Preparation and self-healing behaviors of poly(acrylic acid)/cerium ions double network hydrogels

Highly Self-Healable Polymeric Coating Materials Based on Charge Transfer Complex Interactions with Outstanding Weatherability

In this study, we prepare highly self-healable polymeric coating materials using charge transfer complex (CTC) interactions. The resulting coating materials demonstrate outstanding thermal stability (1 wt% loss thermal decomposition temperature at 420 °C), rapid self-healing kinetics (in 5 min), and high self-healing efficiency (over 99%), which is facilitated by CTC-induced multiple interactions between the polymeric chains. In addition, these materials exhibit excellent optical properties, including transmittance over 91% and yellow index (YI) below 2, and show enhanced weatherability with a ΔYI value below 0.5 after exposure to UV light for 72 h. Furthermore, the self-healable coating materials developed in this study show outstanding mechanical properties by overcoming the limitations of conventional self-healing materials.

Hydrogels with Reversible Crosslinks for Improved Localised Stem Cell Retention: A Review

Successful stem cell applications could have a significant impact on the medical field, where many lives are at stake. However, the translation of stem cells to the clinic could be improved by overcoming challenges in stem cell transplantation and in vivo retention at the site of tissue damage. This review aims to showcase the most recent insights into developing hydrogels that can deliver, retain, and accommodate stem cells for tissue repair. Hydrogels can be used for tissue engineering, as their flexibility and water content makes them excellent substitutes for the native extracellular matrix. Moreover, the mechanical properties of hydrogels are highly tuneable, and recognition moieties to control cell behaviour and fate can quickly be introduced. This review covers the parameters necessary for the physicochemical design of adaptable hydrogels, the variety of (bio)materials that can be used in such hydrogels, their application in stem cell delivery and some recently developed chemistries for reversible crosslinking. Implementing physical and dynamic covalent chemistry has resulted in adaptable hydrogels that can mimic the dynamic nature of the extracellular matrix.

Highly stretchable and self-healing cellulose nanofiber-mediated conductive hydrogel towards strain sensing application

HYPOTHESIS

Hydrogel-based sensors have attracted considerable attention due to potential opportunities in human health monitoring when both mechanical flexibility and sensing ability are required. Therefore, the integration of excellent mechanical properties, electrical conductivity and self-healing properties into hydrogels may improve the application range and durability of hydrogel-based sensors.

EXPERIMENTS

A novel composite hydrogel composed of polyaniline (PANI), polyacrylic acid (PAA) and 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNFs) was designed. The viscoelastic, mechanical, conductive, self-healing and sensing properties of hydrogels were studied.

FINDINGS

The TOCNF/PANI/PAA hydrogel exhibits a fracture strain of 982%, tensile strength of 74.98 kPa and electrical conductivity of 3.95 S m^-1, as well as good mechanical and electrical self-healing properties within 6 h at ambient temperature without applying any stimuli. Furthermore, owing to the high sensitivity of the TOCNF/PANI/PAA-0.6 hydrogel-based strain sensor (gauge factor, GF = 8.0), the sensor can accurately and rapidly detect large-scale motion and subtle localized activity. The proposed composite hydrogel is as a promising material for use as soft wearable sensors for health monitoring and smart robotics applications.

Dually Crosslinked Polymer Networks Incorporating Dynamic Covalent Bonds

Covalent adaptable networks (CANs) are polymeric networks containing covalent crosslinks that are dynamic under specific conditions. In addition to possessing the malleability of thermoplastics and the dimensional stability of thermosets, CANs exhibit a unique combination of physical properties, including adaptability, self-healing, shape-memory, stimuli-responsiveness, and enhanced recyclability. The physical properties and the service conditions (such as temperature, pH, and humidity) of CANs are defined by the nature of their constituent dynamic covalent bonds (DCBs). In response to the increasing demand for more sophisticated and adaptable materials, the scientific community has identified dual dynamic networks (DDNs) as a promising new class of polymeric materials. By combining two (or more) distinct crosslinkers in one system, a material with tailored thermal, rheological, and mechanical properties can be designed. One remarkable ability of DDNs is their capacity to combine dimensional stability, bond dynamicity, and multi-responsiveness. This review aims to give an overview of the advances in the emerging field of DDNs with a special emphasis on their design, structure-property relationships, and applications. This review illustrates how DDNs offer many prospects that single (dynamic) networks cannot provide and highlights the challenges associated with their synthesis and characterization.

