Recent advances in self-actuation and self-sensing materials: State of the art and future perspectives

Journal Pre-proof Recent advances in self-actuation and self-sensing materials: State of the art and future perspectives Yushu Liu, Yunhao Zhong, Chengyin Wang PII:

S0039-9140(20)30099-0

DOI:

https://doi.org/10.1016/j.talanta.2020.120808

Reference:

TAL 120808

To appear in:

Talanta

Received Date: 5 November 2019 Revised Date:

3 February 2020

Accepted Date: 4 February 2020

Please cite this article as: Y. Liu, Y. Zhong, C. Wang, Recent advances in self-actuation and selfsensing materials: State of the art and future perspectives, Talanta (2020), doi: https://doi.org/10.1016/ j.talanta.2020.120808. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Recent

Advances

in

Self-Actuation

and

Self-Sensing

Materials: State of the Art and Future Perspectives Yushu Liua,b,§ Yunhao Zhonga,§ Chengyin Wanga,*

a

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu

Province, 225000, China b

School of Computer Science and Technology, Harbin Institute of Technology, Weihai,

Shandong Province, 264209, China

§

These authors contributed equally to this work.

* Corresponding author. E-mail address: [email protected] (Chengyin Wang). Fax: 86-514 87975244; Tel: 86-514 87990926

1

TOC

2

ABSTRACT The contradiction between human’s strong demand of fossil fuels and their limited reserves becomes increasingly severe. Without external power input, intelligent materials responding sharply and reversibly to various external stimuli are the topic of intense research these years, especially the self-actuation and self-sensing materials. The promising family of these materials will play a significant role in energy-saving, low-cost and environment-friendly intelligent systems in the future. This review summarizes the latest advances in self-actuation and selfsensing materials. The synthetic strategies, morphologies and performance of these materials are introduced, as well as their applications in energy harvest, self-powering sensors, wearable devices, etc. Finally, tentative conclusions and assessments regarding the opportunities and challenges for the future development of these materials are presented.

Keywords: self-actuation self-sensing piezoelectric materials chemical sensors biosensors

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CONTENT 1. Introduction ................................................................................................................................. 5 2. Self-Actuation Materials ............................................................................................................. 8 2.1. Spontaneously-Proceeding Actuation .............................................................................. 8 2.2. Electric Field-Induced Actuation ..................................................................................... 8 2.2.1. Piezoelectric Materials .......................................................................................... 8 2.2.2. Electrostrictive Materials .................................................................................... 10 2.3. Magnetic Field-Induced Actuation ................................................................................ 12 2.3.1. Magnetic Shape-Memory Materials ................................................................... 12 2.3.2. Magnetic Nanomaterials ..................................................................................... 13 2.3.3. Magnetic Fluids .................................................................................................. 14 2.4. Illumination-Induced Actuation ..................................................................................... 15 2.4.1. Photoelectric Materials ....................................................................................... 15 2.4.2. Photostrictive Materials ...................................................................................... 17 2.5. Temperature-Induced Actuation .................................................................................... 18 2.6. Humidity-Induced Actuation ......................................................................................... 19 2.7. Applications of Self-Actuation Materials ...................................................................... 21 3. Self-Sensing Materials .............................................................................................................. 22 3.1. Resistance-Respond Self-Sensing Materials.................................................................. 23 3.2. Voltage-Respond Self-Sensing Materials ...................................................................... 24 3.3. Fluorescence-Respond Self-Sensing Materials.............................................................. 25 3.3.1. Fluorescent fibers ................................................................................................ 25 3.3.2. Mechanophore..................................................................................................... 25 3.4. Brief Summary ............................................................................................................... 26 3.5. Applications of Self-Sensing Materials ......................................................................... 26 3.5.1. Kinetic Parameter Sensors .................................................................................. 26 3.5.2. Thermodynamic Parameter Sensors ................................................................... 29

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3.5.3. Chemical Parameters Sensors ............................................................................. 30 4. Self-Actuation and Self-Sensing Materials ............................................................................... 36 4.1. Piezoelectric Materials ................................................................................................... 36 4.1.1. Piezoelectric Multi Crystals ................................................................................ 37 4.1.2. Piezoelectric Single Crystals............................................................................... 38 4.1.3. Piezoelectric Polymers ........................................................................................ 39 4.1.4. Piezoelectric Compounds.................................................................................... 39 4.2. Shape Memory Materials ............................................................................................... 40 4.3. Novel Self-Actuation and Self-Sensing Materials ......................................................... 42 4.4. Applications of Self-Actuation and Self-Sensing Materials .......................................... 42 5. Conclusions and Outlook .......................................................................................................... 43

