# Mechanism and manufacturing of 4D printing: derived and beyond the combination of 3D printing and shape memory material

Pei Feng, Feng Yang, Jiye Jia, Jing Zhang, Wei Tan, Cijun Shuai

► To cite this version:

Pei Feng, Feng Yang, Jiye Jia, Jing Zhang, Wei Tan, et al.. Mechanism and manufacturing of 4D printing: derived and beyond the combination of 3D printing and shape memory material. International Journal of Extreme Manufacturing, 2024, 6, pp.062011. 10.1088/2631-7990/ad7e5f. hal-05237543

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TOPICAL REVIEW • OPEN ACCESS

# Mechanism and manufacturing of 4D printing: derived and beyond the combination of 3D printing and shape memory material

To cite this article: Pei Feng et al 2024 Int. J. Extrem. Manuf. 6 062011

View the [article online](#) for updates and enhancements.

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Topical Review

# Mechanism and manufacturing of 4D printing: derived and beyond the combination of 3D printing and shape memory material

Pei Feng<fnref n="1" /> ID, Feng Yang<fnref n="1" />, Jiye Jia<fnref n="1" />, Jing Zhang<fnref n="2" />, Wei Tan<fnref n="3" />,* and Cijun Shuai<fnref n="1" />,4,5,* ID

E-mail: tanwei819@126.com and shuai@csu.edu.cn

Received 24 April 2024, revised 25 June 2024

Accepted for publication 23 September 2024

Published 8 October 2024

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CrossMark

Abstract

Four-dimensional (4D) printing is regarded as a methodology that links 3D printing to time, which is characterized by the evolution of predetermined structures or functions for the printed object after applying stimulation. This dynamic feature endows 4D printing the potential to be intelligent, attracting wide attention from academia and industry. The transformation of shape and function is both obtained from the programming of the object endowed by the intrinsic characteristics of the material or by the manufacturing technology. Therefore, it is necessary to understand 4D printing from the perspective of both mechanism and manufacturing. Here, the state-of-the-art 4D printing polymer was summarized, beginning with the classifications, and leading to the mechanisms, stimulations, and technologies. The links and differences between 4D printing polymer and shape memory polymer, between 4D printing and 3D printing were highlighted. Finally, the biomedical applications were outlined and the perspectives were discussed.

Keywords: 4D printing, mechanism, manufacturing, biomedical application

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<footnote n="1">¹ State Key Laboratory of Precision Manufacturing for Extreme Service Performance, College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, People's Republic of China</footnote>

<footnote n="2">² LaTIM, INSERM-UMR1101, University of Brest, Brest 29200, France</footnote>

<footnote n="3">³ Department of Spine Surgery, The Third Xiangya Hospital of Central South University, Changsha 410013, People's Republic of China</footnote>

<footnote n="4">⁴ Institute of Additive Manufacturing, Jiangxi University of Science and Technology, Nanchang 330013, People's Republic of China</footnote>

<footnote n="5">⁵ College of Mechanical Engineering, Xinjiang University, Urumqi 830017, People's Republic of China</footnote>

<footnote>* Authors to whom any correspondence should be addressed.</footnote>

<footnote>Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.</footnote>
# 1. Introduction

The concept of 4D printing was first proposed based on the case of 3D printing shape memory polymer (SMP) object, where the object could change shape in response to external stimulation [1]. Nowadays, the raw materials for 4D printing have been extended to metals [2–6], polymers [7–9] and ceramics [10–12], and developed rapidly in various fields especially in biomedical applications [13–18]. The type of shape transformation for the 4D printed object becomes more complex and diverse, and extends to the transformation of function. The key step throughout the whole 4D printing is ‘program-stimulate’: ‘program’ determines the shape or function of the object before and after transformation, and it can be carried out during the manufacturing process or independently of the manufacturing process; ‘stimulate’ is carried out after the fabrication of the object, and it reflects the mechanism of 4D printing and affects the application of 4D printing.

‘Program-stimulate’ is just the icebergs on the sea, and below the sea is the ‘mechanism-manufacturing’, which is derived from the basis of 4D printing: smart materials and 3D printing. However, 4D printing is not equal to smart materials fabricated by 3D printing. The main difference between 4D printing and 3D printing is the programming of printed objects by using the intrinsic properties of smart materials or manufacturing technology. The ‘mechanism’ involves the development of smart materials and research on the relationship between the process parameters and the properties of the printed objects, which mainly involves materials, chemistry and mechanics. The ‘manufacturing’ involves the design and fabrication of various structures and the development of manufacturing technologies, which mainly involve manufacturing and computer science. Therefore, the integration of multiple disciplines is inevitable for the understanding of 4D printing.

Previous review articles on 4D printing were mainly available based on the perspective of ‘mechanism’ or ‘manufacturing’. Some emphasized the perspective of smart materials, discussed the synthesis and mechanisms of smart materials and then explored their application in 4D printing [19–21]. Others focused on the perspective of manufacturing technologies or the structure design of printed objects [22–24]. There were also review articles highlighting the perspective of stimulation or the application of 4D printing [25–27]. In other words, the previous reviews mapped 4D printing mainly deriving from the perspective of smart materials or 3D printing.

This work is a comprehensive and systematic review of 4D printing by analyzing the latest research status, based on the ‘mechanism-manufacturing’ from the perspective of 4D printing itself with polymer as raw material. The main purpose of this work is to clarify the following three questions: what is the 4D printed polymer object, why does the 4D printed polymer object achieve the evolution of predetermined structure or function, and how to achieve the 4D printed polymer object? In section 2, the difference between direct 4D printing and general 4D printing is emphasized for the first time, and the characteristics of two-way 4D printing are comprehensively

summarized for the first time to distinguish the concepts of two-way shape transformation and formal reversible transformation of shape. In section 3, the mechanisms of 4D printed polymer objects are comprehensively and systematically summarized, from structural design to shape memory properties, from conventional SMP to thermadapt SMP and supramolecular SMP. In section 4, the types of stimulation in 4D printing are summarized as the reflection of the mechanism and the application of 4D printing. In section 5, based on 3D printing technologies, three different preparation strategies in 4D printed polymer are proposed for the first time, and the influence of different manufacturing processes on 4D printing properties is emphasized. In section 6, the application of 4D printing in biomedical field is mainly discussed with three typical tissues, including vessel with characteristics of micro-structure, heart with characteristics of elastic property, and bone with characteristics of load-bearing function. It is hoped that this work will provide a direct understanding of 4D printed polymer, and clarify the links and differences between 4D printing and smart materials, and between 4D printing and 3D printing.

The overview of this review is displayed in figure 1. Specifically, the types of 4D printing are first introduced by drawing on the classification of SMP, and a brief discussion on the theoretical models of 4D printing is also presented. Then the mechanisms of 4D printing are focused, and three categories, including mechanisms of 4D printing based on stress or material heterogeneity, based on stress relaxation, and based on entropy elasticity, are introduced to highlight the links and differences between 4D printing and smart materials. The type of stimulation is briefly introduced by being divided into heat-related and heat-unrelated. After that, the manufacturing technologies are emphatically discussed according to three categories: technologies of 4D printing based on SMP, based on process parameter-encoded, and based on multi-material printing, to highlight the similarities and differences between 3D printing and 4D printing. The discussion on the biomedical applications of 4D printing is then presented. Finally, the perspectives of 4D printing are proposed.

# 2. Classifications

The whole process of 4D printing can be considered as transformations between multiple shapes or functions of the same polymer object (caused by ‘program-stimulate’), where permanent and temporary shapes are the two basic constituents. There is only one permanent shape during a transformation (more precisely, a complete ‘program-stimulate’ process), but it is possible to exhibit multiple temporary shapes. It should be noted that some polymers (such as polymers containing dynamic covalent bonds) possess the ability to reconstruct permanent shapes, but the changes of permanent shape are independent of the 4D printing. The 4D printed polymer object can realize the transformation of ‘permanent shape-temporary shape-permanent shape’, while the 3D printing of recyclable or reprocessable polymer object can only realize
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Overview of this review of 4D printing polymer.</figcaption></figure>

the transformation of one permanent shape to another permanent shape [28–31]. This is attributed to that there is a component in the former remaining unchanged and being used to remember the permanent shape of the object, while no component remembers the original shape of the object in the latter, making the shape change irreversible. This manifests that different permanent shapes can not exist in the same complete ‘program-stimulate’ process of 4D printing, and the permanent shape in the 4D printing process can still be regarded as only one.

Numerous temporary shapes and the many possible transformation types between permanent and temporary shapes extend the classifications of 4D printing. By drawing on the classification of SMP [32–36], 4D printing can be divided into two categories according to whether two-way shape transformations can be made between different shapes (or functions) of the object: one-way 4D printing and two-way 4D printing, as shown in figure 2, in which they can be further divided into dual 4D printing and multi 4D printing, from the perspective of the total amount of permanent and temporary shapes in a transformation of the 4D printing process.

## 2.1. One-way 4D printing

One-way means that the shape transformation of the object occurs when applying the stimulation, and the object remains the transformed shape after the stimulation is removed. Among them, one-way dual 4D printing is the most common, containing one permanent shape and one temporary shape during a complete ‘program-stimulate’ process. In general, the temporary shape can be programmed to any shape, where the shape transformation is based on the properties of raw materials, usually SMP [37–39]. The printed object is in a permanent shape and can be programmed to any temporary shape. Generally, the ‘program-stimulate’ of 4D printing is shown in figure 3(a). The object can carry out several ‘program-stimulate’ processes to achieve many transformations of ‘permanent shape-temporary shape-permanent shape’. Although the temporary shape varies in different transformation processes, it still belongs to dual 4D printing because there is only

one temporary shape during a complete ‘program-stimulate’ process. For example, Fang et al [39] fabricated a polymer elephant-like object in that the shape of the elephant’s trunk could be programmed to any temporary shape during different programming processes and then return to its original shape when subjected to thermal stimulation, as shown in figure 3(c). In this scenario, the programming and manufacturing processes are separated, so that not only can 4D printing be realized based on 3D printing manufacturing technology thereby reducing the difficulty of manufacturing, but also the diversity of temporary shapes makes the application more widely and expands the flexibility of 4D printing.

For one-way dual 4D printing, there is a special case that is commonly referred to as direct 4D printing [40–43, 48–64], and the ‘program-stimulate’ is shown in figure 3(b). Specifically, the temporary shape and permanent shape of the object have been determined during the manufacturing process by the process parameters and material properties. The obtained object is in a temporary shape which will be transformed to a permanent shape after applying stimulation, and then the ‘program-stimulate’ process can no longer be carried out. For tuning the process parameter, Wang et al [40] realized direct 4D printing of polymer objects by changing the printing rates in fused deposition modeling (FDM), as shown in figure 3(d). The upper and lower layers of the original right triangle structure, which consisted of four isosceles blades, were constructed at different rates. The blades would be transformed into windmill shapes when heated. Similarly, van Manen et al [41] achieved complex 4D printing deformation of circular tubes by changing the print path in FDM, as shown in figure 3(e). For tuning the distribution of materials, Ding et al [42] realized the preparation of 1D composite rods by 3D printing, as shown in figure 3(f). Due to the different storage strains of the two components in the composite rods caused by material characteristics during the manufacturing process, the fabricated 1D rod mesh was transformed into a spherical shape after being heated. Zhang et al [43] reported an interesting 4D printing strategy that transformed a flat structure into facial masks by precisely controlling the distribution of different materials, as shown in figure 3(g).
<figure><img src="image_5.png" /><figcaption>**Figure 2.** The diagram of 4D printing. (a) One-way 4D printing. (b) Two-way 4D printing.</figcaption></figure>

It is worth noting that not all direct 4D printing is one-way, and a few can be two-way under certain stringent requirements, which will be discussed in section 2.2. An important feature of direct 4D printing is that the temporary and permanent shapes can only be programmed in the manufacturing process, which leads to more stringent requirements for the manufacturing technologies to obtain the required shape. Specifically, the technologies used in the research of Wang et al [40] and van Manen et al [41] were based on process parameter-encoded, more specifically, changing the orientation of molecular chains by changing the printing path, which in turn led to the shape transformation of printed objects under external stimulations through the heterogeneity of residual stress generated during the manufacturing process. The 4D printing technologies used in the research of Ding et al [42] and Zhang et al [43] were based on multi-material printing using the heterogeneity of material properties to achieve the shape transformation of printed objects in response to external stimulations. The mechanism of direct 4D printing will be summarized in section 3.1, and the manufacturing techniques used for direct 4D printing will be summarized in sections 5.2 and 5.3, where the strategies to achieve the evolution of predetermined structure or function for printed object will be highlighted.

One-way multi 4D printing refers to the transformation of a permanent shape and multiple temporary shapes during a complete 'program-stimulate' process. It can be achieved by printing a plurality of different materials with a one-way dual-shape transformation capability, or by printing a single SMP with a

one-way multi-shape transformation capability [65–69]. For example, Peng et al [65] developed a polymer with two distinct temporary networks and a broad transition window and fabricated an object with triple-shape memory using 3D printing. And a block polymer object prepared by Wang et al [66] also exhibited triple-shape memory. The 'multi-shape' capability is derived from SMP, and in fact, a large number of related studies of SMP have been reported, which lays a rich foundation for one-way multi-shape 4D printing. One-way multi-4D printing embodies the trait of sequential deformation [44, 45, 70, 71], specifically, the shape of the object will be changed correspondingly when applying different types of stimulation because the stimulation conditions required for different temporary shape transformations are not the same. As shown in figure 3(h), Chen et al [44] used FDM to fabricate objects with quadruple shape memory, where the petals or arms were spread out successively when the flower-like object and the three-arm-like object were immersed in water at 65 °C, 80 °C and 95 °C. Wan et al [45] prepared an object with a quintuple shape memory, which was programmed four times and sequentially recovered to the shape before the previous programming until the initial permanent shape when subjected to alternating magnetic field and warm water, as shown in figure 3(i).

It can be found that dual and multi 4D printing focuses on reflecting the overall shape transformation of the object. For the local transformation in the object, there are two kinds of deformation worth discussing: gradient deformation [46, 72] and selective deformation [47]. The gradient deformation
<figure><img src="image_6.png" /><figcaption>**Figure 3.** One-way 4D printing. The diagram of the ‘program-stimulate’ process for (a) general 4D printing, and (b) direct 4D printing. (c) One-way dual 4D printing with different temporary shapes during different programming processes. Reprinted from [39], © 2022 Elsevier B.V. All rights reserved. Direct 4D printing realized by (d) changing the printing rates in FDM. [40] John Wiley & Sons. © 2022 Wiley-VCH GmbH. (e) Changing the print path in FDM. Reproduced from [41]. CC BY 4.0. (f)–(g) Controlling the distribution of different materials. Reproduced with permission from [42]. © 2017 The Authors. Published by Elsevier Ltd. CC BY-NC-ND 4.0. [43] John Wiley & Sons. © 2021 Wiley-VCH GmbH. Multi 4D printing with (h) quadruple shape memory. Reproduced from [44] with permission from the Royal Society of Chemistry. (i) Quintuple shape memory. Reprinted from [45], © 2022 Elsevier B.V. All rights reserved. Four-dimensional (4D) printing with (j) gradient deformation. Reprinted from [46], © 2023 Published by Elsevier B.V. (k) Selective deformation. Reprinted from [47], © 2024 Published by Elsevier Ltd.</figcaption></figure>
refers to the different positions of the object gradually deforming in sequence, and usually occurs when the degree of stimulation on different positions of the object is different, in other words, the intensity of stimulation is different on different positions of the object (this is equal to that the intensity of the stimulation is the same but the time of applying is different). For example, Zhang *et al* [46] prepared a polymer object with an electrical response, and the different parts of the object possessed different abilities to generate Joule heat by controlling the content of the embedded conductive layer, as shown in figure 3(j). And there was a temperature gradient in different parts of the object after electrification, only when the temperature rose to glass transition temperature (T<sub>g</sub>), the corresponding part could be recovered from the temporary shape to the original shape. Selective deformation refers to the stimulation acting only locally on the object and making it deform, while the rest of the object does not deform until being stimulated. For example, Lin *et al* [47] reported an octopus-like polymer object fabricated by FDM, which recovered the temporary shape to its permanent shape when light hit the specific antennae of an octopus, while the unilluminated antennae remained temporary until exposed to light, as shown in figure 3(k). This kind of selective deformation puts a high demand on the spatiotemporal controllability of stimulation.

## 2.2. Two-way 4D printing

Two-way 4D printing possesses the following three characteristics: (I) two-way 4D printing is a type of 4D printing, that undergoes the process of 'program-stimulate'; (II) the temporary shape of the 4D printed object can be fixed after removing the stimulation and external force; (III) the shape transformation of the 4D printed object occurs when applying a stimulation, and the object will return to the shape before being stimulated when the stimulation is removed, where this shape transformation can be achieved repeatedly. The first characteristic is used to distinguish two-way 4D printing from simple shape deformation such as thermal expansion and contraction, which will be discussed in section 3.1 of this manuscript. The combination of the second and third characteristics is used to distinguish two-way 4D printing from formal reversible transformation such as magnetic stimulation and swelling/deswell-ing structures, which will be discussed through case analysis in this section. The third characteristic is used to distinguish two-way 4D printing from one-way 4D printing, and the classifications of two-way 4D printing will be discussed in this section. It should be noted that the shape change of two-way 4D printing is referred to as two-way shape transformation rather than reversible transformation in this section.

The whole process of two-way 4D printing is displayed in figure 2(b). To the best of the authors’ knowledge, regarding the two-way transformation of two shapes in 4D printing, 4D printed shape memory alloy can realize the two-way shape transformation between a permanent shape and a temporary shape, while 4D printed SMP can only realize the two-way shape transformation between two temporary shapes. In addition, two-way shape transformation between temporary

and permanent shapes of 4D printed polymer object can be achieved by designing structures or multi-material distributions. Herein, two-way 4D printing is classified as quasi-two-way 4D printing, non-programmable stress-free two-way 4D printing, and programmable stress-free two-way 4D printing. The difference between quasi-two-way and stress-free two-way is whether an external force is needed in the two-way shape transformation process, and the difference between programmable and non-programmable is whether the two shapes that undergo the two-way shape transformation can be reprogrammed. It should be noted that the changes in the degree of deformation for 4D printed polymer object due to changes in the intensity of the stimulation are still classified as non-programmable.

The two-way shape transformation is mainly determined by material characteristics and structure design and has nothing to do with the manufacturing method. Therefore, much research has been done on developing new materials for two-way SMP, but the case of 4D printing is very few. Whether or not it belongs to 4D printing, the two-way shape transformation is achieved mainly by crystallization-induced elongation (CIE) and melting-induced contraction (MIC), and the key is to construct the oriented crystallization of anisotropy polymer network [73, 74]. This phenomenon was first discovered in liquid crystal elastomer (LCE) [75]. And for the polymer, this key can be achieved by a constant external load. In this scenario, the steps are the same as that of one-way 4D printing except for one difference, where the loaded external force during programming to make the object transformation needs to be maintained throughout the subsequent 'program-stimulate' process. Under the constant external force, the polymer chain will be reoriented along the direction of force in the melting state, and crystallize to maintain the orientation below transition temperature (T<sub>trans</sub>). And reversible shape transformation can be obtained due to CIE and MIC. As shown in figure 4(a), Bonetti *et al* [76] reported that polymer objects prepared by FDM possessed significant CIE and MIC effects, and exhibited reversible shape transformation under constant external force. As a result of the requirement for a constant external force to achieve two-way 4D printing, this situation is also called quasi-two-way 4D printing. However, this requirement limits its application, requiring the construction of a stress-free two-way 4D printing.

An internal stress can replace the constant external force to realize the construction of an anisotropic polymer network thus the two-way shape transformation. A common approach is to construct a bilayer laminate structure object, which typically includes an elastomer and a pre-stretched SMP [77, 82, 83]. Although the bilayer structure object is stress-free to achieve two-way 4D printing, the manufacturing process is relatively complex, and the deformation mode (such as bending or buckling) is very simple. More importantly, the two reversible transformation shapes cannot be programmed later because the internal stress of the object has been determined during the manufacturing process or through material properties differences, therefore, these cases are classified as non-programmable stress-free two-way 4D printing.
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Two-way 4D printing and SMP. Two-way 4D printing achieving by (a) constant external force. Reproduced from [76]. CC BY 4.0. (b) Constructing bilayer laminate structure. Reprinted from [77], © 2021 Elsevier Ltd. All rights reserved. (c) Semi-crystalline polymers. Reprinted from [78], © 2021 Elsevier B.V. All rights reserved. Two-way shape transformation of polymer with (d) multiphase material with two $T_m$. [79]. John Wiley & Sons. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Single-phase material with a wide $T_m$. Reproduced with permission from [80]. Freely available online through the PNAS open access option. (f) Two-way shape transformations of SMP attributed to converse MIC and CIE. Reproduced from [81] with permission from the Royal Society of Chemistry.</figcaption></figure>

Here, the two-way shape transformation is mainly achieved by the structure design, and the detailed process and mechanism of reversible shape transformation will be discussed in the next chapter. As shown in figure 4(b), Wang and Li [77] fabricated a bilayer laminated structure object containing a layer of elastomer and a layer of SMP, in which the direction of pre-strain deformation stored in the SMP layer could be programmed by the print path of FDM, thus obtaining a relative complex reversible shape transformation. Many similar bilayer laminate structure objects can achieve two-way shape transformation sharing the same mechanism for SMP, including bilayer of pre-stretched elastomers and SMP, double-layer structures of SMP, and core-shell structures of SMP and elastomers, which are well documented in these works of literature [84–89].

