Shape Memory Polymers Based on Supramolecular Interactions

Review www.acsami.org

Shape Memory Polymers Based on Supramolecular Interactions Zhi-Chao Jiang,† Yao-Yu Xiao,† Yang Kang,‡ Min Pan,† Bang-Jing Li,*,‡ and Sheng Zhang*,† †

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China Chengdu Institute of Biology, Chinese Academy of Science, Chengdu 610041, China

‡

ABSTRACT: Shape memory polymers (SMPs), with the capability to change from one or more temporary shapes to predetermined shapes in response to an external stimulus, have attracted much interest from both academia and industries. When introducing supramolecular interactions that have been featured as dynamic and reversible into the design of novel SMPs, intriguing and unique functionalities have been engendered and thereby broaden the potential applications of the SMPs to new territories. In this review, we summarize recent progress made in SMPs based on supramolecular interactions, provide insight into the material design and shape memory mechanism, elucidate and evaluate their properties and performance, and point out opportunities and applications of SMPs. KEYWORDS: shape memory polymers, supramolecular interactions, hydrogen bonding, host−guest interaction, hydrophobic interaction, metal−ligand coordination, ionic interaction

1. INTRODUCTION Shape memory polymers (SMPs) are intelligent materials capable of changing from one or more temporary shapes to a predetermined shape in response to an external stimulus.1 As fascinating, smart, stimuli-responsive materials inspired by nature, SMPs have attracted considerable attention from both academia and industries due to their great potential for applications in biomedical devices, media recorders, sensors, actuators, and textiles.2−4 In the past decade in particular, the SMPs field has undergone explosive development. Many innovative SMPs, such as multishape, multiresponsive, and multifunctional SMPs, have emerged.5−7 Recently, several excellent reviews have been published, which have focused on SMP composites, triple-shape SMPs, multifunctional SMPs, general aspects of SMPs, applications of SMPs, and advances in shape memory hydrogels.8−13 However, recent progress in SMPs based on supramolecular interactions (SMPs-SI) has not been systematically reviewed. It should be noted that a recent review, which elaborately summarized supramolecular shape memory hydrogels with shape memory ability at ambient temperature, has been reported by Chen and co-workers.14 In contrast, our review includes all kinds of SMPs (both shape memory hydrogels and dry state SMPs) that contain supramolecular interactions as molecular switches and/or netpoints. Furthermore, both thermal- and athermal-stimuli-enabled shape memory behaviors of SMPs are reviewed. In principle, SMPs should possess at least two different kinds of segments: netpoints for stabilizing the whole material and determining the permanent shape and reversible segments for switching on/off the molecular mobility so as to freeze/release the temporary shape. The netpoints can be chemical crosslinks, physical cross-links (e.g., molecular entanglement), or © 2017 American Chemical Society

crystallites with a high melting point. The switching segments are generally segments that show reversible melting transitions or glass transitions at low temperature.15−17 Very recently, supramolecular interactions have been increasingly designed as molecular switches to construct novel SMPs (Figure 1). Because of their unique features, such as various stimuliresponsiveness and dynamic reversibility,14 supramolecular switches have greatly broadened the list of stimuli triggered by SMPs. Furthermore, the novel triggering methods are expected to significantly expand the applications of SMPs. Additionally, with strong supramolecular interactions serving as permanent netpoints, SMPs-SI could be realized with additional interesting and useful properties, such as self-healing and reprocessability. In this review, we outline recent progress in SMPs based on supramolecular interactions, provide insights into the material design and shape memory mechanism, and point out future opportunities. We hope this paper provides guidance for people who are interested in the field of SMPs and supramolecular chemistry and stimulates more meaningful work in the years ahead.

2. GENERAL ASPECTS OF SMPS-SI From an energy perspective, the shape memory effect of polymeric materials is an entropic phenomenon. In a typical one-way dual-shape memory process, the initial SMP sample is rigid and hard to deform. The molecular chains in this initial Received: March 13, 2017 Accepted: May 29, 2017 Published: May 29, 2017 20276

DOI: 10.1021/acsami.7b03624 ACS Appl. Mater. Interfaces 2017, 9, 20276−20293

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ACS Applied Materials & Interfaces

Figure 1. Schematic plot of the shape memory process of SMPs-SI.

Figure 2. (A) Shape-memory response curve of an elastomer containing 2 mol % of UPy pendant side groups. (B) Cartoon of proposed shapememory mechanism involving thermo-reversible H-bonding.23 Reprinted in part with permission from ref 23. Copyright 2007, John Wiley and Sons.

quantifies the ability of the material to recover its original shape. The most common measure employed is Rf (%) = εu/εm and Rr = (εm − εp)/εm.19 To date, most stimuli applied in cyclic tests are heat or light. However, many SMPs-SI are triggered by other stimuli, such as pH or special chemical agents. For those triggered stimuli, the tensile measurements are hard to perform. The bending test is a good alternative measurement for determination of the shape memory effect.19 In the course of the bending test, after the molecular switches were opened by a stimulus, a sample is bent at a given angle θi. Removing the stimulus to close the molecular switches, the so-deformed sample is fixed at angle θf. The Rf is defined as θf/θi. Once the molecular switches are opened again under the stimulus, the sample recovers its permanent shape. The residual angle after recovery is recorded as θp. The Rr is calculated from (θf − θp)/θf. Supramolecular chemistry deals with noncovalent interactions including hydrogen bonding, metal coordination, host− guest interactions, ionic attractions, hydrophobic interactions, and so forth (Figure 1). The nature of these interactions always determines the shape memory behavior of SMPs. Therefore, the subtopics in the main text are organized according to the type of supramolecular interaction.

shape are at a higher entropy (lower energy) state. Upon applying stimuli, the sample becomes soft because the mobility of some molecular chains are activated; as a result, the sample is easy to deform to a temporary shape when an external force is applied. This deformation leads to an entropic change (energy state raises). When depressing the mobility of chains by stimuli, the deformation imposed to the sample can be maintained even after removal of the external force. At the same time, the entropic energy in the system is stored. Once the mobility of chains is again activated, the entropic energy is released, driving the system back to its higher entropy (lower energy) state. Thus, the sample recovers to its original shape.18 Cyclic mechanical investigation is the typical measurement used to quantify the shape memory effect. A typical test protocol is as follows: first, the test piece is treated with external stimuli to open the molecular switch and is stretched to the maximum strain εm. Then, the stimuli is removed to close the molecular switch. As a consequence, the temporary stretched shape is fixed under constant stress σm. Afterward, the stress is retracted. The strain of the sample is reduced to the εu until the free-stress state is reached. Finally, the molecular switch is opened again, and the sample contracts to recover the original shape. The strain after recovery is defined as εp. The shape fixity rate (Rf) describes the ability of the reversible segments to fix the mechanical deformation, and the shape recovery rate (Rr) 20277


