Invited Feature Article pubs.acs.org/Langmuir
Polymeric Actuators Leonid Ionov* Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden, Germany ABSTRACT: Actuators are materials and devices that are able to change their shape in response to changes in environmental conditions and perform mechanical work on nano-, micro-, and macroscales. Among the huge variety of different actuators, polymer-based ones are highly attractive because of a number of properties such as sensitivity to a broad range of stimuli and good mechanical properties. The goal of this review is to provide a general picture of different mechanisms and working principles of polymeric actuators as well as to show a palette of their applications.
1. INTRODUCTION Actuators are materials and devices that are able to change their shape in response to changes in environmental conditions and thus perform mechanical work on the nano-, micro-, and macroscales.1−3 Actuators find very broad application in microfabrication, microelectronics, medicine, and lab-on-a-chip systems.4 Because of their complexity, the development of actuators includes both material and engineering aspects. This review focuses solely on the material aspect of the development of actuators and does not touch engineering (design of devices). In other words, this review discusses materials that are intrinsically able to move. There are plenty of examples of actuators based on metals, metal oxides, and organic materials such as shape memory metals and polymers, bimetal strings, and hydrogels.1,5 Among a huge variety of different actuator materials, polymer-based ones are very attractive for their spectrum of different polymer properties.2,6−9 The polymers can be soft (viscoelastic state) and hard (glassy state) depending on their chemical and physical structure that allows the design of soft actuators for handling soft living tissues and hard actuators for handling metals.10 There are many polymers sensitive to different stimuli that allows the design of actuators that can be controlled by temperature, pH, biosignals, or light.11 Many polymers are biocompatible and biodegradable, which allows the integration of polymeric actuators in living systems and their resorption there. The goal of this review is to provide an overview of different mechanisms and working principles of polymeric actuators. The review is organized as follows. First, the stimuli-responsive properties of polymers are discussed. Second, polymeric actuators are classified according to different mechanisms of their actuation. Finally, examples of the application of polymeric actuators are given.
Figure 1. Four main classes of polymeric actuators based on different actuation mechanisms: (a) relaxation after deformation (shape memory polymers, reproduced with permission from ref 16, copyright WileyVCH Verlag GmbH & Co. KGaA); (b) change in order (liquidcrystalline polymers, reproduced with permission from ref 17, copyright Wiley-VCH Verlag GmbH & Co. KGaA); (c) volume change (hydrogels, reproduced from ref 18, copyright (2010) Royal Society of Chemistry); and (d) surface-tension-driven actuators (reproduced with permission from ref 15 with kind permission from Springer Science + Business Media).
are, for example, dielectric elastomeric actuators that are elastomers between two electrodes.12 Applying voltage leads to an electrostatic attraction between the electrodes and to
2. CLASSIFICATIONS OF POLYMERIC ACTUATORS The polymeric actuators can be artificially divided into several groups. The first group of actuators comprises those based on the elastic relaxation of shape after deformation (Figure 1a). These © XXXX American Chemical Society
Received: August 28, 2014 Revised: October 10, 2014
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transition temperature (Figure 2b to Figure 2c). Because of the reduced mobility of polymer chains, the shape of the object cannot be changed even if the proper stimulus is applied (where the direct transition of Figure 2c to Figure 2a is not possible). There are two ways how a frozen shape (Figure 2c) can be converted to the original shape (Figure 2a). First, the polymer chains can be unfrozen (Figure 2c to Figure 2b) and then the stimuli can be applied to change the shape (Figure 2b to Figure 2a). The second possibility is to apply the stimuli (Figure 2c to Figure 2d) to generate the stress. The stress cannot be converted to deformation because of the reduced mobility of polymer chains. The chains can then be unfrozen either by increased temperature or by un-cross-linking that leads to a relaxation of the stress and deformation (Figure 2d to Figure 2a). An example of two-step switching could be either a pH-responsive hydrogel with a variable cross-linking density or a hydrogel on a substrate. In the first case, the hydrogel with a very high cross-linking density cannot change its volume when the pH is changed, and a decrease in the cross-linking density unfreezes the responsive properties. In the second case, the swelling of the hydrogel located on a substrate is restricted in the x and y directions. Removal of the hydrogel from the substrate unfreezes the swelling in these directions. 2.1. Shape Memory Polymers (SMPs). There are numerous SMP-based systems described in the literature, varying both in their chemical composition and the stimulus to which they are sensitive.19 Shape memory polymers are either chemically or physically cross-linked networks that consist of one or two polymers (Figure 3). Physical cross-linking may be
