Search

One-dimensional photonic crystals or multilayer films produce colors that change depending on viewing and light illumination angles because of the periodic refractive index variation in alternating layers that satisfy Bragg's law. Recently, we have developed multilayered photonic hydrogels of two distinct bulk geometries that possess an alternating structure of a rigid polymeric lamellar bilayer and a ductile polyacrylamide (PAAm) matrix. In this paper, we focus on fabrication of composite gels with variable photonic band gaps by controlling the PAAm layer thickness. We report programmable angle-dependent and angle-independent structural colors produced by composite hydrogels, which is achieved by varying bulk and internal geometries. In the sheet geometry, where the lamellae are aligned parallel to the sheet surface, the photonic gel sheet exhibits strong angle-dependent colors. On the other hand, when lamellae are coaxially aligned in a cylindrical geometry, the gel rod exhibits an angle-independent color, in sharp contrast with the gel sheet. Rocking curves have been constructed to justify the diverse angle-dependent behavior of various geometries. Despite varying the bulk geometry, the tunable photonic gels exhibit strong mechanical performances and toughness. The distinct angle dependence of these tough photonic materials with variable band gaps could benefit light modulation in displays and sensor technologies.

We study the effect of dehydration on the structure and mechanical properties of anisotropic lamellar hydrogels, consisting of alternative stacking of several thousands of nanoscale rigid bilayers from amphiphilic poly(dodecyl glyceryl itaconate) (PDGI) and submicroscale soft hydrogel layers from hydrophilic polyacrylamide (PAAm) networks. We found that the layered microstructure is well preserved with dehydration, and a ductile–brittle transition occurs at the critical water content. This transition is related to the rubbery–glassy transition of the PAAm layers, which occurs at 58 wt % water content and is much higher than 26 wt % of bulk PAAm hydrogels. Such specific behavior of the lamellar hydrogels indicates that the dynamics of the submicroscale PAAm hydrated layer intercalated between the rigid bilayers are very different from its bulk state.

Novel, supramolecular, anisotropic hydrogels (called MC-PDGI gels) are presented in this study. These MC-PDGI gels consist of multi-cylindrical lipid bilayers aligned in a uniaxial manner and embedded in a soft hydrogel matrix. The bilayers and the hydrogel interact weakly due to hydrogen bonding. These MC-PDGI gels swell after exposure to water, which causes their volume and diameter to increase while simultaneously causing their length to decrease. This anisotropic swelling-induced contraction behavior is the result of competition between the isotropic elasticity of the hydrogel matrix and the interfacial tension of the lipid bilayers. Moreover, the MC-PDGI gels exhibit unique quasi one-dimensional diffusion behavior owing to the difficulty of molecular penetration through the multi-layered lipid bilayers. These materials would be useful for prolonged drug release or as an actuator.

We report that polymers can support bilayer membranes to form physical hydrogels of self-healing and tunable isotropic/anisotropic structure. The system consists of poly(dodecyl glyceryl itaconate) (PDGI) which forms lamellar bilayers and polyacrylamide (PAAm) which adsorbs on the bilayer surfaces via hydrogen bond formation. Adsorption of PAAm brings two effects: disturbs the bilayer packing and causes bending of the bilayers; increases the effective thickness of the bilayers and enhances the repulsion between the bilayers due to excluded volume effect. Competition of these two effects brings about sharp superstructure transition from isotropic multilayer foam phase to unidirectionally aligned lamellar phase. Accompanied by this structure transition, the bulk hydrogel exhibits isotropic/anisotropic swelling. The physical gels exhibit high tensile strength and self-healing properties that can be understood by the sacrificial bonds mechanism.

Tough double network (DN) hydrogels are a kind of interpenetrating network (IPN) gels with a contrasting structure; they consist of a rigid and brittle 1st network with dilute, densely cross-linked short chains and a soft and ductile 2nd network with concentrated, loosely cross-linked long chains. In this work, we focus on how the brittle gel changes into a tough one by increasing the amount of ductile component. By comparing the molecular structures of the individual first network and second network gels, we found that the true key mechanical factor that governs the brittle ductile transition is the fracture stress ratio of the two networks, sigma(f,2)/sigma(f,1). This ratio is related to the density ratio of elastically effective polymer strands of the two networks, nu(e,2)/nu(e,1), where the inter-network topological entanglement makes dominant contribution to nu(e,2). When nu(e,2)/nu(e,1) < k = 3.8-9.5, the second network fractures right after the fracture of the first network, and the gels are brittle. When nu(e,2)/nu(e,1) > k, only the first network fractures. As a result, the brittle first network serves as sacrificial bonds, imparting toughness of DN gels. The study also confirms that the load transfer between the two networks is via inter-network topological entanglement. This result provides essential information to design tough materials based on the double network concept. (C) 2014 Elsevier Ltd. All rights reserved.