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3. SMPS BASED ON HYDROGEN BONDING Hydrogen bonding (H-bonding), one of the most widely used supramolecular interactions for building supramolecular polymers,20,21 has been applied for the construction of SMPs over the last few decades. As weak interactions, the strength and reversibility of H-bonding can be modulated by temperature, pH, or solvent. Therefore, they are ideal molecular switches acting as the switching segments of SMPs. To date, a variety of H-groups have been used for SMPs, including pyridine moiety, ureidopyrimidine units (UPy), carboxylic acids groups, DNA’s double helix, and so forth.10,22 3.1. Thermally Stimulated SMPs Based on H-Bonding. At elevated temperature, the strength of H-bonding decreases because high temperature can accelerate the rupture of Hbonding. Upon cooling, the ruptured bonds can reform along with the restoration of the bonding strength. Because of this thermally induced reversibility, in most cases H-bonds in an SMP are exploited as reversible molecular switches. In 2007, Anthamatten and co-workers reported a thermally triggered SMP based on a chemically cross-linked poly(butyl acrylate) elastomer functionalized with the UPy moieties in the side chain.23 UPy groups contain a linear array of four Hbonding sites. As shown in Figure 2, at a relatively low temperature, two UPy moieties can form strong quadruple hydrogen bonds with dimerization constants Kdim of up to 6 × 107 M−1; therefore, the resulting strong H-bonding could act as cross-links to fix a deformed shape (Rf ≈ 90%). At elevated temperature, the quadruple hydrogen bonds of UPy moieties dissociate, allowing recovery of the temporary shape (Rr ≈ 100%). This work demonstrated, for the first time, that Hbonds can effectively be used to stabilize mechanically strained states of a lightly cross-linked elastomer, forming a new class of SMPs. The creep experiments indicated that the mechanical activation energy of this material was the same order of magnitude as the activation energy of chemical dissociation of a traditional elastomer. Besides acting as the switching segment of dual-shape SMPs, H-bonding is also a good candidate for serving as molecular switches for multishape SMPs. Multishape SMPs are an emerging class of materials that have the capability to remember many metastable shapes. Since Lendlein’s group first reported triple-shape memory polymers (with two temporary shapes) in 2006,24 a substantial endeavor has been dedicated to the development of multishape SMPs because they meet the more complex requirements of the application. Generally, the multishape SMPs contain at least two independent switching domains or a broad thermal-transition range.25 In early work about multishape SMPs, the second switching segment was introduced by chemical synthesis, such as covalently linking two polymers or synthesizing multiblock copolymers. However, these chemical synthesis methods always require complex procedures. H-bonding interactions offer a controllable and simple method to construct additional molecular switches in SMPs, thus endowing SMPs with multishape functionality.8 For example, Voit and co-workers synthesized triple-SMPs comprised of both covalent cross-links and H-bond cross-links via a one-pot method.26 The combined effect of the glass transition of (meth)acrylate copolymers and dissociation of the hydrogen bond of UPy units provides the material with two transition temperatures. As shown in Figure 3, the sample recovered from the bent and textured shapes to the twisted and

Figure 3. (A) Schematic of triple SMP programming and recovery cycles of the acrylic system. (B) Sequential photographs taken of the recovery of the system of acrylic triple SMP demonstrating the tripleshape memory effect. Reproduced with permission from ref 26. Copyright 2012, American Chemical Society.

textured shapes by heating to 37 °C and to the flat and smooth shapes by subsequently heating to 90 °C. Zhou et al. developed a triple-shape memory supramolecular composite through blending mesogenic units into polyurethane (PU) with dualshape memory properties.27 The mesogenic unit, named cholesteryl isonicotinate (INChs), is a liquid crystal compound with an electron-rich pyridine ring. The H-bonding interactions between pyridine rings and carboxyl groups in the chains of PU acted as one kind of switching segment, and the PCL crystalline region in PU acted as another kind of switching segment. The two kinds of reversible switches formed two distinct switching domains that could trigger thermosensitive triple-shape memory behavior. Apart from being a reversible switch in SMPs, H-bonding can also be exploited as netpoints, holding the permanent shape within a certain temperature range. Hu and co-workers prepared a series of pyridine-containing supramolecular PUs from N,N-bis(2-hydroxylethyl) isonicotinamine, hexamethylene diisocyanate, and 1,4-butanediol.28 The strength of H-bonds between the urethane group is stronger, acting as permanent cross-links with low sensitivity to temperature, whereas weaker H-bonds between the pyridine rings serve as thermally sensitive molecular switches. These supramolecular materials showed a good thermally induced shape memory effect. The sample containing 53.7% of BINA showed Rf of ∼98% and Rr of ∼96%. Furthermore, Wang et al. successfully prepared a novel shape memory tough hydrogel through simply mixed poly(vinyl alcohol) (PVA) with tannic acid (TA).29 The TA can form multiple strong H-bonds with PVA. This strong H-bonding can not only endow the PVA/TA hydrogel with high tensile strength (∼2.88 MPa) and elongation (up to 1100%) but also functions as a permanent cross-link. Meanwhile, the hydroxyl of PVP can form weak H-bonding, which, causing reversible breakage and formation, imparts this hydrogel with an outstanding thermally responsive shape memory effect. The recovery progress of the PVA/TA hydrogel is rapid. The sample with a temporary shape can recover to its original shape in 2 s when immersed in 60 °C water. The combination of a semi-interpenetrating structure (semiIPN) or an interpenetrating structure (IPN) and H-bonding can also be applied to realize a thermally responsive shape memory effect. In 2004, Peng and co-workers reported a type 20278


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Figure 4. (A) Schematic structure of the PCL−PTMEG dynamic network, the triple-shape memory effect, and the self-healing capacity of PCL− PTMEG. Reproduced with permission from ref 33. Copyright 2015, American Chemical Society. (B) Illustrative structure of the elastic network of PU with twin-switches (left) and state transitions of the composites under different humidity and temperature conditions (right). Reprinted in part with permission from ref 45. Copyright 2011, John Wiley and Sons.

coexistence of an interpenetrating network as well as Hbonding can provide firm permanent netpoints. Thus, the PCU/PPDO IPNs showed excellent triple-shape properties. Compared with chemical cross-links, physical cross-links may not only serve as permanent netpoints in the shape-changing process32 but also endow reprocessability and a self-healing feature to the SMPs. More recently, Wang and co-workers prepared a dynamic network via self-complementary Hbonding through Upy-telechelic poly(tetremethylene ether) glycol (PTMEG) and four-arm star-shaped PCL precursors.33 These networks possessed two discrete thermal transitions belonging to the melting of the PTMEG and PCL domains, respectively, which acted as the switching segments, and the Hbonds of Upy dimers served as cross-links in the materials. Hence, this material showed excellent triple-shape memory functionality. In the triple-shape memory test, the first and second Rf are 98 and 91%, respectively, and the two Rr values are 95 and 71%, respectively (Figure 4A). Furthermore, the

of hydrogen-bonded semi-IPN composed of poly(methacrylic acid comethyl methacrylate) (P(MAA-co-MMA)) network and linear poly(ethylene glycol) (PEG).30 Because of the Hbonding interactions, PAA and linear PEG formed fully miscible complexes. The amorphous PAA−PEG chain domains, which could transfer from glass state to rubbery state during the heating process, acted as switching segments, and the chemical cross-linked points served as netpoints. Consequently, the complexes showed a thermally stimulated shape memory effect. On the basis of the same principle, poly[(methyl methacrylate)-co-(vinyl-2-pyrrolidone)] (P(MMA-co-VP)/PEO with a semi-IPN structure utilizing Hbonding interactions also exhibits a good thermally induced shape memory effect. Furthermore, Wang and co-workers developed a poly(ε-caprolactone)/poly(p-dioxanone) (PCU/ PPDO) IPN via cross-linking star-shaped Upy-functionalized PCU and PPDO.31 These IPNs had two well-separated switching segments (crystallites of PCU and PPDO), and the 20279


