Evolution of 3D printed soft actuators

Sensors and Actuators A 250 (2016) 258–272

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Review

Evolution of 3D printed soft actuators Ali Zolfagharian a , Abbas Z. Kouzani a , Sui Yang Khoo a , Amir Ali Amiri Moghadam b,c , Ian Gibson a , Akif Kaynak a,∗ a b c

School of Engineering, Deakin University, Geelong, Victoria 3216, Australia Department of Radiology, Weill Cornell Medicine, New York, NY, USA Dalio Institute of Cardiovascular Imaging—Presbyterian Hospital & Weill Cornell Medicine, New York, NY, USA

a r t i c l e

i n f o

Article history: Received 24 May 2016 Received in revised form 5 September 2016 Accepted 19 September 2016 Available online 6 October 2016 Keywords: 3D printing Soft actuator Soft robot Rapid prototyping

a b s t r a c t Developing soft actuators and sensors by means of 3D printing has become an exciting research area. Compared to conventional methods, 3D printing enables rapid prototyping, custom design, and singlestep fabrication of actuators and sensors that have complex structure and high resolution. While 3D printed sensors have been widely reviewed in the literature, 3D printed actuators, on the other hand, have not been adequately reviewed thus far. This paper presents a comprehensive review of the existing 3D printed actuators. First, the common processes used in 3D printing of actuators are reviewed. Next, the existing mechanisms used for stimulating the printed actuators are described. In addition, the materials used to print the actuators are compared. Then, the applications of the printed actuators including softmanipulation of tissues and organs in biomedicine and fragile agricultural products, regenerative design, smart valves, microfluidic systems, electromechanical switches, smart textiles, and minimally invasive surgical instruments are explained. After that, the reviewed 3D printed actuators are discussed in terms of their advantages and disadvantages considering power density, elasticity, strain, stress, operation voltage, weight, size, response time, controllability, and biocompatibility. Finally, the future directions of 3D printed actuators are discussed. © 2016 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Fabrication processes of 3D printed soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Soft actuators developed via 3D printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 3.1. Semi 3D printed soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 3.2. 3D printed soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 3.2.1. Shape memory polymer soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 3.2.2. Photopolymer/light activated polymers soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 3.2.3. Hydraulics and pneumatics soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 3.2.4. Electroactive polymer soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 3.2.5. Magnetic soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 3.2.6. Printed active composite soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 Application of 3D printed soft actuators in soft robotics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 4.1. Origami, self-folding, or self-assembly actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 4.2. Self-healing actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 4.3. Biomedical soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 4.3.1. The technical processes of 3D bio printed soft actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 4.3.2. PLA actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 4.3.3. Hydrogel actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

∗ Corresponding author. E-mail address: [email protected] (A. Kaynak). http://dx.doi.org/10.1016/j.sna.2016.09.028 0924-4247/© 2016 Elsevier B.V. All rights reserved.

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

5. 6. 7.

259

Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Biography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

1. Introduction Soft robots are being developed to tackle the shortcomings of conventional robots in interacting with humans and fragile biological objects. They consist of a collection of subsystems such as sensors, actuators, and controllers that together form a robot that can safely handle fragile and sensitive matters. Focusing on the key component of soft robots, actuators, since conventional rigid actuators are unsuitable for use in soft robots, soft actuators are being developed to safely manipulate delicate objects. Therefore, developing soft actuators to satisfy both mechanical and control properties of soft robots has become a key target of many researchers. Unlike conventional actuators, soft actuators produce flexible motion due to the integration of microscopic changes at the molecular level into a macroscopic deformation of the actuator material [1]. This can manifest as deflection or volumetric change. Soft actuators can be classified into several subgroups such as thermo-driven [2], electro-driven [3], pH-driven [4], light-driven [5], and magnetodriven [6]. Also, different materials like polymers [7], hydrogels [8], and elastomers [9], have been used for construction of soft actuators. Moreover, majority of the existing soft actuators are fabricated using multistep low yield process such as micro-moulding [10] and [11], solid freeform fabrication [12], and mask lithography [13]. However, these methods require manual fabrication of devices, post processing/assembly, and lengthy iterations until maturity in the fabrication is achieved. To avoid the tedious and time-consuming aspects of the current fabrication processes, researchers are exploring an appropriate manufacturing approach for effective fabrication of soft actuators. Therefore, special soft systems that can be fabricated in a singlestep by rapid prototyping methods, such as 3D printing, are being investigated. Such methods narrow the gap between the design and implementation of soft actuators, making the process faster, less expensive, and simpler [14] and [15]. They also enable incorporation of all actuator components into a single structure eliminating the need to use external joints, adhesives, and fasteners. These result in a decrease in the number of discrete parts, post-processing steps, and fabrication time. In addition, soft actuators with submillimetre features can be produced with high accuracy. Whilst the 3D printing approach has been widely applied to the fabrication of sensor devices [16] and [17], the use of this approach for production of soft actuators is a growing field of research. The soft actuators that are produced by the 3D printing method are referred to as 3D printed actuators. This paper for the first time provides a comprehensive review of the emerging field of 3D printed soft actuators. It describes different methods: for fabrication of the actuators using 3D printers, and discusses their actuation mechanism, power density, reversibility, strain, stress, operation voltage, weight, size, response time, controllability, and biocompatibility. The paper is organised as follows. Section 2 describes the technical process of printing soft actuators using the 3D printing techniques, and gives a classification of the existing 3D printers. Section 3 categorises the current 3D printed soft actuators into two groups: semi 3D printed and 3D printed soft actuators. Section 4 presents the applications of the 3D printed soft actuators. Section 5 gives a comparison of the current 3D printed soft actuators fol-

lowed by a discussion of their and advantages and disadvantages. Finally, Section 6 provides the concluding remarks and the future directions of this research field.

2. Fabrication processes of 3D printed soft actuators Producing any 3D printed soft actuator requires 4 steps, the first being the definition of the problem that is the clarification of the need for using a 3D printer for constructing the actuator. Secondly, the advantages of using 3D printer over conventional manufacturing techniques should be justified. Thirdly, the appropriate materials that can accomplish the desired characteristics of the final product should be selected meticulously. Finally, the method of printing should be decided according to the material properties, size of structure, and vertical movement of nozzle and the thickness of layers. The primary procedure of 3D printing starts from CAD design where the desired model is envisaged and drawn in a CAD software. Then, the 3D designed model is divided into multiple 2D stack layers piled up by means of printer jet nozzle depending on the precision of 3D printer in z axis (Fig. 1) [18]. Thus far, 3D printers have used various technologies for constructing the 3D products. In one common type of 3D printing known as spray forming, liquid moulding materials are sprayed using nozzle jet to form the layers of structure [19]. A similar 3D printer technique called spraying moulding with adhesive, utilizes adhesive materials that are sprayed on a layer of powder material in order to form strong bonds between two successive vertical layers [20]. In selective laser melting (SLM), structures are built up layer by layer by depositing a thin layer of metal powder, followed by selectively lasing to achieve the pattern needed in that section. The lasing causes a phase transition in metals; the particles are completely melted for just a fraction of a second during which they bind to the existing structure below [21]. Further, laser technology is applied instead of adhesive materials to fuse the powder materials by means of heat generated by the laser beam known as laser powder sintering moulding [22]. Stereolithography (SLA) is another technique that relies on a photosensitive monomer resin. A polymer is formed and then solidified when exposed to ultraviolet (UV) light while all these processes occur near the surface of the product [23]. Photosensitive polymer curing is another technique where light sources are adopted for 3D printers that use photosensitive polymers. In these printers, the photosensitive polymers are melted using light sources and then solidified rapidly [24]. In order to make 3D structures using polymeric materials, some 3D printers input polymers as filament using the extrusion technique under the pressure and heat to pile up the 3D structure layer by layer which is called material extrusion moulding [25]. Fused deposition Modelling (FDM) uses extrusion to lay down thin lines of a thermoplastic material in the shape of the object being manufactured [26]. Direct metal laser sintering is based on the principles of atomic diffusion at low speed and low temperature [27]. The materials commonly used in extrusion based 3D printers are acrylonitrile butadiene styrene (ABS), nylon, silicon and other thermoplastics [28]. In sinter/melt processing based printers aluminium, stainless steel, and titanium are the most common materials in use [29] and [30]. Also, 3D bio-printers can fabricate stents and heart valve replacements using Teflon, hydrogels and

