In the contemporary era, novel manufacturing technologies like additive manufacturing (AM) have revolutionized the different engineering sectors including biomedical, aerospace, electronics, etc. Four-dimensional (4D) printing aka AM of smart materials is gaining popularity among the scientific community, which has the excellent ability to make soft structures such as soft robots, actuators, and grippers. These soft structures are developed by applying various stimuli such as pH, temperature, magnetic field, and many combinations onto soft materials. Stimuli in 3D printing permit various shape-morphing behaviors such as bending, twisting, folding, swelling, rolling, shrinking, origami, or locomotion. A wide variety of soft magnetic structures can be fabricated through the incorporation of soft or hard magnetic particles into soft materials resulting in magneto-active soft materials (MASMs). With this integration, magneto-thermal coupling actuation allows diverse magneto-deformations, facilitating the development of personalized devices that are capable of enhanced deformation. In this review, guidelines are provided on the 3D printing for MASMs such as magneto-active polymers (MAPs), magneto-active composites, and magneto-active hydrogels (MAHs) on the booming development of various smart and flexible devices such as soft robots, wearable electronics, and biomimetic devices. Moreover, 3D-printed soft robotics have an outstanding capacity to adapt to complicated situations for many advanced actuating applications. Finally, some current challenges and emerging areas in this exciting technology have been proposed. Lastly, it is anticipated that technological advancements in developing smart and intelligent magneto-active structures will have a significant impact on the design of real-world applications. 

3D printing of magneto-active smart materials for advanced actuators and soft robotics applications

Abstract 4D printing technology combines 3D printing and stimulus-responsive materials, enabling construction of complex 3D objects efficiently. However, unlike smart soft materials, 4D printing of ceramics is a great challenge due to the extremely weak deformability of ceramics. Here, we report a feasible and efficient manufacturing and design approach to realize direct 4D printing of ceramics. Photocurable ceramic elastomer slurry and hydrogel precursor are developed for the fabrication of hydrogel-ceramic laminates via multimaterial digital light processing 3D printing. Flat patterned laminates evolve into complex 3D structures driven by hydrogel dehydration, and then turn into pure ceramics after sintering. Considering the dehydration-induced deformation and sintering-induced shape retraction, we develop a theoretical model to calculate the curvatures of bent laminate and sintered ceramic part. Then, we build a design flow for direct 4D printing of various complex ceramic objects. This approach opens a new avenue for the development of ceramic 4D printing technology. 

Direct 4D printing of ceramics driven by hydrogel dehydration


 Multimaterial (MM) 3D printing shows great potential for application in metamaterials, flexible electronics, biomedical devices and robots, since it can seamlessly integrate distinctive materials into one printed structure. Among numerous MM 3D printing technologies, digital light processing (DLP) MM 3D printing is compatible with a wide range of materials from hydrogels to ceramics, and can print MM 3D structures with high resolution, high complexity and fast speed. This paper introduces the fundamental mechanisms of DLP 3D printing, and reviews the recent advances of DLP MM 3D printing technologies with emphasis on material switching methods and material contamination issues. It also summarizes a number of typical examples of DLP MM 3D printing systems developed in the past decade, and introduces their system structures, working principles, material switching methods, residual resin removal methods, printing steps, as well as the representative structures and applications. Finally, we provide perspectives on the directions of the further development of DLP MM 3D printing technology.

Digital light processing based multimaterial 3D printing: challenges, solutions and perspectives

Polymer-derived ceramic (PDC) combined with 3D printing technology allows for the fabrication of various complex 3D structures and components that cannot be achieved through conventional ceramic preparation methods. However, current 3D printing PDC technologies suffer from low accuracy, poor mechanical and low ceramic yield, which severely limit their application values. Here, we report a polysiloxane-based preceramic polymer (PCP) precursor capable of producing high-precision and high-strength SiOC ceramics with ceramic yield as high as 56.9% after pyrolysis at 1100℃ in vacuum. Adding butyl acrylate into the precursor significantly improves printing resolution. Through special material design and printing parameter optimization, we fabricate various complex structures with overall sizes ranging from submillimeters to centimeters and printing resolution as high as 5 μm via vat photopolymerization 3D printing. The compressive strength of SiOC ceramic with I-WP structure reaches up to 240 MPa while its density is only 0.367 g/cm3, corresponding to a specific strength as high as 6.54 × 105 N·m/kg. The printing resolution, specific strength and ceramic yield are all at the top level compared with previously reported works. The proposed approach will greatly promote the industrial application of 3D printing PDC technology in engineering fields and extreme environments. 

