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.

     

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.

     

An Amino‐Acid‐Based Self‐Healing Hydrogel: Modulation of the Self‐Healing Properties by Incorporating Carbon‐Based Nanomaterials

An amino‐acid‐based (11‐(4‐(pyrene‐1‐yl)butanamido)undecanoic acid) self‐repairing hydrogel is reported. The native hydrogel, as well as hybrid hydrogels, have been thoroughly characterized by using various microscopic techniques, including transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, fluorescence spectroscopy, FTIR spectroscopy, X‐ray diffraction, and by using rheological experiments. The native hydrogel exhibited interesting fluorescence properties, as well as a self‐healing property. Interestingly, the self‐healing, thixotropy, and stiffness of the native hydrogel can be successfully modulated by incorporating carbon‐based nanomaterials, including graphene, pristine single‐walled carbon nanotubes (Pr‐SWCNTs), and both graphene and Pr‐SWCNTs, within the native gel system. The self‐recovery time of the gel was shortened by the inclusion of reduced graphene oxide (RGO), Pr‐SWCNTs, or both RGO and Pr‐SWCNTs. Moreover, hybrid gels that contained RGO and/or Pr‐SWCNTs exhibited interesting semiconducting behavior.     

Biocatalytic Self‐Assembly of Supramolecular Charge‐Transfer Nanostructures Based on n‐Type Semiconductor‐Appended Peptides

The reversible in situ formation of a self‐assembly building block (naphthalenediimide (NDI)–dipeptide conjugate) by enzymatic condensation of NDI‐functionalized tyrosine ( NDI‐Y ) and phenylalanine‐amide ( F‐NH₂ ) to form NDI‐YF‐NH₂ is described. This coupled biocatalytic condensation/assembly approach is thermodynamically driven and gives rise to nanostructures with optimized supramolecular interactions as evidenced by substantial aggregation induced emission upon assembly. Furthermore, in the presence of di‐hydroxy/alkoxy naphthalene donors, efficient charge‐transfer complexes are produced. The dynamic formation of NDI‐YF‐NH₂ and electronic and H‐bonding interactions are analyzed and characterized by different methods. Microscopy (TEM and AFM) and rheology are used to characterize the formed nanostructures. Dynamic nanostructures, whose formation and function are driven by free‐energy minimization, are inherently self‐healing and provide opportunities for the development of aqueous adaptive nanotechnology.     

Small‐Peptide‐Based Organogel Kit: Towards the Development of Multicomponent Self‐Sorting Organogels

The results presented here highlight the extremely useful nature of ultra‐short peptides as building blocks in the development of smart multicomponent supramolecular devices. A facile bottom‐up strategy for the synthesis of a small library of stimuli‐responsive smart organogelators has been proposed based on the predictive self‐assembly of ultra‐short peptides. More importantly, the narcissistic self‐sorting of the gelators has been evaluated as a simple method for the efficient co‐assembly of a donor–acceptor dual‐component gel, allowing the investigation of possible future applications of similar systems in the development of a supramolecular photo‐conversion device. Interestingly, it was observed that the self‐organization of the components can lead to highly ordered systems in which discrimination between compatible and non‐compatible building blocks directs the effective organization of the chromophores and gives rise to the formation of an excited‐state complex with exciplex‐like emission. The current report may prove important in the development of organogel‐based multicomponent smart devices.     

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.     

pH‐Responsive Self‐Assembly by Molecular Recognition on a Macroscopic Scale

Macroscopic pH‐responsive self‐assembly is successfully constructed by polyacrylamide(pAAm)‐based gels carrying dansyl (Dns) and β‐cyclodextrin (βCD) residues, which are represented as Dns‐gel and βCD‐gel, respectively. Dns‐gel and βCD‐gel assemble together at pH ≥ 4.0, but disassemble at pH ≤ 3.0. The adhesion strengths for pairs of Dns‐gel/βCD‐gel increase with increasing pH. The fluorescence study on the model system of pAAm modified with 1 mol% Dns moieties (pAAm/Dns) reveals that Dns residues are protonated at a lower pH, which results in the reduction in binding constant (K) for Dns residues and βCD.

     

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.     

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.     

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.

     

Boronic Acids in Molecular Self‐Assembly

The reversibility of boronic acid and diol interaction makes it an ideal candidate for the design of self‐assembled molecular structures. Reversibility is required to ensure that the thermodynamically most stable structure is formed. Reversibility also ensures that any errors produced during the assembly process are not permanent.     

