DN Gels Research Articles

Spatially-selective enhancement of mechanical strength through localized cross-linking of double network hydrogels

To advance the development of flexible materials for soft robotics applications, it is crucial to enhance the elastic modulus and breaking the stress of soft materials, such as hydrogels. Double network hydrogels (DN gels) have displayed remarkable mechanical strength owing to their unique network structure composed of two types of polymer networks. However, current fabrication methods for DN gels entail cross-linking two distinct hydrogel polymers throughout the entire hydrogel matrix. In this study, we focused on employing multi-photon polymerization (MPP) with femtosecond laser pulses as a cross-linking method for hydrogels to achieve spatially selective formation of DN gel structures at the micrometer scale, along with the consequent improvement in local mechanical strength. We assessed the mechanical properties of the fabricated structures and confirmed that the mechanical strength varied within the regions where DN gel was locally formed.

Development of soft inchworm robot with friction control of feet using double-network gel

In this study, we investigate the development of an inchworm-inspired soft robot with active friction control of its feet. The focus is on its design, modeling, and locomotion principle. Friction control is achieved dynamically by using a unique material called double-network hydrogel (DN gel) to form the legs of the inchworm robot. DN gels are soft but tough materials, and they exhibit the unique characteristic of frictional difference on the surface under an applied voltage. This property is exploited to realize locomotion. A wide range of robot speeds can be realized through appropriate control of the voltage applied to its legs. Motion strategies with simultaneous control of actuation force and leg friction allow for greater flexibility in locomotion. The behavior of inchworm locomotion with change in the frictional difference between legs and actuator force was predicted using the developed mathematical model and was confirmed through experiments. Three different inchworm robot prototypes were developed and studied. The inchworm robot is characterized by low cost and inherent compliance, and it is easily disposable.

High-strength shape-memory organohydrogel based on two orthogonal supramolecular effect: Gelator-induced phase-transition and metal-coordination interaction

• The organohydrogels can be synthesized by simple two-step procedures. • Our SN organohydrogels show high mechanical performances, comparable to DN gels. • The gelator endows liquid oil phase with switchable phase transition properties. • The orthogonal networks endow the gel with a dual-programming shape-memory property. • The shape-morphing behavior with high strength can be used for liquid transport. Shape-memory hydrogels are extensively applied in various fields. However, the existing shape-memory hydrogels with simple shape morphing, such as tensile and bending deformations, cannot satisfy complex and diverse requirements. When the hydrophilic and lipophilic structures were integrated into the same gel network, the heterogeneous structures can be separately controlled by two independent supramolecular interactions. Here, we prepared an organohydrogels with two orthogonal supramolecular networks (metal-coordination interaction of hydrogel framework and gelator-induced phase-transition of organogels inclusions) via photoinitiated polymerization and soaking in Fe 3+ solution. Importantly these two non-interfering orthogonal supramolecular networks endow the gel excellent mechanical properties and a unique dual-programmed shape-memory behavior, enabling highly accessible complex shape deformation, such as kirigami and multidimensional deformations. The organohydrogels can be used to fabricate smart devices for multidimensional on-demand fluid delivery. Consequently, our organohydrogel materials, with sophisticated and high-strength shape-memory properties, can further promote the practical application of gel materials in complex environments.

Tough and self‐healable carrageenan‐based double network microgels enhanced physical hydrogels for strain sensor

AbstractDouble network hydrogels (DN gels) demonstrate extremely high strength and toughness; however, conventional DN gels are fully chemically crosslinked, which are often softening and lack self‐healing. Here, a new type of fully physical DN (P‐DN) gels, that is, K+ ions crosslinked κ‐carrageenan (K+C) microgel (MG) composite hydrophobically crosslinked polyacrylamide (HPAAm) gels or K+C‐MG/HPAAm gels, in which the HPAAm gel was physically crosslinked by hydrophobic interactions as well as topologically crosslinked by P‐DN microgels, were prepared by a facile one‐pot method. At the optimal condition, K+C‐MG/HPAAm gel exhibited high strength (0.36 MPa) and high toughness (968 J/m2). In addition, it can rapidly recover from softening and display good fatigue resistance. Notably, K+C‐MG/HPAAm gel also demonstrated self‐healing properties on account of its physical nature. Because there are anionic κ‐carrageenan and KCl, K+C‐MG/HPAAm gels are ionic conductive, which were used as strain sensors to monitor the human motions. We believe our K+C‐MG/HPAAm gels provide a new insight in the field of DN gels.

