Poly(N,N'-Diethylacrylamide)-Based Thermoresponsive Hydrogels with Double Network Structure.

Double – network hydrogel based on exopolysaccharides as a biomimetic extracellular matrix to augment articular cartilage regeneration

SANS and NMR study on nanostructure of thermoresponsive double network hydrogels

To develop hydrogels with high mechanical strength for cell encapsulation and three-dimensional culture is a critical challenge for cartilage tissue engineering. In this study, novel double network (DN) hydrogels were fabricated and phosphate buffer solution (PBS) was used as the solvent to facilitate cell encapsulation. The tough DN hydrogels were prepared through a two-step photopolymerization using neutral oligo(2,2-dimethyltrimethylene carbonate)-poly(ethylene glycol)-oligo(2,2-dimethyltrimethylene carbonate)-diacrylate (DPD-DA) chains for the first network and methacrylated hyaluronic acid (HA-GMA) chains for the second network. The fracture stress values of the optimized and cell-laden DN hydrogels were 8.38 ± 0.67 MPa and 6.28 ± 1.26 MPa, respectively, which showed comparable mechanical strength with natural articular cartilage tissue. The live/dead cell viability assay of cell-laden DN hydrogels demonstrated the tough DN hydrogels have comparable cytocompatibility with PEG hydrogels. After long-term culture, cartilage-specific extra cellular matrix accumulated in these DN hydrogels. The results indicated the potential of the novel DN hydrogels as cartilage tissue engineering scaffolds.

The success of complex tissue and internal organ reconstruction relies principally on the fabrication of a 3D vascular network, which guarantees the delivery of oxygen and nutrients in addition to the disposal of waste. In this study, a rapidly forming cell-encapsulated double network (DN) hydrogel is constructed by an ultrasonically activated silk fibroin network and bioorthogonal-mediated polyethylene glycol network. This DN hydrogel can be solidified within 10 s, and its mechanical property gradually increases to ∼20 kPa after 30 min. This work also demonstrates that coencapsulation of human umbilical vein endothelial cells (HUVECs) and umbilical cord-derived mesenchymal stem cells (UCMSCs) into the DN hydrogel can facilitate the formation of more mature vessels and complete the capillary network in comparison with the hydrogels encapsulated with a single cell type both in vitro and in vivo. Taking together, the DN hydrogel, combined with coencapsulation of HUVECs and UCMSCs, represents a strategy for the construction of a functional vascular network.

Since cleaning of artworks may cause undesirable physicochemical alterations and is a nonreversible procedure, it is mandatory to adopt the proper cleaning procedure. Such a procedure should remove undesired materials whilst preserving the original surface. In this regard, numerous gels have been developed and exploited for the cleaning of various artwork surfaces. Lately, agarose (AG) and polyvinyl alcohol-borax (PVA-B) hydrogels have been widely employed as cleaning tools by conservators. Both hydrogels show some limitations in specific cleaning practices. In this work, we investigated the influence of including increased levels of agarose into PVA-B systems. For this reason, we performed a detailed characterization on the double network (DN) hydrogel including the chemical structure, the liquid phase retention, the rheological behavior, and the self-healing behavior of various PVA-B/AG double network hydrogels. These new hydrogels revealed better properties than PVA-B hydrogels and obviated their limitations. The inclusion of AG into PVA-B systems enhanced the liquid retention capacity, shape-stability, and mechanical strength of the blend. Furthermore, AG minimized the expelling/syneresis issue that occurs when loading PVA-B systems with low polarity solvents or chelating agents. The resultant double network hydrogel exhibits relevant self-healing properties. The PVA-B/AG double network is a new and useful cleaning tool that can be added to the conservators’ tool-kit. It is ideal for cleaning procedures dealing with porous and complex structured surfaces, vertical surfaces and for long time applications.

