Direct Writing Corrugated PVC Gel Artificial Muscle Via Multi-Material

Article Direct Writing Corrugated PVC Gel Artiﬁcial Muscle via Multi-Material Printing Processes

Bin Luo 1,2,3,*, Yiding Zhong 2, Hualing Chen 1,2,*, Zicai Zhu 2 and Yanjie Wang 4

1 State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an 710049, China 2 School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (Y.Z.); [email protected] (Z.Z.) 3 School of Mechanical and Energy Engineering, Shaoyang University, Shaoyang 422000, China 4 Changzhou Campus, School of Mechanical and Electrical Engineering, Hohai University, Changzhou 213022, China; [email protected] * Correspondence: [email protected] (B.L.); [email protected] (H.C.)

Abstract: Electroactive PVC gel is a new artificial muscle material with good performance that can mimic the movement of biological muscle in an electric field. However, traditional manufacturing methods, such as casting, prevent the broad application of this promising material because they cannot achieve the integration of the PVC gel electrode and core layer, and at the same time, it is difficult to create complex and diverse structures. In this study, a multi-material, integrated direct writing method is proposed to fabricate corrugated PVC gel artificial muscle. Inks with suitable rheological properties were developed for printing four functional layers, including core layers, electrode layers, sacrificial layers, and insulating layers, with different characteristics. The curing conditions of the printed CNT/SMP inks under different applied conditions were also discussed. The structural parameters were optimized to improve the actuating performance of the PVC gel artificial Citation: Luo, B.; Zhong, Y.; Chen, muscle. The corrugated PVC gel with a span of 1.6 mm had the best actuating performance. Finally, H.; Zhu, Z.; Wang, Y. Direct Writing we printed three layers of corrugated PVC gel artificial muscle with good actuating performance. The Corrugated PVC Gel Artificial Muscle proposed method can help to solve the inherent shortcomings of traditional manufacturing methods via Multi-Material Printing Processes. Polymers 2021, 13, 2734. https:// of PVC gel actuators. The printed structures have potential applications in many fields, such as soft doi.org/10.3390/polym13162734 robotics and flexible electronic devices.

Academic Editor: Houwen Keywords: direct writing; PVC gel; artiﬁcial muscle; rheological behavior; integrated printing Matthew Pan

Received: 16 July 2021 Accepted: 13 August 2021 1. Introduction Published: 15 August 2021 Artificial muscle is a material or structure that can undergo various forms of deformation under external excitations (i.e., electricity [1–5], heat [6–8], light [9,10], magnetic Publisher’s Note: MDPI stays neutral field [11,12], fluid pressure [13–15], and chemical stimulation [16]) and output strain and with regard to jurisdictional claims in stress. Artificial muscle is named for its ability to achieve biological muscle-like function, published maps and institutional affil- and it has broad application prospects in fields such as robots [4,13,17–19], flexible elec- iations. tronics [20,21], smart textiles [22], and medical rehabilitation [23,24]. Among the various actuating modes of artificial muscles defined according to the external excitation, the electrical actuating mode has advantages of easy control and a wide application range. Electroactive polymers (EAPs) are a class of electrically actuated artificial muscle materials Copyright: © 2021 by the authors. that are of great interest to researchers. Typical representatives of EAPs include ionic poly- Licensee MDPI, Basel, Switzerland. mer/metal composites (IPMCs), dielectric elastomers (DEs), and electroactive polyvinyl This article is an open access article chloride (PVC) gel. Among these materials, IPMC has advantages of low operating voltage distributed under the terms and (1–3 V) and a response time of seconds; however, its actuating force is limited, the working conditions of the Creative Commons energy density is less than other EAPs, and its durability is poor in dry environments [25]. Attribution (CC BY) license (https:// DE has the advantage of a fast response and can work in air; however, DEs generally creativecommons.org/licenses/by/ 4.0/). require operating voltages of up to several kV to meet electric field strength requirements,

Polymers 2021, 13, 2734. https://doi.org/10.3390/polym13162734 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 16

air; however, DEs generally require operating voltages of up to several kV to meet electric field strength requirements, which increases the risk of electrical breakdown. How- Polymers 2021, 13, 2734 ever, PVC gel has advantages of high output stress and strain, quick response, 2good of 15 thermal stability, moderate working voltage, low power consumption, and a long cycle life [5], as well as the advantages of DE and IPMC. Therefore, it has more potential in the whichapplication increases [26]. the risk of electrical breakdown. However, PVC gel has advantages of high outputDeformation stress principle and strain, of quickPVC-gel response, actuator good is shown thermal in stability,Figure 1; moderate in an applied working elec- voltage,tric field, low PVC power molecules consumption, migrate andto the a longanode, cycle the lifegel [is5], polarized, as well as and the advantagesa Maxwell force of DE is andgenerated, IPMC. Therefore,which results it has in morecreep potential deformation in the of application PVC gel near [26 ].the anode and compression Deformationof PVC gel in principle the thickness of PVC-gel direction. actuator The is PVC shown gel in can Figure return1; in to an its applied original electric shape ﬁeld,under PVC its own molecules elasticity migrate when the to the electric anode, field the is gelturned is polarized, off [27]. and a Maxwell force is generated, which results in creep deformation of PVC gel near the anode and compression of PVC gel in the thickness direction. The PVC gel can return to its original shape under its own elasticity when the electric ﬁeld is turned off [27].

Figure 1. Deformation principle of PVC-gel actuator. (a) Discharge (b) Charge. Figure 1. Deformation principle of PVC-gel actuator. (a) Discharge (b) Charge. The PVC-gel actuators in application include the traditional mesh anode-planar PVC gel-planarThe PVC-gel cathode structureactuators (referredin application to as the incl planarude the PVC traditional gel actuator) mesh [5, 24anode-planar] and, more recently,PVC gel-planar the planar cathode electrode-corrugated structure (referred PVC to gelas the structure planar (referred PVC gel toactuator) as the corrugated [5,24] and, PVCmore gel recently, actuator) the [20 planar,28]. For electrode-corrugated any of the structures, PVC the gel basic structure actuating (referred unit is theto as sandwich the cor- structurerugated PVC as electrode gel actuator) layer–core [20,28]. layer–electrode For any of the layer.structures, A key the common basic actuating feature of unit the is two the typicalsandwich actuators structure is the as existenceelectrode of layer–core pore structures, layer–electrode which are layer. necessary A key for common the actuators feature to deformof the two effectively; typical actuators for example, is the the existence migration of pore of the structures, peak material which to are the necessary valley floor for the or theactuators transfer to of deform planar effectively; materials into for the example, mesh pores. the migration of the peak material to the valleyThe floor current or the manufacturing transfer of planar method materials of PVC into gel the is to mesh mix PVC,pores. dibutyl adipate (DBA), and tetrahydrofuranThe current manufacturing (THF) together method to form aof precursor PVC gel material is to mix and PVC, then castdibutyl the materialadipate into(DBA), molds and totetrahydrofuran obtain films with (THF) designed together shapes to form [5]. a Toprecursor make a material PVC gel and actuator, then cast the traditionalthe material method into molds also requiresto obtain the films fabricated with designed PVC gel shapes films to [5]. be cutTo make and then a PVC manually gel ac- stackedtuator, the with traditional metal electrodes. method Obviously, also requires the castingthe fabricated method PVC is only gel suitable films to for be manufac- cut and turingthen manually simple 2D stacked structures, with and metal it is difficultelectrodes to. produce Obviously, complex, the casting diversified, method flexible is only 3D structures.suitable for To manufacturing overcome this simple issue, additive2D structur manufacturinges, and it is difficult is a possible to produce choice. complex, Rossiter anddiversified, co-workers flexible proposed 3D structures. a filament To additive overcome manufacturing this issue, additive technique manufacturing to create complex is a (3D)possible PVC choice. gel structures. Rossiter and The co-workers 3D printing pr processoposed was a filament introduced, additive where manufacturing a precursor materialtechnique (PVC to create precursor complex powder (3D) mix PVC with gel DIDA) structures. is extruded The into3D printing a thermoplastic process filament was in- fortroduced, 3D printing where [29 a]. precursor However, material the electrode (PVC printing precursor process powder results mix in with many DIDA) difficulties is ex- becausetruded into most a thermoplastic electrode materials filament are for not 3D thermoplastic, printing [29]. However, which isessential the electrode for PVC-gel printing as actuators. Recently, multiple reports have shown that a wide variety of materials have been used in 3D direct writing, including hydro-gels [30], nano-particles [31], polyelectrolytes, ceramics [32], shape memory polymers, and carbon materials [33]. Direct writing is a layer- by-layer assembly technique in which shear-thinning inks are extruded through a nozzle Polymers 2021, 13, 2734 3 of 15

