Self-Correction Strategy for Precise, Massive, and Parallel Macroscopic Supramolecular Assembly

Adaptisorption of Nonporous Polymer Crystals

Although our knowledge and understanding of adsorptions in natural and artificial systems has increased dramatically during the past century, adsorption associated with nonporous polymers remains something of a mystery, hampering applications. Here we demonstrate a model system for adaptisorption of nonporous polymers, wherein dative B-N bonds and host-guest binding units act as the kinetic and thermodynamic components, respectively. The coupling of these two components enables nonporous polymer crystals to adsorb molecules from solution and undergo recrystallization as thermodynamically favored crystals. Adaptisorption of nonporous polymer crystals not only extends the types of adsorption in which the sorbate molecules are integrated in a precise and orderly manner in the sorbent systems, but also provides a facile and accurate approach to the construction of polymeric materials with precise architectures and integrated functions.

Reconfiguring hydrogel assemblies using a photocontrolled metallopolymer adhesive for multiple customized functions

Stimuli-responsive hydrogels with programmable shape changes are promising materials for soft robots, four-dimensional printing, biomedical devices and artificial intelligence systems. However, these applications require the fabrication of hydrogels with complex, heterogeneous and reconfigurable structures and customizable functions. Here we report the fabrication of hydrogel assemblies with these features by reversibly gluing hydrogel units using a photocontrolled metallopolymer adhesive. The metallopolymer adhesive firmly attached individual hydrogel units via metal-ligand coordination and polymer chain entanglement. Hydrogel assemblies containing temperature- and pH-responsive hydrogel units showed controllable shape changes and motions in response to these external stimuli. To reconfigure their structures, the hydrogel assemblies were disassembled by irradiating the metallopolymer adhesive with light; the disassembled hydrogel units were then reassembled using the metallopolymer adhesive with heating. The shape change and structure reconfiguration abilities allow us to reprogramme the functions of hydrogel assemblies. The development of reconfigurable hydrogel assemblies using reversible adhesives provides a strategy for designing intelligent materials and soft robots with user-defined functions.

Precise and selective macroscopic assembly of a dual lock-and-key structured hydrogel

Macroscopic assembly offers immense potential for constructing complex systems due to the high design flexibility of the building blocks. In such assembly systems, hydrogels are promising candidates for building blocks due to their versatile chemical compositions and ease of property tuning. However, two major challenges must be addressed to facilitate application in a broader context: the precision of assembly and the quantity of orthogonally matching pairs must both be increased. Although previous studies have attempted to address these challenges, none have successfully dealt with both simultaneously. Here, we propose topology-based design criteria for the selective assembly of hydrogel building blocks. By introducing the dual lock-and-key structures, we demonstrate highly precise assembly exclusively between the matching pairs. We establish principles for selecting multiple orthogonally matching pairs and achieve selective assembly involving simple one-to-one matching and complex assemblies with multiple orthogonal matching points. Moreover, by harnessing hydrogel tunability and the abundance of matching pairs, we synthesize complementary single-stranded structures for programmable assembly and successfully assemble them in the correct order. Finally, we demonstrate a hydrogel-based self-assembled logic gate system, including a YES gate, an OR gate, and an AND gate. The output is generated only when the corresponding inputs are provided according to each logic.

Polyelectrolyte chain conformation matters in macroscopic supramolecular self-assembly

We demonstrate molecular-conformation-dependent macroscopic supramolecular self-assembly (MSA) driven by electrostatic interactions. Evidence from single molecular force spectroscopy reveals that polyelectrolytes modified on MSA component surfaces make MSA possible with a loop conformation, while those with a flat conformation lead to no assembly, which is attributed to distinct molecular mobility. We believe that this finding is also applicable in fundamental phenomena such as surface adsorption and adhesion regarding polymers.

Stabilizing Liquids Using Interfacial Supramolecular Assemblies

Stabilizing liquids based on supramolecular assembly (non-covalent intermolecular interactions) has attracted significant interest, due to the increasing demand for soft, liquid-based devices where the shape of the liquid is far from the equilibrium spherical shape. The components comprising these interfacial assemblies must have sufficient binding energies to the interface to prevent their ejection from the interface when the assemblies are compressed. Here, we highlight recent advances in structuring liquids based on non-covalent intermolecular interactions. We describe some of the progress made that reveals structure-property relationships. In addition to treating advances, we discuss some of the limitations and provide a perspective on future directions to inspire further studies on structured liquids based on supramolecular assembly.