1. Introduction

Natural creatures could alter their structure and functionality on demand in response to various external stimuli. For instance, sea cucumbers could change their stiffness rapidly when confronted by danger and flytraps could close their leaves in response to touches on their trigger hair. These provide an endless source of inspiration and have produced tremendous interest recently driven by increasing demand for intelligent materials. Scientists and engineers are motivated to mimic the response capabilities of natural creatures. During the past few decades, materials science has witnessed the emergence of smart materials with one or more properties that could respond to small external variations in environmental conditions [1,2]. They might undergo reversible changes due to physical (heat, light, electric and magnetic fields and mechanical input) and chemical stimuli (pH and specific target molecule). There is a wide range of smart materials. Among them are self-actuation and self-sensing materials, which have now

5

come into use in various fields, including construction [3,4] and medical treatment [5,6]. Analogous to our nervous system, self-actuation and self-sensing materials could sense and respond to certain stimuli and therefore, could be exerted in intelligent devices and sensing apparatus [7]. The problem of the high energy consumption and complicated structure of traditional actuation devices is particularly evident in modern instrumental analysis. Self-actuation materials could generate mechanical actuation induced by external stimuli, which opens up new avenues for practical remote actuation and remote control. Meanwhile, self-sensing means the ability to sense its conditions without an external sensor. Self-sensing materials are excellent for in-situ, real-time, and continuous sensing [8,9]. Furthermore, it is advantageous to combine the actuating and sensing functionalities of a single device. These materials could translate the surrounding stimuli into energy for their actuation, and then they could sense the stimuli to realize the selfpowered-sensing ability. In other words, they could detect the external change powered by the generated signal instead of the external power supply. Self-actuation and self-sensing materials have attracted significant interest in recent years, as shown in Fig. 1. The bar chart illustrates changes with the number of the publications on these materials from 2015 to 2019, showing a general increasing trend. Their smart features enable versatile applications not only in traditional fields but also in some extended areas. Unfortunately, current self-actuation and self-sensing materials are still in their initial stage of research with challenges for the actual utilization. The related study on this aspect is very sparse and far from a systematic understanding. Although there are some reviews on self-sensing materials [10,11], the focus is usually on one specific material (such as carbon materials), and there is a lack of a general introduction of materials with both self-actuation and self-sensing

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ability. Summarizing the universalities can provide essential instructions for further investigations. Thus, we wish to offer a comprehensive summary of these materials.

Fig. 1. The number of publications on self-actuation and self-sensing materials in recent years (left) and the mechanism of self-actuation and self-sensing materials (right). These materials undergo reversible changes due to physical or chemical stimuli.

In this review, we emphasize recent progress made in self-actuation and self-sensing materials. We discuss the material synthesis briefly, focusing mainly on their responsive property. Moreover, we consider in detail how such responses work, and several substantial improvements are presented in sequence. Meanwhile, their potential applications are also mentioned with an evaluation of their advantages and limitations. Also, our research group’s recent work on self-actuation and self-sensing materials is introduced as well. Finally, we conclude with a perspective of challenges along with development prospects in this field.

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2. Self-Actuation Materials 2.1. Spontaneously-Proceeding Actuation

These materials could proceed spontaneously without external force or induction, which is mainly based on spontaneous physical or chemical processes. For instance, superhydrophobic and superoleophilic meshes, which are usually referred to as “oil-removing” or “waterremoving” materials, could let either water or oil pass through. Song et al. presented a super hydrophilic mesh-capped container with special wettability [12], which could filter oil/water mixtures and collect oil simultaneously (Fig. S1 (a)). Wang’s group made the superhydrophobic fabric in the form of a boat to clean up the crude oil spill efficiently [13]. The as-fabricated boat could be applied in large-scale oil spills on water. Fig. S1 (b) exhibits the in-situ oil spill collecting process. However, other self-actuation materials need the ambient stimulus (such as electric or magnetic fields, light, moisture, or heat) as induction so that they could actuate automatically.

2.2. Electric Field-Induced Actuation 2.2.1. Piezoelectric Materials

Piezoelectric materials can generate internal mechanical strain with an electric field applied to it, which have attracted enough attention in recent decades. Among them, lead zirconate titanate (PZT) ceramic, the dominant one with outstanding electromechanical and mechanical properties, might be toxic to the environment and human health.

