Synergistic Inclusion Effects of Hard Magnetic Nanorods on the Magnetomechanical Actuation of Soft Magnetic Microsphere-Based Polymer Composites.

The magnetomechanical actuation of micropillars is developed for the contactless manipulation of miniaturized actuators and microtextured surfaces. Anisotropic geometry of micropillars can significantly enhance the magnetic actuation compared with their isotropic counterparts by directional stress distributions. However, this strategy is not viable for triangular micropillars owing to insufficient anisotropy. In this study, a significant improvement in the magnetic actuation of triangular micropillars using composite magnetic particles is reported. A minute and optimal amount of hard magnetic gamma-ferrite nanorods are hybridized with soft magnetic iron microspheres to generate synergistic effects of magnetic coupling and percolation phenomenon on the magnetic actuation of polymer composites. The addition of 1 wt% face-centered cubic-phased gamma-ferrite nanorods suppresses the magnetic coupling interference of body-centered cubic-phased iron microspheres. Furthermore, the nanorods reduce the percolation threshold by participating in the percolation of the microspheres. A systematic compositional study on the magnetization and magnetorheological properties reveals that the coupling effect dominates the percolation effect at a low magnetic field, whereas the percolation effect governs the magnetic actuation at a high magnetic field. This hybrid approach can help in designing material constituents for effective magnetic actuators and robotic systems that can sensitively respond to an external magnetic field.

Programming Anisotropic Functionality of 3D Microdenticles by Staggered-Overlapped and Multilayered Microarchitectures.

Natural sharkskin features staggered-overlapped and multilayered architectures of riblet-textured anisotropic microdenticles, exhibiting drag reduction and providing a flexible yet strong armor. However, the artificial fabrication of three-dimensional (3D) sharkskin with these unique functionalities and mechanical integrity is a challenge using conventional techniques. In this study, it is reported on the facile microfabrication of multilayered 3D sharkskin through the magnetic actuation of polymeric composites and subsequent chemical shape fixation by casting thin polymeric films. The fabricated hydrophobic sharkskin, with geometric symmetry breaking, achieves anisotropic drag reduction in frontal and backward flow directions against the riblet-textured microdenticles. For mechanical integrity, hard-on-soft multilayered mechanical properties are realized by coating the polymeric sharkskin with thin layers of zinc oxide and platinum, which have higher hardness and recovery behaviors than the polymer. This multilayered hard-on-soft sharkskin exhibits friction anisotropy, mechanical robustness, and structural recovery. Furthermore, coating the MXene nanosheets provides the fabricated sharkskin with a low electrical resistance of ≈5.3 Ω, which leads to high Joule heating (≈229.9 °C at 2.75 V). The proposed magnetomechanical actuation-assisted microfabrication strategy is expected to facilitate the development of devices requiring multifunctional microtextures.

Steerable and Agile Light-Fueled Rolling Locomotors by Curvature-Engineered Torsional Torque.

On-demand photo-steerable amphibious rolling motions are generated by the structural engineering of monolithic soft locomotors. Photo-morphogenesis of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs) is designed from spiral ribbon to helicoid helices, employing a 270° super-twisted nematic molecular geometry with aspect ratio variations of azo-LCN strips. Unlike the intermittent and biased rolling of spiral ribbon azo-LCNs with center-of-mass shifting, the axial torsional torque of helicoid azo-LCNs enables continuous and straight rolling at high rotation rates (≈720 rpm). Furthermore, center-tapered helicoid structures with wide edges are introduced for effectively accelerating photo-motilities while maintaining directional controllability. Irrespective of surface conditions, the photo-induced rotational torque of center-tapered helicoid azo-LCNs can be transferred to interacting surfaces, as manifested by steep slope climbing and paddle-like swimming multimodal motilities. Finally, the authors demonstrate continuous curvilinear guidance of soft locomotors, bypassing obstacles and reaching desired destinations through real-time on-demand photo-steering.

