Liquid Crystal Elastomer Artificial Tendrils with Asymmetric Core-Sheath Structure Showing Evolutionary Biomimetic Locomotion.

The sophisticated and complex haptonastic movements in response to environmental-stimuli of living organisms have always fascinated scientists. However, how to fundamentally mimic the sophisticated hierarchical architectures of living organisms to provide the artificial counterparts with similar or even beyond-natural functions based on the underlying mechanism remains a major scientific challenge. Here,  liquid crystal elastomer (LCE) artificial tendrils showing evolutionary biomimetic locomotion are developed following the structure-function principle that is used in nature to grow climbing plants. These elaborately designed tendril-like LCE actuators possess an asymmetric core-sheath architecture which shows a higher-to-lower transition in the degree of LC orientation from the sheath-to-core layer across the semi-ellipse cross-section. Upon heating and cooling, the LCE artificial tendril can undergo reversible tendril-like shape-morphing behaviors, such as helical coiling/winding, and perversion. The fundamental mechanism of the helical shape-morphing of the artificial tendril is revealed by using theoretical models and finite element simulations. Besides, the incorporation of metal-ligand coordination into the LCE network provides the artificial tendril with reconfigurable shape-morphing performances such as helical transitions and rotational deformations. Finally, the abilities of helical and rotational deformations are integrated into a new reprogrammed flagellum-like architecture to perform evolutionary locomotion mimicking the haptonastic movements of the natural flagellum.

Selective Laser Sintering of Polydimethylsiloxane Composites.

Conductive silicone elastomer carbon nanotubes (CNTs) composites possess potential applications in a variety of fields, including electronic skin, wearable electronics, and human motion detection. Based on a novel self-made covalent adaptable network (CANs) of polydimethylsiloxane (PDMS) containg dynamic steric-hindrance pyrazole urea bond (PDMS-CANs), CNTs wrapped PDMS-CANs (CNTs@PDMS-CANs) powders were prepared by a liquid phase adsorption and deposition, and were successfully used for selective laser sintering (SLS) three-dimensional printing. SLS-printed PDMS-CANs/CNTs nanocomposites possess high electrical conductivity and low percolation threshold as SLS is one kind of quasi-static processing, which leads to the formation of conductive segregated CNTs network by using the PDMS powders with special CNTs wrapped structure. The introduction of dynamic pyrazole urea bond endows the materials self-healing capability under electrothermal and photothermal stimulus. In addition, due to the resistance difference of the damaged and intact areas, crack diagnosing can be realized by infrared thermograph under electricity. In an application demonstration in strain sensor, the composite exhibits a regular cyclic electrical resistance change at cyclic compression and bending, indicating a relative high reliability.

Robust and recyclable graphene/chitosan composite aerogel microspheres for adsorption of oil pollutants from water.

Despite recent progress in graphene-based aerogels, challenges such as low mechanical strength and adsorption efficiency are still remaining. Here the reduced graphene oxide (rGO)/chitosan (CS) composite aerogel microspheres (rGCAMs) with center-diverging microchannel structures were developed by electrospraying and freeze-drying method. The optimized rGCAMs exhibit a high Young's modulus of 197 kPa and can support ~75,000 times its own weight, due to the cross-linking of CS by glutaraldehyde. Meanwhile, the rGCAMs can maintain high adsorption capacity for 15 cyclic tests due to its excellent mechanical strength. The oil adsorption kinetics and isotherms of rGCAMs follow the pseudo-second-order kinetic equation and the Langmuir model, respectively. The whole adsorption process is influenced by the oil diffusion in the liquid matrix and also in the intra-particle of aerogel microspheres. Moreover, rGCAMs can also be used to separate both surfactant-stabilized water-in-oil and oil-in-water emulsions through demulsification. The high-strength, recyclable and separation-efficient rGCAMs can be a potential candidate for oily wastewater treatment.

High Silica Content Graphene/Natural Rubber Composites Prepared by a Wet Compounding and Latex Mixing Process.

The reduced graphene oxide (rGO) modified natural rubber composite (NR) filled with high contents of silica was prepared by a wet compounding and latex mixing process using a novel interface modifier cystamine dihydrochloride (CDHC) with coagulation ability. CDHC acts as a coagulation agent through electrostatic interaction with rGO, SiO2, and latex rubber particles during the latex-based preparation process, while in the obtained silica/graphene/natural rubber composites, CDHC acts as an interface modifier. Compared with the composites prepared by the conventional mechanical mixing method, the dispersion of both rGO and SiO2 in the composites made by a wet compounding and latex mixing process is improved. As a result, the obtained silica/graphene/natural rubber composite prepared by this new method has good comprehensive properties. A Dynamic Mechanical Test suggests that the tan δ values of the composites at 60 °C decrease, indicating a low rolling resistance with increasing the graphene content at a low strain, but it increases at a higher strain. This unique feature for this material provides an advantage in the rubber tire application.