compression of the elastomer. The shape of the elastomer is recovered if the voltage is removed. In fact, the electric field here is simply the method of deformation of elastomer that also can be done by hand. Elastic shape recovery also occurs if the materials have an inhomogeneous cross-linking density.13 Shape memory polymers are another very important example of actuators based on relaxation. These polymers are deformed and temporarily “frozen” by cooling to low temperature. They become flexible and move at elevated temperature. The classical shape memory polymers are irreversible, but there are few examples of reversible actuation of shape memory polymers. The second class of actuators is liquid-crystalline (LC) actuators, which are based on the change of orientation of mesogen groups (Figure 1b).14 This can be either a transition between different ordered states or a transition from an ordered state to an unordered one. LC actuators are typically reversible. The actuators of the third group are based on a reversible change in the volume (Figure 1c). These actuators can be the hydrogels, which swell reversibly in water.2 Similar to metals, the temperature expansion of polymers can be used for actuation. The temperature expansion coefficient is typically very small and does not allow large deformation. However, the change in volume during the first-order transition phase (melting/ crystallizaton) is larger and allows for larger deformations. All of these actuators are also reversible. The fourth group is actuators where the driving force is surface tension (Figure 1d). These actuators are based on crystalline polymers with a moderate melting point.15 Typically, such actuators are small because they are able to function when the surface tension is considerable. Surface-tension actuators are irreversible. In fact, an actuation can occur in either one or two steps. In a one-step procedure, the stress is immediately converted in deformation (Figure 2a to Figure 2b). Examples of this actuation
Figure 3. Working principle of shape memory polymers (reproduced with permission from the National Academy of Sciences, ref 20).
provided by the local crystallization of one of the polymers. Chemical or physical cross-linking determines the permanent shape. Heating the polymer brings it to the flexible state where it can be easily deformed. The deformed shape can be frozen by cooling to below the melting point or glass-transition point when the mobility of polymer chains or polymer segments is restricted. Subsequent heating leads to unfreezing of the polymer chains and the restoration of the initial shape. Common elastic rubber is an example of the shape of a memory polymer. A piece of rubber can be stretched at room temperature and cooled in liquid nitrogen. The piece will keep its deformed shape until the temperature is kept low. Heating to room temperature increases the mobility of polymer chains, and that leads to a restoration of the initial shape. Common conventional SMP systems include cross-linked polyethylene and polyethylene/nylon-6 graft copolymers, styrene-based polymers, acrylate-based polymers, polynorbornene, epoxy-based polymers, and thioene-based polymers.21 SMPs have been reported to be thermally induced, light-induced,
Figure 2. Shape-state diagram of polymer actuators. (a) The actuator has its equilibrium shape and no stimulius is applied. (b) Application of the stimulus leads to a change in the shape. (c) The shape can be temporary fixed either by cooling or by cross-linking. (d) Applying stimuli to the frozen actuator induces chemical changes but does not lead to movement; the actuator is internally stressed. Unfreezing (d) leads to a restoration of the initial shape (a).
are the swelling/shrinking of hydrogels caused by a change in pH or temperature and a bending of liquid-crystalline polymers caused by the photoisomerization of chemical groups. One-step actuation can be converted to two-step actuation if the system can be frozen either by physical or chemical crosslinking or by cooling to below the melting point or glassB
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electroactive, water/moisture/solvent-induced, pH-sensitive, and magnetically sensitive on the basis of their triggering stimulus.22 A very interesting application of shape memory polymers was demonstrated by Dickey et al.23 They used commercially available prestressed thin polystyrene films, known as Shrinky-Dinks, to create various 3D objects by temperatureinduced self-folding. Here, black hinges are printed on the surface of the polystyrene sheets with a desktop printer. The desired 2D shape was then manually cut off and illuminated with visible light. The polymer directly underneath the ink heated rapidly to exceed the glass-transition temperature. As a result, the hinged regions relaxed and bent the sheet. The direction of folding (toward the light source or away from it) could be controlled by patterning either the top or the bottom side of the sheet, respectively.23 Classical shape memory polymers have two shapes: permanent, which is defined by cross-linking, and temporary, which is defined by deformation. As a structural concept for triple-shaped polymers, polymer networks that are able to form at least two segregated domains were also used.24 Although the original shape is defined by net points resulting from the crosslinking reaction, temporary shapes are created by a two-step thermomechanical programming process. The first temporary shape is determined by physical cross-links associated with the highest transition temperature, and the second temporary shape relates to the second-highest transition temperature. In the beginning of the two-step programming process for creating shapes, the polymer network is heated to Th (Figure 3), at which the material is in an elastic state and is deformed. When the material is cooled to Ttrans (Figure 3) and external stress is maintained, physical cross-links due to either crystallization or transition in the glassy state are established. Releasing the external stress results in a first temporary shape. In the second step, a second temporary shape is created. The sample, which presently is in the first temporary shape, is further deformed at Ttrans (Figure 3). Cooling under external stress to Tl (Figure 3) leads to a second set of physical net points. Heating to Ttrans (Figure 3) leads first to the disappearance of the second set of physical net points and second to the formation of the first temporary shape. Heating to Thigh leads to the disappearance of the first set of physical net points and the formation of the original shape. There are many advantages of shape memory polymeric actuators that make them highly attractive: (i) they are able to act in dry and wet environments; (ii) the transition temperature can be tuned by the proper choice of polymers or composition; and (iii) there are biocompatible and biodegradable shape memory polymers that allow for their use in medicine. Unfortunately, the actuation of most shape memory polymers is irreversible. This problem was solved in recent works by Lendlein et al. (section 2.3.2). Reversible SMPs are based on a slightly different principle. 