Highly Sensitive and Cost-Effective Polymeric-Sulfur-Based Mid-Wavelength Infrared Linear Polarizers with Tailored Fabry-Pérot Resonance.

Inverse-vulcanized polymeric sulfur has received considerable attention for application in waste-based infrared (IR) polarizers with high polarization sensitivities, owing to its high transmittance in the IR region and thermal processability. However, there have been few reports on highly sensitive polymeric sulfur-based polarizers by replication of pre-simulated dimensions to achieve a high transmission of the transverse magnetic field (TTM ) and extinction ratio (ER). Herein, a 400-nanometer-pitch mid-wavelength infrared bilayer linear polarizer with self-aligned metal gratings is introduced on polymeric sulfur gratings integrated with a spacer layer (SM-polarizer). The dimensions of the SM-polarizer can be closely replicated using pre-simulated dimensions via a systematic investigation of thermal nanoimprinting conditions. Spacer thickness is tailored from 40 to 5100 nm by adjusting the concentration of polymeric sulfur solution during spin-coating. A tailored spacer thickness can maximize TTM in the broadband MWIR region by satisfying Fabry-Pérot resonance. The SM-polarizer yields TTM of 0.65, 0.59, and 0.43 and ER of 3.12 × 103 , 5.19 × 103 , and 5.81 × 103 at 4 µm for spacer thicknesses of 90, 338, and 572 nm, respectively. This demonstration of a highly sensitive and cost-effective SM-polarizer opens up exciting avenues for infrared polarimetric imaging and for applications in polarization manipulation.

Applications of biomass-derived solvents in biomass pretreatment - Strategies, challenges, and prospects.

Biomass pretreatment is considered a key step in the 2nd generation biofuel production from lignocellulosic biomass. Research on conventional biomass pretreatment solvents has mainly been focused on carbohydrate conversion efficiency, while their hazardousness and/or carbon intensity were not comprehensively considered. Recent sustainability issues request further consideration for eco-friendly and sustainable alternatives like biomass-derived solvents. Carbohydrate and lignin-derived solvents have been proposed and investigated as green alternatives in many biomass processes. In this review, the applications of different types of biomass pretreatment solvents, including organic, ionic liquid, and deep eutectic solvents, are thoroughly discussed. The role of water as a co-solvent in these pretreatment processes is also reviewed. Finally, current research challenges and prospects of utilizing biomass-derived pretreatment solvents for pretreatment are discussed. Given bioethanol's market potential and increasing public awareness about environmental concerns, it will be a priority adopting sustainable and green biomass pretreatment solvents in biorefinery.

Multimodal collective swimming of magnetically articulated modular nanocomposite robots.

Magnetically responsive composites can impart maneuverability to miniaturized robots. However, collective actuation of these composite robots has rarely been achieved, although conducting cooperative tasks is a promising strategy for accomplishing difficult missions with a single robot. Here, we report multimodal collective swimming of ternary-nanocomposite-based magnetic robots capable of on-demand switching between rectilinear translational swimming and rotational swimming. The nanocomposite robots comprise a stiff yet lightweight carbon nanotube yarn (CNTY) framework surrounded by a magnetic polymer composite, which mimics the hierarchical architecture of musculoskeletal systems, yielding magnetically articulated multiple robots with an agile above-water swimmability (~180 body lengths per second) and modularity. The multiple robots with multimodal swimming facilitate the generation and regulation of vortices, enabling novel vortex-induced transportation of thousands of floating microparticles and heavy semi-submerged cargos. The controllable collective actuation of these biomimetic nanocomposite robots can lead to versatile robotic functions, including microplastic removal, microfluidic vortex control, and transportation of pharmaceuticals.

On-Demand Dynamic Chirality Selection in Flower Corolla-like Micropillar Arrays.