A similar process may also be presented in direct 4D printing, as mentioned above, a few cases of direct 4D printing can also be two-way. Since there is the possibility that the object may be recovered to its printed shape by other means after being stimulated and may be deformed again by applying stimulation. For example, Lee *et al* [90] prepared a bilayer structural object containing TangoBlackPlus and VeroWhitePlus by polyjet and used the asymmetric expansion of the two materials in heated ethanol to achieve deformation, and then restored the original shape of the object by dry heating, with complex stimulation steps compare to only changing temperature. Although the permanent and temporary shapes of the object could not be changed, the object did achieve reversible switching between the two shapes under complicated conditions. In detail, at room temperature, the TangoBlackPlus
with a high swelling ratio exhibited a lower stiffness than the VeroWhitePlus, resulting in little deformation when placing the printed bilayer structural object in ethanol (this process was more clearly described in another related work by Lee *et al* [83]). When the temperature increased, the stiffness of TangoBlackPlus changed little, while the stiffness of VeroWhitePlus decreased and was significantly lower than that of TangoBlackPlus, resulting in significant deformation due to the difference in swelling ratio. At this point, an external force was applied to maintain a temporary shape of the object, and the temperature was lowered to room temperature, then the external force was removed, and the temporary shape of the object was maintained due to the shape memory effect (SME) and the stiffness recovery of VeroWhitePlus. When the temperature rose again and the ethanol evaporated, the shape of the object recovered to its permanent shape due to the reverse stiffness effect (RSE) and the deswelling of material. Based on the above analysis, the following three elements are needed to achieve the two-way shape transformation between a permanent shape and a temporary shape of a 4D printed object: (I) there is a large difference in the swelling ratio of the two materials that constitute the object, which can cause deformation; (II) RSE; (III) material with higher stiffness at room temperature possessed SME.

To distinguish between two-way shape transformation and formal reversible transformation of shape, two points in particular need to be made here. For one thing, the object of two-way 4D printing possesses the ability to maintain a temporary shape after the removal of external stimulation and external force. This ability is also an important difference between SMP and other smart materials such as magnetostriction and piezoelectric materials, where the latter two exhibit pulse-like transformation of shape or function in response to external stimulation. For example, a magnetostrictive object will undergo volume-preserving deformation when stimulated by an alternating magnetic field, and the deformation will disappear once the alternating magnetic field is withdrawn. For another thing, the shape transformation of the 4D printed object occurs when applying a stimulation, and the object will return to the shape before being stimulated when the stimulation is removed. And this shape transformation can be achieved repeatedly with a constant force (quasi-two-way) or without external force (stress-free two-way). Some magnetic materials and swelling/deswelling structures can change shape in response to external stimulation [91–93], but they cannot maintain temporary shapes after the removal of external stimulation, making them outside the scope of two-way. A typical case that does not fit this situation is magneto-thermomechanically reprogrammable mechanical metamaterials reported by Zou *et al* [94]. To realize the repetitive reversible transformation of two shapes, a static magnetic field must be applied to provide the deformation force of the object when programmed and then withdrawn to make the recovery of shape, thus making it outside of the scope of two-way. However, there is no doubt that Zou *et al*'s work provided a valuable approach to achieving a reversible transformation of object in a contactless way.

For non-programmable stress-free two-way 4D printing, the two shapes of the two-way shape transformation cannot be programmed later. The introduction of programmable internal stress to construct an anisotropic polymer network is an effective way to solve the above problems, and this scenario is categorized as programmable stress-free two-way 4D printing. Semi-crystalline polymers are usually used as raw materials [78, 79, 95–101]. Initially, Behl *et al* [79] found that copolymers containing two different melting temperatures ($T_m$, denoted as $T_{m-low}$ and $T_{m-high}$, and the corresponding polymer networks denoted as network A and network B, respectively) exhibited two-way shape transformation without external force, as shown in figure 4(d). It was found that when the temperature rose between $T_{m-low}$ and $T_{m-high}$, network A appeared in a melting state while network B was still in the ordered crystalline state. Due to the restriction of network B, the polymer chains of network A still maintained orientation in the melting state, where network B played the same role of constant external force in quasi-two-way 4D printing. Then the two-way shape transformation could be achieved by MIC and CIE when the temperature changes. Besides, the internal anisotropic polymer network was eliminated when the temperature rose above $T_{m-high}$, thus could be reprogrammed to change the temporary shape of two-way shape transformation. The currently reported cases of two-way 4D printing are mainly based on the above principle, and a typical case is the polymer object with two crystalline phases reported by Shi *et al* [78], as shown in figure 4(c). The object exhibited a reversible shape transformation when the temperature was switched between 0 °C and 50 °C, and when the temperature heating exceeded the higher crystallization temperature, the above reversible shape transformation was erased and could be reprogrammed to achieve a reversible shape transformation between the new two shapes. Furthermore, the same effect could be achieved using polymer networks with a wide $T_m$ range [80], where the wide $T_m$ could be seen as a continuous distribution of a series of different $T_m$, as shown in figure 4(e). To the best of our knowledge, there are currently no corresponding cases in 4D printing. It is worth noting that in addition to CIE and MIC, there are also cases where two-way shape transformations are achieved by crystallization-induced contraction and melting-induced elongation. This interesting converse MIC and CIE was achieved by Chen *et al* [81] through the design of dynamic covalent networks, as shown in figure 4(f).

In summary, two-way 4D printing has the advantages of both programmability and reversible deformation. One-time stimulation corresponding to one-time programming in one-way 4D printing has been extended to the multiple repeated stimulations corresponding to one programming in two-way 4D printing, which may meet the needs of more complex applications.

## 2.3. *Theoretical models*

The evolution behavior of the polymer object in 4D printing is complex and diverse, and the corresponding theoretical models can predict the evolution process and provide a
design basis. Some works have been done to fill this gap, such as constitutive models related to SMP [2, 3, 102–110] and finite element analysis (FEA) with more general applicability, which can be used to optimize design parameters for 4D printing more efficiently and economically [24, 111–129]. The constitutive models of SMP include three types: the macro mechanical constitutive model based on viscoelastic theory, the micromechanical constitutive model based on phase evolution theory, and the combination of the above two [33, 34]. Regarding FEA, topology optimization, machine learning, etc, are often integrated into the design of 4D printing.

The viscoelastic model, also known as the rheological model takes into account the viscoelastic and rate-dependent SMP, and can clearly describe the creep and stress relaxation behaviors of SMP, with the basic elements including spring element and dashpot element. The simplest viscoelastic models are the Maxwell model and Kelvin model, in which a spring element and a dashpot element are constructed in series and parallel, respectively. Earlier studies mainly used the standard linear solid model and a series of generalized Maxwell models have been developed based on the standard linear solid model [33, 34]. In addition, slip elements caused by internal friction are added to the linear viscoelastic equation, which was first proposed by Tobushi et al [102] in the four-element model. The phase evolution model describes the process of transformation in the microstructure by defining the frozen phase volume fraction and stored strain, which is phenomenological since there is no real phase in the polymer. The earliest phase evolution model was proposed by Liu et al [103] in 2006, which assumed that the model consisted of an active phase and a frozen phase, and introduced the frozen phase volume fraction to characterize the shape memory behavior of SMP. Specifically, when the temperature decreased, the molecular chain was frozen and the active phase was transformed into the frozen phase with the elastic strain of the active phase transformed into the storage strain of the frozen phase, that was, the inelastic strain of the frozen phase. When the temperature was increased, the frozen phase would be transformed into the active phase, and the storage strain would disappear. The phase transition model can relate the molecular segments to the phase transition of SMP, and the SME of SMP based on entropy elasticity is well explained. The rheological model and phase evolution model possess their characteristics, among which the former can well describe the mechanical behavior of SME, while the latter exhibits more explicit physical significance. And the researchers gradually begin to elaborate the shape memory mechanism of SMP more clearly by combining the two models, and successively propose three-phase constitutive models such as series and parallel [104–107]. Here a brief introduction to constitutive models is presented, and more detailed information can be found in the following works of literature [108–110]. The construction of the constitutive model can not only improve the understanding of the nature of SME behavior, but also make reasonable predictions of 4D printing behavior, and provide a theoretical basis for the analysis of 4D printing by FEA.

The FEA shows great advantages in predicting the transformation of 4D printing, optimizing the structure of object and reducing the experimental cost, especially in predicting the deformation of object in multi-material 4D printing. Many algorithms, such as topology optimization [111–116] and machine learning-based or other algorithms [43, 117–129], have been developed to achieve forward prediction of shape transformations in 4D printing and provide effective methods for reverse design in 4D printing. Some typical researches regarding FEA-based 4D printing are shown in figure 5. For the forward prediction, Zhang et al [43] used FEA to predict the deformation of a complex structure with two different material distributions, as shown in figure 5(a), and then used FEA to guide the design of more complex structural deformation. Yu et al [117] introduced isogeometric analysis into FEA to predict the deformation behavior caused by residual stress in mesh-like structures based on FDM printing, as shown in figure 5(b). Compared with the experimental results, the FEA model demonstrated high speed (real-time: 0.93 s) and truthfulness (accuracy: 99.89%). de Kergariou et al [118] combined the FEA with the printing path as input of the model to predict the deformation of the structure, as shown in figure 5(c). In addition, Hosseinzadeh et al [119] developed a ‘central composite design’ using the response surface method mathematical model for predicting the torsional and bending deformation of the printed structure by several parameters of layer height, printing speed, and nozzle diameter during FDM printing. For the guide of inverse design, Athinarayanarao et al [111] proposed a computational framework that fused the advantages of topology optimization to achieve reverse optimal material distribution design for target shape changes. The gene coding and case of the proposed computational framework are shown in figure 5(d). Peng et al [124] developed an optimized integrated framework of genetic algorithm to guide the reverse design of the active composite structure for 4D printing, as shown in figure 5(e), and the experimental results were in good agreement with the simulation results. Benyahia et al [125] proposed a computational design approach to convert multi-material 4D objects into suitable interlocking blocks, which could build any voxel-based structure, as shown in figure 5(f), and this approach exhibited good versatility and efficiency. Moreover, as shown in figure 5(g), Sun et al [129] developed a machine learning model combined with recurrent neural networks, which could effectively predict the forward deformation and reverse material design of voxelized beam structures.

## 3. Mechanisms

The focus of direct 4D printing is on how to achieve the evolution of an object to a predetermined shape or function, which relies on a combination of structural design and differences in material properties. For other types of 4D printing, the focus is on how to make the object achieve the fixation of temporary shape and the recovery of permanent shape, which depends on its shape memory properties. Polymer objects share the same mechanisms of shape transformation despite the variety of 4D printing types. To highlight the links and differences between 4D printing polymer and SMP, here the mechanisms
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Theoretical models of 4D printing. The basic process and case for forward prediction by FEA of 4D printing according to the parameter of (a) multimaterial. [43] John Wiley & Sons. © 2021 Wiley-VCH GmbH. (b) Residual stress. Reproduced with permission from [117]. © 2022 The Author(s). Published by Elsevier B.V. CC BY-NC-ND 4.0. (c) Printing path. Reproduced from [118]. CC BY 4.0. The basic process and case for the guide of inverse design by FEA of 4D printing based on (d) topology optimization. Reproduced from [111]. CC BY 4.0. (e) Genetic algorithm. Reprinted from [124], © 2023 Elsevier Ltd. All rights reserved. (f) Interlocking blocks assembly. Reprinted from [125], © 2022 Elsevier B.V. All rights reserved. (g) Forward prediction and the guide of inverse design by FEA of 4D printing based on machine learning model. [129] John Wiley & Sons. © 2021 Wiley-VCH GmbH.</figcaption></figure>

of 4D printing polymer objects are classified into three types: 4D printing based on stress or material heterogeneity, 4D printing based on stress relaxation, and 4D printing based on entropy elasticity. The former includes 4D printing mainly

determined by the manufacturing process, and direct 4D printing is based on this principle. The latter two are related to the shape memory properties of polymers, which are also the most common. It should be noted that other materials with shape
memory properties, such as LCE and hydrogels, possess sim-
ilar or even identical mechanisms of 4D printing to those of
polymers.

## 3.1. Mechanisms of 4D printing based on stress or material heterogeneity

A polymer object without SME can be deformed when applying stimulation, due to an imbalance of different parts of the object in the response of the structure or function to the stimulation. The imbalance can be obtained by controlling process parameters and material distribution due to the high flexibility of manufacturing technologies. The influence of manufacturing technology on 4D printing is explained in detail in section 5, and the mechanism is discussed here. For 4D printing based on stress heterogeneity, the stress heterogeneity can result from residual stresses generated during manufacturing, typically FDM and digital light processing (DLP) where one involves heating when manufacturing and the other does not. For FDM, the post-printed layer tends to shrink but is constrained by the previous layer and partial strain is stored. The stored strain can be programmed by changing the process parameters such as printing speed, and will gradually release as the temperature rises, leading to the achievement of 4D printing [40–42, 48–50, 57, 64]. Based on the above mechanism, van Manen et al [57] obtained the pre-programmed 3D shape by changing the porosity and the thickness, and achieved the sequential closure of the rose-flower-like object, as shown in figure 6(a). Similar cases were also well illustrated through the discussion of the two examples in figures 6(d) and (e). For DLP, the internal stress results from the shrinkage of the resin during curing. Based on this principle, Zhao et al [130] proposed a spontaneous origami structure object by adding a light absorbent to the resin to magnify the difference of light intensity at different depths, due to the limitation of the substrate and the upper layer of cured resin, the object could not release the internal stress during the manufacturing process. And this was different from the thermal deformation of FDM object, but the object was deformed after the environment changes. Liu et al [131] introduced dynamic covalent bonds into a polymer to endow objects with the ability to reconfigure the shape after forming. The unconstrained shape programming by light was realized with the ability to use the stress-induced deformation generated during the printing process, reducing the processing difficulty, as shown in figure 6(b). Besides, it could not only realize the shape transformation of the object on the macro level but also on the micro level.

The heterogeneity can also be obtained by artificially apply-
ing an external force to the printed part of the object dur-
ing the manufacturing process to make it pre-deformed and
then continuing to print or assemble the remaining part while
maintaining its pre-stretched state. After the completion of
manufacturing, the recovery trend of the pre-stretched part is
balanced with that of the undeformed part, and the restoring
force of the pre-deformed part can be used as part of the driv-
ing force of the shape transformation, such as the two-way
4D printing using the bilayer laminate structure object men-
tioned in section 2.2. The detailed process and mechanism of

reversible shape transformation are as follows: (I) the layer of
SMP is stretched above its T<sub>trans</sub> and then cooled down to get
a stable pre-stretched layer, then a layer of elastomer is con-
structed (below the T<sub>trans</sub> of SMP) to obtain the bilayer lam-
inate structure object, and the obtained stable shape is defined
as shape A; (II) as the temperature gradually increases until
it exceeds T<sub>trans</sub>, the modulus of polymer decreases and the
previous equilibrium state is broken, and the bilayer lamin-
ate structure object bends towards the pre-stretched SMP layer
until the recovery force of the pre-stretched SMP is equal to
the resistance of the elastomer while the SMP layer is still in
the stretched state. An equilibrium state in high temperature is
obtained and a large amount of strain energy is stored in the
elastomer, and the stable shape is defined as shape B; (III) as
the temperature decreases, the modulus of polymer increases
and the pre-stretched SMP layer will not be sufficient to res-
ist the reverse deformation caused by the stored energy in the
elastomer layer, that is, recovery deformation to the shape A.
The bilayer laminate structure object will return to the cryo-
genic equilibrium state, and the obtained shape is defined as
shape C, which is a shape between shape A and shape B; (iv)
a reversible transformation between shape C and shape B can
be achieved with temperature increasing and decreasing. The
reason for using SMP is that there is no tendency for SMP to
recover at a pre-stretched state when cooled, making it easy to
process.

For 4D printing based on material heterogeneity, the prop-
erties of different parts of the object can be changed by
adjusting process parameters [51, 54–56], which can also be
achieved by distributing different materials [43, 58–61, 63]. A
difference in the degree of response after the external stim-
ulation can be obtained due to the differences in the prop-
erties such as thermal expansion rate and water absorption
rate in different parts of the object, resulting in the deform-
ation of the object to achieve 4D printing. For example,
Moosabeiki et al [64] recently reported a defect-based 4D
printing for curvature adjustment. The custom bending geo-
metry was obtained by changing the distribution of residual
stresses or different materials, and the combinatorial effects
of the distribution of residual stresses and different materials
on the deformation of the printed object were also studied,
as shown in figure 6(c). More information will be discussed
in section 5. Besides, there are special cases of 4D printing
using phase-changing materials. For example, Shao et al [132]
introduced methanol with low boiling point into polymer net-
works containing graphene plates, and the composite object
was fabricated, as shown in figure 6(d). The graphene plates
enabled the object good photothermal effect, so that meth-
anol could undergo liquid–gas conversion to change the shape
of the object to achieve 4D printing. It is worth noting that
there are also many physical phenomena in nature that change
with the external environment, such as the effect of heat con-
traction and cold expansion, which may also be found in the
3D printed object. However, it is not classified as 4D printing
here, a key factor is the lack of programming for the deforma-
tion of the printed object, which is also the key factor that we
distinguish between these simple deformations and 4D print-
ing. In summary, for 4D printing based on stress or material
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Mechanisms of 4D printing based on stress or material heterogeneity. (a) Stress heterogeneity generated during manufacturing by FDM: (i) the schematic of stress generation, (ii) the basic structure of 4D printing, and (iii) the structure cases of 4D printing. Reproduced from [57]. **CC BY 3.0.** (b) Stress heterogeneity generated manufacturing by DLP: (i) the schematic of stress generation, (ii) 4D printing in macro and, (iii) 4D printing in micro. [131] John Wiley & Sons. © 2021 Wiley-VCH GmbH. (c) Stress and material heterogeneity generated during manufacturing by FDM: (i) the schematic of stress generation, (ii) 4D printing based on stress heterogeneity, (iii) 4D printing based on material heterogeneity, and (iv) 4D printing based on the combinatorial effects of stress and material heterogeneity. Reproduced from [64]. **CC BY 4.0.** (d) Four-dimensional (4D) printing using phase-changing materials: (i) the schematic of phase-changing, and (ii) the structure cases of 4D printing. Reprinted from [132], © 2022 Elsevier B.V. All rights reserved.</figcaption></figure>
heterogeneity, an important feature is that the permanent shape and temporary shape of the object are determined after the manufacturing is completed, and cannot be programmed anymore. In contrast, this limitation is not suitable for 4D printing based on the stress relaxation and entropy elasticity, which all belong to SME.

## 3.2. Mechanisms of 4D printing based on stress relaxation

Nguyen et al [133] proposed that SME could be attributed to the structural and stress relaxation of polymer. It indicated that the mobility of molecular chains varied by temperature, and the polymer chain segments could locally rearrange the macromolecular structure and obtain stress response to achieve equilibrium, which supported the thermally activated shape memory phenomenon of SMP. In detail, the structure relaxed to equilibrium instantly at temperatures higher than $T_{\text{trans}}$, but lowly at temperatures lower than $T_{\text{trans}}$, indicating that macroscopic cooling could freeze the structure in a non-equilibrium configuration below $T_{\text{trans}}$, allowing the polymer object to maintain its temporary shape. By reheating above $T_{\text{trans}}$, the viscosity of the polymer chains was reduced, and the structure relaxed to its equilibrium configuration, resulting in shape recovery. It is worth noting that for the thermotropic phase transition SMP, the ‘program’ process is often accompanied by an increase in temperature above $T_{\text{trans}}$, which is due to the polymer being prone to deformation in the molten or highly elastic state. Temperature management by thermal programming can be time-consuming and energy-consuming, and integral heating is often used to the detriment of local programming. Therefore, it makes sense to be able to program SMP below $T_{\text{trans}}$, which is known as cold programming for the whole process. The cold programming requires sufficient deformation of SMP to cause yield and plastic deformation under large stress. Both thermosetting and thermoplastic polymers can be cold programmed, while the thermoplastic polymer is more susceptible to deformation in the glass or crystalline state than thermosetting polymer, so many researchers have focused on studying the cold programming behavior of the thermoplastic polymer. The process of cold programming and the corresponding thermomechanical curve are shown in figures 7(a) and (b), and the details are introduced as follows [134, 135]:

(I) Cold deformation: an external force is applied to SMP below $T_{\text{trans}}$ to make it plastic deformation; (II) stress relaxation: in this stage, the SMP keeps in the plastic deformation in a constant strain with the help of the external force; (III) unloading: removing the external force; (iv) structural relaxation: the SMP will exhibit a certain degree of viscoelastic springback with unloading; (v) stimulation: the SMP in an unconstrained state will recovery to its original shape when heated above the $T_{\text{trans}}$. Specifically, in step (I), the SMP undergoes elastic deformation and successively yields and plastic deformation with continuous external forces. Conformational changes of the polymer occur concomitantly, where the segmental motion and chemical bond length changes occur in the amorphous polymer and stress-induced crystallization occurs in the crystalline polymer, thus the internal energy of the polymer is

increased and stored. In step (II), part of the stress stored in the SMP during the cold deformation disappears, leading to a reversible inelastic deformation of the SMP. In general, the time of stress relaxation is negatively correlated with the viscoelastic springback in the step (iv), and a longer stress relaxation time generally results in a higher shape fixing rate and shape recovery rate of SMP. After unloading, the polymer chains or segments without constraint will instantly recover to the equilibrium state resulting in elastic springback, those with weak constraint will gradually recover to the equilibrium state resulting in viscoelastic springback, and those with strong constraint will be locked thus the internal energy is stored. The structural relaxation takes a certain time, and the plastic deformation will be locked since the SMP is in the glassy or crystalline state, thus obtaining a temporary shape in the non-equilibrium state. When the temperature rises above $T_{\text{trans}}$, the constraint among polymer chains or segments becomes weaker, and the stored internal energy is released, resulting in the shape recovery of SMP.

