ChemComm This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online FEATURE ARTICLE View Journal | View Issue Propagating MOF flexibility at the macroscale: the case of MOF-based mechanical actuators Cite this: Chem. Commun., 2023, Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. 59, 1744 Javier Troyano *ab and Daniel Maspoch *cde Shapeshifting materials have captured the imagination of researchers for their myriad potential appli- cations, yet their practical development remains challenging. These materials operate by mechanical actuation: their structural responses to external stimuli generate mechanical work. Here, we review progress on the use of flexible metal–organic frameworks (MOFs) in composite actuators that shapeshift in a controlled fashion. We highlight the dynamic behaviour of flexible MOFs, which are unique among materials, even other porous ones, and introduce the concept of propagation, which involves the effi- Received 26th October 2022, cient transmission of flexible MOF deformations to the macroscale. Furthermore, we explain how Accepted 13th January 2023 researchers can observe, measure, and induce such effects in MOF composites. Next, we review pio- DOI: 10.1039/d2cc05813h neering first-generation MOF-composite actuators that shapeshift in response to changes in humidity, temperature, pressure, or to other stimuli. Finally, we allude to recent developments, identify remaining rsc.li/chemcomm R & D hurdles, and suggest future directions in this field. 1. Introduction a Inorganic Chemistry Department, Autonomous University of Madrid, 28049 Madrid, Spain. E-mail: javier.troyano@uam.es In the quest to create shapeshifting systems, researchers have b Institute for Advanced Research in Chemical Sciences (IAdChem), Autonomous explored diverse classes of materials for use in mechanical University of Madrid, 28049 Madrid, Spain actuators, devices whose structural response to external stimuli c Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and the is converted into mechanical work. More than 2 decades ago, Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Kitagawa and co-workers predicted a third generation of metal– Barcelona, Spain. E-mail: daniel.maspoch@icn2.cat d Departament de Quı́mica, Facultat de Ciències, Universitat Autònoma de organic frameworks (MOFs), defining them as porous crystal- Barcelona, 08193 Bellaterra, Spain line solids capable of reversibly undergoing structural changes e ICREA, Pg. Lluı́s Companys 23, 08010 Barcelona, Spain in response to external stimuli.2 Soon afterwards, they and Javier Troyano received his PhD in Daniel Maspoch is an ICREA Chemistry and Material Science at Research Professor at the Institut Autonomous University of Madrid Català de Nanociència i Nanotec- in 2015. Then he worked as a nologia (ICN2). He received his BS Postdoctoral Researcher in Prof. degree at the Universitat de Girona Daniel Maspoch’s group at and his PhD degree at the Catalan Institute of Nanoscience Universitat Autònoma de Barcelona and Nanotechnology (ICN2) in & Institut de Ciència de Materials de Barcelona (Spain). In 2019, he Barcelona. He worked as a post- joined Prof. Shuhei Furukawa’s doctoral fellow at Northwestern group in iCeMS at Kyoto University. His research interests University (Japan) as a JSPS include reticular materials Javier Troyano postdoctoral fellow. In 2022, he Daniel Maspoch (MOFs, COFs and MOPs) and was awarded a Marı́a Zambrano delivery systems. grant from the Spanish Government and joined the Prof. Félix Zamora’s group. His research mainly focuses on coordination and supramolecular chemistry at the macro and nanoscale for the design of novel functional materials. 1744 | Chem. Commun., 2023, 59, 1744–1756 This journal is © The Royal Society of Chemistry 2023 View Article Online Feature Article ChemComm other groups reported the first examples of MOFs exhibiting on the development of MOF-based mechanical actuators. reversible structural transformations upon adsorption of guest Firstly, we explain how researchers can observe and measure molecules.3,4 Since then, the list of flexible MOFs (also called meso- and macroscale deformations in flexible MOFs that soft porous crystals5) has steadily grown.6,7 Research on flexible occur in response to external stimuli. The examples provide This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. MOFs has led to the discovery of diverse response mechanisms the basis for understanding the potential of flexible MOFs for based on variations in temperature,3,8 light,9–11 or mechanical creating autonomous soft materials. Next, we present the most stress,12,13 and of fascinating phenomena such as negative relevant advances in the fabrication of the first flexible MOF- thermal expansion,14 negative gas-adsorption,15 negative linear based actuators, discussing the pioneering work that has been compressibility,16 and shape-memory effect.17 done on asymmetric flexible-MOF composite films. Finally, we The emergence of flexibility in MOFs can be explained by the allude to recent developments, identify remaining R & D Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. combination of soft coordination bonds, weak intermolecular hurdles, and suggest future directions in this field, including interactions, and the presence of internal voids, which provide the potential of porous materials analogous to MOFs. more freedom for structural dynamics. Likewise, the high versatility in composition, topology, pore functionalisation, and crystal size/shape of MOFs all favour the diversity of 2. Deformations of flexible MOFs possible structural transformations. Accordingly, much across different length-scales research is currently devoted to obtaining theoretical insight to understand the flexibility of MOFs and to develop tools for Among the main challenges for the practical application of flexible material design.18–21 MOFs in mechanical actuation is to adapt their mechanical The capacity of flexible MOFs to exhibit such a wide range of response to the required scale of work. In this sense, many of the dynamic behaviour differs markedly from the case of conven- potential applications of flexible MOFs can demand response at the tional porous materials, which are typically rigid. Consequently, macroscopic level, meaning that the MOF dynamics must be researchers have a unique vision of MOFs and their potential successfully transferred from the length-scale of individual crystals applications, as evidenced by reports published over the past (mesoscale) to the macroscale. This concept, named as propaga- decade.6,22 For instance, flexible MOFs are excellent candidates tion, is not