in a programmable pattern, upon which the inks rapidly solidify via gelation, evaporation, or temperature-induced phase change [32]. There are few reports with respect to PVC-gel solution printing by direct writing. Meanwhile, electrode printing, when adopting the direct writing process, has been successful in the presence of a conductive solution that can be cured, such as carbon nanotubes (CNTs) doped polymer and ionic gels [33]. In this work, integrated printing process of electrodes and core layers for fabricating PVC gel artificial muscle were proposed. CNTs doped shape memory polymers (CNT/SMP) were designed for PVC-gel electrode. Sacrificial materials (F127) were used for creating different PVC core structure. Inks with suitable rheological properties were developed for printing four functional layers, including core PVC layers, electrode layers, sacrificial layers, and insulating layers, with different characteristics. The corrugated PVC gel structural parameters were designed and optimized to improve the actuating performance of the PVC gel artificial muscle. Several techniques were used to evaluate the feasibility of the process, including conductivity tests, rheology measurements, and electro-mechanical tests.

2. Experimental Section 2.1. Materials Preparation and Structure Design Polyvinyl chloride (PVC, degree of polymerization 4000) was purchased from Sci- entific Polymer Products Inc., New York, NY, USA. Dibutyl adipate (DBA) was obtained from Hubei Jusheng Technology Co., Ltd., Tianmen, P.R. China. Tetrahydrofuran (THF) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd., Tianjin, P.R. China. Pluronic F-127 was purchased from Sigma-Aldrich, St. Louis, MO, USA. The multi-walled CNTs (Time Nano China, TSW3, diameter 10–20 nm, length 0.5–2 µm, >98%) were from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences, Beijing, P.R. China. Dimethy- lacetamide (DMAC) was obtained from Aladdin Reagent Co., Ltd., Shanghai, P.R. China. Shape memory polymer (SMP, DiAPLEX MM4520) was from Mitsubishi Co., Ltd., Tokyo, Japan. Ecoflex 00-20 was purchased from Smooth-On Inc., Macungie, PA, USA. As shown in Figure2a, CNT doped shape memory polymers (CNT/SMP) were chosen for printing PVC-gel electrode because the electrodes’ hardness is suitable [31]. The Ecoflex silicone materials, which possess very high insulation performance, were designed for connecting PVC-gel actuator units. To fabricate the flexible artificial muscle structure, it was first necessary to prepare inks suitable for direct writing and to satisfy the functional requirements for core layers, sacrificial layers, electrode layers, and insulating layers. Using a printing device with a pneumatic ink extrusion system (Figure S1), each functional layer was printed in sequence with the prepared inks to produce the composite structure. In particular, pluronic F-127 is a polyethylene oxide (PEO)-polypropylene oxide (PPO)- polyethylene oxide (PEO) triblock copolymer. As shown in Figure2b, F-127 has thixotropic properties. When the ink was prepared, the molecules cross-linked with each other to form gel networks. When the gel was subjected to shear stress, the networks broke down and the ink became fluid-like so it could be conveniently extruded through a nozzle. After the shear stress was removed, the ink quickly turned to gel with rigidity that could help maintain the shape of the printed structure. In this paper, the F-127 gel inks were printed for constructing a corrugated PVC-gel core structure. PolymersPolymers 20212021, 13,,13 x ,FOR 2734 PEER REVIEW 4 of4 16 of 15

(a)

(b)

FigureFigure 2. 2.(a)( aSchematic) Schematic diagram diagram of ofthe the printing printing proce processss of ofthe the artificial artiﬁcial muscle muscle structure. structure. (b) ( bSchematic) Schematic diagram diagram of ofthe the thixotropicthixotropic property property of ofthe the F-127 F-127 gel gel ink. ink.

TheThe PVC PVC gel gel ink ink consisted consisted of ofthree three constitu constituentent materials: materials: PVC, PVC, DBA, DBA, and and THF. THF. For For preparationpreparation of ofthe the gel gelink, ink, the thethree three constituen constituentsts were were proportionally proportionally mixed mixed (referring (referring to researchto research on the on relationship the relationship between between the prop theortion proportion of PVC of gel PVC and gel its and properties its properties in lit- in eratureliterature [34,35], [34, 35the], mass the mass ratio ratio used used in this in this paper paper was was PVC:DBA:THF PVC:DBA:THF = 1:7:12), = 1:7:12), and and then then thethe solution solution was was stirred stirred by by a a magnetic magnetic stirrer stirrer for 2–32–3 daysdays until until the the gel gel became became homogenous, homogenous,colorless, colorless, and and transparent. transparent. F-127 F-127 gel inkgel ink was was comprised comprised of F-127 of F-127 and and deionized deionized water. water.During During the preparationthe preparatio process,n process, F-127 F-127 was was mixed mixed with with deionized deionized water water in proportion in pro- portionand then and thethen mixture the mixture was stirred was stirred for 15 minfor 15 at min 2000 at rpm 2000 by rpm a planetary by a planetary mixer (HM800)mixer (HM800)to obtain to theobtain colorless, the colorless, transparent transpar gel.ent When gel. preparing When preparing CNT/SMP CNT/SMP composite composite ink, SMP ink,was SMP ﬁrst was mixed first withmixed DMAC, with DMAC, which which served served as a solvent, as a solvent, and the and mixture the mixture was heatedwas heated(60 ◦ C)(60 and°C) magneticallyand magnetically stirred stirred (600 (600 rpm) rpm) for approximatelyfor approximately 4 h 4 such h such that that the the SMP SMPparticles particles were were completely completely dissolved dissolved to form to form a transparent a transparent solution. solution. Subsequently, Subsequently, CNTs CNTswere were added added proportionally proportionally to the to solution the solu andtion theand mixture the mixture was dispersed was dispersed by ultrasound by ul- trasoundfor approximately for approximately 3 h until 3 h the until carbon the carb nanotubeson nanotubes were evenly were evenly dispersed dispersed in the solution.in the ◦ solution.Finally, Finally, using a using magnetic a magnetic stir heater stir (60heaterC, (60 600 °C, rpm), 600 therpm), solution the solution was concentrated was concen- for tratedapproximately for approximately 5 h to produce 5 h to produce ink containing ink containing 30 wt.% 30 solid wt.% phase solid thatphase could that becould used be for used for printing. Ecoflex silicone consisted of two components: Ecoflex-A and Ecoflex-B.

