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Adhesives typically fall into two categories: those that have high but irreversible adhesion strength due to the formation of covalent bonds at the interface and are slow to deploy, and others that are fast to deploy and the adhesion is reversible but weak in strength due to formation of noncovalent bonds. Synergizing the advantages from both categories remains challenging but pivotal for the development of the next generation of wound dressing adhesives. Here, we report a fast and reversible adhesive consisting of dynamic boronic ester covalent bonds, formed between poly(vinyl alcohol) (PVA) and boric acid (BA) for potential use as a wound dressing adhesive. Mechanical testing shows that the adhesive film has strength in shear of 61 N/cm² and transcutaneous adhesive strength of 511 N/cm², generated within 2 min of application. Yet the film can be effortlessly debonded when exposed to excess water. The mechanical properties of PVA/BA adhesives are tunable by varying the cross-linking density. Within seconds of activation by water, the surface boronic ester bonds in the PVA/BA film undergo fast debonding and instant softening, leading to conformal contact with the adherends and reformation of the boronic ester bonds at the interface. Meanwhile, the bulk film remains dehydrated to offer efficient load transmission, which is important to achieve strong adhesion without delamination at the interface. Whether the substrate surface is smooth (e.g., glass) or rough (e.g., hairy mouse skin), PVA/BA adhesives demonstrate superior adhesion compared to the most widely used topical skin adhesive in clinical medicine, Dermabond.

Dynamic bonds can facilitate reversible formation and dissociation of connections in response to external stimuli, endowing materials with shape memory and self-healing capabilities. Temperature is an external stimulus that can be easily controlled through heat. Dynamic covalent bonds in response to temperature can reversibly connect, exchange, and convert chains in the polymer. In this review, we introduce dynamic covalent bonds that operate without catalysts in various temperature ranges. The basic bonding mechanism and the kinetics are examined to understand dynamic covalent chemistry reversibly performed by equilibrium control. Furthermore, a recent synthesis method that implements dynamic covalent coupling based on various polymers is introduced. Dynamic covalent bonds that operate depending on temperature can be applied and expand the use of polymers, providing predictions for the development of future smart materials.

A novel self‐healing poly(vinyl alcohol) (PVA)‐based hydrogel is developed by cross‐linking PVA chains through multi dynamic covalent bonds by use of a small cross‐linker composed by 4‐formylphenylboric acid (FPBA) and lysine (Lys). The dynamic borate‐imine‐imine‐borate bond structure between PVA chains endows the hydrogel excellent stretchability and ultra‐fast self‐healing ability without external stimulation. The self‐healing efficiency can attain 94% and the elongation at break can reach up to near 1000% after only 3 min healing. Moreover, the self‐healing of the hydrogel through the contact of two faces from both the same cut position and different cut positions has similar excellent efficiency. The hydrogel with the unusual self‐healing performance and stretchability is used as an ideal material in strain sensors monitoring human movement and tiny vibrations caused by human voice. Interestingly, the sensor can continue to function normally after self‐healing for only ≈3 s. It is expected that this simple strategy of fabricating self‐healing hydrogels with multi dynamic bonds will provide new opportunities in the design and preparation of PVA‐based hydrogels to expand their potential applications in sensors and other various fields. The PFL hydrogel based on multi dynamic covalent bond cross‐linking has ultra‐fast self‐healing ability and excellent stretchability, which can be demonstrated by the stress–strain curves and the balloon inflation test. Interestingly, the sensor used in a finger bending can continue to function normally after self‐healing for only ≈3 s.

Schiff base, an important family of reaction in click chemistry, has received significant attention in the formation of self-healing hydrogels in recent years. Schiff base reversibly reacts even in mild conditions, which allows hydrogels with self-healing ability to recover their structures and functions after damages. Moreover, pH-sensitivity of the Schiff base offers the hydrogels response to biologically relevant stimuli. Different types of Schiff base can provide the hydrogels with tunable mechanical properties and chemical stabilities. In this review, we summarized the design and preparation of hydrogels based on various types of Schiff base linkages, as well as the biomedical applications of hydrogels in drug delivery, tissue regeneration, wound healing, tissue adhesives, bioprinting, and biosensors.

Polyborosiloxane gel (PBS-gel) with shear hardening properties was prepared by cross-linking boric acid and hydroxyl-terminated polydimethylsiloxane through B–O–Si dynamic covalent bonding. The prepared PBS gel was mixed with methyl vinyl silicone rubber (MVQ), and a benzoyl peroxide (BPO) cross-linking agent was added to vulcanize the silicone rubber. At the same time, the gel molecules were co-vulcanizing with MVQ to produce molecular cross-linking. The effects of PBS-gel on the damping properties of silicone rubber were analyzed by dynamic rheological test, Fourier transform infrared spectroscopy and dynamic mechanical analysis. The results demonstrated that the damping performance of MVQ/PBS rubber is greatly improved and the rubber has a tanδ > 0.3 in the range of −25~125 °C. The shear-hardening gel is uniformly dispersed in the system, due to the combined action of covalent bonds and intermolecular forces, which act as an active molecular chain that can efficiently dissipate and transfer energy inside the silicone rubber.