merely a question of translating the mechanical for use in separation processes, exhibiting highly selective response to a larger scale but also involves establishing control adsorption properties.23 Another potential application of flexible over both the response time and the magnitude of the resultant MOFs lies in their ability to sense guest molecules.24 Likewise, deformation. their capacity to capture/release guest molecules in response to In this section, we outline how the events that lead to structural an external stimulus makes them ideal vehicles for controlled transformations at the molecular scale can be observed and drug delivery.25 quantified at the mesoscale, and used for operating at the Another way to harness the structural transformations of macroscale. flexible MOFs is to exploit the mechanical stress generated by adsorption/desorption of guest molecules. Namely, when a 2.1 Deformations of flexible MOF crystals: from the molecular flexible MOF experiences structural changes at the molecular to the mesoscopic scales scale, its crystal dimensions undergo mesoscale variations. The development of methods for the characterisation of MOF Thus, if the generated stress is efficiently transferred, then a structures have made enabled molecular studies of the structural macroscale mechanical response can be obtained. Briefly, transformations of flexible MOFs, whether on bulk solids or flexible MOFs can be employed to convert chemical energy into single crystals. However, only a few studies have addressed the a mechanical response. Thus, over the past few years, researchers effects of such transformations at the mesoscale, by analysing have created MOF-based mechanical actuators capable to change the morphological changes experienced by flexible MOF crystals. their shape in response to variations in environmental conditions. Pioneering studies in the 2000s first observed reversible macro- Mechanical actuators are materials that perform mechanical scopic morphological changes in single MOF crystals using work by means of differential swelling/shrinking throughout a optical microscopy upon adsorption/desorption of solvent guest heterogeneous non-homogenous structure. Thus, the mismatch molecules.28 However, it was not until the mid-2010s that these in the generated strain forces the actuator to adopt the most transformations began to be studied at the mesoscale. The first energetically-favourable shape. Importantly, to enable predictable direct evidence of such mesoscopic shape changes was reported shapeshifting, the swellable material must be properly distribu- by Coronas et al.29 With the help of an environmental scanning ted. In this sense, flexible MOFs are an ideal platform on which to electron microscope (ESEM), they were able to track the geo- develop shapeshifting materials. Unlike other kinds of materials metric changes of a 1-micron-sized single crystal of MIL-53(Al) used for creating soft actuators, such as polymeric films26 or upon desorption, caused by the electron beam and subsequent hydrogels,27 MOFs are crystalline, thus offering the possibility to temperature increase. Initially, MIL-53(Al) crystals were cova- finely tune their porosities and morphologies, and consequently, lently attached to a glass support to correctly measure the their behaviour. temporal evolution in their shape. This strategy enabled the Here, we aim to review how the flexibility of MOFs can be determination of the time-resolved extent of deformation of a transformed into useful work, introducing the recent progress single crystal (Fig. 1a). This work nicely illustrated how the This journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 1744–1756 | 1745 View Article Online ChemComm Feature Article This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. Fig. 1 (a) In situ FESEM images showing variations in the single-crystal dimensions of MIL-53(Al) over time. Scale bars = 500 nm. (b) In situ, humidity- controlled FESEM images of a single crystal of MIL-88A at different levels of relative humidity (RH). Top row: adsorption (from 20% RH up to 90% RH); Bottom row: desorption (from 90% RH back down to 20% RH). Scale bar = 5 mm. Adapted, with permission, from ref. 29 and 30. effects of molecular transformations of flexible MOFs could results to computer simulations, which enabled them to pre- be investigated at the mesoscale. However, it was limited to the cisely determine the structural changes induced by the water analysis of the kinetics of single-crystal shape changes upon adsorption/desorption. desorption. Taking this work as a reference, our group (the Maspoch group) later explored the use of a humidity-controlled 2.2. Transferring MOF flexibility to the macroscale: ESEM for studying the geometric changes in a 1-micron-sized propagation single crystal of MIL-88A (Fe) upon adsorption/desorption of water The concept of propagation in flexible MOF crystals can be in near-equilibrium conditions.30 As depicted in Fig. 1b, as the clearly illustrated by the work reported by Kaskel et al.1 In this relative humidity (RH) increased from 20% to 90%, corresponding research, the authors activated a defined quantity of flexible to the adsorption phase, the crystal length gradually decreased MIL-53(Al) ([Al(OH)(bdc)]n; bdc = 1,4-benzenedicarboxylate) (B18%) and its width increased (B33%), leading to an overall MOF powder, and then compacted it in a force-measuring increase of B45% in crystal volume at 90% relative humidity. bench equipped with both a force-transmission stamp and a These results matched with the previously described structural height-measuring device. Next, they exposed the MOF sample transformation of MIL-88A, whereby for each cell, the a-parameter to methanol or ethanol, which led to the expansion of the gradually increases while the c-parameter decreases. Likewise, crystals and consequently, to the expansion of the compressed when the relative humidity dropped back down to 20%, corres- powder, which acted as a spring that pushed the stamp ponding to the desorption phase, the crystal recovered its initial (Fig. 2a). Using this configuration, Kaskel and co-workers were dimensions, which confirmed the reversibility of the adsorption/ able to measure the expansion forces and, by extension, eval- desorption (i.e. swelling/shrinkage) process. This work not only uate the effects of the packing density on the mechanical provided a full picture of the morphologic changes undergone by response (Fig. 2b). They identified three different behaviours, a single crystal of a flexible MOF, but also enabled us to quantify according to the packing density. Thus, at low