2.2. Liquid-Crystalline Actuators. Liquid-crystal polymer networks are another interesting class of stimuli-responsive actuators that are able to work in a solvent-free environment.25 Such materials are fabricated by using reactive liquid-crystalline molecules whose orientation can be manipulated using alignment layers and fixed by photopolymerization, resulting in hierarchical materials with a defined microstructure over macroscopic length scales. In a liquid-crystal polymer network, the material stretches or shrinks anisotropically along the director orientation as the order parameter changes, for instance, as a result of a change in temperature or chemical environment (Figure 4).
Figure 4. Scheme of the liquid-crystalline actuator. In the LC phase, the polymer backbones experience an anisotropic environment that leads to an extended chain conformation. At the phase transition to the isotropic phase, the polymer regains its coiled conformation, giving rise to a macroscopic shape change. (Reproduced with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA25).
There are many reported liquid-crystalline polymer networks that include samples with unidirectional alignment so they contract in the direction parallel to the alignment director upon application of a stimulus. Other reported films have a molecular director that varies in depth, that is, twisted nematic and splayed configurations; these samples bend26 or curl27 when a stimulus is applied.28 Recently, a more exotic deformation of liquid-crystal polymer films, with a circular director profile in the plane of the polymer film, was demonstrated. These samples showed deformation into cone and anticone shapes upon heating.17 There the director microstructure was uniform through the sample thickness and thus essentially two-dimensional. More complex reversible deformation can be achieved by imposing a 3D director profile on the film, with variation through the thickness as well as in the plane of the polymer film. Recently, shape memory materials have been reported with alternating twisted nematic and monodomain sections.29 Along this line, Broer et al. have prepared patterned liquid-crystal actuators with a 3D director variation.30 These films have a discrete alternating striped or checkerboard director profile in the plane and a 90° twist through the depth of the film. These samples show complex reversible deformation driven by their director profile. Upon heating, the striped samples undergo a reversible transition from a long, flat ribbon to a bent/folded state resembling an accordion fold and therefore achieve a large lateral displacement between the end points of the film. The actuators with the checkerboard pattern of twist domains buckle out of plane. It was also shown that an accordion-like deformation can be achieved in an aqueous environment by striped patterned polymer actuators that respond to pH changes. A very intelligent way of controlling the direction and degree of orientation of mesogenic groups is by their photoisomerization. It was shown that a single film of a liquid-crystal network containing an azobenzene chromophore can be repeatedly and precisely bent along any chosen direction by using linearly polarized light.14 This striking photomechanical effect results from a photoselective volume contraction and may be useful in the development of high-speed actuators for microscale or nanoscale applications, for example, in microrobots in medicine or optical microtweezers. In a similar way, polymer LC microparticles can undergo reversible changes. The advantages of liquid-crystalline actuators are their ability to act in dry and wet environment and their reversibility of actuation. The design of biodegradable liquid-crystalline actuators is, however, not a trivial task and limits their in vivo applications. C
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2.3. Actuators Based on Volume Changes. There are two kinds of actuators based on volume changes. The first kind is hydrogels, which are able to swell and shrink. These actuators are able to actuate either in an aqueous environment or in humid air. Volume-changing actuators of the second kind are based on the change in volume due to thermal expansion/shrinking or melting/crystallization. 2.3.1. Wet Hydrogel Actuators. Hydrogels are 3D polymer networks imbibed with aqueous solutions and are able to produce macroscopic actuation upon swelling and shrinking.31 The swelling degree of hydrogels is defined by the balance of free energy for the network expansion, which depends on the crosslinking density and the molar free energy of mixing, determined by the interactions between polymer chains and solvent, as well as the mixing entropy (Flory−Rehner Theory). Whereas the cross-linking density of a hydrogel remains constant, the energy and entropy components depend strongly on temperature, solvent quality, and pH. Thus, by changing the temperature and pH one can affect the swelling degree of hydrogels. A very complex deformation of thermoresponsive hydrogels can be achieved by using eletroheating or light, which generates high temperature locally.32,33 It was observed that homogeneous hydrogels typically undergo a homogeneous change in volume