A Photoresponsive Polymeric Actuator Topologically Cross-Linked by Movable Units Based on a [2]Rotaxane

Boosting Ion Conductivities: Light-Modulated Azobenzene-Based Ionic Liquids in Vertical Nanochannels

Exploring stimuli-responsive ion-conductive materials is a challenging task, but it has gained increasing attention because of their enormous potential applications in actuators, sensors, and smart electronics. Here, we demonstrate a distinctive photoresponsive ion-conductive device that utilizes azobenzene-based ionic liquids ([AzoCnMIM][Br], where n = 2, 6, and 10), confined in nanochannels of anodic aluminum oxide (AAO) templates for photoisomerization. The structure of [AzoCnMIM][Br] comprises photoresponsive and hydrophobic azobenzene moieties, hydrophilic imidazolium cations, and negatively charged bromide ions. Therefore, [AzoCnMIM][Br] can form micelles and exhibit photoresponsive ion conductivities. The nanochannels of AAO templates exhibit a confinement effect on the formation of azobenzene-based ionic liquid micelles due to the pore size, thereby preventing the formation of larger micelles that could lead to a decrease in conductivity. Consequently, the ion conductivities of the azobenzene-based ionic liquids are higher in the nanochannels of the AAO templates. The effects of the length of carbon chains on the azobezene-based ionic liquids and the pore size of the AAO templates have also been investigated. Additionally, through irradiation with UV/vis light, [AzoCnMIM][Br] can undergo reversible isomerization, thereby reversibly changing the sizes of the micelles and subsequently altering the ion conductivities.

Exploration of molecular machines in supramolecular soft robotic systems

Soft robotic system, a new era of material science, is rapidly developing with advanced processing technology in soft matters, featured with biomimetic nature. An important bottom-up approach is through the implementation of molecular machines into polymeric materials, however, the synchronized molecular motions, acumination of strain across multiple length-scales, and amplification into macroscopic actuations remained highly challenging. This review presents the significances, key design strategies, and outlook of the hierarchical supramolecular systems of molecular machines to develop novel types of supramolecular-based soft robotic systems.

Supramolecular topological hydrogels: from material design to applications

Supramolecular topological hydrogels are constructed by introducing different dynamic topological structures into polymeric networks and thus exhibit a wide variety of stimuli-responsive properties and versatile applications.

Poly(β-Cyclodextrin) Prepared by Ring-Opening Metathesis Polymerization Enables Creation of Supramolecular Polymeric Networks

The controlled synthesis of polymers containing densely grafted cyclodextrin units has proven challenging due to the steric hindrance of these cyclic oligosaccharides. In this study, we report the controlled synthesis of poly(β-cyclodextrin) [poly(β-CD)] through ring-opening metathesis polymerization (ROMP) using Grubbs third-generation catalyst. Molecular weights of >10⁵ g/mol were obtained with dispersity values (Đ) of ≤1.2. In aqueous solution, β-cyclodextrin forms a host-guest complex with adamantyl groups (Ad). These interactions were utilized to prepare supramolecular polymer networks (SPNs) made by adding poly(β-CD) to α,ω-adamantyl-functionalized poly(2-hydroxyethyl acrylate) (Ad-PHEA-Ad). These poly(β-CD)/Ad-PHEA-Ad SPNs were prepared in aqueous solution and then dried to make homogeneous, transparent films. Varying the ratios of the two components enabled structure-property studies via tensile measurements.