On the Synergistic Effect of Multi-Walled Carbon Nanotubes and Graphene Nanoplatelets to Enhance the Functional Properties of SLS 3D-Printed Elastomeric Structures.

Elastomer-based porous structures realized by selective laser sintering (SLS) are emerging as a new class of attractive multifunctional materials. Herein, a thermoplastic polyurethane (TPU) powder for SLS was modified by 1 wt.% multi-walled carbon nanotube (MWCNTs) or a mixture of MWCNTs and graphene (GE) nanoparticles (70/30 wt/wt) in order to investigate on both the synergistic effect provided by the two conductive nanostructured carbonaceous fillers and the correlation between formulation, morphology, and final properties of SLS printed porous structures. In detail, porous structures with a porosity ranging from 20% to 60% were designed using Diamond (D) and Gyroid (G) unit cells. Results showed that the carbonaceous fillers improve the thermal stability of the elastomeric matrix. Furthermore, the TPU/1 wt.% MWCNTs-GE-based porous structures exhibit excellent electrical conductivity and mechanical strength. In particular, all porous structures exhibit a robust negative piezoresistive behavior, as demonstrated from the gauge factor (GF) values that reach values of about -13 at 8% strain. Furthermore, the G20 porous structures (20% of porosity) exhibit microwave absorption coefficients ranging from 0.70 to 0.91 in the 12-18 GHz region and close to 1 at THz frequencies (300 GHz-1 THz). Results show that the simultaneous presence of MWCNTs and GE brings a significant enhancement of specific functional properties of the porous structures, which are proposed as potential actuators with relevant electro-magnetic interference (EMI) shielding properties.

Novel Poly(vinyl alcohol)/Chitosan/Modified Graphene Oxide Biocomposite for Wound Dressing Application.

Rapid absorption of wound exudate and prevention of wound infection are prerequisites for wound dressing to accelerate wound healing. In this study, a novel kind of promising wound dressing is developed by incorporating polyhexamethylene guanidine (PHMG)-modified graphene oxide (mGO) into the poly(vinyl alcohol)/chitosan (PVA/CS) matrix, conferring the dressing the required mechanical properties, higher water vapor transmission rate (WVTR), less swelling time, improved antibacterial activity, and more cell proliferation compared to the PVA/CS film crosslinked by genipin. In vivo experiments indicate that the PVA/CS/mGO composite film can accelerate wound healing via enhancement of the re-epithelialization. PVA/CS/mGO composite film with 0.5 wt% mGO sheets displays the best wound healing properties, as manifested by the 50% higher antibacterial rate compared to GO and the wound healing rate of the mouse using this dressing is about 41% faster than the control group and 31% faster than the pure PVA/CS dressing. The underlying mechanism of the accelerated wound healing properties may be a result of the improved antibacterial ability to eradicate pathogenic bacteria on the wound area and maintain an appropriate moist aseptic wound healing environment to accelerate re-epithelialization. These findings suggest that this novel composite PVA/CS/mGO film may have promising applications in wound dressing.

Constructing 3D Graphene Network in Rubber Nanocomposite via Liquid-Phase Redispersion and Self-Assembly.

A three-dimensional graphene (GE) segregated network structure is of significance for improving the conductivity of composites. However, constructing such a GE network structure in composites still remains a challenge. Here, we demonstrate a facile process, that is, liquid-phase redispersion and self-assembly (LRS) to prepare polymer nanocomposites with graphene segregated networks. High shear liquid-phase mixing accompanied by the diffusion of dissolved polymer chains into the interstices and voids of the loose graphene powders can lead to redispersion of GE in polymer solution. Once the stirring is stopped, the self-assembly and segregation of redispersed GE occurs in a poor solvent driven by π-π interaction. After solvent evaporation, the GE assembly structures are retained as networks in the GE/polymer composite prepared by hot pressing. The graphene/(isobutylene-isoprene rubber) nanocomposite (GE/IIR) was investigated as a demonstration for the advantages of the LSR method. The morphologies of GE assemblies in the liquid phase and GE networks in the solid composite were observed. Due to the existence of the homogeneously distributed graphene segregated networks, the tensile strength and elongation at break for GE/IIR nanocomposites increase by ∼410 and ∼126%, respectively, and the electrical conductivity reaches ∼100 S m-1 at a GE content of 3.76 vol %. The LRS method was also successfully tried for systems with different polymer matrixes and different solvents, suggesting the robustness of the proposed method. The prepared flexible GE/IIR nanocomposites with GE networks are sensitive to tiny strain and can be applied in wearable sensors for the detection of human physiological signals.