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printing. Ecoflex silicone consisted of two components: Ecoflex-A and Ecoflex-B. To prepare the Ecoflex ink, the A and B components were mixed at a mass ratio of 1:1 and stirred.

2.2. Performance Characterization of the Inks When analyzing the electrical properties of the CNT/SMP composites, CNT/SMP composites with different CNT contents were made into long strips (40 mm × 5 mm × 1 mm). The resistance of the strips was measured using a source meter (2450 SourceMe- ter, Keithley, Tektronix, Cleveland, OH, USA) and the conductivity of composites with different proportions was calculated. The rheological behaviors of the prepared inks were measured by a rheometer (MCR 302, Anton Paar, Graz, Austria) fitted with a parallel-plate geometry (PP35Ti, diameter of 35 mm and gap of 0.6 mm) at 25 ◦C. The apparent viscosity, storage modulus (G0), and loss modulus (G00) of the PVC gel ink, F-127 gel ink, CNT/SMP composite ink, and Ecoflex ink were measured as a function of the angular frequency from 0.628 rad/s to 62.8 rad/s under fixed strain (1%) and shear stress (from 0.1 to 100 Pa) in oscillatory mode at 1 Hz.

2.3. Direct Writing Process In this paper, the 3D printing used a multi-nozzle direct writing device (Figure S1). In the ﬁrst step, the 3D models were established in CAD software (SolidWorks 2016) and then imported software to generate G codes. The second step was to install the syringe with ink for printing to the designed position in the direct writing device. In the third step, the designed structure was printed layer-by-layer by controlling the movement of the 3D positioning stage according to G codes and adjusting the output air pressure to control the extrusion speed of the ink.

2.4. Post-Processing of the Printed Structures After printing the composite structures of the PVC gel actuators, it was necessary to immerse them in water to remove the sacrificial material and obtain the designed corrugated PVC gel. Post-processing by curing was also needed for the printed CNT/SMP composites (for both the curing of the shape and the formation of conductive network char- acterized by improved conductivity). Therefore, the curing characteristic of the CNT/SMP composite ink and the relationships between the conductivity and time at different curing temperatures were studied. We used 10 wt.% CNT/SMP ink to produce rectangular sheet samples (40 mm × 20 mm × 0.4 mm) that were heated in a vacuum oven or stored at room temperature. At regular intervals, the shape curing was observed, and the resistance was measured to determine the relationship between the conductivity and time. To study integrated printing of PVC gel artificial muscles, we combined the above two post-processing methods, proposing a method for placing the printed structure at room temperature first and then heating in a water bath, and studied the effects of this method on the dissolution of the sacrificial layers and the curing of the CNT/SMP composite ink.

2.5. Performance Characterization of the PVC Gel Artificial Muscle The actuating performance of the PVC gel artificial muscle was measured by a test system (Figure S2, Supporting Information). The displacement of the artificial muscle with a laser sensor (LK-G500, KEYENCE, Osaka, Japan) was measured using a signal generator (DG4062, RIGOL, Beijing, P.R. China) and voltage amplifier (MODEL 20/20C, TREK, Waterloo, WI, USA) to apply voltage to the electrode layers of the artificial muscle.

3. Results and Discussion 3.1. Printability of Printing Inks Rheology measurements were conducted for evaluating printability of inks. As shown in Figure3a,b. The apparent viscosity of the prepared inks decreased with an increase in the angular frequency, proving that they were shear-thinning non-Newtonian ﬂuids Polymers 2021, 13, x FOR PEER REVIEW 6 of 16

crease in the angular frequency, proving that they were shear-thinning non-Newtonian fluids appropriate for direct writing. The G’ of the PVC gel ink was greater than its G”, and their difference decreased with an increase in angular frequency. Therefore, when the shear effect was strong and the angular frequency was high, the PVC gel ink had excellent fluidity and, thus, good filling performance. Alternatively, when the shear effect was weaker and the angular frequency decreased, the difference between G’ and G” increased, which resulted in a decrease in fluidity; therefore, the printed shape was easy to maintain. These characteristics are also demonstrated in Figure 4a. The PVC-gel ink exhibited solid-like gel behavior (G’ > G’’) at stresses (0.1– 29.81 Pa) and liquid-like gel behavior (G’ < G’’) at stresses >29.81 Pa, which indicated that this ink is suitable for direct writing [36]. As for F-127 gel ink, at the same angular frequency, with an increase in F-127, the viscosity of the F-127 gel ink increased, and the printability increased. The G’ of each experimental group was greater than G’’, indicating that the material was in a non-flowing state. With the increase in F-127 content, this behavior was more obvious, and thus, the shape retention ability of the printed structure and the printing accuracy were better. Additionally, 40 wt.% F-127 gel ink storage and loss modulus decreased steadily when the stress was increased from a very low stress of 0.1 Pa up to 10 Pa in Figure 4b,and the 40 wt.% F-127 gel ink exhibited solid-like gel be- Polymers 2021, 13, 2734 havior (G’ > G’’) at stresses (0.1–7.41 Pa) and liquid-like gel behavior (G’ < G’’) at stresses6 of 15 >7.41 Pa, which indicated the thixotropic properties. As the main purpose of using this material was to obtain the required pore structure in the core layers with high printing appropriateprecision, in for the direct following writing. study, The G 400 of wt.% the PVCF-127 gel gel ink ink was was greater used thanto print its Gthe00, andsacrificial their differencelayers. decreased with an increase in angular frequency.

Figure 3. Preparation and performance characterization of the inks. (a) Viscosity and (b) moduli (storage modulus G0 and Figure 3. Preparation and performance characterization of the inks. (a) Viscosity and (b) moduli (storage modulus G’ and loss modulus G00) of the PVC gel ink, F-127 gel ink with different concentrations, pure SMP ink, CNT/SMP ink, and Ecoflex loss modulus G’’) of the PVC gel ink, F-127 gel ink with different concentrations, pure SMP ink, CNT/SMP ink, and Eco- inkflex as ink a function as a function of the of angular the angular frequency. frequency. Therefore, when the shear effect was strong and the angular frequency was high, the PVC gel ink had excellent fluidity and, thus, good filling performance. Alternatively, when the shear effect was weaker and the angular frequency decreased, the difference between G0 and G00 increased, which resulted in a decrease in fluidity; therefore, the printed shape was easy to maintain. These characteristics are also demonstrated in Figure4a. The PVC-gel ink exhibited solid-like gel behavior (G0 > G00) at stresses (0.1–29.81 Pa) and liquid-like gel behavior (G0 < G00) at stresses >29.81 Pa, which indicated that this ink is suitable for direct writing [36]. As for F-127 gel ink, at the same angular frequency, with an increase in F-127, the viscosity of the F-127 gel ink increased, and the printability increased. The G0 of each experimental group was greater than G00, indicating that the material was in a non-flowing state. With the increase in F-127 content, this behavior was more obvious, and thus, the shape retention ability of the printed structure and the printing accuracy were better. Additionally, 40 wt.% F-127 gel ink storage and loss modulus decreased steadily when the stress was increased from a very low stress of 0.1 Pa up to 10 Pa in Figure4b, and the 40 wt.% F-127 gel ink exhibited solid-like gel behavior (G0 > G00) at stresses (0.1–7.41 Pa) and liquid-like gel behavior (G0 < G00) at stresses >7.41 Pa, which indicated the thixotropic properties. As the main purpose of using this material was to obtain the required pore structure in the core layers with high printing precision, in the following study, 40 wt.% F-127 gel ink was used to print the sacrificial layers. PolymersPolymers2021 2021, 13, 13, 2734, x FOR PEER REVIEW 77 of of 15 16