Cyclic Macroscopic Assembly and Disassembly Driven by Ionic Strength Fuel: A Waste-Free Approach

Nonequilibrium assembling systems developed so far have relied on chemical fuels to drive the programmable pH cycles, redox reactions, and metastable bond formations. However, these methods often result in the unwanted accumulation of chemical waste. Herein, we present a novel strategy for achieving cyclic and waste-free nonequilibrium assembly and disassembly of macroscopic hydrogels, utilizing an ionic strength-mediated approach. Our strategy involves using ammonium carbonate as a chemical fuel to temporally regulate the attractions between oppositely charged hydrogels via ionic strength-controlled charge screening and hydrogel elasticity changes. This chemical fuel effectively mediates the assembly/disassembly processes and prevents waste accumulation, as ammonium carbonate can completely decompose into volatile chemical waste. The cyclic and reversible assembly process can be achieved without significant damping due to the self-clearance mechanism, as long as the chemical fuel is repeatedly supplied. This concept holds promise for creating macroscopic and microscopic nonequilibrium systems and self-adaptive materials.

Robust Electrostatically Interactive Hydrogel Coatings for Macroscopic Supramolecular Assembly via Rapid Wet Adhesion

A macroscopic supramolecular assembly (MSA) refers to non-covalent interactions between building blocks over a micrometer scale, which provides insights into bio-/wet adhesion, self-healing, and so on and new fabrication strategies to heterogeneous structures and bio-scaffolds. The key to realize the MSA of rigid materials is pre-modifying a compliant coating known as a "flexible spacing coating" beneath the interactive moieties. However, available coatings are limited to polyelectrolyte multilayers with shortcomings of tedious fabrication, weak adhesion to substrates, susceptibility to external reagents, and so on. Here, we develop a facile method to induce a new "flexible spacing coating" of a poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel with electrostatic interactions to achieve MSA of diverse rigid materials (quartz, metal, rubber, and plastics). Selective self-assembly of positive-negative charged surfaces is observed by the naked eye under 3 min of shaking in water, providing strategies to rapid wet adhesion. The interfacial binding force between positive-negative interacted surfaces is 1018.1 ± 299.2 N/m², which is over two magnitudes larger than that of control groups, that is, positive-positive (24.4 ± 10.0 N/m²) and negative-negative (67.5 ± 16.7 N/m²) interacted surfaces. In situ force measurements and control experiments of identically charged building blocks have strongly supported the improved binding strength and chemical selectivity between interactive building blocks. The coating is advantageous with a simple fabrication, strong adhesion to materials, robust solvent tolerance to assembly solutions, and feasibility of photo-patterning. We envision that the above strategy would broaden the material choices of flexible spacing coatings for efficient MSA and new methods for rapid interfacial adhesion.

Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices. Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces, but also demands the overall device to be flexible in response to external fields. However, flexible energy storage devices inevitably occur mechanical damages (extrusion, impact, vibration)/electrical damages (overcharge, over-discharge, external short circuit) during long-term complex deformation conditions, causing serious performance degradation and safety risks. Inspired by the healing phenomenon of nature, endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices. Herein, this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices. Firstly, the main intrinsic self-healing mechanism is introduced. Then, the research situation of electrodes, electrolytes, artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed. Finally, the current challenges and perspective are provided. We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

Organic‒inorganic semi-interpenetrating networks with orthogonal light- and magnetic-responsiveness for smart photonic gels

Living matter has the ability to perceive multiple stimuli and respond accordingly. However, the integration of multiple stimuli-responsiveness in artificial materials usually causes mutual interference, which makes artificial materials work improperly. Herein, we design composite gels with organic‒inorganic semi-interpenetrating network structures, which are orthogonally responsive to light and magnetic fields. The composite gels are prepared by the co-assembly of a photoswitchable organogelator (Azo-Ch) and superparamagnetic inorganic nanoparticles (Fe₃O₄@SiO₂). Azo-Ch assembles into an organogel network, which shows photoinduced reversible sol-gel transitions. In gel or sol state, Fe₃O₄@SiO₂ nanoparticles reversibly form photonic nanochains via magnetic control. Light and magnetic fields can orthogonally control the composite gel because Azo-Ch and Fe₃O₄@SiO₂ form a unique semi-interpenetrating network, which allows them to work independently. The orthogonal photo- and magnetic-responsiveness enables the fabrication of smart windows, anti-counterfeiting labels, and reconfigurable materials using the composite gel. Our work presents a method to design orthogonally stimuli-responsive materials.

