Recent Progress of Soft Electrothermal Actuators

Abstract

Developing soft electrothermal actuators (ETAs) has drawn extensive concern in recent years. This article presents a comprehensive review on recent progress of soft ETAs through five sections: device design on structure and materials, property, fabrication methods, applications, and prospects. It's found that the fabrication process can be divided into standard surface complementary metal oxide semiconductor technology, novel laser scribing, and inkjet printing method. Moreover, current applications involve three aspects: mechanical applications, optical applications, and biomimetic applications. It will develop in the direction of increasing electrothermal efficiency and response speed emphatically. This review encourages achievement of its higher performance and broad applications in the future.

Introduction

Soft actuators have attracted much attentions from researchers all over the world in recent years for their promising applications in many important situations, such as soft robots,^1–14 artificial muscles,^15–17 cell scaffolds,^18,19 manipulators,^20–22 tune of lighting,²³ and sensing.^24,25 In addition, they can even be a promising candidate for minimally invasive surgery and have the potential to revolutionize health care.^26–31 Among all kinds of soft actuators, electrothermal actuators (ETAs) show better performance in large deformation, fast response, light weight, simple fabrication, flexibility of substrate, low driving voltage, as well as no electrolyte.^32–35 Therefore, the study on soft ETA is of greater significance.

This article for the first time provides a comprehensive review of the emerging field of soft ETAs. The remaining parts of the article are arranged as follows (Fig. 1). Device Design: Structure and Materials section describes the device design on structure and materials of soft ETAs. Property section summarizes the property, and Fabrication Methods section introduces a review of a variety of fabrication methods applied in soft ETAs. Applications section presents the current applications of the soft ETAs in detail from three aspects: mechanical applications, optical applications, and biomimetic applications. Finally, some future directions of this research field and a brief conclusion are outlined in Prospects for Future Soft ETAs section and Conclusions section, respectively.

FIG. 1.

An Overview diagram of this review. LRGO, laser reduced graphene oxide.

Device Design: Structure and Materials

General structure and working mechanisms

A typical soft ETA is composed of two material layers with a great difference in coefficient of thermal expansion (CTE), as shown in Figure 2. When applied with voltages, the conductive layer with a lower CTE generates the joule heat to heat up the whole ETA. On account of the difference in CTE, the substrate with higher CTE will expand more, yielding a bending deformation to the lower-CTE layer. The deformation here is usually expressed in bending curvature and angular and vertical displacement at the end of the ETA. The angle and displacement can be directly measured, while curvature should be derived by solving the following equation: cos (lk) = 1 − dk,³⁶ where l is the length of actuator, d is the displacement, and k is curvature.

FIG. 2.

Schematic diagram of structure and mechanism of soft ETAs. (a) Initial state, and (b) bending state. ETAs, electrothermal actuators.

Materials

The choice of materials has a significant effect on functional effectiveness and actuation properties of soft ETAs. Benefit from the high thermal conductivity and outstanding mechanical performance, carbon nanomaterials, including carbon nanotube (CNT),^37–39 and graphene^40–42 have been widely used in soft ETAs as the flexible heater to introduce temperature change by joule heating. And flexible polymer substrates gain popularity in advantages of getting larger deformation and easy spin-coated and insulated, without necessity of additional insulation around the heater. Among flexible polymer substrates, it is widely selected from the materials with larger CTE, smaller Young's modulus elasticity, good flexibility, chemical inertness, remarkable durability against repeated deformation, and resistance to high temperature, such as polydimethylsiloxane (PDMS) elastomer and polyimide (PI). Table 1 shows a comparison of thermal properties for several typical homogenous materials at room temperature.^43–52

Table 1.