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Figure 5. (A) Schematic of the pH sensitivity of the PEG4000-MDI-BIN polymer. Reproduced with permission from ref 46. Copyright 2014, Royal Society of Chemistry. (B) Synthesis of a nucleic acid-functionalized acrylamide copolymer and its cross-linking in a mold to yield a triangle-shaped hydrogel at pH 5.0 by the formation of i-motifs and a duplex structure (top) and photographs showing the pH-stimulated shape memory behavior (bottom). Reprinted in part with permission from ref 48. Copyright 2014, John Wiley and Sons.

can break the H-bonding between GO and PVA and serve as plasticizers to decrease the glass transition. Other nanocomposites, such as PU/cellulose,41,42 polylactide/cellulose,43 and PVA/Al2O3 nanoparticles44 were also reported to show a water-induced shape memory effect. It should be noted that the introduction of nanofillers could also clearly improve the mechanical properties of the materials. In general, the water-induced shape memory effect can also be triggered by heating because they are controlled by the same switches. Interestingly, Hu and co-workers developed a novel SMP composite featuring heterogeneous twin switches.45 As shown in Figure 4B, this composite was an elastic network of PU, which simultaneously contained thermally sensitive PCL crystals and a water-sensitive H-bonding network of cellulose whiskers. When sequentially triggered by temperature and water, the composite recovered from temporary shape A to temporary shape B and eventually to original shape C. This multiresponsiveness and multishape change capacity is believed to meet the complex requirements of the applications. 3.3. pH-Stimulated SMPs Based on H-Bonding. Except for the extensively studied thermally induced SMPs, pHtriggered SMPs, which have unique potential applications such as in vivo, have also been developed. In contrast to the inconvenience of heating for certain biomedical applications, pH is an ideal stimulus because of the varied pH levels in different sites of the human body. To date, several works regarding pH-sensitive SMPs have been reported. H-bonding interactions are one candidate for molecular switches for the design of pH-induced SMPs-SI. Zhou and co-workers introduced pyridine rings onto the backbone of PU and synthesized a highly pH-sensitive shape memory hydrogel termed PEG4000-MDI-BIN.46 The pH sensitivity of the material resulted from the formation of Hbonding between the N atom of the pyridine ring and the H−N of urethane in neutral or alkaline environments and disruption of these H-bonds in acidic environment (Figure 5A). In a shape memory cycle, the Rf and Rr can reach 83 and 81%, respectively.

damage of the sample can be self-healed by gently warming at 40 °C for 48 h due to the dissociation and reassociation of the quadruple hydrogen bonding during the healing process. Zhao and co-workers reported thermally reversible rubbers constructed by the thiol−ene-functionalized polybutadiene (PB) oligomers via dynamic ionic hydrogen bonds and covalent cross-links.34 The resulting polymers exhibit self-healing and shape memory functions owing to the reversible ionic hydrogen bonds. It should be noted that the self-healing resulted from reversible supramolecular interactions needing a low density of cross-linking bonds to show good self-healing efficiency. However, the shape memory effect often requires a certain amount of covalently cross-linking bonds to act as netpoints and improve the mechanical strength. Therefore, for most selfhealing and shape memory materials, these two contradictory properties should be properly balanced. 3.2. Water-Stimulated SMPs Based on H-Bonding. When SMPs based on H-bonding are transferred to aqueous medium and H-bonding motifs are not surrounded by a hydrophobic microenvironment, the water competes with the H-bonding array and could significantly decrease the strength and stability of the interaction.35Therefore, a water-stimulated shape memory effect is triggered. For instance, Huang and co-workers found that water could be utilized as the plasticizer of some commercial PU due to the weakened H-bonding between N−H and CO groups in water.36 Consequently, the Tg of these PUs was decreased below the ambient temperature, and the shape recovery was triggered. Other water-stimulated SMPs, such as PU-containing pyridine,37 cross-linked poly(vinyl alcohol) (PVA),38 and multiblock thermoplastic PU39 were also reported. By introducing nanofillers into a polymer matrix through hydrogen bonding, it is easy to obtain water-responsive shape memory nanocomposites. For example, Fu and co-workers reported a nanocomposite by introducing graphene oxide (GO) into PVA.40 The resulting materials showed waterinduced shape memory behavior because the water molecular 20280


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Figure 6. (A) CO2-triggered shape memory mechanism of a PVD hydrogel. (B) Actual observation of the shape memory effect of a PVD hydrogel. Reprinted in part with permission from ref 50. Copyright 2015, John Wiley and Sons.

Figure 7. (A) Molecular mechanism of the shape memory effect of a partial α-CD−PEG inclusion complex. (B) Mechanism of the shape memory property of PC16 DMAEMA-AM/α-CD. Reprinted in part with permission from ref 63. Copyright 2015, John Wiley and Sons.

In addition, this pH switching effect was also used to control drug delivery. Later, Zhou and co-workers prepared a pHstimulated SMP nanocomposite by blending pyridine-functionalized cellulose nanocrystals (CNCs) with PEG−PCL-based PU.47 At a high pH value, H-bonding was formed between the pyridine moieties and hydroxyl units, whereas these interactions were weakened or eliminated by protonating the pyridine groups at low pH. Furthermore, the mechanical properties of SMPs were also significantly improved through the addition of CNCs. With increasing CNCs, the Young’s modulus of this composite increased from ∼9.6 to ∼54.2 MPa. DNA was also a kind of pH-sensitive switch unit in which self-recognition of the complementary base pairs by H-bonding leads to self-assembly of the double helix. Willner and coworkers developed a kind of DNA-based pH-stimulated SMP via copolymerizing acrylamide with two different kinds of acrydite-modified nucleic acids (Figure 5B).48 The nucleic acids strand (1) included the cytosine-rich sequence self-assembled at pH 5.0 into an i-motif structure but dissociated at pH 8.0 into a random coil conformation. The nucleic acids strand (2) exhibited self-complementarity and cannot be dissociated. By dissociation of the i-motif units at pH 8.0, the DNA-hydrogel dissolves into a “quasi-liquid”, where the duplex structure network provides the shape-memory code for the recovery of the material at pH 5.0. By cyclic switching of the pH between 8.0 and 5.0, the SMPs undergo reversible transitions between quasi-liquids and predesigned shaped structures.

In the switchable cycles, pH-stimulated shape memory processes normally require repeated addition of acids and bases, which has an inevitable disadvantage due to the accumulation of salt and contamination of the system. CO2 is an alternative stimulus instead of adding chemical agents because it can also react with responsive groups such as amines or amidines and could be clearly moved by purging with inert gases or heating.49 Liu and co-workers constructed a double Hbonding PVD hydrogel by copolymerization of hydrophobic monomers (2-vinyl-4,6-diamino-1,3,5-trizaine) with hydrophilic monomers (N,N-dimethyl-acrylamide) with PEG diacrylates.50 This hydrogel showed CO2-triggered shape memory behavior owing to the reversible destruction and reconstruction of diaminotriazine (DAT) H-bonding. As shown in Figure 6, the hydrogel strip was softened in CO2-treated water because DAT moieties can be protonated in weak acidic solution with double H-bonding disruption. Deforming this sample into a spiral and removing CO2, the deformation was fixed because the DAT moieties deprotonated along with the reformation of the Hbonding. By bubbling CO2 again, the spiral could switch back to a rectangular shape. In summary, H-bonding is a good candidate as a molecular switch of SMPs. Because the strength of H-bonding can be modulated not only by temperature but also by other stimuli like pH or solvent, multiresponsive shape memory effects are expected for the SMP based on H-bonding. Furthermore, with rational material design, H-bonding can be used as permanent 20281


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Figure 8. Molecular mechanism of the pH-induced shape-memory effect of β-CD-Alg/DETA-Alg. Reprinted in part with permission from ref 65. Copyright 2012, John Wiley and Sons.

