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Actuating fibers: design and applications Georgi Stoychev, and Leonid Ionov ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07374 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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ACS Applied Materials & Interfaces

Actuating fibers: design and applications Georgi V. Stoychev,a Leonid Ionov a,* a

College of Engineering, College of Family and Consumer Sciences, University of Georgia,

Athens, GA, 30602, USA KEYWORDS: fibers, actuators, smart materials, artificial muscles, smart textiles ABSTRACT: Actuators are devices capable of moving or controlling objects and systems by applying mechanical force on them. Among all kinds of actuators with different shapes, fibrous ones deserve particular attention. In spite of their apparent simplicity, actuating fibers allow for very complex actuation behavior. This review discusses different approaches for the design of actuating fibers, and their advantages and disadvantages. We also discuss the prospects for the design of fibers with advanced architectures and complex actuation behavior. Introduction Actuators are devices capable of moving or controlling objects and systems by applying mechanical force on them.1 Different materials have been used for the design of actuators, including polymers,1-2 carbon nanotubes (CNTs) and graphene,3-5 and polymer/inorganic material composites5. These materials can be used in different forms: bulky pieces (3D objects), films (quasi 2D objects), and fibers (quasi 1D objects). Actuating fibers is a particularly attractive strategy, because fibers are characterized by flexibility and high anisotropy of properties. Moreover, movement of animals is provided by contraction and

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elongation of muscles, which have fibrous structure. The actuation of muscles is generated by step-like movement of myosin motor proteins on actin filament.6-7 While, these very simple nanoscale

steps

of

motor

proteins

are

converted

in

very

simple

microscopic

contraction/elongation of muscles, a variety of different movements, which muscles can make, is possible (Figure 1). This example from nature clearly shows us that even simple change of length of fibers can cause very complex movement that makes them practically very attractive. In this paper, we attempted to overview different approaches for the design of synthetic fibrous actuators starting from the molecular mechanism of actuation towards the possibilities to generate complex motion. Polymer-based actuators are strongly represented mainly due to the large number of available polymers capable of actuation under different conditions and stimuli.2 Moreover, polymer materials provide a straight-forward approach for manufacturing of such actuators, as polymers can quite often be readily melt- or solventextruded into fibers. The actuating properties of these fibers are a result of the properties of the polymer itself. However, other systems have been reported that are based on inorganic, electrical conductive materials, such as carbon nanotubes (CNTs), graphene, and metal nanowires. Polymer/inorganic composites have also been employed for fiber actuators. These systems are also reviewed here as some of them show performance comparable to or even exceeding of the polymer-based ones. We also try to give a short characteristic of each approach for the design of fibrous actuators from point of view of their advantages and disadvantages.


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Figure 1. Natural fibrous actuator – muscle tissue. Muscles have fibrous structure and contract and expand due to step-like motion of myosin motor proteins. (Reproduced with permission of Nature Publishing Group, Inc from Ref. 8)

Classification of fibrous actuators. Actuation of monofilament fibers is achieved solely due to a change in the properties of the constituting material. Here, changes in the environmental conditions (temperature, presence of solvent) can lead to changes in the molecular order of the material, or its volume. Normally, these two phenomena (order and volume change) are observed simultaneously but with ACS Paragon Plus Environment


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different contribution to actuation. Therefore, actuators can be separated into two main groups (Figure 2) where either volume or order change dominates. Actuators of the first group are those where the volume change dominates (Figure 2a). The volume of the sample can change due to mass transport (hydrogels and similar),9 due to thermal volume expansion,10 or due to phase transitions like melting and crystallization. Actuators of the second group are those where the actuation is caused by a change in the molecular order (changes of volume are small, Figure 2b). These are, for example, shape memory polymers,11-13 where the polymer chains are brought to an unfavorable conformation by mechanical deformation at increased temperatures, which is then temporarily fixed by cooling down. A following temperature increase leads to the relaxation of the chains and restoration of the original shape. In these examples, the sample must be first deformed in order to change the conformation of the polymer chains. Actuation of liquid crystal elastomers is also caused by the change of molecular order, but in this case, it occurs without external stretching of the sample.14 Another example is dielectric elastomers sandwiched between two electrodes; switching an electric field between the electrodes on and off results in a reversible deformation of the polymer.15 For a yarn, i.e. for a fiber consisting of multiple thinner fibers, a third mechanism is possible, namely actuation through a change in the distance between the constituting fibers. (Figure 2c). It is only applicable if the material consists of a large number of fibers. Changing the distance between the individual fibers, for example by inducing an electromagnetic attraction between them, results in actuation. Actuation through this particular mechanism oftentimes is accompanied by volume changes in the material. However, we distinguish it from the pure volume change-based actuation because it is driven by different forces, resulting in dissimilar actuation outcomes. For example, a hydrogel-based fiber would increase in length and thickness upon swelling (accumulation of solvent) due to homogeneous expansion in all