Comparison of Thermal Properties for Typical Materials at Room Temperature

Materials	Coefficient of thermal expansion (10^–6/K)	Thermal conductivity (W/mK)	Young's modulus (GPa)	Preference
Aluminum	24	237	70	⁵¹
Copper	16.4	398	110	⁵¹
Intrinsic graphene	∼−7	5300	>10³	^43–45,47
Single-wall CNT	−1.5	3500	>10³	^47,49,50
PDMS	60	0.15	4	⁵¹
PMMA	75	0.2	2.5	⁵¹
PP	120	0.12	2	⁵¹
Epoxy	55	0.2	2	⁵¹
PI	50	0.16	2.34	^46,48,52

CNT, carbon nanotube; PDMS, polydimethylsiloxane; PI, polyimide; PMMA, polymethyl methacrylate; PP, polypropylene.

In recent years, more and more composite materials have been applied in the research of high-performance soft ETAs. Wang et al. combined the laser reduced graphene oxide and Ag particle, the heater resistance decreased effectively, and the driving voltage of the ETA was reduced.⁵³ Kim et al. presented a high-performance transparent flexible film heater based on a hybrid of CNTs and silver nanowires (AgNWs).⁵⁴ Kang et al. introduced highly transparent and flexible conductive films based on a hybrid structure of graphene and an Ag-grid.⁵⁵ Yao et al. proposed the hybrid heater with the patterned AgNWs embedded below the surface of PDMS, and corresponding soft ETA achieved a maximum curvature of 2.6 cm⁻¹ upon applying a DC voltage of 4.5 V.³⁶ Wang et al. reported a soft actuator based on PI film with a heater of graphite nanoplates and sodium carboxymethyl cellulose anisotropic low-cost nanocomposite, which reached low resistance and low-voltage operation.⁵⁶

In the future, with the rapid development of material science, many more ideal and attractive materials will keep being explored not just from a performance standpoint but also along with its extremely low cost, ease of fabrication, environmental friendliness, and so on.

Property

Compared with other electrical-drive actuators, ETA shows high performance in large deformation, fast response, low driving voltage, light weight, simple fabrication, as well as no electrolyte. An electrothermomechanical lumped element model was earlier proposed in 2008 to be used in describing the behavior of an electrothermal actuator from the perspective of the thermo-electro-mechanical coupling domains.⁵⁷ It demonstrates that in the electrical domain, an input voltage or current causes the electrical resistor to dissipate power and raise the actuator temperature in the thermal domain, delivering an applied moment and angular rotation to the actuator in the mechanical domain. For this, it can help predicting the deformation of ETA in response to an applied power, current, or voltage. So the properties of soft ETA can also be divided into two aspects: mechanical bending property and electrothermal property.

In the mechanical bending property, soft ETAs perform their functions in the form of its deformation, namely displacement, curvature, or bending angle. As mentioned above, the ETA works by the principle of thermal expansion. And take the maximum bending angle, for example, it has a positive linear relationship with the CTE difference of the two materials, temperature increments and geometrical dimensions of the ETA.³⁶ Therefore, a large difference in the CTE between two materials can help lead to a large bending angle under the same temperature. And the desired deformation of a given ETA can be obtained by adjusting the electric input parameters to increase temperature. Generally, in the experimental tests, deformation changes over time under a certain voltage are recorded by a camera to get its maximum deformation and responding time.

The study on electrothermal property of ETA can help understand the bending behavior further from the perspective of electrothermal mechanism. Generally, the saturation temperature versus the input power density can be fitted into a linear relationship.³⁶ Hence driving voltage (mostly) or power is an important parameter to affect the bending property of ETA by changing temperature. To get a lower driving voltage under the same power, doping conductive materials into heater can be adopted to increase its conductivity. In addition, to improve the deformation speed, heating and cooling rates could be improved by choosing the materials with high thermal conductivity. Besides, for resistive heating, higher current or more distributed heating networks are likely to provide faster material expansion. For cooling, an optimized design of the actuator geometry and surface area may facilitate faster cooling rates. Generally, in the experimental tests, temperature changes over time under a certain voltage, as well as temperature changes over driving voltage, are recorded by an infrared thermal imager. And the temperature distribution of soft ETA can also be predicted by finite element analysis. Table 2 summarizes some typical device composition and performance of soft ETAs in recent years.^{36,53,56,58–61}

Table 2.