CD−PEG IC were over 95%.59 This approach introduced the permanent segment through physical self-assembly, which is in contrast to the conventional chemical approaches. Furthermore, this approach is also expected to provide shape memory function to several other classical polymers. In the later study, we prepared biodegradable SMPs based on partial inclusion between α-CD with degradable and hydrophobic PCL60,61 and a supramolecular network composed of γ-CD and PEG.62 The partial α-CD−PCL not only showed good shape memory properties with the recovery ratio exceeding 90% but also good degradability in the presence of lipase. It was demonstrated that the formation of inclusion segments significantly accelerated the degradation of materials. In the γ-CD−PEG network, the PEG chains are cross-linked by interlocking γ-CDs rather than covalent bond or traditional physical attractive interaction. In this case, the PEG/γ-CD inclusion cross-links and the PEG crystallites account for the fixing and reversible cross-links, respectively. By adjusting the mass content of inclusion segments, the shape fixed at 100% and the shape recovery at >95% were observed. In addition, Zhang and co-workers designed a thermally responsive SMP utilizing H-bonding between α-CDs.63 This SMP was prepared by block copolymerization of acrylamide(AM) and dimethyhexadecyl [2-(dimethylamino)ethyl-methacrylate] ammonium bromide (C16DMAEMA) in the presence of α-CD. As shown in Figure 7B, the crystalline domains, induced by the H-bonding between α-CDs threaded on the hydrophobic side units of the polymer chains, can reversibly melt and crystallize in response to temperature. As a result, the material showed excellent shape memory properties. Manufacturing SMP using CD−polymer interactions is a very simple way to endow shape memory function to classical polymers. However, to date, this kind of SMP has only been triggered by temperature. 4.2. SMPs Based on CD Units−Hydrophobic Group Interactions. It has been demonstrated that the ICs between CDs and various small molecular guests can be modulated by

cross-links as well, which endows reprocessability and a selfhealing feature to the SMPs.

4. SMPS BASED ON HOST−GUEST INTERACTIONS Host−guest interactions are the driving force for building a large number of intelligent response materials such as multiresponsive porous polymers,51 macroscopic self-assemblies,52 self-healing materials,53,54 and artificial muscles.55 Cyclodextrins (CDs) are the most widely used hosts owing to their extremely low toxicity and wide availability. They are cyclic oligomers composed of D-glucose units linked by α-1,4glycosidic linkages. The three-dimensional structure of CDs looks like a truncated cone having a hydrophobic inner cavity. CDs are capable of including a variety of compounds ranging from small molecules and ions to polymers in their cavities with high selectivity to form inclusion complexes (ICs). 56 Furthermore, the inclusion complexation between CDs and many guests is sensitive to external stimuli, such as pH, light, and redox. Thus, a number of SMPs based on CD inclusion complexation have been developed. 4.1. SMPs Based on CD−Polymer Interactions. It is known that CDs are able to form ICs with different kinds of polymer chains.57 The CD−polymer ICs show a thermally stable crystalline structure due to the strong hydrogen-bonding formation between adjacent CDs. Generally, the complete CD−polymer ICs have no melting behavior but only decompose above 300 °C.58 Our group has utilized this feature as a physical way to introduce netpoints into SMPs. For instance, we prepared partial α-CD−PEG ICs by controlled the molecular weight of PEG and the ratio of α-CD/PEG through the casting method. PEG has only a melting transition and does not meet the SMP requirement. However, partial α-CD−PEG IC shows shape memory behavior because it contains both thermally stable CD−PEG inclusion crystallites as netpoints and thermally sensitive naked PEG crystallites as switching segments (Figure 7A). The fix and recovery ratios of partial α20282


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ACS Applied Materials & Interfaces external stimuli. In the past decade, introducing CDs and guest moieties into polymer chains has become a very effective method for building intelligent responsive materials.64 Our group first used the host−guest interactions to design the SMPs.65 As shown in Figure 8, a pH-sensitive SMP was prepared by cross-linking the β-CD-modified alginate and diethylenetriamine (DETA)-modified alginate. In a basic medium (pH 11.5), the DETA groups showed a hydrophobic nature and formed IC with β-CD. In a neutral medium, the DETA became hydrophilic due to the ionization of the amine and resulted in dissociation of the β-CD−DETA IC. This pHdependent reversible formation/dissociation of β-CD−DETA IC switched off/on the molecular mobility so as to freeze/ release the temporary shape, and the cross-linked alginate chains served as fixing netpoints for stabilizing the permanent shape. It was shown that this material can be processed into a temporary shape as needed at pH 11.5 and recover to its initial shape at pH 7.0. Both the recovery and fixity ratios are around 95%. Furthermore, this material showed good degradability and biocompatibility; thus, it has a high potential for medical applications. The interactions between CD and guests may also be sensitive to other stimuli, such as light and redox. Therefore, using host−guest interactions as molecular switches, we can design a variety of new SMPs with novel triggers other than heating. For instance, in our later study, a redox-induced SMP was developed by cross-linking β-CD-modified chitosan (βCD-CS) and ferrocene (Fc)-modified branched ethylene imine polymer (Fc-PEI).66 Because β-CDs are able to interact with Fc groups to form ICs, but dissociate with oxidized Fc, the β-CDCS/Fc-PEI sample could recover to its original shape after immersion in an oxidant solution. Intriguingly, if glucose oxidase (GOD) was immobilized within the β-CD-CS/Fc-PEI network, the resultant material could show a shape memory effect in response to glucose. This phenomenon was due to the fact that GOD oxidized glucose to produce hydrogen peroxide and then oxidized Fc. Harada and co-workers further employed two different kinds of host−guest ICs of β-CD with adamantane (Ad) and Fc to bind polymers together to form a supramolecular hydrogel (β-CD−Ad-Fc gel).67 We also prepared a tristimuli-responsive SMP by introducing α-CD and azobenzene (Azo) into a poly(acrylate acid)/alginate (PAA/ Alg) network.68 The host−guest interactions between α-CD and Azo acted as molecular switches in the SMPs. Because the formation/dissociation of α-CD−Azo ICs can be modulated by temperature, light, or chemicals, this material can be processed into a temporary shape as needed and recover its initial shape under UV irradiation, heating, or the addition of competitive molecules. Ritter and co-workers69 prepared another SMP based on CD inclusion by incorporating 2-(N′-(adamantan-1-yl)ureido)ethyl methacrylate (AdMA, as hydrophobic monomer) and N,Ndiethylacrylamide (DAM, as thermosensitive monomer) into the cross-linked polymer networks. The resulted hydrogel can be programmed to form a temporary shape due to the thermosensitive DAM-based segments and recover to its original shape after the addition of β-CD. This inclusioncomplex-driven shape memory effect is due to the fact that the hydrophobic Ad domains would switch to hydrophilic ICs by the addition of a sufficient amount of free β-CD. Yan and coworkers report a poly(ionic liquid) gel (PIL), which showed shape memory behavior because of a similar mechanism.70

In summary, the diversified stimuli of CD−hydrophobe interactions provide a good opportunity to develop SMPs with novel stimulus sensitivities other than thermal induction. However, it should be noted that most SMPs based on CD ICs are hydrogels because the inclusion between CDs and guest groups always requires the help of water.71