if there is no gradient in concentration or temperature.34 Applying a gradient of stimuli (temperature, pH, light, etc.) to homogeneous hydrogels can lead to their bending.35 Very recently, it was demonstrated that one can achieve a very complex deformation of homogeneous hydrogels using a local deposition of heavy metal ions in charged hydrogels (ionoprinting). This method is based on applying electrodes to intrinsically homogeneous hydrogels. One of the electrodes is made of copper, and an electrochemical reaction leads to the formation of copper ions, which diffuse in the hydrogel and change their swelling properties. As a result, hydrogels bend. Complex deformation (formation of hinges, bidirectional bending) is achieved using multiple electrodes.36 Bending can also be achieved by using inhomogeneous materials that consist of hydrogels with different swelling properties.37 Clearly, gradients in concentrations can be applied to inhomogeneous materials that must allow more complex deformation. The deformation of inhomogeneous hydrogels is a very interesting topic. It was found that inhomogeneous hydrogels can move in a variety of complex trajectories and are able to form complex 3D shapes. Even simple bilayers (Figure 5a), where one layer is a stimuli-responsive hydrogel, are not only able to form simple shapes such as tubes and scrolls38 (Figure 5b) but also can result in the formation of complex structures. For example, capsules are formed if the bending curvature is large (Figure 5c, left images). The folding of a similar bilayer with a smaller bending curvature is more complex and occurs in a multistep manner (Figure 5c, right images). In particular, homogeneous bilayer films can undergo sequential steps of folding by forming various 3D shapes with sharp hinges such as a pyramid.39 The step-by-step folding of different elements of self-folding films can also be achieved by the local activation of selected areas of selffolding films by light.40 Structural inhomogeneity plays a very important role and, along with the shape, defines the character of deformation. This mechanism of controlling the deformation character is implemented in nature. For example, pine cone scales and wheat awn are formed from two kinds of tissues. In one kind of tissue, cellulose fibers are oriented along a scale that allows
Figure 5. Different scenarios of bilayer deformation based on poly(Nisopropylacrylamide)-based hydrogel (a) depending on the presence of a substrate (b) and the shape of the film (c). Scale bars are 200 μm. (b) Reprinted from ref 38, copyright (2012) American Chemical Society. (c) Reprinted from ref 18, copyright (2011) Royal Society of Chemistry and ref 39, copyright (2013) Wiley-VCH Verlag GmbH & Co. KGaA). Scale bars are 200 μm.
bending. The orchid tree seedpod is also formed from a two-layer structure, and the cellulose fibers in each layer are oriented at some angle with respect to each other that allows twisting. These nature-inspired mechanisms of deformation control were realized from the example of thermoresponsive hydrogels with incorporated magnetic plates, which are oriented by the magnetic field in a specific direction during the preparation of the hydrogel. The magnetic plates fill the role of cellulose fibers in plants; they provide anisotropy of mechanical properties. Similar to natural examples, hydrogels are able to bend and twist depending on the orientation of the magnetic plates.41 In other reports, Hayward and Santangelo demonstrated that patterned polymer films consisting of patches with different degrees of swelling are able to fold in a very complex way and that the character of folding depends on the pattern. (Figure 6a).42 When swollen in an aqueous medium, a film consisting of stripes with one high- and one low-swelling hydrogel layer does not bend to the side of the less-swollen component as in the case of a “classical” bilayer but rolls into a 3D shape consisting of two nearly cylindrical regions connected by a transitional neck (Figure 6b). Films with a radial gradient of swelling properties, where the outer part swells more strongly than the central one, fold in response to a change in temperature and form complex structures (Figure 6c). Kumacheva et al. investigated the folding of rectangular films formed by stripes of two or three polymers with different swelling properties.43 Such films are unfolded when all polymers have the same swelling properties (either D
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Figure 6. Temperature-responsive swelling of bistrips of poly(Nisopropylacrylamide)-based hydrogel: (a) scheme, (b) folding of a simple shape, and (c) folding of a complex shape with a radial gradient of swelling properties. When the temperature of the aqueous medium is increased, the bistrip shrinks, unrolls, and finally recovers a flat shape by 50 °C as the temperature is decreased.44 The scale bar is 200 μm. (Reprinted from ref.,44 Copyright (2012) Royal Society of Chemistry and ref42 Copyright (2012) AAAS). Figure 7. Scheme of shape memory polymers with reversible actuation (a): After deformation at Treset, the skeleton domains (red), that determine the shape-shifting geometry are crystallized by cooling (programming). The shape memory polymers are triggered by the reversible crystallization and melting of oriented actuator domains (green). Black dots: cross-links. (b) Series of photograph showing shape memory polymers of a polymer ribbon (40 mm × 4 mm × 0.4 mm) from polypentadecalactone−polycaprolactone.75 The bowed shape was obtained after programming by deformation in a helixlike shape at Treset, cooling to Tlow, and subsequent heating to Thigh. The shape memory polymers occurred as a reversible shift between shape A (bow) at Thigh and shape B (helix) at Tlow. The sample was reprogrammed by Treset into an open shape (new shape A), which could be shifted reversibly to a folded shape (new shape B). Reference 45, copyright (2013) Wiley-VCH Verlag GmbH & Co. KGaA.
