Self‐Healing Colloidal Crystals

Healing hands : A complex interplay between colloidal and polymeric energetics in microgel self‐assembly behavior results in soft colloidal assemblies with self‐healing properties. Repulsive soft spheres can adopt highly compressed conformations in colloidal crystalline lattices without directly contacting the nearest neighbors (see picture). This distant action is directly responsible for the self‐healing of the assemblies.

     

An Injectable Self‐Healing Hydrogel Based on Chain‐Extended PEO‐PPO‐PEO Multiblock Copolymer

Injectable hydrogels have been commonly used as drug‐delivery vehicles and tried in tissue engineering. Injectable self‐healing hydrogels have great advantage over traditional injectable hydrogels because they can be injected as a liquid and then rapidly form bulk gels in situ at the target site under physiological conditions. This study develops an injectable thermosensitive self‐healing hydrogel based on chain‐extended F127 (PEO₉₀‐PPO₆₅‐PEO₉₀) multi‐block copolymer (m‐F127). The rapid sol–gel transition ability under body temperature allows it to be used as injectable hydrogel and the self‐healing property allows it to withstand repeated deformation and quickly recover its mechanical properties and structure through the dynamic covalent bonds. It is hoped that the novel strategy and the fascinating properties of the hydrogel as presented here will provide new opportunities with regard to the design and practical application of injectable self‐healing hydrogels.

     

A Multiple‐Responsive Self‐Healing Supramolecular Polymer Gel Network Based on Multiple Orthogonal Interactions

Supramolecular polymer networks have attracted considerable attention not only due to their topological importance but also because they can show some fantastic properties such as stimuli‐responsiveness and self‐healing. Although various supramolecular networks are constructed by supramolecular chemists based on different non‐covalent interactions, supramolecular polymer networks based on multiple orthogonal interactions are still rare. Here, a supramolecular polymer network is presented on the basis of the host–guest interactions between dibenzo‐24‐crown‐8 (DB24C8) and dibenzylammonium salts (DBAS), the metal–ligand coordination interactions between terpyridine and Zn(OTf)₂, and between 1,2,3‐triazole and PdCl₂(PhCN)₂. The topology of the networks can be easily tuned from monomer to main‐chain supramolecular polymer and then to the supramolecular networks. This process is well studied by various characterization methods such as ¹H NMR, UV–vis, DOSY, viscosity, and rheological measurements. More importantly, a supramolecular gel is obtained at high concentrations of the supramolecular networks, which demonstrates both stimuli‐responsiveness and self‐healing properties.

     

Light‐Switchable Self‐Healing Hydrogel Based on Host–Guest Macro‐Crosslinking

A self‐healing hydrogel is prepared by crosslinking acrylamide with a host–guest macro‐crosslinker assembled from poly(β‐cyclodextrin) nanogel and azobenzeneacrylamide. The photoisomerizable azobenzene moiety can change its binding affinity with β‐cyclodextrin, therefore the crosslinking density and rheology property of the hydrogel can be tuned with light stimulus. The hydrogel can repair its wound autonomously through the dynamic host–guest interaction. In addition, the wounded hydrogel will lose its ability of self‐healing when exposed to ultraviolet light, and the self‐healing behavior can be recovered upon the irradiation of visible light. The utilizing of host–guest macro‐crosslinking approach manifests the as‐prepared hydrogel reversible and light‐switchable self‐healing property, which would broaden the potential applications of self‐healing polymers.

     

An “Ingredients” Approach to Functional Self‐Synthesizing Materials: A Metal‐Ion‐Selective,Multi‐Responsive,Self‐Assembled Hydrogel

New methodology for making novel materials is highly desirable. Here, an “ingredients” approach to functional self‐assembled hydrogels was developed. By designing a building block to contain the right ingredients, a multi‐responsive, self‐assembled hydrogel was obtained through a process of template‐induced self‐synthesis in a dynamic combinatorial library. The system can be switched between gel and solution by light, redox reactions, pH, temperature, mechanical energy and sequestration or addition of Mg^II salt.     