Macroscopic Supramolecular Assembly of Rigid Building Blocks Facilitated by Layer-By-Layer Assembled Microgel Film

Macroscopic supramolecular assembly (MSA) of building blocks larger than 1 μm provides new methodology for fabrication of functional supramolecular materials and a platform for mechanism investigation of interfacial phenomena. Most reports on MSA are restricted to soft hydrogels, and supramolecular groups can be directly integrated into a hydrogel matrix to generate sufficient attraction for maintaining macroscopic assemblies. For non-hydrogel stiff building blocks, two layer-by-layer modification processes consisting of flexible spacing coating and additional interacting groups are necessary to enable MSA, which is laborious and time-consuming. Approaches for highly efficient MSA based on flexible spacing coating are desired. In this work, MSA of polydimethylsiloxane (PDMS) building blocks is demonstrated by inducing microgel films that serve as both flexible spacing coating and surface functional groups, thus avoiding a two-step LbL modification process. By the varying bilayer number of microgel films, the MSA probability of modified PDMS increases from 54% at 3 bilayers to 100% at 6 bilayers. Control experiments and in situ force measurement strongly support the obtained MSA results and verify the dominant role of the microgel film as a flexible spacing coating and a supramolecularly interactive layer in achieving MSA. Moreover, the underlying mechanism is interpreted as low Young's modulus microgel films rendering surface groups highly mobile to enhance the multivalent interfacial binding. Taken together, this work has demonstrated the feasibility of MSA of rigid building blocks assisted by microgel films as flexible spacing coating and supramolecularly interactive layer simultaneously, which may extend the application fields of microgel materials to interfacial adhesion and advanced manufacturing with MSA methodology.

Self-sorting in macroscopic supramolecular self-assembly via additive effects of capillary and magnetic forces

Supramolecular self-assembly of μm-to-mm sized components is essential to construct complex supramolecular systems. However, the selective assembly to form designated structures at this length scale is challenging because the short-ranged molecular recognition could hardly direct the assembly of macroscopic components. Here we demonstrate a self-sorting mechanism to automatically identify the surface chemistry of μm-to-mm components (A: polycations; B: polyanions) based on the A-B attraction and the A-A repulsion, which is realized by the additivity and the competence between long-ranged magnetic/capillary forces, respectively. Mechanistic studies of the correlation between the magnetic/capillary forces and the interactive distance have revealed the energy landscape of each assembly pattern to support the self-sorting results. By applying this mechanism, the assembly yield of ABA trimers has been increased from 30%~40% under conventional conditions to 100% with self-sorting. Moreover, we have demonstrated rapid and spontaneous self-assembly of advanced chain-like structures with alternate surface chemistry.

Enhancing the interfacial binding strength between modular stretchable electronic components

Stretchable electronics are emerging for personalized and decentralized clinics, wearable devices and human-machine interactions. Nowadays, separated stretchable functional parts have been well developed and are approaching practical usage. However, the production of whole stretchable devices with full functions still faces a huge challenge: the integration of different components, which was hindered by the mechanical mismatch and stress/strain concentration at the connection interfaces. To avoid connection failure in stretchable devices, a new research focus is to improve the interfacial binding strength between different components. In this review, recent developments to enhance interfacial strength in wearable/implantable electronics are introduced and catalogued into three major strategies: (i) covalent bonding between different device parts, (ii) molecular interpenetration or mechanical interlocking at the interfaces and (iii) covalent connection between the human body and devices. Besides reviewing current methods, we also discuss the existing challenges and possible improvements for stretchable devices from the aspect of interfacial connections.

Codes in Code: AIE Supramolecular Adhesive Hydrogels Store Huge Amounts of Information

With the continuous advancement of information technology, the requirements for the information storage capacity of materials are getting higher and higher. However, information code materials usually only store a single piece of information. In order to improve their storage capacity, aggregation-induced emission (AIE) supramolecular adhesive hydrogels with different fluorescent colors are prepared, and a "Codes in Code" method is used to demonstrate the storage capacity for large amounts of information. Four kinds of poly(vinyl alcohol) (PVA) supramolecular hydrogels with different fluorescent colors are prepared; based on the hydrogen bonds on the hydrogel surface, these hydrogels can be assembled into a hydrogel, G5, which shows multiple fluorescent colors under the irradiation of UV light. When many 1D barcode patterns or/and 2D code patterns are incorporated into G5, not only a kind of 3D information but also plenty of 1D or/and 2D information can be stored. Therefore, the information codes prepared by the "Codes in Code" method can store a large amount of information.

Precise macroscopic supramolecular assembly of photopatterned hydrogels

Here we demonstrate that a precise macroscopic supramolecular assembly (MSA) can be achieved using a surface photopatterning strategy. The electrostatic interaction of the photopatterned polyelectrolytes drives hydrogel cuboids to form a stable MSA on a millimeter scale and the spatial controllability of light enables the hydrogels to be assembled into complex supramolecular architectures.