Chiral morphology has been intensively studied in various fields including biology, organic chemistry, pharmaceuticals, and optics. On-demand and dynamic chiral inversion not only cannot be realized in most intrinsically chiral materials but also has mostly been limited to chemical or light-induced methods. Herein, we report reversible real-time magneto-mechanical chiral inversion of a three-dimensional (3D) micropillar array between achiral, clockwise, and counterclockwise chiral arrangements. Inspired by the flower corolla, achiral arrays of five and six radially arranged semicylindrical micropillars were employed as model systems to investigate the dynamic symmetry properties of arrays consisting of odd and even numbers of micropillars, respectively. Each micropillar underwent twisting actuation with a different twisting angle depending on the angle with the magnetic field direction and magnetic flux density, thereby collectively changing the chirality from the achiral to chiral state. Importantly, the morphological handedness of the micropillars was inverted within a few seconds by manipulating the direction of the magnetic field. A chiral morphology consisting of magnetically twisted micropillars was shape-fixed by the introduction of a polymeric binder. This binder could be simply washed off to return the shape-fixed twisted micropillars to their initial straight state. Magnetically programmable and reproducible 3D flower corolla-like micropillar arrays are expected to expand the potential of shape-reconfigurable devices that require real-time chiral manipulation in ambient environments.

Multi-Modal Locomotion of Caenorhabditis elegans by Magnetic Reconfiguration of 3D Microtopography.

Miniaturized untethered soft robots are recently exploited to imitate multi-modal curvilinear locomotion of living creatures that perceive change of surrounding environments. Herein, the use of Caenorhabditis elegans (C. elegans) is proposed as a microscale model capable of curvilinear locomotion with mechanosensing, controlled by magnetically reconfigured 3D microtopography. Static entropic microbarriers prevent C. elegans from randomly swimming with the omega turns and provide linear translational locomotion with velocity of ≈0.14 BL s-1 . This velocity varies from ≈0.09 (for circumventing movement) to ≈0.46 (for climbing) BL s-1 , depending on magnetic bending and twisting actuation coupled with assembly of microbarriers. Furthermore, different types of neuronal mutants prevent C. elegans from implementing certain locomotion modes, indicating the potential for investigating the correlation between neurons and mechanosensing functions. This strategy promotes a platform for the contactless manipulation of miniaturized biobots and initiates interdisciplinary research for investigating sensory neurons and human diseases.

Confinement-Induced Fabrication of Liquid Crystalline Polymeric Fibers.

In aqueous media, liquid crystalline droplets typically form spherical shapes in order to minimize surface energy. Recently, non-spherical geometry has been reported using molecular self-assembly of surfactant-stabilized liquid crystalline oligomers, resulting in branched and randomly oriented filamentous networks. In this study, we report a polymerization of liquid crystalline polymeric fibers within a micro-mold. When liquid crystal oligomers are polymerized in freely suspended aqueous media, curvilinear and randomly networked filaments are obtained. When reactive liquid crystalline monomers are oligomerized in a micro-channel, however, highly aligned linear fibers are polymerized. Within a top-down microfabricated mold, a bottom-up molecular assembly was successfully achieved in a controlled manner by micro-confinement, suggesting a unique opportunity for the programming architecture of materials via a hybrid approach.

Height-Tunable Replica Molding Using Viscous Polymeric Resins.

Replica molding is one of the most common and low-cost methods for constructing microstructures for various applications, including dry adhesives, optics, tissue engineering, and strain sensors. However, replica molding provides only a single-height microstructure from a mold and master molds produced by an expensive photolithography process are required to prepare microstructures with different heights. Herein, we present a strategy to control the height of micropillars from the same mold by varying the cavity size of the micromold and the viscosity of the photocurable polyimide resin. The height of the constructed micropillar decreases in the case of small microcavities or high viscosity resin. In addition, the height of the micropillar arrays could be arbitrarily patterned by applying a masking technique. We believe that this cost-effective technique can be applied to metasurfaces for manipulation of electromagnetic signal or in biomedical applications including cell-culture and stem-cell differentiation.

Programmable Stepwise Collective Magnetic Self-Assembly of Micropillar Arrays.