260

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

Fig. 1. Fabrication steps of 3D printed soft actuators.

biogases while recently silicon nitride was tested for implants as well [31] and [32]. 3. Soft actuators developed via 3D printing Here we classify the current 3D printed soft actuators into two main groups namely “semi 3D printed soft actuators” and “3D printed soft actuators”. The reason for such classification is to distinguish between the printed soft actuators that are fabricated by means of 3D printing process in whole and the soft actuators whose parts are made by 3D printers and post processed subsequently. This classification helps to clarify the advantages of 3D printed soft actuators over the semi 3D printed soft actuators due to their capability of operating without the need of any further assembly. 3.1. Semi 3D printed soft actuators 3D printing technology has helped to create and embed functional components of actuators such as external soft parts, linkages, and motors and subsequently assembled them together with other components into a working actuator. The soft actuators developed in this way are classified as semi 3D printed soft actuators. There are several applications for these soft actuators in soft robotics as actuated robotic limbs, or smart structures with embedded components such as fibres, hydraulics, and shape memory alloy materials. One of the pioneering work for developing a 3D printed actuator was reported in [33] where the authors devised an actuated finger by Polyjet 3D printing process. They embedded the photopolymer fibre within the 3D printing process (Fig. 2A). However, several limitations were reported in this method such as the need of an expert operator for pausing the printing process and embedding the fibres. Also, the fibres could only be placed in the planar direction and not spatially. A 3D printed bellows actuator, namely Printable Hydraulics was fabricated in a single step process while printing solid and liquid materials concurrently using separate nozzles [34]. A 5-head inkjet 3D printer was employed to deposit three different polymers and non-cured liquids concurrently. As an example, a hexapod robot could be printed in one step, requiring only a single added direct current (DC) motor. The fluid is pumped through the robot’s body causing the legs to move. Shape memory alloys (SMA) are classified as metallic materials that can be programmed to return to their original shape at certain temperatures. Super elasticity character of SMA is a leading cause of its widespread application in soft robotics and actuators [35]. Association of 3D printers and SMA led to the development of new fast and micro size semi 3D printed soft actuators. A tentacle like structure fabricated from SMA and casting silicone elastomer material by a 3D printer was devised and proposed [36] as a prototype of printable robots. A steerable wormlike 3D printed soft robot was developed by utilizing antagonistic SMA coil at rear part

of the actuator [37]. Further, layer by layer additive manufacturing was used in incorporation of the embedded SMA to fabricate actuated compliant joint [38]. However, the application of SMA in bioengineering and medical devices has some drawbacks; one is that a high current supply is needed in order for SMA to be stimulated by heating. Also, the activation temperature is often beyond the threshold domain for human-robot interaction. Shape memory strip particularly Ni-Ti alloy along with silicone rubber materials such as Ecoflex-30 and poly di methyl siloxane (PDMS) are utilized to fabricate a soft humanoid robot finger made of ABS [39]. A Ni-Cr resistance wire is used for thermal stimulus of SMA actuators. A piezoelectric flexor sensor is placed between two layers of shape memory alloys, which are enclosed by 3D printed mould parts. The mould has a perforated structure, which includes an array of fins in order for SMA to dissipate the heat at highest rate. The researchers were able to achieve an acceptable response time of 11 s at 1 A current input with the utmost input voltage of 7 V. Also, a layered reinforcement soft actuator is fabricated using 3D printer and SMA alloy to generate bend-twist motion for applied currents ranging from 1.2 to 2.8 A (Fig. 2B) [40]. Soft Robotic Actuators (SRA) driven by pneumatics is another interesting domain for the application of 3D printed soft actuators. 3D printed finger nails were fabricated and controlled based on a sensor embedded in the top layer of actuator [41]. Furthermore, a 3D printer can be used for the fabrication of geometrically complex moulds of pneumatically actuated soft manipulators. A pneumatic actuator made of PDMS polymer was developed by means of a 3D printed mould that made the fabrication process faster and capable of constructing more complicated geometrical shapes (Fig. 2C) [42]. Ionic polymer-metal composite (IPMC) soft actuators are also utilized to manufacture semi 3D printed soft actuators [43]. Soft silicone tube was printed first and then biaxial IPMC actuators embedded into it. The final product was presented as a multi degree of freedom soft actuator (Fig. 2D). 3.2. 3D printed soft actuators 3D printed soft actuators is defined by fabrication of the entire actuator directly by a 3D printer without any post-process assembly. The main advantages of 3D printed soft actuators are that these actuators can be fabricated in a single step process and are consequently more economical and accessible for custom design applications compared to semi 3D printed counterparts that need skilled operators and post processing assembly. The development of 3D printed soft actuators has already started but still is at its early stages. Recent research findings are presented in the following. 3.2.1. Shape memory polymer soft actuators Shape memory polymer (SMP) actuators are the most similar to our muscles, providing response to a range of stimulus such as light,

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

261

electrical, magnetic, heat, pH, and moisture changes. They have some deficiencies including fatigue and high response time that have been improved through introduction of smart materials and combination of different materials by means of advanced fabrication technology. The advent of 3D printers has made a new pathway for fabricating low-cost and fast response SMP actuators. The process of receiving external stimulus like heat, moisture, electrical input, light or magnetic field by SMP is referred to as shape memory effect (SME). SMP could be an appropriate alternative to improve the pitfalls of SMAs. SMP exhibits some rewarding features such as low density, high strain recovery, biocompatibility, and biodegradability. A 3D printed SMP actuator was designed and made of epoxy based ultra violet (UV) curable polymer materials [14]. These actuators react to thermal stimulus through changing the composition of printed materials at different positions of the actuator (Fig. 3A). A simple gripper was made of SMP using 3D FDM (Fig. 3B) [46]. Different levels of nozzle temperature control and filament speed at extrusion process were studied to optimize the quality of the actuator in terms of part density, dimensional accuracy, and surface roughness. Also, various deformations called predetermined selfevolving including linear stretching, circular expansion, and folding were achieved by 3D printed polymer materials after immersing in water (Fig. 3C) [7] and [47]. Bi-stability is another property of SMP that enables a structure with zero degree of freedom (DOF) to have two (or even more) stable positions under certain circumstances, and the structure is able to switch from one stable position to another if properly loaded to induce slight deformation. A prime sample of a 3D printed bi-stable actuator made from nylon was also fabricated (Fig. 3D) [15]. 3.2.2. Photopolymer/light activated polymers soft actuators Photopolymer/light activated polymers (LAP) are another type of SMP that are activated by light stimuli. The LAP actuators can be controlled remotely with instant response and, without any physical contact, only with variation of light frequency or intensity. The main drawback of the LAP actuators is the requirement of preloading that act simultaneously with irradiation to cause shape variation or curvature of the actuator [48]. However, there have not been many investigations in literature on the fabrication of the LAP actuators by means of 3D printing. An optical physically unclonable function (PUF) was patterned via a commercially-available inkjet 3D printer [49]. A photonic nanomaterial like quantum dots (QD)polymer composite is used as the material that can be deposited selectively. The designers succeeded to embed security features within their models with the simple selection of the material type so that they are not visible to the naked eye while can be validated using fluorescence microscopes. Also, a 3D printed photopolymer actuator was developed [5] that could be stimulated via unfocused light emission. In fact, the ink pattern printed on polystyrene prestrained sheets generated heat by absorbing light (Fig. 3E). Due to the effective light absorption by the ink, the polymer under the black ink heated up faster than the rest of the polymer.

Fig. 2. (A) CAD model and fabricated finger actuator with multiple joints and embedded. Reproduced from [33]. (B) CAD model and 3D printed PDMS pre-polymer poured and cured around the replica mould with size of 30mm × 7mm × 2mm. Reproduced from [42] with the permission from Elsevier. (C) 3D printed SMA actuator. Reproduced from [40] with the permission from Nature Publishing Group. (D) Four IPMC tubes embedded actuator. Reproduced from [43] with the permission from SPIE.