Topological cyclodextrin nanoparticles as crosslinkers for self-healing tough hydrogels as strain sensors

Hydrogels have been widely used for various applications, and thus addressing the challenges associated with the design of sustainable hydrogels has become an important issue. However, little attention has been devoted toward the design of crosslinkers which are often toxic, lack self-healing capabilities, and derived from petrochemicals. Herein, novel cyclodextrin topological nanoparticles (TNPs) have been constructed. These TNPs were found to possess crosslinking capabilities and the corresponding TNPs-crosslinked hydrogels showed excellent mechanical performances with a high stretchability of 1860 % and stress of 180 kPa and good anti-fatigue abilities. These hydrogels could be readily recycled and used for modular assembly and disassembly in various shapes and could serve as flexible strain sensors to monitor human activities with a sensing range of 0-1800 %, controllable sensitivity, and good fatigue resistance. These topological nanoparticles can inspire the design of novel physical crosslinkers for novel flexible strain sensors, tough and self-healing hydrogels, and soft robotics.

Fabrication of 3D Ordered Structures with Multiple Materials via Macroscopic Supramolecular Assembly

Integration of diverse materials into 3D ordered structures is urgently required for advanced manufacture owing to increase in demand for high-performance products. Most additive manufacturing techniques mainly focus on simply combining different equipment, while interfacial binding of distinctive materials remains a fundamental problem. Increasing studies on macroscopic supramolecular assembly (MSA) have revealed efficient interfacial interactions based on multivalency of supramolecular interactions facilitated by a "flexible spacing coating." To demonstrate facile fabrication of 3D heterogeneous ordered structures, the combination of MSA and magnetic field-assisted alignment has been developed as a new methodology for in situ integration of a wide range of materials, including elastomer, resin, plastics, metal, and quartz glass, with modulus ranging from tens of MPa to over 70 GPa. Assembly of single material, coassembly of two to four distinctive materials, and 3D alignment of "bridge-like" and "cross-stacked" heterogeneous structures are demonstrated. This methodology has provided a new solution to mild and efficient assembly of multiple materials at the macroscopic scale, which holds promise for advanced fabrication in fields of tissue engineering, electronic devices, and actuators.

Macroscopic Self-Assembly of Gel-Based Microfibers toward Functional Nonwoven Fabrics

Macroscopic self-assembly has increasingly attracted numerous concerns because of the facile fabrication of complex structures and diversified morphologies. Key challenges still remain to design high-performance building blocks to increase the efficiency and diversity of macroscopic self-assembly. Here, we designed triple noncovalent interactions (carboxyl-Zn²⁺ coordination, host-guest interactions, and hydrogen bonding interactions) to enhance the interactions between self-healing fibers, constructing multidimensional nonwoven fiber-based fabrics through macroscopic self-assembly without further postprocessing. Profiled from the strong interactions generated from triple noncovalent interactions, ordered two-dimensional plane and three-dimensional spiral gel fabrics were fabricated using polyvinyl pyrrolidone/gel-based fibers as building blocks toward a human motion sensor. Moreover, we demonstrated that the macroscopic self-assembly strategy is universal to construct three-dimensional film-based fabrics toward wound dressing based on the triple noncovalent interactions between two-dimensional films. This macroscopic self-assembly approach provides an alternative strategy to fabricate gel fabrics for various applications.

A Functioning Macroscopic "Rubik's Cube" Assembled via Controllable Dynamic Covalent Interactions

The dynamic behavior of a macroscopic adhered hydrogel stabilized through controllable dynamic covalent interactions is reported. These interactions, involving the cross-linked formation of a hydrogel through reaction of a diacylhydrazine precursor with a tetraformyl partner, increase as a function of time. By using a contact time of 24 h and different compounds with recognized aggregation-induced emission features (AIEgens), it proves possible to create six laminated acylhydrazone hydrogels displaying different fluorescent colors. Blocks of these hydrogels are then adhered into a structure resembling a Rubik's Cube, a trademark of Rubik's Brand Limited, (RC) and allowed to anneal for 1 h. This produces a 3 × 3 × 3 block (RC) wherein the individual fluorescent gel blocks are loosely adhered to one another. As a consequence, the 1 × 3 × 3 layers making up the RC can be rotated either horizontally or vertically to produce new patterns. Ex situ modification of the RC or application of a chemical stimulus can be used to produce new color arrangements. The present RC structure highlights how the temporal features, strong versus weak adhesion, may be exploited to create smart macroscopic structures.