5. SMPS BASED ON HYDROPHOBIC INTERACTIONS In an aqueous medium, hydrophobic molecules or moieties tend to form aggregates to minimize their exposure to water. This tendency, namely hydrophobic interactions, can be efficiently used for preparing supramolecular polymers.72 The free and bonded hydrophobes are in a dynamic equilibrium, which is sensitive to temperature change as a result of the temperature-dependent activation energy.73 Hydrophobes can gradually disengage from the hydrophobic associations with increasing temperature or reassociate at relatively low temperature, enabling a reversible change of the strong and weak hydrophobic interactions. Since Osada and co-workers reported the first shape memory hydrogel based on hydrophobic interactions,74 these interactions have been further studied as reversible switches for SMPs.75−79 A typical example of a shape memory hydrogel based on amorphous aggregation of hydrophobic domains was reported by Inomata and co-workers.75 This hydrogel was formed by cross-linked poly-[N5-(2-hydroxyethyl) L-glutamine] (PHEG) containing alkyl side chains. The temperature-dependent segregation of the hydrophobic alkyl chains endowed thermally induced shape memory properties to the material. The shape memory ability of these hydrogels was enhanced with increasing hydrophobicity. When the alkyl side chain is C18, the Rf and Rr of the hydrogel could reach 73 and 100%, respectively. It should be noted that the shape recovery of the deformed hydrogel was accompanied by the gradual decrease of segregation strength with increasing temperature, which meant that the shape recovery ratio could be controlled by the recovery temperature. Another example of SMPs based on hydrophobic interactions was reported by Okay and co-workers.76 The hydrogels in this study were synthesized via micellar copolymerization of acrylic acid with stearyl methacrylate in the presence of the surfactant CTAB. The physical networks were generated from hydrophobically modified polyelectrolytes (PAA-C18) with oppositely charged surfactants (CTAB) via hydrophobic and electrostatic interactions (as shown in Figure 9). C18 and CTAB form the core of the micelles via hydrophobic interactions. The formation of the electrostatic interactions between the carboxyl of AA units and the CTA counterions stabilize the micelle cross-links in water and act as the physical cross-links to strengthen the hydrogel. Containing only 2 mol % C18, the hydrogels exhibit high tensile strength (0.7−1.7 MPa), Young’s moduli (180−600 KPa), and a stretch at break (800−900%). In particular, the shape fixing and recovery efficiency could be optimized (up to 100%) by reducing the water content of these hydrogels to a critical value. Furthermore, the hydrogel samples could self-heal via heating and surfactant treatment of the damaged areas. A common feature of the shape memory hydrogels is that they behave as hard plastics when they are in the temporary state, which defeats one of the important features of a hydrogel, i.e., softness (relatively low modulus). In contrast, Weiss and co-workers reported a relatively soft and elastic shape memory hydrogel with high mechanical strength and fracture tough20283


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Recently, we reported a pH and thermally sensitive dual/ triple shape memory hydrogel by grafting dansyl moieties into cross-linked polyacrylamide (PAAM).78 The hydrophobic aggregation of dansyl moieties exhibited an aggregationdisaggregation transition in response to the pH or temperature change, acting as reversible switches in the shape memory process. Huang and co-workers developed an interesting shape memory hybrid made of a PCL sponge and cool-melting Poloxamer 407 (P407) gel.79 In contrast to conventional thermo-induced SMPs, the shape memory effect of the PCL/ P407 gel can be induced by cooling. The plastic PCL sponge served as an elastic component, and the P407 gel served as a transition component due to the cooling-induced reversible gel−sol transition property. The compressed PCL/P407 sample was able to maintain its compact shape after removing the constraint at 35 °C due to the fact that the hydrophobic interaction of the PPO block resulted in P407 solution solidification and prevented elastic shape recovery of the elastic sponge. Upon cooling to 5 °C, P407 melted, and elastic recovery of the sponge occurred. As the induced temperature (5−35 °C) matches well with human body temperature, this material shows potential for biomedical applications. The introduction of hydrophobes into the hydrophilic polymer chains of hydrogels would increase the hydrophobicity of the structure. It stands to reason that the water content of these materials would decrease. With increasing the concentration and hydrophobicity of the hydrophobes, if the phase separation is not too severe, higher moduli and Rf would be achieved, and the time-dependent relaxation behavior of the temporary shape would be reduced. On the other hand, this may result in hard plastics. In addition, most of the SMPs based on hydrophobic interactions are to date only triggered by temperature.

Figure 9. Image of a PAA hydrogel sample and its corresponding cartoon representation of the physical network. Reproduced with permission from ref 76. Copyright 2014, American Chemical Society.

ness.77 It was fabricated by chemically cross-linking quadpolymers of hydroxyethyl acrylate, N,N-dimethylacrylamide, 2cinnamoyloxyethyl acrylate, and 2-(N-ethylperfluorooctanesulfonamido)ethyl methacrylate. Microphase separation of hydrophobic FOSM formed physical cross-links in the covalent cross-linked network, resulting in hybrid hydrogels (60−70% water) with high mechanical strength (ultimate tensile stress was 270−540 KPa), extensibility (elongation to break values were 500−580%), and fracture toughness (119− 188 J/m2). The temperature-dependent glassy nanodomains served as switching segments for the shape memory and greatly affect the shape fixing efficiencies, whereas the covalent crosslinks determine most of the shape recovery efficiencies. The weakness of this SMP is some creep relaxation occurring in the fixed shape due to viscoelastic effects of the nanodomain crosslinks, but the Rr of the material is approximately 100%.

Figure 10. (A) Preparation of the metal−ligand PB network (top) and schematic of the shape memory effect of the metal−ligand PB network (bottom). Reproduced with permission from ref 83. Copyright 2011, American Chemical Society. (B) Metallosupramolecular poly(BA-MMA) copolymer containing Zn-Mebip bonds (top) and schematic illustration for thermally induced shape memory-assisted healing of the materials (bottom). Reproduced with permission from ref 84. Copyright 2014, Royal Society of Chemistry. 20284


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Figure 11. (A) Zn2+-induced shape memory effect in hydrogels and (B) schematic illustration of the triple SME. Reprinted in part with permission from ref 88. Copyright 2011, John Wiley and Sons. (C) Schematic diagram delineating the multiple associations and dissociations in P(AN-co-AA)based hydrogels. (D) Real-time observation of the recovery of the box-shaped hydrogel. Adapted with permission from ref 93. Copyright 2014, Nature Publishing Group.

