Hydrogel-polymer hybrids have been widely used for various applications such as biomedical devices and flexible electronics. However, the current technologies constrain the geometries of hydrogel-polymer hybrid to laminates consisting of hydrogel with silicone rubbers. This greatly limits functionality and performance of hydrogel-polymer–based devices and machines. Here, we report a simple yet versatile multimaterial 3D printing approach to fabricate complex hybrid 3D structures consisting of highly stretchable and high–water content acrylamide-PEGDA (AP) hydrogels covalently bonded with diverse UV curable polymers. The hybrid structures are printed on a self-built DLP-based multimaterial 3D printer. We realize covalent bonding between AP hydrogel and other polymers through incomplete polymerization of AP hydrogel initiated by the water-soluble photoinitiator TPO nanoparticles. We demonstrate a few applications taking advantage of this approach. The proposed approach paves a new way to realize multifunctional soft devices and machines by bonding hydrogel with other polymers in 3D forms.

4D printing is an emerging fabrication technology that enables 3D printed structures to change configuration over “time” in response to an environmental stimulus. Compared with other soft active materials used for 4D printing, shape‐memory polymers (SMPs) have higher stiffness, and are compatible with various 3D printing technologies. Among them, ultraviolet (UV)‐curable SMPs are compatible with Digital Light Processing (DLP)‐based 3D printing to fabricate SMP‐based structures with complex geometry and high‐resolution. However, UV‐curable SMPs have limitations in terms of mechanical performance, which significantly constrains their application ranges. Here, a mechanically robust and UV‐curable SMP system is reported, which is highly deformable, fatigue resistant, and compatible with DLP‐based 3D printing, to fabricate high‐resolution (up to 2 µm), highly complex 3D structures that exhibit large shape change (up to 1240%) upon heating. More importantly, the developed SMP system exhibits excellent fatigue resistance and can be repeatedly loaded more than 10 000 times. The development of the mechanically robust and UV‐curable SMPs significantly improves the mechanical performance of the SMP‐based 4D printing structures, which allows them to be applied to engineering applications such as aerospace, smart furniture, and soft robots.

Vitrimers, a type of dynamically crosslinked polymers that combine the solventand heat-resistance of thermosets with the reprocessability of thermoplastics, offer a new solution to the problem of plastic pollution. However, the current recycling approaches of vitrimers greatly constrain the shapes of recycled vitrimers to simple geometries, thus significantly limiting the application scopes of recycled vitrimers. Here, a simple but universal method for upcycling vitrimer wastes is reported by developing a UV curable recycling (UVR) solution system. Conventional unprintable vitrimer powders can be mixed with the UVR solution, and the resulting mixture is compatible with digital light processing based 3D printing to fabricate 3D structures with high resolution (up to 20 µm) and high geometric complexity. Heat treatment triggers bond exchange reactions in the printed structures, and greatly enhances the mechanical properties. This method allows to cyclically print vitrimer wastes multiple times. Moreover, the UVR-vitrimer mixture solution can work as an adhesive to bond printed small parts together to build a larger and more complex structure which cannot be printed. The upcycling method reported in this work extends the application scope of recycled vitrimers and provide a practical solution to address environmental challenges associated with plastic pollution.

There are growing demands for multimaterial three-dimensional (3D) printing to manufacture 3D object where voxels with different properties and functions are precisely arranged. Digital light processing (DLP) is a high-resolution fast-speed 3D printing technology suitable for various materials. However, multimaterial 3D printing is challenging for DLP as the current multimaterial switching methods require direct contact onto the printed part to remove residual resin. Here we report a DLP-based centrifugal multimaterial (CM) 3D printing method to generate large-volume heterogeneous 3D objects where composition, property and function are programmable at voxel scale. Centrifugal force enables non-contact, high-efficiency multimaterial switching, so that the CM 3D printer can print heterogenous 3D structures in large area (up to 180 mm × 130 mm) made of materials ranging from hydrogels to functional polymers, and even ceramics. Our CM 3D printing method exhibits excellent capability of fabricating digital materials, soft robots, and ceramic devices.

Thermally responsive shape memory polymers (SMPs) used in 4D printing are often reported to be activated by external heat sources or embedded stiff heaters. However, such heating strategies impede the practical application of 4D printing due to the lack of precise control over heating or the limited ability to accommodate the stretching during shape programming. Herein, we propose a novel 4D printing paradigm by fabricating stretchable heating circuits with fractal motifs via electric-fielddriven microscale 3D printing of conductive paste for seamless integration into 3D printed structures with SMP components. By regulating the fractal order and printing/processing parameters, the overall electrical resistance and areal coverage of the circuits can be tuned to produce an efficient and uniform heating performance. Compared with serpentine structures, the resistance of fractal-based circuits remains relatively stable under both uniaxial and biaxial stretching. In practice, steady-state and transient heating modes can be respectively used during the shape programming and actuation phases. We demonstrate that this approach is suitable for 4D printed structures with shape programming by either uniaxial or biaxial stretching. Notably, the biaxial stretchability of fractal-based heating circuits enables the shape change between a planar structure and a 3D one with double curvature. The proposed strategy would offer more freedom in designing 4D printed structures and enable the manipulation of the latter in a controlled and selective manner.

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The effects of particle size, temperature, time, and pressure on the mechanical properties of regenerated epoxy-acid vitrimers were investigated, which helped to refine the vitrimer reprocessing condition parameter toolbox.