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directions. A CNT-based twisted yarn, consisting of hundreds or thousands of aligned CNTs can actuate when the distance between the individual CNTs is changed. This can be achieved by employing, for example, electric current and does not result in a homogeneous expansion/shrinking of the fiber in all directions. The twisted structure of the fiber converts this in both rotational and translational motion, in sharp contrast to volume change-based fibers. It should be noted that some systems, like the ion-insertion CNT-based or graphenebased fibers, employ both volume change and distance change. We classify these as distance change-based actuators, because they bear more similarities in terms of structure and performance with the latter than with volume change-based fibers.



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Figure 2. Summary of different mechanisms of actuation of polymer fibrous materials; a) volume change induced by sorption of different species (solvent molecules, ions, gases) in the bulk of the fiber; b) entropy-driven relaxation of stretched amorphous polymers (left); chain conformation changes in shape-memory polymers and dielectric elastomers under external force (middle); phase transition in oriented liquid crystal elastomers (right); c) actuation due to distance change between individual fibrils in a multi-filament fiber.

Monofilament actuators based on volume change

Mass transport. The first group of fiber actuators comprises those based on active mass transport in and out of the polymer, for example of solvent molecules. Hydrogels, where water is used as a solvent, is an attractive group of mass transport actuators. Hydrogels are three-dimensional polymer networks strongly imbibed with water and the amount of water can approach 99 wt % of the hydrogel mass. Because of this property, hydrogels are able to considerably swell and shrink (> 10 times in volume) when the amount of water in the polymer network change, which may occur in response to different stimuli such as temperature,16-17 light,18-20 pH,21-22 ionic strength, magnetic field

23-25

etc. In this respect, the

behavior of hydrogels mimics the hydromorphic movement of plants, where twisting and bending is provided by the change of the amount of water in cells and tissues. Clearly, hydrogels are able to act solely in aqueous media that, on one hand, limits their applications but, on the other hand, opens new perspectives for applications where other kinds of actuators are not desirable, such as in biomedical research, where softness and biocompatibility are of the highest importance. Commonly, the reversible change in the diameter of the hydrogel fiber is detected 26 (Figure 3). However, tensile strains of up to 40% have been reported, with exceptional for hydrogels stresses of several MPa.27 In another example, more complex actuation was observed when hydrogel fibrous actuators were prepared by microfluidic jetting


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of two polymers with different swelling properties. As a result, two-layer structure was formed. Because of unequal swelling of the polymers, the fiber could undergo reversible transition between fully stretched and coiled shapes. 28

Figure 3. Optical micrographs of poly(N-isopropyl acrylamide) microfibers containing iron oxide nanoparticles measured at various temperatures. (Reproduced with permission of Royal Society of Chemistry from Ref 26)

Another example of mass-transport-based actuators is ion-insertion actuators based on red-ox polymers such as polypyrole (PPy) or polyaniline.

29-30

PPy has two electrochemical states

(oxidized and reduced), while polyaniline (PANi) has three: leucoemeraldine, emeraldine, and pernigraniline.31 Not only are ions exchanged, but in aqueous electrolytes there is also a proton exchange, since different states have different pKa.32 Therefore, PANi is considerably



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more complex to use as a material for actuators, since oxidation can result in either expansion or contraction, depending on the pH. Despite that, PANi and composite CNT/PANi fiber actuators have been reported.33 The CNT-reinforced fibers showed remarkable mechanical properties, sustaining stresses up to 120 MPa during electromechanical cycling, with corresponding strains of up to 1%. Both PANi and PPy are hard to process and usually used in the form of blends with aliphatic polymers, further increasing the complexity of the system and reducing the mechanical output,34-35 although pristine conductive polymer fibers have been reported.36 Another option is to combine PPy with graphene to create bilayer actuators. 37-38 Bilayer fibers were produced by electrodeposition of PPy on a graphene fiber. The composite was able to bend in 1M NaClO4 both towards the graphene or the PPy side, depending on the applied potential. Bending angles of almost 70o were observed reproducibly for more than 100 cycles.37 Similar bilayer design was used to create an electrically responsive catheter.39 Recently, polyferrocenylsilane was proposed as an electroactive polymer40. In contrast to PANi and PPy, it is not pH-dependant and is readily soluble in various organic solvents, thus boosting its processability. Electrospun polyferrocenylsilane fibers demonstrated actuation when subjected to voltages as low as 1-2 V, with response times in the vicinity of hundreds of milliseconds. Mass transport comprises a straight-forward approach towards the fabrication of actuators; changes in the quantity of the material are accompanied by corresponding changes in its volume, resulting in actuation. No wonder various actuation schemes were proposed based on this phenomenon. From these, hydrogels stand out in terms of net volume change, which can approximate 1000% in individual cases. Moreover, many hydrogels are biocompatible and biodegradable, and they provide a soft, tissue-friendly environment for bio-applications. However, they come with low modulus and poor mechanical, and sometimes chemical,