Rational programming of polysaccharide-based double network hydrogel with heterogeneous architecture and multifunction via electrical signal/temperature triggered sequential self-assembly

Rational programming of double network hydrogel (DN gel) structure is an effective strategy to acquire satisfied mechanical properties and innovative functions. Herein, we report a novel electrochemically strategy for rational design of a DN gel from the blends of two stimuli-responsive self-assembling polysaccharides chitosan (CS, pH-responsive) and agarose (Agar, thermally responsive) by using externally applied electrical signal/temperature stimulations to cue sequential self-assemblies. Specifically, considering of the remarkable difference in the filed-induced chains' migration behavior at liquid/solid systems, chitosan's electrophoretic deposition (EDP) processes were performed comparably at different temperatures and two types of DN gels were fabricated with dramatically different growing kinetics, tailored crystalline structures and enhanced mechanical properties. Moreover, the bioinspired multilayer structure could be facially imparted in the DN gels by using an oscillating electrical signal, and their great potential applications such as the pH/ethanol-sensitive device or versatile platform for staged drug delivery have been revealed respectively. Eventually, we demonstrated that the electrical signal is highly advantageous in preparing DN gels that able to: (i) provide complex pH cues to guide polymer chains self-assembled with specific patterning structures; (ii) quantitative analysis of the gelation process via transfer charges; (iii) offer great possibilities for hydrogel construction via electric filed-induced chain migration. Therefore, we believe this electro-biochemically synthesizing approach could spur new insight into the multifunctional hydrogel materials constructions and biomedical related applications.

Mechanical and microstructural studies in a polysaccharide-acrylate double network hydrogel

Polymeric hydrogels continue to find a wide range of applications. However, a major drawback of hydrogels is the lack of mechanical strength. In this regard, “Double Network Hydrogels” (DN) have shown great promise recently. The toughness in DN hydrogels originates from the synergistic effect of two polymeric networks. In this work, we have synthesized a DN hydrogel consisting of a tightly cross linked carboxymethylcellulose (CMC) as the first network and loosely cross linked poly(hydroxyethylacrylate) (PHEA) as a second network (CMC-PHEA-DN). The required flexibility in the second network (PHEA) was induced by the presence of a small amount of stearyl methacrylate (SM) as a co-monomer in hydroxyl ethyl acrylate (HEA). The compressive strength of the CMC-PEHA-DN hydrogel was found to be 280 times more than that of CMC-SN hydrogel, and the presence of SM in DN hydrogels showed better recovery after deformation. Cell viability studies showed the biocompatibility of DN hydrogels. The micro-structural analysis of DN xerogels by 3D X-ray Microtomography indicated the presence of oriented pores in size range of 30–40 μm. To the best of our knowledge, Microtomography was used for the first time to study the DN gels. These hydrogels can be used to develop implants that can withstand prolonged stress and expand the life span of implants.

Improving the strength and toughness of macroscale double networks by exploiting Poisson\u2019s ratio mismatch

We propose a new concept that utilizes the difference in Poisson's ratio between component materials as a strengthening mechanism that increases the effectiveness of the sacrificial bond toughening mechanism in macroscale double-network (Macro-DN) materials. These Macro-DN composites consist of a macroscopic skeleton imbedded within a soft elastic matrix. We varied the Poisson's ratio of the reinforcing skeleton by introducing auxetic or honeycomb functional structures that results in Poisson’s ratio mismatch between the skeleton and matrix. During uniaxial tensile experiments, high strength and toughness were achieved due to two events: (1) multiple internal bond fractures of the skeleton (like sacrificial bonds in classic DN gels) and (2) significant, biaxial deformation of the matrix imposed by the functional skeleton. The Macro-DN composite with auxetic skeleton exhibits up to 4.2 times higher stiffness and 4.4 times higher yield force than the sum of the component materials. The significant improvement in mechanical performance is correlated to the large mismatch in Poisson's ratio between component materials, and the enhancement is especially noticeable in the low-stretch regime. The strengthening mechanism reported here based on Poisson's ratio mismatch can be widely used for soft materials regardless of chemical composition and will improve the mechanical properties of elastomer and hydrogel systems.

Three-Dimensional Printing of Double-Network Hydrogels: Recent Progress, Challenges, and Future Outlook.