Chain-like magnetic self-organizations have been documented for micron/submicron-scale magnetic particles. However, the positions of the particles are not stationary in a sustaining fluid owing to Brownian translational motion, resulting in irregular magnetic self-assembly. Toward the development of a programmable and reversible magnetic self-assembly, we report a stepwise collective magnetic self-assembly with periodic polymeric micropillar arrays containing magnetic particles. Under an external magnetic field, the individual micropillar acts as a micromagnet; magnetic polarities of embedded ferromagnetic particles are arranged in the same direction. The nearest pillar tops undergo a pairwise assembly owing to the anisotropic quadrupolar interaction, whereas the pillar bases remain stationary because of the presence of a magnetically inert substrate. By increasing the magnetic flux density, a collective quad-body assembly of vicinal paired micropillars is accomplished, finally leading to long-range connectivity of the pillar tops. Simple evaporation of the polymeric solution yields shape-fixation of the connected micropillar architectures even after magnetic fields are removed. We investigate geometric effects on this stepwise collective magnetic self-assembly using rectangular, square, and circular micropillars. Also, we demonstrate spatially selective magnetic self-assembly (e.g., arbitrary letters) using a masking technique. Finally, we demonstrate on-demand programming of bidirectional liquid spreading through long-range ordered magnetic self-assembly.

Intermolecular Interactions and Intramolecular Motions in Photomechanical Effect: Nonlinear Thermo- and Photomechanical Behaviors of Azobenzene-Functionalized Amide-Imide Block Copolymers.

To discern multiple intertwined effects, a set of azobenzene-functionalized amide-imide block copolymers, azo(PA-co-PI)-x, where x is amide-block content, viz., [azoPA] = 25, 50, 75 mol %, was synthesized from 2,2-bis{4-[4-(4-aminophenyldiazenyl)phenoxy]phenyl}propane(azoBPA), 4,4'-oxydibenzoyl chloride (ODBC), and 4,4'-oxydiphthalic anhydride (OPDA). Including homopolymers (azoPA and azoPI), this series of amorphous azopolymers possesses a high glass-transition temperature (Tg > 210 °C) and a modulus (E' ∼ 1.23-2.50 GPa). Their photobending (ca. 23-90°) and photostress (ca. 250-380 kPa) were assessed in the form of cantilevers with a linearly polarized 445 nm light. Nonlinear composition/[azoPA] dependencies of the thermo- and photomechanical properties are correlated. As [azoPA] increases from 0 mol %; Tg, E', photostress, and photobending angle initially decrease to reach four separate minima for azo(PA-co-PI)-50; and then all increase with a higher [azoPA]. The trend considerations of film density, dynamic thermomechanical, Fourier transform infrared (FT-IR), and ultraviolet-visible (UV-vis) measurements implicate that (i) intermolecular association and intramolecular segmental mobility collectively influence the photomechanical outcomes and (ii) two types of hydrogen bonding (HB), namely, amide-amide [HB-AA] and amide-imide [HB-AI] coexist in azo(PA-co-PI)-x copolymers, with [HB-AI] being largely responsible for photomechanical outcomes of azo(PA-co-PI)-x with [azoPA] <40-50 mol %, and [HB-AA] for [azoPA] >40-50 mol %. We hypothesize that the "U-shaped" photomechanical effect apparently stems from the cooperative "unzipping" of H bonds in the [HB-AA]* excited state with H bonds in [HB-AI]* being stabilized by electrostatic interactions inherent in an excited intermolecular complex.

Programmable Liquid Crystal Defect Arrays via Electric Field Modulation for Mechanically Functional Liquid Crystal Networks.

The arrangement of mesogenic units determines mechanical response of the liquid crystal polymer network (LCN) film to heat. Here, we show an interesting approach to programming three-dimensional patterns of the LCN films with periodic topological defects generated by applying an electric field. The mechanical properties of three representative patterned LCN films were investigated in terms of the arrangement of mesogenic units through tensile testing. Remarkably, it was determined that LCN films showed enhanced toughness and ductility as defects increased in a given area, which is related to the elastic modulus mismatch that mitigates crack propagation. Our platform can also be used to modulate the frictional force of the patterned LCN films by varying the temperature, which can provide insight into the multiplex mechanical properties of LCN films.

