Effect of the Second Network on First Network Rupture and the Origin of Energy Dissipation in Double Network Hydrogels

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

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.

Temperature response of double network (DN) hydrogels composed of thermoresponsive poly(N,N′-diethylacrylamide) (PDEAAm) and hydrophilic polyacrylamide (PAAm) or poly(N,N′-dimethylacrylamide) (PDMAAm) was studied by a combination of swelling measurements, differential scanning calorimetry (DSC) and 1H NMR and UV-Vis spectroscopies. Presence of the second hydrophilic network in DN hydrogels influenced their thermal sensitivity significantly. DN hydrogels show less intensive changes in deswelling, smaller enthalpy, and entropy changes connected with phase transition and broader temperature interval of the transition than the single network (SN) hydrogels. Above the transition, the DN hydrogels contain significantly more permanently bound water in comparison with SN hydrogels due to interaction of water with the hydrophilic component. Unlike swelling and DSC experiments, a rather abrupt transition was revealed from temperature-dependent NMR spectra. Release study showed that model methylene blue molecules are released from SN and DN hydrogels within different time scale. New thermodynamical model of deswelling behaviour based on the approach of the van’t Hoff analysis was developed. The model allows to determine thermodynamic parameters connected with temperature-induced volume transition, such as the standard change of enthalpy and entropy and critical temperatures and characterize the structurally different states of water.

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

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.

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

SANS and NMR study on nanostructure of thermoresponsive double network hydrogels

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.

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.

Tensile mechanics of double-network (DN) hydrogels synthesized from 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and acrylamide (AAm) with different cross-link density of the first networks was studied after applying the freeze/thaw (F/T) process. When a DN hydrogel which exhibits necking upon applying a tensile load undergoes F/T process, necking is suppressed and strain hardening is observed in a tensile deformation. The area under the stress–strain curve which measures the work of deformation and is a criterion of toughness also is reduced after the F/T process. The reduction of toughness in a DN hydrogel with more brittle first network is more pronounced. The toughness of samples reduced very markedly after the first F/T cycle; however, the toughness of the samples was very close after the second and third F/T cycles. The reduction of mechanical properties of DN hydrogels upon applying the F/T process is attributed to the damage induced by freezing of water to the internal structure of hydrogel by...

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.