Cold programming has also been extended from SMP to 4D printing [136–146]. For example, Dong et al [136] designed and fabricated continuous fiber-reinforced lattice structures by 4D printing based on cold-programmed induced SME, as shown in figure 7(c), and the results showed that the prepared structure possessed a reusable energy absorption capacity and could be applied to energy absorption devices. Similarly, Namvar et al [137] prepared a reconfigurable mechanical metamaterial by 4D printing. As shown in figure 7(d), the obtained AuxHex structure underwent cold programming when subjected to impact, exhibited good energy absorption capacity and could recover to its original shape after heating, making it able to be applied to reversible energy absorption and dissipation devices. Mehrpouya et al [138] prepared sandwich structures by 4D printing, as shown in figure 7(e), which could be applied to reusable energy absorbers. Aberoumand et al [139] prepared polyvinyl chloride objects by FDM, and conducted cold, warm and hot programming of the objects respectively, as shown in figure 7(f). The results showed that the samples undergoing cold programming possessed lower shape fixation rate, but had better shape recovery rate. In addition to using the object based on cold-programmed for energy absorption devices, Yue et al [140] recently designed a double-layer hinge module based on the cold-programmed strategy by 4D printing. As shown in figure 7(g), through the design optimization of the distribution of heterogeneous materials, it was possible to easily deform with small forces and also to design temperature-responsive multistage deforming structures. The cold programming may cause irreversible deformation or damage to the object because the cold programming is based on energy-driven, requiring very large forces to deform the SMP from its permanent shape to a temporary shape. In addition, it is often very complicated to obtain the desired stable temporary shape due to the inevitable structural relaxation. The SMP will gradually recover from the temporary shape to the permanent shape even without raising the temperature. These place extremely high demands on the mechanical properties of SMP and the precise design of structures, as well as the applied external force during cold programming.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Mechanisms of 4D printing based on stress relaxation. (a) The process of cold programming, and (b) the corresponding thermomechanical curve. Reproduced from [135]. CC BY 4.0. Cold programming in 4D printing for (c) reusable energy absorption. Reprinted from [136], © 2023 Elsevier Ltd. All rights reserved. (d) Reconfigurable mechanical metamaterial. Reproduced from [137]. © The Author(s). Published by IOP Publishing Ltd. CC BY 4.0. (e) Sandwich structures. Reproduced from [138]. CC BY 4.0. (f) Comparison of cold programming with hot programming in 4D printing. Reproduced from [139]. CC BY 4.0. (g) Cold programming in 4D printing temperature-responsive multistage deforming structures. Reproduced from [140]. CC BY 4.0.</figcaption></figure>
### 3.3. Mechanisms of 4D printing based on entropy elasticity

The commonly used SMP is mainly based on entropy elasticity, including SMP based on phase transition and SMP based on dynamic bond. The mechanisms of entropy elasticity are introduced as follows [35, 73, 147]: the polymer chains are arranged randomly with the optimal energy and the highest entropy in the permanent shape; when applying an external force, the SMP is in a metastable state with entropy decreasing, and the polymer chain deforms along the force direction; with the withdrawal of external force, the SMP tends to recover to the most favorable state in energy, namely permanent shape with entropy increasing. Therefore, polymers generally possess the ability to remember shapes. A typical case is that the polymer film is stretched by an external force, and the original shape is recovered after the external force is withdrawn, which is often accompanied by creep and stress relaxation. However, this does not mean that the SME is universal in the polymer. A major feature of entropic elastic SMP is that its chain network consists of two components: the switching segment and the netpoint [148, 149]. The switching segment in SMP mainly includes phase transition and molecular switch (such as a reversible bond, supramolecular bond, etc), which plays a role in fixing or recovering the temporary shape of the SMP under external stimulation. The netpoint in SMP includes covalent crosslinking, crystalline entanglement and interpenetrating network, which is used to remember the permanent shape of the SMP.

The switching segment and the netpoint endow the SMP with the capacity of ‘program-stimulate’. Specifically, when the switch segment is activated, the object can be programmed, and the molecular chains in the switch segment are arranged along the force direction, while the molecular chains in the netpoint remain unchanged due to the higher energy required to deform, which ensures that the macromolecular chains do not slip as a whole. The object is in a metastable state with a shape change from a permanent shape to a temporary shape and the entropy decreases. Then when the switch segment is suppressed, the movement of the molecular chain is limited, forming a dynamic potential barrier in a high-energy state, and the temporary shape of the object can be kept even if the external force is withdrawn. When the switch segment is activated again by external stimulation, the dynamic potential barrier is broken with entropy increases, and the energy stored in the netpoint is released, leading to a spontaneous shape change to the permanent shape in a low-energy state, therefore a complete ‘program-stimulate’ process of SMP is achieved. Different types of switch segments and netpoints in 4D printing polymer objects will be introduced in terms of phase transition and dynamic bond.

#### 3.3.1. Phase transition.
It should be noted that the phase transitions indicate the transition of the aggregated structure of the polymer, which is different from the phase change (liquid-solid-gas). For the SMP based on phase transition, the phase transition of the polymer corresponds to the activation and deactivation of the switching segment, and the crosslinking of the polymer chain corresponds to the netpoints, as shown

in figure 8(a). Since the phase transition of a polymer is usually thermally induced, the whole process is called hot programming, echoing cold programming. The processes of hot programming are as follows: (I) heating, the SMP is heated to above the transition $T_{\text{trans}}$; (II) deformation, the SMP is then programmed to obtain a temporary shape with the help of an external force; (III) cooling, the temporary shape of the SMP is maintained under an external force and the temperature is cooled below the $T_{\text{trans}}$; (iv) fixed, the external force is removed indicating that the programming process is completed, and the SMP maintains the temporary shape at this time; (v) stimulation, the temperature is raised again above the $T_{\text{trans}}$, and the SMP will change from the temporary shape to the permanent shape in an unconstrained state. The crosslinking forms of polymer chains can be divided into chemical crosslinking and physical crosslinking, corresponding to thermoset and thermoplastic polymers, respectively. Due to strong covalent bonding, the thermosetting SMP exhibits excellent shape recovery rate and shape fixation rate, and can avoid the slip between chain segments. While the thermoplastic SMP has the ability to change permanent shape and is easy to process. The phase transition of polymer in the ‘program-stimulate’ process corresponds to the change of its mechanical state, and the aggregated structure of the polymer can be mainly divided into crystalline and amorphous states. Generally, the transition between crystalline and molten states in the crystalline structure, and the transition between glassy and rubbery states in the amorphous structure are the switching segments of the SMP, and the corresponding $T_m$ and $T_g$ are generally referred to as $T_{\text{trans}}$. Almost all of the thermotropic SMP is based on the melt and glass transitions mentioned above, and various types of thermotropic SMP can be obtained by combining different switching segments and netpoints.

Thermotropic phase transition SMP can be divided into two main categories [34, 154–157]. One possesses a stable two-phase in the whole ‘program-stimulate’ process, whose temporary shape and permanent shape have only a single stable phase state. The potential energy difference between the two stable phase states endows SMP with the ability to spontaneously recover from the metastable state (temporary shape) to the stable state (permanent shape), while the temperature rise compensates for the potential energy difference and activates the polymer chain to induce shape recovery. These are often found in thermosetting polymers, and the molecular mechanisms are exhibited in figure 8(b) (case 1 and case 2). In detail, for case 1 and case 2, the netpoints of both are chemical crosslinks, and the switching segments are the transition between glassy and rubbery states, and the transition between crystalline and molten states, respectively. The other is a common polyphase material such as block polymers, grafted polymers, etc, and there are two or more phase states in the temporary shape: reversible phase (also known as soft domain, which acts as switching segment) and fixed phase (also known as hard domain, which acts as netpoint). Compared with the first category, the second one exhibits that at least one phase does not undergo phase transition in the ‘program-stimulate’ process, and is more diverse and more widely used due to the existence of multiple reversible phases. The molecular mechanisms are
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Mechanisms of 4D printing based on entropy elasticity in terms of phase transition. (a) The netpoints and switching segments in 4D printing. (b) The molecular mechanisms of 4D printing. (c) Four-dimensional (4D) printing of thermosetting polymer: (i) the diagram of the formation of the chemical crosslinking network, and (ii) the structure cases of 4D printing. Reprinted from [150], © 2023 Elsevier B.V. All rights reserved. (d) Four-dimensional (4D) printing of thermoplastic polyphase polymer with blending: (i) the polyphase structure in printed object, and (ii) the corresponding diagram of the 4D printing process. Reproduced from [151]. CC BY 4.0. (e) Four-dimensional (4D) printing of thermoplastic polyphase polymer with interpenetrating network: (i) the diagram of the formation of interpenetrating network, and (ii) the structure cases of 4D printing. Reprinted from [152], © 2021 Elsevier B.V. All rights reserved. (f) Four-dimensional (4D) printing of thermoplastic polyphase polymer with macro-layered structure: (i) the polyphase structure in the printed object, and (ii) the corresponding diagram of the 4D printing process. Reproduced with permission from [153]. © 2024 The Authors. Published by Elsevier B.V. CC BY-NC-ND 4.0.</figcaption></figure>
exhibited in figure 8(b) (case 3 to case 6). For case 3, case 4, case 5 and case 6, the netpoints are macromolecular entangle- ments, crystalline phase, crystalline phase and macromolecu- lar entanglements, respectively. And the switching segments are the transition between crystalline and molten states, the transition between crystalline and molten states, the transition between glassy and rubbery states, and the transition between glassy and rubbery states, respectively. The polyphase struc- ture can be obtained by blending two different SMP, in which the one with the higher $T_{\text{trans}}$ acts as the fixed phase and the one with the lower $T_{\text{trans}}$ acts as the reversible phase, and there are two special structures for the polyphase structure by blen- ded: interpenetrating network [158, 159] and macro-layered structure [160–164], which still follow the mechanisms men- tioned above.

Four-dimensional (4D) printing based on the phase trans- ition is the most common type. For the 4D printing of ther- mosetting polymer, Zhou *et al* [150] reported the 4D print- ing of a benzoxazine/epoxy thermosetting object, in which the ultraviolet curing and thermal curing were utilized to achieve chemical crosslinking of benzoxazine and epoxy resin. As shown in figure 8(c), the obtained object showed good shape memory properties, and the covalent crosslinking in the poly- mer network acted as the netpoints, while the epoxy resin chain acted as the switching segments. For the 4D printing of ther- moplastic polymer, three common cases of polyphase poly- mers including blending, interpenetrating network and macro- layered structure are discussed. Our previous study [151] reported 4D printing of poly (L-lactic acid)/thermoplastic polyurethane object, in which poly (L-lactic acid) served as the switching segments and thermoplastic polyurethane served as the netpoints, as shown in figure 8(d). In addition, poly (L- lactic acid) and thermoplastic polyurethane were immiscible, and a co-continuous structure was formed with the help of fillers, which effectively improved the shape memory proper- ties of the object. Blending is a simple and effective method, which can enable the complementary advantages between materials. Interpenetrating polymer networks are often used for 4D printing to strengthen polymer object [66, 152, 165]. Zhou *et al* [150] reported a 4D printed object based on a cyanate ester interpenetrating polymer network, as shown in figure 8(e). The interpenetrating polymer network effect- ively improved the transition temperature, tensile strength and thermal stability of the object, and the object showed good shape memory properties. The macro-layered structure can also be regarded as a special polyphase structure [153, 166]. Recently, our team [153] prepared the layered co-continuous object of poly (L-lactic acid) and thermoplastic polyurethane, as shown in figure 8(f). The poly (L-lactic acid) was used as the switching segment and thermoplastic polyurethane as the netpoint, the results indicated that the layered structure exhib- ited better electro-responsive shape memory properties than the blending one. However, even though 4D printing based on the phase transition is the most common type, the single type of heat-related stimulus limits its wider application.

### 3.3.2. *Dynamic bond.*
Dynamic bond can be broken and recombined under specific conditions, which makes poly- mer possible to exhibit SME. In addition, it also endows the polymer with other characteristics, such as remolding, self- healing, and so on. More importantly, compared with the SMP based on phase transition which can be stimulated only by temperature, there are various kinds of dynamic bonds, which can respond to external stimulations including tem- perature, light and pH, extending the application range of SMP [167, 168]. Lehn *et al* [169] first proposed the concept of ‘dynamer’ for polymers containing dynamic bonds, and the dynamic bonds could be divided into dynamic covalent bonds and dynamic non-covalent bonds. For polymers con- taining dynamic covalent bonds, Kloxin *et al* [170] proposed the concept of ‘covalent adaptable network’. Montarnal *et al* [171] proposed the concept of ‘vitrimer’ in 2011 based on the polymer with covalent crosslinking networks. Zou *et al* [172] proposed the concept of ‘thermadapt SMP’ for the SMP containing dynamic covalent bonds in 2017. For another, the dynamic non-covalent bonds are ubiquitous in polymer, and the dynamic non-covalent bonds which are prominent in dir- ectivity, selectivity and bond strength are defined as supra- molecular interactions, and the corresponding SMP is called supramolecular SMP [173–175].

Compared with SMP based on thermal phase transition by changing the aggregated structure to change the polymer chain movement ability, SMP based on the dynamic bond changes the intramolecular or intermolecular interactions and the topological structure of molecules through external stim- ulation to change the polymer network movement ability [167, 168, 176]. The reversible transformation of dynamic bonds is based on the association or dissociation mechan- isms, as shown in figure 9(a), the break and formation of bonds occur almost simultaneously in the association pro- cess, while the first bond break is followed by recombina- tion in the dissociation [176]. The dynamic bond exchange through association exhibits a weak temperature dependence, and the enthalpy of the process is close to zero, while the dynamic bond exchange through dissociation is often accom- panied by the destruction of polymer network integrity and a sharp decline in viscosity. There are many kinds of dynamic bonds. As shown in figure 9(b), dynamic covalent bonds mainly include the ester/lmine bond, disulfide bond, Diels- Alder bond, oxime bond, benzoxaborole, and so on [177–180]. Supramolecular interactions mainly include hydrogen bond- ing, host-guest interaction, metal coordination, hydrophobic interactions, ionic attractions, and so on [173–175, 181–183]. The dynamic bonds can act as switching segments, and also can act as netpoints when there are other switching segments in the SMP. The strength of dynamic covalent bonds is higher than that of supramolecular interactions which makes it stable, and a relatively harsh condition is required for the reversible transformation of dynamic covalent bonds [184–187]. While the supramolecular interactions can be reversibly transformed under a relatively mild environment even in the absence of
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Mechanisms of 4D printing based on entropy elasticity in terms of dynamic bond. (a) Mechanisms of reversible transformation of dynamic bonds. Reproduced from [176]. CC BY 4.0. (b) Type of dynamic covalent bonds and supramolecular interactions. Reprinted (adapted) with permission from [181]. Copyright (2017) American Chemical Society. (c) Four-dimensional (4D) printing polymer object containing dynamic covalent bonds with repairing property: (i) the structure case of repairing, and (ii) the structure case of 4D printing. [191] John Wiley & Sons. © 2022 Wiley-VCH GmbH. (d) Four-dimensional (4D) printing polymer object containing dynamic covalent bonds with regenerative living property: (i) the mechanism of the regenerative living, and (ii) the structure case of regenerative living 4D printing. [192] John Wiley & Sons. © 2023 Wiley-VCH GmbH. (e) Four-dimensional (4D) printing polymer object containing supramolecular interactions with high energy density: (i) the mechanism of the strain-induced formation of supramolecular nanostructures, and (ii) the structure case of high energy density. [193] John Wiley & Sons. © 2024 Wiley-VCH GmbH. (f) Four-dimensional (4D) printing polymer object containing supramolecular interactions with temporal programming property: (i) the mechanism of temporal programming, and (ii) the structure case of temporal programming. [194] John Wiley & Sons. © 2021 Wiley-VCH GmbH. (g) Four-dimensional (4D) printing polymer object containing dynamic covalent bonds and supramolecular interactions with self-healing and recyclable properties: (i) the structure case of 4D printing, (ii) the structure case and mechanism of self-healing, and (iii) the structure cases of recyclable. [195] John Wiley & Sons. © 2022 Wiley-VCH GmbH.</figcaption></figure>
stimulation, endowing the SMP with special features such as temporal programmable ability, and so on [188–190].

Recently, the polymer containing different dynamic bonds is constantly being developed and widely used in 4D printing, and endowing 4D printing other significant characteristics, including repairing, regenerative living properties, high energy density, temporal programming, self-healing, recyclability, and so on [191–204]. For dynamic covalent bonds, Cui *et al* [191] introduced dynamic thiocarbamate bonds into a polymer to achieve polythiamine 4D printing with repairing properties, as shown in figure 9(c), which could be reprinted on damaged surfaces, and exhibited high shape fixity and shape recovery. Xu *et al* [192] proposed to use of thiol-ene click chemistry for photoradical 3D printing. As shown in figure 9(d), the printed objects could grow by soaking in 1,5,7-triazabicyclo[4.4.0] dec-5-ene catalyst, and degrow by soaking in excessive n-butylamine toluene solution, which endowed the objects the ability to regenerate and improve the flexibility and sustainability for 4D printing. For supramolecular interactions, as shown in figure 9(e), Chen *et al* [193] reported a strain-induced formation of hierarchical hydrogen bonding supramolecular network to achieve high energy density polymer with an unprecedented energy density of 24.5 MJ·m⁻³, which could be used for high energy density actuator. Peng *et al* [194] introduced urea-pyrimidone supramolecular units into SMP, endowing the SMP with a strong time-temperature dependence, and the printed object possessed a spatiotemporal programmable capability of autonomous deformation paths, as shown in figure 9(f). For the combination of dynamic covalent bonds with supramolecular interaction, as shown in figure 9(g), Yang *et al* [195] introduced dynamic boroxine bonds and hydrogen bonds into the thermosetting material to engender self-healing and recyclable characteristics of the 4D printing object, which could realize repeatable encryption/decryption while providing strong protection for encrypted data, and was expected to develop a high-level dual-encryption platform. In summary, 4D printing based on dynamic bond not only expands the type of stimulation but also endows other additional functions on the printed object. If the problem of its universality is solved, it will greatly promote the development of 4D printing.

## 4. Stimulations

The stimulations are the bridge from temporary shape to permanent shape of 4D printed polymer object, and it is necessary to choose the appropriate stimulation type according to different application scenarios. The types of stimulations have evolved from initial direct heating to different environmental stimulations that trigger changes in the shape or function of the polymer object through physical, chemical and biological reactions. Combined with the content of the previous chapter, the stimulations are divided into two types: heat-related and heat-unrelated stimulations. The heat-related stimulations are the most widely used method, and the 4D printed polymer object based on different mechanisms in the previous chapter all contain cases of heat-related stimulations. While

the 4D printed polymer object based on stress or material heterogeneity and based on the entropy elasticity of dynamic bonds contain cases of heat-unrelated stimulations. The basic principles and characteristics of different stimulations are introduced as follows.

### 4.1. Heat-related stimulations

The heat-related stimulations are derived from the heat conduction from outside to the polymer object or the heat generated inside the object, and the shape or function of the object can be changed by the release of residual thermal stress, phase transition or the association and dissociation of dynamic bonds. It can be divided into direct thermal stimulation and indirect thermal stimulation according to the mode of heat generation. Direct thermal stimulation refers to the use of heat transfer in the medium (gas, liquid, or even solid) to rise the temperature of the polymer object, such as the use of hot water or hot air directly heating the object, which is the most traditional method of stimulation. However, the response rate of the object is limited by the thermal conductivity, which is often slow. More importantly, direct thermal stimulation makes it difficult to achieve accurate temperature management and cannot be controlled remotely, thus limiting its application [38–40, 205]. Therefore, a series of indirect thermal stimulations have gradually evolved, usually adding functional fillers in the polymer object to improve heat conversion efficiency. It mainly includes electrothermal stimulation, magnetothermal stimulation, and photothermal stimulation. The suffix ‘thermal’ is added after the stimulation mode to distinguish because some external stimulations can also trigger the response in a heat-unrelated way.

Electrothermal stimulation refers to Joule heat generated by the collision of free electrons passing through the material under the action of an external voltage, which indirectly raises the temperature to trigger the response of the polymer object. Compared with direct thermal stimulation, electrothermal stimulation has the advantages of rapid heating and remote control [46, 53, 206–208]. For example, Wang *et al* [206] integrated continuous carbon fiber into the hinge of the origami structure, and the shape transformation of the hinge could be obtained based on the SMP in which activated by Joule heating, as shown in figure 10(a). In fact, conductive fillers need to be added to construct conductive networks in the object since it does not conduct electricity. Our previous study [207] enhanced the electrically driven shape memory properties of biopolymer objects by constructing a 3D conductive network in the SMP object by self-assembling multiwall carbon nanotube onto ZnO tetrapod to form interwoven layers. An inherent defect of electrothermal stimulation is that an external voltage should be configured and thus cannot realize remote non-contact control. In contrast, the magnetothermal stimulation can achieve non-contact remote control. Magnetothermal stimulation refers to the magnetic filler generating induced heat due to hysteresis loss and eddy current loss under the action of an external alternating magnetic field, thus indirectly heating to trigger the response of polymer object. A remarkable feature of magnetothermal stimulation is
<figure><img src="image_13.png" /><figcaption>**Figure 10.** Stimulations of 4D printing. Heat-related stimulations of (a) electrothermal stimulation. Reproduced from [206]. CC BY 4.0. (b) Magnetothermal stimulation. Reprinted from [209], © 2022 Elsevier Ltd. All rights reserved. (c) Photothermal stimulation. Reproduced from [212]. CC BY 4.0. Heat-unrelated stimulations of (d) solution stimulation. Reproduced from [217] with permission from the Royal Society of Chemistry. (e) Photochemical stimulation. Reproduced from [218]. CC BY 4.0. (f) pH stimulation. Reprinted with permission from [219]. Copyright (2023) American Chemical Society.</figcaption></figure>

that it can tune the intensity of stimulation and the position of action by controlling the alternating magnetic field, exhibiting high flexibility [209–211]. For example, Liu *et al* [209] introduced $Fe_3O_4$ nanoparticles into SMP to endow it with magnetothermal responsiveness, and the results showed that the flower-like object could complete shape transformation in 80 s under an alternating magnetic field, as shown in figure 10(b). However, magnetothermal stimulation is limited by the limited variety of magnetic fillers. The types of functional fillers for photothermal stimulation are extensive, including metal-based nanofillers, carbon-based nanofillers, inorganic semiconductor nanomaterials [191, 212–216]. The temperature of polymer object containing metal-based nano-fillers is raised by the effect of local surface plasmon resonance. And the polymer objects containing carbon-based nano-fillers or inorganic semiconductor nanomaterials absorb the energy of photons through the electrons in the nanomaterials, so that the electrons are in an unstable stimulation state, and the photothermal effect will be generated during the process of attenuation back to the ground state, thereby making the temperature rise. For

example, Paunović *et al* [212] added Au nanorods to a SMP object, and the results indicated that the object possessed good photothermal performance under near-infrared light, and was rapidly heated 50 °C within 1 min thus triggering the shape transformation, as shown in figure 10(c). The photothermal stimulation exhibits the advantage of high spatiotemporal controllability, but it has the inherent defect of limited penetration.

