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 fractures at a relatively low strain, while the soft, stretchable network serves as hidden length that sustains stress by large extension afterwards. The internal fracture process of the brittle network strongly depends on the relative strength of the two networks. In this study, we study the internal fracturing process of typical DN gels that show yielding or necking under uniaxial stretching, using in situ small-angle X-ray scattering.Two samples consisting of the same brittle first network from poly(2-acrylamido-2methylpropanesulfonic acid) but stretchable second network from poly(N,N-dimethylacrylamide) of different concentrations were adopted. We found that (1) the brittle network shows non-affine deformation even far below the yield strain by local fracture; (2) for the sample of low second network concentration, significant strain amplification occurs around the submicron-scale voids (defects) preexisting in the brittle network, which induces the fracture percolation of brittle network from voids to show the necking phenomenon; (3) the strain amplification at voids is suppressed in the sample of high second network concentration, and fracture of brittle network occurs dispersedly, showing yielding without necking.

Bone tissues possess excellent mechanical properties such as compatibility between strength and flexibility and load bearing owing to the hybridization of organic/inorganic matters with anisotropic structure. To synthetically mimic such an anisotropic structure of natural organic/inorganic hybrid materials, we carried out hydroxyapatite (HAp) mineralization in stretched tough double network (DN) hydrogels. Anisotropic mineralization of HAp took place in stretched hydrogels, as revealed by high brightness synchrotron X-ray scattering and transmission electron microscopic observation. The c-axis of mineralized HAp aligned along the stretching direction, and the orientation degree S calculated from scattering profiles increased with increasing in the elongation ratio λ of the DN gel, and S at λ = 4 became comparable to that of rabbit tibial bones. The morphology of HAp polycrystal gradually changed from spherical to unidirectional rod-like shape with increased elongation ratio. A possible mechanism for the anisotropic mineralization is proposed, which would be one of the keys to develop mechanically anisotropic organic/inorganic hybrid materials.

Inspired from toughening mechanism of bone tissues, hydrogels, toughened by low crsytalline hydroxyapatite as sacrificial bonds, were created.

We report a method to create anisotropic double-network (DN) hydrogels, through the controlled orientation of a physical sacrificial network. A cross-linked polyacrylamide hydrogel is synthesized from a solution containing a semirigid anionic polyelectrolyte. Subsequently, the gel is stretched to orient the semirigid polyelectrolyte, which does not relax in the stretched state because of the high contour length in comparison to the mesh size of the polyacrylamide network. The polyelectrolyte is then physically cross-linked with a multivalent cation, ZrCl2O, to fix the anisotropy. Anisotropy was visualized by observing birefringence and quantified by small-angle X-ray scattering. By comparing the scattering in the oriented direction versus perpendicular to the oriented direction, a structural anisotropy factor was calculated. Uniaxial tensile testing was performed on samples of varying prestretch, both parallel and perpendicular to the stretching direction. Young’s modulus, fracture stress, fracture strain, and work of extension were characterized, and the resulting mechanical anisotropy was compared to the structural anisotropy factor. We find that the anisotropy of Young’s modulus and fracture stress is directly controlled by the anisotropy of the sacrificial network, while fracture strain and work of extension show little influence from structural anisotropy. The results of this work demonstrate that prestretching of a physical sacrificial network is a controllable and simple method to create anisotropic DN hydrogels.