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stability. Actuation times are limited by solvent diffusion, which is at least on the order of several seconds. These limitations make them an impractical choice for applications beyond the lab-bench. Electroactive polymers have arguably the fastest response-times, but are plagued by technical difficulties in the processing step. Some of them, like PANi and PPy, are also pH-sensitive, making their actuating behavior complicated and less predictable. However, all polymer actuators can be easily and cheaply synthesized; many are based on commercially available polymers. This potential for scalability leaves them a viable candidate for future research. Monofilament actuators based on order change.

Temperature contraction of amorphous polymers. One way to achieve actuation is to use temperature-dependence of the stress of stretched amorphous polymer networks. In fact, the stretching of amorphous rubber leads to its partial crystallization and decrease of internal entropy.41 The enthalpy changes only slightly during stretching. Thus, there is a driving force (increase of entropy) that is leads to the relaxation of the polymer chains. This driving force increases with temperature as the contribution of the entropic component into the Gibbs energy (T∆S) also increases with temperature. Thus, the increase in temperature increases the stress in a stretched amorphous fiber and it contracts stronger at higher temperatures.42 Exactly this effect was used to design actuators based on fishing line, proposed by Haines et al.

43

They designed actuators by twisting polyethylene and nylon fibers to obtain coiled

structures as shown in Figure 4. Both polymers exhibited reversible volume change of around 2% due to phase transition when the temperature was cycled between 25oC and 150oC. The insertion of twists in the fiber boosted the coaxial contraction of the fibers up to 49% in some cases. The reversibility of the system was tested for over 1 million cycles.43 Fishing-line based actuators sport a number of highly relevant advantages that place them among the best in their class in terms of performance and durability. While strains of several tens of percent



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are not exceptional in the field, they are far better than values for other systems and are more than necessary for many potential applications, including artificial muscles. The stick-toitiveness of the materials is outstanding, and the design is as simple as it gets – a twisted fiber. Scalability is also not an issue, as cheap, commercially available fibers (fishing lines) can be used. One restriction of the broader application of this particular design could be the relatively large temperature intervals that are needed for efficient operation. This can pose additional technical challenges for heat transfer as the system is scaled up.

Figure 4. Entropy driven temperature contraction of twisted amorphous polymer fibers; a) temperature dependence of the strain for different fibers; b) images of relaxed and contracted over-twisted fishing line actuators; the graph shows actual strains. (Reproduced with permission of AAAS from Ref 43)

Dielectric elastomer actuators are non-conductive elastic polymers between two electrodes. 15, 44

Applying an electric potential leads to the formation of opposite charges on the

electrodes. The oppositely charged electrodes attract each other and squeeze the elastomer. Removal of the field results in the relaxation of elastomer and restoration of the initial shape.1


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This approach was also realized on the examples of multilayer core-shell fibers where the inner fiber and the most outer shell are conducive and the elastomer is between them. For example, a multilayer coaxial fiber was proposed by Kofod et al45, where up to three conductive layers were separated by soft isolating layers (Figure 5). Applying a high voltage to the electrodes led to a decrease it the diameter and a corresponding elongation of the fiber due to electrostatic attraction between the conductive layers. Maximal strain of approximately 7% at 10 kV was reported.