3.2.3. Hydraulics and pneumatics soft actuators A need for soft, light-weight and biocompatible soft actuators in soft robotics has influenced researchers for devising pneumatic soft actuators because of their intrinsic compliance nature and ability to produce muscle tension [50]. A soft skin gripper was designed and fabricated in which all the soft skin modules are constructed by a 3D printer [50]. Pneumatic sensors are employed in order to send feedback information via air pressure to robot’s controller. Further, Inflatable tiles of soft materials are also 3D printed and applied as a pneumatic actuator [51]. 2D sheets of elastomeric materials are initially constructed and then assembled using the soft joinery method (Fig. 3F). Researchers also succeeded to combine the use of hydraulic system and 3D printing, where elastic elements, inte-

262

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

Fig 3. (A) Spontaneous and sequential shape recovery process of the helical SMP component with graded hinge section. Reproduced from [44] with the permission from Nature Publishing Group (B) SMP gripper heated above Tg . Reproduced from [46] with the permission from Springer. (C) Initial joint and its folding with their corresponding springmass systems. Reproduced from [7] with the permission from Nature Publishing Group. (D) 3D printed nylon bi-stable structure using compliant mechanism. Reproduced from [15] with the permission from Springer. (E) 3D printed photopolymer actuators. Reproduced from [5] with the permission of The Royal Society of Chemistry. (F) Cubic soft actuators. The actuators under atmospheric pressure ( , P = 0), negative pressure ( , −P) and positive pressure ( , +P). Reproduced from [51] with the permission from John Willey and Sons (G) Antagonistic 3D printed Pneumatics actuator with outer diameter of 20mm; left chamber pressurized. Reproduced from [53] with the permission from © IOP Publishing. (H) 3D printed electromechanical key. Reproduced from [54] with the permission from John Willey and Sons. (I) Deflection of the printed IPMC under various voltage. Reproduced from [55] with the permission from © IOP Publishing. (J) Inkjet printer micro pump actuator based on piezoelectric polymers. Reproduced from [66] with the permission from Elsevier. (K) Various 3D structures fabricated by DOPsL. Reprinted with permission from [67]. Copyright (2014) American Chemical Society. (L) 3D printed Polymagnet. Reproduced from [68] © 2016 Correlated Magnetics Research.

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

Fig 3. (Continued)

263

264

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

Fig 3. (Continued)

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

265

drawback of IPMCs is the use of complicated chemical fabrication processes that require a wet laboratory for their synthesis. A 3D printer was employed [55] for fabricating an electroactive polymer namely 3D soft IPMC electroactive structure. A 3D solid structure was first modelled by means of CAD software and input to a 3D printer. Then, the printed model was activated by a layer of platinum electrodes (Fig. 3I). The results were compared with a commercial Nafion IPMC. A derivative of EAP namely, Bucky gel actuator, was devised using layer by layer 3D printing [56]. Bucky gel is composed of ionic fluids, carbon nanotubes, and polymers and claimed to possess the ability of being driven in dry environment with low voltage stimulation. Adopting ionic liquids as electrolyte added the capability of working in dry environment to the Bucky gel actuators.

Fig. 4. (A) 3D printing of active composite materials. Reproduced from [32] with the permission from AIP Publishing LLC. (B) Active and passive phases for a fibre orientation of ␪ ◦ . Reproduced from [32] with the permission from AIP Publishing LLC. (C) The snapshots of multi-material gripper grabbing an object. Reproduced from [73] with the permission from Nature Publishing Group.

grated fluid circuits, and embedded joints/links, were developed in a single manufacturing process [52]. In another similar project, a pneumatically driven soft actuator was developed in a single process using SLA. Antagonistic manipulation like octopus tentacles was achieved using this artificial muscle like actuator (Fig. 3G) [53]. 3.2.4. Electroactive polymer soft actuators Polymers such as dielectric elastomers (DE), ionic polymer metal composites (IPMC), ionic electro active polymers, polyelectrolyte gels, and gel-metal composites are common materials to form 3D layered structures that can be tailored to work as soft actuators. EAP actuators are categorised as 3D printed soft actuators that respond to electrical excitation as deformation in their shape. A thermally sensitive actuator that worked as a physical electrical circuit key was presented, the functionality of which was such that the initially open electrical circuit would be closed upon heat induced deformation (Fig. 3H) [54]. 3.2.4.1. Ionic polymer-metal composite soft actuators. IPMCs are considered as a subgroup of EAPs that are known for their acceptable response properties and durability. Nonetheless, the main

3.2.4.2. Dielectric elastomer soft actuators. DE actuators, like silicone and acrylic elastomers, are other types of soft actuators that can be constructed using 3D printing. These soft actuators can exhibit high strain of up to almost 200% due to their low Young’s modulus and high elastic energy density. A two-layer DE actuator was developed [57] using 3D printing in acrylic-based photopolymer. A facial robotic system was devised using 3D printed DEA to mimic real facial muscles. The voltage applied for displacement tests of this actuator was up to 3.6 KV. Various commercial KE1283 silicone elastomers were utilized to fabricate the test object. A generator was developed [58] by means of 3D printed dielectric elastomer parts. The proposed system could be employed for further applications such as energy harvesting in human body in customizable orthotic shoe inserts in an effort to correct an over pronating gait and optimally harvest energy as the shoe strikes the ground. However, the main shortcoming of these soft actuators is the requirement of high voltage compared to the voltage range in human scale interactions [59]. A multi DOF hexapod robot was fabricated by 3D printed DE soft actuators. All parts of the hexapod were made by 3D printing, and only electrical wiring was done after assembling [60]. Also, application of pre-strain was needed to enhance the performance of DE soft actuators. Several methods: were used to overcome these deficiencies but the proposed soft actuators either deviated from the concept of soft robotic and flexibility [61] or were labour intensive [62].

3.2.4.3. Piezoelectric soft actuators. Soft piezoelectric materials also known as piezoelectric polymers are being used in soft actuators because of their flexibility, softness, transparency, and lightness [63]. Dielectric property of such materials enables them to generate a mechanical strain in response to electrical voltage. Most of the printed piezoelectric actuators have been fabricated through electrospinning [64]. However, a Polyvinylidene fluoride (PVDF) based actuator was fabricated entirely by means of inkjet printing [65] and [66]. This actuator was then used as a reciprocating membrane pump in a smart lab-on-a-chip system (Fig. 3J). Also, a low cost approach for fabrication of active piezoelectric nanoparticles using microscale dynamic optical projection stereolithography (DOPsL) was developed [67]. This method improves the printing resolution and throughput over SLA for 3D printing of piezoelectric polymers (Fig. 3K).

3.2.5. Magnetic soft actuators One of the most recent 3D printed soft actuators is Polymagnet actuator. The novel features of this magnetic printed actuator is that both north and south magnetic poles can be placed on one face of the magnet which in turn leads to a tight magnetic field instead of a long field near the magnet surface (Fig. 3L). This feature avoids energy waste in some industrial applications and can

266

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

replace mechanical springs with a higher efficiency and better control accuracy [68]. 3.2.6. Printed active composite soft actuators The technology of metamaterial polymer 3D printers has made it feasible to place different phases of a material within a composite structure in order to develop 3D printed soft actuators (Fig. 4A). A novel actuator named printed active composite (PAC) was devised [69]. Also, layers of laminate and lamina made of glassy active fibre composites were imbued to fabricate a 3D printed thermomechanical actuator (Fig. 4B) [70]. A 3D multilateral polymer printer was employed to form the layers of composite. A 3D printed PLA light-weight composite sheet that remains stable at room temperature and start actuating when exposed to hot environments was fabricated [71]. Further applications of these 3D printed thermo-actuator such as self-folding and opening-box were also investigated. Other PAC actuators were fabricated using a multimaterial 3D printer (Objet260 Connex, Stratasys Inc, Edina, MN, USA) and digital SMP fibres with different glass transition temperatures [14] and [72]. One of the recently developed soft actuators was reported to have shape memory effects in the temperature range between ∼20 ◦ C and ∼70 ◦ C [72]. Moreover, a soft actuator was developed by projection microstereolithography (P␮SL) using compositions of photo-curable SMP materials (Fig. 4C) [73]. The printed actuator demonstrated a high strain failure compared to similar actuators. Biopolymers reinforced with natural fibres were also used by FDM 3D printer to develop a hydromorphic bio-composite that deflected in the presence of moisture. The topological aspects of the proposed soft actuators were considered and optimised. Different printing orientation of filaments were tested and the results were compared in terms of water uptake and swelling with manufactured compressed samples [74]. 4. Application of 3D printed soft actuators in soft robotics 4.1. Origami, self-folding, or self-assembly actuators The viscoelastic behaviour of polymers and advent of accessible commercial 3D printers have allowed the researcher to develop 4D origami and self-folding. Origami and self-folding soft actuators are made of 3D printed structural polymers that respond to SME based on time lapse and intensity of stimulus [75]. SMPs are precisely printed using 3D printers in an elastomeric matrix as intelligent active hinges to enable 4D origami folding patterns [75]. These systems have many potential applications in small volume photovoltaic solar cells [76], and biomedical industry [77]. 3D printers can deposit SMP in a sophisticated fashion at pivotal locations or throughout an entire structure in such a way that a temporary shape of an arbitrary form can be achieved by applying a stimuli at predefined conditions (Fig. 5A) [75]. Also, a novel method of sequence folding with temporal sequencing of activation when the structure was subjected to a uniform temperature was proposed [45]. In another application of 3D printed soft actuators a new 4D material that contained discrete information and computational abilities that enabled self-assembly via external stimulation was proposed [78]. A series of material elements each with a simple switch mechanism that responded to loading conditions were programmed. Once an external load is applied to the structure, the switch mechanism was activated and configured based on the extent of the applied load (Fig. 5B). Composite structures in the form

