Longer and stronger; stiff but not brittle Hydrogels are highly water-swollen, cross-linked polymers. Although they can be highly deformed, they tend to be weak, and methods to strengthen or toughen them tend to reduce stretchability. Two papers now report strategies to create tough but deformable hydrogels (see the Perspective by Bosnjak and Silberstein). Wang et al . introduced a toughening mechanism by storing releasable extra chain length in the stiff part of a double-network hydrogel. A high applied force triggered the opening of cycling strands that were only activated at high chain extension. Kim et al . synthesized acrylamide gels in which dense entanglements could be achieved by using unusually low amounts of water, cross-linker, and initiator during the synthesis. This approach improves the mechanical strength in solid form while also improving the wear resistance once swollen as a hydrogel. —MSL

The reversible and click nature of Diels−Alder (DA) reactions has made them ideal candidates to design materials with nonconventional properties. Most commonly, the reversibility of DA is utilized for designing thermosets that can be liquefied for reprocessing and self-healing, yet the dynamic equilibrium nature has been largely neglected. In this work, shape memory polymers (SMP) containing DA moieties in the networks were synthesized. In addition to its remoldability at the liquid state at sufficiently high temperatures (above 110°C), we show uniquely and surprisingly that such a network can undergo plastic deformation in its solid state at intermediate temperatures (60−100°C) by taking advantage of its dynamic equilibrium for network topological rearrangement. The liquid state remoldability and solid state plasticity represent two distinct yet complementary mechanisms to manipulate the permanent shape of an SMP, leading to unprecedented versatility that can benefit a variety of applications in the future.T he ability of shape memory polymer (SMP) to fix a temporary shape and recover to its permanent shape has shown unique advantages toward various applications, including aerospace structures and biomedical devices. 1−6 Expanding the material properties beyond the traditional shape memory effect can drastically open up new opportunities. On this front, much of the recent progress is centered on the temporary shape fixing (e.g., multishape memory effect 7 ) and the reversibility of the polymer shape memory effect. 8−10 In contrast, the attention on the permanent shapes has been largely nonexistent beyond the simple distinction between thermoset SMP and thermoplastic SMP, with the latter remoldable for permanent shape resetting. This is in sharp contrast to the requirement of many real world device applications that often demand sophistication in manipulating the permanent geometry.Plasticity in thermoset SMP allows covalent bond exchange in the network, consequently, permanent shape reconfiguration without melting through the network topological rearrangement. 2−5,11−16 Thus, the permanent shape can be reconfigured repetitively without using mold, more importantly, this offers a new way to access extremely sophisticated permanent shapes that cannot be made otherwise. 2 However, the types of permanent shapes accessible via plastic deformation are limited to the solid state deformation of the prior permanent shape. In this work, we reveal that the permanent shape of an SMP network containing Diels−Alder (DA) adducts can be redefined by both liquid molding and solid state plasticity in two different temperature ranges by taking advantage of their dynamic equilibrium nature.The click and reversible nature of DA reactions has historically attracted much attention for designing nonconventional thermosets. 17−22 Typically, the network material consisting of furan/maleimide adducts is heated to a high temperature (above 110°C) to trigger the reverse Diels−Alder reaction (retro-DA) such that it can reach a liquid state t...