Recent advances in dynamic covalent bond-based shape memory polymers

1 Introduction

As a most important soft smart material, the shape memory polymer (SMP) can be designed for various temporary shapes in a specific environment and due to their fast speed of shape recovery triggered by external stimuli (1,2). Different from shape memory alloys (3), which are rigid and can only respond to heat, SMPs are soft and can respond to diverse external stimuli (such as temperature (1,2), light (4), electricity (5,6), magnetism (7), pH value (8), humidity (9,10), bi/multivalent metal ions (11), etc.) which have attracted more and more attention.

Compared with common covalent bonds which cannot open easily due to their strong bond force, dynamic covalent bonds (DCBs) can open and reform reversibly in response to various external stimuli owing to their relatively weak bond force, which is similar to dynamic noncovalent bonds (12,13) with weak supramolecular interaction. Therefore, DCBs can be introduced in SMPs, and DCB-SMPs obtain their flash point and set off a new wave of research. Classic DCBs, including ester bonds, imine bonds, boronic acid bonds, oxime bonds, hydrazine bonds, alkoxyamine bonds, disulfide bonds, even light-responsive spiropyran bonds (14), and Diels–Alder (DA) bonds (15), can provide fast responsive speed for SMPs under relatively mild conditions (Scheme 1).

Scheme 1

General illustration of SMPs based on diverse DCBs including DCB-SMP solids, DCB-SMP hydrogels, and 3D-printing DCB-SMPs.

DCB-SMPs have broad application prospects, and on account of diverse DCBs, can be utilized to design SMPs and can achieve specific shape memory behaviors (15). First of all, introducing DCBs into shape memory networks can endow SMPs with new functions and properties, including self-healing (16), reprocessability (17), and shape reconfiguration (18), without sacrificing high-efficiency of fixation, high strength, and high speed of shape recovery. Second, DCB-SMPs can easily create permanent shapes with complex 1D, 2D, and 3D geometry (19) and without designing complicated molds. Furthermore, besides DCB-SMP solids, DCB-SMP hydrogels (20) have also been explored in most recent years, which have a unique biocompatible soft/wet hydrogel structure for biomedicine, biosensors, and biomimetic actuations. Last but not least, the novel 3D-printing technique also has been introduced to extend the applications of DCB-SMPs (21), owing to the convenient and programmable design of the original shapes by this technique (22).

Although DCB-SMPs are very similar to the supramolecular SMPs (23,24,25), which can provide reversible and dynamic cross-linking for the design of various complex original shapes, the bond strength of DCBs, generally, can be more precisely programmed and more easily controlled by external stimuli than by the supramolecular interaction in SMPs. The shortcoming of these DCB-SMPs is that the development of them is still at a primary stage and especially in the very few kinds of DCBs used in the SMPs. However, nowadays there are no special reviews for the DCB-SMPs, which is urgently needed for the related research studies in the future. Herein, recent progresses in the DCB-SMPs, including DCB-SMP solids, DCB-SMP hydrogels, and the introduction of 3D-printing using them have been summarized and analyzed by polymeric structures. Furthermore, the prospect and growing trend of this field also has been predicted.

2 DCB-SMP solids

Compared with common SMPs, strongly cross-linked by covalent bonds, the DCB-SMPs can achieve more special properties by dynamically combining and dissociating DCBs. Introducing DCBs in main chains or branched chains of polymers can endow the SMPs with repeatable processing, self-healing, remolding, and recyclable characteristics, which can also significantly prolong the service life of SMPs, which, the same as most of common SMPs, can still keep a good fixation rate, high strength, and high speed of the shape-recovering process.