FigureFigure 4. 4.Plot Plot of of the the storage storage modulus modulus andand lossloss modulusmodulus as a function of shear stress. stress. (a (a) )PVC-gel PVC-gel ink, ink, (b (b) )F-127 F-127 gel gel ink, ink, (c) Ecoflex ink, (d) CNT/SMP composite ink. (c) Ecoﬂex ink, (d) CNT/SMP composite ink.

TheThe rheological rheological properties properties of of the the Ecoflex Ecoflex ink ink were were measured measured after after storage storage at at room room temperaturetemperature (25 (25◦ C)°C) for for 20 20 min. min. The The ink ink reached reached a a semi-cured semi-cured state state since since the the molecular molecular chainschains of of the the two two components components had had gradually gradually crosslinked, crosslinked, and and the Gthe0 of G’ the of Ecoflex the Ecoflex ink was ink greaterwas greater than G than00. Therefore, G’’. Therefore, the ink the can ink be can used be to used print to stable print lines.stable Forlines. both For pure both SMP pure inkSMP and ink CNT/SMP and CNT/SMP ink with ink 10 with wt% 10 carbon wt% carbon nanotubes, nanotubes, their G their00 were G’’ greater were greater than their than respectivetheir respective G0, demonstrating G’, demonstrating strong strong fluidity flui suitabledity suitable for printing for printing the planar the structureplanar struc- in thisture paper in this that paper can alsothat producecan also goodproduce surface good quality. surface Additionally, quality. Additionally, after CNT after loading, CNT theloading, viscosity the and viscosity moduli and (G moduli0 and G 00(G’) of and the G’’) ink wereof the greater ink were than greater that of than pure that SMP of ink,pure showingSMP ink, that showing CNT had that a goodCNT had thickening a good effect. thickening The 10 effect. wt% CNT/SMPThe 10 wt% composite CNT/SMP ink’s com- fluidityposite ink’s is indicated fluidity in is Figure indicated4d; itsin lossFigure modulus 4d; its wasloss highermodulus than was the higher storage than modulus the stor- atage stresses modulus (0.1–10 at stresses Pa). We (0.1–10 systematically Pa). We system evaluatedatically the evaluated printability the of printability inks by process of inks experiments.by process experiments. The influence The of influence the extrusion of th pressuree extrusion (P) pressure and nozzle (P) and moving nozzle speed moving (V) onspeed thefilament (V) on the width filament of the width printed of the inks printed are listed inks in are Figure listed5. Atin Figure the lowest 5. At extrusion the lowest pressureextrusion and pressure highest nozzleand highest moving nozzle speed, movi a minimumng speed, fiber a minimum diameter wasfiber achieved diameter with was theachieved nozzle diameterwith the (0.29nozzle mm) diameter for each (0.29 ink. mm) The processfor each parameters ink. The process used in parameters the subsequent used printingin the subsequent experiment printing are as follows: experiment PVC-gel are inksas follows: (P = 20 PVC-gel KPa, V = inks 12 mm/s); (P = 20 F127-gelKPa, inks (PV = = 37012 mm/s); KPa, V F127-gel = 12 mm/s); inks (P CNT/SMP = 370 KPa, inks V = 12 (P mm/s); = 260 KPa, CNT/SMP V = 6 mm/s);inks Ecoflex inks (P(P = = 180 260 KPa, KPa, V V = = 10 6 mm/s); mm/s). Ecoflex inks (P = 180 KPa, V = 10 mm/s).

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FigureFigure 5. 5. InfluenceInﬂuence of of the the extrusion extrusion pressure pressure and and nozzl nozzlee moving moving speed speed on on the the printed printed diameter diameter (a) PVC gel ink, (b) F-127 (agel) PVC ink, gel (c) ink, CNT/SMP (b) F-127 ink, gel ( dink,) Ecoﬂex (c) CNT/SMP ink. ink, (d) Ecoflex ink.

3.2.3.2. Research Research Results Results of of Curing Curing Methods Methods TheThe solidification solidification of of electrode electrode and and the the integrated integrated structure structure shown shown in in Figure Figure 22 is is the the keykey to to the the success success of of printing printing PVC-gel PVC-gel actuat actuators.ors. The The curing curing properties properties of of the the CNT/SMP CNT/SMP inkink were were analyzed analyzed (including(including bothboth shape shape curing curing and and conductive conductive network network formation formation char- characterizedacterized by by improved improved conductivity) conductivity) at at different different temperatures. temperatures. After heating at at 80 80 °C◦C forfor 10 10 min, min, 60 60 °C◦ Cfor for 1515 min, min, or orstorage storage at room at room temperature temperature (25 (25°C)◦ forC) for48 h, 48 the h, theshape shape of theof thecomposites composites was was completely completely cured cured and and their their surface surface was was uniform uniform and and dense. dense. The The conductivity–timeconductivity–time relationships relationships of of the the CNT/SM CNT/SMPP composite composite ink ink at at different different temperatures temperatures areare shown shown in in Figure Figure 6a,b.6a,b. As As shown shown in in the the figu figure,re, higher higher temperatures temperatures resulted resulted in in a a faster faster ◦ increaseincrease in in conductivity conductivity and and a higher a higher final final conductivity. conductivity. After After heating heating at 80 at °C 80 forC 3 forh, the 3 h, ◦ conductivitythe conductivity tended tended to be tostable be stable up to up5.06 to S/m. 5.06 After S/m. heating After heating at 60 °C at 60for 4C h, for the 4 h,con- the ductivityconductivity reached reached 3.61 S/m. 3.61 At S/m. room At temperat room temperature,ure, the curing the curingspeed speedwas too was slow too to slow meet to themeet requirements the requirements of this of paper. this paper. However, However, because because of compatibility of compatibility issues issues of of multiple multiple differentdifferent material material systems, systems, the the layer-by-layer layer-by-layer thermal thermal curing curing method method was was not not suitable suitable for for thethe integrated integrated printing printing of of multi-material multi-material co compositemposite structures. structures. For For example, example, the the thermal thermal curingcuring of of the the upper upper electrode electrode layer layer may may cause cause th thee sacrificial sacrificial layer layer to to lose lose moisture moisture quickly quickly andand melt melt before before the the shape shape of of the the upper upper electrod electrodee layer layer is is solidified, solidified, leading leading to to collapse collapse of of the overall structure. Similarly, repeated heating at high temperature for a long time may also affect the performance of PVC gel core layers. Therefore, we proposed a step-wise

