Intrinsic self-healing polymeric materials are substances that relieve external stress and restore their original mechanical properties after extreme damage via dynamic covalent bonding in the polymeric structure or the reversible association of supramolecular motifs. Boronic ester-based dynamic covalent bonds formed between boronic acids and diols impart excellent self-healing properties to the hydrogels, organic gels, elastomers, and plastics depending on the characteristics of the corresponding polymer. In addition, these bonds induce internal reorganization in the chemical/physical structure in response to changes in the biological signals and biomaterials, such as hydrophilicity, pH, and the presence of glucose. This multi-responsiveness to stimuli as well as the self-healing, injectability, and biocompatibility of boronic ester-based polymers have led to several technological achievements with applicability in biomedical fields, including drug delivery, medical adhesion, bioimplants, and healthcare monitoring. This review provides an overview of various self-healing polymeric materials based on reversible B–O bonds, with a focus on boronic/boronate esters, borax, and benzoxaborole and their properties, responses to external stimuli, and applications.

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Recent progress in self-healing polymers and hydrogels based on reversible dynamic B–O bonds: boronic/boronate esters, borax, and benzoxaborole

As an important part of artificial intelligence, electronic skin has received more and more attention recently. However, two serious issues, slow self-healing and lack of direction recognition, have limited the burgeoning of electronic skin largely. Herein, for the first time we report a dual network flexible hydrogel, which was synthesized via cross-linking polyvinyl alcohol (PVA) and polyethylenimine (PEI) with 4-formylbenzoboric acid (Bn) to form a polymer network and then incorporating MXene into the polymer network. Due to the synergy of multiple reversible dynamic covalent bonds and supramolecular interactions, the PVA/Bn/PEI/MXene (PBPM) hydrogel exhibits direction-aware and ultrafast self-healing abilities (self-healing time ∼0.06 s) as well as rapid response performance (signal response time ∼0.12 s). Furthermore, an electronic skin strain sensor assembled by using the PBPM hydrogel can not only efficiently detect the movements in different parts of the prosthetic person body but also specifically identify the directions of the movements including head-down/up and wrist-down/up. The flexible PBPM hydrogel in this work has shown great potential in the applications of artificial skin, soft robots, health monitoring and human-machine exchange interfaces.

A direction-aware and ultrafast self-healing dual network hydrogel for a flexible electronic skin strain sensor

Polyurethane or polyurea elastomers with superb mechanical strength and toughness, good self-recoverability and healable characteristics are of key significance for practical applications. However, some mutually exclusive conflicts among these properties make it challenging to optimize them simultaneously. Herein, we report a facile strategy to fabricate a colorless healable poly(urethane–urea) elastomer with the highest reported mechanical toughness and recoverable energy dissipation capability (503.3 MJ m−3 and 37.3 MJ m−3 recovered after 7× stretching). These results were achieved via implanting a large number of irregularly arranged urea H-bonds into units of hard domains of weak and soft, self-healing polymer, which led to a dramatic increase in the Young's modulus, tensile strength, toughness, and fracture energy, while maintaining dynamic adaptiveness and responsiveness. Similar to other external stimuli, such as heat, light, or electricity, etc., trace solvent is capable of dissociating noncovalent crosslinks, promoting the mobility of polymer chains surrounding the fracture surface, and thus endowing the elastomer with healability. Impressively, this elastomer possessed outstanding impact-resistance and energy-absorbing ability, even under relatively high temperature. Moreover, it recovered this functionality even after severe deformation or accidental mechanical damage.

Molecular engineering of a colorless, extremely tough, superiorly self-recoverable, and healable poly(urethane-urea) elastomer for impact-resistant applications.

