polymers 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 defor- mation 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 elec- tric 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 compres- sion 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