Hydrogels are widely used materials due to their biocompatibility, their ability to mimic the hydrated and porous extracellular microenvironment, as well as their ability to tune both mechanical and biochemical properties. However, most hydrogels lack mechanical toughness, and shaping them into complicated three-dimensional (3D) structures remains challenging. In the past decade, tough and stretchable double-network hydrogels (DN gels) were developed for tissue engineering, soft robotics, and applications that require a combination of high-energy dissipation and large deformations. Although DN gels were processed into simple shapes by using conventional casting and molding methods, new 3D printing methods have enabled the shaping of DN gels into structurally complex 3D geometries. This review will describe the state-of-art technologies for shaping tough and stretchable DN gels into custom geometries by using conventional molding and casting, extrusion, and optics-based 3D printing, as well as the key challenges and future outlook in this field.

Antioxidant and multi-sensitive PNIPAAm/keratin double network gels for self-stripping wound dressing application.

Hydrogel is a potential wound dressing material due to its ability to maintain a humid environment, the strong absorptive capacity of exuded tissue fluid, and gas exchange function. Herein, poly(N-isopropyl acrylamide)/keratin double network (PNIPAAm/keratin DN) gels were fabricated through covalent and ionic double cross-linking strategy. The effects of PNIPAAm/keratin ratios on the morphology and swelling rate of gels were characterized. The DN gels could swell up from 2600% to 4600% in proportion to the keratin content, demonstrating their great ability to absorb tissue fluid. The gels possessed thermo-sensitiveness, imparting self-stripping property. Moreover, the antibacterial chlorhexidine acetate (CHX) was loaded into gels with a post-fabrication drug-loading strategy. The release behavior showed that CHX-loaded DN gels exhibited multiple responsive characteristics (temperature, pH, and ROS). Furthermore, the drug-loaded gels showed greater antibacterial activity than free CHX due to the sustained drug release effect. Meanwhile, the antioxidant efficiency of PNIPAAm/keratin DN gels was ca. 33.1%, while the PNIPAAm gel was just ca. 18.2%, indicating the strong oxidation resistance of DN gels. In the Sprague Dawley (SD) rat skin defect model, the hydrogel had better tissue regeneration ability than the commercial film. Taken together, the multifunctional PNIPAAm/keratin DN gels are potential candidates for clinical wound treatment.

Development of the network alteration theory for the Mullins softening of double-network hydrogels

Double-network hydrogels (DN gels) with excellent mechanical properties have great application prospects as an innovative soft material. The Mullins effect originally discovered in rubber materials is also observed in this material. The network alteration theories have achieved many successes in capturing this stress-softening behavior of DN gels and exploring the underlying damage mechanisms. In this paper, we develop a network alteration theory to account for the softening behaviors of DN gels under both uniaxial tension and pure shear and simultaneously explore more about the damage origins. The advantage of the use of worm-like chain model for DN gels is illustrated. New network evolutions of the network alteration parameters are further developed on a basis of an existing network alteration theory. The model predictions of the new model for the experimental results of DN gels under both uniaxial tension and pure shear are compared with those of the old one. The results demonstrate that the new model is more physically sound and offers much better predictive capabilities. It is found that the dangling chains, chain disentanglements and the degradation of the interactions between the two networks during deformation play significant roles in the Mullins softening of DN gels. This paper gives new insights into the microdamage origins of DN gels and significantly improve the network alteration theory for DN gels.

Polyzwitterions as a Versatile Building Block of Tough Hydrogels: From Polyelectrolyte Complex Gels to Double-Network Gels.

The high water content of hydrogels makes them important as synthetic biomaterials, and tuning the mechanical properties of hydrogels to match those of natural tissues without changing chemistry is usually difficult. In this study, we have developed a series of hydrogels with varied stiffness, strength, and toughness based on a combination of poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS), a strong acidic polyelectrolyte, and poly-N-(carboxymethyl)-N,N-dimethyl-2-(methacryloyloxy) ethanaminium) (PCDME), a polyzwitterion with a weak acidic moiety. We demonstrate that modifying the true molar ratio, R, of PCDME to PAMPS results in four unique categories of hydrogels with different swelling ratios and Young's moduli. When R < 1, a negatively charged polyelectrolyte gel (PE) is formed; when 1 < R < 3, a tough and viscoelastic polyelectrolyte complex gel (PEC) is formed; when 3 < R < 6.5, a conventional, elastic interpenetrating network gel (IPN) is formed; and when R > 6.5, a tough and stiff double-network gel (DN) is formed. Both the PEC and DN gels exhibit high toughness and fracture stress, up to 1.8 and 1.5 MPa, respectively. Importantly, the PEC gels exhibit strong recovery properties along with high toughness, distinguishing them from DN gels. Without requiring a change in chemistry, we can tune the mechanical response of hydrogels over a wide spectrum, making this a useful system of soft and hydrated biomaterials.