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Fig. 2. (a) The d*33 values for various BNKT based ceramics compounds. Reproduced by permission of Journal of the Korean Physical Society from [14]; (b) Models and photographs of the millimeter-scale gel walkers fabricated by Morale et al. Reproduced by permission of Royal Society of Chemistry from [15].

To

address this problem,

some other piezoelectric ceramics,

such

as

some

Bi0.5(Na,K)0.5TiO3 (BNKT) ceramics, could have notable potential for replacing PZT materials. Several peers attempted to modify these materials to control the phase transition, and thus, the electric-field induced strain could be obtained. Nevertheless, the properties of BNKT ceramics are still inferior to those of PZT ceramic, so recent researches are also focused on single crystals, with small hysteresis and unprecedented piezoelectric ability. We compared the max electricfield induced strain under the max electric-field intensity in Table 1. The ratio of the max electric-field induced strain with the max electric-field intensity was shown to illustrate the piezoelectric ability. Moreover, the d*33 values for various BNKT based ceramics compounds were shown in Fig. 2 (a) [14].

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2.2.2. Electrostrictive Materials

Due to high structural flexibility and extraordinary electromechanical properties, carbon materials could deform in an electric field. For instance, Galineau et al. loaded conductive carbon black on a Pu membrane to achieve electric-field induced actuation capabilities [16]. Compared with carbon black, carbon nanotube (CNT) with higher structural flexibility, more extraordinary mechanical strength along with multi-functionality has been studied recently. Misra’s group concluded the interaction between the electrically induced strain of a cellular CNT with the electric field [17]. Nevertheless, currently, the required electric-field for high-degree deformation seems to be extremely high. Gowda’s group found that the actuation in the same direction with the applied electric field is much larger than that in a perpendicular direction and they designed a CNT cellular-based structure with high polarity independence, which could deform up to 30 % in a 4.2 kV/m electric field [18]. Some liquid metal droplets or gels also exhibit electrostrictive ability due to an asymmetrical structure with easy charge flow. Tang et al. demonstrated liquid metal droplets coated with WO3 nanoparticles, which could migrate along the surface of liquid metals freely to break its original symmetry in an electric field and result in actuation behaviors [19]. Thus, they could move in a solution when a voltage is applied. Morale et al. fabricated millimeter-scale gel walkers to transduce applied electrical stimuli into mechanical motions [15]. In Fig. 2 (b), their two legs were made of gels with opposite charges, which could deform in the opposite direction under an electric field, and thus they could walk without external mechanical input. This method offers us a chance of self-actuation materials.

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Table 1. Comparation of Smax, Emax and d*33 (Smax/ Emax) of various Piezoelectric Materials. Classification

Smaxa)

Emaxb)

d*33c)

(%)

(KV/cm)

(pm/V)

System

Refs

Piezoelectric Organics

x-PVDFd)

3

100

30

[20]

Piezoelectric Composite

Pb(Zr0.52Ti0.48)O3/PVDFd)

5

50

87

[21]

Piezoelectric Organics

PTFEe)

10

50

200

[22]

Piezoelectric Polycrystals

BiAlO3-doped Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3

21

70

300

[23]

Piezoelectric Composite

PZTf)/epoxy

108

270

400

[24]

Piezoelectric Polycrystals

0.85Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3

46

75

613

[25]

Piezoelectric Polycrystals

Bi0.5(Na,K)0.5TiO3

40

60

667

[14]

Piezoelectric Polycrystals

(Ba0.70Sr0.30)TiO3-Modified Bi0.5(Na0.80K0.20)0.5TiO3

40

55

728

[26]

Piezoelectric Polycrystals

(94-x)(Na1/2Bi1/2)TiO3-6BaTiO3-x(K1/2Na1/2)NbO3

57

70

814

[27]

Piezoelectric Polycrystals

(Bi1/2Na1/2)0.935Ba0.065Ti1−x(Fe1/2Nb1/2)xO3

42

50

840

[28]

Piezoelectric Polycrystals

LiNbO3-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3

60

70

857

[29]

Piezoelectric Polycrystals

Pr3+-modified 0.93(Bi0.5Na0.5)TiO3-0.07BaTiO3

43

50

860

[30]

Piezoelectric Polycrystals

0.985[(0.94-x)Bi0.5Na0.5TiO3-0.06BaTiO3-xSrTiO3]0.015LiNbO3

44

50

880

[31]