Fig. 5. (A) Self-assemble origami box and pyramid. Reproduced from [75] with the permission of © IOP Publishing (B) A representation of programmable structure with external loading and their resultant connections/disconnections for topological structural optimization. Reproduced from [78] with the permission from Emerald Group Publishing Limited. (C) 3D printed PLA actuators with medical applications. Reproduced from [96] © 2014 by MDPI. (D) Programmable printed capsule and its

rupturing via radiation. Reprinted with permission from [102]. Copyright (2014) American Chemical Society. (E) 3D printed hydrogel specimen swollen in water at different temperatures. Reproduced from [103] with the permission from John Willey and Sons. (F) 4D printed valve operating at different temperatures. Reproduced from [104] with the permission from Cambridge University Press.

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

of printed porous foams were produced to prevent crack propagation with applications in robotics, bioengineering, and food printing [79]. Volumetric pixel changes in structures were also suggested by [80]. In early stages of research a composition of hard and soft materials were applied in order to construct a 3D printed soft structure changing locomotion in the presence of pneumatic pressure. The volumetric changes of structure with periodic pneumatic pressure could be illustrated. This technology could be applied in regenerative design for online analysis of structures under dynamic loading like earthquake or different loads applied on flexible robotic arms.

267

to jet the materials to substrate [91]. Crosslinking supported 3D printers are more advanced extrusion based 3D bio-printers [92]. Coaxial 3D printer is a modified and improved version of extrusion printing, used in bio-printing in order to fabricate hollow filaments. In this technique, the first material is dispensed through the outer tube of the coaxial nozzle, while the second material is dispensed through the inner section of the coaxial nozzle and the diffusion or crosslinking occurs once the two flows contact [93]. ABS and PLA are the most common materials used for making prosthetic skeletal organs like hands and fingers while Nylons, flexible rubbers and carbon fibres as well as silicone are also used [94] and [95].

4.2. Self-healing actuators The application of 3D bio-printing of hydrogel materials for self-healing also named as supramolecular 3D printing is another area of research that is booming these days. The high resolution of 3D printing is a breakthrough for creating complex chemical and cellular compositions of hydrogels [81]. Sacrificial microvascular network and capsule based self-healing are two prevalent approaches that 3D printer has found its place [82]. 3D printed poly lactic acid (PLA) is known as an appropriate sacrificial material for constructing the sacrificial microvascular network so that it vaporizes at high temperature polymerisation [83]. 4.3. Biomedical soft actuators 3D printing in medicine has the potential to reduce the burden on the healthcare system by providing more effective treatments. 3D printed soft actuators applied in prosthetics and body organs can help older people to stay healthy and to live independently for longer. The technology of using 3D printers for constructing the biological implants of human tissues is still at its infancy stage but is expected to grow drastically in a few years through replacing the human organs by 3D printed active biological organs. The combination and synergy of electricity and muscle cells is a new paradigm that empowers the researchers to control over the human muscular movement (i.e. muscle-tendon relationship). Constructing glaucoma implants are another application of 3D bio-printers that allow scientists and surgeons to find the causes of diseases through a process of trial and error [84]. Covered stents in thin film Ni-Ti is a new application of 3D bio-printers in medicine. Covered stents replace conventional metal ones and mainly used in stenting vessels that are at risk for rupturing. The next generation of 3D printed stents are not only modified in terms of the biological response, but they also allow for the production of very low live tissue cells profile. Soft tissue organs of human body like heart valves could directly be constructed using biological ink printer and biologically active cells. The combination of 3D bio-printing and bioreactor could be a form of bio-printing so that the change in tissues/organ can be programmed [85]. 4.3.1. The technical processes of 3D bio printed soft actuators Up until now there are three dominant kinds of 3D bio-printers. The most common type and broadly used 3D bio-printer is extrusion based in which mechanical forces exerted from pneumatic pumps are used to extrude the materials through a nozzle [86] and [87]. Inkjet 3D bio-printers are the second common type, which are basically designed for low viscosity materials. In these printers the droplets of the materials are dispensed onto a substrate drop by drop till the next layer in vertical dimension. Various jetting techniques such as piezoelectric ink jetting [88], and electro hydrodynamic jetting [89] are adopted in these printers. Laser based 3D bio-printers are another type that employ two different technologies. One is called vat polymerization where photopolymers are cured by means of laser beam [90]. Laser induced forward transfer (LIFT) is other laser based 3D bio-printer that utilizes laser energy

4.3.2. PLA actuators PLA is one of the most popular filaments used in 3D bio-printing nowadays. It has applications as surgical staples and minimally invasive surgery. An example of 3D printed PLA as surgical staple is fabricated in [96] (Fig. 5C). Another application of 3D printed PLA soft actuators is in minimally invasive embolization procedure. A particular design of spiral springs of 3D printed PLA is constructed for minimum invasive embolization surgery purpose where it can be embedded in a catheter and be activated via thermal stimulus (Fig. 5C) [96]. 4.3.3. Hydrogel actuators The synergetic product of smart hydrogels and 3D printers are quite new generation of 4D systems that respond to external stimulus such as electrical excitation [97], magnetic field [98], pH variation [99], temperature and moisture variation [100], or photon induced crosslinking [101]. The common hydrogel types used in these systems are, alginate, polyethyleneglycol (PEG), polyethyleneoxide (PEO), pluronic, methylcellulose, chitosan, and agarose [92]. The main advantages of hydrogel compared to conventional counterparts are the high water content that mimics natural tissue environment, shape memory character and controllable sol-gel transition [87]. A 3D printed stimuli-responsive core/shell capsule was devised for programmable release of multiplexed gradients within hydrogel matrices [102]. The system was capable of selective rupturing of capsule and drug release by means of laser wavelength. The first layer of capsule was aqueous core while the outer layer was formed of a solid stimuli-responsive shell that could be accurately programmed and irradiated for scheduled rupturing and drug release (Fig. 5D). Also, a fast reactor muscle like linear actuator was 3D printed from hydrogel materials [103]. The novelty of this research was to introduce a temperature sensitive hydrogel that showed large volume deflection to low temperature increments (Fig. 5E). The proposed model was fabricated as a smart valve that controlled the flow of water by closing at exposure to hot water and opening in cold water exposure [104] (Fig. 5F). Also, a biobot made of hydrogel was fabricated using SLA 3D printer [105]. A skeletal muscle was designed in a way that was powered by collagen, fibre, and matrix proteins when stimulated by electrical signal. 4D origami or self-folding is another interesting domain in which hydrogels play a significant role [106]. Furthermore, a biomimetic, programmable 3D printed hydrogel composite that was excited by immersing in water and formed its programmed pattern was designed and fabricated [107]. Anisotropic swelling feature of the proposed functional materials was used to control the desired curvature of printed object. 5. Discussions An exploration of 3D printed soft actuators fabricated by 3D printers and their applications are provided. The new generation of soft actuators fabricated by 3D printing technology is introduced at earlier sections. Soft actuators are then categorised into main groups of semi 3D printed soft actuators and 3D printed soft

