Jian Ping Gong

Why are double network hydrogels so tough

Double-network (DN) gels have drawn much attention as an innovative material having both high water content (ca. 90 wt%) and high mechanical strength and toughness. DN gels are characterized by a special network structure consisting of two types of polymer components with opposite physical natures: the minor component is abundantly cross-linked polyelectrolytes (rigid skeleton) and the major component comprises of poorly cross-linked neutral polymers (ductile substance). The former and the latter components are referred to as the first network and the second network, respectively, since the synthesis should be done in this order to realize high mechanical strength. For DN gels synthesized under suitable conditions (choice of polymers, feed compositions, atmosphere for reaction, etc.), they possess hardness (elastic modulus of 0.1–1.0 MPa), strength (failure tensile nominal stress 1–10 MPa, strain 1000–2000%; failure compressive nominal stress 20–60 MPa, strain 90–95%), and toughness (tearing fracture energy of 100∼1000 J m−2). These excellent mechanical performances are comparable to that of rubbers and soft load-bearing bio-tissues. The mechanical behaviors of DN gels are inconsistent with general mechanisms that enhance the toughness of soft polymeric materials. Thus, DN gels present an interesting and challenging problem in polymer mechanics. Extensive experimental and theoretical studies have shown that the toughening of DN gel is based on a local yielding mechanism, which has some common features with other brittle and ductile nano-composite materials, such as bones and dentins.

Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity

Hydrogels attract great attention as biomaterials as a result of their soft and wet nature, similar to that of biological tissues. Recent inventions of several tough hydrogels show their potential as structural biomaterials, such as cartilage. Any given application, however, requires a combination of mechanical properties including stiffness, strength, toughness, damping, fatigue resistance and self-healing, along with biocompatibility. This combination is rarely realized. Here, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and viscoelastic hydrogels with multiple mechanical properties. The randomness makes ionic bonds of a wide distribution of strength. The strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds reversibly break and re-form, dissipating energy. These physical hydrogels of supramolecular structure can be tuned to change multiple mechanical properties over wide ranges by using diverse ionic combinations. This polyampholyte approach is synthetically simple and dramatically increases the choice of tough hydrogels for applications.

Soft and Wet Materials: Polymer Gels

A polymer gel is a soft and wet material capable of undergoing large deformation. A deformed gel, in turn, changes its chemical potential, behaving as an energy transducer. Thus, a polymer gel shows a variety of stimuli-responsive actions, responding to external environmental changes. In this article unique electrical, thermal, and chemical responses of polymer gels are described. Recently observed frictional specificities of gels are also briefly introduced.

Oppositely Charged Polyelectrolytes Form Tough, Self‐Healing, and Rebuildable Hydrogels

A series of tough polyion complex hydrogels is synthesized by sequential homopolymerization of cationic and anionic monomers. Owing to the reversible interpolymer ionic bonding, the materials are self-healable under ambient conditions with the aid of saline solution. Furthermore, self-glued bulk hydrogels can be built from their microgels, which is promising for 3D/4D printing and the additive manufacturing of hydrogels.

Friction and lubrication of hydrogels—its richness and complexity

Biological connective tissues, such as cartilage and corneal stroma, are essentially hydrogels consisting of fibrous collagen and proteoglycans. Little is known of the surface properties of the hydrogel, although we observe fascinating tribological behavior in biological soft tissues, such as extremely low friction between animal cartilages. We consider that the role of the solvated polymer network existing in the extracellular matrix as a gel state is critically important in the specific frictional behavior of cartilages. In order to elucidate the general tribological features of a solvated polymer matrix, the friction of various kinds of hydrogels has been investigated, and very rich and complex frictional behaviors are observed. The friction force and its dependence on the load differ with the chemical structure of the gels, surface properties of the opposing substrates, and the measurement conditions, which are totally different from those of solids. Most importantly, the coefficient of friction of gels, , varies over a wide range and exhibits very low values (≈ 10-10), which cannot be obtained from the friction between two solid materials. A repulsion-adsorption model has been proposed to explain the gel friction, which says that the friction is due to lubrication of a hydrated layer of polymer chains when the polymer chain of the gel is non-adhesive (repulsive) to the substrate, and the friction is due to elastic deformation of the adsorbed polymer chain when it is adhesive to the substrate.