Physical and Chemical Factors Influencing the Printability of Hydrogel-based Extrusion Bioinks

Bioprinting researchers agree that "printability" is a key characteristic for bioink development, but neither the meaning of the term nor the best way to experimentally measure it has been established. Furthermore, little is known with respect to the underlying mechanisms which determine a bioink's printability. A thorough understanding of these mechanisms is key to the intentional design of new bioinks. For the purposes of this review, the domain of printability is defined as the bioink requirements which are unique to bioprinting and occur during the printing process. Within this domain, the different aspects of printability and the factors which influence them are reviewed. The extrudability, filament classification, shape fidelity, and printing accuracy of bioinks are examined in detail with respect to their rheological properties, chemical structure, and printing parameters. These relationships are discussed and areas where further research is needed, are identified. This review serves to aid the bioink development process, which will continue to play a major role in the successes and failures of bioprinting, tissue engineering, and regenerative medicine going forward.

Self-healing stable polymer hydrogel for pH regulated selective adsorption of dye and slow release of graphene quantum dots

Developing composite hydrogels with excellent self-healing ability and reasonable mechanical strength is extremely challenging. Herein, to overcome this difficulty, we identify the importance of balancing the ratio between the components of a composite hydrogel based on graphene oxide and poly(acrylic acid-acrylamide) [GOxAAM]. The gel exhibits enhanced mechanical strength, excellent self-healing ability, and extraordinary swelling capacity (a swelling ratio of 732) at room temperature and neutral pH. Further, the dye adsorption of the composite hydrogel has been investigated. The hydrogel could selectively adsorb organic cationic dyes (methylene blue vs. rhodamine B) from contaminated water with remarkable efficiency (90-95%). The kinetics investigation suggests that the dye adsorption on this composite hydrogel follows a pseudo-second-order model. The reusability of the hydrogel has been demonstrated by repeating the adsorption-desorption process over 10-14 cycles with almost identical results in the adsorption efficiency. Moreover, the hydrogel is utilized as a solvent-induced slow release source for graphene quantum dots (GQDs). The results taken together suggest that the (GOxAAM) composite gel can be a promising candidate for developing multifunctional gel materials.

Recent advances in soft functional materials: preparation, functions and applications

Synthetic materials and biomaterials with elastic moduli lower than 10 MPa are generally considered as soft materials. Research studies on soft materials have been boosted due to their intriguing features such as light-weight, low modulus, stretchability, and a diverse range of functions including sensing, actuating, insulating and transporting. They are ideal materials for applications in smart textiles, flexible devices and wearable electronics. On the other hand, benefiting from the advances in materials science and chemistry, novel soft materials with tailored properties and functions could be prepared to fulfil the specific requirements. In this review, the current progress of soft materials, ranging from materials design, preparation and application are critically summarized based on three categories, namely gels, foams and elastomers. The chemical, physical and electrical properties and the applications are elaborated. This review aims to provide a comprehensive overview of soft materials to researchers in different disciplines.

Polyethylenimine-Based Nanogels for Biomedical Applications

Nanogels (NGs) are 3-dimensional (3D) networks composed of hydrophilic or amphiphilic polymer chains, allowing for effective and homogeneous encapsulation of drugs, genes, or imaging agents for biomedical applications. Polyethylenimine (PEI), possessing abundant positively charged amine groups, is an ideal platform for the development of NGs. A variety of effective PEI-based NGs have been designed and much effort has been devoted to study the relationship between the structure and function of the NGs. In particular, PEI-based NGs can be prepared either using PEI as the major NG component or using PEI as a crosslinker. This review reports the recent progresses in the design of PEI-based NGs for gene and drug delivery and for bioimaging applications with a target focus to tackle the diagnosis and therapy of cancer.

Development of Self-Healing Double-Network Hydrogels: Enhancement of the Strength of Wheat Gluten Hydrogels by In Situ Metal-Catechol Coordination

Wheat gluten, a byproduct of the wheat starch industry, is widely used as a dough strengthener and gelling agent. In this research, we developed novel double-network hydrogels by gelation of gluten using in situ metal-catechol coordination. The first network consisted of physically associated gluten molecules, while the second network consisted of Fe³⁺-cross-linked proanthocyanidins (PACs). Dynamic shear rheology experiments suggested that coordination of Fe³⁺ and PACs greatly enhanced the mechanical properties of the gluten hydrogels. The double-network hydrogels exhibited a 3-fold higher shear modulus than pure gluten hydrogels. The formation of bis- and tris-catechol-Fe³⁺ complexes between Fe³⁺ and PACs in the hydrogels was confirmed by ultraviolet-visible spectrometry and isothermal titration calorimetry (ITC). The ITC measurements of Fe³⁺ binding to PACs indicated a molar stoichiometry of 1:4 and a dissociation constant ( K_D) of 24.9 × 10^-9. When subject to repeated shear deformation-compression cycles, the hydrogels exhibited strong and rapid recovery of their rheological properties. The strong, self-healing characteristics of the double-network gluten hydrogels produced in this study may be useful for certain applications in the food, agriculture, biomedicine, and tissue-engineering industries.

