressed through an interpolation scheme of the strain energy known as the fictitious domain method. Four numerical design examples are presented to demonstrate the validity and effectiveness of the proposed algorithm in significantly reducing local distortion energy concentration. One of these examples involves design of highly stretchable metamaterial where the optimized design is shown to sustain three times the finite strain of its base material under uniaxial tension without failure. Therefore, the proposed method has great potential in designing extremely stretchable materials such as stretchable electronics in the future.

Stretchable and patchable composite electrode with trimethylolpropane formal acrylate-based polymer

With the huge variety of wearable/stretchable electronic devices that require materials with different properties depending on the applications, securing the large reserves of stretchable material systems is crucial. In this study, trimethylolpropane formal acrylate (TFA)-based polymer films are explored to develop stretchable composite electrodes. The TFA resin containing 1-hydroxycyclohexyl phenyl ketone as a photoinitiator is UV curable and shows a high optical transmittance of ∼93.0 ± 0.3% and a low haze of ∼0.29 ± 0.03%. The TFA films can withstand a tensile strain up to 100% and show excellent mechanical reliability during 30 cycles of tensile tests. Furthermore, the elastomeric TFA films are attachable to polymer or glass substrates, and during multiple adhesion tests, show a lower degradation of the adhesion properties than that showed by typical adhesive materials such as scotch tapes. The electrodes fabricated by embedding Ag nanowires in the surface of the TFA films (TFA/Ag nanowire electrodes) function as stretchable/patchable conductors under severe mechanical deformation such as stretching and crumpling. Furthermore, a patchable/stretchable supercapacitor comprising the TFA/Ag nanowire electrodes as a current collector is demonstrated that shows a reliable electrochemical performance under bending, stretching, and twisting conditions.

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.

Waterproof and stretchable triboelectric nanogenerator for biomechanical energy harvesting and self-powered sensing

We introduce a waterproof and stretchable triboelectric nanogenerator (TENG) that can be attached on the human body, such as fingers and the wrist, to harvest mechanical energy from body movement. The whole device is composed of stretchable material, making it able to endure diverse mechanical deformations and scavenge energy from them. Under gentle mechanical motions of pressing, stretching and bending, the device with an effective area of 1 × 2 cm2 can generate the peak-to-peak output current of 257.5 nA, 50.2 nA, and 33.5 nA, respectively. Besides, the TENG is tightly encapsulated, enabling it to avoid the influence of the external environment like humidity changes and harvest energy under water. Particularly, owing to the thin and soft properties of the encapsulation film, the device can respond to weak vibrations like the wrist pulse and act as a self-powered pulse sensor, which broadens its application prospects in the field of wearable energy harvesting devices and self-powered sensing systems.

Operationalizing low-cost three-dimensional printing in planning for complex congenital cardiac interventions

ObjectivesThis study aims to evaluate the feasibility and efficacy of commercially available 3-D printing technology for anatomical modeling. BackgroundAdditive manufacturing has been widely talked about in the context of personalized medical care and interventional and surgical planning for the past decade. Early studies have shown reduction in procedure time and optimization of device deployment by improving anticipation of potential obstacles by bench testing reconstructed models. However, commercially printed 3-D models are costly, ranging between $ 1,500–$ 3500 per model. Our goal is to produce heart models of acceptable quality for as low as $ 1–$ 20 and to demonstrate their utility in planning and carrying out complex congenital heart interventions. MethodsLow-and mid-tier fused deposition modeling (FDM) printers able to utilize flexible filament along with a sophisticated modeling software were used in our practice to create congenital heart models for bench testing. For cases that required softer and more stretchable material than the available flexible filaments offered, molding and casting with dissolvable filament and silicone was carried out. Results15 models were constructed and 13 were bench tested (8 commercial models), preliminary data gathered by questionnaires completed by treating physicians before and after obtaining the 3D models show equal improvement in pre-procedure planning and increased confidence about the procedure with both model types. ConclusionsThe use of commercially available reasonably priced 3–D printing technology is feasible for printing complex congenital heart disease models in a cost-effective manner. These models were not deemed to be inferior to commercially printed models that are up to 100 times more expensive.