Unidirectional Alignment of Lamellar Bilayer in Hydrogel: One-Dimensional Swelling, Anisotropic Modulus, and Stress/Strain Tunable Structural Color

[∗] Dr. T. Kurokawa , Prof. J. P. Gong Faculty of Advanced Life Science Graduate School of Science Hokkaido University Sapporo, 060–0810 (Japan) E-mail: gong@sci.hokudai.ac.jp Dr. T. Kurokawa Creative Research Initiative Sousei Hokkaido University Sapporo, 001–0021 (Japan) M. A. Haque , G. Kamita Division of Biological Sciences Graduate School of Science Hokkaido University Sapporo, 060–0810 (Japan) Prof. K. Tsujii , Nanotechnology Research Center Research Institute for Electronic Science Hokkaido University (Retired) Sapporo, 001–0021 (Japan)

Materials both Tough and Soft

Tough elastomers are created by adapting an approach previously used for hydrogels. [Also see Report by Ducrot et al.] Hydrogels and elastomers are soft materials that have similar network structures but very different affinities to water. Consisting mostly of water, hydrogels resemble biological soft tissues and have great potential for use in biomedical applications; they tend to be very brittle, like fragile jellies. Elastomers are formed of nonhydrated polymer networks and are widely used as load-dispersing and shock-absorbing materials. They are stretchable but break easily along a notch. On page 186 of this issue, Ducrot et al. (1) show that the toughness of elastomers can be improved substantially by combining two different network materials, an approach previously applied to hydrogels.

Gel friction: A model based on surface repulsion and adsorption

A model describing the frictional force produced when a polymer gel is sliding on a solid surface has been proposed from the viewpoint of solvated polymer repulsion and adsorption theory at a solid surface. General relations for the frictional force f expressed as functions of the normal loading P, sliding velocity ν, the polymer volume fraction φ, or the elastic modulus E of the gel, etc., have been derived by applying scaling relations to the model. For the repulsive case, f is ascribed to the viscous flow of solvent at the interface and f is theoretically demonstrated to be proportional to the sliding velocity ν and the normal pressure P when the pressure is smaller than the elastic modulus of the gel. For the attractive case, in addition to the hydrodynamic friction, the force to detach the adsorbing chain from the substrate appears as friction. When ν is not very large, f∝ν. At an intermediate velocity, f has a velocity dependence less than linear, depending on the strength of adsorption. At a higher...

Mechano-actuated ultrafast full-colour switching in layered photonic hydrogels

Photonic crystals with tunability in the visible region are of great interest for controlling light diffraction. Mechanochromic photonic materials are periodically structured soft materials designed with a photonic stop-band that can be tuned by mechanical forces to reflect specific colours. Soft photonic materials with broad colour tunability and fast colour switching are invaluable for application. Here we report a novel mechano-actuated, soft photonic hydrogel that has an ultrafast-response time, full-colour tunable range, high spatial resolution and can be actuated by a very small compressive stress. In addition, the material has excellent mechanical stability and the colour can be reversibly switched at high frequency more than 10,000 times without degradation. This material can be used in optical devices, such as full-colour display and sensors to visualize the time evolution of complicated stress/strain fields, for example, generated during the motion of biological cells.

Characterization of internal fracture process of double network hydrogels under uniaxial elongation

Previously we revealed that the high toughness of double network hydrogels (DN gels) derives from the internal fracture of the brittle network during deformation, which dissipates energy as sacrificial bonds. In this study, we intend to elucidate the detailed internal fracture process of DN gels. We quantitatively analysed the tensile hysteresis and re-swelling behaviour of a DN gel that shows a well-defined necking and strain hardening, and obtained the following new findings: (1) fracture of the 1st network PAMPS starts far below the yielding strain, and 90% of the initially load-bearing PAMPS chains already break at the necking point. (2) The dominant internal fracture process occurs in the necking and hardening region, although the softening mainly occurs before necking. (3) The internal fracture efficiency is very high, 85% of the work is used for the internal fracture and 9% of all PAMPS chains break at sample failure. (4) The internal fracture is anisotropic, fracture occurs perpendicular to the tensile direction, in preference to the other two directions, but the fracture anisotropy decreases in the hardening region. Results (1) and (2) are in agreement with a hierarchical structural model of the PAMPS network. Based on these findings, we present a revised description of the fracture process of DN gels.