Figure 5. Actuating coaxial dielectric elastomer fiber. Schematic representation of the structure of a co-axial, dielectric elastomer-base fiber actuator (top), and its performance at different pre-strain values and voltages (bottom). (Reproduced with permission of Elsevier from Ref 15)



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Dielectric elastomer actuators possess some luring qualities, making them a potential candidate for commercialization. One of their main advantages is the electrically-enabled actuation. This means that they can be easily integrated with existing and well-established technologies. Moreover, they sport simple designs and can be manufactured from cheap, readily available materials, giving them an edge in scalability. However, there are some set-backs of the approach that have to be solved before these actuators could be implemented in end-product devices. One of them is the relatively high voltages on the order of thousands of volts that are necessary for actuation. This brings the risk of dealing with high voltages. Another problem is their actual performance, as the reported elongations of several percent are by far surpassed by many other existing systems. The durability of the system has to be tested, too, as only around several tens of cycles have been reported. Shape memory polymers are another group of actuators based on relaxation. Shape memory polymers are either chemically or physically crosslinked networks, which consist of one or two polymers11, 13. Physical crosslinking may be provided by local crystallization of one of the polymers. Chemical or physical crosslinking determines the permanent shape. Heating the polymer brings it to the visco-elastic state where it can be easily deformed. The deformed shape can be frozen by cooling 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 to restoration of the initial shape.12, 46 Common elastic rubber is an example of a shape memory polymer47-48. A piece of rubber can be stretched at room temperature and cooled down in liquid nitrogen. The piece will keep its deformed shape until the temperature is kept low. Heating up to room temperature increases the mobility of polymer chains and that leads to restoration of the initial shape. Classical shape memory polymers

46 49-50

have two shapes: permanent, which is defined by crosslinking, and


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temporary, which is defined by deformation. This principle was recently realized on fibers, which are able to make self-tightening knot

49

, and degradable self-tightening sutures

51

(Figure 6)

Figure 6. Actuating fibers based on shape memory polymers – heating results in the tightening of a knot. (Reproduced with permission of AAAS from Ref 49)

Shape memory fibers and yarns have also been used in the design of smart textiles, providing the fabric with shape –change ability

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, or advanced controllable vapor permeability54.

SMP-based fabrics were also proposed for applications in biomedical research24. Shapememory polymers are among the few actuators systems that have made it all the way to commercialization. This is probably due to their reproducible, straight-forward actuation mechanism, as well as relatively low manufacturing costs. However, one fundamental challenge is the reversibility of actuation. Even in advanced SMPs, external force has to be applied to the material in order to bring it back to its original shape, rendering them unsuitable for applications such as artificial muscles.

Liquid crystal elastomers. Another approach to the design of actuators is melting liquidcrystalline polymers.

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For example, Naciri et. al. designed fibers from comb-like nematic

polymers.56 The fibers were prepared by melt spinning, and since a shear force was applied ACS Paragon Plus Environment


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during preparation of the fibers, one can expect uniaxial orientation of mesogens. Increasing the temperature leads to isotropization of the polymer, whereupon its conformation changes to a random coil, and an anisotropic shape change occurs –a decrease of length and an increase of width (Figure 7). The amplitude of actuation (length change) depends on the structure of the polymer and can be up to 400%.

Figure 7. Liquid crystalline actuators. a) schematic representation of the phase transition in oriented liquid-crystalline polymer fibers; b) contraction of LC fiber actuator upon heating; c) mechanical performance of the actuator under different loads. (Reprinted (adapted) with permission from Naciri, J.; Srinivasan, A.; Jeon, H.; Nikolov, N.; Keller, P.; Ratna, B. R., Nematic Elastomer Fiber Actuator. Macromolecules 2003, 36, 8499-8505. Copyright 2003 American Chemical Society. 56)


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A different method for the preparation of highly oriented LC fibers was introduced by Ohm et al57. In their approach, polymer LC solution was injected in laminar co-flowing silicon oil, resulting in the formation of oriented LC fibers, with director aligned parallel to the fiber axis. Fibers with thicknesses between 20 and 50 um could be produced with this method. Under constant load, varying the temperature between values below and above the transition temperature of the polymer resulted in the reversible elongation and contraction of the fibers, respectively (Figure 8).

Figure 8. Reversible actuation of LC fibers under constant load. a) the fiber under mechanical tension at temperatures below the clouding point of the liquid-crystalline polymer; b) increasing the temperature leads to a phase transition in the liquid-crystalline phase that results in contraction (shortening) of the fiber; c) decreasing the temperature returns the fiber to its initial state. (Reproduced with permission of Royal Society of Chemistry from Ref 58)

Mechanical characterization of the fibers revealed maximum elastic modulus of around 0.75 MPa, with corresponding strains of up to 90%. The reversibility of actuation and its kinetics were also tested. Under an applied load, relaxation time on the order of one hour was observed at 40oC. Increasing the temperature to 80 degrees shortened the relaxation time to less than 15 seconds.