Covalent organic frameworks (COFs) are newly emerged crystalline porous polymers with well-defined skeletons and nanopores mainly consisted of light-weight elements (H, B, C, N and O) linked by dynamic covalent bonds. Compared with conventional materials, COFs possess some unique and attractive features, such as large surface area, pre-designable pore geometry, excellent crystallinity, inherent adaptability and high flexibility in structural and functional design, thus exhibiting great potential for various applications. Especially, their large surface area and tunable porosity and π conjugation with unique photoelectric properties will enable COFs to serve as a promising platform for drug delivery, bioimaging, biosensing and theranostic applications. In this review, we trace the evolution of COFs in terms of linkages and highlight the important issues on synthetic method, structural design, morphological control and functionalization. And then we summarize the recent advances of COFs in the biomedical and pharmaceutical sectors and conclude with a discussion of the challenges and opportunities of COFs for biomedical purposes. Although currently still at its infancy stage, COFs as an innovative source have paved a new way to meet future challenges in human healthcare and disease theranostic.

The accumulation of discarded petroleum-based plastics causes serious environmental crises. Currently, recyclable polymers with neutrality in thermodynamics, such as polyesters, polycarbonates, and polyolefins, have been developed as promising alternatives to traditional petroleum-based polymers. However, the chemical recycle of these polymers usually requires high energy input and expensive catalysts. Dynamic covalent bonds, such as thioester and disulfide bonds, have emerged as building blocks for constructing recyclable polymers that can be rapidly degraded/recycled under mild conditions. In this review, we introduce representative studies on recyclable polythioesters and polydisulfides with respect to their synthetic strategies, thermodynamic manipulation, physicochemical properties, and preliminary applications. We also highlight the important role of kinetic factors played in the design of recyclable polymers. Finally, major challenges, perspectives, and future opportunities in the synthesis and applications of polythioesters/polydisulfides are discussed.

The integration of polymer materials with self-healing features into advanced lithium batteries is a promising and attractive approach to mitigate degradation and, thus, improve the performance and reliability of batteries. Polymeric materials with an ability to autonomously repair themselves after damage may compensate for the mechanical rupture of an electrolyte, prevent the cracking and pulverization of electrodes or stabilize a solid electrolyte interface (SEI), thus prolonging the cycling lifetime of a battery while simultaneously tackling financial and safety issues. This paper comprehensively reviews various categories of self-healing polymer materials for application as electrolytes and adaptive coatings for electrodes in lithium-ion (LIBs) and lithium metal batteries (LMBs). We discuss the opportunities and current challenges in the development of self-healable polymeric materials for lithium batteries in terms of their synthesis, characterization and underlying self-healing mechanism, as well as performance, validation and optimization.

Dynamic covalent bond-based shape memory polymers (DCB-SMPs) are one of most important SMPs which have a wide potential application prospect. Different from common strong covalent bonds, DCBs own relatively weak bonding energy, similarly to the supramolecular interactions of noncovalent bonds, and can dynamically combine and dissociate these bonds. DCB-SMP solids, which can be designed to respond for different stimuli, can provide excellent self-healing, good reprocessability, and high mechanical performance, because DCBs can obtain dynamic cross-linking without sacrificing ultrahigh fixing rates. Furthermore, besides DCB-SMP solids, DCB-SMP hydrogels with responsiveness to various stimuli also have been developed recently, which have special biocompatible soft/wet states. Particularly, DCB-SMPs can be combined with emerging 3D-printing techniques to design various original shapes and subsequently complex shape recovery. This review has summarized recent research studies about SMPs based on various DCBs including DCB-SMP solids, DCB-SMP hydrogels, and the introduction of new 3D-printing techniques using them. Last but not least, the advantages/disadvantages of different DCB-SMPs have been analyzed via polymeric structures and the future development trends in this field have been predicted.

Implantable electronics have recently been attracting attention because of the promising advances in personalized healthcare. They can be used to diagnose and treat chronic diseases by monitoring and applying bioelectrical signals to various organs. However, there are challenges regarding the rigidity and hardness of typical electronic devices that can trigger inflammatory reactions in tissues. In an effort to improve the physicochemical properties of conventional implantable electronics, soft hydrogel-based platforms have emerged as components of implantable electronics. It is important that they meet functional criteria, such as stretchability, biocompatibility, and self-healing. Herein, plant-inspired conductive alginate hydrogels composed of “boronic acid modified alginate” and “oligomerized epigallocatechin gallate,” which are extracted from plant compounds, are proposed. The conductive hydrogels show great stretchability up to 500% and self-healing properties because of the boronic acid-cis-diol dynamic covalent bonds. In addition, as a simple strategy to increase the electrical conductivity of the hydrogels, ionically crosslinked shells with cations (e.g., sodium) were generated on the hydrogel under physiological salt conditions. This decreased the resistance of the conductive hydrogel down to 900 ohm without trading off the original properties of stretchability and self-healing. The hydrogels were used for “electrophysiological bridging” to transfer electromyographic signals in an ex vivo muscle defect model, showing a great bridging effect comparable to that of a muscle-to-muscle contact model. The use of plant-inspired ionically conductive hydrogels is a promising strategy for designing implantable and self-healable bioelectronics.