Light triggered self-healing of polyacrylate polymers crosslinked with 7-methacryloyoxycoumarin crosslinker

Crosslinked acrylate polymers with coumarin crosslinker displayed a light (UV) only triggered self-healing property.

Self-healing polymeric materials for membrane separation: an example of a polybenzimidazole-based membrane for pervaporation dehydration on isopropanol aqueous solution

The demonstration of a self-healing material based separation membrane for pervaporation dehydration on liquid–liquid mixtures.

Rheology of nitrile rubber with hybrid crosslinked network composed of covalent bonding and hydrogen bonding

A hybrid crosslinked network composed of covalent bonding and non-covalent bonding was constructed in nitrile rubber (NBR) by using a compound crosslinking agents dicumyl peroxide (DCP) and N,N-methylenebis acrylamide (MBA).

Self-Healing Hydrogels

Over the past few years, there has been a great deal of interest in the development of hydrogel materials with tunable structural, mechanical, and rheological properties, which exhibit rapid and autonomous self-healing and self-recovery for utilization in a broad range of applications, from soft robotics to tissue engineering. However, self-healing hydrogels generally either possess mechanically robust or rapid self-healing properties but not both. Hence, the development of a mechanically robust hydrogel material with autonomous self-healing on the time scale of seconds is yet to be fully realized. Here, the current advances in the development of autonomous self-healing hydrogels are reviewed. Specifically, methods to test self-healing efficiencies and recoveries, mechanisms of autonomous self-healing, and mechanically robust hydrogels are presented. The trends indicate that hydrogels that self-heal better also achieve self-healing faster, as compared to gels that only partially self-heal. Recommendations to guide future development of self-healing hydrogels are offered and the potential relevance of self-healing hydrogels to the exciting research areas of 3D/4D printing, soft robotics, and assisted health technologies is highlighted.

Multifunctional Energy Storage and Conversion Devices

Multifunctional energy storage and conversion devices that incorporate novel features and functions in intelligent and interactive modes, represent a radical advance in consumer products, such as wearable electronics, healthcare devices, artificial intelligence, electric vehicles, smart household, and space satellites, etc. Here, smart energy devices are defined to be energy devices that are responsive to changes in configurational integrity, voltage, mechanical deformation, light, and temperature, called self-healability, electrochromism, shape memory, photodetection, and thermal responsivity. Advisable materials, device designs, and performances are crucial for the development of energy electronics endowed with these smart functions. Integrating these smart functions in energy storage and conversion devices gives rise to great challenges from the viewpoint of both understanding the fundamental mechanisms and practical implementation. Current state-of-art examples of these smart multifunctional energy devices, pertinent to materials, fabrication strategies, and performances, are highlighted. In addition, current challenges and potential solutions from materials synthesis to device performances are discussed. Finally, some important directions in this fast developing field are considered to further expand their application.

Design Paradigm Utilizing Reversible Diels-Alder Reactions to Enhance the Mechanical Properties of 3D Printed Materials

A design paradigm is demonstrated that enables new functional 3D printed materials made by fused filament fabrication (FFF) utilizing a thermally reversible dynamic covalent Diels-Alder reaction to dramatically improve both strength and toughness via self-healing mechanisms. To achieve this, we used as a mending agent a partially cross-linked terpolymer consisting of furan-maleimide Diels-Alder (fmDA) adducts that exhibit reversibility at temperatures typically used for FFF printing. When this mending agent is blended with commercially available polylactic acid (PLA) and printed, the resulting materials demonstrate an increase in the interfilament adhesion strength along the z-axis of up to 130%, with ultimate tensile strength increasing from 10 MPa in neat PLA to 24 MPa in fmDA-enhanced PLA. Toughness in the z-axis aligned prints increases by up to 460% from 0.05 MJ/m(3) for unmodified PLA to 0.28 MJ/m(3) for the remendable PLA. Importantly, it is demonstrated that a thermally reversible cross-linking paradigm based on the furan-maleimide Diels-Alder (fmDA) reaction can be more broadly applied to engineer property enhancements and remending abilities to a host of other 3D printable materials with superior mechanical properties.

An adhesive elastomeric supramolecular polyurethane healable at body temperature

In this paper, we report the synthesis and healing ability of a non-cytotoxic supramolecular polyurethane network whose mechanical properties can be recovered efficiently (>99%) at the temperature of the human body (37 °C). Rheological analysis revealed an acceleration in the drop of the storage modulus above 37 °C, on account of the dissociation of the supramolecular polyurethane network, and this decrease in viscosity enables the efficient recovery of the mechanical properties. Microscopic and mechanical characterisation has shown that this material is able to recover mechanical properties across a damage site with minimal contact required between the interfaces and also demonstrated that the mechanical properties improved when compared to other low temperature healing elastomers or gel-like materials. The supramolecular polyurethane was found to be non-toxic in a cytotoxicity assay carried out in human skin fibroblasts (cell viability > 94% and non-significantly different compared to the untreated control). This supramolecular network material also exhibited excellent adhesion to pig skin and could be healed completely in situ post damage indicating that biomedical applications could be targeted, such as artificial skin or wound dressings with supramolecular materials of this type.