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Melt-extrusion was also applied for the production of oriented LCP fibers59. Here, tri-block copolymer with liquid-crystalline main chain was utilized. Acrylate monomer was included in the main chain to enable photo-crosslinking. The polymer exhibited transition temperature of around 100oC. Heating of a fiber, based on this material, above the transition temperature resulted in maximal elongations of around 400%, with corresponding stresses up to 60 kPa. The reversibility of the process was shown for only several heating/cooling cycles at much lower strains. LCP-based actuators have many promising features, but at the same time suffer some serious drawbacks that, if not addressed, could potentially hinder their application on a larger than laboratory extent. One major impediment is the time-scale of actuation. For most practical applications, from artificial muscles to robotics, fast actuator responses on the order of one second or less are required. So far, the best reported non-equilibrium response for an LCP system was several seconds, with equilibrium state reached after almost 15 seconds, an order of magnitude slower than needed. A second obstacle is that big temperature differences in the order of several tens of degrees are necessary for actuation. This not only puts strain on the practicality of these systems in terms of energy efficiency, but also severely limits its operating conditions. In order to incorporate LCP-based actuators in an end-product, the active part has to be thermally isolated from its environment, and heat sources and dumps have to be added for the device to operate properly. The typical transition temperatures of LCPs is above 50oC, leaving them unsuitable for most bio-applications. Moreover, the speed of heating/cooling of bulk constructions will further slow the actuation, as it decreases with size and is additionally limited by the thermal conductivity of the material. Despite the aforementioned problems, LCP actuators sport some luring characteristics. Arguably the most significant one is their immediate scalability. LCPs are readily synthesized through standard chemical routes and can be drawn into oriented fibers using commercially


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established methods. Another virtue is the outstanding strains of up to 400% they exhibit, a number yet to be achieved by other systems. These unmet strains come in hand with reasonable strength that, albeit falling somewhat below the average for artificial actuators, still outperforms skeletal muscles by a factor of 10. LCP-based actuators surely are in their infancy; only a handful of publications on the topic can be found in the scientific literature. We have shown that they suffer from some drawbacks, which are counterbalanced by their scalability and performance, rendering them a viable and promising candidate for future development. Multifilament actuators based on the change of distance between fibers Volume and order change provide straightforward rules for the design of monofilament actuators. The use of multifilament fibers allows also for the design of actuators based on the change of distance between the filaments. Melting/crystallization. One possibility to reversibly change the distance between the filaments is to impregnate the fiber with fusible material, which can change its volume due to melting and crystallization. Indeed, melting and crystallization have been used to design electrically- and light- powered torsional actuators based on a twisted CNTs yarn impregnated with paraffin (Figure 9). In this particular approach, CNT forests were drawn into fibers by twisting. Overtwisting of the fibers led to the formation of coils. These overtwisted yarns were then infiltrated with paraffin. The actuator was heated by electric current or by light, which led to melting of the paraffin at 60°C, so that its volume increased by 20%. 60-61 Further heating of the melted paraffin brought additional 10% of volume expansion, totaling 30%. Cooling down led to crystallization of the paraffin and to its shrinking. Since wax and similar materials are polycrystalline, the shape change upon melting is isotropic, so the volume change is uniform in all directions. If only wax was to be considered, this would have



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translated into a simple elongation of the fiber by around 9%, with corresponding thickening on the same scale. However, due to the twisted nature of the supporting yarn, more complex actuation occurs. Tensile actuation of up to -7% was observed, i.e. the yarn contracted when wax melted. This is due to the fact that, in the particular configurations used, the yarn uptwisted when the wax melted.

Figure 9. Scheme of torsional actuators based on twisted CNTs yarns; a) micro-structure of a typical CNT-based yarn; b) schematic representation of the effect of volume change on the yarn geometry and twisting before (left) and after (right) volume increase; c) temperaturedependent tensile actuation of CNT yarn before (black) and after (red) wax infiltration. (Reproduced with permission of AAAS from Refs 60, 62)

Promising results were demonstrated by this approach, with reported tensile actuation of up to 7% and rotation at an average 11,500 revolutions per minute. Moreover, actuation times as low as 25 milliseconds were achieved, far outperforming the rest of the proposed competitor systems. It should be noted that slightly different yarn-architectures63, as well as alternative conductive materials64 have been proposed. While the introduction of Nb-nanowires improved the ultimate tensile strength of the set-up and provided higher conductivity, other important


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characteristics of the system, including torsional actuation speed and torque-to-mass ratio, declined. The alternative yarn architecture didn’t bring any improvements in the performance.