Modelling the mechanical behaviors of double-network hydrogels

Double-network hydrogels (DN gels), consisting of a rigid, highly cross-linked short-chain network and a flexible, loosely cross-linked long-chain network, are a kind of promising soft and tough material with superior mechanical properties. In order to capture the mechanical behaviors of DN gels more accurately and with more physical depth than some existing models, a constitutive formulation is developed in this paper. The Helmholtz free energy function is decomposed into three parts accounting for the free energies of each network and the mixing of all the components. The eight-chain worm-like chain (WLC) based model is then employed to describe the deformation of each network with two network structure parameters: the chain density and the average chain length. Based on the existing experimental investigations of the internal fracture process of DN gels, a detailed and clear physical picture is illustrated. According to this damage picture, the two network structure parameters of the short-chain network are regarded as two internal variables. Within the framework of general thermodynamics, the energy dissipation is defined through the variation of the internal variables, which allows one to obtain the evolutions of internal variables from analyzing the macroscopic dissipated behaviors of DN gels. For the cases without energy dissipation, the network structure parameters are constant and can be determined directly through the stress-strain relation at one typical point on the stress-strain curve and the stored Helmholtz free energy of each state. These determined values of the parameters are then used to validate the mass conservation. For the cases with energy dissipation, the evolution equations of the two internal variables are determined through the analysis of the energy dissipation and the mass conservation. As a result, not only the unloading stress-stain curves but also the primary loading curve can be theoretically predicted by the constitutive formulation. The mechanical behaviors of both t-DN gels and c-DN gels before necking are analyzed. The comparisons of the theoretical results with the experimental data show sufficiently good agreements, demonstrating the ability and efficiency of the formulation to capture the Mullins effect of DN gels.

Effect of Relative Strength of Two Networks on the Internal Fracture Process of Double Network Hydrogels As Revealed by in Situ Small-Angle X-ray Scattering

Double network hydrogels (DN gels) exhibit extraordinarily high strength and toughness by interplay of the two contrasting networks: the rigid, brittle network serves as a sacrificial bond that fra...

A multifunctional metal-biopolymer coordinated double network hydrogel combined with multi-stimulus responsiveness, self-healing, shape memory and antibacterial properties

Outfitted with abundant hydrogen bonding and coordination active groups, carboxymethyl chitosan (CMC) possesses a class of naturally occurring ligands for coordination with metal ions, establishing its excellent potential for various fields. Herein, by incorporating the naturally derived CMC into a thermally reconfigurable agarose (Agar) gel medium, a novel type of metal-biopolymer coordinated double network hydrogel (DN gel) was successfully fabricated via the strong coordination interactions. The interpenetrated CMC was confirmed to retain its excellent chelating abilities within the bulk gel matrix, which resulted in a series of metal-coordinated DN gels through spontaneous self-associative complexation with metal ions such as Cu2+, Zn2+, Ni2+, Co2+, Fe3+, and Cr3+. Moreover, these two types of physical cross-links are functionally independent and reversible, which enables the programming of the hydrogel with multi-functionality, including pH-regulated shape memory behavior, multi-staged self-healing properties and durable antibacterial activities. Thus, we believe that the successful preparation of such a coordination-driven DN gel will lead to the development of biopolymer-based multifunctional hydrogels, as well as provide new insight into nanocomponent assembly and soft electronic biosensing systems for biomedical applications.

Natural protein-based hydrogels with high strength and rapid self-recovery

Natural protein hydrogels are considered as the promising candidates for biomaterials. However, natural protein hydrogels often exhibit poor mechanical properties. Herein, using bovine serum albumin (BSA) as natural protein model, a new kind of D/C-hybrid DN gels, consisting of tetrakis(hydroxymethyl)phosphonium chloride (THPC) cross-linked BSA (THPC-BSA) as dynamic covalent bond cross-linked first network and covalently cross-linked polyacrylamide (PAAm) as second network, were successfully synthesized by a facile method. Different from fully chemical DN gels without recovery, the optimized THPC-BSA/PAAm D/C-hybrid DN gel not only demonstrated excellent tensile properties, but also displayed extremely fast self-recovery property and fatigue resistance. More importantly, various natural proteins could be also used to prepare natural protein-based D/C-hybrid DN gels, and all of them showed improved mechanical properties and fast self-recovery properties. The results indicate our new strategy to fabricate recoverable natural protein-based D/C-hybrid DN gels is general. We hope our new strategy as well as our natural protein-based D/C-hybrid DN gels will provide a new avenue to prepare and study high performance natural protein hydrogels.