4D Printing of a Liquid Crystal Elastomer with a Controllable Orientation Gradient.

Liquid crystal elastomers (LCEs), a class of soft materials capable of a large and reversible change in the shape under the trigger of external stimuli, can be fabricated into diverse architectures with complicated deformation modes through four-dimensional (4D) printing. However, the printable LCE ink is only in the form of monomeric precursors and the deformation mode is limited to contraction/extension deformation. Herein, we report a novel approach to break through these limitations. We achieved 4D printing of a single-component liquid crystal polymer ink in its isotropy state through direct ink writing (DIW) technology. The drawing force imposed by the movement of nozzle in the extruded printing process was able to align the mesogen units along the specific printing path. An orientation gradient perpendicular to the printing direction was obtained due to the existence of a temperature gradient between the two sides of printed samples and could be further fixed by post-photo-cross-linking treatment through the dimerizable groups in the LCE, realizing a new actuation mode in the field of extrusion-based printing of LCE actuators. The printed film was able to change reversibly from a strip to a tightly hollow cylinder and could reversibly lift up an object with roughly 600 times its own weight. The orientation gradient can be patterned through liquid-assistant printing or programmed structure design to integrate both bending and contraction actuation modes on the same printed sample, leading to complex deformation and two-dimensional (2D) planar porous structure to three-dimensional (3D) porous cylinder transition. This study opens up a new prospect to directly print a wide variety of LCE actuators with versatile actuation modes.

Pt Nanoparticle-Loaded Graphene Aerogel Microspheres with Excellent Methanol Electro-Oxidation Performance.

Platinum-decorated graphene aerogel microspheres were fabricated through a combined electrospraying, freeze-casting, and solvothermal process. Platinum nanoparticles with a narrow size distribution are evenly anchored on the graphene aerogel microspheres without agglomeration benefitting from the distinct center-diverging microchannel structure of the graphene aerogel microspheres, which results in the as-prepared catalysts presenting excellent electrocatalytic performance including high electrocatalytic activity and high poison tolerance toward methanol electro-oxidation, showing great potential for direct methanol fuel cell anode catalysts. In particular, the platinum-decorated graphene aerogel microspheres exhibit an extremely high mass activity of 1098.9 mA mg-1 toward methanol oxidation as well as excellent antipoisoning ability, which are dramatically enhanced compared with Pt particles dispersed on graphene oxide and commercial carbon black supports.

HIFU induced particles redistribution in polymer matrix via synchrotron radiation X-ray microtomography.

High-intensity Focused Ultrasound (HIFU) was used to stimulate the embedded copper sulfate (CuSO4) particles to release from the crosslinked poly (methyl methacrylate-co-butyl acrylate) copolymer solid matrix. In order to better understand the ultrasound release mechanism for drug/polymer delivery systems, the synchrotron radiation X-ray computed microtomography (SR-CT) was used to non-destructively investigate the structure of drug/polymer delivery systems after different HIFU treatment time. For the first time, we clearly demonstrate that ultrasonic waves can overcome the constraints of the polymer chain and drive the filler to move from the strong region to the weak region in the solid polymer matrix, thus resulting in a change in distribution of the filler in solid polymers. This result also demonstrates that SR-CT is a powerful technique which can be used to quantitatively study the 3D structure of fillers/polymers composite as it can take a broader and overall view than the conventional localized two-dimensional analysis method such as SEM, TEM.

Polydimethylsiloxane incorporated with reduced graphene oxide (rGO) sheets for wound dressing application: Preparation and characterization.

Toward fabricating a novel multifunctional wound dressing material, we incorporated a series of contents of reduced graphene oxide (rGO) sheets into polydimethylsiloxane (PDMS) matrix to prepare the rGO-PDMS composite membrane and be used for wound dressing. The pore structure, dispersion of rGO, physical properties, water vapor transmission rate (WVTR), cytotoxicity and antibacterial activity were studied. Finally, the effect of the rGO-PDMS composite membrane on wound healing was investigated on a murine full-thickness skin wound model. The rGO-PDMS composite membrane exhibited bionic performance (ordered pore structure and suitable WVTR), improved mechanical properties, good compatibility and effective antibacterial activity. In vivo experiment indicated that the rGO-PDMS composite membrane could accelerate wound healing via enhancement of the re-epithelialization and granulation tissue formation. These findings suggest that rGO doping PDMS uniquely resulted in a multifunctional material for potential use in wound dressing.

Liquid-Crystalline Dynamic Networks Doped with Gold Nanorods Showing Enhanced Photocontrol of Actuation.

