Controlled Assembly of Liquid Metal Inclusions as a General Approach for Multifunctional Composites

Soft composites that use droplets of gallium-based liquid metal (LM) as the dispersion phase have the potential for transformative impact in multifunctional material engineering. However, it is unclear whether percolation pathways of LM can support high electrical conductivity in a wide range of matrix materials. This issue is addressed through an approach to LM composite synthesis that focuses on the interrelated effects of matrix curing/solidification and droplet formation. The combined influence of LM concentration, particle size, and sedimentation is explored. By developing this approach, the functionalities that have been demonstrated with LM composites can be generalized to other matrix materials that impart additional functionality. Specifically, composites are synthesized using a biodegradable/reprocessable plastic (polycaprolactone), a hydrogel (poly(vinyl alcohol)), and a processable rubber (a styrene–ethylene–butylene–styrene derivative) to demonstrate wide applicability. This method enables synthesis of composites: i) with high stretchability and negligible electromechanical coupling ( > 600% strain); ii) with Joule-heated healing and reprocessability; iii) with electrical and mechanical self-healing; and iv) that can be printed. This approach to controlled assembly represents a widely applicable technique for creating new classes of LM composites with unprecedented multifunctionality.     

Light‐Responsive Hierarchically Structured Liquid Crystal Polymer Networks for Harnessing Cell Adhesion and Migration

Extracellular microenvironment is highly dynamic where spatiotemporal regulation of cell‐instructive cues such as matrix topography tightly regulates cellular behavior. Recapitulating dynamic changes in stimuli‐responsive materials has become an important strategy in regenerative medicine to generate biomaterials which closely mimic the natural microenvironment. Here, light responsive liquid crystal polymer networks are used for their adaptive and programmable nature to form hybrid surfaces presenting micrometer scale topographical cues and changes in nanoscale roughness at the same time to direct cell migration. This study shows that the cell speed and migration patterns are strongly dependent on the height of the (light‐responsive) micrometer scale topographies and differences in surface nanoroughness. Furthermore, switching cell migration patterns upon in situ temporal changes in surface nanoroughness, points out the ability to dynamically control cell behavior on these surfaces. Finally, the possibility is shown to form photoswitchable topographies, appealing for future studies where topographies can be rendered reversible on demand.     

Pulsed Laser Modulated Shock Transition from Liquid Metal Nanoparticles to Mechanically and Thermally Robust Solid–Liquid Patterns

In this work, a general mechanism is discovered to form liquid‐metal‐based, stable and stretchable conductive patterns on rigid and soft substrates. It is discovered that pulsed laser irradiation of liquid metal nanoparticles (LMNPs) with tunable conditions can induce transformation to stable and stretchable solid–liquid (S–L) dual phases on various surfaces. Formation of this unique solid–liquid composite phase is the key to change the wetting behavior of the conductive patterns on various substrates and enables mechanically stable patterns on various substrates. Pulsed‐laser‐driven thermo‐mechanical shock momentum is important for rupture and joining of the LMNPs, providing much better control than the traditional mechanical sintering. The solid nanophase forms a nanoporous matrix filled with and wetted by the LM, thereby providing a stabilization mechanism for the S–L composite patterned thin film. The mechanical and thermal reliability of the solid–liquid patterns is investigated. The S–L patterns can stretch up to 30% strain and cycle stably for 7000 cycles. It can be heated up to 177 °C with an input power of 0.58 W. The solid–liquid composite film provides great opportunity for various applications as a flexible conductor with unique mechanical and physics properties and further inspires design of LM devices for completely exposed applications.     

Nematic DNA Thermotropic Liquid Crystals with Photoresponsive Mechanical Properties

Over the last decades, water‐based lyotropic liquid crystals of nucleic acids have been extensively investigated because of their important role in biology. Alongside, solvent‐free thermotropic liquid crystals (TLCs) from DNA are gaining great interest, owing to their relevance to DNA‐inspired optoelectronic applications. Up to now, however, only the smectic phase of DNA TLCs has been reported. The development of new mesophases including nematic, hexagonal, and cubic structures for DNA TLCs remains a significant challenge, which thus limits their technological applications considerably. In this work, a new type of DNA TLC that is formed by electrostatic complexation of anionic oligonucleotides and cationic surfactants containing an azobenzene (AZO) moiety is demonstrated. DNA–AZO complexes form a stable nematic mesophase over a temperature range from ?7 to 110 °C and retain double‐stranded DNA structure at ambient temperature. Photoisomerization of the AZO moieties from the E‐ to the Z‐ form alters the stiffness of the DNA–AZO hybrid materials opening a pathway toward the development of DNA TLCs as stimuli‐responsive biomaterials.     