Different from common thermoset SMPs cross-linked with strong covalent bonds, DCB-SMPs own unique thermadapt behaviors, the same as supramolecular SMPs (26,27), which can provide both intrinsic plasticity and elasticity change. For thermoset SMPs, introducing DCBs can achieve the permanent shape change and temporary shape change, corresponding to plasticity and elasticity at different temperatures, which is named thermadapt SMPs (28,29). For example, Zhao et al. (30) have researched about the relationship between the cumulative plasticity effect and the shape memory effect. They constructed a polycaprolactone (PCL)-based SMP network with crystalline phase transition which is a compound of PCL-diacrylate and pentaerithrytol tetrakis(3-mercaptopropionate) (PETMP) by a radical initiated reaction. In this system, the transesterification reaction was carried out in the presence of a catalyzer at a high temperature (130°C), endowed complex 3D shape changes. Because of the rearrangement of the topological cross-linking network, it could achieve the permanent change of solid plasticity. Setting the same deformation and recovery temperature as 80°C, PCL-based SMPs achieved fixing and recovery ratios even above 98%, and the reason of this change is the temperature heated up to chain movement temperature, thereby realizing shape memory behavior, plasticity, and elasticity change clearly transforming at different temperatures.

Many kinds of thermadapt DCB-SMPs (31,32,33,34,35,36,37,38,39,40,41,42,43) have been developed based on the design of the topology network transition temperature and the glass transition temperature (T _g) (Figure 1). For example, based on cross-linking of dynamic ester bonds, Yang et al. (44) developed a new DCB-SMP with triple shape memory, which can achieve good adjustment of deformation temperature and still keep high mechanical performance. On account of the presence of zinc acetate catalysts, the epoxy group of epoxy soybean oil (ESO) reacted with the carboxyl group from fumaropimaric acid (FPA) to generate new ester bonds, further exchanging them with ester groups. Such transesterification determined the rearrangement of the cross-linking network, shape memory behaviors were achieved, and even self-healing and reprocessing properties were observed. Because of these two characteristic temperatures, the researchers changed its permanent shape at 160°C and fixed the temporary shape at 80°C, thereby transforming it twice when the temperature increased from room temperature to the T _g and then up to the topology network transition temperature. It realized a triple-shape transformation. For another example, Wu et al. also did a similar work and got decent biobased vitrimers (45). Practical vitrimers with dynamic ester bonds obtained reprocessability and unique shape memory behaviors and even with the intrinsic healing capability (46). Such many biobased SMPs showed that DCB-SMPs for green recycle would become the future trend, with the development for environment friendly, cellulose (47,48,49) has been the general addition agent to endow SMPs with reprocessability and recyclability (50,51,52,53,54,55), even cellulose can improve shape memory performance (56,57). There were some research studies about DCB-SMPs with cellulose, which can be a pretty green material (58,59,60). For example, Zhao et al. (61) have formed a new kind of DCB-SMP using lignin as the cross-linker to achieve high mechanical performance, when the lignin content was 50%, tensile strength could be high as 11.6 MPa. The dynamic covalent ester bonds in polyhydroxyurethane endowed this DCB-SMP excellent recyclability, shape memory, and self-healing. Adding various types of cellulose into the dynamic covalent network could be a strategy to develop DCB-SMPs. Besides ester bonds, imine bonds also have contributed their energy in SMPs, and Yang et al. (62) prepared a one-pot method at room temperature, adding imine-coordinated boroxine to fabricate malleable and high-strength polybutadiene-based (PB) transparent materials without any catalyst. It can be recyclable to obtain shape memory behavior. They fixed its shape and released around the T _g. Owning to the movement and exchange of the chain segment in the network, it can fix its shape under stress and immobilize its complex shape because of the frozen polymer chain. When the temperature is reheated above T _g, the solid polymer chain was activated again, and it would release the locked chain, restoring its original shape. Similarly, the dynamic disulfide bond also has been introduced into polyurethane (63,64,65,66,67,68), which owns huge potential applications in biomedical field, on account of not only good shape memory effects but also excellent seal-healing performance (69). Shaabani and Sedghi (70) reported their work on conductive polyurethanes, which had shape memory behaviors and can even be self-healing. The disulfide bonds would reestablish between thiol groups and disulfide bonds to realize self-healing. In this study, shape memory behavior and self-healing based on disulfide bonds reinforced each other. It with great bio-compatibility and low cytotoxicity can be used in osteogenic materials. They expected that this thermadapt DCB-SMP can applied in minimally invasive surgery helping the growth of bones. PCL also has been applied in medical disposable materials using hindered urea dynamic bonds to achieve outstanding shape memory behavior (71) (Figure 2).