equally swollen or equally shrunk) and roll in different directions when the polymers swell unequally. The main advantage of hydrogel-based actuators is considerable volume changes. Hydrogel actuators with different shapes and trajectories of movement can easily be fabricated using photolithography and molding. Moreover, there are many biocompatible and biodegradable hydrogels, which opens broad perspectives for their application in medicine. In contrast to shape memory actuators, hydrogels require an aqueous environment or humid air to act. 2.3.2. Dry Actuators Based on Thermal Expansion and Melting. Recently, Lendlein reported very interesting reversible shape memory actuation (Figure 7). These actuators consist either of two cross-linked polymers with different melting points45 or one cross-linked polymer with a broad melting range.46 Similarly to all shape memory polymers, these polymers can be deformed at elevated temperature and formed into a temporary shape that is stable after cooling. Heating to a temperature above the melting point of the polymer with the highest melting point results in complete restoration of the original shape. Reversible actuation is observed when the sample is heated to above the melting point of the polymer with a lower melting point but below the melting point of the polymer with a higher melting point. This reversibility of actuation is caused by reversible volume changes upon melting/crystallization of one of the polymers. A similar effect was recently reported by Sheiko et al.47 A very interesting example of polymeric actuators based on thermal expansion and shrinking was demonstrated by Baughman et al. Although a change in the length of polymer fibers upon heating is very small (10−15% upon heating from room temperature to 150 °C), special waving of these fibers allows the design of the muscle that can contract by ca. 50%.48 This kind of actuator is a substantial step in the development of shape memory polymers and allows us to solve the problem of reversibility. Development methods for the miniaturization of such actuators would also be very desirable.
2.4. Surface-Tension-Driven Actuators. Typically, actuators employ a change in the internal properties of polymers in order to produce movement because the contribution of surface tension remains negligible on the macroscale. As soon as the size of the actuators is diminished to the microscale and nanoscale, the role of surface tension becomes larger, which allows the design of surface-tension actuators. The simplest case of surface tension actuators is nonspherical fusible particles, which become softer and adopt an equilibrium spherical shape upon melting or glass transition.49 One can also use particles made from very viscous polymers such as PLGA. Because of the high viscosity, the particles are unable to change their shape in a reasonable period of time. However, the viscosity can be changed by applying stimuli such as pH, which leads to quick relaxation of the shape to a spherical one.49 Gracias et al. used surface-tension-driven shape changes for the design of self-folding polymer containers (Figure 8). In the unfolded state, the self-folding containers are six squares made from an SU-8 photoresist that are connected to each other by patches of polycaprolcatone (PCL), which has a low melting point (Tm = 60 °C). PCL forms droplets upon melting, which moves the SU-8 parts and forces them to fold into a cube.15 E
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Figure 9. Imaging devices based on hydrogel actuators: (a) smart optical lenses with tunable focal lengths based on poly(N-isopropylacrylamide)-based hydrogel (reprinted from ref 53, copyright (2006) Nature Publishing Group); (b) artificial skin with a tunable topography device, which contains 4225 actuators within an area of 37.7 mm × 37.7 mm (ref 54, copyright (2009) Wiley-VCH Verlag GmbH & Co. KGaA).