A Novel Design of Multi‐Mechanoresponsive and Mechanically Strong Hydrogels

A newly developed polyacrylamide‐co ‐methyl acrylate/spiropyran (SP) hydrogel crosslinked by SP mechanophore demonstrates multi‐stimuli‐responsive and mechanically strong properties. The hydrogels not only exhibit thermo‐, photo‐, and mechano‐induced color changes, but also achieve super‐strong mechanical properties (tensile stress of 1.45 MPa, tensile strain of ≈600%, and fracture energy of 7300 J m^?2). Due to a reversible structural transformation between spiropyran (a ring‐close) and merocyanine (a ring‐open) states, simple exposure of the hydrogels to white light can reverse color changes and restore mechanical properties. The new design approach for a new mechanoresponsive hydrogel is easily transformative to the development of other mechanophore‐based hydrogels for sensing, imaging, and display applications.     

Printable Superelastic Conductors with Extreme Stretchability and Robust Cycling Endurance Enabled by Liquid‐Metal Particles

Stretchable conductors are vital and indispensable components in soft electronic systems. The development for stretchable conductors has been highly motivated with different approaches established to address the dilemma in the conductivity and stretchability trade‐offs to some extent. Here, a new strategy to achieve superelastic conductors with high conductivity and stable electrical performance under stretching is reported. It is demonstrated that by electrically anchoring conductive fillers with eutectic gallium indium particles (EGaInPs), significant improvement in stretchability and durability can be achieved in stretchable conductors. Different from the strategy of modulating the chemical interactions between the conductive fillers and host polymers, the EGaInPs provide dynamic and robust electrical anchors between the conductive fillers. A superelastic conductor which can achieve a high stretchability with 1000% strain at initial conductivity of 8331 S cm^?1 and excellent cycling durability with about eight times resistance change (compared to the initial resistance at 0% strain before stretching) after reversibly stretching to 800% strain for 10 000 times is demonstrated. Applications of the superelastic conductor in an interactive soft touch device and a stretchable light‐emitting system are also demonstrated, featuring its promising applications in soft robotics or soft and interactive human–machine interfaces.     

A Highly Stretchable Liquid Metal Polymer as Reversible Transitional Insulator and Conductor

Materials with a temperature‐controlled reversible electrical transition between insulator and conductor are attracting huge attention due to their promising applications in many fields. However, most of them are intrinsically rigid and require complicated fabrication processes. Here, a highly stretchable (680% strain) liquid metal polymer composite as a reversible transitional insulator and conductor (TIC), which is accompanied with huge resistivity changes (more than 4 × 10⁹ times) reversibly through a tuning temperature in a few seconds is introduced. When frozen, the insulated TIC becomes conductive and recovers after warming. Both the phase change of the liquid metal droplets and the rigidity change of the polymer contribute directly to transition between insulator and conductor. A simplified model is established to predict the expansion and connection of liquid metal droplets. Along with high stretchability, straightforward fabrication methods, rapid triggering time, large switching ratio, good repeatability, the TIC offers tremendous possibilities for numerous applications, like stretchable switches, semiconductors, temperature sensors, and resistive random‐access memory. Accordingly, a system that can display numbers and letters via converting alternative TIC temperature to a binary signal on a computer is conceived and demonstrated. The present discovery suggests a general strategy for fabricating and stimulating a stretchable transitional insulator and conductor based on liquid metal and allied polymers.     

Dynamic Control of Light Direction Enabled by Stimuli‐Responsive Liquid Crystal Gratings

The ability to control light direction with tailored precision via facile means is long‐desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next‐generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli‐controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.     

Colloid‐in‐Liquid Crystal Gels that Respond to Biomolecular Interactions

This paper advances the design of stimuli‐responsive materials based on colloidal particles dispersed in liquid crystals (LCs). Specifically, thin films of colloid‐in‐liquid crystal (CLC) gels undergo easily visualized ordering transitions in response to reversible and irreversible (enzymatic) biomolecular interactions occurring at the aqueous interfaces of the gels. In particular, LC ordering transitions can propagate across the entire thickness of the gels. However, confinement of the LC to small domains with lateral sizes of ～10 μm does change the nature of the anchoring transitions, as compared to films of pure LC, due to the effects of confinement on the elastic energy stored in the LC. The effects of confinement are also observed to cause the response of individual domains of the LC within the CLC gel to vary significantly from one to another, indicating that manipulation of LC domain size and shape can provide the basis of a general and facile method to tune the response of these LC‐based physical gels to interfacial phenomena. Overall, the results presented in this paper establish that CLC gels offer a promising approach to the preparation of self‐supporting, LC‐based stimuli‐responsive materials.