High crystallinity of tunicate cellulose nanofibers for high-performance engineering films.

Tunicate cellulose nanofibers (CNFs) have received widespread attention as renewable and eco-friendly engineering materials because of their high crystallinity and mechanical stiffness. Here, we report the effects of disintegration process conditions on structure-property relationships of tunicate CNFs. By varying the hydrolysis time, we could establish a correlation between crystallinity of the CNFs with linearity and stiffness, which produces different molecular ordering within their nanostructured films. Despite having identical raw materials, tensile strength and thermal conductivity of the resulting layered films varied widely, ranging from 95.6 to 205 MPa and from 1.08 to 2.37 W/mK respectively. Furthermore, nanolayered CNF films provided highly anisotropic thermal conductivities with an in- and through-plane ratio of 21.5. Our systematic investigations will provide general and practical strategies in tailoring material properties for emerging engineering applications, including flexible paper electronics, heat sink adhesives and biodegradable, implantable devices.

High-Speed Production of Crystalline Semiconducting Polymer Line Arrays by Meniscus Oscillation Self-Assembly.

Evaporative self-assembly of semiconducting polymers is a low-cost route to fabricating micrometer and nanoscale features for use in organic and flexible electronic devices. However, in most cases, rate is limited by the kinetics of solvent evaporation, and it is challenging to achieve uniformity over length- and time-scales that are compelling for manufacturing scale-up. In this study, we report high-throughput, continuous printing of poly(3-hexylthiophene) (P3HT) by a modified doctor blading technique with oscillatory meniscus motion-meniscus-oscillated self-assembly (MOSA), which forms P3HT features ∼100 times faster than previously reported techniques. The meniscus is pinned to a roller, and the oscillatory meniscus motion of the roller generates repetitive cycles of contact-line formation and subsequent slip. The printed P3HT lines demonstrate reproducible and tailorable structures: nanometer scale thickness, micrometer scale width, submillimeter pattern intervals, and millimeter-to-centimeter scale coverage with highly defined boundaries. The line width as well as interval of P3HT patterns can be independently controlled by varying the polymer concentration levels and the rotation rate of the roller. Furthermore, grazing incidence wide-angle X-ray scattering (GIWAXS) reveals that this dynamic meniscus control technique dramatically enhances the crystallinity of P3HT. The MOSA process can potentially be applied to other geometries, and to a wide range of solution-based precursors, and therefore will develop for practical applications in printed electronics.

Enhancement of Magneto-Mechanical Actuation of Micropillar Arrays by Anisotropic Stress Distribution.

Magnetically active shape-reconfigurable microarrays undergo programmed actuation according to the arrangement of magnetic dipoles within the structures, achieving complex twisting and bending deformations. Cylindrical micropillars have been widely used to date, whose circular cross-sections lead to identical actuation regardless of the actuating direction. In this study, micropillars with triangular or rectangular cross-sections are designed and fabricated to introduce preferential actuation directions and explore the limits of their actuation. Using such structures, controlled liquid wetting is demonstrated on micropillar surfaces. Liquid droplets pinned on magnetic micropillar arrays undergo directional spreading when the pillars are actuated as depinning of the droplets is enabled only in certain directions. The enhanced deformation due to direction dependent magneto-mechanical actuation suggests that micropillar arrays can be fundamentally tailored to possess application specific responses and opens up opportunities to exploit more complex designs such as micropillars with polygonal cross sections. Such tunable wetting of liquids on microarray surfaces has potential to improve printing technologies via contactless reconfiguration of stamp geometry by magnetic field manipulation.

Light-driven complex 3D shape morphing of glassy polymers by resolving spatio-temporal stress confliction.