density, the voids the extent of deformation at the mesoscale. between the crystals allowed them to expand freely, and the Importantly, ESEM is not the only microscopy technique researchers only observed negligible forces. However, as the that researchers have used to analyse the morphologic trans- density increased, the voids decreased in size, thereby forcing formations of single particles of flexible MOFs. Gianneschi, the crystals to expand by pressing the stamp. In this domain, as Paesani et al.31 employed environmental transmission electron the packing density increased, so did the registered expansion microscopy (ETEM) to monitor, in situ, the flexible behaviour of force. Finally, at the highest densities, the powder reached MIL-53(Cr) individual crystals upon water sorption and upon maximum compression, at which point it exhibited back- temperature changes. They correlated their experimental pressure to the stamp, generating detectable expansion force 1746 | Chem. Commun., 2023, 59, 1744–1756 This journal is © The Royal Society of Chemistry 2023 View Article Online Feature Article ChemComm This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. Fig. 2 Schematic of the experimental set-up for estimating the force exerted by compressed MIL-53(Al) powder upon adsorption (left), and a plot of the observed expansion force profile vs. MOF packing density (right).1 even before any solvent was added. Thus, Kaskel and colleagues flexible MOF powders presents several limitations. In this not only demonstrated that variations in the crystal dimensions sense, the fabrication of composites of flexible-MOFs and of MOFs could be transferred (and measured) at larger scales, organic binders has proven fruitful for producing macroscopic but also evidenced that controlling the spatial distribution of objects with responsive behaviour. In this section, we cover MOF crystals is essential to attaining macroscopic responses. recent progress in this area. Despite the obvious potential of flexible MOFs in mechanical actuation, their use as powders demands careful consideration 3.1 Flexible MOF crystals as responsive fillers for various reasons. Firstly, and as described above, to achieve The limitations of using bare flexible MOF powders for a mechanical response at the macroscale, inter-crystal voids mechanical actuation described in the previous section have ideally should be removed to allow for the transmission of the motivated researchers to explore the incorporation of flexible individual crystal forces across the entire volume. This agglom- MOF crystals, as responsive fillers, within a continuous matrix. eration can be done through direct compression of the MOF This entails using a binder to eliminate the inter-crystal voids, to powder, resulting in a densely-packed bulk object. However, this in turn allow for the transmission of mechanical forces. Moreover, processing method will still be limited by the resistance of the the incorporation of the binder also confers the resultant compo- MOF to external mechanical stress, which can lead to structural site with mechanical strength and cohesion. Obviously, the binder collapse. Thus, this approach demands a compromise between material must exhibit several characteristics that ensure opera- the elimination of the inter-crystal voids and the loss of the tional functionality, which we outline below. initial MOF properties. Secondly, the use of pressed flexible MOF Firstly, the binder must be mechanically suitable to transfer pellets involves the loss of their active form upon expansion, as the mechanical stress caused by the deformation of the MOF. crystal expansion will create new voids, thus requiring that the Thus, it must be sufficiently flexible to be compressed by MOF agglomeration step be repeated to enable further use. Finally, crystals, which will lead to the propagation of the stress across and contrary to what is observed for a single MOF crystal, the entire system, while being sufficiently elastic under working whereby changes in unit-cell parameters involve deformations stress conditions, to enable a reversible response via expansion with preferential directions, the use of MOF powders, in which and contraction cycles. High compatibility between the MOF crystals are randomly distributed, implies the isotropic deforma- particles and the binder is critical to guarantee good contact, by tion of the formed object. Thus, one immediate consequence of reducing the presence of voids, as these would result in futile moving from the mesoscale to the macroscale is a loss of MOF deformation. The influence of the interaction between the directionally, which, as we discuss in the following section, binder and the MOF on its flexible behaviour has been evi- implies certain design considerations. denced by Zhao et al., who used the flexible [Cu(dhbc)2(bpy)]n (dhbc = 2,5-dihydroxybenzenedicarboxylate, bpy = 4,4 0 - bipyridine) MOF.32 Intriguingly, after they had embedded this 3. Flexible MOF composites as actuators MOF into a polymeric (Pebax) matrix and heat-activated the resultant ensemble at 120 1C, the MOF retained its open-pore In the previous section, we described how the structural configuration; contrariwise, when they heat-activated the pris- transformations of flexible MOFs can be converted into useful tine bulk powder at 70 1C, it adopted the closed-pore configu- work at the macroscale. However, the practical use of bare ration. Zhao and co-workers attributed this discrepancy to the This journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 1744–1756 | 1747 View Article Online ChemComm Feature Article strong interactions between the Pebax polymer groups (i.e. composite with better cohesion. Moreover, for conventional –NH, –CO, –OH) and the free –OH groups at the surface of applications of MOF-based MMMs, such as separation, optimal the MOF crystals. performance of the membranes requires an even distribution Secondly, since most of the reported structural transforma- of the crystals. However, as we mentioned before, such homo- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. tions of flexible MOFs are based on host–guest interactions, the geneous distribution of the crystals is expected to provide an binder material must allow for molecular transport throughout isotropic deformation, which is incompatible with mechanical the entire MOF volume to enable MOF–adsorbate interactions. actuation. Thus, conventional methods for fabricating MOF- Great progress has been made in the design and fabrication of based MMMs must be adapted to create composite membranes MOF-based, mixed-matrix membranes (MMMs), which now in which an inhomogeneous distribution of the MOF particles provide a foundation for developing efficient composite mem- is achieved. branes capable of actuation.33–36 For instance, methods for Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. improving the MOF–polymer interaction, such as MOF surface 3.2 Asymmetric flexible-MOF films as mechanical actuators functionalisation or pre-polymer coating, have been demon- The simplest way to create a mechanical actuator is probably to strated to reduce the interfacial voids, thereby conferring the construct a bilayer comprising two materials with different Fig. 3 Schematic of the folding mechanism in asymmetric, flexible-MOF, composite films (a). Schematic of the syntheses, and photographs illustrating the reversible bending, of asymmetric HKKUST-1@PIL (b), MIL-88A@PVDF (c), MIL-88A@TPU (d) (scale bars = 2 mm). Adapted, with permission, from ref. 30, 38 and 40. 