268

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

Table 1 3D printed soft actuators in terms of used materials. Materials

Stimuli

Pros and Cons

References

SMA

Heat

[35,37] and [39]

Pneumatic

Air pressure

Hydraulic

Fluids pressure

SMP

Electrical Heat Magnetic Chemical-Reaction

LAP

Light

IPMC

Electrical

DE

Electrical

Piezoelectric

Electrical

Magnetic

Magnetic field

PLA

Heat

Hydrogel

Moisture Heat pH Magnetic Electrical

High power density High reversibility Low operating voltage Medium strain and stress Medium response velocity Light weight Inflatable and hollow structures Biocompatible Needs external air pump Difficult controlling of force Complex structure of force transmitting fluid High force transmission Easy controlling of force Needs external hydraulics pump Medium strain and stress Slow response velocity Low-medium power density Crystallization UV curing required No need to physical stimulation Too sensitive to thermal noise High response velocity High deflection Low operating voltage Low-medium power density Lightweight Medium power density Requires electrolyte layer Fast response High strain High elastic energy density High longevity High operating voltage Pre-strain required in fabrication Fast response High longevity High input voltage Not biocompatible Low strain Large hysteresis Remotely control low energy loss Brittle and hard to print Locking mechanism required Biodegradability Low cost Flimsiness Nozzle clogging Low voltage Biocompatible High water content Slow response Low mechanical strength

actuators. Semi 3D printed soft actuators are constructed from several separate parts that are printed and then assembled together including other non-printed components at the final process of manufacturing. Some fibre embedded, SMA, hydraulic and pneumatic soft actuators are categorized in this class [33,34,40,42]. 3D printed soft actuators are considered to be more sophisticated and have the potential to make a breakthrough in the field of soft robotics. The whole fabrication process is done only by means of 3D printers, and the actuators can be fabricated in a single step process by smart materials that are programmed, controlled and activated by external stimulus like heat, moisture, light and electric or magnetic field [5,14,44,45,54] and [72]. Semi 3D printed soft actuators can only be printed in complex shapes and geometries while the 3D printed soft actuators are capable of even changing the initial printed shape and dynamic behaviour without any further assembly. Thus, there is neither a need for an expert to assemble the parts nor for shipment of different parts for actuators assembly.

[51] and [50]

[34] and [42]

[5,14,44,45,54,74] and [75]

[48] and [49] [55] and [56]

[58,59] and [62]

[66] and [67]

[68] and [108]

[109,110] and [111]

[92,97] and [103]

It has been observed that one of the main features of the 3D printing compared to conventional manufacturing is the layer-bylayer fabrication of 3D printing. Using traditional manufacturing methods, fibres can only be embedded within a single plane while no part of the fibre can stick out above the 2D plane of the part that is being fabricated. Moreover, when it comes to soft robotics and smart materials, many conventional strategies of fabrication to construct 3D structures particularly in small scales are inconvenient and useless. Further, the conventional linkers such as screws, bolts, and even welding cannot be applied in a broad range of purposes in small size soft actuators to bond components of actuators. One prevalent solution has been the use of adhesive materials instead of rigid fasteners because of their effectiveness on broad types of materials. However, adhesive materials are highly depended on the surface area contact between the components, thus may not be suitable for diverse cases and sizes of actuators.

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

A comparison of distinct features of 3D printed soft actuators in terms of used materials is given in Table 1. It was found that each of those actuators has their own advantages and disadvantages. SMP can cover some deficiencies of semi 3D printed soft actuators because of its inherent advantages such as low density, high strain recovery, biocompatibility, and biodegradability while low to medium power density and crystallization at high temperatures could be deemed as main pitfall of 3D printed SMP soft actuators. 3D printed LAP soft actuators are valuable because of the capability to be controlled remotely while the interception of heat generated by emitted light is a deficiency. 3D printed hydraulic soft actuators are appropriate devices for the systems on the chip purposes, and they are easily controllable in wake of force control. However, they need bulky external pumps, which could be a disadvantage in sub-millimetre size systems. 3D printed pneumatic soft actuators are considered as lightweight, soft, and biocompatible while they require air compressors that are challenging to control. 3D printed EAP soft actuators including IPMC, dielectric elastomers, piezoelectric materials are other kinds of soft actuators that are stimulated by electrical field. 3D printed IPMC soft actuators have been reported to be much faster than SMP, LAP, and hydrogel actuators. However, the literature does not provide the actual actuation speeds of such actuator for a proper comparison. On the other hand, the fabrication process of the 3D printed IPMC soft actuators is more complex than that of the other actuators because of the need for conductive layer coating [55,56,44] and [75]. Also, IPMC soft actuators typically require a humid environment or electrolyte layers to operate. 3D printed dielectric elastomer soft actuators are useful for high strain and high elastic energy applications at the expense of high input voltage [58,59] and [62]. It is also deduced from literature that there are not many 3D printed piezoelectric soft actuators that can be related to high level technological construction requirements for devising such actuators. The 3D printing technology can now produce Polymagnet soft actuators that possess both north and south magnetic poles on one face of the magnet. This feature can bring numerous applications of such actuators in remotely controlled and low energy dissipating devices. 3D printed composite soft actuators are also evolving in line with other types of above-mentioned actuators while they use a combination of material properties in different layers contributing to the mechanical strengths and shape memory effects of 3D printed soft actuators [14,72] and [73]. The applications of the 3D printed soft actuators are increasingly growing. Origami, self-folding, or self-assembly 3D printed soft actuators have found some applications as intelligent active hinges to enable folding patterns [75] and [80]. Further, ongoing research for their development on active structures, digital materials, and smart structures as 3D printed soft actuators are being carried out. Healthcare and biomedical sectors will greatly benefit from 3D printed soft actuators. One prime application is 3D printed drug release capsule [102]. Self-healing structures is another interesting domain for 3D printed soft actuators that have capability of being utilised as sacrificial material for constructing the sacrificial microvascular network [82]. Active Polymers and hydrogels are the most common materials used in these fields that are able to print complex and sophisticated biodegradable structures. With the introduction of new 3D bio-printing technology, the low mechanical strength of hydrogel materials could be improved. Fabricating composite hydrogels is also an ongoing investigation that has resulted in improvements in the mechanical properties of conventional hydrogels [106] and [107].

6. Future directions Despite the advancement in the 3D printed soft actuators, there still exist several potential untouched areas for future studies. 3D