Vat photopolymerization 3D printing of polymer-derived SiOC ceramics with high precision and high strength

Ceramic materials are highly regarded for their exceptional chemical and mechanical stability; however, their inherent brittleness limits their resistance to impact fractures. Researchers have investigated the design of biomimetic ceramic structures to overcome this problem, demonstrating the ability to toughen ceramics and expand their applications. However, the complexity of these structures makes the implementation of traditional processing methods challenging. To address this problem, a new preparation method for high-strength and high-toughness biomimetic ceramic structures is proposed. First, the primitive structure with the best mechanical performance was selected as the experimental object. Second, the vat photopolymerization printing process was optimized; further, primitive structures were designed with different wall thicknesses to investigate the effects of graded and variable-graded structures on the performance of triply periodic minimal surface structures. Finally, a polymer was used to impregnate the ceramic parts to prepare biomimetic structures. In the experiments, the biomimetic structures outperformed their pure ceramic counterparts in terms of toughness and avoided catastrophic failure. In particular, when the ceramic had graded structures, the energy absorption capability of the part increased by 202%. Finite-element modeling was used to analyze the stress concentration and distribution. Peridynamic simulations provided insights into the strengthening and toughening mechanisms at the micro level and guidance for developing damage-resistant, high-energy-absorbing materials. 

Method for preparing biomimetic ceramic structures with high strength and high toughness

Ultrastrong and damage-tolerant ceramic architectures via 3D printing

Efficient toughening of the interlaminar fracture toughness for CFRP composites without sacrificing in-plane mechanical performance remained an unresolved challenge. Here, we present a synergistic toughening strategy by construction of hierarchical architecture, within which carbon nanotubes (CNTs) in-depth stitched into the nano-channel between neighboring carbon fibers at the interlaminar region. Mode I fracture test revealed that, even at low concentration of CNTs (0.3 wt%), a considerable improvement (50%) on mode I fracture energy GIc of composites can be realized, from 1098.6 to 1647.8 J/m2 which is nearly two times greater than that of most aerospace CFRP laminates (∼500 J/m2). The excellent fracture toughness is predominantly attributed to the desired hierarchical architecture, which simultaneously triggered the intrinsic toughening by CNTs nano-bridging and extrinsic toughening mechanisms due to carbon fiber bridging, as was demonstrated by SEM images of fracture surfaces and further verified by finite element simulations. These findings offer significant guidelines for designing CFRP composites with high fracture toughness by application of low content CNTs using cost-effective resin mixing process. 

Synergistic toughening on CFRP via in-depth stitched CNTs

Abstract Active metamaterials with shapes or mechanical properties that can be controlled remotely are promising candidates for soft robots, flexible electronics, and medical applications. However, current active metamaterials often have long response times and short ranges of linear working strains. Here, we demonstrate magnetoactive microlattice metamaterials constructed from 3D-printed, ultra-flexible polymer shells filled with magnetorheological (MR) fluid. Under compressive stress, the magnetorheological fluid develops hydrostatic pressure, allowing for a linear compression strain of more than 30% without buckling. We further show that under a relatively low magnetic field strength (approximately 60 mT), the microlattices can become approximately 200% stiffer than those in a relaxed state, and the energy absorption increases ~16 times. Furthermore, our microlattices showed an ultra-low response time with “field on” and “field off” times of ~200 ms and ~50 ms, respectively. The ability to continuously tune the mechanical properties of these materials in real time make it possible to modulate stress‒strain behavior on demand. Our study provides a new route toward large-scale, highly tunable, and remotely controllable metamaterials with potential applications in wearable exoskeletons, tactile sensors, and medical supports. 

Magnetoactive microlattice metamaterials with highly tunable stiffness and fast response rate

Gradient evolution in graphene reinforced carbon/carbon composites

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Ultrastrong and damage-tolerant ceramic architectures via 3D printing

Synergistic toughening on CFRP via in-depth stitched CNTs

Magnetoactive microlattice metamaterials with highly tunable stiffness and fast response rate

Gradient evolution in graphene reinforced carbon/carbon composites