1748 | Chem. Commun., 2023, 59, 1744–1756 This journal is © The Royal Society of Chemistry 2023 View Article Online Feature Article ChemComm expansion coefficients that are bound together. In response to high dispersibility of the crystals as well as optimal homogeneity external stimuli, such an array undergoes swelling mismatch of the resultant membrane. between the layers, which forces the entire structure to fold. Consequently, thinking in the opposite direction, using Although this approach is simple, it can entail certain draw- MOFs with a broad size distribution could provide the level of This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. backs, such as a lack of compatibility between the layers, or their heterogeneity needed for actuation. To illustrate this principle, potential separation due to the stress generated under working our group produced asymmetric composite films by combining conditions.37 These limitations have motivated researchers to pre-synthesised MIL-88A ([Fe3OH(X)2(fumarate)3]n; X = Cl, OH) develop single-layered systems, in which actuation is induced by MOF crystals having a broad size distribution (0.5 mm to 9.0 mm), asymmetric expansion across the thickness. Such asymmetry can with PVDF (polyvinylidene difluoride) as polymeric binder. This be achieved by various means, such as by generating different heterogeneity that we achieved resulted in differential sedimen- Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. levels of crosslinking, localizing distinct crystalline phases, or tation during solvent evaporation, causing the larger crystals to creating a compositional gradient via embedding of extrinsic accumulate at the bottom, which in turn generated a vertical fillers. Correspondingly, MMMs can be designed to present a gradient of MOF-crystal distribution across the thickness vertical flexible-MOF gradient, such that the greater swelling of (Fig. 3c). We chose MIL-88A owing to its well-known swelling the MOF-rich side will cause the membrane to fold (Fig. 3a). behaviour, which involves the gradual enlargement of the MOF This concept was first demonstrated by Yuan et al., who unit cell caused by the changes in pore-size during adsorption/ reported the fabrication of an asymmetric composite film desorption. Additionally, and as we mentioned in Section 2.1, constructed from HKUST-1 ([Cu3(btc)2]n; btc = 1,3,5-benze- the swelling behaviour of a single crystal of MIL-88A had netricarboxylate) and poly(ionic liquid) (PIL) polymer, via previously been studied by ESEM, and its structural transforma- coordination-driven pseudomorphic replacement.38 First, they tion was found to be guest-selective: when the MOF crystals were formed an asymmetric PIL film with an ionic cross-linking exposed to non-polar molecules, no response was observed. gradient, in which they employed H3btc/btc3 linker as anionic Moreover, the shape, size, and size dispersity of MIL-88A crystals component. Next, they replicated the asymmetric structure by can be precision-tuned by changing the synthetic conditions. immersing the film into a Cu(NO3)2 solution, which led to the Besides, PVDF exhibits several features, such as high chemical formation of HKUST-1 crystals. Thus, the asymmetric structure stability and excellent mechanical properties, which have been of the initial polymeric film was maintained, yielding the exploited to produce various MOF-based MMMs. asymmetric composite film, HKUST-1@PIL, which contained As we had expected from its asymmetric structure, our MIL- a higher concentration of HKUST-1 crystals at one side (Fig. 3b). 88A@PVDF film exhibited reversible self-folding upon immer- This asymmetry conferred the film with actuation capacity in sion in water and different organic solvents. To confirm that response to NH3 vapor, which the authors attributed to the the driving force responsible for such behaviour was indeed expansion of HKUST-1 crystal volume upon adsorption of NH3. the reversible expansion/contraction of MIL-88A crystals, we Additionally, Yuan and colleagues were able to recover the fabricated and subsequently tested bare polymeric film, and original shape of the membrane by heating the film; however, non-flexible MOF (MIL-100, [Fe3O(OH)(btc)2]n; and MOF-801, after a few adsorption/desorption cycles, they observed a [Zr6O4(OH)4(fumarate)6]n) composite films with asymmetrically decrease in the curvature, which they attributed to the high distributed MOF crystals. Importantly, none of them showed affinity of NH3 gas for HKUST-1. Their report clearly evidenced any response. Furthermore, we observed that the solvents that the capacity of flexible MOFs to form asymmetric structures did (or did not) induce folding of the films were the same ones capable of folding in response to chemical stimuli. However, that did (or did not) cause swelling of MIL-88A crystals, which the mechanical response of HKUST-1 crystals upon adsorption is corroborated that MOF crystals were indeed the component modest, and implies only very slight structural deformations.39 behind the actuation. Moreover, the in situ formation of the MOF inside the membrane Having ascertained the role of the MOF in the folding imposes more limitations on control over the crucial aspects of response, we next studied the relationship between the MOF the crystals, such as their size, shape, and defects, and demands and macroscopic phenomena that we had observed, by tracking that the conditions used for MOF synthesis be compatible with the changes in its water-vapour uptake, XRPD spectra, and the pre-formed asymmetric membrane. Reflecting on the above folding response, upon increasing (adsorption) and then limitations, our group conceived of a more-general strategy to decreasing (desorption) the levels of relative humidity (RH) produce asymmetric composite films, based on drop-casting, a (Fig. 4). These experiments revealed how the adsorption profile method that had already been widely used for preparing MOF- of the MOF corresponds to structural transformations of the based MMMs,35,36 due to its simplicity and versatility. Briefly, crystals, which simultaneously correspond to the macroscopic pre-formed MOF crystals are dispersed into an organic polymer deformation of the films. As the RH gradually increased (up to solution, the resultant dispersion is deposited into a substrate, 90%), so did the folding response. Interestingly, when the RH and the solvent is evaporated off to yield a composite film. decreased, the films did not begin to unfold until ca. 40% RH. Typically, such MOF-based MMMs are intended for separation This observed shape-memory effect can be explained by the applications, for which a homogenous MOF dispersion is crucial hysteresis on the respective water-sorption isotherms for bare for optimal performance. Therefore, control over both the size MIL-88A crystals and for MIL-88A@PVDF films. Thus, due to and the monodispersity of the MOF particles is critical to ensure the high affinity of the MOF network for water, desorption at This journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 1744–1756 | 1749 View Article Online ChemComm Feature Article This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. Fig. 4 (a) Photographs of an asymmetric MIL-88A@PVDF film after 15 min under different levels of relative humidity (RH): from 20% up to 90% (adsorption), and from 90% back down to 20% (desorption). (b) Plots of changes in the folding-angle of MIL-88A@PVDF films in function of RH, from 20% to 90% (adsorption; solid dots) and then decreasing back to 20% (desorption; outlined dots). (c) In situ XRPD of a MIL-88A@PVDF film undergoing adsorption and desorption of water-vapour. (d) Water-vapour adsorption isotherm for an asymmetric MIL-88A@PVDF film at 25 1C (solid dots: adsorption; outlined dots: desorption). Adapted, with permission, from ref. 30. room temperature is less favoured at medium-high RH levels, polar solvents. Interestingly, the MIL-88C@P(VDF-CTFE) films resulting in a lack of mechanical response (shrinkage). Briefly, exhibited the opposite behaviour: they did not respond to water, the mismatch between the adsorption and desorption branches but did bend when treated with toluene or dichloromethane. of the MOF isotherm (i.e. hysteresis) translates into an equivalent Yet another method for the preparation of MIL-88 MOF mismatch between adsorption and desorption folding-response composite films has been conceived of by Guo et al.,41 who (i.e. shape-memory). This work offered a simple, adaptable, and employed a different polymeric matrix to produce MIL-88A@ efficient strategy to construct actuators by exploiting the flexibility TPU (TPU = thermoplastic polyurethane) composite films. of MOFs. In principle, this method enables the incorporation of Firstly, they prepared MIL-88A crystals with a more narrow size any flexible MOF into a polymeric matrix, whereby the control distribution, by following a surfactant-assisted method.42 Then, over the size and dispersity of the MOF crystals is crucial for they studied the effects of increasing the MOF-loading, by accessing self-folding capabilities. Furthermore, the response of preparing composite films in which the MIL-88A crystals were asymmetric flexible-MOF composite films is determined by the homogeneously distributed. Despite this symmetric MOF MOF-crystal deformations, which in turn depend on the specific arrangement, the resulting films still folded in response to type of host–guest interactions. This scenario makes the design of water. Guo and co-workers attributed this behaviour to the actuators with specific features possible through the rational vertical-gradient distribution of the TPU phase, which was selection of flexible MOFs. located mainly at the bottom layer (Fig. 3d). Thus, in this The versatility of the concept described above has been further denser region, MIL-88A crystals could transfer the mechanical demonstrated by Zheng et al., who prepared an asymmetric, stress to the surrounding medium more effectively, thus caus- flexible-MOF composite film, similar to MIL-88A@PVDF, but ing the film to fold towards the upper part. This example of film instead based on the MOF MIL-88C ([Fe3OH(X)2(2,6-naphthalene actuation operates by a mechanism distinct from that of the dicarboxylate)3]n; X = Cl, OH).40 MIL-88C MOF is isoreticular to previously cited examples of MIL-88A@PVDF and MIL-88C@ MIL-88A, exhibiting larger, more-hydrophobic pores, due to the P(VDF-CTFE). Nonetheless, in all three cases, the guiding prin- naphthalene-based linker. Thus, the resulting responsive asym- ciple remains the strategic use of vertical gradients, such that metric MIL-88C@P(VDF-CTFE) (PCTFE = polychlorotrifluoroethy- swelling of the flexible MOF triggers a mechanical response. lene) film not only functioned as an actuator, but also exhibited The examples that we have described in this section not only the same guest-response selectivity as bare MIL-88C crystals do. evidence that MOF-crystal deformations can be exploited for Therefore, whilst MIL-88A@PVDF films exhibited actuation when mechanical actuation, but they also illustrate how the beha- exposed to water or polar organic solvents, they did respond to less- viour of the resultant responsive films could be controlled. 1750 | Chem. Commun., 2023, 59, 1744–1756 This journal is © The Royal Society of Chemistry 2023 View Article Online Feature Article ChemComm Thus, the nature of the flexible MOF structural transformation, The first example of a programmable, 3D, self-shaping, the size and dispersion of the crystals, and the polymer used as a MOF-based film was developed by our group.53 Our strategy binder will each influence the response. However, an important involves the controlled chemical etching of non-responsive limitation of the first-generation films described here is that they homogenous MIL-88A@PVDF films, which enables the creation This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. only exhibit vertical gradients; as such, their actuation capacities of MOF-containing and MOF-free domains by patterning (Fig. 5). are limited to uniaxial deformations, thus narrowing their We initially employed the surfactant-assisted method42 to pro- potential applications. Accordingly, the next advance for research- duce smaller, more monodisperse MIL-88A crystals than those ers was to engineer new MOF-based composite films capable of obtained with other methods, and then combined these crystals performing more-complex 2D-to-3D shape transformations. with PVDF to generate homogenous MIL-88A@PVDF composite films. As we had expected, these films did not function as 3.3 Controlling the spatial distribution of flexible MOF Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. actuators, due to their lack of directionality. Next, following crystals: patterning our controlled-etching strategy, we exposed one side of the film Smart films that could undergo predictable, reversible, 2D-to- to HCl gas at room temperature for a period of time (Fig. 5a). 