Cracking effects in squashable and stretchable thin metal films on PDMS for flexible microsystems and electronics

Flexible multicolor biaxial sensor for strain direction identification based on sandwich-structured mechanoluminescent materials

Strain sensors capable of recognizing the direction of strain are crucial in applications such as robot attitude adjustment and detection of strain states in complex structures. In this study, a sandwich-structured flexible biaxial strain sensor was developed using polydimethylsiloxane as the substrate, mechanoluminescent materials as the luminescent elements, and rubber-ink as the light-blocking layer. By correlating the emitted light color with the stretching state, precise identification of the applied strain direction is achieved. Additionally, the mechanoluminescence of the sensor is collected by a photodiode, generating photocurrent that can be analyzed. This provides a solution for practical applications of sensor.

Periodic liquid crystalline waveguiding microstructures

Different methods allowing for creating optical waveguides with liquid-crystal (LC) cores, in which molecules form periodic patterns with precisely controlled periods, are reported. The first one is based on reversible photoalignment with high-resolution selective illumination and allows to control the period of LC molecules inside silica microcapillaries. The second method employs microstructures formed in PDMS, allowing to obtain both: LC-core waveguides and a set of specially designed periodic microelectrodes used for the periodic reorientation of molecules. Using both methods, we successfully controlled the period of the patterned alignment in the range from about 500 µm and scaled it down to as small as 20 µm. We performed experimental studies on waveguiding phenomenon in such structures, in view to obtain transmission spectra typical to optical fiber gratings. Since the results achieved in experimental conditions differed from those expected, the additional numerical simulations were performed to explain the observed effects. Finally, we obtained the waveguiding in a blue phase LC, characterized by naturally created three-dimensional periodicity with periods smaller than one micrometer. In such a structure, we were able to observe first-order bandgap, and moreover, we were able to tune it thermally in nearly the whole visible spectral range.

Wrinkled Thermo-Electric Meander-Shaped Element on a Thin Freestanding PDMS Membrane

Natural wrinkling of metal films on silicone substrates can appear by means of the metal sputtering process and can be described by the continuous elastic theory and non-linear wrinkling model. Here, we report the fabrication technology and behavior of thin freestanding Polydimethylsiloxane (PDMS) membranes equipped with thermo-electric meander-shaped elements. The Cr/Au wires were obtained on the silicone substrate by magnetron sputtering. We observe wrinkle formation and suppose furrows appear once PDMS returns to its initial state after the thermo-mechanical expansion during sputtering. Although the substrate thickness is usually a negligible parameter in the theory of wrinkle formation, we found that the self-assembled wrinkling architecture of the PDMS/Cr/Au varies due to the membrane thickness of 20 µm and 40 µm PDMS. We also demonstrate that the wrinkling of the meander wire affects its length, and it causes a 2.7 times higher resistance compared to a calculated value. Therefore, we investigate the influence of the PDMS mixing ratio on the thermo-electric meander-shaped elements. For the stiffer PDMS with a mixing ratio of 10:4, the resistance due to wrinkle amplitude alterations is 25% higher compared to the PDMS of ratio 10:1. Additionally, we observe and describe a thermo-mechanically induced motion behavior of the meander wires on completely freestanding PDMS membrane under applied current. These results can improve the understanding of wrinkle formation, which influences thermo-electric characteristics and may promote the integration of this technology in applications.

Flexible and Robust Triboelectric Nanogenerators with Chemically Prepared Metal Electrodes and a Plastic Contact Interface Based on Low-Cost Pressure-Sensitive Adhesive