Post-synthesis modification of slide-ring gels for thermal and mechanical reconfiguration

Ring-sliding behavior in polyrotaxanes imbues gels, elastomers, and glasses with remarkable stress-dissipation and actuation properties. Since these properties can be modulated and tuned by structural parameters, many efforts have been devoted to developing synthetic protocols that define the structures and properties of slide-ring materials. We introduce post-synthetic modifications of slide-ring gels derived from unmodified α-cyclodextrin and poly(ethylene glycol) polyrotaxanes that enable (i) actuation and control of the thermo-responsive lower critical solution temperature (LCST) behavior of ring-modified slide-ring hydrogels, and (ii) chemically bonding separate gels into hybrid or shape-reconfigured macro-structures with a slide-ring adhesive solution. The mechanical properties of the post-modified gels have been characterized by shear rheology and uniaxial tensile tests, while the corresponding xerogels were characterized by wide-angle X-ray scattering. These demonstrations show that post-synthetic modification offers a practical solution for re-configuring the properties and shapes of slide-ring gels.

MXene-Based Humidity-Responsive Actuators: Preparation and Properties

Water is a significant and abundant resource as well as a pure natural energy source. Many researchers have been reported on humidity-responsive actuators that mimick the humidity responsive behavior that widely exists in nature. Benefiting from advantages such as hydrophilicity, high electrical conductivity, and good dispersibility, MXenes (Ti₃ C₂ T_x ) show promising performance when applied to humidity-responsive actuators. This Minireview describes the preparation methods and structural characteristics of MXenes, and the mechanism of humidity-responsive actuators. Recent important advances of MXene materials in actuators are objectively reviewed and evaluated, and existing issues are discussed. In addition, the development of these systems is outlined from the aspects of MXene preparation, structure control, design and assembly, and applications, and provides new ideas and guidance for the development of the next generation of high-performance MXene-based humidity-responsive actuators.

Rotaxane-Based Difunctional Nitrile N-Oxide Crosslinker: Synthesis and Direct Introduction of Movable Crosslinking Points into Ethylene-Propylene-Butadiene Monomer (EPDM) Rubber

Incorporation of rotaxane scaffolds into the crosslinking points of polymer networks significantly affects their rheological and mechanical properties. The present study involves the synthesis of a new rotaxane-type crosslinker containing nitrile N-oxide functional groups on both the axle and wheel components. The prepared crosslinker is highly reactive; however, it can be isolated and applied in the crosslinking reaction of a commercially important polymer, namely ethylene-propylene-butadiene monomer rubber (EPDM), in the absence of additives and catalysts. Tensile tests reveal that compared to a network containing conventional crosslinking points, both breaking strength and strain of the network structure prepared herein are improved due to the incorporation of movable crosslinking points. The synthesized network structure also exhibits five times higher fracture energy. The developed post-crosslinking methodology for the direct introduction of movable crosslinking points into pre-formed polymers will be valuable in the production of rotaxane materials for various applications.

Controlled Sol-Gel and Diversiform Nanostructure Transitions by Photoresponsive Molecular Switching of Tetraphenylethene- and Azobenzene-Functionalized Organogelators

The implementation of stimuli-responsive materials with dynamically controllable features has long been an important objective that challenges chemists in the materials science field. We report here the synthesis and characterization of [2]rotaxanes (R1 and R1-b) with a molecular shuttle and photoresponsive properties. Axles T1 and T1-b were found to be highly efficient and versatile organogelators toward various nonpolar organic solvents, especially p-xylene, with critical gelation concentrations as low as 0.67 and 0.38 w/v %, respectively. The two molecular stations of switchable [2]rotaxanes (R1 and R1-b) can be revealed or concealed by t-butylcalix[4]arene macrocycle, thus inhibiting the gelation processes of the respective axles T1 and T1-b through the control of intermolecular hydrogen-bonding interactions. The sol-gel transition of axles T1 and T1-b could be achieved by the irradiation of UV-visible light, which interconverted between the extended and contracted forms. Interestingly, the morphologies of organogels in p-xylene, including flakes, nanobelts, fibers, and vesicles depending on the molecular structures of axles T1 and T1-b, were induced by UV-visible light irradiation. Further studies revealed that acid-base-controllable and reversible self-assembled nanostructures of these axle molecules were mainly constructed by the interplay of multi-noncovalent interactions, such as intermolecular π-π stacking, CH-π, and intermolecular hydrogen-bonding interactions. Surprisingly, our TPE molecular systems (R1, R1-b, T1, and T1-b) are nonemissive in their aggregated states, suggesting that not only fluorescence resonance energy transfer but also aggregation-caused quenching may have been functioning. Finally, the mechanical strength of these organogels in various solvents was monitored by rheological experiments.