Thermally Conductive-Silicone Composites with Thermally Reversible Cross-links

Thermally conductive-silicone composites that contain thermally reversible cross-links were prepared by blending diene- and dienophile-functionalized polydimethylsiloxane (PDMS) with an aluminum oxide conductive filler. This class of thermally conductive-silicones are useful as thermal interface materials (TIMs) within Information Technology (IT) hardware applications to allow rework of valuable components. The composites were rendered reworkable via retro Diels-Alder cross-links when temperatures were elevated above 130 °C and required little mechanical force to remove, making them advantageous over other TIM materials. Results show high thermal conductivity (0.4 W/m·K) at low filler loadings (45 wt %) compared to other TIM solutions (>45 wt %). Additionally, the adhesion of the material was found to be ∼7 times greater at lower temperatures (25 °C) and ∼2 times greater at higher temperatures (120 °C) than commercially available TIMs.

Spontaneously Healable Thermoplastic Elastomers Achieved through One-Pot Living Ring-Opening Metathesis Copolymerization of Well-Designed Bulky Monomers

We report here a series of novel spontaneously healable thermoplastic elastomers (TPEs) with a combination of improved mechanical and good autonomic self-healing performances. Hard-soft diblock and hard-soft-hard triblock copolymers with poly[exo-1,4,4a,9,9a,10-hexahydro-9,10(1',2')-benzeno-l,4-methanoanthracene] (PHBM) as the hard block and secondary amide group containing norbornene derivative polymer as the soft block were synthesized via living ring-opening metathesis copolymerization by use of Grubbs third-generation catalyst through sequential monomer addition. The microstructure, mechanical, self-healing, and surface morphologies of the block copolymers were thoroughly studied. Both excellent mechanical performance and self-healing capability were achieved for the block copolymers because of the interplayed physical cross-link of hard block and dynamic interaction formed by soft block in the self-assembled network. Under an optimized hard block (PHBM) weight ratio of 5%, a significant recovery of tensile strength (up to 100%) and strain at break (ca. 85%) was achieved at ambient temperature without any treatment even after complete rupture. Moreover, the simple reaction operations and well-designed monomers offer versatility in tuning the architectures and properties of the resulting block copolymers.

Microcapsule-Type Organogel-Based Self-Healing System Having Secondary Damage Preventing Capability

We have developed a novel microcapsule-type organogel-based self-healing system in which secondary damage does not occur in the healed region. A mixture of an organogelator, poor and good solvents for the gelator is used as the healing agent; when the good solvent evaporates from this agent, a viscoelastic organogel forms. The healing agent is microencapsulated with urea-formaldehyde polymer, and the resultant microcapsules are integrated into a polymer coating to prepare self-healing coatings. When the coatings are scratched, they self-heal, as demonstrated by means of corrosion testing, electrochemical testing, optical microscopy, and scanning electron microscopy (SEM). After the healed coatings are subjected to vigorous vibration, it is demonstrated that no secondary damage occurs in the healed region. The secondary damage preventing capability of the self-healing coating is attributable to the viscoelasticity of the organogel. The result can give insight into the development of a "permanent" self-healing system.

Chelation-assisted CuAAC in star-shaped polymers enables fast self-healing at low temperatures

The achievement of self-healing (SH) under ambient conditions (low temperature, no external input of energy) still presents a significant area of research, and is enabledvia“click”-type crosslinking reactions.

A mild, efficient and catalyst-free thermoreversible ligation system based on dithiooxalates

We introduce dithiooxalates as efficient and catalyst-free thermoreversible hetero Diels–Alder linkers for applications in self-healing materials, organic sheets, mild ligation or complex architecture design.

Amphiphilic functional block copolymers bearing a reactive furfuryl group via RAFT polymerization; reversible core cross-linked micelles via a Diels–Alder “click reaction”

Amphiphilic BCPs, PFMA-b-PPEGMA were prepared via RAFT polymerization. They were self-assembled into micelles in aqueous medium with a hydrophobic PFMA core and hydrophilic PPEGMA corona. Core cross-linked micelles were prepared via the DA reaction.

A self-healing, re-moldable and biocompatible crosslinked polysiloxane elastomer

The self-healable polysiloxane elastomers cross-linked with DA bonds show high healing efficiency, good mechanical properties and good biocompatibility.