Lightweight polymeric materials are highly attractive platforms for many potential industrial applications in aerospace, soft robots, and biological engineering fields. For these real-world applications, it is vital for them to exhibit a desirable combination of great toughness, large ductility, and high strength together with desired healability and biocompatibility. However, existing material design strategies usually fail to achieve such a performance portfolio owing to their different and even mutually exclusive governing mechanisms. To overcome these hurdles, herein, for the first time a dynamic hydrogen-bonded nanoconfinement concept is proposed, and the design of highly stretchable and supratough biocompatible poly(vinyl alcohol) (PVA) with well-dispersed dynamic nanoconfinement phases induced by hydrogen-bond (H-bond) crosslinking is demonstrated. Because of H-bond crosslinking and dynamic nanoconfinement, the as-prepared PVA nanocomposite film exhibits a world-record toughness of 425 ± 31 MJ m-3 in combination with a tensile strength of 98 MPa and a large break strain of 550%, representing the best of its kind and even outperforming most natural and artificial materials. In addition, the final polymer exhibits a good self-healing ability and biocompatibility. This work affords new opportunities for creating mechanically robust, healable, and biocompatible polymeric materials, which hold great promise for applications, such as soft robots and artificial ligaments.

Dynamic Nanoconfinement Enabled Highly Stretchable and Supratough Polymeric Materials with Desirable Healability and Biocompatibility.

Self-healing soft electronic material composition is crucial to sustain the device long-term durability. The fabrication of self-healing soft electronics exposed to high moisture environment is a significant challenge that has yet to be fully achieved. This paper presents the novel concept of a water-assisted room-temperature autonomous self-healing mechanism based on synergistically dynamic covalent Schiff-based imine bonds with hydrogen bonds. The supramolecular water-assisted self-healing polymer (WASHP) films possess rapid self-healing kinetic behavior and high stretchability due to a reversible dissociation-association process. In comparison with the pristine room-temperature self-healing polymer, the WASHP demonstrates favorable mechanical performance at room temperature and a short self-healing time of 1 h; furthermore, it achieves a tensile strain of 9050%, self-healing efficiency of 95%, and toughness of 144.2 MJ m-3 . As a proof of concept, a versatile WASHP-based light-emitting touch-responsive device (WASHP-LETD) and perovskite quantum dot (PeQD)-based white LED backlight are designed. The WASHP-LETD has favorable mechanical deformation performance under pressure, bending, and strain, whereas the WASHP-PeQDs exhibit outstanding long-term stability even over a period exceeding one year in a boiling water environment. This paper provides a mechanically robust approach for producing eco-friendly, economical, and waterproof e-skin device components.

Super Tough and Spontaneous Water-Assisted Autonomous Self-Healing Elastomer for Underwater Wearable Electronics.

The synthesis of polymeric materials that simultaneously possess multiple excellent mechanical properties and high-efficient self-healability at room temperature is always a huge challenge. Here, we report the synthesis of a transparent polyurea material that can self-heal at room temperature with the aid of water and, meanwhile, has multiple remarkable mechanical performances, including super-high strength, excellent toughness, and large stretchability. Thanks to the synergistic enhancement of both dynamic imine bonds and hierarchical hydrogen bonds within the networks, the resulting polyureas have a world-record tensile strength of 41.2 MPa when compared with other polyurethanes that can self-heal at room temperature and, at the same time, a large breaking strain of 823.0% and a superior toughness of 127.2 MJ/m3. Besides the influence of imine bonds, the mechanical properties of the polyureas are also strongly related to the density and strength of the hierarchical hydrogen bonds within the polyurea networks, and these two factors could be finely controlled by adjusting the mass ratio of the soft segments with different chain lengths and the types of diisocyanates used for polyurea synthesis, respectively. More importantly, the highly dynamic characteristic of both imine bonds and hierarchical hydrogen bonds within the polyureas endows the materials with repeated water-enabled room-temperature self-healing capacity with a high healing efficiency of 92.2%. Moreover, the polyureas can also be recycled or remolded under mild conditions by the hot-pressing or dissolution/casting process. The synthesized polyureas also show great potential in damping applications with a loss factor larger than 0.3 over the temperature range from 12 to 75 °C. It is believed that polyureas with super-high and well-tunable mechanical properties and high-efficient room-temperature self-healing ability have great potential to substitute traditional irreparable polymers in diverse practical applications.

Water-Enabled Room-Temperature Self-Healing and Recyclable Polyurea Materials with Super-Strong Strength, Toughness, and Large Stretchability.

Review of: "Strength criterion of rock mass considering the damage and effect of joint dip angle"

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Water-Enabled Room-Temperature Self-Healing and Recyclable Polyurea Materials with Super-Strong Strength, Toughness, and Large Stretchability.

Review of: "Strength criterion of rock mass considering the damage and effect of joint dip angle"