Amino‐Acid‐Based Block Copolymers by RAFT Polymerization

This review summarizes recent advances in the design and synthesis of amino‐acid‐based block copolymers by reversible addition–fragmentation chain transfer (RAFT) polymerization of amino‐acid‐bearing monomers. We will mainly focus on stimuli‐responsive block copolymers, such as pH‐, thermo‐, and dual‐stimuli‐responsive block copolymers, and self‐assembled block copolymers, including amphiphilic and double‐hydrophilic block copolymers having tunable chiroptical properties. We will also highlight recent results in RAFT synthesis of amino‐acid‐based copolymers having various properties, such as catalytic and optoelectronic properties, cross‐linked block copolymer micelles, unimolecular micelles, and organic–inorganic hybrids.     

Self‐Healing,Expansion–Contraction,and Shape‐Memory Properties of a Preorganized Supramolecular Hydrogel through Host–Guest Interactions

Supramolecular materials cross‐linked between polymer chains by noncovalent bonds have the potential to provide dynamic functions that are not produced by covalently cross‐linked polymeric materials. We focused on the formation of supramolecular polymeric materials through host–guest interactions: a powerful method for the creation of nonconventional materials. We employed two different kinds of host–guest inclusion complexes of β‐cyclodextrin (βCD) with adamantane (Ad) and ferrocene (Fc) to bind polymers together to form a supramolecular hydrogel (βCD‐Ad‐Fc gel). The βCD‐Ad‐Fc gel showed self‐healing ability when damaged and responded to redox stimuli by expansion or contraction. Moreover, the βCD‐Ad‐Fc gel showed a redox‐responsive shape‐morphing effect. We thus succeeded in deriving three functions from the introduction of two kinds of functional units into a supramolecular material.     

Fluorescence Detection of Microcapsule‐Type Self‐Healing,Based on Aggregation‐Induced Emission

An extrinsic self‐healing coating system containing tetraphenylethylene (TPE) in microcapsules was monitored by measuring aggregation‐induced emission (AIE). The core healing agent comprised of methacryloxypropyl‐terminated polydimethylsiloxane, styrene, benzoin isobutyl ether, and TPE was encapsulated in a urea‐formaldehyde shell. The photoluminescence of the healing agent in the microcapsules was measured that the blue emission intensity dramatically increased and the storage modulus also increased up to 10⁵ Pa after the photocuring. These results suggested that this formulation might be useful as a self‐healing material and as an indicator of the self‐healing process due to the dramatic change in fluorescence during photocuring. To examine the ability of the healing agent to repair damage to a coating, a self‐healing coating containing embedded microcapsules was scribed with a razor. As the healing process proceeded, blue light fluorescence emission was observed at the scribed regions. This observation suggested that self‐healing could be monitored using the AIE fluorescence.

     

Biomimetic Self‐Healing

Self‐healing is a natural process common to all living organisms which provides increased longevity and the ability to adapt to changes in the environment. Inspired by this fitness‐enhancing functionality, which was tuned by billions of years of evolution, scientists and engineers have been incorporating self‐healing capabilities into synthetic materials. By mimicking mechanically triggered chemistry as well as the storage and delivery of liquid reagents, new materials have been developed with extended longevity that are capable of restoring mechanical integrity and additional functions after being damaged. This Review describes the fundamental steps in this new field of science, which combines chemistry, physics, materials science, and mechanical engineering.     

Chemistry of Crosslinking Processes for Self‐Healing Polymers

Recent developments in material design have seen an exponential increase of polymers and polymer composites that can repair themselves in response to damage. In this review, a distinction is made between extrinsic materials, where the self‐healing property is obtained by adding healing agents to the material to be repaired, and intrinsic materials, where self‐healing is achieved by the material itself through its chemical nature. An overview of the crosslinking chemistries used in self‐healing materials will be given, discussing the advantages and drawbacks of each system. The review is not only aiming to enable researchers to compare their ongoing research with the state‐of‐the‐art but also to serve as a guide for the newcomers, which allows for a selection of the most promising self‐healing chemistries.     