Ultra-high strength poly(N-(2-hydroxyethyl)acrylamide)/chitosan hydrogel with “repelling and killing” bacteria property

Hydrogels with good mechanical properties are promising for various applications. In this work, a simple yet effective method for preparing a novel double-network hydrogel was reported. First, nonfouling polymer, poly(N-(2-hydroxyethyl)acrylamide) (PHEAA), was crosslinked through covalent bonds. Antibacterial polysaccharide, chitosan (CS), was then crosslinked by chelation between the N-glucosamine units on the CS and citrate or sulfate ions. The poly(N-(2-hydroxyethyl)acrylamide)/chitosan double-network hydrogels (PHEAA/CS DN hydrogels) exhibited high tensile strength (3.8 MPa), strong elastic modulus (0.6 MPa). And the dynamic ionic crosslinking in CS network provided the DN gels with fast self-recovery ability as well as excellent fatigue resistance. Furthermore, the mechanical properties of the DN gels were enhanced after stretching and relaxing because of the molecular orientation and reconstruction of chitosan network. More importantly, the hydrogels have excellent antifouling and antibacterial properties, which is called “repelling and killing”, making them competitive candidates for applications in the biomedical field.

Self‐recovery and fatigue of double‐network gels with permanent and reversible bonds

ABSTRACTDouble‐network (DN) gels subjected to cyclic deformation (stretching up to a fixed strain followed by retraction down to the zero stress) demonstrate a monotonic decrease in strain with time (self‐recovery). Observations show that the duration of total recovery varies in a wide interval (from a few minutes to several days depending on composition of the gel), and this time is strongly affected by deformation history. A model is developed for the kinetics of self‐recovery. Its ability to describe stress–strain diagrams in cyclic tests with various periods of recovery is confirmed by comparison with observations on several DN gels. Numerical simulation reveals pronounced enhancement of fatigue resistance in multi‐cycle tests with stress‐ and strain‐controlled programs when subsequent cycles of deformation are interrupted by intervals of recovery. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019,57, 438–453

Double network hydrogel of sodium alginate/polyacrylamide cross-linked with POSS: Swelling, dye removal and mechanical properties.

In this work, for the first time, we presented a novel design strategy to fabricate a new type of Octa-aminopropyl polyhedral oligomeric silsesquioxane hydrochloride salt (OA-POSS)-based Alg/PAAm DN gels, consisting of PAAm gel as the first chemical network and Alg/OA-POSS gel as the second chemical-physical network. Incorporation of OA-POSS nanoparticles with various feed ratio into the second network not only led to form strong coordination interactions but also in comparison to the other Alg/PAAm gels demonstrated superior mechanical properties. pH, temperature, contact time and salt effects parameters were studied on the swelling and dye removal properties of the synthesized DN hybrid hydrogels. The adsorption experiments exhibited that 20 mg of Alg/PAAm/OA-POSS1 showed the maximum adsorption capacity for methylene blue (MB) at pH 9 and 11. Dye adsorption kinetics were modeled by the pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetic equations under different conditions. Additionally, adsorption isotherms of the adsorption of MB were analyzed.

Double network hydrogels with controlled shape deformation: A mini review

ABSTRACTDouble network hydrogels (DN gels), consisting of two networks with strongly asymmetric network structures and properties, are one of most investigated high strength hydrogels. In most cases, the first network of DN gels is rigid, brittle and tightly crosslinked, while the second network is soft, ductile and loosely crosslinked. Because of the tunable and diverse network structures, DN gels with controlled shape deformation have attracted great attention in recent years. The shape deformation of DN gels can be controlled by first network, second network, or both networks. In this mini review, the shape deformation of DN gels via different networks will be summarized, and the application and future perspectives also are discussed. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1351–1362

(Invited) Distinctive Characteristics of Internal Fracture in Tough Double Network Hydrogels Revealed By Various Modes of Stretching

(Invited) Distinctive Characteristics of Internal Fracture in Tough Double Network Hydrogels Revealed By Various Modes of Stretching