There are many other indirect thermal stimulation methods, such as microwave-thermal stimulation, ultrasonic-thermal stimulation, etc, which can be applied to 4D printing [220, 221]. Microwave-thermal stimulation possesses high penetration ability, which belongs to electromagnetic wave. It refers to the temporary dipole movement of molecules under the action of alternating electromagnetic field to generate heat to indirectly raise the temperature of polymer object. The heating effect of microwave-thermal stimulation depends on the dielectric properties of the material. The microwave absorption capacity of polymer is weak, so microwave-sensitive particles such as CNT and other nanomaterials are usually introduced to enhance the microwave absorption performance
of SMP. However, microwave-thermal stimulation also has the problem of relatively poor spatiotemporal controllability. Ultrasonic-thermal stimulation has the advantages of remote non-contact, strong penetration and strong spatiotemporal controllability, and the vibration energy generated by mechanical waves is absorbed by the medium and converted into heat. The ultrasonic-thermal stimulation has a good application prospect. In summary, indirect thermal stimulations usually involve the addition of functional fillers, so when the intensity of external stimuli applied is evenly distributed, the heat generation of the polymer object is affected by the distribution of fillers. This characteristic can be used to achieve sequential deformation of the polymer object, as mentioned in section 2.1.

## 4.2. Heat-unrelated stimulations

There are two main approaches for the 4D printing polymer object triggered by heat-unrelated stimulations: difference of response degree due to material heterogeneity and reversible responses based on dynamic bonds. This is usually the intrinsic response of the polymer object to stimulations, as opposed to stimulations that are indirectly thermally driven by functional fillers. In other words, the stimulations in the former directly act on the polymer object itself, while the stimulations in the latter act on the functional filler and then indirectly act on the polymer object. Heat-unrelated stimulations mainly include solution stimulation, photochemical stimulation and pH stimulation.

Solution stimulation, also known as humidity stimulation, is a common type that can trigger the response of the polymer object through two main mechanisms [217, 222, 223]. One is to use the heterogeneity of object material to cause shape change in solution expansion/swelling, it is worth noting that this property is more obvious in hydrogels, and many studies focus on 4D printing combining hydrogels and polymers. The other is to use the interaction between water molecules and polymer functional groups (mainly hydrogen bonding) to reduce the $T_g$ of the polymer to the ambient temperature thus triggering the response, which is also known as the plasticization effect, but the process is often very slow. Our study [217] reported a water-responsive shape memory object that was plasticized due to hydrogen bond interactions between water and SMP molecular chains, enabling the object to undergo shape transformation at 37 °C, as shown in figure 10(d). Photochemical stimulation is the use of photosensitive functional groups in polymers, such as cinnamyl, azobenzene and spiropyran, to undergo reversible changes such as photoisomerization, reversible cycloaddition reactions under the action of light, thus triggering the response of polymer object [218, 224, 225]. Müller et al [218] grafted azobenzene photochromic material onto the surface of cellulose nanocrystals and introduced it into SMP, using the trans-cis photoisomerization of azobenzene to endow the object photoresponse, as shown in figure 10(e). Notably, light can also reduce the $T_g$ of polymers by interacting with their functional groups (usually spiropyran groups), which is also known

as photoplasticization [226]. However as mentioned previously, an inherent flaw of light stimulation is poor penetration. There are two main types of pH stimulations [71, 219, 227]. The first type is that the shape of the polymer object changes in response to the change of pH due to the presence of pH-sensitive functional groups in the polymer, such as –COOH, –$SO_3H$, and –$NH_2$. Pan and Goto [219] developed a pH-responsive block polymer and fabricated an object using 3D printing, as shown in figure 10(f). The object exhibited different deformability when countering different pH environments and could be used as an actuator. The second type is the case when the switching segment is pH-sensitive such as multiple hydrogen bonds, boric ester bonds and other dynamic bonds, and is more common in hydrogels. Both pH stimulation and solution stimulation belong to direct contact stimulations, which can be applied in specific environments (such as the physiological environment of the human body) to realize the self-adaptation of the polymer object. However the response rate depends on the diffusion rate of stimulators in the medium, and they can not be controlled remotely.

Different stimulations have their characteristics, and it is necessary to choose the appropriate stimulations according to the application scenarios. With the continuous progress of 4D printing, the development of 4D printed polymer object that can respond to multiple stimulations has improved adaptability and intelligence, which is helpful for application in complex scenarios. The multiple stimulations response feature mainly has the following two advantages: first, it can select the appropriate stimulations according to different environments, improve the application range of 4D printed polymer object, and improve its robustness so that it can be applied in complex changing environments. The second is that it can endow the polymer object with more abundant conversion forms, such as selective deformation or sequential deformation, which significantly improves the intelligent characteristics of the polymer object. According to the previous chapter, there are mainly two types of polymer objects to realize the response to multiple stimulations. One is based on the combination of different 4D printing mechanisms. The other is based on the entropy elastic mechanism, and there are multiple switching segments in the polymer network that can respond to different stimulations. Finally, it is worth noting that in the relevant research on bioprinting, the controllable contraction force generated by cells in response to single or multiple physical and chemical external stimulations drives the deformation of polymer object. And the deformation can be controlled by cell distribution, which can be seen as the 'program' in 4D printing. Therefore, bioprinting can also be regarded as a special case of 4D printing to a certain extent, but will not be discussed here. The research on bioprinting can refer to the following literature [228–233].

# 5. Technologies

The technologies used for 4D printing are the same as those used for 3D printing in terms of principle [36, 234–236]. Some printed polymer objects have been considered as special
cases of 3D printing before 4D printing is defined but are essentially 4D printing [237, 238]. Recently new manufacturing technologies have been developed rapidly, which substantially promote the application prospect of 4D printing. Three-dimensional (3D) printing can be divided into seven categories according to ISO/ASTM 52900:2021. Among them, directed energy deposition is used for the processing of metal materials [239], and there is almost no research on the 4D printed polymer object by binder jetting or sheet lamination. Thus, this chapter mainly focuses on the other four categories. Furthermore, in order to highlight the similarities and differences between 4D printing and 3D printing in the manufacturing process, here the technologies are divided into three categories, based on the influence degree of the manufacturing process on the ‘program-stimulate’ in 4D printing: 4D printing based on SMP (i.e. 3D printing SMP), 4D printing based on process parameter-encoded, and 4D printing based on multi-material printing. The ‘program’ is completely unaffected by the manufacturing techniques in the former scenario, and partially or even completely affected in the latter two scenarios.

## 5.1. Technologies of 4D printing based on SMP

In this scenario, the ‘program-stimulate’ in 4D printing is implemented based on the shape memory properties of SMP, which is the most common in 4D printing. The type of manufacturing technologies can follow from that of 3D printing, including material extrusion (ME), vat photopolymerization (VP), powder bed melting (PBF), and material jetting (MJ), each of which contains subtypes. Characteristics of common manufacturing technologies are summarized in table 1 [22, 240, 241].

ME is the most economical and easy to operate and mainly contains FDM (also known as fused filament fabrication) [242–252] and direct ink writing (DIW) [45, 253–261, 287, 288]. The manufacturing process of FDM and DIW is similar, and the main difference lies in the feed state and the curing mechanism. Besides, the process of DIW does not involve heating compared with FDM, making it more suitable for polymer processing. VP contains the most subtypes, which can be mainly divided into stereolithography (SLA) [262–265], DLP [266–274] and two-photon polymerization (TPP, also known as direct laser writing) [241, 275–280, 289–292]. Many advanced manufacturing technologies are also included, such as continuous liquid interface production [293, 294], computed axial lithography [295], and projection micro stereolithography (PµSL) [296–300]. The subtypes share the same curing mechanism but the manufacturing principle and process. VP possesses many advantages such as high resolution and high precision, while the limitation of raw material on photocuring resins narrows down its application, especially in the biomedical field. The only subtype of PBF suitable for polymer processing is selective laser sintering (SLS) [151, 207, 281, 282, 301–306]. The range of raw materials suitable for SLS is wide and the printed polymer object exhibits superior mechanical properties, compared to VP and ME. However, the process of SLS involves heating, the need to maintain an inert atmosphere to prevent oxidation of polymer and the high cost,

making its research and application in 4D printing relatively fewer. MJ [283–286] mainly contains continuous ink jet (CIJ) and drop on demand (DOD), in which the main difference between the two subtypes is the generation mode of ink. A suitable ink with a certain viscosity is essential for MJ, ensuring the smooth production of droplets and possessing the self-supporting ability.

Four-dimensional (4D) printing has developed rapidly since it was proposed, owing to the profound accumulation in 3D printing and the extensive research and development of smart materials. Various structures from macro to micro which are realized by 3D printing, have been carried forward in the field of 4D printing. Moreover, the dynamic properties of 4D printing pave the way for the subliming of printed object from structural and functional characteristics to intelligent characteristics [23, 307–316]. In micro, as shown in figure 11(a), Spiegel et al [315] prepared a microscope double platform structure (20 µm × 20 µm × 18 µm) and a box-like structure (20 µm × 20 µm × 8 µm) with a smooth surface, and the SME of the structures had been verified. Li et al [244] prepared films containing microlevel cylindrical microarrays with different spacing by photolithography technology, as shown in figure 11(b). The microarrays possessed a SME, leading to the transition of the films from hydrophobe to superhydrophobic in macroscopic. In macro, as shown in figure 11(c), Deng et al [256] fabricated a mangrove structure-like cartilage object (in the maximum size of about 14 mm) with good shape memory properties. And Peng et al [247] used FDM to prepare anastomosis rings with shape memory capability, and the anastomotic ring exhibited a lattice structure with a maximum size of about 15 mm, as shown in figure 11(d). For metamaterials, Wang et al [250] used FDM to prepare tubular chiral metamaterials, which showed a good thermomechanical response, as shown in figure 11(e). And Wu et al [271] prepared a modular chiral mechanical metamaterial by DLP, whose basic component was a compression-torsion unit with a size of about 15 mm × 10 mm, and a macro-assembly with a size of 180 mm × 80 mm could be obtained by mass production. The shape transition between the original configuration and temporary chiral deformation was realized based on the shape-locking mechanism of SMPs, as shown in figure 11(f). Besides, for the functional transformation in 4D printing, as shown in figure 11(g), Zhang et al [314] explored the 4D printing of SMP at sub-micron scale by TPP, and achieving large visual changes through the deformation of nanoscale structures. Deng et al [266] explored the switch of optical properties of printed objects by taking advantage of the different transparency properties of polymers in crystalline and molten states, as shown in figure 11(h). Wang et al [313] added thermochromic dyes to SMP and used FDM to realize the 4D printing of polymer objects with shape changing and color transition, as shown in figure 11(i).

## 5.2. Technologies of 4D printing based on process parameter-encoded

It is common that the properties of printed polymer object can be varied by changing the process parameters during
<table><caption>**Table 1.** Characteristics of common manufacturing technologies for 4D printing based on SMP. Reprinted from [22], © 2023 Elsevier Ltd. All rights reserved. Reproduced from [240]. CC BY 4.0.</caption><thead><tr><th>Categories</th><th>Diagram</th><th>Process</th><th>Curing mechanism</th><th>Typical materials</th><th>Advantage</th><th>Disadvantage</th><th>Typical case</th></tr></thead><tbody><tr><td>FDM</td><td></td><td>Point-line-surface-body</td><td><ul><li>Physical process</li><li>Crystallization and macromolecules of polymer</li></ul></td><td>Thermoplastic polymers</td><td><ul><li>Economical</li><li>Easy to operate</li></ul></td><td><ul><li>Low resolution</li><li>High anisotropy of printed objects</li></ul></td><td>[242-252]</td></tr><tr><td>DIW</td><td></td><td>Point-line-surface-body</td><td><ul><li>Physical or chemical processes</li><li>Solvent volatilization, thermal curing, photocuring</li></ul></td><td>Liquid polymer</td><td><ul><li>Good multi-material printing ability</li><li>High resolution</li></ul></td><td><ul><li>High requirements on raw materials</li><li>High anisotropy of printed objects</li></ul></td><td>[45, 253-261]</td></tr><tr><td>SLA</td><td></td><td>Point-line-surface-body</td><td><ul><li>Chemical processes</li><li>Photopolymerization and crosslinking</li></ul></td><td>Photocuring resins (epoxy or acrylate-based resin)</td><td><ul><li>High resolution</li><li>High printing speed</li></ul></td><td><ul><li>Low mechanical strength of printed objects</li><li>High cost</li></ul></td><td>[262-265]</td></tr></tbody><tfoot><tr><td colspan="8" style="text-align: right;">(Continued.)</td></tr></tfoot></table>
<table><caption>**Table 1.** (Continued.)</caption><thead><tr><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td>DLP</td><td>Surface-body</td><td><ul><li>Chemical processes</li><li>Photopolymerization and crosslinking</li></ul></td><td>Photocuring resins (epoxy or acrylate-based resin)</td><td><ul><li>High resolution</li><li>High printing speed</li></ul><ul><li>High cost</li><li>Limited raw materials</li></ul></td></tr><tr><td>TPP/MPP</td><td>Point-line-body</td><td><ul><li>Chemical processes</li><li>Photopolymerization and crosslinking</li></ul></td><td>Photocuring resins (acrylate-based resin)</td><td><ul><li>High resolution</li></ul><ul><li>High cost</li><li>Small build volume</li></ul></td></tr><tr><td>SLS</td><td>Point-line-surface-body</td><td><ul><li>Physical process</li><li>Crystallization and macromolecular entanglements of polymer</li></ul></td><td>Thermoplastic polymers</td><td><ul><li>High resolution</li><li>High mechanical strength of printed objects</li></ul><ul><li>High cost</li><li>Poor multi-material printing ability</li></ul></td></tr><tr><td>CIJ/DOD</td><td>Point-line-surface-body</td><td><ul><li>Chemical processes</li><li>Photopolymerization and crosslinking</li></ul></td><td>Photocuring resins</td><td><ul><li>Good multi-material printing ability</li><li>Large build volume</li><li>High resolution</li></ul><ul><li>Low printing speed</li></ul></td></tr></tbody></table>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>
<figure><img src="image_18.png" /><figcaption>**Figure 11.** Technologies of 4D printing based on SMP. Four-dimensional (4D) printing in micro structure of (a) microscope double platform structure and box-like structure. Reproduced from [315]. CC BY 4.0. (b) Cylindrical microarrays. Reprinted from [244], © 2022 Elsevier Inc. Four-dimensional (4D) printing in macro structure of (c) mangrove structure-like object. Reprinted with permission from [256]. Copyright (2023) American Chemical Society. (d) Lattice structure. [247] John Wiley & Sons. © 2023 Wiley-VCH GmbH. Four-dimensional (4D) printing metamaterials of (e) tubular chiral metamaterial. Reproduced with permission from [250]. © 2023 The Author(s). Published by Elsevier Ltd. CC BY-NC-ND 4.0. (f) Modular chiral mechanical metamaterial. [271] John Wiley & Sons. © 2023 Wiley-VCH GmbH. Functional transformation in 4D printing for (g) color change. Reproduced from [314]. CC BY 4.0. (h) Transparency change. [266] John Wiley & Sons. © 2021 Wiley-VCH GmbH. (i) Shape and color change. [313] John Wiley & Sons. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.</figcaption></figure>

manufacturing [22, 317, 318]. For example, the degree of crys-
tallization, crosslinking and orientation of molecular polymer
is affected by printing temperature, exposure light dose and
printing speed, respectively. And the difference in microstruc-
ture will lead to the inconsistency of properties in macro.
The inconsistency can be amplified when the process para-
meters are carefully encoded, resulting in a macroscopical
response of the printed object due to the large differences in
response at different locations when applying external stimu-
lation. Therefore, in the case of 4D printing based on process
parameter-encoded, the microstructure including orientation
of molecular chains/fillers, and degree of crosslinking/crys-
tallization are constantly changing with the variation of the
process parameters. And the ‘program’ in 4D printing is also
conducted at the same time.

5.2.1. Orientation of molecular chains/fillers. Inducing the orientation of molecular chains/fillers by process parameter-encoded is a promising strategy to achieve 4D printing, which is generally found in ME. In detail, the molecular chains/fillers tend to align in the printing path due to shear force, therefore, the orientation of molecular chains/fillers can be tailorable by design the direction or density of the printing path, to achieve the construction of ordered structures and the predetermined deformation of 4D printed polymer object [41, 48–50, 52, 319–322]. This situation is particularly prominent in FDM as compared to DIW, because FDM involves heating and the strains generated by thermal expansion and cold contraction are partly stored in the polymer object and released again when stimulated by heat, which is also the main cause of warpage deformation in 3D printing. If the printing path
is carefully coded, the warpage deformation can occur in a predetermined form to achieve 4D printing [41, 48–50, 52, 319–322]. As shown in figure 12(a), Koh and Sutradhar [49] obtained basic structures with anisotropic expansion rates by changing the printing path, in which the anisotropic expansion was based on the strain stored in the object printed by FDM being released upon thermal stimulation. And the combination of different basic structures is used to realize different deformation types, such as multi-layer deformation structures. In fact, the printing path mainly affects the deformation direction of the polymer object, while the deformation degree of the object is also affected by other process parameters. Specifically, the diameter of the nozzle affects the width of the polymer chain and thus the number of polymer chains, the printing speed affects the shear force and thus the degree of polymer chain alignment, as well as the printing and ambient temperature affect the crystallization of the polymer and thus the stored strain [50, 323].

In contrast, the printing path in DIW that does not involve heating possesses a limited effect on the alignment of the polymer chains. It should be noted that the commonly used materials in 4D printing are hydrogels or LCE [321, 322, 331, 332], which is attributed to two factors: (I) the strong dependence of the DIW process on the rheological properties of materials and the relatively narrow processing window of DIW; (II) hydrogels with strong water absorption capacity and LCE with strong anisotropic molecular chain alignment are easy to deform. The anisotropic differences in polymer object can be amplified by the introduction of fillers, where the shear forces enable the fillers’ alignment and the printing path induces the orientation of aligned fillers [324, 333–335]. Wang et al [333] prepared objects with adjustable off-center positions of continuous fiber-reinforced LCE by DIW. The objects would bend (instead of anisotropic contraction of pure LCE) when applied thermal stimulation, due to the difference in the coefficients of thermal expansion between the fiber and LCE. Li et al [324] reported a highly anisotropic hydrogel object fabricated by DIW using shear to induce the orientation of short fibers in the hydrogel, as shown in figure 12(b). The object with different patterns could transform into curled, bent or twisted configurations by coding the printing path, and more complex structures such as butterflies could also be obtained by encoding multiple shape-changing domains.

The aligned fillers in the printed object can also be realized by field assistance. The magnetic field is the most commonly used to redirect the magnetic fillers in the polymer matrix, and the printed objects with programmed magnetic domains are obtained by switching the printing path or the direction of the magnetic field [325, 336]. In addition, anisotropic deformation will be found in the object when applying the stimulation of a magnetic field. As shown in figure 12(c), Kim et al [325] reported a strategy for achieving programmable magnetic domains in polymer objects using magnetic field-assisted DIW. The magnetic particles in the polymer were redirected by the external magnetic field during the manufacturing process, and the complex magnetic domains could be encoded by switching the magnetic field direction and the printing path. The prepared objects could be transformed into a complex

structure after being subjected to external magnetic stimulation. The assistance of the field makes it easy to enable the fillers aligned in printed object in other printing technologies besides ME, especially for VP [51, 326, 337–339]. Because the fillers in liquid polymers are susceptible to the influence of field before curing. A special case proposed by Deng et al [326] was the magnetic particle redirected in the prepared object using the magnetic field-assisted laser direct writing, as shown in figure 12(d). The magnetic particles encapsulated would be aligned with the magnetic field orientation after laser scanning under the combined effect of polymer melting and the magnetic field. Arbitrarily fine magnetic domains can be created by changing the laser scanning path, which allows the object to obtain a predetermined complex shape transformation after being stimulated by an external magnetic field. It should be noted that the fixation of temporary shape and the recovery of permanent shape are achieved based on the shape memory properties of the polymer object itself. The magnetic field applied after the object is manufactured (named magnetic field II) provides the force that deforms the object, while the role of the magnetic field applied when the object being manufactured (named magnetic field I) is to redirect the magnetic fillers to obtain a bigger force when affected by magnetic field II, and to control the distribution of the magnetic filler thus the direction and degree of deformation of the printed object. In addition to the magnetic field, electric and sound fields are also commonly used to achieve aligned fillers in polymer object [301, 327, 340–343]. For example, Tang et al [327] developed an LCE object with an array of anisotropy-arranged nanofillers by using electric-field-assisted DLP, as shown in figure 12(e). The degree of anisotropy of nano-fillers could be controlled by changing the strength of the electric field and the applied time, and then the deformation of the object could be controlled. Besides, the angle between the orientation of the nano-filler and the orientation of LCE could be controlled by changing the printing path, thus the object could be deformed along the predetermined direction to obtain the complex structure. More methods for obtaining the aligned fillers can be found in this paper summarized by Xu et al [344].