Triboelectric nanogenerators (TENGs) are devices that can harvest energy from mechanical motions; such devices can be used to power wearable sensors and various low-power electronics. To increase the lifetime of the device, scientists mainly use the method of making TENG in a hard skeleton to simplify the complex possible relative movements between two triboelectric parts. However, the hard skeletons cannot be embedded in soft and lightweight clothing. To make matters worse, the materials used in the garments must be able to withstand high mechanical forces when worn, such as the pressure of more than 100 KPa exerted by body pressure or everyday knocks. Notably, the TENGs are usually made of fragile materials, such as vacuum-evaporated metal electrodes and nano-sized coatings, on the contact interface; these electrodes and coatings often chip or wear off under the action of external loads. In this work, we succeeded in creating a thin, light-weight, but extremely robust garment-integrated triboelectric nanogenerator (G-TENG) that can be embedded in clothing and pass the water wash test. First, we chemically deposited a durable electrode with flexible properties for G-TENG using a novel technique called polymer-assisted metal deposition (PAMD). The as-formed metal electrodes are firmly bonded to the plastic substrate by a sub-10 nm adhesive polymer brush and can withstand a pressure of 22.5 MPa and a tear force of 0.7 MPa. We then removed the traditionally used fragile nanoparticle materials and the non-durable poly-dimethylsiloxane (PDMS) layer at the triboelectric interface, and then used a cost-effective, durable and slightly flowable pressure-sensitive adhesive to form a plastic contact interface. Such a soft plastic interface can ensure full contact of the triboelectric materials, which is excellent in complex environments and ultimately improves the power generation efficiency of the devices. The as-formed low-cost energy harvesting device could become an industry standard for future smart clothing.

Advances in Biodegradable Electronic Skin: Material Progress and Recent Applications in Sensing, Robotics, and Human-Machine Interfaces

The rapid growth of the electronics industry and proliferation of electronic materials and telecommunications technologies has led to the release of a massive amount of untreated electronic waste (e-waste) into the environment. Consequently, catastrophic environmental damage at the microbiome level and serious human health diseases threaten the natural fate of the planet. Currently, the demand for wearable electronics for applications in personalized medicine, electronic skins (e-skins), and health monitoring is substantial and growing. Therefore, "green" characteristics such as biodegradability, self-healing, and biocompatibility ensure the future application of wearable electronics and e-skins in biomedical engineering and bioanalytical sciences. Leveraging the biodegradability, sustainability, and biocompatibility of natural materials will dramatically influence the fabrication of environmentally friendly e-skins and wearable electronics. Here, the molecular and structural characteristics of biological skins and artificial e-skins are discussed. The focus then turns to the biodegradable materials, including natural and synthetic-polymer-based materials, and their recent applications in the development of biodegradable e-skin in wearable sensors, robotics, and human-machine interfaces (HMIs). Finally, the main challenges and outlook regarding the preparation and application of biodegradable e-skins are critically discussed in a near-future scenario, which is expected to lead to the next generation of biodegradable e-skins.

A Novel Approach for the Creation of Electrically Controlled LC:PDMS Microstructures

This work presents research on unique optofluidic systems in the form of air channels fabricated in PDMS and infiltrated with liquid crystalline material. The proposed LC:PDMS structures represent an innovative solution due to the use of microchannel electrodes filled with a liquid metal alloy. The latter allows for the easy and dynamic reconfiguration of the system and eliminates technological issues experienced by other research groups. The paper discusses the design, fabrication, and testing methods for tunable LC:PDMS structures. Particular emphasis was placed on determining their properties after applying an external electric field, depending on the geometrical parameters of the system. The conclusions of the performed investigations may contribute to the definition of guidelines for both LC:PDMS devices and a new class of potential sensing elements utilizing polymers and liquid crystals in their structures.

Cracking of Colloidal Films to Generate Rectangular Fragments

Cracks are common in nature. Cracking is known as an irreversible and uncontrollable process. To control the cracking patterns, many researchers have proposed methods to prepare notches for stress localization on films. In this work, we investigate a method of controlling cracks by making microscale pyramid patterns that have notches between the pyramids. After preparing pyramid patterns consisting of colloidal particles with organic residue, we annealed them to induce volume shrinkage and cracking between the pyramids. We studied the effect of film thickness on cracking and the generation of rectangular fragments consisting of multiple pyramids. The area of rectangular fragments was in good agreement with the results of scaling analysis. The concept of controlling cracks by imprinting notches on a film and the relationship with the film thickness can guide the study of cracking phenomena.

