A soft robotic fish can quickly swim and turn with a fully integrated onboard system for power and remote control.

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3D printing technology has been widely explored for the rapid design and fabrication of hydrogels, as required by complicated soft structures and devices. Here, a new 3D printing method is presented based on the rheology modifier of Carbomer for direct ink writing of various functional hydrogels. Carbomer is shown to be highly efficient in providing ideal rheological behaviors for multifunctional hydrogel inks, including double network hydrogels, magnetic hydrogels, temperature-sensitive hydrogels, and biogels, with a low dosage (at least 0.5% w/v) recorded. Besides the excellent printing performance, mechanical behaviors, and biocompatibility, the 3D printed multifunctional hydrogels enable various soft devices, including loadable webs, soft robots, 4D printed leaves, and hydrogel Petri dishes. Moreover, with its unprecedented capability, the Carbomer-based 3D printing method opens new avenues for bioprinting manufacturing and integrated hydrogel devices.

Hydrogels—natural or synthetic polymer networks that swell in water—can be made mechanically, chemically, and electrically compatible with living tissues. There has been intense research and development of hydrogels for medical applications since the invention of the hydrogel contact lenses in 1960. More recently, functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various substrates. Hydrogel-coated substrates combine the advantages of hydrogels, such as lubricity, biocompatibility, and anti-biofouling properties, with the advantages of the substrates, such as stiffness, toughness, and strength. In this review, we focus on three aspects of functional hydrogel coatings: (1) applications and functions enabled by hydrogel coatings, (2) methods of coating various substrates with different functional hydrogels with tough adhesion, and (3) tests to evaluate the adhesion between functional hydrogel coatings and substrates. Conclusions and outlook are given at the end of this review.