Figure 1

Synthesis and structure property of disulfide bond dynamic epoxy resins. Copyright 2021, Elsevier (35).

Figure 2

Manipulation of PCL-based polymer via plasticity, followed by conventional shape fixing and recovering. Copyright 2018, John Wiley & Sons – Books (71).

Most recently, oxime bonds as a new kind of DCBs, it is a new way to provide self-healing and shape memory behaviors. While researching on polyurethane, Luo et al. (72) combined graphene oxide with oxime carbamate dynamic bonds, which greatly improved shape memory performance and self-healing ability. The network structure would change due to the exoteric temperature, when temperature was increased high up to 100°C, the oxime carbamate bond split into isocyanate and hydroxyl groups, and it formed again when the temperature dropped to 60°C. Besides self-healing, the mechanical properties could adjust by the addition amount of graphene oxide. Regrettably, the shape memory effect of this thermadapt DCB-SMP was not particularly good, with only 55.2% shape fixation ratio which was caused by low cross-linking density (Figure 3).

Figure 3

Schematic illustration for the preparation of PU-GO composites. Copyright 2021, John Wiley & Sons – Books (72).

Furthermore, a few of photothermal-responsive DCB-SMPs have been developed which can respond to light, especially in long wavelength visual light and near-infrared light (NIR), because of the photothermal conversion (73,74). For instance, Lee et al. (75) have explored a new photothermal-responsive DCB-SMP with multishape memory behavior based on the dynamic disulfide bond. They researched the polycondensation between elemental sulfur and p-diiodo-benzene (DIB) during the presence of silicone oil at 230°C and then created elemental sulfur-derived poly(phenylene polysulfide) networks (PSNs). It can respond to intrinsic NIR and transform shapes without any additional photothermal agents. This is the biggest highlight in this work. Because of the intrinsic NIR-induced photothermal effect of this material and plasticity and elasticity for the PSN, the complex shape fixation of 3D multishape memory structures based on PSN could realize spatiotemporal control (Figure 4). In another instance, recently, a new photothermal-responsive DCB-SMP based on dynamic ester bonds in polyurethane with good thermochromism, photochromism, and malleability has been reported (76).

Figure 4

Demonstration of photocontrolled assembly, reconfiguration, shape fixing/recovery, and repair of PSN-based 3D multishape memory structures, mimicking the constitution and motion of Venus flytrap (scale bar = 2 cm). Copyright 2020, John Wiley & Sons – Books (75).

Although a few kinds of photothermal-responsive DCB-SMPs have been developed (77), many other kinds of DCBs should been introduced in the design of new SMPs, and some nanoparticles with high-efficiency of photothermal conversion can be composited to enhance the responsive speed of the photothermal-responsive DCB-SMPs.