3D shape-transformations would have countless applications These experiments yielded responsive asymmetric composite in soft robotics,43–46 (bio)sensing,47–49 biomedicine,50,51 and films in which the MIL-88A crystals located at the exposed side engineering,52 among other fields. However, as we previously were removed, while the rest remained untouched. As the mentioned, the construction of flexible-MOF composite exposure time increases, so does the volume of HCl that diffuses films that contain only vertical gradients limits access to more- into the composite film, and consequently, the amount of MOF complex 3D structures. To achieve such 3D self-shaping, the that is etched; thus, the degree of etching can be controlled by actuator must present multi-axial deformation, which means selective use of exposure time. Remarkably, the different ratios that MOF composite films must exhibit both vertical and lateral between the MOF and the non-MOF domains leads to distinct heterogeneity. Therefore, programmable 2D-to-3D shape trans- types of bending. Thus, as the vertical etching increased, so did formations would demand having control over the spatial dis- the self-folding capacity, until eventually reaching a performance tribution of the responsive MOF filler over the entire film. To maximum; in fact, any further increases in etching actually led to date, this challenge has been addressed by following multi-step a reduction in folding response. We rationalised this observation methods, due to the difficulty in precisely localising MOF by considering the balance between two antagonistic effects of particles within the polymeric matrix during its formation. the etching: on one hand, the creation of an asymmetric Fig. 5 (a) Schematic representation of the patterning of MIL-88A@PVDF films by chemical etching with HCl. (b) Schematic of four-pointed star patterned MIL-88A@PVDF structure (left) and photographs of its 2D to 3D shape transformation upon exposure at 90% RH (right). Scale bars = 1 cm. (c) Schematic of seven-petal flower patterned MIL-88A@PVDF structure (left) and photographs of its 2D to 3D shape transformation upon exposure at 90% RH (right). Scale bars = 5 mm. (d) Schematic of open cube patterned MIL-88A@PVDF structure (left) and photographs of its 2D to 3D shape transformation, lifting cargo that is five-times heavier upon exposure at 90% (right). Scale bars = 5 mm. (e) Self-folding claw application of patterned MIL- 88A@PVDF seven-petal flower structure, gripping a modelling-clay disc immersed in warm water. Adapted, with permission, from ref. 53. This journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 1744–1756 | 1751 View Article Online ChemComm Feature Article structure, which favours actuation; and on the other hand, the by fabricating an open-cube structure capable of lifting cargo removal of active fillers, which disfavours actuation. 20-times its own weight (Fig. 5d). Another mechanical function Having demonstrated the efficiency of this method for that we explored with flexible-MOF composite films was uni- generating controlled vertical gradients, our next step was to directional walking. Thus, we prepared a worm-like crawling This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. create lateral gradients. To this end, we developed a mask- actuator comprising an active domain (etched; 60% of the total patterning method by attaching an adhesive Kapton tape mask length) and a passive domain (non-etched; 40% of the total to the film surface, and then further treating the film with HCl. length) with a fully etched region. We exposed the film to This procedure enabled us to create active domains (i.e. the alternating UV/Vis irradiation cycles under constant relative exposed areas), in which the removal of MOF crystals induced humidity, which led to differential responses across the film folding by generating hinges, and passive domains (i.e. the length, thereby resulting in unidirectional motion. Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. unexposed areas), which did not exhibit any folding response. Researchers have explored other, non-etching based patterning The demonstration of these predictable 2D-to-3D shapeshifts techniques to confer MOF composite films with mechanical opened a whole range of possibilities for fabricating complex functions. For instance, Guo et al.41 devised template patterning 3D structures with well-defined functionalities inspired by and laser-ablation patterning. In the former, they casted a origami and kirigami designs. Thus, by following the same solution of MOF (MIL-88C) crystals and P(VDF-CTFE) polymer patterning strategy, we were able to design and fabricate a set of onto a microchannel-patterned PDMS (polydimethylsiloxane) various programmable self-shaping films that executed differ- template, and then dried it to obtain a film with microchannels. ent mechanical tasks such as lifting, grabbing, and walking. In the latter method, they applied femtosecond laser radiation to First, we demonstrated controllable 2D-to-3D self-shaping a pre-formed MIL-88C@P(VDF-CTFE) film, creating micro- for MIL-88A@PVDF by fabricating a patterned film capable of structured patterns with precise control over the pattern width reversibly assembling into a 3D four-pointed star (Fig. 5b). To and depth. Next, Guo and co-workers demonstrated that each of this end, we patterned two plus-signs, oriented at 451 to each their two methods could be used to pattern films that could other, at both sides of a square film (i.e. one plus-sign per side), undergo programmable and reversible 2D-to-3D shapeshifting. which enabled upwards and downwards folding. Next, we Thus, similarly to our work with self-shaping MIL-88A@PVDF prepared a seven-petal flower, based on active (etched) petals films, they fabricated a six-petal gripping-flower and a unidirec- that fold towards a passive (non-etched) centre, in what we tional walking device, both of which actuated in response to called reverse-blooming. We then extended this process to the dichloromethane. mechanical task of grabbing: by exposing the seven-petal flower to water, it worked a mechanical claw, gripping a modelling- 3.4 Alternative designs clay disc 15-times heavier than itself (Fig. 5e). We further Two-dimensional films are not the only functional composites demonstrated the working capacity of MIL-88A@PVDF films that have been generated by exploiting MOF flexibility. Fig. 6 (a) Schematic of the fabrication of the MIL-88A@PVDF/PDMS fibres (left). SEM image of a MIL-88A@PVDF/PDMS fibre, revealing the embedding of MIL-88A within the PVDF polymer matrix (right). Scale bar (1 mm). (b) Photographs of the deformation of MIL-88A@PVDF/PDMS fibre upon exposure to different levels of relative humidity (RH): from 20% up to 90% (adsorption), and from 90% back down to 20% (desorption). Adapted, with permission, from ref. 54. 