As a new class of high-strength hydrogels, we designed a tetra-PEG gel by combining two symmetrical tetrahedron-like macromonomers of the same size. Because the nanostructural unit of the gel network was defined by the length of the tetrahedral PEG arm, the gel had a homogeneous structure and resultant high mechanical strength comparable to that of native articular cartilage. Furthermore, since the gel was formed by mixing two biocompatible macromonomer solutions, the gelation reaction itself and the resultant gel were also biocompatible. The breaking strength had local maxima at the overlap concentration of the macromonomers (C*) and at 2C*. Dynamic light scattering measurement indicated the near absence of inhomogeneities in the network at C*. Thus, we successfully designed and fabricated a high-strength hydrogel by controlling the homogeneity of network structure for the first time, which will lead to multiplied effects, i.e., contributing to the understanding of ideal networks, providing a universal strategy for designing high-strength gels, and opening up the biomedical application of hydrogels.

Optimizing Injectable Hydrogels: Injectable hydrogels are showing promise as scaffolds in regenerative medicine because they can be injected in liquid form and transform in situ into the gel state. However, when exposed to ionic solutions, such as those found in the body, hydrogels can increase in volume by a factor of 2, which can weaken the material. Kamata et al. (p. 873 ) added a thermoresponsive component to a hydrogel so that the thermoresponsive component would tend to collapse in shape when heated and counteract the hydrogel's tendency to swell. Indeed, the resulting gel retained its unswollen volume following immersion in a physiological solution and retained its mechanical strength during repeated stretching or compression.