A Summary of Some Typical Device Composition and Performance of Soft Electrothermal Actuators in Recent Years

Device composition	Maximum curvature	Maximum angle	Driving voltage	References
RGO-Ag particle/PI	—	192°	28 V	⁵³
CNT(BP)/PDMS	—	180°	25 V	⁶⁰
SACNT/polymer	—	730°	40 V	⁶¹
SG/PDMS	1.2 cm⁻¹	—	10 V	⁵⁸
SACNT/BOPP	1.03 cm⁻¹	389°	5 V	⁵⁹
PI/AgNW/PDMS	2.6 cm⁻¹	720°	4.5 V	³⁶
GN-CMC/PI	2.2 cm⁻¹	270°	12.5 V	⁵⁶

AgNW, silver nanowire; CMC, carboxymethyl cellulose; RGO, reduced graphene oxide.

Fabrication Methods

Different technologies have been introduced to fabricate a heating layer on various substrates, including standard surface complementary metal oxide semiconductor (CMOS) technology,^42,62 novel laser scribing process,⁶³ and inkjet printing method.⁶⁴ Figure 3 shows different processes of typical fabrication methods.

FIG. 3.

Different processes of typical fabrication methods. (a) Standard surface CMOS micro-machining technology with sacrificial layer process, (b) standard surface CMOS micro-machining technology with induced diminished adhesion process, (c) laser scribing pattern process, and (d) ink-jet printing water-based ink onto paper and schematic representation of the printed electrothermal actuator. BOE, buffered oxide etch; GO, graphene oxide; PI, polyimide. Color images are available online.

Corresponding to the fabrication methods shown above, the standard surface CMOS can be divided into two types according to the method of separating the ETA from the supporting silicon substrate, namely sacrificial layer process and peeling off directly by induced diminished adhesion. These two processes have advantages of high precision, high repeatability, and mass production, but the experimental conditions are harsh. Direct drop-coating process, which consists of spin coating, direct laser cutting, and patterning, is particularly appealing because it enables rapid and simple fabrication with more custom designs. As for the last one, inkjet printing needs to base on paper, which is simple to prepare but limits the application scenarios.

Applications

Mechanical applications

Gripper

Actuators have been widely studied and applied in the field of gripper, so have soft ETAs. In 2011, Chen et al. fabricated a new bending actuator by embedding super-aligned CNT sheets into a polymer matrix.³⁷ This design remarkably reduced the CTE of the polymer matrix, leading to a large bending. It demonstrated a potential application of a tiny gripper manipulating a small object, as shown in Figure 4a. In 2015, Zeng et al. proposed a bimetallic ETA using multiwalled CNTs with the matrix of waterborne polyurethane or silicone rubber,³⁹ which could be used as a gripper to lift samples more than four times heavier than itself up.

FIG. 4.

Mechanical applications of soft ETAs. (a) A tiny gripper manipulating a small object. (b) Grabbing and releasing process of multifinger gripper. (c) Moving process of a weight lifting walking robot.

Recently, gripper has been combined into more complex structures, such as a multifinger gripper designed by Yao et al. in 2017 to imitate the grabbing and releasing process of a human hand, as shown in Figure 4b.³⁶ Besides, the gripper also involves biological cell micromanipulation in solution-based environments with development of biocompatible materials. In 2003, Li's group proposed a novel micropolymer-based actuator for grasping and holding a cell in place during a cell injection process.¹⁸ The group also developed a nanometer scale “nano-cutter” to be used for cell probing and cutting. By combining those technologies, microcellular surgery may become feasible in the future.