5.2.2. *Degree of crosslinking/crystallization.* Changing the degree of crosslinking/crystallization by process parameter-encoded is a simple and efficient strategy to achieve 4D printing, which is generally found in VP [56, 72, 345–352]. For one thing, the polymer object printed by VP is generally easy to deform due to its relatively poor mechanical properties. For another thing, the polymer network with different crosslinking densities can be obtained by changing the wavelength, intensity and exposure time of the light source in VP, which is easy to operate with high resolution. For the process parameter-encoded in VP, a common method is obtained by gradient printing of material through gray exposure to achieve 4D printing, which is also referred to as grayscale 4D printing [54, 328]. As shown in figure 12(f), Kuang et al [328] fabricated an object with adjustable mechanical properties gradient using grayscale DLP, and the results indicated that the degree of crosslinking and modulus of the object decreased
<figure><img src="image_19.png" /><figcaption>**Figure 12.** Technologies of 4D printing based on process parameter-encoded. (a) Inducing orientation of molecular chains by changing printing path in FDM. Reprinted from [49], © 2022 Elsevier B.V. All rights reserved. Inducing orientation of fillers by (b) shear forces in DIW. Reprinted from [324], © 2023 Elsevier B.V. All rights reserved. Field assistance of (c) magnetic field in DIW. Reproduced from [325], with permission from Springer Nature. (d) Magnetic field in TPP. Reproduced from [326]. **CC BY 4.0**. (e) Electric field in DLP. [327] John Wiley & Sons. © 2022 Wiley-VCH GmbH. Changing the degree of crosslinking by (f) grayscale light. Reproduced with permission from [328] Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (**CC BY-NC**). (g) Changing exposure time. Reproduced from [329]. **CC BY 4.0**. (h) Changing wavelengths. Reproduced from [330]. **CC BY 4.0**.</figcaption></figure>
with the increase of gray percentage, thus affecting the $T_g$ and achieving sequential deformation of the object. In fact, the degree of crosslinking affects the entropy elasticity of the polymer object. Therefore, when the polymer object exhibits a gradient degree of crosslinking, it will lead to the difference of deformation degree in different parts of the object, and more importantly, affect the difference of the $T_g$ thus the response speed of the object to the external stimulus, realizing sequential deformation. Changing the exposure time is another common method to achieve the above purposes [55, 329, 353–355]. Nojoomi et al [329] developed a hydrogel object that could adjust the degree of crosslinking at different positions of the object by varying the exposure time, as shown in figure 12(g), thus controlling the speed and degree of expansion and contraction. And the obtained object could be deformed sequentially after being stimulated by heat. Then they introduced the concept of cone singularities to control exposure time, increasing the accessible space of 3D shapes [353]. Four-dimensional (4D) printing can also be achieved by changing the wavelength of the light source to change the curing sequence of different materials. For example, Schwartz and Boydston [330] explored the use of different light sources in DLP, as shown in figure 12(h). The light source with different wavelengths would affect the polymerization and curing of the material thus its crosslinking degree, and finally realize the control of the spatial heterogeneity of the object to achieve 4D printing.

Compared with changing the degree of crosslinking, it is difficult to achieve 4D printing by changing the degree of crystallization of polymer object through parameter code, which is attributed to two factors: (I) continuous and rapid changes in the printing and cooling temperature are essential to obtain gradient degree of crystallization, which is difficult to achieve in technically; (II) temperature changes will change the bond strength between the layers of the printed polymer object, and more importantly, affect the residual stress in the object, easily leading to manufacturing failure or making the shape change difficult to be predicted. Therefore, there is almost no relevant research on 4D printing based on parameter-encoded by changing the degree of crystallization of the polymer object via temperature changes. We hypothesize that by changing the molecular weight of the polymer and controlling the distribution of the material, it would be more convenient to make the polymer object possess a gradient degree of crystallization, thus achieving 4D printing.

In summary, it is a general and economical strategy to achieve 4D printing by encoding the process parameters, making full use of existing printing technology. It is worth noting that when subjected to external stimulation: if the force driving the deformation of the object comes from the stress stored in the object during the manufacturing process, the deformation of the object is irreversible, and the printed structure is a temporary shape of the object; if the force driving the deformation of the object comes from the difference in properties (such as water absorption, thermal expansion, etc), the deformation of the object is irreversible, and the printed support structure is a permanent shape. For the 4D printing based on process

parameter-encoded, the challenge is to improve the consistency between the pre-designed deformed shape of the polymer object and the actual results, which requires understanding the relationship among process parameters, microstructures and macroproperties of the object. However, the diversity of printing technologies and materials undoubtedly further increases the complexity, and the application of these polymer objects is limited due to the differences in the properties of different parts.

### 5.3. Technologies of 4D printing based on multi-material printing

The inconsistency of properties at different locations of the object can be realized via 3D printing which possesses the ability to precisely distribute different materials, thus leading to a macroscopical response to achieve 4D printing similar to that by process parameter-encoded. Moreover, more complex structures can be obtained based on multi-material printing, but this also requires the manufacturing technology to possess multi-material printing capabilities [18, 356]. Two approaches can be employed to achieve this capability: the combination of a variety of 3D printing technologies (namely hybrid additive manufacturing), and single 3D printing technology to print multi-materials. The former can exert the advantages of different printing technologies, and a variety of hybrid additive manufacturing technologies have been developed [357–364]. However, there are few reports of 4D printed object by hybrid additive manufacturing, which may be partly due to the relatively high cost and low printing efficiency. A typical case is that Peng et al [361] developed a hybrid 3D printing technology combining DIW and DLP, as shown in figure 13(a). This hybrid additive manufacturing fully realized the complementary advantages of DIW and DLP, and obtained the freestanding structure combining functionally freestanding structure (printed by DIW) and supporting structures (printed by DLP), which would collapse and deform after being heated. In contrast, multi-material printing by single printing technology is blossoming in the field of 4D printing.

The complexity of polymer objects' structure constructed by different 3D printing technologies is different for multi-material printing. Specifically, the multi-material printing ability of SLS is relatively poor, which can only control the distribution of different materials in different layers to prepare multi-layer composite structures [153]. FDM, DIW, MJ or VP can control the distribution of different materials in the same printing layer, possessing relatively good multi-material printing capability. For FDM, Liu et al [365] fabricated objects with heterogeneous structures by FDM that equipped multiple nozzles, as shown in figure 13(b). The objects were deformed in solution due to the difference in water absorption capacity of different materials, and the predetermined deformation of objects could be achieved by controlling the distribution positions of different materials. DIW exhibits strong spatial distribution configuration of heterogeneous materials, enabling 4D printing of the polymer object with complex structures. A variety of multi-material manufacturing technologies based
<figure><img src="image_20.png" /><figcaption>**Figure 13.** Technologies of 4D printing based on multi-material printing. (a) Diagram and structure cases for 4D printing using hybrid additive manufacturing. [361] John Wiley & Sons. © 2022 Wiley-VCH GmbH. Diagram and structure cases for 4D printing using single printing technology of (b) FDM equipped multiple nozzles. Reproduced from [365]. **CC BY 4.0.** (c) DIW with rotational co-extrusion ability. Reprinted from [61], © 2023 Elsevier B.V. All rights reserved. (d) MJ equipped multiple nozzles. Reproduced with permission from [58]. Copyright © 2017, The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited. (e) PµSL by exchanging photosensitive resins. Reproduced from [60]. **CC BY 4.0.** (f) VP using centrifugal force to achieve material switching. Reproduced from [59]. **CC BY 4.0.**</figcaption></figure>
on DIW have been developed, and among them a simple and
effective way to realize 4D printing is configuring many ink
accumulators or nozzles [336, 366-368], as reported by Choi
et al [366]. And many more sophisticated methods have also
been developed [61, 369, 370]. As shown in figure 13(c),
Ren et al [61] developed a novel rotational co-extrusion DIW
technique to achieve twisted structural filaments with a con-
trolled distribution of heterogeneous materials. By controlling
the spatial distribution configuration of different materials, the
obtained objects could deform when applying thermal stim-
ulation. MJ can also be equipped with multiple ink nozzles
for simple and efficient multi-material printing. As shown in
figure 13(d), Ding et al [58] used polyJet (one of the MJ
subtypes) to spray multi-material ink droplets along prede-
termined printing paths to obtain an object with a heterogen-
eous structure. And the object would obtain a predetermined
deformed structure after applied thermal stimulation, due to
the coefficient of thermal expansion mismatch between dif-
ferent materials. For the multi-material manufacturing tech-
nologies based on VP [43, 59, 60], 4D printing of the poly-
mer object can be achieved by switching tanks containing dif-
ferent photosensitive resins during the manufacturing process,
but there is a risk of feedstock cross-contamination. As shown
in figure 13(e), Ge et al [60] achieved multi-material manu-
facturing using PµSL technology by exchanging photosensit-
ive resins during the manufacturing process. The thermo-
mechanical behavior of the obtained object was controlled
by controlling the materials distribution, and the object was
sequentially deformed after being heated. Recently, Chen et al
[59] reported a multi-material manufacturing process based on
VP that used centrifugal force to achieve material switching,
as shown in figure 13(f), achieving efficient control of the spa-
tial arrangement of each material voxel.

A pivotal feature is that the direction and degree of deform-
ation of polymer object under external stimulation are determ-
ined by the distribution of materials, for the 4D printing based
on multi-material printing. And the shape of the polymer
object will gradually return to its original printed shape when
the external stimulation is removed again. Therefore, SMP is
usually used as one of the raw materials, using its characterist-
ics to fix the temporary shape. In summary, the development
of multi-material printing technology promotes the complex-
ity of the printed structure and the ability to construct the het-
erogeneous polymer object more simply and efficiently, which
can greatly promote the development and application of 4D
printing.

6. Biomedical applications

Personalization is an increasingly important feature of implant
in biomedical applications, which is paid more and more atten-
tion to realizing the customization of implant according to the
individual and the injury part. Specifically, the personaliza-
tion of an implant is not only reflected in the perfect match
between the implant and the injury part in the structure but
also the precise control of the composition of the different pos-
itions of the implant [371–377]. The concept of 4D printing

is put forward based on 3D printing, and the technologies
used for 4D printing are the same as those used for 3D print-
ing in terms of principle as mentioned above, therefore 4D
printing can also control the structure and composition of the
implant on the micro and macro scales, making the 4D prin-
ted implant personalized [25, 378–382]. For example, Zhao
et al [378] proposed the concept of personalized 4D printing
of bioinspired tracheal implants with optimal hole size and
high supporting capacity. The 4D printed tracheal implant
could match the geometry of the trachea and exhibit better
supporting and fixation. Li et al [379] reported 4D-printed
dual-responsive implants for treating critical-sized irregular
bone defect, the obtained implants could realize the minim-
ally invasive implantation and precise navigation in irregular
bone defect and improve osteogenesis and angiogenesis. Bond
et al [381] reported a 4D printed hierarchically porous struc-
ture by DLP, allowing for precise control over elastic modulus,
porosity, structure and transition temperature for reducing the
invasiveness of implantation.

Moreover, the dynamic properties of the 4D printed poly-
mer object exhibit superior and unique features in biomed-
ical applications compared to the static properties of the 3D
printed polymer object [8, 13, 240, 383–385]. The structure or
function of the 4D printed object will be maintained after the
external stimulation is removed, which is different from the
pulse-like transformation caused by other material character-
istics (e.g. piezoelectric properties). Therefore, the 4D prin-
ted object is suitable for minimally invasive and possesses a
significant advantage in easily obtaining uniform cell distri-
bution in irregular structures, making its application in ves-
sel (required micro-structure), heart (required elastic prop-
erty), bone (required load-bearing function) and other tissues
blossoming.

6.1. Vessel

Vascular implant is one of the earliest applications of 4D print-
ing in the medical field [386, 387]. The commonly employed
drug-eluting vascular implant is non-degradable, leading to
endovascular restenosis and late thrombosis. The concept of
biodegradable vascular implant has been proposed to tackle
these drawbacks. However, the biodegradable implant is diffi-
cult to implant through minimally invasive surgery. This chal-
lenge is expected to be overcome by 4D printing.

The vascular implant exhibits a simple tubular structure and
can be obtained by crimp of the 4D printed membrane. And
the membrane structure can obtain fine surface micro-patterns
more easily than that of the tubular structure, which facilitates
the interaction of vascular implant with surrounding tissues
[257, 388–390]. For example, Kirillova et al [388] reported a
polymer film with a crosslinking gradient that would self-fold
to form a hollow tube in seconds when exposed to water or
phosphate buffer solution. The self-folding tube with the low-
est average internal tube diameter of 20 µm, about the same
size as the minimum vessel diameter, was fabricated by DIW.
And the self-folding tube could support cell survival without
reducing cell viability. Moreover, the 4D printed vascular
implant also could remotely control self-expansion [251, 259,
260, 391–394]. As shown in figure 14(a), Zhang *et al* [260] developed a shape memory mesh vascular implant with a suitable modulus fabricated by DIW. The *in vitro* experiment resulted demonstrated that the implant imposed force on the vessel wall and enlarged the diameter when recovering its permanent shape from a temporary one. And the result of the *in vivo* vascular graft experiment demonstrated that the implant provided a biocompatibility environment for endothelial cell attachment and growth, facilitating endothelial growth and vascular remodeling. Shin *et al* [394] fabricated a diffuser-shaped SMP tube with a diameter of about 2 mm by 3D printing. The tube recovery to its original diameter by local heating which could reduce tissue damage during deployment, and the elastic eigenvalue of the tube could minimize the obstruction of cyclic contraction of arteries. The *in vivo* experiment resulted showed that the patency of the transplanted pig blood vessels was improved with occlusive vascular remodeling. The 4D printed implant also possessed other functions [395, 396], such as blocking blood flow and playing the opposite function of unclogging blood vessels. As shown in figure 14(b), Wang *et al* [396] reported a shape-adaptive 4D microcoil to treat artery aneurysms by causing thrombosis and abnormal blood flow. The result of *in vivo* experiment showed that the microcoil possessed a high biocompatibility and thrombosis capacity, which could promote the growth of new endothelial cells and collagen fibers to cure aneurysms.

## 6.2. Heart

The cardiac patch exhibits a curved and ordered structure, which is difficult to precisely fabricate by 3D printing due to the layer-by-layer characteristics. However, this challenge can be easily solved by 4D printing [269, 397, 401–405]. As shown in figure 14(c), Cui *et al* [397] prepared a cross-linked gradient cardiac patch by SLA, which was able to be self-adaptable in an aqueous solution to achieve a conformation identical to the curvature of the heart. The patch showed good cellular compatibility, and the cells were uniformly and orderly distributed on its surface. The result of *in vivo* experiment demonstrated that the patch possessed a strong vascularization ability and could improve heart function for the treatment of myocardial infarction. The occlusion devices are another potential application of 4D printing in heart tissue [245, 246]. As shown in figure 14(d), Lin *et al* [246] developed a 4D printed shape memory occlusion device with biodegradability, which could be deployed under magnetic field stimulation. The occluder exhibited good cytocompatibility and histocompatibility, and could promote the regeneration of vessels and endothelium, which provided a new method for the treatment of atrial septal defect.

## 6.3. Bone

The application of 4D printing in bone tissue engineering also exhibits great advantages [17]. At present, 3D-printed artificial bone transplantation is mainly operated through traditional open surgery, which has challenges such as a long time, a large wound, a large amount of blood loss during surgery,

long postoperative recovery time, and easy-to-reappear wound infection and other complications. It is viable to apply the minimally invasive concept to bone transplantation by 4D printing. In detail, the artificial bone implant is programmed to a small temporary shape and then switched to its permanent shape with a large dimension when exposed to specific stimulation after transplantation. The adaptive ability of the 4D printed bone implant can accurately match the irregular bone defect area, and the contact force between the implant and the surrounding tissue can be adjusted to avoid bone nonunion [398, 399, 406–412]. Zhang *et al* [398] developed a shape memory bone implant with near-infrared responsiveness by ME, as shown in figure 14(e). The compressed implant would recover to its original shape under the stimulation of near-infrared light after being implanted in the body and realizing close contact with the surrounding tissues. The implant showed good biocompatibility, and the result *in vivo* experiment showed more new tissue regeneration with enhanced bone integration. As shown in figure 14(f), You *et al* [399] proposed a bilayer membrane with multiple response-ability for bone defect repair fabricated by DLP. The membrane would be deformed under the stimulation of the physiological environment to locate the defective bone after implantation. And the microstructures of the membrane could switch from a flat state (temporary shape) to a columnar array (permanent shape) under thermal stimulation, thus realizing a controlled transition of the function of the membrane from promoting cell adhesion to osteogenic differentiation. The results of *in vitro* and *in vivo* experiments indicated that the membrane possessed the ability to remotely regulate stem cell fate and accelerate bone healing.

However, a big challenge remains for the 4D printed bone implant based on SMP: the large difference in the strength of bone implant below and above the $T_g$. If the $T_g$ of the bone implant is below the body temperature, the implant can not provide adequate strength. If the $T_g$ of the bone implant is above the body temperature, it is easy to damage the surrounding tissue when being applied thermal stimulation. In this case, developing a water-triggered stiffening of polymer will be a possible solution [365, 413].

## 6.4. Others

The applications of 4D printing in other tissues, including nerves, eyes, muscles and trachea, etc, have also been extensively studied [205, 247, 254, 263, 265, 400, 414–425]. As shown in figure 14(g), Sun *et al* [400] proposed a strategy based on a 4D-oriented dynamic implant to remotely regulate the behavior and fate of neurons for nerve regeneration. The implant presented dynamic micromorphic surfaces under external stimulation, and the mechanical stimulation caused by micromorphologic changes could further promote neuronal differentiation and maturation, etc. The result of *in vivo* experiment showed that the implant could facilitate the repair of large segmental nerve defects, with a good therapeutic effect comparable to that of autologous nerve transplantation. As shown in figure 14(h), Deng *et al* [254] reported a 4D printed orbital implant with shape memory properties for the
<figure><img src="image_21.png" /><figcaption>**Figure 14.** Biomedical applications of 4D printing. The schematic and *in vivo* results of vascular implants used for (a) unclogging blood vessels. Reprinted from [260], © 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. (b) Blocking blood flow. [396] John Wiley & Sons. © 2023 Wiley-VCH GmbH. The schematic and *in vivo* results of 4D printing in heart applications including (c) cardiac patch. Reproduced with permission from [397]. Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). (d) Occlusion devices. [246]. John Wiley & Sons. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. The schematic and *in vivo* results of bone implants used for (e) realizing close contact. Reproduced with permission from [398]. © 2022 The Authors. Published by KeAi Communications Co., Ltd. CC BY-NC-ND 4.0. (f) Regulate stem cell. [399] John Wiley & Sons. © 2021 Wiley-VCH GmbH. The schematic and *in vivo* results of 4D printing implant used in (g) nerves application. [400] John Wiley & Sons. © 2023 Wiley-VCH GmbH. (h) Eyes application. Reprinted from [254], © 2022 Elsevier Ltd. All rights reserved.</figcaption></figure>
treatment of enophthalmic invagination. The implant could recover to its permanent shape under thermal stimulation after precise minimally invasive deployment, and exhibited a good volume-filling capacity. Besides, the result of *in vivo* experiment showed that the implant possessed good biosafety and was conducive to the growth of orbital tissue. Weems *et al* [265] developed a minimally invasive 4D implant fabricated by DLP with a self-fitting property for the repairing of adipose tissue. The implant took on the void geometry with low expansion force, keeping the surrounding tissue from deforming. And result of the *in vivo* experiment exhibited that the implant was conducive to neovascularization.

## 7. Conclusions and perspectives

In this review, based on the viewpoint that the main difference between 4D printing and 3D printing lies in the smart materials and the influence of the manufacturing process on structure, 4D printing polymer object is introduced from the perspective of ‘mechanism-manufacturing’, and the biomedical applications are briefly analyzed. Four-dimensional (4D) printing is complex, which is not only embodied before the object is fabricated, but also during the fabrication, and more importantly after it has been fabricated. Before fabrication, it is necessary to design the material molecular structure of the polymer and select the appropriate material system to ensure that the printed object possesses the ability to transform into a specific shape or function. During fabrication, the influence of process parameters on the properties of the object needs to be considered, in order to ensure that the printed object can make precise shape or function transformations in response to external stimulation. After fabrication, the ‘program-stimulate’ progress needs to be carried out to ensure that the object can be applied to a specific scenario. Although the concept of 4D printing has been around for more than ten years and has been explored and applied in many different fields such as aerospace, robotics and biomedicine, many challenges remain urgently to be conquered in terms of materials, manufacturing technologies and integration with existing technologies.