Besides photothermal-responsive thermadapt DCB-SMP-based photothermal conversion, photochemical-responsive DCB-SMPs are another light-responsive DCB-SMPs based on the photochemical reaction. For example, the DA chemistry as the classic dynamic cross-link reaction applied in shape memory materials (78,79,80,81,82). Wu et al. (83) made a dual cross-linking network by maleic anhydride-modified ethylene-vinyl acetate copolymers through reactive melt processing. The crystallization temperature and melting temperature of the pure ethylene-vinyl acetate, respectively, are 44°C and 77°C, and these acted as the switches for shape changing. Owning to the dual reversible cross-linking networks, the polymer was strengthened and the networks endowed the material reprocessability and shape memory properties. The peculiarity of the reversible DA reaction was that the material adapted to high temperature changes and the deformation temperature decreased up to 6°C helping shape recovery, the 100% tensile stress was increased from 3.8 to 5.6 MPa, and the fracture strength and elongation were maintained at 30.3 MPa and 486%, respectively. Furthermore, multifunctional-responsive DCB-SMPs also have been explored in most recent years based on DA “click” chemistry (78,84,85). Self-healing is the commonest property that can be observed in DCBs, so researchers made some attempts to connect shape memory, self-healing, and light-induced responsive behaviors. Yang et al. (86) prepared photoactive shape memory assisted self-healing polymer composites by utilizing polydopamine particles as fillers in dynamic cross-linked polyurethane containing DA bonds. On account of polydopamine particles having great photothermal effects, they caused the temperature to rise, which triggered shape memory and self-healing. The material can be immediately closed after irradiating with NIR when it was cracked, the crack closure resulted on account of its shape memory function, and the dynamic DA reaction resulted in healing after the crack closure. This work also confirmed that self-healing was achieved with the help of shape memory, because it would shrink when irradiated with NIR, while the temperature does not rise up to the self-healing temperature. Researchers have tried to form network by multiple DCBs (87–90). For example, Yue et al. (89) have explored the properties of triple DCBs (ester bonds, urethane bonds, and thiourea bonds) in SMPs. DCB-SMPs with high transparency and excellent mechanical properties, the maximum tensile stress can be up to 29.7 MPa, have near 144% reprocessing efficiency. Under the combination of solid-state plasticity and shape memory property, DCB-SMPs can achieve a complex original shape and multiple shape memory performance. DCB-SMPs have the potential to contribute to sustainable development (Figure 5).

Figure 5

Chemical structure and bond exchange mechanisms for the dynamic network. Copyright 2021, Elsevier (89).

Zeng et al. (90) were the first to construct a covalent adaptable network of dual dynamic connectomes including DA chemistry and boronic ester bonds (Figure 6). The reason for the excellent dynamic properties came at the expense of mechanical properties, they made flexible, reprocessable, and superior mechanically tough semi-interpenetrating polymer networks (IPNs), which were synthesized by a continuous synthetic pathway. The main network was a kind of covalent adaptable network which was from using boronic esters as cross-linkers and utilized the guest polymer to increase remarkable flexibility. This attempt first perfected mechanical performance of the semi-IPNs because of the robust features of covalent adaptable networks and improvement of thermoplastic polyurethanes. Second, shape memory, even self-healing and welding, can be acquired though exchange-induced network rearrangement at high temperatures due to dynamic DA reactions and boronic ester exchange reactions. Besides DA reaction-based photochemical-responsive DCB-SMPs, a new DCB-SMP based on dynamic open-close spiropyran groups in the polymeric main chain also has been designed with both photochemical-responsiveness and photochemical color change (14). Because the spiropyran would change its form from a closed spiro form to an open zwitterionic merocyanine form under UV light irradiation, ethylene-vinyl acetate copolymers (EVA) as the general heat triggered resin were used, and introduction of spiropyran groups into EVA achieved light-induced DCB-SMPs.

Figure 6

(a) Schematic representation of the DA reaction (right) and the transesterification reaction of boronic ester linkages (right); (b) optical images of welding behaviors of 6% CPSFTPU₁ sample, optical microscopy images of the 6% CPSFTPU₁ and TPU fracture surface before and after self-healing: (c) for 6% CPSFTPU₁ and (d) for TPU. Copyright 2021, Molecular Diversity Preservation International (90).

Above all works based on SMP solids, the shape memory behaviors depended on the T _g, and utilizing DCBs can develop the topological freezing transition temperature to be multiple SMPs or as the critical bonding to develop the self-healing property. The DCB-SMP solids have excellent mechanical properties and pretty good fixation ratios, and the existing research studies of photothermal conversion of DCB-SMPs can inspire the design for intelligent materials.