Soft robots

With more and more new applications having emerged in the age of artificial intelligence, soft ETAs have also been demonstrated in the applications of soft robotics by virtue of the large and continuum deformation with flexibility. In 2012, Liang et al. demonstrated a mini-robot based on graphene-PDA actuator with controllable motion, fast response rate, and high-frequency resonance,³⁵ which provided a new avenue for actuation applications. After that, the ETAs applied in soft robotics have appeared successively. Among them, the most common is walking robot. In 2015, Chen et al. designed a weight lifting walking robot based on a large-deformation curling actuator.⁵⁹ Figure 4c showed its moving process of lifting a foam sample. Another novel and interesting walking robot is one self-folding paper robot created by printing method, which was put forward by Shigemune et al.⁶⁴ Such as these, soft ETAs show more potential in emerging field of soft robots.

Summary and prospects

Gripper, as a typical application of soft ETAs in the mechanical field, has been developing from initial single unit into more complex combined structures, such as a multifinger gripper to imitate a human hand. Predictably, different grippers with diverse structures will appear in large quantities in the future. In addition, to grasp more equivalently the weight of the object in practical applications, the gripper with larger output force will be in need. That can be achieved considering by optimizing the geometric dimension of ETA to reduce its length, as well as increase width or thickness, or by increasing the Young's modulus of the two layers. Besides, with development of biocompatible materials, the gripper with good biological compatibility will be deeply explored in the medical field, which could expand its application scenario further. It could also move toward miniaturization or even encapsulation for more complex tasks in the future.

For soft robots, various walking or weight lifting robots based on soft ETAs have appeared, which show more potential and provide a new research avenue. However, it is obvious that those mentioned above are just prototypes, and soft robots present a special design challenge in that their actuation and sensing mechanisms often are highly integrated with the robot body and overall functionality. So if one wants to implement system-level applications, there is still a long way to go for soft ETAs.

Optical applications

Tuning of light transmission path

Soft ETAs with large deformation driven by electricity make adjusting optical path easy. Therefore, they have started exploring in optical applications. In 2017, Sang et al. proposed its application in tuning of light transmission path based on a ring-shaped ETA.²³ As shown in Figure 5a, when the electric current was applied on the ring-shaped actuator, the majority current would pass the inner reduced graphene oxide layer through the minimum distance between two electrodes. Then the current generated Joule heating subsequently resulted in the asymmetric expansion of the PDMS, which induced the deformation of the ring-shaped actuator. Therefore, it could tune a light transmission by varying an actuator aperture. This new designed structure of ETA explores one more potential application in a way.

FIG. 5.

Optical applications of soft ETAs. (a) Tuning of light transmission path based on a ring-shaped ETA. (b) Synchronous performances in color change and bending deformation. PDMS, polydimethylsiloxane; RGO, reduced graphene oxide. Color images are available online.

Smart chromatic display

As an important development direction of flexible wearable devices in the future, smart color-changing display has become a hotspot and especially resistance–heat driven thermochromic ones. By combining the thermochromic pigment or inks with the soft ETAs, chromatic display and actuation deformation can be achieved simultaneously. For example, Fan et al. just proposed electrothermal chromatic actuators by screen printing a thermochromic ink with a transition temperature of 45°C on the other side of the AgNW/PDMS film in 2017.⁶⁵ As shown in Figure 5b, the electrothermal chromatic actuator shows obviously synchronous performances in reversible color change from blue to pink and bending deformation simultaneously at different applied currents.

Visible scenes of transparent soft ETAs

With the emergence of transparent soft ETAs, new applications on how to combine transparency and actuation deformations of soft ETAs well together are being explored now. A good example of these is transparent wipers,³³ which provide a promising solution to drivers' obstructed vision and safety concerns. Especially, there is a new type of transparent soft ETA with great switchable transparency. It can be fabricated into a smart window,⁶⁶ which has switchable transparency and can be opened under an external voltage applied.