Double-network hydrogels for biomaterials: Structure-property relationships and drug delivery

Cartilage-like tribological performance of charged double network hydrogels

Enhanced strength and toughness of κ-carrageenan/polyacrylic acid physical double-network hydrogels by dual cross-linking of the first network

Chemically cross-linked double-network (DN) hydrogels undergo irreversible softening and weakening after yielding due to internal fracture, which restricts their reusability. In contrast, many biological tissues can repair internal damage through ion-mediated metabolic process, such as the healing of bone fractures. Inspired by this phenomenon, we demonstrate that specific multivalent metal ions can repair chemically cross-linked DN hydrogels after yielding. Repair is achieved by soaking the yielded hydrogels in metal ion solutions, where strong coordination interactions between metal ions and sulfonate side groups of the first network reconnect broken chains after fracture. As a result, the repaired DN hydrogels not only regain their energy dissipation capacity but can even surpass the original samples. The repair efficiency is influenced by ion concentration, preyield strain, first-network cross-linking density, and second-network concentration. Furthermore, the incorporation of dynamic metal-coordination bonds transforms the originally elastic DN hydrogels into viscoelastic and self-recoverable materials, enabling them to endure continuous and variable loading. This repair strategy, achieved without altering the classical DN hydrogel composition, expands the application potential of a system previously regarded primarily as a model for theoretical studies.

The novel double-network (DN) hydrogels were prepared using the chitosan-g-poly(acrylic acid) as the first network, and polyacrylamide as the second. The effects of the concentrations of the second network on chemical structure, intermolecular interactions and mechanical properties for the DN gels were investigated. The DN hydrogels had decreased swelling capacities and significantly improved glassy modulus and strength with the increase of the acrylamide concentration, owing to the enhanced intermolecular interaction and physical entanglement, and reduced molecular motion. It is worth noting that DN hydrogels with 5.50 mol/L acrylamide content had the greatest mechanical strength and still relatively high water content (~82 wt%), resulting from the effectiveness of the intermolecular penetration and intermolecular interactions between two independent polymer networks. Therefore, the reported novel chitosan-based DN hydrogels exhibit good potentials in some applications, for example biomedical engineering applications.

Several soft tissues residing in the living body have excellent hydration lubrication properties and can provide effective protection during relative motion. In order to apply this advantage of soft matters in practical applications and try to avoid its disadvantage, such as swelling and weakening in water, a design strategy of a soft/hard double network (DN) hydrogel microsphere modified ultrahigh molecular weight polyethylene (UHMWPE) composite is proposed in this study. A series of microspheres of urea-formaldehyde (UF), polyacrylamide (PAAm) hydrogel, UF/PAAm double network, and their composites were prepared. The mechanical properties, swelling, wettability, friction properties, and the lubrication mechanisms of the composites were investigated. The results show that DN microspheres can have an excellent stability and provide hydration lubrication. The performance of 75 DN-1 composite was superior to others. This finding will provide a novel strategy for the development of water-lubricated materials and have wide application in engineering fields.

Polymer hydrogels attract attention as excellent soft and wet materials. However, common hydrogels are mechanically too soft and brittle to be used as load-bearing substances. By mimicking the structure of the articular cartilage, which is one of the native tough hydrogels, double network (DN) hydrogel with extremely high mechanical performance has been developed. Having high water content (about 90 wt%), DN gels consist of two types of polymer components with opposite physical natures: the minor component (the first network) abundantly cross-linked polyelectrolytes, and the major component (the second network) comprised of poorly cross-linked neutral polymers. Under suitable conditions, DN gels exhibit 0.1–1 MPa of elastic modulus, 20–60 MPa of compressive fracture stress, 1,000–2,000% of tensile strain, and 100–1,000 J m−2 of fracture energy. These excellent mechanical properties are comparable to those of rubber and natural bio-tissues. This paper reviews the main principle of DN gels, including their preparation method, mechanical feature, and toughening mechanism. The processability and the applicability of DN hydrogels as biomaterials and as conductive materials are also discussed.

