Tough Bonding of Liquid Metal-Elastomer Composites for Multifunctional Adhesives

Fully Recycled Polyolefin Elastomer-Based Vitrimers with Ultra-High, Universal, Stable, and Switchable Adhesion

Achieving both robust adhesion to arbitrary surfaces and thermal-switchable/recyclable properties has proven challenging, particularly for commodity polyolefins. Herein, a simple and effective route is reported to transform polyolefins elastomer (POE) into a fully recycled epoxy-functionalized POE vitrimers (E-POE vit) with ultra-high, universal, stable, and switchable adhesion via facile free radical grafting and dynamic cross-linking. The resultant E-POE vit exhibits increase in adhesion strength on glass exceeding three to ten times compared to those commonly used polymers, due to the synergy of dense hydrogen (H)-bonds and strong interfacial affinity. In addition, E-POE vit also displays strong adhesion on diverse surfaces ranging from inorganic to organic while maintaining good stability in various harsh environments. More importantly, temperature-sensitive H-bonds allow E-POE vit to switch between attachment-detachment at alternating temperatures, resulting in reversible adhesion without adhesion loss, even after 10 cycles. Moreover, E-POE vit is able to be fully recycled and reused more than ten times via thermo-activated transesterification reactions with negligible change in structure and performance. This work may unlock strategies to fabricate high-performance commercial polymer-based adhesives with adhesion and recyclable features for intelligent and sustainable applications.

Room-Temperature Liquid Metals for Flexible Electronic Devices

Room-temperature gallium-based liquid metals (RT-GaLMs) have garnered significant interest recently owing to their extraordinary combination of fluidity, conductivity, stretchability, self-healing performance, and biocompatibility. They are ideal materials for the manufacture of flexible electronics. By changing the composition and oxidation of RT-GaLMs, physicochemical characteristics of the liquid metal can be adjusted, especially the regulation of rheological, wetting, and adhesion properties. This review highlights the advancements in the liquid metals used in flexible electronics. Meanwhile related characteristics of RT-GaLMs and underlying principles governing their processing and applications for flexible electronics are elucidated. Finally, the diverse applications of RT-GaLMs in self-healing circuits, flexible sensors, energy harvesting devices, and epidermal electronics, are explored. Additionally, the challenges hindering the progress of RT-GaLMs are discussed, while proposing future research directions and potential applications in this emerging field. By presenting a concise and critical analysis, this paper contributes to the advancement of RT-GaLMs as an advanced material applicable for the new generation of flexible electronics.

Directional Adhesion of Monodomain Liquid Crystalline Elastomers

Pressure-sensitive adhesives (PSAs) are widely employed in consumer goods, health care, and commercial industry. Anisotropic adhesion of PSAs is often desirable to enable high force capacity coupled with facile release and has typically been realized through the introduction of complex surface and/or bulk microstructures while also maintaining high surface conformability. Although effective, microstructure fabrication can add cost and complexity to adhesive fabrication. Here, we explore aligned liquid crystalline elastomers (LCEs) as directional adhesives. Aligned LCEs exhibit direction-dependent stiffness, dissipation, and nonlinear deformation under load. By varying the cross-link content, we study how the bulk mechanical properties of LCEs correlate to their peel strength and peel anisotropy. We demonstrate up to a 9-fold difference in peel force measured when the LCE is peeled parallel vs perpendicular to the alignment axis. Opportunities to spatially localize adhesion are presented in a monolithic LCE patterned with different director orientations.

Highly Stable Liquid Metal Conductors with Superior Electrical Stability and Tough Interface Bonding for Stretchable Electronics

Ga-based liquid metal stretchable conductors have recently gained interest in flexible electronic devices such as electrodes, antennas, and sensors. It is essential to maintain electrical stability under strain or cyclic strain for reliable data acquisition and exhibit tough interfacial bonding between liquid metal and polymers to prevent performance loss and device failure. Herein, a highly stable conductor with superior electrical stability and tough interface bonding is introduced by casting curable polymers and a peeling-activated process from liquid metal particles. Based on the compensating effect of liquid metal, similar to the recharge relationship of water between rivers and lakes in nature, the conductor is not only strain-insensitive (ΔR/R₀ < 10% for 100% strain) but also immune to cyclic deformation (ΔR/R₀ < 7% with 5000 stretching cycles at 50% strain). Embedding liquid metal within the elastomer to create stretchable conductors effectively improves interfacial adhesion properties (the fluid-solid interfacial adhesion force increases from 0.48 to 0.62 mN/mm²). The constructed tough interface could even withstand sonication treatment. Finally, by combining strategies in material design and fabrication, an integrated array composed of vertical interconnect access and robust electrodes is fabricated, which simultaneously holds tough interfacial bonding with the upper and lower layers.

Liquid metal enabled conformal electronics

The application of three-dimensional common electronics that can be directly pasted on arbitrary surfaces in the fields of human health monitoring, intelligent robots and wearable electronic devices has aroused people's interest, especially in achieving stable adhesion of electronic devices on biological dynamic three-dimensional interfaces and high-quality signal acquisition. In recent years, liquid metal (LM) materials have been widely used in the manufacture of flexible sensors and wearable electronic devices because of their excellent tensile properties and electrical conductivity at room temperature. In addition, LM has good biocompatibility and can be used in a variety of biomedical applications. Here, the recent development of LM flexible electronic printing methods for the fabrication of three-dimensional conformal electronic devices on the surface of human tissue is discussed. These printing methods attach LM to the deformable substrate in the form of bulk or micro-nano particles, so that electronic devices can adapt to the deformation of human tissue and other three-dimensional surfaces, and maintain stable electrical properties. Representative examples of applications such as self-healing devices, degradable devices, flexible hybrid electronic devices, variable stiffness devices and multi-layer large area circuits are reviewed. The current challenges and prospects for further development are also discussed.

X-ray scattering as an effective tool for characterizing liquid metal composite morphology

Quantitative analysis of particle size and size distribution is crucial in establishing structure-property relationships of composite materials. An emerging soft composite architecture involves dispersing droplets of liquid metal throughout an elastomer, enabling synergistic properties of metals and soft polymers. The structure of these materials is typically characterized through real-space microscopy and image analysis; however, these techniques rely on magnified images that may not represent the global-averaged size and distribution of the droplets. In this study, we utilize ultra-small angle X-ray scattering (USAXS) as a reciprocal-space characterization technique that yields global-averaged dimensions of eutectic gallium indium (EGaIn) alloy soft composites. The Unified fit and Monte Carlo scattering methods are applied to determine the particle size and size distributions of the liquid metal droplets in the composites and are shown to be in excellent agreement with results from real-space image analysis. Additionally, all methods indicate that the droplets are getting larger as they are introduced into composites, suggesting that the droplets are agglomerating or possibly coalescing during dispersion. This work demonstrates the viability of X-ray scattering to elucidate structural information about liquid metal droplets for material development for applications in soft robotics, soft electronics, and multifunctional materials.