3 DCB-SMP hydrogels

Besides common dual-layer shape memory hydrogels (91,92) or cross-linking by supramolecular interaction (93,94), DCB-SMP hydrogels as important intelligent materials have attracted great attention from researchers. A hydrogel matrix can be introduced with DCBs to realize the shape memory behavior of uncoupling and recoupling. Because of the great soft property of hydrogels and their nice hydrophilic networks, DCB-SMP hydrogels can be used in many fields, especially in biomedicine, biosensors, and biomimetic actuations.

Chen et al. have done outstanding work in DCB-SMP hydrogels. First, they fabricated a kind of hydrogel with pH- and sugar-induced shape memory behaviors (95). The hydrogel changed its shapes though the interaction of dynamic phenylboronic acid (PBA)–diol. They formed the network by the reaction between PBA-modified sodium alginate (Alg-PBA) and polyvinyl alcohol (PVA). The permanent shape of the hydrogel was mounded by Alg–Ca²⁺ cross-links where alginate complexes with Ca²⁺. In this network, the dynamic PBA–diol ester bonds acted as temporary cross-linkers, rapidly stabilizing the temporary shape of the hydrogel. Based on the pH-responsiveness of the PBA–diol dynamic ester bond, they explored the response of the hydrogel under acidic conditions and aqueous solutions of glucose and fructose (Figure 7). Using phenylboronic acid groups to achieve shape immobilization, they tried another way to synthesize benzene boronic acid ester bonds by acrylamide, bis-acrylamide, PVA, and borax (96), and a hydrogel with quite good shape memory ratios/rates was constructed. The research group reported a multifunctional shape memory hydrogel (97) later. Similar to the former work, they observed the hydrogel which was formed by dopamine-grafted sodium alginate and Alg-PBA. The hydrogel also permanently was cross-linked because of the reaction from Ca²⁺–alginate binding. The pH-responsiveness of PBA–catechol interactions caused the hydrogel to develop an impressive pH-induced shape memory behavior. This work glistened because of the self-healing feature and good adhesive properties. The dynamic PBA–catechol interactions endowed self-healing in the hydrogel, and catechol moieties endowed it good adhesive properties that seemed like the mussel.

Figure 7

The shape memory mechanism of Alg-PBA-PVA hydrogels. Copyright 2015, John Wiley & Sons – Journals (95).

Most recently, depending on various DCBs, including boric acid ester bonds, stretchable hydrogels with triple shape memory (98) or tunable mechanical properties, multishape memory hydrogels (99,100), and even hydrogels with self-healing (101,102) have been produced. But for these DCB-SMP hydrogels, multiple shape memory behaviors were realized by means of some supermolecule interactions, such as metal coordination and hydrogen bonds. There are some shape memory hydrogels with high mechanical properties. Xiao et al. (103) designed poly(aminobenzene boronic acid)-cellulose nanocrystal (PABA@CNC)-mediating hydrogels. The special structure endowed the hydrogel NIR-triggered self-healing and pH-triggered shape memory behaviors. PABA@CNCs in the hydrogel acted as highly efficient cross-linkers and photothermal converter and enhancer agents. Due to the presence of boronic bonds, the temporary shape of the hydrogel could be induced by a change in pH, and the network could be firmly stabilized. Microcrystallization between PVA chains in the double cross-link network served as permanent cross-linking networks to avoid collapse. With the development of DCB-SMP hydrogels, cellulose may be a great choice to solve the problem of mechanical performance (104–106). Most recently, the Schiff base bond has been applied in biomedical field, especially for medical suture lines (107). Mao et al. (108) constructed an oxidized starch/gelatin-based (OSG) shape memory hydrogel, and it can expectantly serve as a self-shrinking wound dressing to achieve noninvasive healing (Figure 8). In this work, Schiff base reactions were used as the shape memory net-point to stabilize structure and enhance the mechanical properties. When the stretched OSG hydrogel temporarily fixed its shape and attached to the wound skin, it is stimulated when the temperature exceeded the transition temperature, and the self-shrinking property of the hydrogel made the wound lips noninvasively close. In addition, OSG hydrogels could promote wound healing because the moisture content in the wound microenvironment was well maintained. Research on disulfides in shape memory hydrogels is still in the development stage (109). The disulfide bonds endowed keratin hydrogels self-shrinking ability, which is utilized for wound repair.