1752 | Chem. Commun., 2023, 59, 1744–1756 This journal is © The Royal Society of Chemistry 2023 View Article Online Feature Article ChemComm For example, Wan et al. fabricated a humidity-sensitive fibrous stretched, due to the high affinity of MIL-88A crystals for these composite, MIL-88A@PVDF/PDMS, in which they supported a solvents. However, when they soaked the fibres in non-polar MIL-88A@PVDF layer onto a core PDMS fibre (Fig. 6a).54 To solvents (e.g. toluene and trichloromethane), which cannot prepare the composite fibres, they first produced PDMS fibres induce shapeshifting of MIL-88A, the fibres underwent irregu- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. by wet-spinning. Next, they evenly drop-coated a DMF solution lar changes in shape. The authors attributed this phenomenon of MIL-88A crystals, which were very heterogeneous in size, and to the swelling of the inner hydrophobic PDMS core, which PVDF onto the obtained PDMS fibre. After drying, the analysis exhibits greater affinity for those solvents. Finally, when soaked of the resulting composite fibre revealed that the MOF crystals in acetone, the fibres initially bent and subsequently stretched. had distributed broadly across the external PVDF layer. This dual response stemmed from the combined swelling of Although these MIL-88A@PVDF/PDMS composites could be initial (external) MIL-88A crystals and subsequent (internal) Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. considered to be analogous to the aforementioned MIL- PDMS layers. Wan and colleagues further corroborated the role 88A@PVDF films, their 1D fibrous structure, as well as their of PDMS on the observed responses of their MIL-88A@PVDF/ swellable polymeric core, together created a distinct actuation PDMS fibres, using tensile stress–tensile strain measurements. response. The clearest difference between the two materials is Interestingly, whereas the fibres soaked in acetone, toluene, or that deformation in the 1D fibrous actuators is less restricted trichloromethane suffered a marked deterioration in their than in 2D films, since fibres can undergo axial extension and mechanical properties, those soaked in ethanol or water exhib- twisting, radial expansion, bending, or some combination ited minimal or negligible damage, respectively. thereof.55 Otherwise, the responsiveness of MIL-88A@PVDF/ The aforementioned work by Wan and co-workers clearly PDMS upon water sorption is analogous to that of MIL- illustrated that the incorporation of an additional component 88A@PVDF films. Thus, when exposed to increasing humidity into flexible-MOF composites could confer them with added levels from 10% to 90% RH, the fibres gradually stretched functionalities and thus, broader perspectives for applications. according to the swelling properties of MIL-88A. Correspond- Accordingly, researchers have developed materials that, in ingly, when RH was reduced from 90% to 10% RH, the fibres addition to mechanical actuation, exhibit complementary shrank, due to the desorption of water by the MIL-88A crystals responsive behaviour. For example, Jia et al. reported a dual- and their consequent shrinkage (Fig. 6b). However, this shrink- response, MIL-88A/CB@PVDF (where CB = carbon black) ing process upon desorption did not allow the fibres to return composite-film actuator (Fig. 7a).56 Here, in addition to achiev- to their initial state. Thus, the responsive behaviour of MIL- ing a folding response through asymmetric distribution of MIL- 88A@PVDF/PDMS fibres is analogous to that observed for MIL- 88A crystals, the authors also generated a reverse opal structure 88A@PVDF films, in which a similar shape-memory effect was to provide the film with photonic properties. To this end, they described (see above). Finally, Wan et al. also investigated the drop-casted a solution of MIL-88A, CB and PVDF onto a SiO2 response of their composite fibres to immersion in different photonic crystal template, and then heated it to remove the organic solvents, registering three different solvent-dependent solvent. Next, they exposed the side containing the SiO2 tem- responses. As expected, when they soaked the fibres in polar plate to HF vapour to induce chemical etching. This process solvents (e.g. water, ethanol, or acetonitrile), the fibres simultaneously yielded two outcomes: (i) the formation of a Fig. 7 (a) Schematic of the fabrication of the MIL-88A/CB@PVDF, inverted-opal, dual-response film. (b) SEM image of a cross-section of a MIL-88A/ CB@PVDF reverse-opal film, showing the inverted-opal structure (top of film); the region where the MIL-88A was partially etched (middle of film); and the intact structure of MIL-88A/CB@PVDF (bottom of film). (c) Photographs of a MIL-88A/CB@PVDF eight-petal flower, which blooms in a methanol atmosphere. (d) Photographs of a dual-response, MIL-88A/CB@PVDF, worm-like crawling actuator in a methanol atmosphere. Adapted, with permission, from ref. 56. This journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 1744–1756 | 1753 View Article Online ChemComm Feature Article reverse opal structure, due to the removal of the SiO2 template, parameters to enable systematic analysis of multiple device and (ii) the asymmetric distribution of MIL-88A crystals across parameters (e.g. MOF type, linker type, MOF-loading, curvature, the film thickness. The morphological analysis of these films thickness, etc.), the assessment of long-term stability, as well revealed three consecutive layers: a reverse opal layer; an as performance reproducibility, will be invaluable for further This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. etched-MOF layer; and a non-etched-MOF layer. The etched advances. MIL-88A/CB@PVDF films exhibited excellent actuation perfor- We recognise that the functionalities of