Dynamic ionic crosslinks enable high strength and ultrastretchability in a single elastomer

Maintenance-free self-healing elastomers that switch their mechanical properties on demand would be extremely useful materials for improving the functionalities, safety, energy efficiency, and lifetimes of many kinds of products and devices. However, strength and stretchability are conflicting properties for elastomers because the inherent crosslinking density of a polymeric network is unchangeable. For example, heavily crosslinked elastomers are strong, but poorly stretchable. Here we report an ionically crosslinked polyisoprene elastomer in which the ionic moieties are continually hopping between ionic aggregates at room temperature. Thus, the network is dynamic. This elastomer spontaneously self-heals without the input of external energy or healing agents. Furthermore, it behaves like a strong elastic material under rapid deformation, but acts like a highly stretchable and viscoelastic material under slow deformation. Our ionic elastomer shows a variety of notable mechanical properties, including high fracture strength (≈7 MPa), good toughness (≈70 MJ m−3), and ultrastretchability (>13,400%).

Touch Locating and Stretch Sensing Studies of Conductive Hydrogels with Applications to Soft Robots.

Soft robots possess great potential in environmental adaptations, while their environmental sensing abilities are critical. Conductive hydrogels have been suggested to possess sensing abilities. However, their application in soft robots is lacking. In this work, we fabricated a soft and stretchable gel material, introduced its sensing mechanisms, and developed a measurement setup. Both experimental and simulation studies indicate strong nonlinearity of touch locating on a square touch panel with Cartesian coordinates. To simplify the touch locating, we proposed a touch locating system based on round touch panels with polar coordinates. Mathematical calculations and finite element method (FEM) simulations showed that in this system the locating of a touch point was only determined by its polar radius. This was verified by experimental studies. As a resistor, a gel strip’s resistance increases with stretching. To demonstrate their applications on soft robots, a 3D printed three-fingered soft gripper was employed with gel strips attached. During finger bending for rod grasping, the resistances of the gel strips increased, indicating stretching of the soft material. Furthermore, the strain and stress of a gel strip increased with a decrease of the rod diameter. These studies advance the application of conductive hydrogels on soft robots.

New stretchable material inspired by octopus camouflage

New stretchable material inspired by octopus camouflage

Stretchable scintillator composites for indirect X-ray detectors

Abstract Flexible and stretchable materials are increasingly being investigated for future technological platforms, polymer based materials being the most suitable candidates for those emerging technologies. This work reports on polymer based scintillator composites based on the thermoplastic elastomer Styrene-Ethylene/Butadiene-Styrene (SEBS) and Gd2O3:Eu3+ scintillator nanoparticles, to form a polymer-based flexible and stretchable material for X-ray indirect detectors. Further, visible light yield under X-ray irradiation was improved by the inclusion of 2,5 dipheniloxazol (PPO) and (1,4-bis (2-(5-phenioxazolil))-benzol (POPOP) within the polymer matrix. Together with high levels of stretchability, with deformations up to 100%, the films exhibit suitable performance with low mechanical hysteresis (less than 1.5 MJ/m3 for cycles up to 100% of strain) and reproducibly such as a scintillator material for the conversion of X-ray radiation into visible radiation. The decrease of just ∼13% of the X-ray radiation into visible light upon stretching up to 100% is attributed to a reduction of the effective filler concentration and proves the suitability of the developed materials for large area and stretchable X-ray radiation detectors.

Slippery when stretched: New soft, stretchable material provides tunable wetting characteristics

Researchers have designed a soft, elastic material whose surface texture – and thus wetting properties – changes upon stretching and relaxing.