Figure 8

Schematic representation of OSG hydrogel synthesis. Copyright 2020, Elsevier (108).

Furthermore, multiple shape memory effects also have been constructed in the DCB-SMP hydrogels, which greatly extend the application prospect of them. Therefore, SMPs with triple/multiple shape memory functions have attracted widespread attention (110–113). By introducing two or more reversible interactions that do not interfere with each other into a unified system, two or more temporary shapes can be memorized and triple or multiple shape memory effects can be achieved under mild conditions. Tang et al. (114) manufactured a shape memory hydrogel which can change its shape by four types of stimulations to achieve programing sequential quadruple shape memory. It is easy to obtain the hydrogel only by simply mixing PVA and cysteamine-grafted alginate at room temperature. The temporary shape could be first immobilized through the chelation between Ca²⁺ and alginate, second due to the formation of disulfide, and lastly by formation of hydrogen bonding among PVA. Such hydrogels would have many potentially useful applications, for drug carriers to achieve sequential release of multiple payloads and so on. For example, Zhang et al. (115) synthesized a series of dual network hydrogel though cryogenic technology, and the first network was formed by oligoethylene glycol (OEG)-based dendronized polymers based on dynamic Schiff base bonds, and the other network was constructed from covalent cross-linked polyacrylamide. The ultimate hydrogel with IPNs got enhancing mechanical properties and multiple functions. Due to dendritic OEG moieties and Schiff base bonds, great thermoresponsive properties and pH-responsive properties were added in the hydrogel, respectively (Figure 9).

Figure 9

(a) pH-responsive shape memory effects and (b) temperature-responsive shape memory effects. Copyright 2020, Elsevier (115).

While SMP hydrogels with DCBs are already presented and have many potential application, based on the soft and wet hydrogels, there are still various stimuli-induced conditions to be achieved, and mechanical performance is still the key difficulty to break though; improvement in increasing their efficiency also can be a research point. As one of the soft intelligent materials, the DCB-SMP hydrogel has significant potential applications.

4 3D-printing DCB-SMPs

The emerging technology (116–120) is hot nowadays, DCB-SMPs also can be combined with 3D printing (121,122), as shown in Figure 10. For instance, Davidson et al. (123) synthesized one type of liquid crystal elastomers (LCE)-based inks. The ink was compounded by liquid crystal mesogens polymerized with an allyl dithiol chain extender containing dynamic bonds. The inks based on dynamic bonds can be printed though locally programing their direct alignment. Simultaneously, it can use UV light to enable controlled network reconfiguration without any imposed mechanical field. The LCEs can reversibly conduct shape change when the temperature is cycled above and below their nematic-to-isotropic transition temperature and the activate states can be locked again via high-temperature UV irradiation.

Figure 10

Overview of light-based 3D printing of resins, including dynamic boronate esters. Left: Monomers (diallyl boronate (DABo), PETMP, and diallyl phthalate (DAP)). Center: Graphical representation of a printer. Right: Schematic of the network structure containing a DABo ester and static DAP cross-links. Copyright 2021, American Chemical Society (121).