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.

Our recent study has revealed that neutral polyampholytes form tough physical hydrogels above a critical concentration Cm,c by forming ionic bonds of wide strength distribution. In this work, we systematically investigate the behavior of a polyampholyte system, poly(NaSS-co-DMAEA-Q), randomly copolymerized from oppositely charged monomers, sodium p-styrenesulfonate (NaSS) and acryloyloxethyltrimethylammonium chloride (DMAEA-Q) without and with a slight chemical cross-linking. A phase diagram of formulation has been constructed in the space of monomer concentration Cm and cross-linker density CMBAA. Three phases are observed for the as-synthesized samples: homogeneous solution at dilute Cm, phase separation at semi-dilute Cm, and homogenous gel at concentrated Cm. Above a critical Cm,c, the polyampholyte forms a supramolecular hydrogel with high toughness by dialysis of the mobile counter-ions, which substantially stabilizes both the intra- and inter chain ionic bonds. The presence of the chemical cross-linker (CMBAA > 0) brings about a shift of the tough gel phase to lower Cm,c. The tough polyampholyte gel, containing [similar]50 wt% water, is highly stretchable and tough, exhibits fracture stress of σb [similar] 0.4 MPa, fracture strain of εb [similar] 30, and the work of extension at fracture Wext [similar] 4 MJ m−3. These values are at the level of most tough soft materials. Owing to the reversible ion bonds, the poly(NaSS-co-DMAEA-Q) gels also exhibit complete self-recovery (100%) and high fatigue resistance upon repeated large deformation.

We demonstrate, for the first time, a distinct change of the bulk properties of a hydrogel based on the lamellar–micelle transition. The hydrogel consists of thousands of periodical stacks of bilayer membranes, poly(dodecyl glyceryl itaconate) (PDGI), inside a dilute polyacrylamide network. Along with a bright structure color, the hydrogel exhibits a strong anisotropy in its bulk properties. By grafting a short chain polymer lipid, poly(ethylene glycol) dodecyl ether (C12EO23), to the bilayer membrane, sharp changes in the structure color, swelling and modulus of the hydrogel are observed in a narrow C12EO23 concentration range. These changes are ascribed to the structure transition of the single domain lamellar phase to the multi-domain micelle phase of PDGI, induced by the excluded volume effect of two dimensionally grafted poly(ethylene glycols) (PEG). By controlling the water permeable nano-channels formed during the structure transition, a specific molecular diffusion and recognition are demonstrated. This lamellar bilayer hydrogel could be a new insight into the lipid bilayer system to investigate the nature of biological membranes by the bulk behaviors of hydrogels.

The double network (DN) technique, developed by authors’ group, provides an innovative and universal pass way to fabricate hydrogels with super high toughness comparable to rubbers. The excellent mechanical performances of DN hydrogels originate from the specific combination of two networks with contrasting structures. The first brittle network serves as sacrificial bonds, which breaks into small clusters to efficiently disperse the stress around the crack tip into the surrounding damage zone, while the second ductile polymer chains act as hidden length, which extends extensively to sustain large deformation. Based on the principle of DN hydrogel, the author’s group recently has developed several novel systems and techniques, which has greatly expanded the practical accessibility of DN technique for practical use. The DN principle and the DN gel have already attracted much attention in the soft matter community. Inspired by the DN principle, many research groups have also designed and developed some innovative hydrogels with large enhancement in their mechanical strength and toughness. Some tough hydrogels fabricated by the DN technique also exhibit good biocompatibility and low friction resistance with promising prospective in industrial and medicine fields, especially for load-bearing artificial soft tissues such as artificial cartilage. In this feature article, we address the major concept and toughening mechanism of DN gel, then we describe some recent novel hydrogel systems based on the DN concept, and finally the applicability of DN gel as soft biomaterials is discussed.