269

printed soft actuators commonly possess both soft and flexible attributes that make them a multi DOF system. Thus, this innate characteristic of 3D printed soft actuators that demands more research particularly on their control aspect. System identification and dynamics modelling of 3D printed soft actuators are deemed to be other areas of research that still need further work and research. Accurate models that describe the large-deflection dynamics of such actuators in real world such as efficient model order reduction or inverse dynamics modelling are current techniques that could be elaborated further. Developing and investigating the self-folding structures with either sequential or spontaneous deformations or combination of them and the study of automatic control aspects in order to avoid self-collision or self-blocking may be an interesting research domain. High resolution 3D printing in conjunction with novel biocompatible materials, a mixture of bio-gel materials such as collagens, alginates and fibrins, and composite layering of diverse materials during the printing process can be used in order to enhance the mechanical properties such as Young’s modulus. With the use of 3D scanners, the processing time of fabricating a 3D printed soft actuator, e.g. prosthetic finger, could be significantly reduced while recreating the unconventional and sophisticated geometries. 7. Conclusions This review provides researchers and engineers with new insights and research concepts associated with the use of 3D printing technology for developing soft actuators with applications in soft robotics and biomedicine. An investigation into the current 3D printed soft actuators including common processes used in printing of actuators, existing mechanisms used for stimulating the actuators, materials used to form the actuators, applications of the actuators, discussions on the characteristics of the actuators, and future directions of the printed actuators technology were presented. 3D printing of soft actuators results in a decrease in the number of discrete parts, post-processing steps, and time associated with the fabrication of such actuators. In addition, soft actuators with sub-millimetre features can be produced with a high accuracy. The key finding of this study is that the application of the 3D printing technology into soft actuators manufacturing enables new possibilities in the production of customized soft actuators for a range of real-world applications. References [1] K. Asaka, H. Okuzaki, Soft Actuators: Materials, Modeling, Applications, and Future Perspectives, Springer, 2014. [2] W.J. Zheng, N. An, J.H. Yang, J. Zhou, Y.M. Chen, Tough Al-alginate/poly (N-isopropylacrylamide) hydrogel with tunable LCST for soft robotics, ACS Appl. Mater. Interfaces 7 (3) (2015) 1758–1764. [3] E. Palleau, D. Morales, M.D. Dickey, O.D. Velev, Reversible patterning and actuation of hydrogels by electrically assisted ionoprinting, Nat. Commun. 4 (2013). [4] C. Yu, P. Yuan, E.M. Erickson, C.M. Daly, J.A. Rogers, R.G. Nuzzo, Oxygen reduction reaction induced pH-responsive chemo-mechanical hydrogel actuators, Soft Matter 11 (40) (2015) 7953–7959. [5] Y. Liu, J.K. Boyles, J. Genzer, M.D. Dickey, Self-folding of polymer sheets using local light absorption, Soft Matter 8 (6) (2012) 1764–1769. [6] T. Mitsumata, A. Nagata, K. Sakai, J. i. Takimoto, Giant complex modulus reduction of ␬-carrageenan magnetic gels, Macromol. Rapid Commun. 26 (19) (2005) 1538–1541. [7] D. Raviv, W. Zhao, C. McKnelly, A. Papadopoulou, A. Kadambi, B. Shi, et al., Active printed materials for complex self-evolving deformations, Sci. Rep. 4 (2014) 7422. [8] D. Morales, E. Palleau, M.D. Dickey, O.D. Velev, Electro-actuated hydrogel walkers with dual responsive legs, Soft Matter 10 (9) (2014) 1337–1348. [9] J. Biggs, K. Danielmeier, J. Hitzbleck, J. Krause, T. Kridl, S. Nowak, et al., Electroactive polymers: developments of and perspectives for dielectric elastomers, Angew. Chem. Int. Ed. 52 (36) (2013) 9409–9421. [10] G.-H. Feng, S.-C. Yen, Micromanipulation tool replaceable soft actuator with gripping force enhancing and output motion converting mechanisms, 18th

270

[11]

[12] [13]

[14]

[15]

[16]

[17]

[18] [19] [20] [21]

[22] [23] [24] [25]

[26] [27]

[28]

[29] [30]

[31] [32] [33]

[34]

[35] [36]

[37]

[38]

[39] [40] [41]

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272 International Conference in Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS) (2015) 1877–1880. E.T. Roche, R. Wohlfarth, J.T. Overvelde, N.V. Vasilyev, F.A. Pigula, D.J. Mooney, et al., A bioinspired soft actuated material, Adv. Mater. 26 (8) (2014) 1200–1206. E. Malone, H. Lipson, Freeform fabrication of ionomeric polymer-metal composite actuators, Rapid Prototyping J. 12 (5) (2006) 244–253. P. Kerdlapee, A. Wisitsoraat, D. Phokaratkul, K. Leksakul, R. Phatthanakun, A. Tuantranont, Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process, Microsyst. Technol. 20 (1) (2014) 127–135. Y. Mao, Z. Ding, C. Yuan, S. Ai, M. Isakov, J. Wu, et al., 3D printed reversible shape changing components with stimuli responsive materials, Sci. Rep. 6 (2016). Y. Zhou, W.M. Huang, S.F. Kang, X.L. Wu, H.B. Lu, J. Fu, et al., From 3D to 4D printing: approaches and typical applications, J. Mech. Sci. Technol. 29 (10) (2015) 4281–4288. H. Ota, S. Emaminejad, Y. Gao, A. Zhao, E. Wu, S. Challa, et al., Application of 3D printing for smart objects with embedded electronic sensors and systems, Adv. Mater. Technol. (2016). J.T. Muth, D.M. Vogt, R.L. Truby, Y. Mengüc¸, D.B. Kolesky, R.J. Wood, et al., Embedded 3D printing of strain sensors within highly stretchable elastomers, Adv. Mater 26 (36) (2014) 6307–6312. G.G. Wallace, R. Cornock, C. O’Connell, S. Bernie, S.M. Dodds, F. Gilbert, 3D BioPrinting: Printing Parts for Bodies, 2014. A. Rosochowski, A. Matuszak, Rapid tooling: the state of the art, J. Mater. Process. Technol. 106 (1–3) (2000) 191–198. D.J. Dippenaar, K. Schreve, 3D printed tooling for vacuum-assisted resin transfer moulding, Int. J. Adv. Manuf. Technol. 64 (5–8) (2013) 755–767. F. Abe, K. Osakada, M. Shiomi, K. Uematsu, M. Matsumoto, The manufacturing of hard tools from metallic powders by selective laser melting, J. Mater. Process. Technol. 111 (1–3) (2001) 210–213. S. Upcraft, R. Fletcher, The rapid prototyping technologies, Assembly Autom. 23 (4) (2003) 318–330. D.T. Pham, R.S. Gault, A comparison of rapid prototyping technologies, Int. J. Mach. Tools Manuf. 38 (10–11) (1998) 1257–1287. P.J. Bartolo, G. Mitchell, Stereo-thermal-lithography: a new principle for rapid prototyping, Rapid Prototyping J. 9 (3) (2003) 150–156. E. Sachs, M. Cima, P. Williams, D. Brancazio, J. Cornie, 3-dimensional printing—rapid tooling and prototypes directly from a cad model, J. Eng. Ind.—Trans. Asme 114 (4) (1992) 481–488. E. Bassoli, A. Gatto, L. Iuliano, M.G. Violante, 3D printing technique applied to rapid casting, Rapid Prototyping J. 13 (3) (2007) 148–155. M.W. Khaing, J.Y.H. Fuh, L. Lu, Direct metal laser sintering for rapid tooling: processing and characterisation of EOS parts, J. Mater. Process. Technol. 113 (1–3) (2001) 269–272. S. Adams, A.Z. Kouzani, M. Mohammed, C. Usma, S.J. Tye, 3D printed biocompatible enclosures for an implantable DBS microdevice, Procedia Technol. 20 (2015) 155–161. A. Ambrosi, M. Pumera, 3D-printing technologies for electrochemical applications, Chem. Soc. Rev. (2016). V. Gupta, M. Talebi, J. Deverell, S. Sandron, P.N. Nesterenko, B. Heery, et al., 3D printed titanium micro-bore columns containing polymer monoliths for reversed-phase liquid chromatography, Anal. Chim. Acta (2016). S. Jana, A. Lerman, Bioprinting a cardiac valve, Biotechnol. Adv. 33 (8) (2015) 1503–1521. Q. Ge, H.J. Qi, M.L. Dunn, Active materials by four-dimension printing, Appl. Phys. Lett. 103 (13) (2013) 131901. L. Stiltner, A. Elliott, C. Williams, A method for creating actuated joints via fiber embedding in a Polyjet 3D printing process, Solid Freeform Fabrication Symp. Proc. (2011). R. MacCurdy, R. Katzschmann, Y. Kim, D. Rus, Printable hydraulics: a method for fabricating robots by 3D co-printing solids and liquids, arXiv preprint arXiv:1512.03744, (2015). J. Mohd Jani, M. Leary, A. Subic, M.A. Gibson, A review of shape memory alloy research, applications and opportunities, Mater. Des. 56 (2014) 1078–1113. P. Walters, D. McGoran, Digital fabrication of smart structures and mechanisms-creative applications in art and design, In: NIP & Digital Fabrication Conference, Society for Imaging Science and Technology. (2011). T. Umedachi, B.A. Trimmer, Design of a 3D-printed soft robot with posture and steering control, in Robotics and Automation (ICRA), 2014 IEEE International Conference on, 2014, IEEE. N.A. Meisel, A.M. Elliott, C.B. Williams, A procedure for creating actuated joints via embedding shape memory alloys in PolyJet 3D printing, J. Intell. Mater. Syst. Struct. 26 (12) (2015) 1498–1512. Y. She, C. Li, J. Cleary, H.-J. Su, Design and fabrication of a soft robotic hand with embedded actuators and sensors, J. Mech. Rob. 7 (2) (2015) 021007. S.-H. Song, J.-Y. Lee, H. Rodrigue, I.-S. Choi, Y.J. Kang, S.-H. Ahn, 35 Hz shape memory alloy actuator with bending-twisting mode, Sci. Rep. (2016) 6. J. Morrow, H.-S. Shin, J. Torrey, R. Larkins, S. Dang, C. Phillips-Grafflin, et al., Improving soft pneumatic actuator fingers through integration of soft sensors, position and force control, and rigid fingernails, IEEE International Conference on Robotics and Automation (ICRA) (2016), http://dx.doi.org/10. 1109/ICRA.2016.7487707.