Firstly, material is the main factor that determines the type of deformation and the type of stimulation for a 4D printed polymer object. The classifications of 4D printing are various, among which multi 4D printing, two-way 4D printing and the selective deformation in 4D printing are worth noting due to their potential to be applied to complex scenarios. However, it is difficult to integrate all these features simultaneously, accompanied by consideration of the strength, biosafety, and other characteristics of the material to match the application environment. And the type of stimulation needs to be considered according to the application environment, which undoubtedly further deepens the difficulty of designing the molecular structure of the material and the material system. Besides, degradability and recyclability cannot be ignored for reducing pollution. Based on the above background, it is expected that the development of polymers

containing ‘covalent adaptable networks’ and supramolecular interactions is expected to meet the complex needs of 4D printing. In addition, the dynamic bonds are rich in types and can also endow printed objects with new functions, which will promote the development of 4D printing.

Secondly, the development of manufacturing technologies is an important factor in achieving the 4D printed polymer object out of the laboratory. On the one hand, the current printing speed of additive manufacturing is still slow, and it is still difficult to achieve large-scale production of product. On the other hand, the manufacturing technologies determine the accuracy of the printed structure, thus determining whether the response of the implant is accurate and whether it can accurately play its function. Therefore, the current researches mainly focus on improving the flexibility of manufacturing technologies and improving the accuracy and complexity of printed objects. To improve the flexibility of manufacturing technologies, it is necessary to reconstruct or design according to the processing principles and processing control processes of different technologies. To improve the accuracy and complexity of the printed object, one promising strategy is the development of volumetric additive manufacturing. Volumetric additive manufacturing can realize object fabrication at any point in space, which is no longer constrained by the lack of accuracy caused by the layer-by-layer characteristics and realize the fabrication of any complex structure. The key to volumetric additive manufacturing is the choice of medium that reacts with the material. Light carries a lot of information including phase, light intensity, amplitude, frequency, etc. The different information carried by light in different positions of space will lead to differences in the reaction with the material, to realize the forming of the object in the selected space and the precise control of the properties of different parts of the printed object so that the reaction with the material is different. While light has poor penetration and is easy to generate heat, thus ultrasound can be used as a medium to achieve volume printing. The technology of volumetric additive manufacturing is still in its infancy, and it is expected to promote the breakthrough and development of manufacturing technologies in 4D printing.

Finally, it is essential for the integration of 4D printing and existing technology. The whole 4D printing includes the design of materials, the design of multi-scale structures and the manufacture and application of objects, involving various disciplines including materials, chemistry and mechanics, manufacturing, computer science, and so on. The interdisciplinarity integration is indispensable to achieve the guidance for the design of 4D printed objects and the prediction of the dynamic characteristics of 4D printed objects. Besides, the 4D printed object as a module or voxel, combined with modern electronic technology, computer technology, etc, to achieve the integration of their characteristics, is an important factor in making 4D printed polymer objects intelligent. More researchers with interdisciplinarity experience are needed to apply 4D printing for their work, and is believed to accelerate the functionality and utility of 4D printing for various real-life applications.
## Acknowledgments

This work was supported by the following funds: (1) The Natural Science Foundation of China (52275393, 51935014, 82072084); (2) Jiangxi Provincial Natural Science Foundation of China (20224ACB204013); (3) The Project of State Key Laboratory of Precision Manufacturing for Extreme Service Performance; (4) National Key Research and Development Program of China (2023YFB4605800).

## ORCID iDs

Pei Feng https://orcid.org/0000-0003-0400-2110

Cijun Shuai https://orcid.org/0000-0002-2029-5112

## References

[1] Tibbits S 2013 The emergence of “4D printing” (available at: www.ted.com/talks/skylar_tibbits_the_emergence_of_4d_printing)

[2] Ma J, Franco B, Tapia G, Karayagiz K, Johnson L, Liu J, Arroyave R, Karaman I and Elwany A 2017 Spatial control of functional response in 4D-printed active metallic structures *Sci. Rep.* **7** 46707

[3] Kim D, Ferretto I, Leinenbach C and Lee W 2022 3D and 4D printing of complex structures of Fe-Mn-Si-based shape memory alloy using laser powder bed fusion *Adv. Mater.* *Interfaces* **9** 22020171

[4] Caputo M P, Berkowitz A E, Armstrong A, Müllner P and Solomon C V 2018 4D printing of net shape parts made from Ni-Mn-Ga magnetic shape-memory alloys *Addit. Manuf.* **21** 579–88

[5] Chen W, Gu D D, Yang J K, Yang Q, Chen J and Shen X F 2022 Compressive mechanical properties and shape memory effect of NiTi gradient lattice structures fabricated by laser powder bed fusion *Int. J. Extrem. Manuf.* **4** 045002

[6] Wei S S, Zhang J L, Zhang L, Zhang Y J, Song B, Wang X B, Fan J X, Liu Q and Shi Y S 2023 Laser powder bed fusion additive manufacturing of NiTi shape memory alloys: a review *Int. J. Extrem. Manuf.* **5** 032001

[7] Zhang J, Xu M K, Zhang N, Wang T M, Zhang Y M, Yang Z H and Wang Q H 2024 Advances and applications of 4D-printed high-strength shape memory polymers *Addit. Manuf. Front.* **3** 200115

[8] González-Henríquez C M, Sarabia-Vallejos M A and Rodriguez-Hernandez J 2019 Polymers for additive manufacturing and 4D-printing: materials, methodologies, and biomedical applications *Prog. Polym. Sci.* **94** 57–116

[9] Chen M, Gao M, Bai L C, Zheng H, Qi H J and Zhou K 2023 Recent advances in 4D printing of liquid crystal elastomers *Adv. Mater.* **35** 2209566

[10] Wang R, Yuan C, Cheng J X, He X N, Ye H T, Jian B C, Li H G, Bai J M and Ge Q 2024 Direct 4D printing of ceramics driven by hydrogel dehydration *Nat. Commun.* **15** 758

[11] Liu G et al 2023 4D additive-subtractive manufacturing of shape memory ceramics *Adv. Mater.* **35** 2302108

[12] Li T, Liu Q Y, Qi H B and Zhai W 2022 Prestrain programmable 4D printing of nanoceramic composites with bioinspired microstructure *Small* **18** 2204032

[13] Wang Y, Cui H T, Esworthy T, Mei D Q, Wang Y C and Zhang L G 2022 Emerging 4D printing strategies for next-generation tissue regeneration and medical devices *Adv. Mater.* **34** 2109198

[14] Wan X et al 2024 Recent advances in 4D printing of advanced materials and structures for functional applications *Adv. Mater.* **36** 2312263

[15] Lyu Z Y, Wang J L and Chen Y F 2023 4D printing: interdisciplinary integration of smart materials, structural design, and new functionality *Int. J. Extrem. Manuf.* **5** 032011

[16] Han X Y, Saiding Q, Cai X L, Xiao Y, Wang P, Cai Z W, Gong X, Gong W M, Zhang X C and Cui W G 2023 Intelligent vascularized 3D/4D/5D/6D-printed tissue scaffolds *Nano-Micro. Lett.* **15** 239

[17] Chen X, Han S Y, Wu W H, Wu Z H, Yuan Y, Wu J and Liu C S 2022 Harnessing 4D printing bioscaffolds for advanced orthopedics *Small* **18** 2106824

[18] Chen A N et al 2024 Multimaterial 3D and 4D bioprinting of heterogenous constructs for tissue engineering *Adv. Mater.* **36** 2307686

[19] Yan S Y, Zhang F H, Luo L, Wang L L, Liu Y J and Leng J S 2023 Shape memory polymer composites: 4D printing, smart structures, and applications *Research* **6** 0234

[20] Mahmood A, Akram T, Shenggui C and Chen H F 2023 Revolutionizing manufacturing: a review of 4D printing materials, stimuli, and cutting-edge applications *Composites B* **266** 110952

[21] Shahbazi M, Jäger H, Ettelaie R, Mohammadi A and Kashi P A 2023 Multimaterial 3D printing of self-assembling smart thermo-responsive polymers into 4D printed objects: a review *Addit. Manuf.* **71** 103598

[22] Ren L, Wang Z G, Ren L Q, Xu C, Li B Q, Shi Y S and Liu Q P 2023 Understanding the role of process parameters in 4D printing: a review *Composites B* **265** 110938

[23] Zhou X L, Ren L, Song Z, Li G, Zhang J, Li B, Wu Q, Li W, Ren L and Liu Q 2023 Advances in 3D/4D printing of mechanical metamaterials: from manufacturing to applications *Composites B* **254** 110585

[24] de Kergariou C, Demoly F, Perriman A, Le Duigou A and Scarpa F 2023 The design of 4D-printed hygromorphs: state-of-the-art and future challenges *Adv. Funct. Mater.* **33** 2210353

[25] Yarali E, Mirzaali M J, Ghalayaniesfahani A, Accardo A, Diaz-payno P J and Zadpoor A A 2024 4D printing for biomedical applications *Adv. Mater.* **36** 2402301

[26] Shi H, Zhang M and Mujumdar A S 2024 3D/4D printed super reconstructed foods: characteristics, research progress, and prospects *Comprehensive Rev. Food Sci. Food Saf.* **23** e13310

[27] Huang J J, Xia S J, Li Z A, Wu X W and Ren J N 2021 Applications of four-dimensional printing in emerging directions: review and prospects *J. Mater. Sci. Technol.* **91** 105–20

[28] Li H G et al 2024 Reconfigurable 4D printing via mechanically robust covalent adaptable network shape memory polymer *Sci. Adv.* **10** eadl4387

[29] Webber M J and Tibbitt M W 2022 Dynamic and reconfigurable materials from reversible network interactions *Nat. Rev. Mater.* **7** 541–56

[30] Machado T O, Stubbs C J, Chiaradia V, Alraddadi M A, Brandolese A, Worch J C and Dove A P 2024 A renewably sourced, circular photopolymer resin for additive manufacturing *Nature* **629** 1069–74

[31] Liang C, Cui J J, Sun Y D, Lu Z, Liu F K, Feng S W, Hu G, Cui J and Zhang B 2024 Digital light 3D printed fast- and controlled degradation of covalent hydrogel networks *Chem. Eng. J.* **480** 148027

[32] Zhao Q, Qi H J and Xie T 2015 Recent progress in shape memory polymer: new behavior, enabling materials, and mechanistic understanding *Prog. Polym. Sci.* **49–50** 79–120
[33] Xia Y L, He Y, Zhang F H, Liu Y J and Leng J S 2021 A review of shape memory polymers and composites: mechanisms, materials, and applications *Adv. Mater.* **33** 2000713

[34] Wang X F, He Y, Liu Y J and Leng J S 2022 Advances in shape memory polymers: remote actuation, multi-stimuli control, 4D printing and prospective applications *Mater. Sci. Eng.* R **151** 100702

[35] Lendlein A and Gould O E C 2019 Reprogrammable recovery and actuation behaviour of shape-memory polymers *Nat. Rev. Mater.* **4** 116–33

[36] Khalid M Y, Arif Z U, Noroozi R, Zolfagharian A and Bodaghi M 2022 4D printing of shape memory polymer composites: a review on fabrication techniques, applications, and future perspectives *J. Manuf. Process.* **81** 759–97

[37] Wang Y, Sachyani Keneth E, Kamyshny A, Scalet G, Auricchio F and Magdassi S 2022 4D multimaterial printing of programmable and selective light-activated shape-memory structures with embedded gold nanoparticles *Adv. Mater. Technol.* **7** 2101058

[38] Tekay E, Aybakan B and Aslan V U 2023 4D printable maleated poly(styrene-*b*-ethylene/butylene- *b*-styrene)/poly(ethylene-*co*-vinyl acetate) thermoplastic elastomer blends; effects of blend composition on shape memory and mechanical properties *Addit. Manuf.* **78** 103870

[39] Fang Z Z, Lu R Z, Chen J D, Zhao Q and Wu J J 2022 Vat photopolymerization of tough glassy polymers with multiple shape memory performances *Addit. Manuf.* **59** 103171

[40] Wang F F, Luo F Z, Huang Y X, Cao X S and Yuan C 2023 4D printing via multispeed fused deposition modeling *Adv. Mater. Technol.* **8** 2201383

[41] van Manen T, Janbaz S, Jansen K M B and Zadpoor A A 2021 4D printing of reconfigurable metamaterials and devices *Commun. Mater.* **2** 56

[42] Ding Z, Weeger O, Qi H J and Dunn M L 2018 4D rods: 3D structures via programmable 1D composite rods *Mater. Des.* **137** 256–65

[43] Zhang Q, Kuang X, Weng S Y, Yue L, Roach D J, Fang D N and Qi H J 2021 Shape-memory balloon structures by pneumatic multi-material 4D printing *Adv. Funct. Mater.* **31** 2010872

[44] Chen S Y, Zhang Q L and Feng J C 2017 3D printing of tunable shape memory polymer blends *J. Mater. Chem. C* **5** 8361–5

[45] Wan X, He Y, Liu Y J and Leng J S 2022 4D printing of multiple shape memory polymer and nanocomposites with biocompatible, programmable and selectively actuated properties *Addit. Manuf.* **53** 102689

[46] Zhang H *et al* 2023 Regulation of electrically responsive shape recovery behavior of 4D printed polymers for application in actuators *Addit. Manuf.* **78** 103836

[47] Lin C, Xin X Z, Tian L F, Zhang D, Liu L W, Liu Y J and Leng J S 2024 Thermal-, magnetic-, and light-responsive 4D printed SMP composites with multiple shape memory effects and their promising applications *Composites B* **274** 111257

[48] Goo B, Hong C H and Park K 2020 4D printing using anisotropic thermal deformation of 3D-printed thermoplastic parts *Mater. Des.* **188** 108485

[49] Koh T Y and Sutradhar A 2022 Untethered selectively actuated microwave 4D printing through ferromagnetic PLA *Addit. Manuf.* **56** 102866

[50] Zarna C, Rodríguez-Fabià S, Echtermeyer A T and Chinga-Carrasco G 2022 Preparation and characterisation of biocomposites containing thermomechanical pulp fibres, poly(lactic acid) and

poly(butylene-adipate-terephthalate) or poly(hydroxyalkanoates) for 3D and 4D printing *Addit. Manuf.* **59** 103166

[51] Ren L Q, Li B Q, Song Z Y, Liu Q P, Ren L and Zhou X L 2019 Bioinspired fiber-regulated composite with tunable permanent shape and shape memory properties via 3D magnetic printing *Composites B* **164** 458–66

[52] Wang Q R, Tian X Y, Huang L, Li D C, Malakhov A V and Polilov A N 2018 Programmable morphing composites with embedded continuous fibers by 4D printing *Mater. Des.* **155** 404–13

[53] Zeng C J, Liu L W, Bian W F, Liu Y J and Leng J S 2020 4D printed electro-induced continuous carbon fiber reinforced shape memory polymer composites with excellent bending resistance *Composites B* **194** 108034

[54] Wu J T, Zhao Z A, Kuang X, Hamel C M, Fang D N and Qi H J 2018 Reversible shape change structures by grayscale pattern 4D printing *Multifunct. Mater.* **1** 015002

[55] Zhang Y, Huang L M, Song H J, Ni C J, Wu J J, Zhao Q and Xie T 2019 4D printing of a digital shape memory polymer with tunable high performance ACS Appl. Mater. Interfaces **11** 32408–13

[56] Peng W J *et al* 2020 Light-coded digital crystallinity patterns toward bioinspired 4D transformation of shape-memory polymers *Adv. Funct. Mater.* **30** 2000522

[57] van Manen T, Janbaz S and Zadpoor A A 2017 Programming 2D/3D shape-shifting with hobbyist 3D printers *Mater. Horiz.* **4** 1064–9

[58] Ding Z, Yuan C, Peng X R, Wang T J, Qi H J and Dunn M L 2017 Direct 4D printing via active composite materials *Sci. Adv.* **3** e1602890

[59] Cheng J X *et al* 2022 Centrifugal multimaterial 3D printing of multifunctional heterogeneous objects *Nat. Commun.* **13** 7931

[60] Ge Q, Sakhaei A H, Lee H, Dunn C K, Fang N X and Dunn M L 2016 Multimaterial 4D printing with tailorable shape memory polymers *Sci. Rep.* **6** 31110

[61] Ren L Q, Li W X, Liu H L, Li B Q, Zhou X L, Ren L, Han Z W, Song Z Y and Liu Q P 2023 Rotational Co-extrusion 4D printing of heterogeneous filaments to enable sophisticated shape morphing *Addit. Manuf.* **73** 103661

[62] Wang W, Li C Z, Cho M and Ahn S H 2018 Soft tendril-inspired grippers: shape morphing of programmable polymer-paper bilayer composites ACS Appl. Mater. Interfaces **10** 10419–27

[63] Yuan C, Wang F F, Qi B Y, Ding Z, Rosen D W and Ge Q 2020 3D printing of multi-material composites with tunable shape memory behavior *Mater. Des.* **193** 108785

[64] Moosabeiki V *et al* 2024 Curvature tuning through defect-based 4D printing *Commun. Mater.* **5** 10

[65] Peng B A, Yang Y C, Gu K, Amis E J and Cavicchi K A 2019 Digital light processing 3D printing of triple shape memory polymer for sequential shape shifting ACS Mater. Lett. **1** 410–7

[66] Wang L L, Zhang F H, Du S Y and Leng J S 2023 4D printing of triple-shape memory cyanate composites based on interpenetrating polymer network structures ACS Appl. Mater. Interfaces **15** 21496–506

[67] Li H Z, Gao X and Luo Y W 2016 Multi-shape memory polymers achieved by the spatio-assembly of 3D printable thermoplastic building blocks *Soft Matter* **12** 3226–33

[68] Rossegger E, Höller R, Reisinger D, Fleisch M, Strasser J, Wieser V, Griesser T and Schlögl S 2021 High resolution additive manufacturing with acrylate based vitrimers using organic phosphates as transesterification catalyst *Polymer* **221** 123631

[69] Rossegger E, Höller R, Reisinger D, Strasser J, Fleisch M, Griesser T and Schlögl S 2021 Digital light processing 3D
printing with thiol-acrylate vitrimers *Polym. Chem.* **12** 639–44

[70] Inverardi N, Pandini S, Bignotti F, Scalet G, Marconi S and Auricchio F 2020 Sequential motion of 4D printed photopolymers with broad glass transition *Macromol. Mater. Eng.* **305** 1900370

[71] Zhou K, Sun R, Wojciechowski J P, Wang R, Yeow J, Zuo Y, Song X, Wang C, Shao Y and Stevens M M 2024 4D multimaterial printing of soft actuators with spatial and temporal control *Adv. Mater.* **36** 2312135

[72] Huang S et al 2023 Digital light processing 4D printing multilayer polymers with tunable mechanical properties and shape memory behavior *Chem. Eng. J.* **465** 142830

[73] Wang K J, Jia Y G, Zhao C Z and Zhu X X 2019 Multiple and two-way reversible shape memory polymers: design strategies and applications *Prog. Mater. Sci.* **105** 100572

[74] Zare M, Prabhakaran M P, Parvin N and Ramakrishna S 2019 Thermally-induced two-way shape memory polymers: mechanisms, structures, and applications *Chem. Eng. J.* **374** 706–20

[75] Finkelmann H, Benné J and Semmler K 1995 Smectic liquid single crystal elastomers *Macromol. Symp.* **96** 169

[76] Bonetti L, Natali D, Pandini S, Messori M, Toselli M and Scalet G 2024 4D printing of semi-crystalline crosslinked polymer networks with two-way shape-memory effect *Mater. Des.* **238** 112725

[77] Wang Y and Li X 2021 4D-printed bi-material composite laminate for manufacturing reversible shape-change structures *Composites B* **219** 108918

[78] Shi Y P, Fang G Q, Cao Z L, Shi F Z, Zhao Q, Fang Z Z and Xie T 2021 Digital light fabrication of reversible shape memory polymers *Chem. Eng. J.* **426** 131306

[79] Behl M, Kratz K, Zotzmann J, Nöchel U and Lendlein A 2013 Reversible bidirectional shape-memory polymers *Adv. Mater.* **25** 4466–9

[80] Behl M, Kratz K, Noechel U, Sauter T and Lendlein A 2013 Temperature-memory polymer actuators *Proc. Natl Acad. Sci. USA* **110** 12555–9

[81] Chen G C, Dong J T, Xu X N, Zou W K, Jin B J, Peng W J, Zhao Q, Xie T and Zheng N 2022 Converse two-way shape memory effect through a dynamic covalent network design *J. Mater. Chem. A* **10** 10350–4

[82] Roach D J, Sun X H, Peng X R, Demoly F, Zhou K and Qi H J 2022 4D printed multifunctional composites with cooling-rate mediated tunable shape morphing *Adv. Funct. Mater.* **32** 2203236

[83] Lee A Y, An J, Chua C K and Zhang Y 2019 Preliminary investigation of the reversible 4D printing of a dual-layer component *Engineering* **5** 1159–70

[84] Wang Y and Li X 2021 4D printing reversible actuator with strain self-sensing function via structural design *Composites B* **211** 108644

[85] Wang X F, He Y and Leng J S 2023 Bionic shape memory polyurethane/Prussian blue nanoparticle-based actuators with two-way and programmable light-driven motions *ACS Appl. Polym. Mater.* **5** 1398–408

[86] Mao Y Q, Ding Z, Yuan C, Ai S G, Isakov M, Wu J T, Wang T J, Dunn M L and Qi H J 2016 3D printed reversible shape changing components with stimuli responsive materials *Sci. Rep.* **6** 24761

[87] Li H, Liang X Y and Song W B 2019 Buckling-controlled two-way shape memory effect in a ring-shaped bilayer *Acta Mech. Sin.* **35** 1217–25

[88] Kim D, Lee H S and Yoon J 2016 Highly bendable bilayer-type photo-actuators comprising of reduced graphene oxide dispersed in hydrogels *Sci. Rep.* **6** 20921

[89] Chen S J, Hu J L and Zhuo H T 2010 Properties and mechanism of two-way shape memory polyurethane composites *Compos. Sci. Technol.* **70** 1437–43