Self-healable poly(acrylic acid-co-maleic acid)/glycerol/boron nitride nanosheet composite hydrogels at low temperature with enhanced mechanical properties and water retention

Many living tissues possess excellent mechanical properties and water retention which enable them to self-heal at room temperature even below the freezing temperature of water. To mimic the unique features of living tissue, a poly(acrylic acid-co-maleic acid) composite hydrogel with enhanced mechanical properties and remarkable water retention was fabricated under accessible conditions. The hydrogel is functionalized by amino group modified boron nitride nanosheets (BNNS-NH2)/glycerol and exhibits self-healing abilities at low temperature. The self-healing process occurs through the re-establishing of hydrogen bonds and metal coordination interactions at the damaged surfaces. Its anti-freezing abilities enable the hydrogel to self-heal at -15 °C, and the self-healing efficiency based on tensile strength reaches up to ∼70%. Moreover, glycerol also endows the hydrogel with long-lasting water retention, which remains a water content of ∼99 wt% for more than 30 days. Meanwhile, the simultaneous introduction of BNNS-NH2 and glycerol significantly improved the mechanical properties of the hydrogel, which displays great stretchability (∼474%), tensile strength (∼151.3 kPa), stiffness (Young's modulus of ∼62.75 kPa) and toughness (∼355.13 kJ m-3). It is anticipated that these novel hydrogels will develop many fields and be exploited for new applications in extensive external environments.

Dynamic and Cell-Infiltratable Hydrogels as Injectable Carrier of Therapeutic Cells and Drugs for Treating Challenging Bone Defects

Biopolymeric hydrogels have been widely used as carriers of therapeutic cells and drugs for biomedical applications. However, most conventional hydrogels cannot be injected after gelation and do not support the infiltration of cells because of the static nature of their network structure. Here, we develop unique cell-infiltratable and injectable (Ci-I) gelatin hydrogels, which are physically cross-linked by weak and highly dynamic host-guest complexations and are further reinforced by limited chemical cross-linking for enhanced stability, and then demonstrate the outstanding properties of these Ci-I gelatin hydrogels. The highly dynamic network of Ci-I hydrogels allows injection of prefabricated hydrogels with encapsulated cells and drugs, thereby simplifying administration during surgery. Furthermore, the reversible nature of the weak host-guest cross-links enables infiltration and migration of external cells into Ci-I gelatin hydrogels, thereby promoting the participation of endogenous cells in the healing process. Our findings show that Ci-I hydrogels can mediate sustained delivery of small hydrophobic molecular drugs (e.g., icaritin) to boost differentiation of stem cells while avoiding the adverse effects (e.g., in treatment of bone necrosis) associated with high drug dosage. The injection of Ci-I hydrogels encapsulating mesenchymal stem cells (MSCs) and drug (icaritin) efficiently prevented the decrease in bone mineral density (BMD) and promoted in situ bone regeneration in an animal model of steroid-associated osteonecrosis (SAON) of the hip by creating the microenvironment favoring the osteogenic differentiation of MSCs, including the recruited endogenous cells. We believe that this is the first demonstration on applying injectable hydrogels as effective carriers of therapeutic cargo for treating dysfunctions in deep and enclosed anatomical sites via a minimally invasive procedure.

Self-Healing Polymeric Hydrogel Formed by Metal-Ligand Coordination Assembly: Design, Fabrication, and Biomedical Applications

Self-healing hydrogels based on metal-ligand coordination chemistry provide new and exciting properties that improve injectability, rheological behaviors, and even biological functionalities. The inherent reversibility of coordination bonds improves on the covalent cross-linking employed previously, allowing for the preparation of completely self-healing hydrogels. In this article, recent advances in the development of this class of hydrogels are summarized and their applications in biology and medicine are discussed. Various chelating ligands such as bisphosphonate, catechol, histidine, thiolate, carboxylate, pyridines (including bipyridine and terpyridine), and iminodiacetate conjugated onto polymeric backbones, as well as the chelated metal ions and metal ions containing inorganic particles, which are used to form dynamic networks, are highlighted. This article provides general ideas and methods for the design of self-healing hydrogel biomaterials based on coordination chemistry.

A self-healing conductive and stretchable aligned carbon nanotube/hydrogel composite with a sandwich structure

Self-healing conductive elastomers have emerged as a class of novel materials that are important for fabricating human-motion sensors, soft robots and healthcare monitoring systems. Herein, we report on a hydrogel of modified poly(γ-glutamic) acid polymer chains crosslinked by coordination complexes, which exhibits good stretchability (1375%), long-term stability (more than 40 days), and self-healing ability (99.0 ± 1.5% in 3 h). Furthermore, a "sandwich" structure composite was fabricated, which is composed of self-healing hydrogels and Au nanograin-decorated aligned multiwall carbon nanotube sheets. It possesses fast self-healing ability, a low stable electronic resistance of 10 ± 1 Ω sq^-1, in the temperature range of -40-90 °C, the humidity range of 10-90%, and a stretching range up to 200%.

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.