6. SMPS BASED ON METAL−LIGAND COORDINATION Metal−ligand coordination bonds, like H-bonds, are highly directive and saturable. The cooperation of the various metal ions and ligands have been widely applied for building metal− organic frameworks, crystal clusters, and supramolecular materials.80,81 Basically, the metal-coordination bonds are stronger than most of supramolecular interactions; meanwhile, they could be reversibly tuned by proper external stimuli (e.g., light, heating, or chemicals).82 Therefore, metal−ligand coordinations are also ideal molecular switches acting as the switching segments of SMPs. SMPs relying on the metal− ligand complex can be divided into dry SMPs and shape memory hydrogels. 6.1. Dry State SMPs Based on Metal Coordination Bonds. For dry SMPs, the metal−ligand complex can serve as a thermoresponsive switch by direct or indirect heating. In addition, certain solvents may act as plasticizer and/or competitive agents to bring about shape memory behavior. For example, Rowan and co-workers reported covalently crosslinked metallosupramolecular polymer films with shape memory properties that could respond to changing temperature, light irradiation, as well as chemical agents.83 As shown in Figure 10A, these materials were prepared by adding metal salts to cross-linkable poly(butadiene) (PB) with metal chelating ligands at the end. The fixed cross-links were achieved by photo cross-linking of the PB with tetrafunctional thiol, and the reversible cross-links were metal−ligand complexes. It was demonstrated that heating, UV irradiation, and methanol were able to soften the metal−ligand interactions and thus enable the shape memory effect. The sample with various cross-linking densities of the soft phase were all found to have shape memory properties with Rf greater than 80% and Rr over 95%. The selection of different metal ions and ligands has a significant effect on the binding strength of metal−ligand complexes. Using the metal−ligand complex to form switching segments, proper dynamic properties should be adopted to achieve high shape fixing efficiency, whereas in another design fashion, stronger metal-coordination bonds can be utilized to serve as fixing cross-links in a noncovalently cross-linked SMP. Xia and co-workers designed a Zn-Mebip complex dynamically cross-linked poly(n-butyl acrylate-co-methyl methacrylate) (poly(BA-MA)) metallosupramolecular polymer.84 The side chain Mebip ligand coordinated with Zn2+ and the Zn-Mebip complex formed separated microphases owing to their

incompatibility. As a result, the polymer in the metal−ligandrich region possessed a higher Tg than in the polymer matrixrich region. Both Tg are below the flow temperature (Tf) associated with decomplexation of the Zn-Mebip complex. Therefore, below Tf, the strong Zn-Mebip bonds formed a robust elastic network along with the two glass transitions, endowing the material with the triple shape memory effect. Moreover, because the Zn-Mebip can be dissociated above Tf and reformed at low temperature, the material also showed good self-healing ability. As shown Figure 10B, under thermal stimulus, the shape memory ability brought the separated damaged area together and then performed self-healing. Along similar lines, Zhou and co-workers synthesized a silver coordination SMP (Ag-PIE) possessing antibacterial action.85 Because of the wide glass transition temperature, excellent shape memory function was observed both under air and physiological conditions for this Ag-PIE. This system can also act as a reservoir for bactericidal Ag ions and shows antibacterial function by releasing a moderate amount of Ag ions in aqueous environments. Further investigation of the same group found that a similar Cu-PIE also exhibited shape memory properties.86 The metal−ligand coordination bonds have a broad bonding strength range; therefore, these bonds can either act as netpoints or reversible cross-links in the SMPs-SI. Basically, most of these systems are all in the form of polymers bearing certain ligands, which always requires complex synthesis procedures to fabricate the sophisticated chemical structures.87 6.2. Shape Memory Hydrogels Based on Metal Coordination Bonds. As described previously, the diversity of choices for the metal ions and ligands provides large tunability for the design of the SMPs. Generally, the metal− ligand complex hydrogels are formed simply by immersing in the metal ion-containing solution and are disrupted either by simply immersing the hydrogels in water or adding certain competitive ligands. Metal ions play an important role in biological activity, e.g., conveying electrons during biological redox reactions, maintaining a balance of water and electrolytes in the organism, and so forth. Therefore, SMP hydrogels based on metal coordination bonds show great potential in biomedical applications. In 2012, Liu and co-workers first reported a Zn2+-induced shape memory effect of CN-CN dipole−dipole reinforced high strength hydrogels.88 As shown in Figure 11A, strong dipole− 20285


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Figure 12. Schematic illustration of a self-healable shape memory supramolecular hydrogel formed by Alg-PBA and PVA. Reproduced with permission from ref 94. Copyright 2014, Royal Society of Chemistry.

Figure 13. (A) Schematic depicting changes in copper-based cross-link density. (B) Demonstration of shape memory for copper cross-linked hydrogels. Reproduced with permission from ref 98. Copyright 2013, American Chemical Society.

reversible formation/dissociation of ion-chelation and dipole− dipole interactions gives rise to shape memory effects as shown in Figure 11D. Additionally, utilizing this shape memory effect, controlled heterogeneous stem cell differentiation could be realized. These results could lead to simple and controllable scaffold formulation for the application of regenerative medicine. Introducing dual reversible interactions into materials may render them self-healing with shape memory properties.94−96 Chen and co-workers reported a self-healable shape memory hydrogel formed from phenylboronic acid-modified sodium alginate (Alg-PBA) and PVA on the basis of reversible PBA-diol ester bond interactions (Figure 12).94 The PBA-diol ester bonds can be associated under basic aqueous conditions, whereas they are disassociated under acid aqueous conditions. This dynamic nature was responsible for the self-healing properties and good deformability. The Ca2+-carboxyl complex served as reversible cross-links and fixed the temporary shapes, and the fast formation of the complex make it so that the temporary shape could be fixed in 30 s with Rf of 100%. Extracting Ca2+ by CO32− or EDTA, the temporary shape recovered to its original state. Moreover, combined with the microcontact printing method, the hydrogel was found to possess microscopic shape memory performance as well. In

dipole interactions were formed in water. In 30% ZnCl2 solution, the hydrogels shrank because Zn(CN)2 complexes were formed as the other physical cross-links to densify the polymeric network. However, when the Zn2+ concentration exceeded a critical value, each Zn2+ only bonded to a single CN group, shielding CN groups from forming dipole−dipole interactions. As a result, the absence of both the dipole−dipole interaction and Zn(CN)2 complexes brought about swelling of the hydrogel. The Young’s modulus of the hydrogels could increase from 7.36 MPa in water to 37.57 MPa in 30% ZnCl2 solution but decreased to 0.053 MPa in 50% ZnCl2 solution. This Zn2+-controlled dynamical dipole−dipole interaction imparted the hydrogel with the triple shape memory effect (as shown in Figure 11B). On the basis of the same principle, the same group further reported a series of metal ion-triggered shape memory hydrogels reinforced by H-bonding or dipole− dipole interactions.89−92 Interestingly, they realized double-ion-triggered shape memory effect of a poly(acrylonitrile-acrylic acid) (p(AN-coAA)) network reinforced by dipole−dipole interactions.93 As shown in Figure 11C, Ca2+ serves as a chelation center among carboxyl groups, resulting in a denser network of hydrogels. Conversely, Zn2+ can selectively dissociate CN-CN dipole− dipole interactions and thus loosens the hydrogel. The 20286


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Figure 14. (A) Molecular structure (left), dynamic mechanical analysis curve (right), and (B) visual demonstration of a quadruple-SME. Adapted with permission from ref 25. Copyright 2010, Nature Publishing Group. (C) Thermomechanical strain response of PFSA between 80 and 120 °C under a constant stress of 0.47 MPa. Reproduced with permission from ref 102. Copyright 2014, American Chemical Society.