Fiber actuators based on the thermal volume change of materials show outstanding properties, including high tensile strength, fast actuation on the order of tens of milliseconds, robustness and reversibility of actuation for millions of cycles. Moreover, the use of light-adsorbing, conductive materials, such as CNTs and Nb-nanowires, allows the use of different stimuli like electric current, light, and temperature, providing those systems with versatility. These qualities may sound like the perfect choice for artificial muscles and other fiber-based actuating systems; however, the scalability of the approach is yet to be demonstrated. Whatever stimulus is adapted to trigger actuation, the underneath mechanism is melting/crystallization of a material, a process connected with the exchange of heat with the environment. This means that the system cannot simply be scaled up, as additional elements for heat transfer will have to be added. Moreover, the fibers should be equipped with an insulating layer in order to sustain the integrity of the actuators when the wax is in the molten state. On the commercial side of the problem, only actuators, based on the costly CNTs or the even more expensive Nb-nanowires, have been proposed. In order to bring down the cost of the actuators, alternative materials need to be sought. Swelling. Alternatively, space between filaments can be filled with a material which swells 65. These fibers sported the same general architecture as the paraffin-infiltrated fibers described earlier, with the exception that the paraffin was substituted with a soft cross-linked PDMSresin. When subjected to different solvents, the resin swelled and the twisted CNT base translated this uniform expansion to both contractive and torsional movement. Strains of up to -45% were achieved by this method, with corresponding stresses of tens of MPa. The system also demonstrated outstanding contraction times of around 0.1 s. However, the reverse action ACS Paragon Plus Environment


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was limited by diffusion of the solvent out of the fiber and occurred much slower, delaying the response of the system to ca. 10 seconds. Adsorption of gases or vapor. In another approach, volume change was achieved by sorptiondesorption of hydrogen in palladium deposited on the surface of CNT-yarns.60 This particular set-up was almost identical to the wax-infiltrated CNT-yarn actuators. However, this system suffered from highly increased reaction times and much lower torsional actuation, compared to the wax counterpart. Graphene oxide (GO) has been suggested as an alternative to CNTs for the design of fiber actuators.66-67 The fibers were prepared by twisting of free-standing GO films. Graphene oxide is hydrophilic, making the fiber a moisture-driven actuator that also doubled as a water vapor sensor. When exposed to high humidity (RH 85%), the fiber absorbed water molecules from the air. This induced a change of distance between the GO-layers, resulting in untwisting (rotational movement) of the fiber. Exposure to dry air reversed the process, with water molecules desorbing and leaving the volume of the fiber, leading to its twisting. The process was repeated up to 500 times without any loss of function. The rotational speed was highly dependent on the degree of the inserted twist, with maximum values of 5000 rpm achieved at 5190 revolutions/meter. Tensile stress of around 110 MPa was measured for this scenario, with tensile strains as high as 4.7%.67 Alternative moisture-driven graphene/graphene oxide fiber actuator was also suggested.

68

Here, asymmetrical patchy fibers were produced by laser-assisted region-specific reduction of GO to graphene.69-71


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Figure 10. Actuation of asymmetrical G/GO-fibers; a) - laser-assisted region-specific reduction of GO; b) - snapshot of partially modified GO fiber showing the color difference between graphene oxide (yellow) and reduced graphene oxide (black); scale bar is 50 µm; c) schematic of different geometries and their corresponding actuation behavior. Reproduced with permission of John Wiley and Sons from Ref. 68 ACS Paragon Plus Environment


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In this particular design, actuation did not occur due to the twisting/untwisting of the fibers as in the previous example. Instead, bending was achieved through the uneven expansion of the different patches, which respond differently to water vapor – GO absorbs water and expands, whereas rGO is more hydrophobic and does not. This mechanism resembles the bending of a bi-metallic strip72 or a polymer bilayer73-74 because it arises from the different expansion of two connected layers. Bending of up to 150o was observed, with an average motion rate of 8o/s. The reversibility of actuation was shown for 1000 cycles.68 Electrostatic repulsion. Another possibility to tune the distance between filaments is to adsorb ions on them that will cause electrostatic repulsion. Such effects are observed when an electric field is applied to conductive fibers. In this case, selective adsorption of ions leads to electrostatic repulsions between the fibers, increasing the volume. For example, Foroughi et al. have demonstrated torsional carbon nanotube artificial muscles formed by a twisted carbon nanotube yarn. In this case, the system acts not only as an actuator but also as a capacitor – energy is stored in the form of an ion bilayer