Figure 8. Fabrication process flow and self-folding mechanism for polymeric containers. (a) (i) A sacrificial layer was spin coated on a clean Si wafer. SU-8 panels were patterned using conventional photolithography. (ii) PCL was deposited in hinge gaps. (iii) 2D templates were lifted off via dissolution of the PVA layer in water, and self-assembly occurred on heating to above 58 °C. (b) (i−iii) Schematic demonstrating the self-folding of a cubic container. External “locking” hinges are colored in pairs to denote corresponding meeting edges. (c) Video capture sequence (over 15 s) showing a 1-mm-sized, sixwindowed polymeric container self-folding at 60 °C (with kind permission from Springer Science + Business Media permission, copyright from ref 15).
change in the volume enclosed by the ring and the change in water droplet volume, causes a change in the pressure difference across the water−oil interface that directly determines the geometry of the liquid meniscus. Swelling/shrinking of hydrogels can also be used to change the topography that can be felt by touching. For examples, Richer et al. designed palpable displays based on an array of thermoresponsive hydrogels.54 They integrated more than 4000 temperature-sensitive controlled actuators, each of them being heated by an individual electroheating element. The designed hydrogel array was able to form a visual and palpable artificial imaging system (Figure 9b). 3.3. Control of Liquid Flow. Controlling liquid flow in microfluidic devices is another potential application for actuators.55−59 The simplest possibility is to use large hydrogel pieces that act as a smart valve (Figure 10). These valves allow the
The main disadvantage of this approach is the irreversibility of actuation. Moreover, the driving force, which is related to interfacial tension, depends strongly on the environment.
3. APPLICATIONS OF ACTUATORS The actuation of polymers is accompanied by the generation of force and a change in their shape. This chapter will describe typical fields of application for different kinds of polymer actuators. Some of the applications such as artificial muscles, swimmers or walkers, and microsurgical devices are based mostly on the generation of force during actuation. Other applications such as sensors, switchable optical devices, active elements of microfluidic devices, and 3D microfabrication are based on the change in shape during actuation. 3.1. Sensors. In a typical example, an actuator applying a stimulus leads to a macroscopic change in the shape that can readily be used for the design of sensors.50,51 Actuating sensors are typically designed using hydrogels because there is a broad variety of hydrogels sensitive to different stimuli such as temperature, pH, and specific ions and chemicals. One of the possible schemes is to use AFM cantilevers, which are coated on one side with a stimuli-responsive hydrogel and are able to bend depending on the swelling state of the hydrogel.51,52 The bending is detected by the change in the reflection of the laser beam from the surface of the cantilever. The advantage of this approach is its simplicity: any AFM device can be used for sensing. 3.2. Imaging Devices. Actuating hydrogels can be used for the design of lenses with tunable focal lengths. For this, the membrane separating water and oil is mounted on a ring formed by macroscopic stimuli-responsive hydrogels (Figure 9a).53 When exposed to an appropriate stimulus, the hydrogel ring underneath the aperture responds by expanding or shrinking. This leads to a change in the volume of the water droplet located in the middle of the ring. The net volume change, which is the
Figure 10. Smart valves for the control of liquid flow based on the poly(2-hydroxyethyl methacrylate)-based hydrogel (reprinted from ref 55, copyright (2001) American Institute of Physics).