Redox-responsive supramolecular polymeric networks having double-threaded inclusion complexes

Stimuli-responsive hydrogels have attracted attention as soft actuators that act similarly to muscles. In this work, hydrogel actuators controlled by host–guest interactions have been developed. The introduction of a 1:1 inclusion complex into a hydrogel is a popular design for achieving a change in cross-linking density. To realize faster and larger deformation properties, the introduction of a 1:2 inclusion complex is effective because the alteration in cross-linking density in a hydrogel with 1:2 complexes is larger than that in a hydrogel with 1:1 complexes. A redox-responsive hydrogel actuator cross-linked with 1:2 inclusion complexes is designed, where γ-cyclodextrin (γCD) and viologens modified with an alkyl chain derivative (VC11) were employed as the host and guest units, respectively. γCD includes two VC11 molecules in its cavity. The obtained γCD–VC11 hydrogel cross-linked with the 1:2 complex showed faster and larger deformation behaviour than the αCD–VC11 and the βCD–VC11 hydrogels cross-linked with a 1:1 complex. The deformation ratio and response speed of the γCD–VC11 hydrogel, which forms a supramolecular cross-linking structure by stimuli, are 3 and 11 times larger, respectively, than those of our previous hydrogel consisting of a βCD/ferrocene 1:1 inclusion complex.

A hydrogel actuator with a 1:2 host–guest complex controlled by redox stimuli has been developed to realize faster and larger deformation.

Visualization of the slide-ring effect: a study on movable cross-linking points using mechanochromism

To evaluate the ‘slide-ring’ effect in a rotaxane cross-linked network, we incorporated mechanochromophores into static and rotaxane cross-linking points and compared the mechanochromisms exhibited by the obtained polymers.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Type III-C rotaxane dendrimers: synthesis, dual size modulation and in vivo evaluation

Type III-C rotaxane dendrimers were synthesized by a divergent approach. Dual shuttling behavior and size modulation were observed from non-methylated/methylated rotaxane dendrimers under the same external stimuli. The biological distribution of dendrimers in C57BL/6J mice determined by MALDI-TOF-MS shows predominant accumulation in the spleen and liver. Drug encapsulations with chlorambucil and lithocholic acid were demonstrated.

Complex systems that incorporate cyclodextrins to get materials for some specific applications

Cyclodextrins (CDs) are a family of biodegradable cyclic hydrocarbons composed of α-(1,4) linked glucopyranose subunits, the more common containing 6, 7 or 8 glucose units are named α, β and γ-cyclodextrins respectively. Since the discovery of CDs, they have attracted interest among scientists and the first studies were about the properties of the native compounds and in particular their use as catalysts of organic reactions. Characteristics features of different types of cyclodextrins stimulated investigation in different areas of research, due to its non-toxic and non-inmunogenic properties and also to the development of an improved industrial production. In this way, many materials with important properties have been developed. This mini-review will focus on chemical systems that use cyclodextrins, whatever linked covalently or mediated by the non covalent interactions, to build complex systems developed mainly during the last five years.