[42] Y. Hwang, O.H. Paydar, R.N. Candler, Pneumatic microfinger with balloon fins for linear motion using 3D printed molds, Sens. Actuators A 234 (2015) 65–71. [43] J. Liu, Y. Wang, D. Zhao, C. Zhang, H. Chen, D. Li, Design and fabrication of an IPMC-embedded tube for minimally invasive surgery applications, in: SPIE Smart Structures and Materials+ Nondestructive Evaluation and Health Monitoring, International Society for Optics and Photonics, 2014. [44] K. Yu, A. Ritchie, Y. Mao, M.L. Dunn, H.J. Qi, Controlled sequential shape changing components by 3D printing of shape memory polymer multimaterials, Procedia IUTAM 12 (2015) 193–203. [45] Y. Mao, K. Yu, M.S. Isakov, J. Wu, M.L. Dunn, H. Jerry Qi, Sequential self-folding structures by 3D printed digital shape memory polymers, Sci. Rep. 5 (2015) 13616. [46] Y. Yang, Y. Chen, Y. Wei, Y. Li, 3D printing of shape memory polymer for functional part fabrication, Int. J. Adv. Manuf. Technol. (2015). [47] Q. Zhang, D. Yan, K. Zhang, G. Hu, Pattern transformation of heat-shrinkable polymer by three-dimensional (3D) printing technique, Sci. Rep. 5 (2015). [48] X. Mu, N. Sowan, J.A. Tumbic, C.N. Bowman, P.T. Mather, H.J. Qi, Photo-induced bending in a light-activated polymer laminated composite, Soft Matter 11 (13) (2015) 2673–2682. [49] O. Ivanova, A. Elliott, T. Campbell, C.B. Williams, Unclonable security features for additive manufacturing, Addit. Manuf. 1–4 (2014) 24–31. [50] S.-H. Ahn, K.-T. Lee, H.-J. Kim, R. Wu, J.-S. Kim, S.-H. Song, Smart soft composite: an integrated 3D soft morphing structure using bend-twist coupling of anisotropic materials, Int. J. Precis. Eng. Manuf. 13 (4) (2012) 631–634. [51] S.A. Morin, S.W. Kwok, J. Lessing, J. Ting, R.F. Shepherd, A.A. Stokes, et al., Elastomeric tiles for the fabrication of inflatable structures, Adv. Funct. Mater. 24 (35) (2014) 5541–5549. [52] J.E. Slightam, V.R. Gervasi, Novel integrated fluid-power actuators for functional end-use components and systems via selective laser sintering Nylon 12, 23rd Ann. Int. Solid Freeform Fabrication Symp. (2012). [53] B.N. Peele, T.J. Wallin, H. Zhao, R.F. Shepherd, 3D printing antagonistic systems of artificial muscle using projection stereolithography, Bioinspiration Biomimetics 10 (5) (2015) 055003. [54] M. Zarek, M. Layani, I. Cooperstein, E. Sachyani, D. Cohn, S. Magdassi, 3D printing of shape memory polymers for flexible electronic devices, Adv. Mater. (2015). [55] J.D. Carrico, N.W. Traeden, M. Aureli, K.K. Leang, Fused filament 3D printing of ionic polymer-metal composites (IPMCs), Smart Mater. Struct. 24 (12) (2015) 125021. [56] N. Kamamichi, T. Maeba, M. Yamakita, T. Mukai, Fabrication of bucky gel actuator/sensor devices based on printing method, In: Intelligent Robots and Systems, IROS 2008. IEEE/RSJ International Conference on, 2008, IEEE 2008. [57] J. Cai, A. Vanhorn, C. Mullikin, J. Stabach, Z. Alderman, W. Zhou, 4D printing of soft robotic facial muscles, 2016 Ann. Int. Solid Free Form Fabrication Symp. (2016). [58] Y. Bar-Cohen, T. McKay, P. Walters, J. Rossiter, B.O’Brien, I. Anderson, 3-dimensional fabrication of soft energy harvesters, 2013, 8687: 86870J. [59] A.J. McDaid, S.Q. Xie, K.C. Aw, A compliant surgical robotic instrument with integrated IPMC sensing and actuation, Int. J. Smart Nano Mater. 3 (3) (2012) 188–203. [60] C.T. Nguyen, H. Phung, H. Jung, U. Kim, T.D. Nguyen, J. Park, et al., Printable monolithic hexapod robot driven by soft actuator, in Robotics and Automation (ICRA), IEEE International Conference on, 2015, IEEE 2015. [61] B. O’Brien, E. Calius, S. Xie, I. Anderson, An experimentally validated model of a dielectric elastomer bending actuator, The 15th International Symposium on: Smart Structures and Materials & Nondestructive Evaluation and Health Monitoring, International Society for Optics and Photonics (2008). [62] F. Carpi, C. Salaris, D. De Rossi, Folded dielectric elastomer actuators, Smart Mater. Struct. 16 (2) (2007) S300. [63] K.S. Ramadan, D. Sameoto, S. Evoy, A review of piezoelectric polymers as functional materials for electromechanical transducers, Smart Mater. Struct. 23 (3) (2014) 033001. [64] V. Cauda, G. Canavese, S. Stassi, Nanostructured piezoelectric polymers, J. Appl. Polym. Sci. 132 (13) (2015). [65] O. Pabst, E. Beckert, J. Perelaer, U.S. Schubert, R. Eberhardt, A. Tünnermann, All inkjet-printed electroactive polymer actuators for microfluidic lab-on-chip systems, SPIE Smart Structures and Materials+ Nondestructive Evaluation and Health Monitoring, International Society for Optics and Photonics (2013). [66] O. Pabst, S. Hölzer, E. Beckert, J. Perelaer, U.S. Schubert, R. Eberhardt, et al., Inkjet printed micropump actuator based on piezoelectric polymers: device performance and morphology studies, Org. Electron. 15 (11) (2014) 3306–3315. [67] K. Kim, W. Zhu, X. Qu, C. Aaronson, W.R. McCall, S. Chen, et al., 3D optical printing of piezoelectric nanoparticle–polymer composite materials, ACS Nano 8 (10) (2014) 9799–9806. [68] R.S. Evans, Magnetic shear force transfer device. Google Patents. 2015. [69] K. Maute, A. Tkachuk, J. Wu, H. Jerry Qi, Z. Ding, M.L. Dunn, Level set topology optimization of printed active composites, J. Mech. Des. 137 (11) (2015) 111402. [70] Q. Ge, X. Luo, E.D. Rodriguez, X. Zhang, P.T. Mather, M.L. Dunn, et al., Thermomechanical behavior of shape memory elastomeric composites, J. Mech. Phys. Solids 60 (1) (2012) 67–83.