liquid to flow when the hydrogel is shrunk and closes the channel when swollen. Another possibility in controlling liquid flow is to use the bending of a hydrogel bilayer. Such a setup was realized by Yu et al.55 The valve consists of a bistrip formed by pHsensitive and pH-indifferent hydrogels. Back pressure closes the leaflets, thereby restricting backflow, whereas forward pressure opens the leaflets and allows fluid to pass. The valve activates and deactivates in response to solution pH because of the use of a pHresponsive hydrogel in the leaflets. At high pH, the valve is functional, and at low pH, the leaflets contract to close the valve. Therefore, the valve not only functions as a one-way check valve but also provides the ability to call the valve into service when desired. Similar check valves were found in mammalian veins.60 Electroactive polymers such as polypyrolle, which is able to swell in one redox state and does not swell in another, can also be F
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applied to the design of smart valves for the control of liquid flow.61 3.4. Walkers and Swimmers. In fact, the deformation of actuators can easily be transformed in walking or swimming by applying cyclical stimuli. In 1992, Osada et al. reported the deformation of a polyelectrolyte hydrogel immersed in surfactant solution, causing swelling of the hydrogel.8 The hydrogel bends toward the cathode that allows control of the direction of bending by the polarity of the applied voltage. Switching of the polarity resulted in a change in the bending direction. Thus, the gel demonstrated bending and stretching upon a cyclic change in the polarity of the applied voltage. The specific deformation of the hydrogel allowed its walking at a speed of 15 mm/min. Sun et al. also developed a macroscopic hydrogel-based actuating system that is able to walk on a ratchet substrate. The system is based on the bending of a hydrophobic−hydrophilic bilayer in response to a change in humidity.62 This bilayer actuator could drive a walking device carrying a load 120 times heavier than the actuator and could walk steadily on a ratchet substrate under the periodic alternation of the relative humidity between 11 and 40%. This concept was further developed in later work. For example, Velev and Dickey demonstrated the walking of an electrically active hydrogel.63 Hydrogel-based actuators are also able to swim when their shape changes cyclically. Different shapes were used to design hydrogel-based swimmers. Lee et al. used the inhomogeneous deformation of hydrogels and fabricated pH-sensitive hydrogel actuators mimicking the shape and swimming motion of an octopus and sperm.64 Such aquabots are able to produce directional motion in response to changes in electrochemical potential and can be potentially used for biomedical applications to sense and destroy certain microorganism. Jager et al. demonstrated more complex actuation via the example of conjugated polymer actuators formed from a bilayer consisting of polypyrrole and metal.65 They showed that such bilayer actuators are able to capture and release particles.7,66 Liquid-crystalline polymers can also be used for the design of motors. Similarly to hydrogels, polymeric LC actuators that are able to bend in one direction or another can be used for the movement. Contrary to hydrogels, this movement can also occur in a dry state. The simple walking of the ratchet surface can be used in a fancier way to design a rotating motor that converts light energy to mechanical work (Figure 11).67 3.5. Three-Dimensional Microfabrication. In fact, in most applications the actuating of actuators is related to the generation of force to move different objects. However, as shown above, actuation can also lead to a considerable change in shape, which is used for the design of complex 2D and 3D systems in the origami-inspired concept (Figure 12).68−70 In fact, the utilization of the folding of actuators for the design of structured materials is very attractive; it allows for a very simple, template-free fabrication of very complex repetitive 2D and 3D patterns, which can hardly be prepared using other very sophisticated methods, such as two-photon and interference photolithography. Three-dimensional microfabrication using actuators is particularly attractive for the design of scaffolds and the fabrication of 3D cellular constructs,15,71 potentially allowing the fabrication of 3D scaffolds for tissue engineering.72 3.6. Smart Textiles. A very promising application of shape memory polymers is the design of textiles with a shape memory effect. Shape memory textiles can be obtained by the incorporation of individual fibers with a shape memory effect. The shrinking of such fibers leads to the folding and shrinking of
Figure 11. Light-driven plastic motor with a polymer liquid-crystalline laminated film. (a) Schematic illustration of the light-driven plastic motor system used in this study, showing the relationship between light irradiation positions and the direction of rotation. (b) Series of photographs showing time profiles of the rotation of the light-driven plastic motor with the LCE laminated film induced by simultaneous irradiation with UV and visible light at room temperature. (reproduced from ref 67, copyright (2008) Wiley-VCH Verlag GmbH & Co. KGaA).
Figure 12. Examples of 2D and 3D microfabricaiton using poly(ethylene glycol) and poly(N-isopropylacrylamide)-based actuators (reproduced from ref 72, copyright (2013) Wiley-VCH Verlag GmbH & Co. KGaA, ref 73, copyright (2013) Wiley-VCH Verlag GmbH & Co. KGaA, reproduced from ref 43, Nature Publishing Group, copyright (2013), reproduced from ref 39, copyright (2013) Wiley-VCH Verlag GmbH & Co. KGaA, reproduced from ref 23, copyright (2010) Royal Society of Chemistry, reproduced from ref 39, Springer Science + Business Media).
a piece of textile. These textiles are able to demonstrate changes in their shape upon heating (hair dryer), which was successfully used for the design of smart cloths.74 For example, a shirt with long sleeves could be programmed so that the sleeves shorten as the temperature increases. The fabric can be rolled up, pleated, creased, and returned to its former shape by using a hair dryer. 3.7. Surgery. One of the big advantages of shape memory polymeric materials is that one can easily design them with biocompatible and biodegradable properties. Another very G
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important feature of shape memory polymers is that the transition temperature is independent of the chemical environment (ionic strength, pH, and presence of bioactive molecules), which is typically not the case when hydrogels are used. Moreover, shape memory polymers have greater Young’s modulus value. All of these properties make shape memory polymers highly attractive for biomedical applications such as minimally invasive surgery, including endovascular clot removal75 (Figure 13a), aneurysm occlusion, fasteners and removable
Figure 13. Examples of microsurgical applications of shape memory actuators: (a) endovascular clot removal (reproduced from ref 75, copyright (2005) Optics Express); (b) stents (reproduced from ref 76, copyright (2007) Science Direct); and (c) sutures (reprinted from ref 77, copyright 1995 AAAS).