Besides these inks, there are other composite polymers which could be printed and could form complex original shapes. For example, Miao et al. (124) formed an active cross-linked (meth)acrylate system which contained dynamic imine bonds and could be used for 4D-printing, as shown in Figure 11. In this system, the amino groups on the hyperbranched cross-linker reacted with aldehyde groups to form dynamic imine bonds; the hyperbranched cross-linker endowed the material great shape memory behavior, and the soft and flexible chain structure also enhanced the toughness. The permanent shape could be reconfigured because of the dynamic change of imine bonds under relatively mild conditions. These 4D-printed structures showed excellent potential in aerospace or soft robots. Most recently, Li et al. (125) made 4D-printing of reprocessing polyurethane, which was fabricated by a kind of polyurethane acrylate oligomer including the DA reaction and reacted with a reactive diluent. Through digital light processing 3D-printing, photopolymers designed into some complicated structures can be printed. To prove that the printed objects had the high shape fixity property and recovery rate, the printed objects were tested for shape memory cycles tests and fold-deploy experiments. In 16 continuous shape memory cyclic tests, the shape fixation rate and shape recovery rate of the printed materials were 96.4% and 99.3%, respectively.

Figure 11

Schematic illustrations of the 4D-printing of shape memory polymer networks. (a) Chemical structures of (meth)acrylate monomers (EGPEA, IBOA, and MEFB), cross-linker (HPASi), and initiators (Irgacure 819 and TPO) in the photopolymer solution. (b) DLP 3D-printing system for printing of 3D structures. (c) Formation of dynamic imine bonds of cross-linked resins through the reaction of aldehyde groups and amino groups on HPASi. Copyright 2019, American Chemical Society (124).

Applied in novel technologies, they provide the complex original shape realizability, but in the matrix choice, the appropriate monomer and DCB intervention are quite important issues to solve. The research studies on combination between DCB-SMPs and 3D-printing are in the starting stage; meanwhile, DCB-SMP hydrogels also can be utilized with 3D-printing but there are no relevant studies.

5 Summary and outlook

In this present review, recent DCB-SMPs have been researched. DCBs can endow the DCB-SMPs with excellent self-healing and remolding properties, because of the reversible and dynamic cross-linking/dis-cross-linking of them. DCBs as distinctive covalent cross-linkers have inferior bond force but can adjust the cross-linking density to realize good fixation effects of the network and promote the development of the shape memory polymers, but there still exists some defects in the progress.

First, for DCB-SMP solids, the superior shape memory behaviors and excellent mechanical properties are prevalent in these materials, facilitating development of self-healing and reprocessing. The re-cross-linking and high cross-linking density of DCBs provide a pretty good mechanical property after self-healing, and the materials can change the permanent shape to design multiple shape memory behaviors on account of the topological freezing temperature from DCBs. However, most shape deformations are heat-induced, and other few materials are light-induced, developing the responses under diverse stimulus requires sustained attentions. Until now, there are many reports about great performance of DCB-SMPs, but integration of all performance into a material is quite difficult. Therefore, developing multifunctional DCB-SMPs, which would be triggered by light, electricity, or magnetism, to fix more complicated situations, would be the future trend.

Second, on account of the soft and wet characteristic from the hydrogel, there are plentiful stimuli-induced environments for DCB-SMP hydrogel deformation. The DCB-SMP hydrogels must be developed with biocompatibility, which requires us to be prudent in the selection of fillers in order to really apply them in biomedical fields such as wound healing, drug delivery, and tissue engineering. Meanwhile, the disadvantage of the hydrogel is its bad mechanical property which will be the research focus in the development, and novel functional DCB-SMP hydrogels seem be the future direction of development, to be conductive, magnetic, and luminous.

In the end, DCB-SMPs applied in emerging technology, 3D-printing, can design complex original shapes and are suitable for real limit situations, but this kind of research is in the beginning stage, and the point is that it is difficult to introduce DCBs into photopolymers. The use of special materials will expand the applications in soft robotics and biomedical area. However, regrettably, the reports about DCB-SMP hydrogels combining 3D-printing are almost not found, we look forward that the relative research studies will be presented in the near future.

All the difficulties will be solved as time goes by, and we believe that with the joint efforts of scientific research personnel in all fields, these challenges in the near future will get a satisfactory solution.