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Polymers 2021, 13, 2734 9 of 15 the overall structure. Similarly, repeated heating at high temperature for a long time may also affect the performance of PVC gel core layers. Therefore, we proposed a step-wise curing method for the printed electrode layer, which included room temperature storage curing method for the printed electrode layer, which included room temperature storage for 48 h to cure the shape and then uniform heat in a water bath after printing. Uniform for 48 h to cure the shape and then uniform heat in a water bath after printing. Uniform heating was used to avoid the effects of repeated heating on the performance of the PVC heating was used to avoid the effects of repeated heating on the performance of the PVC gel core layers. Water bath heating allowed the sacrificial material to be rapidly dissolved gel core layers. Water bath heating allowed the sacrificial material to be rapidly dissolved and helped uniformly heat the electrode. Regarding the heating temperature, long-term and helped uniformly heat the electrode. Regarding the heating temperature, long-term heating of the structure at 80 °C could greatly attenuate the performance of PVC gel core heating of the structure at 80 ◦C could greatly attenuate the performance of PVC gel core layers, while heating the structure at 60 °C had little effect. Therefore, 60 °C was selected layers, while heating the structure at 60 ◦C had little effect. Therefore, 60 ◦C was selected as the heating temperature. For the printed electrode layers, the first step of incubating at as the heating temperature. For the printed electrode layers, the first step of incubating at room temperature for 60 h was to ensure that the shapes of the samples were completely room temperature for 60 h was to ensure that the shapes of the samples were completely cured. As shown in Figure 6c, the conductivity increased slowly at room temperature, cured. As shown in Figure6c, the conductivity increased slowly at room temperature, and and increased rapidly when the samples were heated in a water bath. After the samples increased rapidly when the samples were heated in a water bath. After the samples were heatedwere in heated a water in batha water at 60 bath◦C forat 60 30 °C min, for the 30 min, conductivity the conductivity reached 1.87 reached S/m, 1.87 which S/m, meets which themeets requirements the requirements for conductivity for conductivity of the electrode of the electrode material inmaterial actuators. in actuators. Therefore, Therefore, when separatelywhen separately printing theprinting electrode the electrode layers, the layers, selected the curing selected condition curing wascondition 80 ◦C forwas 3 80 h. For°C for integrated3 h. For printingintegrated of PVCprinting gel actuators,of PVC gel the actu curingators, method the curing for the method electrode for layersthe electrode was firstlayers storage was at first room storage temperature at room fortemperature 60 h until for the 60 shape h until of the newlyshape of printed the newly electrode printed layerelectrode was completely layer was cured,completely printing cured, of theprinting other of layers, the other and layers, after the and shape after ofthe the shape top of electrodethe toplayer electrode was completelylayer was completely cured, heating cured, of the heating entire of structure the entire in a structure 60 ◦C water in a bath 60 °C forwater 30 min. bath for 30 min.

Figure 6. The conductivity of the CNT/SMP composite inks as functions of time (a) at 80 ◦C, 60 ◦C, (b) room temperature Figure 6. The conductivity of the CNT/SMP composite inks as functions of time (a) at 80 °C, 60 °C, (b) room temperature (25 ◦C), and (c) stepwise curing conditions. (25 °C), and (c) stepwise curing conditions.

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3.3. 3D Printing Processes of Flexible PVCPVC Gel ArtificialArtificial Muscle StructuresStructures The printingprinting methodsmethods of of the the core core layers, layers, electrode electrode layers, layers, and and whole whole artificial artificial muscles mus- werecles were studied. studied. Initially, Initially, the printingthe printing method method of the of corrugated the corrugated PVC PVC gel core gel layerscore layers was studied.was studied. A corrugated A corrugated F-127 F-127 gel sacrificial gel sacrificial layer layer was printedwas printed and thenand then the PVC the PVC gel was gel printedwas printed on top, on filling top, filling the pores the ofpores the sacrificial of the sacrificial layer. The layer. PVC The gel wasPVC cured gel was at room cured tem- at ◦ peratureroom temperature (25 C) for (25 approximately °C) for approximately 12 h. The resulting12 h. The composite resulting composite structure was structure immersed was inimmersed water to in remove water to the remove sacrificial the sacrificial material, andmaterial, after and drying, after a drying, corrugated a corrugated PVC gel corePVC layergel core was layer produced, was produced, as shown as in shown Figure 7ina. Figure The fabricated 7a. The corrugatedfabricated corrugated PVC gel core PVC layer gel hadcore goodlayer elasticityhad good and elasticity transparency and transparency with an obvious with corrugatedan obvious shape. corrugated The shape shape. of The the corrugationshape of the incorrugation the core layers in the is core shown layers in Figure is shown7b. Toin Figure study the7b. effectTo study of the the corrugation effect of the scalecorrugation on the actuatingscale on the performance actuating performanc of the PVCe gel of corethe PVC layers, gel wecore designed layers, we and designed printed fiveand groupsprinted of five corrugated groups of PVC corrugate gels withd PVC different gels with spans, different b. The dimensionsspans, b. The of dimensions each group wereof each as group follows: were a = as 0.2 follows: mm, h =a 2= mm,0.2 mm, d = h 0.3 = 2 mm, mm, and d = b 0.3 = 0.8,mm, 1.2, and 1.6, b = 2.0, 0.8, 2.4 1.2, mm. 1.6, The 2.0, × bottom2.4 mm. of The the bottom core layer of the had core a planar layer sizehad ofa planar 40 mm size20 of mm.40 mm × 20 mm.

Figure 7. Cont.

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Figure 7. PrintingPrinting process process and and printed printed structures. structures. (a)( aFinished) Finished corrugated corrugated PVC PVC gel gelcore core layer. layer. (b) (Designedb) Designed shape shape of the of thecorrugation corrugation in core in core layers. layers. (c) Finished (c) Finished CNT/SMP CNT/SMP composite composite electrode electrode layer. layer.(d) Printing (d) Printing process process of a single of a single layer layerPVC PVCgel actuator. gel actuator. (e) Printing (e) Printing process process of a ofmultilayer a multilayer PVC PVC gel gelactuator. actuator.