Mechanical Testing for Stretchable Electronics

Stretchable electronics have been a subject of increased research over the past decade (Lacour, S., et al., 2006, “Mechanisms of Reversible Stretchability of Thin Metal Films on Elastomeric Substrates,” Appl. Phys. Lett., 88(20), p. 204103; Lacour, S., et al., 2004, “Design and Performance of Thin Metal Film Interconnects for Skin-Like Electronic Circuits,” IEEE Electron Device Lett., 25(4), pp. 179–181; and Maghribi, M., et al., 2005, “Stretchable Micro-Electrode Array,” International IEEE-EMBS Conference on Microtechnologies in Medicine and Biology, pp. 80–83.). Although stretchable electronic devices are a relatively new area for the semiconductor/electronics industries, recent market research indicates that the market could be worth more than $900 million by 2023 (PR Newswire, 2015, “Stretchable Electronics Market Worth $911.37 Million by 2023,” PR Newswire, Albuquerque, NM.). This paper investigates mechanical testing methods designed to test the stretching capabilities of potential products across the electronics industry to help quantify and understand the mechanical integrity, response, and the reliability of these devices. Typically, the devices consist of stiff modules connected by stretchable traces (Loher, T., et al., 2006, “Stretchable Electronic Systems,” Electronics Packaging Technology Conference (EPTC '06), pp. 271–276.). They require electrical and mechanical connectivity between the modules to function. In some cases, these devices will be subject to biaxial and/or cyclic mechanical strain, especially for wearable applications. The ability to replicate these mechanical strains and understand their effect on the function of the devices is critical to meet performance, process, and reliability requirements. In this paper, methods for simulating biaxial and out-of-plane strains similar to what may occur in a wearable device on the human body are proposed. Electrical and/or optical monitoring (among other methods) can be used to determine cycles to failure depending on expected failure modes. Failure modes can include trace damage in stretchable regions, trace damage in functional component regions, or bulk stretchable material damage, among others. Three different methods of applying mechanical strain are described, including a stretchable air bladder method, membrane test method, and lateral expansion method.

Arbitrary alignment-angle control method of electrospun fibers: potential for a stretchable electrode material

This paper presents a method by which it is possible to control the align-angle arbitrarily from vertical to horizontal demonstrating a mechanical stretch of as much as 120%. Fibers are aligned directly onto the sample surface.

IMU Sensor-based Electronic Goniometric Glove (iSEG-Glove) for clinical finger movement analysis

Arthritis remains a disabling and painful disease, and involvement of finger joints is a major cause of disability and loss of employment. Traditional arthritis measurements require labour intensive examination by clinical staff. These manual measurements are inaccurate and open to observer variation. This paper presents the development and testing of a next generation wireless smart glove to facilitate the accurate measurement of finger movement through the integration of multiple IMU sensors, with bespoke controlling algorithms. Our main objective was to measure finger and thumb joint movement. These dynamic measurements will provide clinicians with a new and accurate way to measure loss of movement in patients with rheumatoid arthritis. Commercially available gaming gloves are not fitted with sufficient sensors for this particular application, and require calibration for each glove wearer. Unlike these state-of-the-art data gloves, the inertial measurement unit glove uses a combination of novel stretchable substrate material and nine degree of freedom inertial sensors in conjunction with complex data analytics to detect joint movement. Our novel iSEG-Glove requires minimal calibration and is therefore particularly suited to the healthcare environment. Inaccuracies may arise for wearers who have varying degrees of movement in their finger joints, variance in hand size or deformities. The developed glove is fitted with sensors to overcome these issues. This glove will help quantify joint stiffness and monitor patient progression during the arthritis rehabilitation process.

Stretchable and durable superhydrophobicity that acts both in air and under oil

A facile and robust Michael addition reaction is strategically exploited here, to develop a highly stretchable (150% deformation) superhydrophobic material. This material strongly repels aqueous phase both in air and under oil and with impeccable physical/chemical durability, and is appropriate for rapid separation of both heavy and light oils from complex aqueous phases with above 99% efficiency.

Thin film wrinkling by strain mismatch on 3D surfaces

Wrinkling of thin films in micro- and nanoscale has found wide applications in stretchable electronics, optical gratings, material characterization, and biology. However, most wrinkling work is in planar fashion and only limited work on 3D wrinkling has been reported. In this paper, we present experimental demonstration of 3D Cr film wrinkling on PDMS micro-ridges induced by strain mismatch. We first fabricated PDMS micro-ridges using replication from a silicon mold, prepared by conventional microfabrication processes. While pre-stretching the PDMS specimen along the ridge direction, thin Cr film was deposited through physical sputtering. After releasing the prestrain, two wrinkling waves formed on the two inclined surfaces of the PDMS ridges with the same wavelength but opposite phases. These two wrinkling waves twist the top of the ridge into a wavy shape, forming wrinkling waves oscillating in the direction parallel to the substrate plane. In contrast, 2D wrinkling waves oscillate in the direction perpendicular to the substrate plane. Using finite element simulations, we demonstrate that the 180° phase shift between the two sides is favored by the dominant buckling modes. While the 3D wrinkling wavelength is consistent with 2D theoretical prediction, wrinkles on the ridge feature non-uniform wrinkling wave amplitude along the width of the inclined surface. This 3D wrinkling work opens new opportunities in manufacturing curved-shape mold for contact printing and increases surface to volume ratio for wetting and battery applications.