based on ionic interactions are extremely robust and therefore can be used as an alternative to covalent cross-links. In SMPs based on ionic interactions, the strong interactions are usually used as permanent net nodes with relatively high temperature resistance.100 A typical example of an SMP based on ionic interactions is a commercial ionomer, perfluorosulfonic acid (PFSA, also known as Nafion). In 2010, Xie reported that PFSA possessed an attractive multishape memory effect due to its broad Tg (Figure 14A).25 PFSA contains ionic cluster phases because of the presence of associated sulfonate groups. The electrostatic networks of PFSA are stable and can resist relatively high temperature, serving as permanent networks. The broad glass transition from 55 to 130 °C contributes to the multishape memory effects, and the shape memory property is tunable by changing the deformation temperature rather than the material composition. For dual-shape memory effects, nearly 100% shape fixing and recovery performance at any temperature above the onset of the glass transition was observed. The quadruple-shape memory effect of PFSA was shown in Figure 14B. Quantitatively, the Rr values are all above 93%, whereas the first and second Rf are 60%, corresponding to ionomers with broad glass transition.101 Intriguingly, the PFSA also showed a quasi two-way shape memory effect. A two-way (or reversible) shape memory effect refers to the behavior that a material can switch back and forth between two shapes without any change to the imposed external mechanical conditions. It was found that PFSA exhibited strain response under cyclic heating and cooling conditions while the material was placed under a constant external stress (Figure 14C).102 This phenomenon is believed to be due to the orientation/erasing of the ionic phase. Although PFSA is not as competitive as a typical liquid crystalline elastomer-based two-way SMP as a reversible actuator because of the small reversibility ratio and irreversible creep, this study does point to a potential new method for making ionomeric reversible actuators. Apart from neat ionomers, Weiss also developed a series of fatty acid/ionomer SMP composites based on the ionic interaction.103−106 For the zinc oleate (ZnOl)-filled elastomeric ionomers,105 physical cross-links in the ionomer due to interchain ionic interaction contributed to the permanent network, and the dipolar interactions between the ionomer and the dispersed phase of crystalline ZnOl provided a temporary network. For the sodium oleate (NaOl)-filled metal salts of

another report, the PBA-diol ester bonds were chosen to be the reversible cross-links, whereas the Ca2+-carboxyl coordination acted as the permanent cross-links; a pH- and sugar-induced shape memory effect was obtained because the PBA-diol ester bonds disassociated in acidic conditions and an aqueous solution of monosaccharides.95 Metal ions such as copper and iron exhibit redox-state preferences in coordination number, geometry, and ligand type.97 On the basis of their unique redox characteristics, these metal ions have been used to develop redox-induced shape memory materials. In these hydrogels, the oxidized metal ions can form stronger coordination complexes with ligands, thus densifying the polymer network and reinforcing the hydrogels. When reducing the metal ions by adding reducing agents or extracting metal ions by a competitive ligand, the complexes can be disassociated along with a decrease in their mechanical properties, endowing the materials with shape memory capability. For instance, an electroplastic elastomer hydrogel based on Cu2+-pyridine coordination bonds has been prepared by Meyer and co-workers.98 Cu2+ tends to coordinate with pyridine in the oxidized state, whereas the coordination between Cu+ and pyridine is too weak to be an effective reversible cross-link. Measured tensile moduli were as high as 10−18 MPa for Cu2+containing hydrogels, while only 0.15−0.16 MPa for those Cu+doped ones. When applying chemical stimuli, reversible switching between high and low moduli could be accomplished and used to enable a shape memory effect (Figure 13). With this approach, Yang and co-workers reported a redox-induced shape memory hydrogel with ferric-phosphate complex.99 The metal−ligand bond-based shape memory hydrogels can be triggered in athermal ways under relatively mild conditions. However, because the diffusion of the metal ions or other chemical agents is relatively slow, the time for the shape memory cycle may be long, up to a few days. This shape fixing or recovery process may be accelerated by properly improving the temperature or enhancing the trigger conditions, such as increasing the concentration of the metal ions.

7. SMPS BASED ON IONIC INTERACTIONS In a polymer system, the precursor polymers with ionic moieties can form strong ionic interactions with oppositely charged ions, and the binding strength of ionic interactions can be strong enough if multivalent ions are involved. Hydrogels 20287


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Figure 15. Schematic illustration of a triple shape memory programming and recovery cycle of the PAAm-TSMP hydrogel. Reproduced with permission from ref 112. Copyright 2016, Royal Society of Chemistry.

sulfonated poly(ether ether ketone) (M-SPEEK),106 ionic nanodomains due to ionic or dipolar interactions between sulfonated metal served as a permanent network, and two temporary networks were formed: one from the glassy state of the ionomer and the other from the crystalline NaOl. Consequently, the M-SPEEK/NaOl showed a triple shape memory effect. However, for the noncovalently cross-linked fatty acid/ionomer, significant creep of the temporary and/or the permanent network occurred over time. Further experiments found that this could be resolved by covalent crosslinking of the ionomer, which greatly limited the relaxation of the temporary and/or the permanent network.105 Such materials are aiming at aerospace or structural component applications in which higher modulus and switching temperatures of SMPs are required.106,107

responsiveness is mainly determined by the nature of the molecular switches. Our group recently reported a PAAm-based multiresponsive shape memory hydrogel.112 It contains pH-sensitive dansylaggregations and light-responsive azobenzene-CD inclusion complexes as two noninterfering supramolecular switches. Therefore, triple shape memory performances involving pH and light stimulations could be realized with two kinds of supramolecular interactions serving as molecular switches and the chemically bonded network as the netpoints (Figure 15). Furthermore, given the thermal sensitivity of both switches, excellent thermally induced shape memory function was also observed. SMPs with two or more kinds of supramolecular interactions in their structure may provide more complexity and features to the materials. However, there are few relevant studies. The reason for this might be that the incorporation of different kinds of supramolecular interactions requires complex synthesis procedures or that it is difficult to single out the function of individual supramolecular interactions in the complicated system.

8. SMPS BASED ON DUAL SUPRAMOLECULAR INTERACTIONS Most of the existing SMPs-SI rely on one type of supramolecular interaction to switch the shape memory function. Recently, a few studies have tried to combine different supramolecular interactions into single materials. Liu and co-workers reported a high strength shape memory hydrogel based on dipole−dipole and H-bonding interactions.108 On the one hand, the synergistic effect of two kinds of supramolecular interactions greatly reinforced the hydrogel. On the other hand, both interactions are thermosensitive and could act as reversible switches to tune mechanical properties as well as enable a triple shape memory effect. Dual supramoleculuar interactions could serve as molecular switches and permanent cross-links. For example, Chen and coworkers reported SMPs with multishape memory effects based on ionic interactions and H-bonding.109,110 The netpoints are the strong ionic interactions among zwitterionic segments, and the reversible switch is based on the H-bonding between acrylic acid segments. Yang and co-workers developed a dualresponsive shape memory hydrogel composed of acrylamide (AM), acrylic acid (AA), and cationic surfmer.111 Two different reversible cross-links are included in the hydrogel: the ion/ complex bindings between ferric ions (Fe3+) and carboxyl groups of AA segments, and the salt-dependent hydrophobic association from the alkyl chain of the surfmer. The stimuli-

9. APPLICATIONS OF SMPS-SI The exciting properties of SMPs have endowed the materials with desirable utility from industrial and biomedical to aerospace applications.8,10 To date, most of applications are based on classical thermally responsive SMPs. However, along with the development of SMPs-SI, various trigger methods are also being developed, suggesting that more applications not triggered by heat may be realized. Liu and co-workers reported a series of metal ion-triggered SMPs for medical applications. It was demonstrated that employing the complexation and decomplexation of imidazole groups with a low concentration of zinc ions can enable poly(1vinylimidazole-co-acrylonitrile) hydrogels with shape memory properties.90 Zinc ion is a physiologically important trace element and endows the hydrogel with antibacterial and antiinflammatory functions. Furthermore, the hydrogel could encapsulate cells as a cell-carrying 3-D scaffold. It is known that the physical properties of the extracellular matrix could act as crucial factors in regulating the differentiation of stem cells. Interestingly, by utilizing a double-ion20288