Additionally, wewe printedprinted CNT/SMPCNT/SMP compositecomposite electrodeelectrode layerslayers (40(40 mm mm× ×20 20 mmmm× × 0.2 mm)mm) andand thenthen curedcured themthem atat 8080 ◦°CC forfor 33 h.h. StainlessStainless steelsteel wirewire andand conductiveconductive silversilver pastepaste werewere usedused to to wire wire them them together. together. As As shown shown in Figure in Figure7c, the 7c, finished the finished electrode electrode layer hadlayer a had dense a dense and smooth and smooth surface surface withsuitable with suitable rigidity. rigidity. Based on thethe aboveabove research,research, wewe printedprinted thethe artificialartificial musclemuscle structurestructure shownshown inin Figure2 2aa inin an an integrated integrated way. way. From From the the perspective perspective of of manufacturing, manufacturing, the the functional functional layerslayers ofof this this structure structure were were integrated integrated printings printings layer-by-layer layer-by-layer at different at different times to times produce to aproduce complete a complete structure. structure. Regarding Regarding the working the principle, working principle, all of the electrode all of the layerselectrode in contact layers within contact the corrugated with the sidecorrugated of the corrugated side of the PVC corrugated gel core layers PVC shouldgel core be layers connected should to thebe anode,connected while to the all ofanode, the electrode while all layersof the inelectrod contacte layers with thein contact planar sidewith ofthe the planar corrugated side of PVCthe corrugated gel core layers PVC gel should core belayers connected should tobethe connected cathode. to Tothe avoid cathode. mutual To avoid interference mutual orinterference coupling ofor differentcoupling actuatingof different units actuatin wheng powered,units when the powered, insulating the layersinsulating of silicone layers Ecoflexof silicone were Ecoflex placed were between placed adjacent between actuating adjacent units. actuating units. As shown inin FigureFigure7 7d,d, single single layer layer actuators actuators (actuating (actuating units) units) were were first first printed. printed. At At the beginning, the bottom electrode layer was printed and then heated in a vacuum oven the beginning,◦ the bottom electrode layer was printed and then heated in a vacuum oven at 80 °CC for 3 h to completely cure. Subsequent Subsequently,ly, the sacrificialsacrificial layerlayer waswas printedprinted overover thethe bottom electrode layer. The The PVC PVC gel gel core core layer layer was was then then printed printed on on the the sacrificial sacrificial layer layer to tofully fully fill fill and and cover cover the the pores pores of of the the sacrificia sacrificiall layer, layer, and and the the sample sample was stored atat roomroom temperaturetemperature forfor approximately approximately 12 12 h untilh until the the THF THF was was sufficiently sufficiently volatilized volatilized and theand PVC the gel was cured. The top electrode layer was then printed on the core layer, and the sample PVC gel was cured. The top electrode layer was then printed on the core layer, and the was stored at room temperature for 60 h until the top electrode layer was cured. After sample was stored at room temperature for 60 h until the top electrode layer was cured. printing, the following post-processing of the sample was performed. First, the composite After printing, the following post-processing of the sample was performed. First, the structure was heated in a water bath at 60 ◦C for 30 min until the sacrificial layer was composite structure was heated in a water bath at 60 °C for 30 min until the sacrificial completely dissolved. Next, after drying, the edges of the sample were cut to avoid contact layer was completely dissolved. Next, after drying, the edges of the sample were cut to between the two electrode layers at the edges, which may result in short circuit of the avoid contact between the two electrode layers at the edges, which may result in short actuator during testing. Finally, stainless steel wire and conductive silver paste were used circuit of the actuator during testing. Finally, stainless steel wire and conductive silver to create a lead from the two electrode layers. After heating at 60 ◦C for approximately paste were used to create a lead from the two electrode layers. After heating at 60 °C for 15 min to solidify the conductive silver paste, the integrated printed corrugated PVC gel approximately 15 min to solidify the conductive silver paste, the integrated printed cor- actuating unit was complete. rugated PVC gel actuating unit was complete.

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On this basis, we studied the printing method of a multilayer PVC gel actuator (two layers and three layers), and Figure7e illustrates the printing process with a triple layer actuator as an example. Insulating layer 1 was printed on actuating unit 1 and then stored at room temperature for 4 h for complete cure. Similarly, the bottom electrode layer, sacrificial layer, core layer, and top electrode layer were printed in sequence, wherein the electrode layers and core layer were both cured at room temperature before printing the Polymers 2021, 13, x FOR PEER REVIEWnext functional layer, thereby, completing the printing of actuating unit 2. The process12 of 16 continued to superimpose the required number of actuating units. After the overall structure was printed and the electrode layers cured, the entire structure was heated in a water bath at 60 ◦C for 30 min to remove the sacrificial layer. The sample was then trimmed On this basis, we studied the printing method of a multilayer PVC gel actuator and leaded to complete fabrication of the multilayer actuators. (two layers and three layers), and Figure 7e illustrates the printing process with a triple 3.4.layer Performance actuator as Characterization an example. Insulating of the Printed layer Structures 1 was printed on actuating unit 1 and then stored at room temperature for 4 h for complete cure. Similarly, the bottom electrode To test the actuating performance of the printed corrugated PVC gel core layers, three layer, sacrificial layer, core layer, and top electrode layer were printed in sequence, pieces of PVC gel from each of the five experimental groups with varying spans and four wherein the electrode layers and core layer were both cured at room temperature before pieces of zinc foil electrode (40 mm × 20 mm × 0.07 mm) were superimposed to make a metalprinting electrode-based the next functional PVC gel layer, actuator. thereby, At different completing voltages the (400 printing to 800 of V, actuating 1 Hz, 50% unit duty 2. cycleThe process square wave),continued the strainto superimpose of the actuators the requ wasired tested number as a functionof actuating of the units. corrugated After the spanoverall of corestructure layers, was as shown printed in and Figure the8 a.electrode The strain layers of each cured, actuator the entire increased structure with anwas increaseheated in voltage.a water bath As the at corrugated60 °C for 30 span min increased,to remove the the strain sacrificial at each layer. voltage The exhibitedsample was a singlethen trimmed peak form and that leaded increased to complete first and fabrication then decreased. of the multilayer The corrugated actuators. PVC gel with a span of 1.6 mm had the best actuating performance, and the strain of the actuator using this3.4. kind Performance of PVC Characterization gel core layer reached of the Printed 9.9% at Structures 800 V. Overall, the PVC gel printed using the additiveTo test manufacturing the actuating methodperformance described of th ine thisprinted paper corrugated had good electro–deformationPVC gel core layers, performance.three pieces Additionalof PVC gel experiments from each of in the this fi studyve experimental used a corrugated groups PVC with gel varying with a spanspans sizeand 1.6 four mm. pieces of zinc foil electrode (40 mm × 20 mm × 0.07 mm) were superimposed to makeTo a prove metal the electrode-based applicability ofPVC the gel printed actuator. CNT/SMP At different composite voltages electrode, (400 to 800 we V, super- 1 Hz, imposed50% duty three cycle pieces square of wave), corrugated the strain PVC of gel th withe actuators the 1.6 was mm tested span andas a fourfunction pieces of ofthe CNT/SMPcorrugated composite span of core electrode layers, to as form shown a CNT/SMP in Figure composite 8a. The strain electrode-based of each actuator PVC gel in- actuatorcreased andwith tested an increase the strain in voltage. versus voltageAs the corrugated (400 to 800 span V, 1 Hz,increased, 50% duty the cyclestrain square at each wave).voltage Figure exhibited8b shows a single a comparison peak form that of the increased actuating first performance and then decreased. of the CNT/SMP The corru- compositegated PVC electrode-based gel with a span actuator of 1.6 mm and had a metal the best electrode-based actuating performance, actuator with and the the same strain kindof the of actuator core layer. using The this strain kind of theof PVC two gel actuators core layer at the reached same voltage9.9% at were800 V. similar. Overall, The the strainPVC ofgel the printed CNT/SMP using composite the additive electrode-based manufacturing actuator method reached described 10.3% atin 800this V, paper proving had thatgood the electro–deformation performance of the printedperformance. CNT/SMP Addi compositetional experiments electrode wasin this similar study to thatused of a thecorrugated metal electrode, PVC gel meeting with a span the requirements size 1.6 mm. of this study.

Figure 8. Cont.