Programmable 3D shape morphing of hot-drawn polymeric sheets has been demonstrated using photothermal local shrinkage of patterned hinges. However, the hinge designs have been limited to simple linear hinges used to generate in-plane local folding or global curvature. Herein, we report an unprecedented design strategy to realize localized curvature engineering in 3D structures employing radial hinges and stress-releasing facets on 2D polymeric sheets. The shape and height of the 3D structures are readily controlled by varying the number of radial patterns. Moreover, they are numerically predictable by finite elemental modeling simulation with consideration of the spatio-temporal stress distribution, as well as of stress competition effects. Localized curvature engineering provides programming capabilities for various designs including soft-turtle-shell, sea-shell shapes, and saddle architectures with the desired chirality. The results of local curvilinear actuation with quantifiable stress implies options to advance the applicability of self-folded architectures embodying coexisting curved and linear geometric surfaces.

Shape-Programmed Fabrication and Actuation of Magnetically Active Micropost Arrays.

Micro- and nanotextured surfaces with reconfigurable textures can enable advancements in the control of wetting and heat transfer, directed assembly of complex materials, and reconfigurable optics, among many applications. However, reliable and programmable directional shape in large scale is significant for prescribed applications. Herein, we demonstrate the self-directed fabrication and actuation of large-area elastomer micropillar arrays, using magnetic fields to both program a shape-directed actuation response and rapidly and reversibly actuate the arrays. Specifically, alignment of magnetic microparticles during casting of micropost arrays with hemicylindrical shapes imparts a deterministic anisotropy that can be exploited to achieve the prescribed, large-deformation bending or twisting of the pillars. The actuation coincides with the finite element method, and we demonstrate reversible, noncontact magnetic actuation of arrays of tens of thousands of pillars over hundreds of cycles, with the bending and twisting angles of up to 72 and 61°, respectively. Moreover, we demonstrate the use of the surfaces to control anisotropic liquid spreading and show that the capillary self-assembly of actuated micropost arrays enables highly complex architectures to be fabricated. The present technique could be scaled to indefinite areas using cost-effective materials and casting techniques, and the principle of shape-directed pillar actuation can be applied to other active material systems.

Effects of Helix Geometry on Magnetic Guiding of Helical Polymer Composites on a Gastric Cancer Model: A Feasibility Study.

This study investigates the effects of soft-robot geometry on magnetic guiding to develop an efficient helical mediator on a three-dimensional (3D) gastric cancer model. Four different magnetically active helical soft robots are synthesized by the inclusion of 5-µm iron particles in polydimethylsiloxane matrices. The soft robots are named based on the diameter and length (D2-L15, D5-L20, D5-L25, and D5-L35) with samples having varied helical pitch and weight values. Then, the four samples are tested on a flat surface as well as a stomach model with various 3D wrinkles. We analyze the underlying physics of intermittent magnetomotility for the helix on a flat surface. In addition, we extract representative failure cases of magnetomotility on the stomach model. The D5-L25 sample was the most suitable among the four samples for a helical soft robot that can be moved to a target lesion by the magnetic-flux density of the stomach model. The effects of diameter, length, pitch, and weight of a helical soft robot on magnetomotility are discussed in order for the robot to reach the target lesion successfully via magnetomotility.

On-demand orbital maneuver of multiple soft robots via hierarchical magnetomotility.

Magnetic soft robots facilitate the battery-free remote control of soft robots. However, parallel control of multiple magnetic robots is challenging due to interference between robots and difficult maneuvers. Here we present the orbital maneuvering of manifold magnetic soft robots. Magneto-induced motion (magnetomotility) that includes the hierarchy of rotation and resultant revolution allows for the independent control of the robot's velocity and orbital radius. The soft robot achieves a speed of 60 body length (BL) s-1, which is approximately 50, 000 times faster with 1/7 the weight of the current lightest legged soft robot. The hierarchical magnetomotility is suitable for versatile locomotion such as stairs and uphill climbing, underwater and above water swimming. Owing to their swimming functionality, a swarm of such soft robots is capable of transportation of cargo. On-demand orbital maneuvering of magnetic soft robots provides a new methodology for concurrent actuation of multiple robots exhibiting collective behaviors.