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they recently reported a more complex system containing three kinds of switchable interactions,120 i.e., the coordination interactions between acrylic acid and Fe3+ ions, the pHsensitive PBA-diol ester bonds, and the temperature-dependent coil−helix transition of agar. As a result, the hydrogel could respond to Fe3+, pH, and temperature change and showed tunable mechanical properties as well as dual, triple, and even quadruple shape memory effects. Dynamic covalent bonds are generally stronger than most noncovalent bonds, allowing for the production of robust covalent networks with dynamic character. However, the dynamic nature of reversible covalent bonds generally cannot be accessed without a catalyst (or stimulus), whereas supramolecular interactions are relatively weak dynamic bonds that are generally under continuous equilibrium by nature.116,117 This feature of supramolecular interactions may help SMPs-SI to generate the autonomous shape recovery function of the materials without external triggers,113 which is hard to realized by the SMPs based on dynamic covalent bonds. Furthermore, unlike reversible covalent bonds that require specific appropriate conditions to trigger their breaking and reformation, the noncovalent bonds are generally highly susceptible to different kinds of external factors and more likely to result in multiresponsive SMPs.116,117

triggered shape memory effect, differentiation of human mesenchymal stem cells (hMSCs) was guided via the manipulation of cytoskeletal contractility.93 During the fixation of a temporary shape by a dipole−dipole interaction and calcium chelation, hMSCs were seeded on the hydrogel surface. After extracting calcium ions, the hydrogel-hMSCs automatically recovered a cubic box where hMSCs would exhibit heterogeneous differentiation behavior at different growth positions. Additionally, SMPs-SI could also be designed for special applications in closed or inert systems as reported by Hu and Zhou et al.113 They generated a hydrogel through the copolymerization of methacrylic acid (MAAc) and N,Ndimethylacrylamide (DMAA), which yields a loose chemical network integrated with a dense physically bonded network based on the H-bonds between MAAc and DMAA groups. The chemical network could act as a “spring”, and the physical network could regulate the energy release rate and shapeshifting pathway based on the reorganization of the reversible H-bonds without applying an external trigger. Therefore, this dual network hydrogel could realize trigger-free shapeshifting, which may provide applicatons in autonomous actuators, drugrelease systems, and active implants in the future. Despite great interest in the development of SMPs-SI, this area is still in its infancy. Currently, it is still a significant challenge to combine great mechanical properties, fast response rate, high shape fixity/recovery ratios, and multishape memory properties into a single SMP-SI. Therefore, only a few potential applications of the SMPs-SI have been proposed. However, in the future, the applications of SMPs-SI are expected to be diverse and encouraging. The nature of the supramolecular interaction can endow the materials with versatile functions to meet the increasing demand and, in particular, application contexts.

11. SUMMARY AND OUTLOOK We have highlighted a burgeoning trend in SMP design that exploits supramolecular interactions to endow shape memory function to the materials. The nature of supramolecular interactions not only always determines the triggering method of SMPs but also provides some other functions, such as selfhealing ability, to the materials. On the basis of the current research results, there are multiple potential directions of SMPSIs that can be developed in the future: (1) Multiresponsive and multishape SMPs-SI. SMPs are expected to be capable of providing more complex actuation for the application. Compared with conventional SMPs, SMPs-SI have shown more options in terms of triggering mechanism. A number of SMP stimuli types have been developed. Combining different supramolecular interactions into a single material would give the possibility of multiresponsiveness (responding to both thermal and nonthermal stimuli) to SMPs. Furthermore, the multishape memory effect would be in operation and controllable using different molecular switches. In addition, special new molecular switches responding special stimuli, such as a certain biomedical agent or voice, are also expected. (2) Multifunctional SMPs-SI. The dynamic reversibility of supramolecular interactions has not only been utilized to construct SMPs but has also been used to introduce self-healing property to the material. Therefore, supramolecular interactions are promising candidates for constructing materials that exhibit both shape memory and self-healing properties. Furthermore it should be noted that the shape memory function could bring the separated damaged area together automatically, thus assisting the self-healing process. To date, several studies have been reported. However SMPs require the coexistence of permanent and reversible cross-links, which may conflict with the requirement of self-healing. Thus, it is still a challenge to integrate self-healing and shape memory properties with good mechanical performance into a single material. In addition, supramolecular interactions can bond the polymer matrix and functional filler together. The resultant composites are expected

10. COMPARISON OF SMPS-SI AND SMPS BASED ON DYNAMIC COVALENT INTERACTIONS As the other broad category of reversible interactions, dynamic covalent bonds share some similar features with supramolecular interactions. The prominent common point is their reversibility and dynamic property, which is key for producing dynamic networks with distinct properties. On the one hand, dynamic covalent bonds (such as thermally reversible Diels−Alder reaction) could also be introduced into a classical polymer matrix as permanent netpoints, similar to those SMPs-SI with strong supramolecular interactions as permanent cross-links, to produce SMPs with self-healing or reprocessing properties or the capability of reshaping the permanent shape.10,114,115 On the other hand, certain reversible covalent bonds could act as reversible cross-linking points. For example, the use of reversible photodimerization of cinnamic acid derivates or [2 + 2] cycloaddition of coumarins has endowed many materials with a light-responsive shape memory effect.10,12 As reversible segments in polymer networks, both dynamic covalent and noncovalent interactions can play an important role in determining, to a large extent, the shape memory capability, the type of stimuli-responsiveness, as well as the response rate.116,117 In particular, Chen and Zhang et al. reported several triple shape memory hydrogels that have been achieved by combining both reversible covalent and noncovalent interactions into one ensemble, such as the chelation of alginate with Ca2+ and PBA-diol ester bonds118 or the chitosan-metal ions coordination bonds and Schiff base bonds.119 Interestingly, 20289


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to not only show a shape memory effect but also acquire special functions from the fillers. An ordinary blend of thermally induced SMP/fillers always uses the fillers to realize an indirect heating response. Using supramolecular interactions to introduce fillers could not only realize more nonthermal trigger methods, it could also combine the functions (such as photoelectric, electromagnetic, and catalytic performance) of fillers. For example, an electronic SMP component responding to water could be developed. (3) Applications of SMPs-SI. The development of new trigger methods and new functionalities is desirable for broadening the SMPs applications to new territories. It is hard to predict what unprecedented applications will be discovered in the future, but it should be noted that several challenges still remain. For example, compared with the conventional SMP, SMPs-SI always show relatively poor mechanical strength, as the strength of supramolecular interactions is weaker than that of chemical bonds. Therefore, how to further increase the mechanical properties and recovery stress of SMPs-SI to meet the requirements of applications is a big challenge for the design of SMPs-SI. In addition, how to simplify the fabrication of SMPs from a design aspect for realistic applications or to find new high value-add applications are also challenges that require attention. With many advances achieved, supramolecular interactions have become significant candidates for the SMP design. We anticipate that supramolecular interactions will continue to play an important role in the explosive development of SMPs, and the new achievement of supramolecular chemistry will push the design and application of SMPs to a new level.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bang-Jing Li: 0000-0002-0405-6883 Sheng Zhang: 0000-0003-0462-6724 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Project of State Key Laboratory of Polymer Materials Engnieering (Sichuan University), the National Natural Science Foundation of China (Grants 51573187 and 51373174), and the West Light Foundation of CAS. In addition, we are particularly grateful for the assistance of Musong Lin (Guangdong Power Grid Corporation, Electric Power Research Institute, Guangzhou 510080, China) during the process of accessing some related papers and materials.

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REFERENCES

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Shape Memory Polymers Based on Supramolecular Interactions

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