flexible-MOF actua- mance, analogous to that of the aforementioned MIL-88A@PVDF tors are not yet as sophisticated as those of other systems. films, owing to their similar asymmetric structures. Moreover, A major challenge for researchers is to achieve control over the the reverse-opal PVDF photonic structure conferred the resul- spatial location of responsive particles within a fabricated tant film with the ability to change colour under different device. Two main approaches to create asymmetric porous Open Access Article. Published on 16 January 2023. Downloaded on 3/7/2023 2:38:17 PM. viewing angles. Wan and co-workers then demonstrated that materials at the macroscale are of interest here: single-step these two properties could be combined to develop a dual- fabrication, in which all the steps occur simultaneously; and response methanol-vapour sensor. The principle of this device stepwise fabrication, in which the device is built sequentially.57 lies in the dependence of the film curvature on the methanol Likewise, reported strategies for positioning of MOF particles,58 concentration, as well as the change in the film colour due to such as pseudomorphic replication,59 electrodeposition,60 and the change in the curvature. Thus, as the methanol concen- magnetic alignment,61 should be revisited. Here, the enormous tration increases, the film bends further and consequently, the knowledge gained from decades of MOF research offers a colour observed by the naked eye shifts further red to enable solid foundation on which to gain greater control and access detection of the vapour. a wider design catalogue. In this context, we strongly believe that interdisciplinary, especially among experts in Materials Science, Organic & Inorganic Chemistry, and Physical & 4. Conclusions and outlook Chemical Simulations, will be cardinal to success. Although this line of research is in its infancy, several recent The conversion of the swelling or shrinking of a chemically publications point to future directions.62,63 Various groups have responsive material into mechanical work makes it possible to demonstrated that the highly porous and ordered structures of exploit environmental changes for transmitting and/or harvesting MOFs are suitable to develop systems whose response to external energy. Researchers have broadly investigated this concept in the stimuli yields mechanical motion. Such autonomous MOF-based field of soft robotics, in which flexible materials are designed to motors rely on different mechanisms. For example, Matsui et al. execute controlled autonomous motion in response to external reported a device in which the partial decomposition of a MOF, stimuli. Flexible MOFs offer an alternative platform compared to and subsequent release of loaded peptide molecules, triggered purely organic or inorganic materials, not only due to their autonomous swimming motion, via the Marangoni effect.64 outstanding porosity, but also because of the synthetic control Similarly, our group has engineered different types of self- now possible for their composition, pore structure, chemical propelled MOF particles: metallic Janus MOF particles with a functionalisation, and crystal morphology and size. In addition, catalytic Pt coating;65 and hierarchical micro/mesoporous struc- their crystalline nature makes it possible to track the structural tures loaded with an active enzyme.66 An important considera- changes with high precision by using X-ray diffraction techniques. tion is that MOFs can exhibit diverse physical/chemical Flexible MOFs have recently made their foray into soft robotics, properties that could be exploited to achieve work. For instance, demonstrating their potential use in smart devices and systems Wang, Xu et al. recently prepared near-infrared (NIR) light- capable of executing autonomous tasks. The pioneering work in driven, hydrogel actuators, by using MOF particles as photo- this area indicates that the responsive behaviour arising from thermal nanotransducers.67 Likewise, Wang et al. reported that MOF–guest interactions can be transferred across multiple length flexible MOFs could be combined with other materials to gen- scales and that the macroscopic response can be predicted by erate 1D photonic crystals, in which MOF deformations provoke understanding structure–property relationships. This latter aspect an optical response.68 is quite advantageous, since MOF crystalline porous frameworks Overall, we believe that the applications of such MOF-based can be designed to present myriad characteristics by reticular devices can range from highly sensitive/selective chemical chemistry and tuned by post-synthetic modification. For instance, detectors and controlled delivery systems, thanks to their high asymmetric composite films of isoreticular MIL-88A or MIL-88C microporosity with precise pore design, to soft grippers, artifi- respond differently to polar and non-polar guest molecules, due to cial muscles, or flexible triboelectric generators. In addition, their distinct respective linkers. the field of porous-material actuators could be further The field of MOF-based actuators remains embryonic: advanced by encompassing other reticular porous materials, indeed, the number of responsive devices based on flexible such as covalent-organic frameworks (COFs) and molecular MOFs is still limited. The devices that we have cited in this metal organic polyhedra (MOPs), which could be regarded as review had been constructed as asymmetric films or, to a lesser the purely organic or purely molecular analogues of MOFs, extent, 1D composite fibres or 3D bulk-powders. Most of these respectively. In fact, recent examples illustrate that such materials first-generation devices were intended merely for proof-of- can indeed be used for creating mechanically responsive systems. principle or as prototypes. Thus, the establishment of comparative For instance, Yan et al. hybridised MOPs with polymers, thereby 1754 | Chem. Commun., 2023, 59, 1744–1756 This journal is © The Royal Society of Chemistry 2023 View Article Online Feature Article ChemComm creating a gradual change in MOP crosslinking across the film S. Isoda, W. Kosaka, O. Sakata and S. Kitagawa, Science, 2013, 339, thickness, to create soft actuators.69 As for COFs, Zhang et al. has 193–196. 18 L. Sarkisov, R. L. Martin, M. Haranczyk and B. Smit, J. Am. Chem. introduced gas-responsive groups into the COF structure to obtain Soc., 2014, 136, 2228–2231. a new series of responsive materials.70 Interestingly, by incorpor- 19 J. D. Evans, V. Bon, I. Senkovska, H.-C. Lee and S. Kaskel, Nat. 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