Proton‐Coupled Self‐Assembly of a Porphyrin‐Naphthalenediimide Dyad

The construction of an n–p heterojunction through the self‐assembly of a dyad based on tetraphenylporphyrin (TPP) and 1,4,5,8‐naphthalenedimide (NDI) ( 1 ) is described. Proton transfer from the lysine head group of 1 to the porphyrin ring occurs concomitantly with self‐assembly into 1D nanorods in CHCl₃. TEM and AFM studies showed that the nanorods are formed by the lateral and vertical fusion of multilameller vesicles into networks and hollow ribbons, respectively. These intermediate structures transitioned to nanorods over the course of 4–6 days. Time‐resolved spectroscopy revealed that photoinduced charge separation occurs with rate constants that depend on the nature of the aggregation.     

Remarkable Shape‐Sustaining,Load‐Bearing,and Self‐Healing Properties Displayed by a Supramolecular Gel Derived from a Bis‐pyridyl‐bis‐amide of L‐Phenyl Alanine

A series of bis‐amides decorated with pyridyl and phenyl moieties derived from L ‐amino acids having an innocent side chain (L ‐alanine and L ‐phenyl alanine) were synthesized as potential low‐molecular‐weight gelators (LMWGs). Both protic and aprotic solvents were found to be gelled by most of the bis‐amides with moderate to excellent gelation efficiency (minimum gelator concentration=0.32–4.0 wt. % and gel–sol dissociation temperature T_gel=52–110 °C). The gels were characterized by rheology, DSC, SEM, TEM, and temperature‐variable ¹H NMR measurements. pH‐dependent gelation studies revealed that the pyridyl moieties took part in gelation. Structure–property correlation was attempted using single‐crystal X‐ray and powder X‐ray diffraction data. Remarkably, one of the bis‐pyridyl bis‐amide gelators, namely 3,3‐Phe (3‐pyridyl bis‐amide of L ‐phenylalanine) displayed outstanding shape‐sustaining, load‐bearing, and self‐healing properties.     

A Rapidly Self‐Healing Supramolecular Polymer Hydrogel with Photostimulated Room‐Temperature Phosphorescence Responsiveness

Development of self‐healing and photostimulated luminescent supramolecular polymeric materials is important for artificial soft materials. A supramolecular polymeric hydrogel is reported based on the host–guest recognition between a β‐cyclodextrin (β‐CD) host polymer (poly‐β‐CD) and an α‐bromonaphthalene (α‐BrNp) polymer (poly‐BrNp) without any additional gelator, which can self‐heal within only about one minute under ambient atmosphere without any additive. This supramolecular polymer system can be excited to engender room‐temperature phosphorescence (RTP) signals based on the fact that the inclusion of β‐CD macrocycle with α‐BrNp moiety is able to induce RTP emission (CD‐RTP). The RTP signal can be adjusted reversibly by competitive complexation of β‐CD with azobenzene moiety under specific irradiation by introducing another azobenzene guest polymer (poly‐Azo).     

Dextran‐Based Self‐Healing Hydrogels Formed by Reversible Diels–Alder Reaction under Physiological Conditions

A dextran‐based self‐healing hydrogel is prepared by reversible Diels–Alder reaction under physiological conditions. Cytocompatible fulvene‐modified dextran as main polymer chains and dichloromaleic‐acid‐modified poly(ethylene glycol) as cross‐linkers are used. Both macro‐ and microscopic observation as well as the rheological recovery test confirm the self‐healing property of the dextran‐l‐poly(ethylene glycol) hydrogels (“l” means “linked‐by”). In addition, scanning electrochemical microscopy is used to qualitatively and quantitatively in situ track the self‐healing process of the hydrogel for the first time. It is found that the longitudinal depth of scratch on hydrogel surface almost completely healed at 37 °C after 7 h. This work represents a facile approach for fabrication of polysaccharide self‐healing hydrogel, which can be potentially used in several biomedical fields.

     

Conductive Elastomers with Autonomic Self‐Healing Properties

Healable, electrically conductive materials are highly desirable and valuable for the development of various modern electronics. But the preparation of a material combining good mechanical elasticity, functional properties, and intrinsic self‐healing ability remains a great challenge. Here, we design composites by connecting a polymer network and single‐walled carbon nanotubes (SWCNTs) through host–guest interactions. The resulting materials show bulk electrical conductivity, proximity sensitivity, humidity sensitivity and are able to self‐heal without external stimulus under ambient conditions rapidly. Furthermore, they also possess elasticity comparable to commercial rubbers.