Activated anionic ring-opening polymerization for the synthesis of reversibly cross-linkable poly(propylene oxide) based on furan/maleimide chemistry

Controlled anionic copolymerization of propylene oxide and furfuryl glycidyl ether was developed for the synthesis of reversibly cross-linkable polyethers.

Design of new disulfide-based organic compounds for the improvement of self-healing materials

Self-healing materials are a very promising kind of materials due to their capacity to repair themselves. Among others, diphenyl disulfide-based compounds (Ph2S2) appear to be among the best candidates to develop materials with optimum self-healing properties. However, few is known regarding both the reaction mechanism and the electronic structure that make possible such properties. In this vein, theoretical approaches are of great interest. In this work, we have carried out theoretical calculations on a wide set of different disulfide compounds, both aromatic and aliphatic, in order to elucidate the prevalent reaction mechanism and the necessary electronic conditions needed for improved self-healing properties. Two competitive mechanisms were considered, namely, the metathesis and the radical-mediated mechanism. According to our calculations, the radical-mediated mechanism is the responsible for this process. The formation of sulfenyl radicals strongly depends on the S-S bond strength, which can be modulated chemically by the use of proper derivatives. At this point, amino derivatives appear to be the most promising ones. In addition to the S-S bond strength, hydrogen bonding between disulfide chains seems to be relevant to favour the contact among disulfide units. This is crucial for the reaction to take place. The calculated hydrogen bonding energies are of the same order of magnitude as the S-S bond energies. Finally, reaction barriers have been analysed for some promising candidates. Two reaction mechanisms were compared, namely, the [2+2] metathesis reaction mechanism and the [2+1] radical-mediated mechanism. No computational evidence for the existence of any transition state for the metathesis mechanism was found, which indicates that the radical-mediated mechanism is the one responsible in the self-healing process of these materials. Interestingly, the calculated reaction barriers are around 10 kcal mol(-1) regardless the substituent employed. All these results suggest that the radical formation and the structural role of the hydrogen bonding prevale over kinetics. Having this in mind, as a conclusion, some new compounds are proposed for the design of future self-healing materials with improved features.

Self-Propelled Nanomotors Autonomously Seek and Repair Cracks

Biological self-healing involves the autonomous localization of healing agents at the site of damage. Herein, we design and characterize a synthetic repair system where self-propelled nanomotors autonomously seek and localize at microscopic cracks and thus mimic salient features of biological wound healing. We demonstrate that these chemically powered catalytic nanomotors, composed of conductive Au/Pt spherical Janus particles, can autonomously detect and repair microscopic mechanical defects to restore the electrical conductivity of broken electronic pathways. This repair mechanism capitalizes on energetic wells and obstacles formed by surface cracks, which dramatically alter the nanomotor dynamics and trigger their localization at the defects. By developing models for self-propelled Janus nanomotors on a cracked surface, we simulate the systems' dynamics over a range of particle speeds and densities to verify the process by which the nanomotors autonomously localize and accumulate at the cracks. We take advantage of this localization to demonstrate that the nanomotors can form conductive "patches" to repair scratched electrodes and restore the conductive pathway. Such a nanomotor-based repair system represents an important step toward the realization of biomimetic nanosystems that can autonomously sense and respond to environmental changes, a development that potentially can be expanded to a wide range of applications, from self-healing electronics to targeted drug delivery.

A molecular dynamics study of bond exchange reactions in covalent adaptable networks

Covalent adaptable networks are polymers that can alter the arrangement of network connections by bond exchange reactions where an active unit attaches to an existing bond then kicks off its pre-existing peer to form a new bond. When the polymer is stretched, bond exchange reactions lead to stress relaxation and plastic deformation, or the so-called reforming. In addition, two pieces of polymers can be rejoined together without introducing additional monomers or chemicals on the interface, enabling welding and reprocessing. Although covalent adaptable networks have been researched extensively in the past, knowledge about the macromolecular level network alternations is limited. In this study, molecular dynamics simulations are used to investigate the macromolecular details of bond exchange reactions in a recently reported epoxy system. An algorithm for bond exchange reactions is first developed and applied to study a crosslinking network formed by epoxy resin DGEBA with the crosslinking agent tricarballylic acid. The trace of the active units is tracked to show the migration of these units within the network. Network properties, such as the distance between two neighboring crosslink sites, the chain angle, and the initial modulus, are examined after each iteration of the bond exchange reactions to provide detailed information about how material behaviors and macromolecular structure evolve. Stress relaxation simulations are also conducted. It is found that even though bond exchange reactions change the macroscopic shape of the network, microscopic network characteristic features, such as the distance between two neighboring crosslink sites and the chain angle, relax back to the unstretched isotropic state. Comparison with a recent scaling theory also shows good agreement.

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.