[90] Lee A Y, Zhou A W, An J, Chua C K and Zhang Y 2020 Contactless reversible 4D-printing for 3D-to-3D shape morphing *Virtual Phys. Prototyp.* **15** 481–95

[91] Wu S, Ze Q J, Zhang R D, Hu N, Cheng Y, Yang F Y and Zhao R K 2019 Symmetry-breaking actuation mechanism for soft robotics and active metamaterials *ACS Appl. Mater. Interfaces* **11** 41649–58

[92] Jiang S J et al 2019 Multifunctional Janus microplates arrays actuated by magnetic fields for water/light switches and bio-inspired assimilatory coloration *Adv. Mater.* **31** 1807507

[93] Chang L F, Wang D P, Huang Z S, Wang C F, Torop J, Li B, Wang Y J, Hu Y and Aabloo A 2023 A versatile ionomer-based soft actuator with multi-stimulus responses, self-sustainable locomotion, and photoelectric conversion *Adv. Funct. Mater.* **33** 2212341

[94] Zou B H, Liang Z H, Zhong D J, Cui Z M, Xiao K, Shao S and Ju J 2023 Magneto-thermomechanically reprogrammable mechanical metamaterials *Adv. Mater.* **35** 2207349

[95] Gao Y, Liu W F and Zhu S P 2019 Thermoplastic polyolefin elastomer blends for multiple and reversible shape memory polymers *Ind. Eng. Chem. Res.* **58** 19495–502

[96] Song H J, Fang Z Z, Jin B J, Pan P J, Zhao Q and Xie T 2019 Synergetic chemical and physical programming for reversible shape memory effect in a dynamic covalent network with two crystalline phases *ACS Macro Lett.* **8** 682–6

[97] Wong J-W et al 2023 Sustainable approach for the synthesis of a semicrystalline polymer with a reversible shape-memory effect *ACS Macro Lett.* **12** 563–9

[98] Yang G, Li H Q, Xing R Q, Lv M D, Ma C Q, Yan J and Zhuang X P 2023 Thermal-triggered “on-off” switchable triboelectric nanogenerator based on two-way shape memory polymer *Adv. Funct. Mater.* **33** 2214001

[99] Yuan W H, Liu K K, Zhou J, Ni L L, Shan G R, Bao Y Z and Pan P J 2020 Stress-free two-way shape memory effects of semicrystalline polymer networks enhanced by self-nucleated crystallization *ACS Macro Lett.* **9** 1325–31

[100] Li S Y et al 2023 4D printed thermoplastic polyamide elastomers with reversible two-way shape memory effect *Adv. Mater. Technol.* **8** 2202066

[101] Jiang Y X, Leng Q Y, Yan Y J, Ng E L L, Chee H L, Wang F K, Chan S Y, Loh X J, Wang J and Chan B Q Y 2022 4D printing of single-network shape memory polyurethanes with two-way actuation properties *ACS Appl. Polym. Mater.* **4** 8574–83

[102] Tobushi H, Hashimoto T, Hayashi S and Yamada E 1997 Thermomechanical constitutive modeling in shape memory polymer of polyurethane series *J. Intell. Mater. Syst. Struct.* **8** 711–8

[103] Liu Y P, Gall K, Dunn M L, Greenberg A R and Diani J 2006 Thermomechanics of shape memory polymers: uniaxial experiments and constitutive modeling *Int. J. Plast.* **22** 279–313

[104] Zhao F, Zheng X Y, Zhou S C, Zhou B, Xue S F and Zhang Y 2021 Constitutive model for epoxy shape memory polymer with regulable phase transition temperature *Int. J. Smart Nano Mater.* **12** 72–87

[105] Qi H J, Nguyen T D, Castro F, Yakacki C M and Shandas R 2008 Finite deformation thermo-mechanical behavior of thermally induced shape memory polymers *J. Mech. Phys. Solids* **56** 1730–51
[106] Kim J H, Kang T J and Yu W R 2010 Thermo-mechanical constitutive modeling of shape memory polyurethanes using a phenomenological approach *Int. J. Plast.* **26** 204–18

[107] Baghani M, Naghdabadi R, Arghavani J and Sohrabpour S 2012 A thermodynamically-consistent 3 D constitutive model for shape memory polymers *Int. J. Plast.* **35** 13–30

[108] Yarali E, Taheri A and Baghani M 2020 A comprehensive review on thermomechanical constitutive models for shape memory polymers *J. Intell. Mater. Syst. Struct.* **31** 1243–83

[109] Shojaei A, Xu W, Yan C, Yang Q X and Li G Q 2022 Insight in thermomechanical constitutive modeling of shape memory polymers *Front. Mech. Eng.* **8** 956129

[110] Huang R, Zheng S J, Liu Z S and Ng T Y 2020 Recent advances of the constitutive models of smart materials—hydrogels and shape memory polymers *Int. J. Appl. Mech.* **12** 2050014

[111] Athinarayanarao D, Prod'hon R, Chamoret D, Qi H J, Bodaghi M, André J C and Demoly F 2023 Computational design for 4D printing of topology optimized multi-material active composites *npj Comput. Mater.* **9** 1

[112] Sigmund O and Maute K 2013 Topology optimization approaches *Struct. Multidiscip. Optim.* **48** 1031–55

[113] Maute K, Tkachuk A, Wu J T, Qi H J, Ding Z and Dunn M L 2015 Level set topology optimization of printed active composites *J. Mech. Des.* **137** 111402

[114] Lumpe T S and Shea K 2021 Computational design of 3D-printed active lattice structures for reversible shape morphing *J. Mater. Res.* **36** 3642–55

[115] Liu J K et al 2018 Current and future trends in topology optimization for additive manufacturing *Struct. Multidiscip. Optim.* **57** 2457–83

[116] Geiss M J, Boddeti N, Weeger O, Maute K and Dunn M L 2019 Combined level-set-XFEM-density topology optimization of four-dimensional printed structures undergoing large deformation *J. Mater. Des.* **141** 051405

[117] Yu Y X, Qian K R, Yang H, Yao L N and Zhang Y J 2022 Hybrid IGA-FEA of fiber reinforced thermoplastic composites for forward design of AI-enabled 4D printing *J. Mater. Process. Technol.* **302** 117497

[118] de Kergariou C, Kim B C, Perriman A, Le Duigou A, Guessasma S and Scarpa F 2022 Design of 3D and 4D printed continuous fibre composites via an evolutionary algorithm and voxel-based finite elements: application to natural fibre hygromorphs *Addit. Manuf.* **59** 103144

[119] Hosseinzadeh M, Ghoreishi M and Narooei K 2023 4D printing of shape memory polylactic acid beams: an experimental investigation into FDM additive manufacturing process parameters, mathematical modeling, and optimization *J. Manuf. Process.* **85** 774–82

[120] Zolfagharian A, Durran L, Gharaie S, Rolfe B, Kaynak A and Bodaghi M 2021 4D printing soft robots guided by machine learning and finite element models *Sens. Actuators A* **328** 112774

[121] Tang K K, Xiang Y J, Tian J, Hou J X, Chen X Y, Wang X Q and Zhong Z 2024 Machine learning-based morphological and mechanical prediction of kirigami-inspired active composites *Int. J. Mech. Sci.* **266** 108956

[122] Sossou G, Demoly F, Belkebir H, Qi H J, Gomes S and Montavon G 2019 Design for 4D printing: modeling and computation of smart materials distributions *Mater. Des.* **181** 108074

[123] Ji Q L, Chen M, Wang X V, Wang L H and Feng L 2022 Optimal shape morphing control of 4D printed shape memory polymer based on reinforcement learning *Robot. Comput. Integr. Manuf.* **73** 102209

[124] Peng X, Liu G A, Wang J, Li J Q, Wu H P, Jiang S F and Yi B 2023 Controllable deformation design for 4D-printed active composite structure: optimization, simulation, and

experimental verification *Compos. Sci. Technol.* **243** 110265

[125] Benyahia K, Seriket H, Prod'hon R, Gomes S, André J C, Qi H J and Demoly F 2022 A computational design approach for multi-material 4D printing based on interlocking blocks assembly *Addit. Manuf.* **58** 102993

[126] Wang J G, Zhang Y Z, Zheng J C, Zhao X B, Guo H J, Qiu Y, Wang X D, Liu L Q and Yu H B 2023 A 4D-printing inverse design strategy for micromachines with customized shape-morphing *Small* **19** 2302656

[127] Sun X H, Yu L X, Yue L, Zhou K, Demoly F, Zhao R R and Qi H J 2024 Machine learning and sequential subdomain optimization for ultrafast inverse design of 4D-printed active composite structures *J. Mech. Phys. Solids* **186** 105561

[128] Hamel C M, Roach D J, Long K N, Demoly F, Dunn M L and Qi H J 2019 Machine-learning based design of active composite structures for 4D printing *Smart Mater. Struct.* **28** 065005

[129] Sun X H, Yue L, Yu L X, Shao H, Peng X R, Zhou K, Demoly F, Zhao R K and Qi H J 2022 Machine learning-evolutionary algorithm enabled design for 4D-printed active composite structures *Adv. Funct. Mater.* **32** 2109805

[130] Zhao Z, Wu J T, Mu X M, Chen H S, Qi H J and Fang D N 2017 Origami by frontal photopolymerization *Sci. Adv.* **3** e1602326

[131] Liu C, Tan Y Z, He C W, Ji S B and Xu H P 2021 Unconstrained 3D shape programming with light-induced stress gradient *Adv. Mater.* **33** 2105194

[132] Shao Y C, Long F, Zhao Z, Fang M, Jing H, Guo J, Shi X, Sun A, Xu G and Cheng Y 2023 4D printing light-driven soft actuators based on liquid-vapor phase transition composites with inherent sensing capability *Chem. Eng. J.* **454** 140271

[133] Nguyen T D, Qi H J, Castro F and Long K N 2008 A thermoviscoelastic model for amorphous shape memory polymers: incorporating structural and stress relaxation *J. Mech. Phys. Solids* **56** 2792–814

[134] Li G Q and Wang A Q 2016 Cold, warm, and hot programming of shape memory polymers *J. Polym. Sci. B* **54** 1319–39

[135] Shahi K and Ramachandran V 2022 Theoretical and experimental investigation of shape memory polymers programmed below glass transition temperature *Polymers* **14** 2753

[136] Dong K, Wang Y H, Wang Z H, Qiu W L, Zheng P and Xiong Y 2023 Reusability and energy absorption behavior of 4D printed continuous fiber-reinforced auxetic composite structures *Composites A* **169** 107529

[137] Namvar N, Zolfagharian A, Vakili-Tahami F and Bodaghi M 2022 Reversible energy absorption of elasto-plastic auxetic, hexagonal, and AuxHex structures fabricated by FDM 4D printing *Smart Mater. Struct.* **31** 055021

[138] Mehrpouya M, Edelijn T, Ibrahim M, Mohebshahedin A, Gisario A and Barletta M 2022 Functional behavior and energy absorption characteristics of additively manufactured smart sandwich structures *Adv. Eng. Mater.* **24** 2200677

[139] Aberoumand M, Soltanmohammadi K, Rahmatabadi D, Soleyman E, Ghasemi I, Baniassadi M, Abrinia K, Bodaghi M and Baghani M 2023 4D printing of polyvinyl chloride (PVC): a detailed analysis of microstructure, programming, and shape memory performance *Macromol. Mater. Eng.* **308** 2200677

[140] Yue L, Sun X H, Yu L X, Li M Z, Montgomery S M, Song Y Y, Nomura T, Tanaka M and Qi H J 2023 Cold-programmed shape-morphing structures based on
grayscale digital light processing 4D printing *Nat. Commun.* **14** 5519

[141] Soleyman E, Rahmatabadi D, Soltanmohammadi K, Aberoumand M, Ghasemi I, Abrinia K, Baniassadi M, Wang K and Baghani M 2022 Shape memory performance of PETG 4D printed parts under compression in cold, warm, and hot programming *Smart Mater. Struct.* **31** 085002

[142] Soleyman E, Rahmatabadi D, Aberoumand M, Soltanmohammadi K, Ghasemi I, Baniassadi M, Abrinia K and Baghani M 2024 Cold programming of ordered porous PETG 4D printed by material extrusion *Arch. Civ. Mech. Eng.* **24** 67

[143] Rahmatabadi D, Ghasemi I, Baniassadi M, Abrinia K and Baghani M 2023 4D printing of PLA-TPU blends: effect of PLA concentration, loading mode, and programming temperature on the shape memory effect *J. Mater. Sci.* **58** 7227–43

[144] Fallah A, Asif S, Gokcer G and Koc B 2023 4D printing of continuous fiber-reinforced electroactive smart composites by coaxial additive manufacturing *Compos. Struct.* **316** 117034

[145] Bodaghi M, Serjouei A, Zolfagharian A, Fotouhi M, Rahman H and Durand D 2020 Reversible energy absorbing meta-sandwiches by FDM 4D printing *Int. J. Mech. Sci.* **173** 105451

[146] Aberoumand M, Rahmatabadi D, Soltanmohammadi K, Soleyman E, Ghasemi I, Baniassadi M, Abrinia K, Bodaghi M and Baghani M 2023 Stress recovery and stress relaxation behaviors of PVC 4D printed by FDM technology for high-performance actuation applications *Sens. Actuators A* **361** 114572

[147] Delaey J, Dubruel P and Van Vlierberghe S 2020 Shape-memory polymers for biomedical applications *Adv. Funct. Mater.* **30** 1909047

[148] Xie H, Yang K K and Wang Y Z 2019 Photo-cross-linking: a powerful and versatile strategy to develop shape-memory polymers *Prog. Polym. Sci.* **95** 32–64

[149] Hornat C C and Urban M W 2020 Shape memory effects in self-healing polymers *Prog. Polym. Sci.* **102** 101208

[150] Zhou Y F, Wei G, Yuan J Z, Sang X X, Miao J T and Liu R 2023 UV-assisted direct ink writing 4D printing of benzoxazine/epoxy thermosets *Chem. Eng. J.* **477** 147221

[151] Feng P, Zhao R Y, Yang F, Peng S P, Pan H and Shuai C J 2024 Co-continuous structure enhanced magnetic responsive shape memory PLLA/TPU blend fabricated by 4D printing *Virtual Phys. Prototyp.* **19** e2290186

[152] Tang Z Z, Gong J H, Cao P R, Tao L M, Pei X Q, Wang T M, Zhang Y M, Wang Q H and Zhang J Q 2022 3D printing of a versatile applicability shape memory polymer with high strength and high transition temperature *Chem. Eng. J.* **431** 134211

[153] Shuai C J, Xu W T, He H F, Yang F, Liu J L and Feng P 2024 Layered co-continuous structure in bone scaffold fabricated by laser additive manufacturing for enhancing electro-responsive shape memory properties *J. Mater. Res. Technol.* **30** 61–69

[154] Ramaraju H, Akman R E, Safranski D L and Hollister S J 2020 Designing biodegradable shape memory polymers for tissue repair *Adv. Funct. Mater.* **30** 2002014

[155] Liu C, Qin H and Mather P T 2007 Review of progress in shape-memory polymers *J. Mater. Chem.* **17** 1543–58

[156] Hager M D, Bode S, Weber C and Schubert U S 2015 Shape memory polymers: past, present and future developments *Prog. Polym. Sci.* **49–50** 3–33

[157] Behl M, Razzaq M Y and Lendlein A 2010 Multifunctional shape-memory polymers *Adv. Mater.* **22** 3388–410

[158] Qi X, Zhang J C, Zhang L Q and Yue D M 2021 Bio-based, robust, shape memory, self-healing and recyclable elastomers based on a semi-interpenetrating dynamic network *J. Mater. Chem. A* **9** 25399–407

[159] Liu R Y *et al* 2018 Shape memory polymers for body motion energy harvesting and self-powered mechanosensing *Adv. Mater.* **30** 1705195

[160] Zheng Y, Zeng B B, Yang L H, Shen J B and Guo S Y 2020 Fabrication of thermoplastic polyurethane/polycaprolactone multilayered composites with confined distribution of MWCNTs for achieving tunable thermo- and electro-responsive shape-memory performances *Ind. Eng. Chem. Res.* **59** 2977–87

[161] Zheng Y, Qin J X, Shen J B and Guo S Y 2020 Controllable distribution of conductive particles in polymer blends *via a bilayer structure design: a strategy to fabricate shape-memory composites with tunable electro-responsive properties* *J. Mater. Chem. C* **8** 9593–601

[162] Zheng Y, Dong R Q, Shen J B and Guo S Y 2016 Tunable shape memory performances via multilayer assembly of thermoplastic polyurethane and polycaprolactone *ACS Appl. Mater. Interfaces* **8** 1371–80

[163] Zeng B B, Yang L H, Qin J X, Zheng Y and Guo S Y 2020 Nacre-inspired polymeric materials with body heat-responsive shape-memory effect, high optical transparence, and balanced mechanical properties *ACS Appl. Mater. Interfaces* **12** 52008–17

[164] Du J, Armstrong S R and Baer E 2013 Co-extruded multilayer shape memory materials: comparing layered and blend architectures *Polymer* **54** 5399–407

[165] Wang L L, Zhang F H, Du S Y and Leng J S 2022 4D printing of shape-changing structures based on IPN epoxy composites formed by UV post-curing and γ-ray radiation *Composites A* **162** 107146

[166] Wu P, Yu T Y, Chen M J, Kang N and El Mansori M 2024 Electrically/magnetically dual-driven shape memory composites fabricated by multi-material magnetic field-assisted 4D printing *Adv. Funct. Mater.* **34** 2314854

[167] Li Z L, Yu R and Guo B L 2021 Shape-memory and self-healing polymers based on dynamic covalent bonds and dynamic noncovalent interactions: synthesis, mechanism, and application *ACS Appl. Bio Mater.* **4** 5926–43

[168] Xia D Y, Wang P, Ji X F, Khashab N M, Sessler J L and Huang F H 2020 Functional supramolecular polymeric networks: the marriage of covalent polymers and macrocycle-based host-guest interactions *Chem. Rev.* **120** 6070–123

[169] Lehn J M 2005 Dynamers: dynamic molecular and supramolecular polymers *Prog. Polym. Sci.* **30** 814–31

[170] Kloxin C J, Scott T F, Adzima B J and Bowman C N 2010 Covalent adaptable networks (CANs): a unique paradigm in cross-linked polymers *Macromolecules* **43** 2643–53

[171] Montarnal D, Capelot M, Tournilhac F and Leibler L 2011 Silica-like malleable materials from permanent organic networks *Science* **334** 965–8

[172] Zou W K, Dong J T, Luo Y W, Zhao Q and Xie T 2017 Dynamic covalent polymer networks: from old chemistry to modern day innovations *Adv. Mater.* **29** 1606100

[173] Williams G T, Haynes C J E, Fares M, Caltagirone C, Hiscock J R and Gale P A 2021 Advances in applied supramolecular technologies *Chem. Soc. Rev.* **50** 2737–63

[174] Yan X Z, Wang F, Zheng B and Huang F H 2012 Stimuli-responsive supramolecular polymeric materials *Chem. Soc. Rev.* **41** 6042–65

[175] Peng H Q, Zhu W P, Guo W J, Li Q Y, Ma S X, Bucher C, Liu B, Ji X F, Huang F H and Sessler J L 2023 Supramolecular polymers: recent advances based on the
types of underlying interactions *Prog. Polym. Sci.*
**137** 101635

[176] Wanasinghe S V, Dodo O J and Konkolewicz D 2022
Dynamic bonds: adaptable timescales for responsive
materials *Angew. Chem., Int. Ed.* **61** e202206938

[177] Zheng N, Xu Y, Zhao Q and Xie T 2021 Dynamic covalent polymer networks: a molecular platform for designing functions beyond chemical recycling and self-healing *Chem. Rev.* **121** 1716–45

[178] Peng S Y, Sun Y, Ma C M, Duan G G, Liu Z Z and Ma C X 2022 Recent advances in dynamic covalent bond-based shape memory polymers *e-Polymers* **22** 285–300

[179] Zhao X L, Tian P X, Li Y D and Zeng J B 2022 Biobased covalent adaptable networks: towards better sustainability of thermosets *Green Chem.* **24** 4363–87

[180] Cui J J, Liu F K, Lu Z, Feng S W, Liang C, Sun Y D, Cui J and Zhang B 2023 Repeatedly recyclable 3D printing catalyst-free dynamic thermosetting photopolymers *Adv. Mater.* **35** 2211417

[181] Jiang Z C, Xiao Y Y, Kang Y, Pan M, Li B J and Zhang S 2017 Shape memory polymers based on supramolecular interactions *ACS Appl. Mater. Interfaces* **9** 20276–93

[182] Jiang Z, Bhaskaran A, Aitken H M, Shackleford I C G and Connal L A 2019 Using synergistic multiple dynamic bonds to construct polymers with engineered properties *Macromol. Rapid Commun.* **40** 1900038

[183] He X, Lin Y C, Ding Y C, Abdullah A M, Lei Z P, Han Y B, Shi X J, Zhang W and Yu K 2022 Reshapeable, rehealable and recyclable sensor fabricated by direct ink writing of conductive composites based on covalent adaptable network polymers *Int. J. Extrem. Manuf.* **4** 015301

[184] Fan L F, Rong M Z, Zhang M Q and Chen X D 2018
Repeated intrinsic self-healing of wider cracks in polymer
via dynamic reversible covalent bonding molecularly
combined with a two-way shape memory effect *ACS Appl.
Mater. Interfaces* **10** 38538–46

[185] Li X J, Wang L L, Li Y W and Xu S F 2023 Reprocessable, self-healing, thermadapt shape memory polycaprolactone via robust ester–ester interchanges toward kirigami-tailored 4D medical devices *ACS Appl. Polym. Mater.* **5** 1585–95