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272 [71] Q. Zhang, K. Zhang, G. Hu, Smart three-dimensional lightweight structure triggered from a thin composite sheet via 3D printing technique, Sci. Rep. 6 (2016). [72] J. Wu, C. Yuan, Z. Ding, M. Isakov, Y. Mao, T. Wang, et al., Multi-shape active composites by 3D printing of digital shape memory polymers, Sci. Rep. 6 (2016). [73] Q. Ge, A.H. Sakhaei, H. Lee, C.K. Dunn, N.X. Fang, M.L. Dunn, Multimaterial 4D printing with tailorable shape memory polymers, Sci. Rep. 6 (2016) 31110. [74] A. Le Duigou, M. Castro, R. Bevan, N. Martin, 3D printing of wood fibre biocomposites: from mechanical to actuation functionality, Mater. Des. 96 (2016) 106–114. [75] Q. Ge, C.K. Dunn, H.J. Qi, M.L. Dunn, Active origami by 4D printing, Smart Mater. Struct. 23 (9) (2014) 094007. [76] B. Myers, M. Bernardi, J.C. Grossman, Three-dimensional photovoltaics, Appl. Phys. Lett. 96 (7) (2010). [77] K. Chalapat, N. Chekurov, H. Jiang, J. Li, B. Parviz, G.S. Paraoanu, Self-organized origami structures via ion-induced plastic strain, Adv. Mater. 25 (1) (2013) 91–95. [78] S. Tibbits, K. Cheung, Programmable materials for architectural assembly and automation, Assembly Automation 32 (3) (2012) 216–225. [79] J.I. Lipton, H. Lipson, 3D printing variable stiffness foams using viscous thread instability, Sci. Rep. 6 (2016) 29996. [80] J. Hiller, H. Lipson, Automatic design and manufacture of soft robots, Rob. IEEE Trans. 28 (2) (2012) 457–466. [81] C.B. Highley, C.B. Rodell, J.A. Burdick, Direct 3D printing of shear-thinning hydrogels into self-healing hydrogels, Adv. Mater. 27 (34) (2015) 5075–5079. [82] S.Z. Guo, F. Gosselin, N. Guerin, A.M. Lanouette, M.C. Heuzey, D. Therriault, Solvent-cast three-dimensional printing of multifunctional microsystems, Small 9 (24) (2013) 4118–4122. [83] Y. Wang, D.T. Pham, C. Ji, Self-healing composites: a review, Cogent Eng. 2 (1) (2015) 1075686. [84] C. Ross, D. Nguyen, S. Pandav, Y.Q. Li, J. Crowston, M. Coote, A model to measure surgical outflow using a two-tubed experimental glaucoma drainage device, Invest. Ophthalmol. 54 (15) (2013), 4488–4488. [85] J. An, C.K. Chua, V. Mironov, A perspective on 4D bioprinting, Int. J. Bioprint. 2 (2016). [86] T. Billiet, E. Gevaert, T. De Schryver, M. Cornelissen, P. Dubruel, The 3D printing of gelatin methacrylamide cell-laden tissue-engineered constructs with high cell viability, Biomaterials 35 (1) (2014) 49–62. [87] S. Wang, J.M. Lee, W.Y. Yeong, Smart hydrogels for 3D bioprinting, Int. J. Bioprint. (2015). [88] Z.X. Khoo, J.E.M. Teoh, Y. Liu, C.K. Chua, S. Yang, J. An, et al., 3D printing of smart materials: a review on recent progresses in 4D printing, Virtual Phys. Prototyping 10 (3) (2015) 103–122. [89] A. Abeyewickreme, A. Kwok, J.R. McEwan, S.N. Jayasinghe, Bio-electrospraying embryonic stem cells: interrogating cellular viability and pluripotency, Integr. Biol. 1 (3) (2009) 260–266. [90] P. Soman, P.H. Chung, A.P. Zhang, S.C. Chen, Digital microfabrication of user-defined 3D microstructures in cell-laden hydrogels, Biotechnol. Bioeng. 110 (11) (2013) 3038–3047. [91] F. Guillemot, B. Guillotin, A. Fontaine, M. Ali, S. Catros, V. Keriquel, et al., Laser-assisted bioprinting to deal with tissue complexity in regenerative medicine, MRS Bull. 36 (12) (2011) 1015–1019. [92] I.T. Ozbolat, M. Hospodiuk, Current advances and future perspectives in extrusion-based bioprinting, Biomaterials 76 (2016) 321–343. [93] Q. Gao, Y. He, J. -z. Fu, A. Liu, L. Ma, Coaxial nozzle-assisted 3D bioprinting with built-in microchannels for nutrients delivery, Biomaterials 61 (2015) 203–215. [94] J. Gu, J.M. Catchmark, Polylactic acid composites incorporating casein functionalized cellulose nanowhiskers, J. Biol. Eng. 7 (1) (2013). [95] M. Kowalczyk, E. Piorkowska, P. Kulpinski, M. Pracella, Mechanical and thermal properties of PLA composites with cellulose nanofibers and standard size fibers, Composites Part A 42 (10) (2011) 1509–1514. [96] W.G. Yang, H. Lu, W.M. Huang, H.J. Qi, X.L. Wu, K.Y. Sun, Advanced shape memory technology to reshape product design, manufacturing and recycling, Polymers 6 (8) (2014) 2287–2308. [97] N. Jackson, F. Stam, Optimization of electrical stimulation parameters for electro-responsive hydrogels for biomedical applications, J. Appl. Polym. Sci. 132 (12) (2015). [98] G. Giani, S. Fedi, R. Barbucci, Hybrid magnetic hydrogel: a potential system for controlled drug delivery by means of alternating magnetic fields, Polymers 4 (2) (2012) 1157–1169. [99] N.K. Singh, D.S. Lee, In situ gelling pH- and temperature-sensitive biodegradable block copolymer hydrogels for drug delivery, J. Controlled Release 193 (2014) 214–227. [100] M.K. Joo, M.H. Park, B.G. Choi, B. Jeong, Reverse thermogelling biodegradable polymer aqueous solutions, J. Mater. Chem. 19 (33) (2009) 5891–5905. [101] A. Skardal, J.X. Zhang, L. McCoard, X.Y. Xu, S. Oottamasathien, G.D. Prestwich, Photocrosslinkable hyaluronan-gelatin hydrogels for two-step bioprinting, Tissue Eng. Part A 16 (8) (2010) 2675–2685. [102] M.K. Gupta, F. Meng, B.N. Johnson, Y.L. Kong, L. Tian, Y.W. Yeh, et al., 3D printed programmable release capsules, Nano Lett. 15 (8) (2015) 5321–5329.

271

[103] S.E. Bakarich, R. Gorkin 3rd, M. in het Panhuis, G.M. Spinks, 4D printing with mechanically robust, thermally actuating hydrogels, Macromol Rapid Commun 36 (12) (2015) 1211–1217. [104] S.E. Bakarich, R. Gorkin, S. Naficy, R. Gately, G.M. Spinks, 3D/4D printing hydrogel composites: a pathway to functional devices, MRS Adv. 1 (08) (2016) 521–526. [105] C. Cvetkovic, R. Raman, V. Chan, B.J. Williams, M. Tolish, P. Bajaj, et al., Three-dimensionally printed biological machines powered by skeletal muscle, Proc. Natl. Acad. Sci. U. S. A. 111 (28) (2014) 10125–10130. [106] G. Stoychev, N. Puretskiy, L. Ionov, Self-folding all-polymer thermoresponsive microcapsules, Soft Matter 7 (7) (2011) 3277–3279. [107] A.S. Gladman, E.A. Matsumoto, R.G. Nuzzo, L. Mahadevan, J.A. Lewis, Biomimetic 4D printing, Nat. Mater. (2016). [108] Y. Liu, J. Genzer, M.D. Dickey, 2D or not 2D: shape-programming polymer sheets, Prog. Polym. Sci. 52 (2016) 79–106. [109] J. Alam, M. Alam, M. Raja, Z. Abduljaleel, L.A. Dass, MWCNTs-reinforced epoxidized linseed oil plasticized polylactic acid nanocomposite and its electroactive shape memory behaviour, Int. J. Mol. Sci. 15 (11) (2014) 19924–19937. [110] D. Garlotta, A literature review of poly (lactic acid), J. Polym. Environ. 9 (2) (2001) 63–84. [111] I. Ward Small, P. Singhal, T.S. Wilson, D.J. Maitland, Biomedical applications of thermally activated shape memory polymers, J. Mater. Chem. 20 (17) (2010) 3356–3366.

Biographies

Ali Zolfagharian is a PhD student and Casual Lecturer with the School of Engineering at Deakin University.

Prof Abbas Z. Kouzani is a Professor of Microsystems Engineering with the School of Engineering, Deakin University, Australia.

Dr Sui Yang Khoo is a Senior Lecturer in Electrical & Electronics Engineering with the School of Engineering, Deakin University, Australia.

Dr Amir Ali Amiri Moghadam is a Postdoctoral Fellow with the Department of Radiology and Dalio Institute of Cardiovascular Imaging, Weill Cornell Medicine, USA.

272

A. Zolfagharian et al. / Sensors and Actuators A 250 (2016) 258–272

Prof Ian Gibson is the Head of the School of Engineering, Deakin University, Australia.

Prof Akif Kaynak is a leading researcher is electroactive polymers within the School of Engineering, Deakin University, Australia.