Rapid preparation of auto-healing gels with actuating behaviour

Gels with multiple stimuli-responsive actuating behaviour have shown great potential in many applications. Nevertheless, facile approaches to rapidly preparing gel actuators are still highly needed, and obtaining gels possessing both actuating and auto-healing capabilities remains a challenge. Herein, we report the rapid preparation of gel actuators with a self-healing ability. Dual-component gels, composed of poly(BA-co-VI-co-AM) (G-1) and poly(BA-co-AA-co-AM/β-CD) (G-2) (BA = butyl acrylate, VI = N-vinyl imidazole, AM = acrylamide, AA = acrylic acid, β-CD = β-cyclodextrin), are prepared within 10 minutes (min) via biphase frontal polymerization (FP). Both G-1 and G-2 gels show excellent intrinsic self-healing properties based on hydrogen bonds, with healing efficiencies of 91% and 97%, respectively; self-healing between G-1 and G-2 also occurs due to hydrogen bonding and host-guest interactions. Moreover, dual-component gels, in terms of G-1 and G-2 bilayer gel flowers and strips, heterogeneous healed bilayer gel strips, and microfluidic-directed bilayer gel microsphere ensembles, all show actuating behaviour in acidic, alkaline and organic solutions, with actuation degrees up to 96% in 5 min. The actuation mechanism is also proposed. This work might provide new insights into fast synthesis of self-healing dual-component gels towards application in the actuator field.

Disabling Molecular Recognition through Reversible Mechanical Stoppering

Mechanical stoppering of a guest molecule prevents its self-assembly with a macrocycle unit, so that both species coexist in a medium but do not recognize each other. The application of a chemical or physical stimulus reverses mechanical stoppering and subsequently enables molecular recognition. This process, which occurs without cross-reactivity and is perceptible at the macroscopic scale, could facilitate programming on/off states in supramolecular materials and molecular devices.

Composite Nanotube Ring Structures Formed by Two-Step Self-Assembly for Drug Loading/Release

Nanotube rings are barely reported novel structures formed by the self-assembly of soft matter, as compared with nanotube structures and ring structures. The two-step self-assembly of amphiphilic copolymer AB and solvophobic copolymer CDC was studied. We found that nanotube rings can be formed from a certain mass ratio of copolymer CDC to copolymer AB and block D of certain rigidity. More interestingly, we discovered a new strategy for drug loading and release, which is different from the usual strategies reported in the literature. The present study provides a new rationale for the self-assembly of copolymers.

[Functionalization of Cyclodextrin Derivatives to Create Supramolecular Pharmaceutical Materials]

Molecular recognition is useful in creating functional supramolecular materials. Non-covalent bond formations, such as host-guest interactions, hydrogen bonding, and electrostatic interaction, are effective tools for introducing various functions and properties into materials. This review focuses on such macroscopic functions as selective molecular adhesion, self-healing, toughness, and the actuation of supramolecular polymeric materials-materials which have potential in pharmaceutical development. These functions have been achieved using reversible bonding between cyclodextrins (CDs; cyclic host molecules) and guest molecules. For example, macroscopic adhesions between host-modified hydrogels and guest-modified hydrogels have been investigated. CD-modified hydrogels were found to show selective adhesion to a guest hydrogel with an appropriate molecular size for the CD cavity, indicating that the host-guest complex formation between the gels led to the adhesive behavior. Surprisingly, polymeric materials having host-guest cross-linking points show both high toughness and flexibility, unlike conventional covalently cross-linked materials. These materials also exhibited self-healing properties, capable of repairing damage to the materials. Furthermore, the supramolecular materials demonstrated macroscopic rapid expansion and contraction driven by external stimuli under wet or semi-dry conditions, in which the supramolecular gels vary the cross-linking density between the polymers accordingly. Different topological gels are able to vary the length of the polymer chain between cross-linking points to show large deformation. Both types of actuators were found to exhibit externally stimulated flexing behaviors. This review summarizes recent advancements in the development of these supramolecular materials, which appear to be promising new components in pharmaceutical science.

Efficient enantiorecognition of amino acids under a stimuli-responsive system: synthesis, characterization and application of electroactive rotaxane

An electroactive chiral rotaxane, consisting of a polymeric chiral ionic liquid as the flexible axle and 18-crown-6 as the wheel, is synthesized for efficient enantiorecognition of amino acids.