Summary and prospects

In conclusion, soft ETAs in the field of optics have just started recently, mainly involve tuning of light path induced by deformation, smart chromatic display by combining thermochromic pigment with ETAs, and visible scenes based on the transparent ETAs such as transparent wipers and smart window. In the future, the deeper studies in the optical field will go on.

For the tuning of light path, it is promising in lens focusing in the future. Due to high sensitivity of light paths to small changes in angle, soft ETAs with higher stability and higher precision of deformation by adjusting driving are in demand, especially linear adjustment. That will be easier to achieve as the decrease in size of soft ETAs.

For smart chromatic display, due to its easy fabrication and synchronous color changing and actuation deformation, it is possible to be applied into some artificial animals and plants to make the bionic more realistic.

For the visible scenes based on newly designed transparent or transparency-switchable soft ETAs, it is expected to broaden further in more fields, such as smart windows, optical switches, and so on.

Biomimetic applications

Soft ETAs, with characteristics of flexibility, lightweight, biocompatibility, and optical transparency, have also gained extensive concerns on a range of biomimetic applications, including motion of flying insect-like, human tissue, plants, and so on.

Artificial insect organs

Based on the bending performance of soft ETAs, multiform actuations can be achieved through complex predesigned patterns or anisotropy behaviors to imitate the behavior of creatures like insects. For example, Zhu et al. showed a complex functional flytrap-shaped ETA by use of laser-scribing anisotropy inspired by natural flytraps just in 2019.⁶⁷ Besides, by taking advantage of the transparency and mechanical properties of ultrathin carbon and organic materials, artificial wings have been explored. In 2011, Zhu et al. proposed an artificial dragonfly wing based on a graphene-on-organic film hybrid actuator (Fig. 6a).⁴² In that work, a very thin layer of the lithographically defined graphene sheets was monolithic, and its extraordinary properties enabled a large displacement, rapid response, and low power consumption. The reversible mechanical actuation, as well as the transparency of graphene, provided a basis for the design of artificial dragonfly wing in both actuation and structural support. Its flapping and bending movement could be controlled by changing frequency and duration of the applied voltage.

FIG. 6.

Biomimetic applications of soft ETAs. (a) An artificial dragonfly wing based on graphene-on -organic film actuator. (b) Separate movements of five fingers based on hand-shape actuator. (c) An artificial flower blooming induced by electrical voltage or light.

Artificial human tissue

At the very beginning, biomimetic appeared mainly in plants, animals, and their behaviors and ecology in nature. In recent years, biomimetic applications related to human tissues have emerged one after another. In 2013, Litao Sun's group put forward the artificial cilia based on a novel graphene/GO actuator.⁴⁰ The tiny artificial cilia realized controllable movements of objects in a 2D plane with good durability and stability. It is believed to be useful for transporting objects and precisely positioning in MEMS applications. In 2015, Li et al. showed an interesting design of hand-shaped actuator to imitate various gestures based on a highly anisotropic large-area CNT paper.⁶⁰ It realized separate movements of five fingers controlled by five independent circuits (Fig. 6b). Therefore, the ETAs in artful design play significant roles in realizing novel and functional actuations. In 2017, Shou-shan Fan's group proposed an artificial arm based on a helix-shaped arm-like SACNT/polymer ETA.⁶¹ The artificial arm with a rapid response, large gripping force, and good biocompatibility shows a new way for manipulating objects.

Artificial plant

An artificial flower consisting of graphene-based actuators with dual response and large deformation was proposed by Chang et al. in 2019.⁶⁸ Due to photothermal and electrothermal property of graphene, the artificial flower based on this soft ETA showed electrical voltage or light induced blooming with dual responsiveness, as demonstrated in Figure 6c. In addition, with emergence of proposed anisotropic and color-shifting soft ETAs, an artificial color-shifting twining tendril composed of a long string shaped ETA was successfully demonstrated in 2018.⁶⁹ After a heat was applied, the artificial tendril, which was initially flat and black, changed into twisted and bright green and eventually wrapped around the central stem.