62

.Further improvement of this set-up was

brought by the introduction of a second CNT-yarn, which acted as the counter-electrode75. Importantly, this was an all-solid-state fiber, in contrast to other adsorption-based systems. Tensile actuation of up to 0.6% and torsional movement of 50 degrees/mm were demonstrated. Response times were less than one second. Recently, a slightly modified design of a CNT-yarn-based capacitor was proposed. This time, the fiber displayed bending instead of elongation/torsional movements, with actuation times on the order of hundreds of seconds76. Electromechanical actuation. It is not unusual to see a fiber woven from hundreds, even thousands of individual fibrils. This is especially true for carbon-nanotube fibers. With a diameter of a single CNT in the order of several nanometers, it takes tens of thousands of them to form a thread visible to the naked eye. Moreover, CNTs are known for their


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exceptionally high strength. No wonder CNT-based fibers present a compelling candidate for actuators and artificial muscles.77 However, neither volume nor order change can be induced in CNTs for the purpose of actuation. Therefore different strategies need to be sought. One of them makes use of the multitude of CNTs in fibers and their high conductivity. This approach was pursued by the group of Peng.78 Their method is based on the use of CNT-forests, from which a fiber is drawn. An example of a fiber, drawn from CNT-forests is shown in Figure 10. It is the conductivity of the CNTs and their specific arrangement in the fiber that enable actuation when electric current is applied. Specifically, owing to the preparation method, all the CNTs are aligned parallel to each other but at a certain angle, referred to as the helical angle, to the fiber-axis78. In this particular configuration, the CNTs in the spun fiber act like conductive wires when electric current is applied leading to the formation of an electromagnetic field. As a result, attraction force between the CNTs is generated, leading to the twisting of the fiber and its contraction (Figure 11).

Figure 11. Schematic representation of the forces arising in a compound CNT-based fiber when electric current is applied - electromagnetic attraction between individual CNTs results in contraction and twisting of the yarn. (Reproduced with permission of AAAS from Ref 78) Both the current density and the helical angle were shown to influence the actuating of the fibers, with higher currents inducing larger stresses in the material. The helical angle exhibited a more complex influence on the stress, with stress reaching maximum levels at around 30 degrees. Rotational angles of up to 10 degrees at 5 A current were achieved. The



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set-up sustained reproducible response for more than 200 cycles. This mediocre performance of the fiber is mainly due to the fact that only single-ply fibers were used. Increasing the twisting in the fiber can be precisely controlled to result in the formation of coils, called writhes. These loops provide further hierarchical step of complexity and grant superior contractive strain (up to 14%) and enhanced rotation output (up to 135 revolutions per meter) compared to the single-ply fibers79. Moreover, the multi-ply fibers were much thicker, making them easier to handle, a feature used by the authors to manually create actuators on the centimeter scale, which were able to fold and move in various directions depending on their design. Electromechanical CNT-based fiber actuators are new to the field. However, even in the limited number of publications on the topic, they were shown to possess some outstanding properties. The inherent toughness of the CNTs grants them high strength and modulus. More importantly, their actuation is controlled simply by passing electric current through them, making them arguably the most convenient system in terms of control. Another advantage of the electromechanical approach is the fast response of the fibers; actuation times as low as 0.5 seconds were demonstrated. However, there are some drawbacks of this approach. Their contractive strain of 14% and rotational output of less than 1 revolution per 1 cm of fiber leaves them on the lower side for such systems. Another potential issue is the cost. CNTs and especially CNT-forests are expensive to produce, thus limiting scalability. Unless this changes in future, CNT-based fiber actuators will fall short to become price-competitive especially compared to LCP- and, moreover, to fishing-line based systems. Further possible hindrance arises from thermal considerations. Though not commented on in the papers, one could assume that the CNTfibers heat substantially during actuation, when currents as high as 5 A are applied. Unless the


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aforementioned issues are addressed, the CNT-based electromechanical fiber actuators have the potential for only limited and highly specific applications.

Surface tension. It is well known that the interfacial forces play significant role in the mechanics of micro- and nano-sized objects.80 Therefore, the proximity of the individual CNTs in the fibers and yarns to each other naturally leads to the idea of employing these forces for actuation; this was realized on the very same architecture used for electromechanical actuators, which we covered in the previous chapter. Here, again, fibers were drawn from CNT-forests; several fibers were then plied together to form multi-ply fibers (SHFs); further twisting of the multi-ply fibers lead to the formation of writhes, forming hierarchically twisted multi-ply fibers (HHFs).81 When SHFs or HHFs are then wetted, the liquid infiltrates the free space between the fibers and the CNTs. Due to the surface tension of the liquid, an attractive force between elements on all levels of hierarchy arise. This leads to the further twisting and shortening of the corresponding SHF or HHF on the macroscale, i.e. to actuation (Figure 12).