Gelation‐Induced Enhanced Fluorescence Emission from Organogels of Salicylanilide‐Containing Compounds Exhibiting Excited‐State Intramolecular Proton Transfer: Synthesis and Self‐Assembly

Self‐assembly structure, stability, hydrogen‐bonding interaction, and optical properties of a new class of low molecular weight organogelators (LMOGs) formed by salicylanilides 3 and 4 have been investigated by field‐emission scanning electron microscopy (FESEM), X‐ray diffraction (XRD), UV/Vis absorption and photoluminescence, as well as theoretical studies by DFT and semiempirical calculations with CI (AM1/PECI=8) methods. It was found that salicylanilides form gels in nonpolar solvents due to π‐stacking interaction complemented by the presence of both inter‐ and intramolecular hydrogen bonding. The supramolecular arrangement in these organogels predicted by XRD shows lamellar and hexagonal columnar structures for gelators 3 and 4 , respectively. Of particular interest is the observation of significant fluorescence enhancement accompanying gelation, which was ascribed to the formation of J‐aggregates and inhibition of intramolecular rotation in the gel state.     

Fast Self‐Healing of Polyelectrolyte Multilayer Nanocoating and Restoration of Super Oxygen Barrier

A self‐healable gas barrier nanocoating, which is fabricated by alternate deposition of polyethyleneimine (PEI) and polyacrylic acid (PAA) polyelectrolytes, is demonstrated in this study. This multilayer film, with high elastic modulus, high glass transition temperature, and small free volume, has been shown to be a super oxygen gas barrier. An 8‐bilayer PEI/PAA multilayer assembly (≈700 nm thick) exhibits an oxygen transmission rate (OTR) undetectable to commercial instrumentation (<0.005 cc (m⁻² d⁻¹ atm⁻¹)). The barrier property of PEI/PAA nanocoating is lost after a moderate amount of stretching due to its rigidity, which is then completely restored after high humidity exposure, therefore achieving a healing efficiency of 100%. The OTR of the multilayer nanocoating remains below the detection limit after ten stretching‐healing cycles, which proves this healing process to be highly robust. The high oxygen barrier and self‐healing behavior of this polymer multilayer nanocoating makes it ideal for packaging (food, electronics, and pharmaceutical) and gas separation applications.

     

Charge‐Transfer Complex Formation in Gelation: The Role of Solvent Molecules with Different Electron‐Donating Capacities

A naphthalenediimide (NDI)‐based synthetic peptide molecule forms gels in a particular solvent mixture (chloroform/aromatic hydrocarbon, 4:1) through charge‐transfer (CT) complex formation; this is evident from the corresponding absorbance and fluorescence spectra at room temperature. Various aromatic hydrocarbon based solvents, including benzene, toluene, xylene (ortho, meta and para) and mesitylene, have been used for the formation of the CT complex. The role of different solvent molecules with varying electron‐donation capacities in the formation of CT complexes has been established through spectroscopic and computational studies.     

Multicolour Self‐Assembled Fluorene Co‐Oligomers: From Molecules to the Solid State via White‐Light‐Emitting Organogels

Five fluorene‐based co‐oligomers have been prepared to study their self‐assembly in a wide range of concentrations, from dilute solutions to the solid state. Subtle changes to the chemical structures, introduced to tune the emission colours over the entire visible range, induce strong differences in aggregation behaviour. Only two of the fluorescent co‐oligomer derivatives self‐assemble to form soluble fibrils from which fluorescent organogels emerge at higher concentrations. In contrast, the other compounds form precipitates. Mixed fluorescent co‐oligomer systems exhibit partial energy transfer, which allows the creation of white‐light‐emitting gels. Finally, a mechanism for the hierarchical self‐assembly of this class of materials is proposed based on experimental results and molecular modelling calculations.     

Tetrapeptidic Molecular Hydrogels: Self‐assembly and Co‐aggregation with Amyloid Fragment Aβ1‐40

A new family of isomeric tetrapeptides containing aromatic and polar amino acid residues that are able to form molecular hydrogels following a smooth change in pH is described. The hydrogels have been studied by spectroscopic and microscopic techniques showing that the peptide primary sequence has an enormous influence on the self‐assembly process. In particular, the formation of extended hydrophobic regions and the appearance of π‐stacking interactions have been revealed as the driving forces for aggregation. Moreover, the interaction of these compounds with amyloid peptidic fragment Aβ1‐40 has been studied and some of them have been shown to act as templates for the aggregation of this peptide into non‐β‐sheet fibrillar structures. These compounds could potentially be used for the capture of toxic, soluble amyloid oligomers.