Summary and prospects

The research of soft ETAs in the field of bionics is of great significance, especially in the era when human health has become the focus of research. As mentioned above, the related studies are extensive and involve from humans to animals and even plants. Taking full advantage of flexible deformation characteristics, soft ETAs have demonstrated great potential in biomimetic applications. In the future, there is no doubt that the application of soft ETAs in this field will continue to be explored in depth.

For artificial human tissue, its flexibility, biocompatibility, transparency, low weight, and pattern ability are likely to highlight its potential for many other scenes, including biomimetic membranes and skins and artificial muscle in the future. At the same time, on basis of actuation, introducing the sensing of external stimulation becomes the key to development. It can be achieved either by introducing external sensors to constitute a complete system or by exploring the sensing functions of the actuator itself, such as the change of resistance in the bending process, so as to realize the integration of sensing and sensing.

For artificial insects and plants, on the one hand, the developed color-shifting and anisotropic soft ETAs are expected to be further developed into the various scenes of biomimetic field. On the other hand, soft ETAs copied with more other physical quantities such as sound, light, and force are expected to achieve more comprehensive imitation and open new application fields and functionalities overcoming the limitation of current biomimicking applications.

Prospects for Future Soft ETAs

Soft ETAs have developed rapidly in recent years with development of soft robotics. The original purpose of this article is to summarize systematically the recent progress of soft ETAs to point out its development trend. So far as it goes, soft ETAs have not been widely used in industry mainly due to problems such as its low stability and repeatability from the industrial level, low efficiency of electrothermal conversion, lack of batch standardized preparation, inconvenient operation of large power supply, and so on. In view of that, they may be improved in the following aspects: for the component materials, new component materials with greater coefficient of expansion difference and tolerable steady temperature can be tried to achieve performance optimization, such as composite materials and functional materials. For the performance, first, improving the efficiency of electrothermal conversion is still the key. It can be achieved by the suitable material substitution with high thermal conductivity, as well as a new structure design based on the underlying to reduce the heat loss. Second, speeding up the response in the whole cycle can be considered from more distributed heating networks to provide faster material expansion and an optimized design of geometry to facilitate cooling. Third, lowering the driving voltage by decreasing the resistance of the heater is also necessary, which can be achieved through doping or high-quality fabrication process. For the applications, soft ETAs applied on minimally invasive surgery will be an emerging application field in the future due to rapid development of flexible and biocompatible materials and potentials of flexible manipulator or soft robots in the medical field under the background of intelligent times. Based on our continuous in-depth research and the development of industrial conditions, it is believed that the future soft ETAs will be more and more widely used.

Conclusions

It is universally acknowledged that soft ETAs have attracted extensive attention. To have a clear and systematic understanding of that, this article conducts a comprehensive review on recent progress of soft ETAs, including device design on structure and materials, property, fabrication process, and broad applications emerged in the past two decades. They have the advantages of wide-range nonplanar deformation, relative low driving voltage, simple fabrication, and custom design, which enable its traditional applications in mechanical and optical field. Moreover, its tendency of flexibility and biological compatibility makes it possible to expand to some emerging field, such as soft robotics, bionics, and modern medicine. In the future, it will also develop in the direction of increasing its electrothermal efficiency and response speed, which puts forward a test of device structure, material selection, and preparation process. With solution of the problem such as stability and repeatability from the industrial level, lack of batch standardized preparation, and inconvenient operation of large power supply in the industry, it will be more widely used in the future.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported by the National Key R&D Program (2016YFA0200400), National Natural Science Foundation (61434001, 61574083, 61874065, 51861145202), and National Basic Research Program (2015CB352101) of China. The authors are also thankful for the support of the Research Fund from Beijing Innovation Center for Future Chip, Beijing Natural Science Foundation (4184091), and Shenzhen Science and Technology Program (JCYJ20150831192224146).

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