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Figure 8. Performance of the printed structures. (a) The strain–span curves of metal electrode-based PVC gel actuators atFigure different 8. Performance voltages. ( bof) the The printed strain–voltage structures. curves (a) The of thestrain–span CNT/SMP curves composite of metal electrode–based electrode-based actuator PVC gel andactuators a metal at electrode-baseddifferent voltages. actuator (b) The with strain–voltage the same kind curves of core of layer. the CNT/SMP (c) The strain–stress composite curves electrode–based of the PVC gelactuator actuators and witha metal different elec- numberstrode-based of layersactuator under with voltage the same from kind 200 of V core to 800 layer. V. (c) The strain–stress curves of the PVC gel actuators with different numbers of layers under voltage from 200 V to 800 V. For testing the actuating performance of the corrugated PVC gel actuators obtained by integratedTo prove printing, the applicability the strain–voltage of the print (400ed to 800CNT/SMP V, 1 Hz, composite 50% duty cycleelectrode, square we wave) su- perimposedrelationship withoutthree pieces load of and corrugated the strain–load PVC gel (0 towith 70 g)the relationship 1.6 mm span (constant and four application pieces of CNT/SMPof 800 V, 1 Hz,composite 50% duty electrode cycle square to form wave) a CN ofT/SMP the actuators composite with electrode-based varied number ofPVC layers gel (1actuator to 3) were and measured.tested the strain As shown versus in voltage Figure 8(400c, at to the 800 same V, 1 voltage Hz, 50% and duty as thecycle number square wave).of layers Figure increased, 8b shows the strain a comparison gradually decreased:of the actuating at 800 performance V, the strain ofofthe the single CNT/SMP layer compositeactuator was electrode-based as high as 10.9%, actuator the and strain a metal of the electrode-based double layer actuator actuator was with up the to 8.5%,same kindand theof core strain layer. of the The triple strain layer of the actuator two actuators reached at 8.0%. the same Since voltage the insulating were similar. layers The in strainthe multilayer of the CNT/SMP structure increasedcomposite the electrode- total thickness,based actuator the strain reached of the actuators 10.3% at typically 800 V, provingdecreased that to the some performance extent as the of numberthe print ofed layers CNT/SMP increased. composite As the electrode load increased, was similar the tostrain thatgradually of the metal decreased, electrode, and meeting the speed the graduallyrequirements slowed. of this The study. fewer number of layers, the fasterFor testing the decreasing the actuating speed. performance When subjected of the corrugated to the same PVC stress gel (0 actuators to 300 kPa) obtained at the variableby integrated voltage printing, (200 to the 800 strain–voltage V), the strain increased(400 to 800 with V, the1 Hz, number 50% duty of layers. cycle Assquare the wave)number relationship of superimposed without layers load increased, and the thestrain–load total output (0 to force 70 g) of therelationship actuator increased,(constant applicationso the load carrying of 800 V, capacity 1 Hz, increased50% duty and, cycle thus, square the negative wave) of effect the ofactuators load on thewith actuating varied numberstrain decreased of layers accordingly. (1 to 3) were The measured. performance As shown of our in printed Figure actuators8c, at the issame better voltage than thatand asof the actuatorsnumber of made layers by increa castingsed, reported the strain in the gradually literature decreas [37]. ed: at 800 V, the strain of the single layer actuator was as high as 10.9%, the strain of the double layer actuator was 4. Conclusions up to 8.5%, and the strain of the triple layer actuator reached 8.0%. Since the insulating layersIn in summary, the multilayer we used structure direct writing increased to printthe total a series thickness, of flexible the PVCstrain gel of artificialthe actuators mus- typicallycles with gooddecreased actuating to some performance extent as in the the numb integrateder of way.layers Inks increased. with tailored As the rheological load in- propertiescreased, the met strain the correspondinggradually decreased, functional and requirements. the speed gradually PVC gel slowed. ink, F-127 The gel fewer ink, numberCNT/SMP of layers, composite the faster ink, andthe decreasing Ecoflex ink sp wereeed. used When for subjected printing to core the layers,same stress sacrificial (0 to 300layers, kPa) electrode at the variable layers, andvoltage insulating (200 to layers, 800 V), respectively. the strain increased The inks werewith prepared,the number char- of layers.acterized, As andthe evaluatednumber of for superimposed curing. Using layers these increased, inks, multi-material the total output composite force artificial of the

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muscle structures with complex geometries were fabricated. Performance testing of these structures demonstrated the feasibility of the proposed manufacturing method. Our flexible PVC gel artificial muscles with good actuating performance prepared by integrated printing have potential application in soft robotics, medical rehabilitation, and wearable electronic devices. The proposed method of integrated printing artificial muscles intro- duces a new route to solve the inherent problems of the traditional fabricating methods of PVC gel, which lays a foundation for the broad application of this new artificial muscle material in various fields.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/polym13162734/s1, Figure S1. Experimental setup for the direct writing process; Figure S2. Composition of 3D printing system; Figure S3. Schematic illustration of the system used to test the PVC gel artificial muscle actuating performance. Video S1: Actuating performance of the triple layer printed structures. Author Contributions: Writing—original draft preparation, B.L.; writing—review and editing, Y.Z.; supervision, H.C.; formal analysis, Z.Z.; investigation, Y.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded byNational Natural Science Foundation of China, grant number (91648110); National Natural Science Foundation of Hunan Province, grant number (2020jj5520, 2020jj4526); Postdoctoral Science Foundation of China, grant number (2021M690871); Excellent youth project of Hunan Provincial Department of Education, grant number (19B515, 19B516). The APC was funded by National Natural Science Foundation of Hunan Province. Acknowledgments: We thank Ashleigh Cooper for editing the English text of a draft of this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Yan, Y.; Santaniello, T.; Bettini, L.G.; Minnai, C.; Bellacicca, A.; Porotti, R.; Denti, I.; Faraone, G.; Merlini, M.; Lenardi, C. Electroactive ionic soft actuators with monolithically integrated gold nanocomposite electrodes. Adv. Mater. 2017, 29, 1606109. [CrossRef] 2. Kong, L.; Chen, W. Carbon nanotube and graphene-based bioinspired electrochemical actuators. Adv. Mater. 2014, 26, 1025–1043. [CrossRef] 3. Zhao, H.; Hussain, A.M.; Duduta, M.; Vogt, D.M.; Wood, R.J.; Clarke, D.R. Compact dielectric elastomer linear actuators. Adv. Funct. Mater. 2018, 28, 1804328. [CrossRef] 4. Nguyen, C.T.; Phung, H.; Nguyen, T.D.; Lee, C.; Kim, U.; Lee, D.; Moon, H.; Koo, J.; Choi, H.R. A small biomimetic quadruped robot driven by multistacked dielectric elastomer actuators. Smart Mater. Struct. 2014, 23, 065005. [CrossRef] 5. Li, Y.; Hashimoto, M. PVC gel based artificial muscles: Characterizations and actuation modular constructions. Sens. Actuators A 2015, 233, 246–258. [CrossRef] 6. Ongaro, F.; Scheggi, S.; Yoon, C.; Van den Brink, F.; Oh, S.H.; Gracias, D.H.; Misra, S. Autonomous planning and control of soft untethered grippers in unstructured environments. J. Micro-Bio Robot. 2017, 12, 45–52. [CrossRef][PubMed] 7. Wu, J.; Yuan, C.; Ding, Z.; Isakov, M.; Mao, Y.; Wang, T.; Dunn, M.L.; Qi, H.J. Multi-shape active composites by 3D printing of digital shape memory polymers. Sci. Rep. 2016, 6, 24224. [CrossRef][PubMed] 8. Mirvakili, S.M.; Hunter, I.W. Multidirectional artificial muscles from nylon. Adv. Mater. 2017, 29, 1604734. [CrossRef] 9. Yang, H.; Leow, W.R.; Wang, T.; Wang, J.; Yu, J.; He, K.; Qi, D.; Wan, C.; Chen, X. 3D Printed photoresponsive devices based on shape memory composites. Adv. Mater. 2017, 29, 1701627. [CrossRef] 10. Bushuyev, O.S.; Aizawa, M.; Shishido, A.; Barrett, C.J. Shape-shifting azo dye polymers: Towards sunlight-driven molecular devices. Macromol. Rapid Commun. 2018, 39, 1700253. [CrossRef] 11. Zhu, P.; Yang, W.; Wang, R.; Gao, S.; Li, B.; Li, Q. 4D Printing of complex structures with a fast response time to magnetic stimulus. ACS Appl. Mater. Interfaces 2018, 10, 36435–36442. [CrossRef][PubMed] 12. Liu, J.; Jiang, C.; Xu, H. Giant magnetostrictive materials. Sci. China Technol. Sci. 2012, 55, 1319–1326. [CrossRef] 13. Yuk, H.; Lin, S.; Ma, C.; Takaffoli, M.; Fang, N.X.; Zhao, X. Hydraulic hydrogel actuators and robots optically and sonically camouflaged in water. Nat. Commun. 2017, 8, 14230. [CrossRef] 14. Villegas, D.; Van Damme, M.; Vanderborght, B.; Beyl, P.; Lefeber, D. Third-generation pleated pneumatic artificial muscles for robotic applications: Development and comparison with mckibben muscle. Adv. Rob. 2012, 26, 1205–1227. [CrossRef] 15. Hawkes, E.W.; Christensen, D.L.; Okamura, A.M. Design and implementation of a 300% strain soft artificial muscle. In Proceedings of the IEEE International Conference on Robotics and Automation (ICRA), 20 May–5 June 2016, Xi’an, China; pp. 4022–4029. Polymers 2021, 13, 2734 15 of 15