Figure 14. Switchable surfaces based on polymeric actuators: (a) selfregulating oscillating surfaces based on a stimuli-responsive hydrogel with pillars (explanation in the text) and (b) surfaces with switchable adhesion based on arrays of shape memory pillars. Reproduced from ref 79, Nature Publishing Group, copyright (2013); ref 80, copyright (2013) Wiley-VCH Verlag GmbH & Co. KGaA).
stents (Figure 13b),76 degradable sutures77 (Figure 13c), and orthodontic appliances. A very interesting microsurgical application of actuators was suggested by Gracias el al.78 They developed self-folding-hydrogel-based structures that are able to stick to a piece of tissue when stimuli are applied and slowly release a drug. 3.8. Switchable Surfaces. Very recently, Aizenberg et al. demonstrated fascinating self-regulating oscillating hydrogelbased actuators (Figure 13a).79 In their study, arrays of PNIPAM pillars were fabricated with a catalyst as a cap for each pillar. Intense exothermic reactions were localized at the tips of the pillars, also localizing the increased temperature in this region. Once the temperature reached the lower critical solution temperature (LCST), the rods bent into a region of liquid that was designed to contain no reactants, allowing the pillars to cool to below the LCST. This caused the pillars to elongate once more and continue the cycle. Simulation results show that such an oscillation is stable for an extended period of time and could thus continue as long as the supply of reagent is continued. In another example, del Campo et al. developed smart actuating surfaces with switchable adhesion (Figure 13b).80 For this they created an array of vertical pillars made of shape memory polymer. The original surface was adhesive. The pillar can be tilted at elevated temperature when the polymer is soft. The “tilted” configuration can be fixed at room temperature when the polymer chains are immobile. The surface with tilted pillars is less adhesive. Increases in the temperature led to a relaxation of the pillars, and they recovered their original orientation, which resulted in an increase in adhesion.
when a stimulus is applied: polymer actuators. There are several main groups of actuators based on changes in different properties such as swelling, melting, anisotropic expansion, and surface forces. There are actuators that are able to function only in a liquid (water) environment, such as hydrogels. A big advantage of the hydrogels is that they are able to undergo very large changes in volume during swelling and shrinking and that there exist many hydrogels sensitive to different stimuli. Shape memory polymers based on the melting/softening of polymers are able to function in both aqueous and dry environments. Actuators based on the utilization of surface forces also work in dry and wet environments but are able to function only in one direction. The movement of polymers can be induced by direct treatment with a stimulus or by the utilization of cascades of stimuli-induced processes, allowing for the design of actuators sensitive to a broader set of stimuli. In fact, actuation can be induced by a stimulus either in one or two steps. In a one-step procedure, the stress is immediately converted in deformation. In the two-step actuation, the changed shape is frozen either by cross-linking or by reducing the mobility of polymer chains. As a result, the initial shape can be recovered only after a stimulus is applied and the actuator is unfrozen. All of these findings open broad possibilities for the design of actuators sensitive to different combinations of stimuli, actuators that respond directly to stimuli or after addition treatment is applied, and actuators that are able to act in a dry and/or wet and/or physiological environment. Such actuators can be applied for a variety of purposes such as micromanipulators, control of liquid flow, design of sensors,
4. CONCLUSIONS This review presents a brief view of a variety of polymeric materials that are able to change their shape and generate force H
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tactile displays, walkers, smart textiles, microfabrication, and surgery.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biography
Leonid Ionov is a group leader at the Leibniz Institute of Polymer Research (Dresden, Germany). He graduated from Lomonosov Moscow State University (Russia) and received a Ph.D. in polymer chemistry in 2005 from Dresden University of Technology (Germany). He has worked as a guest and postdoctoral researcher at the Eindhoven University of Technology (The Netherlands), Max Planck Institute of Molecular Cell Biology and Genetics (Germany), and at Clarkson University (Potsdam, NY, USA). He is the author of more than 60 publications and several patent applications and book chapters. His research interests lie in the fields of stimuli-responsive and selfassembling materials.
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ACKNOWLEDGMENTS I am grateful to Georgi Stoychev and Ivan Raguzin for fruitful comments on the manuscript and help with the illustrations. My funding source was DFG (IO 68/1-1).
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