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.

3D printing of hydrogels finds widespread applications in biomedicine and engineering. Artificial cartilages and heart valves, tissue regeneration and soft robots, require high mechanical performance of complex structures. Although many tough hydrogels have been developed, complicated synthesis processes hinder their fabrication in 3D printing. Here, a strategy is proposed to formulate hydrogel inks, which can be printed into various strong and tough particle-based double-network (P-DN) hydrogels of arbitrary shapes without any rheological modifiers. These hydrogel inks consist of microgels and a hydrogel precursor. The microgels are individual highly cross-linked networks. They are prepared by swelling dried microparticles in the hydrogel precursor that consists of monomers, initiators, and cross-linkers. Microgels regulate the rheological properties of the hydrogel ink and enable the direct printing. After printing and curing, the precursor forms a sparsely cross-linked network that integrates the microgels, leading to a P-DN hydrogel. The proposed hydrogel inks allow 3D printing of multifunctional hydrogel structures with high mechanical performance and strong adhesion to diverse materials. This strategy will open new avenues to fabricate multifunctional devices in tissue engineering and soft robotics.

Modern medicine, including minimally invasive surgeries, relies on optical fibers to efficiently transmit light into the human body. However, existing optical fibers exhibit poor mechanical compatibility with tissues and insufficient functionality. Herein is shown that a thin layer of hydrogel coating simultaneously improves the performance of optical fibers mechanically, optically, and functionally. By involving the hydrogel as a mechanically friendly skin, the optical fiber achieves surface tissue‐like softness and low friction, without compromising its bulk properties. With high transparency and low refractive index, the hydrogel coating helps form a step‐index profile for low‐loss light transmission. Furthermore, versatile functionalities by hydrogel‐coated optical fibers, like fluorescence generation, photothermal effect, pH sensing, and drug delivery, are conceptually demonstrated. This simple coating/core design that can easily incorporate extraordinary biocompatibility, extreme physical properties, and advanced functionality of hydrogels, will encourage more opportunities for the development of optical fibers in biomedical applications.

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