Gelatin methacryloyl (GelMA), a photocurable hydrogel, is widely used in 3D culture, particularly in 3D bioprinting, due to its high biocompatibility, tunable physicochemical properties, and excellent formability. However, as the properties and performances of GelMA vary under different synthetic conditions, there is a lack of standardization, leading to conflicting results. In this study, a uniform standard is established to understand and enhance GelMA applications. First, the basic concept of GelMA and the density of the molecular network (DMN) are defined. Second, two properties, degrees of substitution and ratio of solid content, as the main measurable parameters determining the DMN are used. Third, the mechanisms and relationships between DMN and its performance in various applications in terms of porosity, viscosity, formability, mechanical strength, swelling, biodegradation, and cytocompatibility are theoretically explained. The main questions that are answered: what does performance mean, why is it important, how to optimize the basic parameters to improve the performance, and how to characterize it reasonably and accurately? Finally, it is hoped that this knowledge will eliminate the need for researchers to conduct tedious and repetitive pre‐experiments, enable easy communication for achievements between groups under the same standard, and fully explore the potential of the GelMA hydrogel.

No abstract

The development of flexible electronics (FEs) has rapidly accelerated in numerous fields due to their exceptional deformability, bending, and stretchability. Room‐temperature gallium‐based liquid metals (LMs) are considered as efficient conductive materials for FEs due to their outstanding electrical conductivity and intrinsic flexibility. Recently, 3D printing has become a promising technique for fabricating FEs. However, the poor printability due to high surface tension and fluidity offers huge challenges in the 3D printing of LMs. This review summarizes the effective strategies to address these challenges. It primarily focuses on three points: 1) how to improve the printability of LM and its wettability with the substrate, 2) how to select the appropriate printing method to improve the printing speed and ensure the resolution of printing structure, and 3) how to provide perfect encapsulation for LM‐based FEs with 3D printing. Following a brief introduction, the mainstream printing technologies and recent developments in the 3D printing of LMs are provided, with an emphasis on the selection of printing method, improvement of printability, encapsulation, and conductivity activation. Then, the revolutionary changes attained after 3D printing of LMs are specifically focused upon. Finally, opinions and potential directions for this thriving discipline are explored.

Insects with small and light bodies possess the capability of agile and fast movement in a small space. Inspired by nature, an insect-like soft robot may update the strategies in many scenarios like exploration, rescue, etc. However, the design and mass manufacture of soft robots combining insect size, fast mobility, good robustness, and impact-perception capability still present great engineering challenges. Herein, we report an insect-scale (15 mm body length (BL), 450 mg body weight) and ultrafast (∼4.0 BL s–1) soft robot. The remarkable motion performance is attributed to the high-frequency (760 Hz) operation as well as the long lifetime (>one million cycles) of its artificial muscle, which is a coil dielectric elastomer actuator (DEA) made by multimaterial coaxial three-dimensional printing with well-designed highly elastic materials and 5-inlets nozzle structure. The current robot is not only the smallest and fastest among the reported DEA-driven robots but also obtains high robustness, good environmental adaptability, and impact-perception capability: It can run on various grounds and complex paths, climb inside small pipes, work in robot swarms, and sustain and perceive the impact of the external environment.