Figure 12. Surface tension-driven actuation of CNT-based fibers; a) a schematic representation of the actuation mechanism; b) rotation and contraction of the fiber when exposed to ethanol. (Reproduced with permission of Nature Publishing Group from Ref 81)



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Due to the inherent hydrophobicity of the CNTs, the fibers could not be wetted by water, so no actuation was observed in this case. Less polar solvents, such as ethanol, acetone, and dichloromethane had to be used instead. Strains of up to 60% were reported, with actuation times for the HHFs on the order of 1 second. The performance stability was tested for more than 50 cycles. In a complementary study from the same group, the fibers were treated with oxygen plasma in order to increase their hydrophilicity, thus enabling actuation by wetting with water. Similar results were obtained, albeit with decreased response times of several seconds.82 At first glance, wetting-driven actuation may sound more like a curious phenomenon, rather than a meaningful approach. However, it possesses some appealing characteristics that make it a viable potential candidate for a fiber actuator with real-world applications. First of all, it is the natural strength and high-modulus of the CNTs, providing the fibers with outstanding mechanical robustness and toughness. Second, wetting with non-water, highly volatile solvents results in actuation times on the scale of 1 second, which, despite lagging behind electromechanical actuation, are still one of the best reported so far. Additionally, the use of highly volatile solvents enables fast reset times. Third, no substantial amounts of heat are produced in the process; neither is heating/cooling necessary for the actuation to occur, and all related difficulties can thus be discarded. On the other side, this approach suffers from the same problems that plague the electromechanical actuators: low contractive strain and high manufacturing costs. The use of potentially non-recyclable solvents also significantly decreases the sustainability of the method, while at the same time it increases its environmental impact and raises safety concerns. Pneumatic actuators. A curious and innovative approach was suggested by Spinks et al83. Fibers, made of CNT-sheets were employed for the purpose. These fibers were immersed in


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NaCl and HCl solutions and electric current was passed through them. It was observed that the thickness of the fibers significantly increased (Figure 13A), while their length by more than 1% in several cases. The reason behind this behavior was the formation and accumulation of molecular hydrogen in the form of nano-bubbles, confined between the CNTsheets. Actuation times of tens of seconds and strains of maximum 2% were achieved by this method. Here, a similar work from the same group should be noted, despite the fact that it doesn’t fall under the description of pneumatic actuators. In this particular case, an organic, more electrochemically-stable solvent (acetonitrile) was used instead of water, while the fibers under study were the same.84 Applying of voltage to the fiber induced actuation but no apparent generation of bubbles. The authors hypothesized that the actuation was the result of charge build-up on the fiber, which led to changes in the C-C bond length in the CNTs. Typical strains for the system were less than 0.5%, with reaction times of less than a second.

Figure 13. Pneumatic actuation of graphene-based fibers. Gas bubbles are formed in the space between the cross-linked graphene layers, expanding the network in the perpendicular



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direction, which results in shortening of the fiber. A) microscope snapshots of expanding fiber; B) schematic representation of the process (Reproduced with permission of John Wiley and Sons from Ref 83)

Summary of actuation mechanisms So far, three distinct mechanisms of actuation were considered, namely volume change, order change, and distance change. They have been implemented in various systems, which are shortly summarized in Table 1, together with the advantages and draw-backs of the corresponding approaches. Two different types of actuation speeds are given in the table, as well as through the whole body of this article. For torsional and bending actuators, the maximum speed of rotation or bending, achieved in the corresponding work, is given. For translational actuation, the time necessary for the actuator to complete a maximum stroke motion is given. For example, hydrogel-based fibers need more than 10 seconds to reach fully swollen equilibrium state, while it takes only a fraction of a second for the wax to melt and the actuator to complete its motion in the case of wax-infiltrated twisted fibers (see Table 1). In this sense, we considered an actuation to be fast if it was completed for 200 milliseconds or less. Fast actuation on this time-scale or faster is generally required for application in robotics and especially in bio- and soft-robotics, the main target areas for polymer-based and carbonbased fiber actuators.85-87 Such speeds are on par with the speeds achieved by human skeletal muscles,88-89 an objective that needs to be met if artificial muscles are to be created. Other applications, like high-speed flow-control in microfluidic systems require even faster reaction-times.90 Thus, the speed of actuation is a highly important parameter of the actuating systems.


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Table 1. Summary of different approaches for the design of actuating fibers

Volume change Approach Mass transport (hydrogels)26-27

Tensile stresses 1 million • • • • 25-50 • • •

Actuating Fibers: Design and Applications - ACS Publications

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