16. Duan, J.; Liang, X.; Zhu, K.; Guo, J.; Zhang, L. Bilayer hydrogel actuators with tight interfacial adhesion fully constructed from natural polysaccharides. Soft Matter 2017, 13, 345–354. [CrossRef] 17. Umedachi, T.; Vikas, V.; Trimmer, B.A. Softworms: The design and control of non-pneumatic, 3D-printed, deformable robots. Bioinspir. Biomim. 2016, 11, 025001. [CrossRef] 18. Li, T.; Li, G.; Liang, Y.; Cheng, T.; Dai, J.; Yang, X.; Liu, B.; Zeng, Z.; Huang, Z.; Luo, Y. Fast-moving soft electronic fish. Sci. Adv. 2017, 3, e1602045. [CrossRef][PubMed] 19. Gu, G.; Zou, J.; Zhao, R.; Zhao, X.; Zhu, X. Soft wall-climbing robots. Sci. Robot. 2018, 3, 2874. [CrossRef] 20. Park, W.H.; Bae, J.W.; Shin, E.J.; Kim, S.Y. Development of a flexible and bendable vibrotactile actuator based on wave-shaped poly (vinyl chloride)/acetyl tributyl citrate gels for wearable electronic devices. Smart Mater. Struct. 2016, 25, 115020. [CrossRef] 21. Zarek, M.; Layani, M.; Cooperstein, I.; Sachyani, E.; Cohn, D.; Magdassi, S. 3D printing of shape memory polymers for flexible electronic devices. Adv. Mater. 2016, 28, 4449–4454. [CrossRef] 22. Xia, H.; Hirai, T. New shedding motion, based on electroactuation force, for micro-and nanoweaving. Adv. Eng. Mater. 2013, 15, 962–965. [CrossRef] 23. Li, Y.; Hashimoto, M. Design and prototyping of a novel lightweight walking assist wear using PVC gel soft actuators. Sens. Actuators A 2016, 239, 26–44. [CrossRef] 24. Li, Y.; Hashimoto, M. PVC gel soft actuator-based wearable assist wear for hip joint support during walking. Smart Mater. Struct. 2017, 26, 125003. [CrossRef] 25. Must, I.; Kaasik, F.; Põldsalu, I.; Mihkels, L.; Johanson, U.; Punning, A.; Aabloo, A. Ionic and capacitive artificial muscle for biomimetic soft robotics. Adv. Eng. Mater. 2015, 17, 84–94. [CrossRef] 26. Kim, S.Y.; Yeo, M.; Shin, E.J.; Park, W.H.; Jang, J.S.; Nam, B.U.; Bae, J.W. Fabrication and evaluation of variable focus and large deformation plano-convex microlens based on non-ionic poly (vinyl chloride)/dibutyl adipate gels. Smart Mater. Struct. 2015, 24, 115006. [CrossRef] 27. Ali, M.; Ueki, T.; Tsurumi, D.; Hirai, T. Influence of plasticizer content on the transition of electromechanical behavior of PVC gel actuator. Langmuir 2011, 27, 7902–7908. [CrossRef] 28. Park, W.H.; Shin, E.J.; Kim, S.Y. Enhanced Design of a soft thin-film vibrotactile actuator based on PVC gel. Appl. Sci. 2017, 7, 972. [CrossRef] 29. Helps, T.; Taghavi, M.; Rossiter, J. Thermoplastic electroactive gels for 3D-printable artificial muscles. Smart Mater. Struct. 2019, 28, 085001. [CrossRef] 30. Guo, S.Z.; Qiu, K.; Meng, F.; Park, S.H.; McAlpine, M.C. 3D printed stretchable tactile sensors. Adv. Mater. 2017, 29, 1701218. [CrossRef] 31. Wang, R.; Yang, W.Y.; Zhu, P.F.; Gao, S.; Li, B.; Li, Q. Creation of 3D terahertz photonic crystals by the direct writing technique with a TiO2 sol-gel ink. J. Am. Ceram. Soc. 2018, 101, 1967–1973. [CrossRef] 32. Walton, R.L.; Brova, M.J.; Watson, B.H.; Kupp, E.R.; Fanton, M.A.; Meyer, R.J.; Messing, G.L. Direct writing of textured ceramics using anisotropic nozzles. J. Eur. Ceram. Soc. 2021, 41, 1945–1953. [CrossRef] 33. Wan, X.; Luo, L.; Liu, Y.J.; Leng, J.S. Direct ink writing based 4D printing of materials and their applications. Adv. Sci. 2020, 7, 2001000. [CrossRef][PubMed] 34. Shin, E.J.; Park, W.H.; Kim, S.Y. Fabrication of a high-performance bending actuator made with a PVC gel. Appl. Sci. 2018, 8, 1284. [CrossRef] 35. Xia, H.; Ueki, T.; Hirai, T. Direct observation by laser scanning confocal microscopy of microstructure and phase migration of PVC gels in an applied electric field. Langmuir 2011, 27, 1207–1211. [CrossRef][PubMed] 36. Smay, J.E.; Gratson, G.M.; Shepherd, R.F.; Cesarano, J.; Lewis, J.A. Directed colloidal assembly of 3D periodic structures. Adv. Mater. 2002, 14, 1279–1283. [CrossRef] 37. Helps, T.; Taghavi, M.; Rossiter, J. Towards electroactive gel artificial muscle structures. Adv. Mater. 2002, 14, 1279–1283.