[186] Miao W S, Zou W K, Jin B J, Ni C J, Zheng N, Zhao Q and Xie T 2020 On demand shape memory polymer via light regulated topological defects in a dynamic covalent network *Nat. Commun.* **11** 4257

[187] Miao W S, Zou W K, Luo Y W, Zheng N, Zhao Q and Xie T 2020 Structural tuning of polycaprolactone based thermadapt shape memory polymer *Polym. Chem.* **11** 1369–74

[188] Meurer J, Hniopek J, Bätz T, Zechel S, Enke M, Vitz J, Schmitt M, Popp J, Hager M D and Schubert U S 2021 Shape-memory metallopolymers based on two orthogonal metal-ligand interactions *Adv. Mater.* **33** 2006655

[189] Ahn D *et al* 2022 Controllable physical synergized triboelectricity, shape memory, self-healing, and optical sensing with rollable form factor by Zn cluster *Adv. Sci.* **9** 2200441

[190] Ni C J *et al* 2023 Shape memory polymer with programmable recovery onset *Nature* **622** 748–53

[191] Cui C H, An L, Zhang Z L, Ji M K, Chen K, Yang Y X, Su Q, Wang F, Cheng Y L and Zhang Y F 2022 Reconfigurable 4D printing of reprocessable and mechanically strong polythiourethane covalent adaptable networks *Adv. Funct. Mater.* **32** 2203720

[192] Xu X N, Fang Z Z, Jin B J, Mu H F, Shi Y P, Xu Y, Chen G C, Zhao Q, Zheng N and Xie T 2023 Regenerative living 4D printing via reversible growth of polymer networks *Adv. Mater.* **35** 2209824

[193] Chen J Y, Wang Z F, Yao B W, Geng Y H, Wang C, Xu J H, Chen T, Jing J J and Fu J J 2024 Ultra-highly stiff and tough shape memory polyurea with unprecedented energy density by precise slight cross-linking *Adv. Mater.* **36** 2401178

[194] Peng W J, Zhang G G, Zhao Q and Xie T 2021 Autonomous off-equilibrium morphing pathways of a supramolecular shape-memory polymer *Adv. Mater.* **33** 2102473

[195] Yang X *et al* 2022 A repeatable dual-encryption platform from recyclable thermosets with self-healing ability and shape memory effect *Adv. Funct. Mater.* **32** 2205177

[196] Miao J T, Ge M Y, Peng S Q, Zhong J, Li Y W, Weng Z X, Wu L X and Zheng L H 2019 Dynamic imine bond-based shape memory polymers with permanent shape reconfigurability for 4D printing *ACS Appl. Mater. Interfaces* **11** 40642–51

[197] Joe J *et al* 2021 A 4D printable shape memory vitrimer with repairability and recyclability through network architecture tailoring from commercial poly(ε-caprolactone) *Adv. Sci.* **8** 2103682

[198] Choi S, Park B, Jo S, Seo J H, Lee W, Kim D G, Lee K B, Kim Y S and Park S 2022 Weldable and reprocessable shape memory epoxy vitrimer enabled by controlled formulation for extrusion-based 4D printing applications *Adv. Eng. Mater.* **24** 2101497

[199] Tran H B D, Vazquez-martel C, Catt S O, Jia Y X, Tsotsalas M, Spiegel C A and Blasco E 2024 4D printing of adaptable “living” materials based on alkoxyamine chemistry *Adv. Funct. Mater.* **34** 2315238

[200] Liu S, Dong X X, Wang Y X, Xiong J, Guo R, Xiao J F, Sun C F, Zhai F and Wang X L 2023 4D printing of shape memory epoxy for adaptive dynamic components *Adv. Mater. Technol.* **8** 2202004

[201] Tournier P, Saint-pé G, Lagneau N, Loll F, Halgand B, Tessier A, Guicheux J, Le Visage C and Delplace V 2023 Clickable dynamic bioinks enable post-printing modifications of construct composition and mechanical properties controlled over time and space *Adv. Sci.* **10** 2300055

[202] Li Z *et al* 2024 Self-healing and shape memory reconfigurable poly(urethane-urea-amide) elastomers containing multiple dynamic bonds for improving performance of 4D printout *Chem. Eng. J.* **485** 149933

[203] Es Sayed J, Khoonkari M, Oggioni M, Perrin P, Sanson N, Kamperman M and Włodarczyk-biegun M K 2022 Multi-responsive jammed micro-gels ink: toward control over the resolution and the stability of 3D printed scaffolds *Adv. Funct. Mater.* **32** 2207816

[204] Wang J, Lin X, Wang R G, Lu Y L and Zhang L Q 2023 Self-healing, photothermal-responsive, and shape memory polyurethanes for enhanced mechanical properties of 3D/4D printed objects *Adv. Funct. Mater.* **33** 2211579

[205] Cheng C Y, Xie H, Xu Z Y, Li L, Jiang M N, Tang L, Yang K K and Wang Y Z 2020 4D printing of shape memory aliphatic copolyester via UV-assisted FDM strategy for medical protective devices *Chem. Eng. J.* **396** 125242

[206] Wang Y H, Ye H T, He J, Ge Q and Xiong Y 2024 Electrothermally controlled origami fabricated by 4D printing of continuous fiber-reinforced composites *Nat. Commun.* **15** 2322

[207] Shuai C J, Wang Z C, Yang F, Zhang H Y, Liu J L and Feng P 2023 Laser additive manufacturing of shape memory biopolymer bone scaffold: 3D conductive network construction and electrically driven mechanism *J. Adv. Res.* accepted (https://doi.org/10.1016/j.jare.2023.11.031)

[208] Wang W H, Wang S, Zhou J Y, Deng H X, Sun S S, Xue T, Ma Y Q and Gong X L 2023 Bio-inspired semi-active
safeguarding design with enhanced impact resistance via shape memory effect *Adv. Funct. Mater.* **33** 2212093

[209] Liu H, Wang F F, Wu W Y, Dong X F and Sang L 2023 4D printing of mechanically robust PLA/TPU/Fe₃O₄ magneto-responsive shape memory polymers for smart structures *Composites B* **248** 110382

[210] Zhang F H, Wen N, Wang L L, Bai Y and Leng J S 2021 Design of 4D printed shape-changing tracheal stent and remote controlling actuation *Int. J. Smart Nano Mater.* **12** 375–89

[211] Zhang F H, Wang L L, Zheng Z C, Liu Y J and Leng J S 2019 Magnetic programming of 4D printed shape memory composite structures *Composites A* **125** 105571

[212] Paunović N *et al* 2022 Digital light 3D printed bioresorbable and NIR-responsive devices with photothermal and shape-memory functions *Adv. Sci.* **9** 2200907

[213] Yang H, Leow W R, Wang T, Wang J, Yu J C, He K, Qi D P, Wan C J and Chen X D 2017 3D printed photoresponsive devices based on shape memory composites *Adv. Mater.* **29** 1701627

[214] Launay V, Wolf R, Dumur F and Lalevée J 2022 Photothermal activation in the near infrared range for 4-dimensional printing using relevant organic dyes *Addit. Manuf.* **58** 103031

[215] Han B, Zhang Y L, Chen Q D and Sun H B 2018 Carbon-based photothermal actuators *Adv. Funct. Mater.* **28** 1802235

[216] Feng S W *et al* 2024 Photo switchable 4D printing remotely controlled responsive and mimetic deformation shape memory polymer nanocomposites *Adv. Funct. Mater.* **34** 2401431

[217] Shuai C J, Wang Z C, Peng S P, Shuai Y, Chen Y W, Zeng D and Feng P 2022 Water-responsive shape memory thermoplastic polyurethane scaffolds triggered at body temperature for bone defect repair *Mater. Chem. Front.* **6** 1456–69

[218] Müller L A E, Zingg A, Arcifa A, Zimmermann T, Nyström G, Burgert I and Siqueira G 2022 Functionalized cellulose nanocrystals as active reinforcements for light-actuated 3D-printed structures *ACS Nano* **16** 18210–22

[219] Pan H M and Goto A 2023 Topology-dependent ph-responsive actuation and shape memory programming for biomimetic 4D printing *Macromol. Rapid Commun.* **44** 2300074

[220] Qiao Y Q *et al* 2020 Treatment of MRSA-infected osteomyelitis using bacterial capturing, magnetically targeted composites with microwave-assisted bacterial killing *Nat. Commun.* **11** 4446

[221] Huang D Q, Wang J L, Song C H and Zhao Y J 2023 Ultrasound-responsive matters for biomedical applications *Innovation* **4** 100421

[222] Sol J A H P, Smits L G, Schenning A P H J and Debije M G 2022 Direct ink writing of 4D structural colors *Adv. Funct. Mater.* **32** 2201766

[223] Cecchini L, Mariani S, Ronzan M, Mondini A, Pugno N M and Mazzolai B 2023 4D printing of humidity-driven seed inspired soft robots *Adv. Sci.* **10** 2205146

[224] Müller L A E, Zimmermann T, Nyström G, Burgert I and Siqueira G 2020 Mechanical properties tailoring of 3D printed photoresponsive nanocellulose composites *Adv. Funct. Mater.* **30** 2002914

[225] Lu X L, Ambulo C P, Wang S T, Rivera-tarazona L K, Kim H, Searles K and Ware T H 2021 4D-printing of photoswitchable actuators *Angew. Chem., Int. Ed.* **60** 5536–43

[226] Zhang X Z, Zhou Q Q, Liu H R and Liu H W 2014 UV light induced plasticization and light activated shape memory of

spiropyran doped ethylene-vinyl acetate copolymers *Soft Matter* **10** 3748–54

[227] Teng X X, Zhang M, Adhikari B and Ma Y M 2024 4D printed pH-responsive labels of methacrylic anhydride grafted konjac glucomannan for detecting quality changes in respiring climacteric fruits *Food Hydrocolloids* **149** 109543

[228] Wang Z J, Liang X, Wang G Y, Wang X H and Chen Y 2023 Emerging bioprinting for wound healing *Adv. Mater.* **11** 2304738

[229] Gao L, Akhtar M U, Yang F, Ahmad S, He J K, Lian Q, Cheng W, Zhang J H and Li D C 2021 Recent progress in engineering functional biohybrid robots actuated by living cells *Acta Biomater.* **121** 29–40

[230] Fu Z Q, Ouyang L L, Xu R Z, Yang Y and Sun W 2022 Responsive biomaterials for 3D bioprinting: a review *Mater. Today* **52** 112–32

[231] Guix M, Mestre R, Patiño T, De Corato M, Fuentes J, Zarpellon G and Sánchez S 2021 Biohybrid soft robots with self-stimulating skeletons *Sci. Robot.* **6** eabe7577

[232] Wang Y X, Pereira R F, Peach C, Huang B Y, Vyas C and Bartolo P 2023 Robotic in situ bioprinting for cartilage tissue engineering *Int. J. Extrem. Manuf.* **5** 032004

[233] Zhou D Z, Dou B H, Kroh F, Wang C Q and Ouyang L L 2023 Biofabrication strategies with single-cell resolution: a review *Int. J. Extrem. Manuf.* **5** 042005

[234] Park S, Shou W, Makatura L, Matusik W and Fu K 2022 3D printing of polymer composites: materials, processes, and applications *Matter* **5** 43–76

[235] Ligon S C, Liska R, Stampfl J, Gurr M and Mülhaupt R 2017 Polymers for 3D printing and customized additive manufacturing *Chem. Rev.* **117** 10212–90

[236] Fu P *et al* 2022 4D printing of polymers: techniques, materials, and prospects *Prog. Polym. Sci.* **126** 101506

[237] Meng H and Hu J L 2010 A brief review of stimulus-active polymers responsive to thermal, light, magnetic, electric, and water/solvent stimuli *J. Intell. Mater. Syst. Struct.* **21** 859–85

[238] Ahn S H, Lee K T, Kim H J, Wu R Z, Kim J S and Song S H 2012 Smart soft composite: an integrated 3D soft morphing structure using bend-twist coupling of anisotropic materials *Int. J. Precis. Eng. Manuf.* **13** 631–4

[239] Luo M J, Li R D, Zheng D, Kang J T, Wu H T, Deng S H and Niu P D 2023 Formation mechanism of inherent spatial heterogeneity of microstructure and mechanical properties of NiTi SMA prepared by laser directed energy deposition *Int. J. Extrem. Manuf.* **5** 035005

[240] Jin Z B Y, Li Y R, Yu K, Liu L X, Fu J Z, Yao X H, Zhang A G and He Y 2021 3D printing of physical organ models: recent developments and challenges *Adv. Sci.* **8** 2101394

[241] Jian B C, Li H G, He X N, Wang R, Yang H Y and Ge Q 2024 Two-photon polymerization-based 4D printing and its applications *Int. J. Extrem. Manuf.* **6** 012001

[242] Chen D B *et al* 2020 4D printing strain self-sensing and temperature self-sensing integrated sensor-actuator with bioinspired gradient gaps *Adv. Sci.* **7** 2000584

[243] Chen Q Y, Han L, Ren J B, Rong L H, Cao P F and Advincula R C 2020 4D printing via an unconventional fused deposition modeling route to high-performance thermosets *ACS Appl. Mater. Interfaces* **12** 50052–60

[244] Li N, Zhao W, Li F F, Liu L W, Liu Y J and Leng J S 2023 A 4D-printed programmable soft network with fractal design and adjustable hydrophobic performance *Matter* **6** 940–62

[245] Lin C, Liu L W, Liu Y J and Leng J S 2021 4D printing of bioinspired absorbable left atrial appendage occluders: a proof-of-concept study *ACS Appl. Mater. Interfaces* **13** 12668–78
[246] Lin C, Lv J X, Li Y S, Zhang F H, Li J R, Liu Y J, Liu L W and Leng J S 2019 4D-printed biodegradable and remotely controllable shape memory occlusion devices *Adv. Funct. Mater.* **29** 1906569

[247] Peng W J, Yin J, Zhang X M, Shi Y P, Che G, Zhao Q and Liu J 2023 4D printed shape memory anastomosis ring with controllable shape transformation and degradation *Adv. Funct. Mater.* **33** 2214505

[248] Ren L, Wang Z, Ren L, Liu Q, Li W, Song Z, Li B, Wu Q and Zhou X 2023 4D printing of shape memory composites with remotely controllable local deformation *Mater. Today Chem.* **29** 101470

[249] Sang L, Wu W Y, Sun Z Q, Wang F, Xu J, Tian J, Zhao Y P and Zhang H 2023 Reusability and energy absorption behavior of 4D-printed heterogeneous lattice structures based on biomass shape memory polyester *J. Mater. Res. Technol.* **27** 1563–78

[250] Wang J J, Hong L Q and Jiao P C 2023 Graphene-reinforced shape memorable chiral metamaterials: theoretical analysis with experimental and numerical validations *Mater. Des.* **226** 111648

[251] Wang X L, Zhang Y, Shen P Q, Cheng Z J, Chu C L, Xue F and Bai J 2022 Preparation of 4D printed peripheral vascular stent and its degradation behavior under fluid shear stress after deployment *Biomater. Sci.* **10** 2302–14

[252] Yang X C, Chen W H, Liu H Y, Yang B S, Xie Y, Wang Y, Lei Y F and Xue L J 2022 Light-driven shape memory of 3D-printed PEEK for programmable actuations *Adv. Mater. Technol.* **7** 2200266

[253] Cen S Y, Li Z H, Guo Z A, Li H R, Shi J Y, Huang X W, Zou X B and Holmes M 2022 4D printing of a citrus pectin/β-CD Pickering emulsion: a study on temperature induced color transformation *Addit. Manuf.* **56** 102925

[254] Deng Y D, Yang B B, Zhang F H, Liu Y J, Sun J B, Zhang S Q, Zhao Y T, Yuan H P and Leng J S 2022 4D printed orbital stent for the treatment of enophthalmic invagination *Biomaterials* **291** 121886

[255] Deng Y D, Zhang F H, Jiang M L, Liu Y J, Yuan H P and Leng J S 2022 Programmable 4D printing of photoactive shape memory composite structures *ACS Appl. Mater. Interfaces* **14** 42568–77

[256] Deng Y D, Zhang F H, Liu Y J, Zhang S Q, Yuan H P and Leng J S 2023 4D printed shape memory polyurethane-based composite for bionic cartilage scaffolds *ACS Appl. Polym. Mater.* **5** 1283–92

[257] Kuang X, Chen K J, Dunn C K, Wu J T, Li V C F and Qi H J 2018 3D printing of highly stretchable, shape-memory, and self-healing elastomer toward novel 4D printing *ACS Appl. Mater. Interfaces* **10** 7381–8

[258] Wei H Q, Cauchy X, Navas I O, Abderrafai Y, Chizari K, Sundararaj U, Liu Y J, Leng J S and Therriault D 2019 Direct 3D printing of hybrid nanofiber-based nanocomposites for highly conductive and shape memory applications *ACS Appl. Mater. Interfaces* **11** 24523–32

[259] Wei H Q, Zhang Q W, Yao Y T, Liu L W, Liu Y J and Leng J S 2017 Direct-write fabrication of 4D active shape-changing structures based on a shape memory polymer and its nanocomposite *ACS Appl. Mater. Interfaces* **9** 876–83

[260] Zhang C, Cai D P, Liao P, Su J W, Deng H, Vardhanabhuti B, Ulery B D, Chen S Y and Lin J 2021 4D printing of shape-memory polymeric scaffolds for adaptive biomedical implantation *Acta Biomater.* **122** 101–10

[261] Wan X, Luo L, Liu Y J and Leng J S 2020 Direct ink writing based 4D printing of materials and their applications *Adv. Sci.* **7** 2001000

[262] Choong Y Y C, Maleksaeedi S, Eng H, Wei J and Su P C 2017 4D printing of high performance shape memory polymer using stereolithography *Mater. Des.* **126** 219–25

[263] Miao S D, Cui H T, Nowicki M, Xia L, Zhou X, Lee S J, Zhu W, Sarkar K, Zhang Z Y and Zhang L G 2018 Stereolithographic 4D bioprinting of multiresponsive architectures for neural engineering *Adv. Biosyst.* **2** 1800101

[264] Kalat M N, Staszczak M, Urbański L, Polvorinos-Fernández C, Vega C A, Cristea M, Ionita D, Lantada A D and Pieczyska E A 2023 Investigating a shape memory epoxy resin and its application to engineering shape-morphing devices empowered through kinematic chains and compliant joints *Mater. Des.* **233** 112263

[265] Weems A C, Arno M C, Yu W, Huckstepp R T R and Dove A P 2021 4D polycarbonates via stereolithography as scaffolds for soft tissue repair *Nat. Commun.* **12** 3771

[266] Deng S H, Huang L M, Wu J J, Pan P J, Zhao Q and Xie T 2021 Bioinspired dual-mode temporal communication via digitally programmable phase-change materials *Adv. Mater.* **33** 2008119

[267] Kuhnt T, Camarero-espinosa S, Takhsha Ghahfarokhi M, Arreguín M, Cabassi R, Albertini F, Nieto D, Baker M B and Moroni L 2022 4D printed shape morphing biocompatible materials based on anisotropic ferromagnetic nanoparticles *Adv. Funct. Mater.* **32** 2202539

[268] Le Fer G and Becker M L 2020 4D printing of resorbable complex shape-memory poly(propylene fumarate) star scaffolds *ACS Appl. Mater. Interfaces* **12** 22444–52

[269] Wang Y, Cui H T, Wang Y C, Xu C Y, Esworthy T J, Hann S Y, Boehm M, Shen Y L, Mei D Q and Zhang L G 2021 4D printed cardiac construct with aligned myofibers and adjustable curvature for myocardial regeneration *ACS Appl. Mater. Interfaces* **13** 12746–58

[270] Wu H Z, Chen P, Yan C Z, Cai C and Shi Y S 2019 Four-dimensional printing of a novel acrylate-based shape memory polymer using digital light processing *Mater. Des.* **171** 107704

[271] Wu Y, Han Y, Wei Z X, Xie Y, Yin J and Qian J 2023 4D printing of chiral mechanical metamaterials with modular programmability using shape memory polymer *Adv. Funct. Mater.* **33** 2306442

[272] Zhang B *et al* 2021 Mechanically robust and UV-curable shape-memory polymers for digital light processing based 4D printing *Adv. Mater.* **33** 2101298

[273] Zhang B, Zhang W, Zhang Z Q, Zhang Y F, Hingorani H, Liu Z J, Liu J and Ge Q 2019 Self-healing four-dimensional printing with an ultraviolet curable double-network shape memory polymer system *ACS Appl. Mater. Interfaces* **11** 10328–36

[274] Zhang H, Huang S, Sheng J, Fan L S, Zhou J Z, Shan M Y, Wei J A, Wang C, Yang H W and Lu J Z 2022 4D printing of Ag nanowire-embedded shape memory composites with stable and controllable electrical responsivity: implications for flexible actuators *ACS Appl. Nano Mater.* **5** 6221–31

[275] Elliott L V, Salzman E E and Greer J R 2021 Stimuli responsive shape memory microarchitectures *Adv. Funct. Mater.* **31** 2008380

[276] Minnick G *et al* 2023 Two-photon polymerized shape memory microfibers: a new mechanical characterization method in liquid *Adv. Funct. Mater.* **33** 2206739

[277] Spratte T, Geiger S, Colombo F, Mishra A, Taale M, Hsu L Y, Blasco E and Selhuber-Unkel C 2023 Increasing the efficiency of thermoresponsive actuation at the microscale by direct laser writing of pNIPAM *Adv. Mater. Technol.* **8** 2200714

[278] Wang J Y, Jin F, Dong X Z, Liu J and Zheng M L 2022 Flytrap inspired Ph-driven 3D hydrogel actuator by