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       Conventional polymers soften above their glass transition temperature—think of familiar plastics like vinyl bags and PET bottles. Now, Jianping Gong and his colleagues, writing in the journal Advanced Materials, describe a polymer that rapidly and reversibly transforms from a soft hydrogel to a hard plastic as the temperature rises.
       Beyond the transition temperature, the material’s stiffness, strength, and toughness increase dramatically while the volume remains constant. The gel changes from a transparent, soft state to an opaque, hard state. At 60°C, a thin sheet of gel can support a weight of 10 kg. This thermal hardening is reversible and can be repeated many times.
       Nonoyama, T., et al., Instantaneous thermal switching from soft hydrogel to hard plastic inspired by thermophilic bacterial proteins. Adv. Mater. https://doi.org/10.1002/adma.201905878 (2019)