Twin-Wire Networks for Zero Interconnect, High-Density 4-Wire Electrical Characterizations of Materials

Four-wire measurements have been introduced by Lord Kelvin in 1861 and have since become the standard technique for characterizing small resistances and impedances. However, high-density 4-wire measurements are generally complex, time-consuming, and inefficient because of constraints on interconnects, pads, external wires, and mechanical contacts, thus reducing reproducibility, statistical significance, and throughput. Here, we introduce, systematically design, analyze, and experimentally validate zero interconnect networks interfaced to external instrumentation by couples of twin wire. 3D-printed holders with magnets, interconnects, nonadhesive layers, and spacers can effortlessly establish excellent electrical connections with tunable or minimum contact forces and enable accurate measurements even for delicate devices, such as thin metals on soft polymers. As an example, we measured all the resistances of a twin-wire 29-resistor network made of silver-nanoparticle ink printed on polyimide, paper, or photo paper, including during sintering or temperature calibration, resulting in an unprecedentedly easy and accurate characterization of both resistivity and its temperature coefficient. The theoretical framework and experimental strategies reported here represent a breakthrough toward zero interconnect, simple, and efficient high-density 4-wire characterizations, can be generalized to other 4-wire measurements (impedances, sensors) and can open the way to more statistically meaningful and reproducible analyses of materials, high-throughput measurements, and minimally invasive characterizations of biomaterials.

Electronically integrated microcatheters based on self-assembling polymer films

Existing electronically integrated catheters rely on the manual assembly of separate components to integrate sensing and actuation capabilities. This strongly impedes their miniaturization and further integration. Here, we report an electronically integrated self-assembled microcatheter. Electronic components for sensing and actuation are embedded into the catheter wall through the self-assembly of photolithographically processed polymer thin films. With a diameter of only about 0.1 mm, the catheter integrates actuated digits for manipulation and a magnetic sensor for navigation and is capable of targeted delivery of liquids. Fundamental functionalities are demonstrated and evaluated with artificial model environments and ex vivo tissue. Using the integrated magnetic sensor, we develop a strategy for the magnetic tracking of medical tools that facilitates basic navigation with a high resolution below 0.1 mm. These highly flexible and microsized integrated catheters might expand the boundary of minimally invasive surgery and lead to new biomedical applications.

Modified cellulose nanocrystals are used to enhance the performance of self-healing siloxane elastomers

Self-healing siloxane elastomers with high ductility can be developed for the manufacture of wearable and flexible electronic devices. However, the poor mechanical properties of self-healing siloxane elastomers limit their industrial applications. Herein, the mechanical and self-healing properties of siloxane elastomers were improved using 3-isocyanatepropyltrimethoxysilane modified (ICNCs) as a crosslinking agent and nanofiller. ICNCs increased the tensile strength and shear strength of siloxane elastomers by 54% and 68.6%, respectively. Significantly, ICNCs decreased the relaxation time for siloxane elastomers and reduced the activation energy for reversible siloxane equilibration from 68 kJ/mol to 38 kJ/mol. Therefore, ICNCs improve the self-healing and welding efficiency of siloxane elastomers. Furthermore, the self-healing siloxane elastomer maintained excellent mechanical properties and thermal properties after three cycles of recycling. Our research reveals that such a material can be applied to the field of recyclable self-healing products and surface adhesives.

Intercalated water mediated electromechanical response of graphene oxide films on flexible substrates

The confinement of water between sub-nanometer bounding walls of layered two-dimensional materials has generated tremendous interest. Here, we examined the influence of confined water on the mechanical and electromechanical response of graphene oxide films, prepared with variable oxidative states, casted on polydimethylsiloxane substrates. These films were subjected to uniaxial strain under controlled humid environments (5 to 90% RH), while dc transport studies were performed in tandem. Straining resulted in the formation of quasi-periodic linear crack arrays. The extent of water intercalation determined the density of cracks formed in the system thereby, governing the electrical conductance of the films under strain. The crack density at 5% strain, varied from 0 to 3.5 cracks mm^-1for hydrated films and 8 to 22 cracks mm^-1for dry films, across films with different high oxidative states. Correspondingly, the overall change in the electrical conductance at 5% strain was observed to be ∼5 to 20 folds for hydrated films and ∼20 to 35 folds for the dry films. The results were modeled with a decrease in the in-plane elastic modulus of the film upon water intercalation, which was attributed to the variation in the nature of hydrogen bonding network in graphene oxide lamellae.

