Stimuli‐responsive polymers (SRPs) are special types of soft materials, which have been extensively used for developing flexible actuators, soft robots, wearable devices, sensors, self‐expanding structures, and biomedical devices, thanks to their ability to change their shapes and functional properties in response to external stimuli including light, humidity, heat, pH, electric field, solvent, and magnetic field or combinations of two or more of these stimuli. In recent years, additive manufacturing (AM) aka 3D printing technology of these SRPs, also known as 4D printing, has gained phenomenal attention in different engineering fields, thanks to its unique ability to develop complex, personalized, and innovative structures, which undergo twisting, elongating, swelling, rolling, shrinking, bending, spiraling, and other complex morphological transformations. Herein, an effort has been made to provide insightful information about the AM techniques, type of SRPs, and their applications including, but not limited to tissue engineering, soft robots, bionics, actuators, sensors, construction, and smart textiles. This article also incorporates the current challenges and prospects, hoping to provide an insightful basis for the utilization of this technology in different engineering fields. It is expected that the amalgamation of 3D printing with SRPs would provide unparalleled advantages in different engineering arenas. 

Recent advances in the additive manufacturing of stimuli‐responsive soft polymers

Study of the bending properties of variable stiffness chain mail fabrics

In this study, three types of cellular metamaterials with negative/zero/positive Poisson's ratio and vibration isolation effects were devised and fabricated based on 4D printing of shape memory polymers. The compressive properties and energy absorption capacity of three cellular metamaterials at variable temperatures were evaluated by finite element simulations and compression experiments. To explore the application possibilities of 4D printed cellular metamaterials in deformable structures, shape recovery experiments were carried out, and the results showed that the three types of cellular metamaterials exhibited a high shape recovery rate. The vibration modes and vibration isolation capabilities of 4D printed cellular metamaterials were investigated for three types of cellular metamaterials with negative/zero/positive Poisson's ratio. In addition, based on the deformation reversible capability of the structure, the vibration isolation capability of three cellular metamaterial structures was investigated under different deformation stages. The results demonstrate that the cellular metamaterials with zero Poisson's ratio possess superior vibration isolation capability compared to negative or positive Poisson's ratio cellular metamaterials at different deformation stages by a comprehensive analysis. The methods and conclusions presented in this study will guide the design of functionalized cellular metamaterials with high energy absorption, strong vibration isolation, and autonomous deformation ability. 

Compression behavior of 4D printed metamaterials with various Poisson's ratios

Four-dimensional (4D) printing has attracted tremendous worldwide attention during the past decade. This technology enables the shape, property, or functionality of printed structures to change with time in response to diverse external stimuli, making the original static structures alive. The revolutionary 4D-printing technology offers remarkable benefits in controlling the geometric and functional reconfiguration, thereby showcasing immense potential across diverse fields, including biomedical engineering, electronics, robotics, and photonics. Here, a comprehensive review of the latest achievements on 4D printing using various types of materials and different additive manufacturing techniques is presented. The state-of-the-art strategies implemented in harnessing various 4D-printed structures are highlighted, which involve materials design, stimuli, functionalities, and applications. The machine learning approach explored for 4D printing is also discussed. Finally, the perspectives on the current challenges and future trends toward further development in 4D printing are summarized. This article is protected by copyright. All rights reserved.

Recent Advances in 4D Printing of Advanced Materials and Structures for Functional Applications.

4D printing was initially defined as multi-material 3D printing that can produce pre-programmed shape changes under external stimuli over time. Thus extensive research relies on multi-material-based design to manipulate the deformation of 4D printed objects. An easy to neglect fact is that printing parameters are also dominant factors affecting the microstructure and properties of the printed objects. Parameter-encoded programming strategy endows 3D printing with exclusive capabilities in building site-specific gradient/ordered structures, and thus confers preprogrammed anisotropic/nonlinear shape morphing responses to printed structures. In this work, we provide a comprehensive review of recent advances in parameter-encoded 4D printing and highlight how the structure-property of the printed materials was refashioned by exerting control over the process parameters. The review expects to shed light on understanding the role of parameters in 4D printing and opens up a new design space through parameter-encoded design, which helps truly unleash the potential of this emerging and promising technology. 

Understanding the role of process parameters in 4D printing: A review

Shear-induced mesogen alignments are commonly used to program the morphing behaviors of liquid crystal elastomers (LCEs) in the material extrusion process of 4D printing. The 4D printing parameters are intimately relevant to the alignment degrees and actuation strains of printed LCEs filaments. Nevertheless, there is still a lack of systematic research on the interaction of 4D printing parameters on shape morphing behavior of LCEs so far. Here, the multi-parameter-encoded 4D printing is proposed for systematically uncovering the process-structure-property relationships and programming the shape morphing behaviors of LCEs. The impact of multiple parameters (print speed, extrusion pressure, printing height, and UV light intensity) on the filament shape and actuation strains are analyzed using experiments and simulations, respectively. The role of UV light intensity on the TNI has been revealed and adopted for programming the threshold temperature, immediate state and actuation strain of printed LCEs. To demonstrate the feasibility of the multi-parameter-encoded 4D printing method, various parameters codes are integrated into the samples with the same geometric features and successfully enable distinct localized shape morphing behaviors. The multi-parameter-encoded 4D printing method allows on-demand modulation of local deformation features of LCEs structures solely through parametric design, thus providing additional design freedom and versatile tools for building soft robotics, artificial muscles, flexible electronics, etc. 

Multi-parameter-encoded 4D printing of liquid crystal elastomers for programmable shape morphing behaviors

Stiffness-tunable and self-sensing integrated soft machines based on 4D printed conductive shape memory composites

/pdf/fiber-dominated-soft-actuators-inspired-by-plant-cell-walls-2ih4f7mb.pdf

Fiber-dominated Soft Actuators Inspired by Plant Cell Walls and Skeletal Muscles

4D Printing of Polymer‐Dispersed Liquid Crystal Elastomers toward Tunable Vibration Isolation and Cushioning Properties

Rotational 3D printing spruce-inspired helical structural material with tunable mechanical properties

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