Three-dimensional (3D) bio-printing has recently emerged as a crucial technology in tissue engineering, yet there are still challenges in selecting materials to obtain good print quality. Therefore, it is essential to study the influence of the chosen material (i.e., bio-ink) and the printing parameters on the final result. The “printability” of a bio-ink indicates its suitability for bio-printing. Hydrogels are a great choice because of their biocompatibility, but their printability is crucial for exploiting their properties and ensuring high printing accuracy. However, the printing settings are seldom addressed when printing hydrogels. In this context, this study explored the printability of double network (DN) hydrogels, from printing lines (1D structures) to lattices (2D structures) and 3D tubular structures, with a focus on printing accuracy. The DN hydrogel has two entangled cross-linked networks and a balanced mechanical performance combining high strength, toughness, and biocompatibility. The combination of poly (ethylene glycol)-diacrylate (PEDGA) and sodium alginate (SA) enables the qualities mentioned earlier to be met, as well as the use of UV to prevent filament collapse under gravity. Critical correlations between the printability and settings, such as velocity and viscosity of the ink, were identified. PEGDA/alginate-based double network hydrogels were explored and prepared, and printing conditions were improved to achieve 3D complex architectures, such as tubular structures. The DN solution ink was found to be unsuitable for extrudability; hence, glycerol was added to enhance the process. Different glycerol concentrations and flow rates were investigated. The solution containing 25% glycerol and a flow rate of 2 mm/s yielded the best printing accuracy. Thanks to these parameters, a line width of 1 mm and an angle printing inaccuracy of less than 1° were achieved, indicating good shape accuracy. Once the optimal parameters were identified, a tubular structure was achieved with a high printing accuracy. This study demonstrated a 3D printing hydrogel structure using a commercial 3D bio-printer (REGEMAT 3D BIO V1) by synchronizing all parameters, serving as a reference for future more complex 3D structures.

Hydrogels with intrinsic high stretchability and flexibility are extremely attractive for soft electronics. However, the existing complicated and laborious methods (such as mold curing) to fabricate microstructured hydrogel (MH) still limit the development of hydrogel-based sensors for flexible devices. Herein, we use digital light processing 3D printing technology to rapidly construct double-network (DN) ionic conductive hydrogel, and then design and print fingerprint-like MH film to manufacture an iontronic pressure sensor. In particular, the DN hydrogel consists of acrylamide/acrylic acid to form a covalently cross-linked network, and magnesium chloride is introduced to form an ionic cross-linked physical network in the hydrogel. The printability (with resolution 150 μm) and mechanical property tunability of DN hydrogel enable the convenient fabrication of sensors. With the biomimetic fingerprint MH film, the iontronic pressure sensor not only has a high sensitivity (0.06 kPa−1), but also has a large detection range (26 Pa–70 kPa) and good stability (200 cycles of pressure loading). We demonstrated that our sensor can be applied to realize tactile sensing in a prosthetic application and detect human motion. With the easy strategy of constructing DN hydrogel with microstructures by 3D printing technology, hydrogel-based sensors are anticipated to be employed in more smart electronics.

Double-network (DN) hydrogels combine two interpenetrating polymer networks with contrasting roles: a brittle, rigid primary network that dissipates energy through sacrificial bond breakage, and a flexible secondary network that maintains integrity. This architecture imparts exceptional toughness, stretchability, and crack resistance, making DN hydrogels valuable for biomedical, soft robotics, and energy-storage applications. Incorporating natural polymers, particularly nanostructured biopolymers like nanocellulose, enhances biocompatibility, sustainability, and tunability. This review outlines DN hydrogel design principles and nanocellulose structures, focusing on their role in polyacrylamide (PAM) and polyvinyl alcohol (PVA) matrices. Emphasis is placed on how cellulose nanofibers (CNF) improve mechanical strength, stability, and dynamic response through entanglement, hydrogen bonding, and nanoconfinement. The review concludes with challenges and future opportunities for nanocellulose-based DN hydrogels in multifunctional and sustainable material design.