Strain-resilient electrical functionality in thin-film metal electrodes using two-dimensional interlayers

Flexible electrodes that allow electrical conductance to be maintained during mechanical deformation are required for the development of wearable electronics. However, flexible electrodes based on metal thin-films on elastomeric substrates can suffer from complete and unexpected electrical disconnection after the onset of mechanical fracture across the metal. Here we show that the strain-resilient electrical performance of thin-film metal electrodes under multimodal deformation can be enhanced by using a two-dimensional (2D) interlayer. Insertion of atomically-thin interlayers - graphene, molybdenum disulfide, or hexagonal boron nitride - induce continuous in-plane crack deflection in thin-film metal electrodes. This leads to unique electrical characteristics (termed electrical ductility) in which electrical resistance gradually increases with strain, creating extended regions of stable resistance. Our 2D-interlayer electrodes can maintain a low electrical resistance beyond a strain in which conventional metal electrodes would completely disconnect. We use the approach to create a flexible electroluminescent light emitting device with an augmented strain-resilient electrical functionality and an early-damage diagnosis capability.

Biodegradable Materials and Green Processing for Green Electronics

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life. However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century. In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods. In response to these issues, the construction of electronic devices from renewable or biodegradable materials that decompose to harmless by-products is becoming a topic of great interest. Such "green" electronic devices need to be fabricated on industrial scale through low-energy and low-cost methods that involve low/non-toxic functional materials or solvents. This review highlights recent advances in the development of biodegradable materials and processing strategies for electronics with an emphasis on areas where green electronic devices show the greatest promise, including solar cells, organic field-effect transistors, light-emitting diodes, and other electronic devices.

Fast room-temperature self-healing siloxane elastomer for healable stretchable electronics

Siloxane elastomers having simultaneously high stretchability, fast and efficient self-healing abilities at room temperature and excellent mechanical properties have broad application prospects in many fields. However, it is still challenging to satisfy this request. In this work, a stretchable, fast self-healing siloxane elastomer was successfully synthesized by introducing aromatic disulfides into a siloxane matrix. The resulting siloxane elastomer exhibited a tensile stress of 0.5 MPa, an elongation at break over 1000%, and a healing efficiency above 95% at room temperature. The healed siloxane elastomer could recover an elongation at break of 357 ± 15% after healing for only one minute at room temperature. A healing efficiency higher than 90% was achieved even after surface aging or by overlap contact, which was due to the presence of the dynamic disulfide bonds. Furthermore, the elastomer was successfully deployed as the substrate for self-healing stretchable electronics. As a proof-of-concept, stretchable electrode and stretchable strain sensors were produced, and they all showed high stretchability, fast self-healing properties at room temperature and high durability and stability, paving the way to promising applications in stretchable and wearable electronics.

Harnessing Multiple Surface Deformation Modes for Switchable Conductivity Surfaces

Surface deformation modes, such as wrinkling, creasing, and cracking, enable a plethora of surface morphologies under mechanical loading, which have been widely exploited to provide flexibility and stretchability to electronic devices. As each phenomenon offers a distinct set of potential advantages, controlling the types and spatial locations of deformation modes is key for their successful application. In this study, we demonstrate a method to simultaneously harness multiple surface deformation modes-wrinkles, creases, and cracks-in patterned multilayer films. The wrinkling of metal-coated stiff patterned films provides flexibility and stretchability, while the reversible formation of creases in the intervening regions of the bare elastomer is used to template the formation of patterned cracks in the metal. While conventional cracks can be difficult to precisely control, the patterned cracks demonstrated here remain straight over long distances and show tunable lateral spacings from hundreds of micrometers to centimeters. Finally, the reversible opening and closing of these cracks under mechanical loading provides mechanically gated electrical switches with small and tunable critical switching strains of 0.05-0.18 and high on/off ratios of >10⁷, enabling the preparation of mechanical NAND and NOR logic gates each composed of multiple patterned switches on a single elastomer surface.

The Use of a Water Soluble Flexible Substrate to Embed Electronics in Additively Manufactured Objects: From Tattoo to Water Transfer Printed Electronics

The integration of electronics into the process flow of the additive manufacturing of 3D objects is demonstrated using water soluble films as a temporary flexible substrate. Three process variants are detailed to evaluate their capabilities to meet the additive manufacturing requirements. One of them, called water transfer printing, shows the best ability to fabricate electronics onto 3D additively manufactured objects. Moreover, a curved capacitive touchpad hidden by color films is successfully transferred onto the 3D objects, showing a potential application of this technology to fabricate fully additively manufactured discrete or even hidden electronic devices.