“Petrificus totalus!” — 3D-printed hydrogel switches from kPa-Soft to GPa-hard on command

A novel 3D-printed hard/soft switchable hydrogel developed by researchers at Zhejiang University exhibits the ability to easily and repeatedly switch its Young’s modulus from kPa to GPa, enabled by the on-demand crystallization of the embedded supercooled hydrate salt solution through seeding. Its softness can also be restored by recovering the supercooled state of the solution.

The work, reported in the International Journal of Extreme Manufacturing, highlights the promising potential of directly using hard hydrogel as a robust industrial material. In its hard state, it achieves a hardness of 86.5 Shore D (comparable to hard plastics), a compression strength of 81.7 MPa, and a Young's modulus of 1.2 GPa—significantly surpassing current 3D-printed hydrogels.

Water (solvent) is one of the core components of hydrogels, and their soft and wet properties make them widely applicable in fields such as flexible electronics, soft robotics, and regenerative medicine. However, due to their loose crosslinking and susceptibility to swelling, the printed hydrogels always suffer from bearing compressive stress and shear stress.

In recent years, significant efforts have been devoted to enhancing the mechanical properties of hydrogels. Despite notable progress in this area, most studies have primarily focused on improving toughness, with less emphasis on hardness (modulus). Hydrogels, with carefully designed polymer networks, typically exhibit an elastic modulus of less than 1 MPa. Therefore, shifting the focus of research to the solvent may provide an effective approach to addressing this challenge.

“We believe that optimizing the properties and behavior of the solvent is key to balancing the hardness and toughness of hydrogels, which is essential for expanding their potential applications,” says corresponding author Yong He.

Herein, a 3D photo-printable hard/soft switchable hydrogel composite is fabricated by infusing phase transition (liquid/solid transition) supercooled hydrated salt solution into the printed hydrogel structures. In the supercooled state, the random arrangement of liquid solvent molecules within hydrogels makes it as soft as conventional hydrogels. Upon artificial seeding of the crystal nucleus, the solvent in hydrogel undergoes rapid crystallization, resulting in the in-situ formation of numerous rigid nanoscale crystals, which hardens the hydrogel.

Taking advantage of soft/hard status switching, a hydrogel smart medical plaster bandage, capable of fitting the shape of limb in the soft state and providing sufficient support for injured sites after 10 minutes of crystallization, was developed to explore the practical applications.

Despite the hydrogel exhibiting extraordinary strength and hardness, its toughness remains insufficient. The researchers are continuing their work, aiming to enhance its toughness while maintaining its strength and hardness.

About IJEM:

International Journal of Extreme Manufacturing (IF: 16.1, consecutive 1st in the Engineering, Manufacturing category) is a multidisciplinary and double-anonymous peer-reviewed journal uniquely publishing original articles and reviews of the highest quality and impact in the areas related to extreme manufacturing, ranging from fundamentals to process, measurement, and systems, as well as materials, structures, and devices with extreme functionalities.

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