Light-Switchable Polymer Adhesive Based on Photoinduced Reversible Solid-to-Liquid Transitions

The development of switchable adhesives for reversible bonding and debonding can overcome the problems associated with conventional adhesives in separating, recycling, and repairing glued surfaces. Here, a photoresponsive azobenzene-containing polymer (azopolymer) is developed for photocontrolled adhesion. The azopolymer P1 (poly(6-(4-(p-tolyldiazenyl)phenoxy)hexyl acrylate)) exhibits photoinduced reversible solid-to-liquid transitions due to trans-cis photoisomerization. Trans P1 is a solid that glues two substrates with a stiffness comparable to that of conventional adhesives. UV light induces trans-to-cis isomerization, liquefies P1, weakens the adhesion, and facilitates the separation of glued substrates. Conversely, visible light induces cis-to-trans isomerization, solidifies P1, and enhances the adhesion. P1 enables photocontrolled reversible adhesion for various substrates with different wettability, chemical compositions, and surface roughness. P1 can also be implemented in both dry and wet environments. Light can control the adhesion process with high spatiotemporal resolution when using P1 as a switchable adhesive. Photoinduced reversible solid-to-liquid transitions represent a strategy for materials recycling and automated production processes that require reversible bonding and debonding.

Microphase separation of a quadruple hydrogen bonding supramolecular polymer: effect of the steric hindrance of the ureido-pyrimidone on their viscoelasticity

Supramolecular polymers based on 2-ureido-4[1H]-pyrimidone (UPy) units with extremely high dimerization constants and adjustable properties have received significant attention. In this work, we attempt to discuss the relationship between the micro-phase separation and the viscoelastic properties of the supramolecular polymers. For this reason, polymers with different UPy moieties structures and different UPy moieties contents were prepared and studied. It was found that the UPy moiety with little hindrance at the six-position of the pyrimidone could self-assemble into a nano-fiber structure and the degree of the micro-phase separation increased with the content of the UPy moiety. With the enlargement of the steric hindrance of the six-position of the pyrimidone, the nano-fiber structure gradually disappeared, meaning the degree of the micro-phase separation decreased astonishingly. More importantly, with the degree of the micro-phase separation increased, the storage modulus or the elasticity modulus increased exponentially and the T_m and the loss modulus area increased linearly. These results would lead a new way to study and develop novel polymeric materials.

The supramolecular polymers based on 2-ureido-4[1H]-pyrimidone (UPy) with different structure were prepared and the effects of microtopography on the viscoelasticity were investigated.

Leeches-Inspired Hydrogel-Elastomer Integration Materials

Inspired by the functions of leeches, for the first time homogeneous materials integrating hydrogels and elastomers were achieved by free radical polymerization. 2-Methoxyethyl acrylate (MEA) was used as elastomer monomer and Pluronics functionalized with vinyl groups acted as cross-linkers to impart the hydrogel property to the materials. The resulting Pluronic/PMEA gels possess a swelling ratio of about 210% and good water-retaining ability. Compression tests of Pluronic/PMEA gels at swelling equilibrium state show a stress up to 1.6 MPa under 85% strain. The gels act as elastomer after dehydration. Uniaxial tensile fracture stress and the elongation reached 1200 kPa and 500%, respectively, and compression stress was above 22 MPa. Furthermore, the Pluronic/PMEA gels also show self-healing properties. Owing to the excellent mechanical performance in both wet and dry conditions, this hydrogel-elastomer integrated material may have potential applications in tissue engineering, soft robotics, and biomedical devices.

High-Performance pH-Switchable Supramolecular Thermosets via Cation-π Interactions

Supramolecular chemistry has provided versatile and affordable solutions for the design of intelligent soft materials, but it cannot be applied in stiff materials. This paper describes a new concept for the design of high-performance supramolecular thermosets by using the noncovalent cation-π interaction as cross-linking. These supramolecular thermosets are a class of infusible and insoluble stiff polymers having excellent mechanical properties even at temperatures exceeding 300 °C. The cation-π interaction can be locally and reversibly installed and removed by aqueous treatments at high or low pH, respectively. Local manipulation of cross-linking confers these thermosets with multiple stimuli-responsive functions, such as recyclability, healability, adhesion, and nondestructive detection of cross-linking and mechanical properties.

Multiple-responsive shape memory polyacrylonitrile/graphene nanocomposites with rapid self-healing and recycling properties

A Diels–Alder covalent bond based crosslinked polyacrylonitrile/graphene nanocomposite is reported, which has the multiple-responsive properties of shape memory, self-healing, and reprocessing in addition to enhanced mechanical property.