A near-infrared-light (NIR)- and UV-light-responsive polymer nanocomposite is synthesized by doping polymer-grafted gold nanorods into azobenzene liquid-crystalline dynamic networks (AuNR-ALCNs). The effects of the two different photoresponsive mechanisms, i.e., the photochemical reaction of azobenzene and the photothermal effect from the surface plasmon resonance of the AuNRs, are investigated by monitoring both the NIR- and UV-light-induced contraction forces of the oriented AuNR-ALCNs. By taking advantage of the material's easy processability, bilayer-structured actuators can be fabricated to display photocontrollable bending/unbending directions, as well as localized actuations through programmed alignment of azobenzene mesogens in selected regions. Versatile and complex motions enabled by the enhanced photocontrol of actuation are demonstrated, including plastic "athletes" that can execute light-controlled push-ups or sit-ups, and a light-driven caterpillar-inspired walker that can crawl forward on a ratcheted substrate at a speed of about 13 mm min-1 . Moreover, the photomechanical effects arising from the two types of light-triggered molecular motion, i.e., the trans-cis photoisomerization and a liquid-crystalline-isotropic phase transition of the azobenzene mesogens, are added up to design a polymer "crane" that is capable of performing light-controlled, robot-like, concerted macroscopic motions including grasping, lifting up, lowering down, and releasing an object.

A Facile Strategy for Self-Healing Polyurethanes Containing Multiple Metal-Ligand Bonds.

A metal-ligand crosslinked internal self-healing polyurethane is developed using low-cost and commercially available compounds. The mechanical, photoluminescent, and self-healing properties can be governed by incorporating multiple metal-ligand crosslinks with weak and strong coordination bonds and varying the metal ion. In-situ attenuated total reflectance Fourier transform infrared spectroscopy reveals that the metal-ligand bond is cleaved during the damage process while metal ion is still coordinated with the ligand by stronger metal-pyridyl interaction. The multiple metal-ligand coordination facilitates the crosslinks to be fully reformed during the repairing process, leading to the superior self-healing property.

Dual water-healable zwitterionic polymer coatings for anti-biofouling surfaces.

Herein, we show for the first time drop-casting zwitterionic polymer colloidal particles onto different surfaces to obtain zwitterionic coatings with highly protein-repelling properties and dual self-healing capabilities. Upon nano/micro mechanical scratches, the coatings self-heal in a NaCl solution which is accompanied by the recovery of the anti-biofouling characteristics. Also under severe macro damage conditions, water will induce the zwitterionic groups buried inside the particles to transfer to the coating surface, and as such regenerate the surface-wetting properties and repair the anti-biofouling properties.

High-intensity focused ultrasound selective annealing induced patterned and gradient crystallization behavior of polymer.

High-intensity focused ultrasound (HIFU) was developed as a spatial selective annealing method to control the crystallization behavior and performance of polymer using amorphous polyethylene terephthalate (PET) as an example for demonstration. The spatial crystallization and morphological details of HIFU induced crystallization areas at the lamellar level and spherulite scale were studied by Micro-Focus hard X-ray diffraction, small angle X-ray scattering and optical microscopy. According to the distribution of crystallinity of PET, we can indirectly detect the history of thermal distribution of the ultrasonic focal point, which is hard to obtain by other methods. The crystallinity and the area of the crystalline region of PET sample increased with ultrasound power or irradiation time. Different from common crystalline structure of polymer materials, HIFU induced crystallinity of PET has a significant gradient distribution. The gradient crystal structure leads to a better mechanical performances, which can realize the good balance between toughness and strength. Ultrasound annealing, as a complement and development of the traditional annealing technology, has the characteristics of high efficient and spatial selectivity, showing great application prospect in post processing field.

Polydopamine Particles Reinforced Poly(vinyl alcohol) Hydrogel with NIR Light Triggered Shape Memory and Self-Healing Capability.

This study focuses on developing a facile approach to prepare biocompatible poly(vinyl alcohol) (PVA) composite hydrogels containing polydopamine particles (PDAPs) with ultrafast near-infrared (NIR) light-triggered shape memory and self-healing capability. The PVA-PDAPs composite hydrogels with excellent mechanical properties can be achieved after freezing/thawing treatment, and the formation of physically cross-linked networks from the hydrogen bonding (H-bonding) between PVA and PDAPs. Due to the excellent photothermal effect of polydopamine, the composited hydrogel can achieve rapid shape recovery and efficient self-healing properties under NIR light exposure in a short time. With the excellent shape memory performance, good biocompatibility, and self-healing property, this hydrogel should have great potential in biomedical fields such as tissue engineering, arthrodial cartilage, and artificial skin.