Piezoelectric Polycrystals

Nb-doped Bi1/2(Na0.84K0.16)1/2TiO3-SrTiO3

44

50

880

[32]

Piezoelectric Single Crystals

Na0.5Bi0.5TiO3-0.12K0.5Bi0.5TiO3

23

30

766

[33]

Piezoelectric Single Crystals

(94-x)(Na1/2Bi1/2)TiO3-6BaTiO3-x(K1/2Na1/2)NbO3

57

60

950

[34]

Piezoelectric Composite

Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3/ epoxy

60

48

1256

[35]

Piezoelectric Single Crystals

Mn-doped (Li,Na,K)(Nb,Ta)O3

42

30

1400

[36]

Piezoelectric Single Crystals

Na1/2Bi1/2TiO3-BaTiO3-K1/2Na1/2NbO3

67

40

1675

[37]

Piezoelectric Single Crystals

xBaZrO3-(0.85-x)BaTiO3-0.15CaTiO3

112

50

2244

[38]

Piezoelectric Single Crystals

0.92(Na0.5Bi0.5)TiO3-0.06BaTiO3-0.02(K0.5Na0.5)NbO3

83

28

2964

[39]

a)

Smax: the maximum electric-field induced strain; b)Emax: the max electric-field intensity; c) d*33: piezoelectric coefficient; d)PVDF: polyvinylidene fluoride; e)PTFE: poly tetrafluoroethylene; f)PZT: lead zirconate titanate.

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2.3. Magnetic Field-Induced Actuation 2.3.1. Magnetic Shape-Memory Materials

Fig. 3. (a) Magnetic susceptibility could be improved with the length of the hysteresis loop. Reproduced by permission of Springer Nature from [40]; (b) A ring-like model made by shapememory polymer nanocomposites. Reproduced by permission of Elsevier from [41]; (c) Transformation temperature could be lowered to improve the magnetic properties. Reproduced by permission of Elsevier from [42].

Magnetic-Responsive materials, especially some magnetic shape-memory materials, could change their shape in a predefined way from a temporary shape to an original state when exposed in a magnetic field. As indicated in Fig. 3 (b), Heuchel et al. proposed a ring-like model made by shape-memory polymer nanocomposites, whose diameter could increase with the magnetic field strength in a controlled way [41]. Moreover, Li’s group reported a more selective self-actuation

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progress with a shape-memory multicomposite, which could exhibit multiple temporary shapes in various alternating magnetic and radiofrequency fields [43] and thus provided a unique and convenient way of self-actuation (Fig. 4 (b)). The mechanism of magnetic shape-memory materials was visualized by Smith and coworkers via a sample of Ni-Mn-Ga [44]. They pointed out that magnetic-field-induced stress is generated when a sufficiently strong magnetic field is exerted to the material, which might rearrange the crystallographic structure of this material.

2.3.2. Magnetic Nanomaterials

Fig. 4. (a) Magnetic building blocks could self-assemble in a magnetic field. Reproduced by permission of John Wiley and Sons from [45]; (b) Magnetic shape-memory materials could change their shape in a magnetic field Reproduced by permission of Royal Society of Chemistry from [43].

Magnetism is inherent in magnetic nanoparticles, which could be controlled remotely in an external magnetic field to generate mechanical force or heat. Fig. 4 (a) indicated that magnetic building blocks could self-assemble in a magnetic field [45]. Several peers dedicated to improving the magnetic-responsive ability. For instance, in Fig. 3 (c), Zeng’s group lowered the 13

martensitic transformation temperature of Mn-Co-Ge alloy significantly via the substitution of Fe for Co atoms in order to improve the magnetic properties [42]. Wong and coworkers investigated the hysteresis loop of magnetic nanomaterials to pursue a better magnetic susceptibility [40] (Fig. 3 (a)), which provided a theory for the characterization of magnetostrictive materials and laid the foundation of more intricate magnetic field-induced effects together with other stimulations, such as illumination.

2.3.3. Magnetic Fluids

Magnetic fluids strongly respond to external magnetic fields while they behave as Newtonian fluids in the absence of magnetic field application. Therefore, it could be said that their rheological behavior changes from a liquid-like to a solid-like one with the application of external fields [46]. Based on these properties, Nokata et al. designed a robot that can move through a confined space with a soft and deformable body [47]. When a magnetic field is applied, the inter magnetic fluids move in the direction of the magnetic field quickly and then the outer object rolls and moves. Therefore, magnetic fluids inside are affected by the external magnetic field and push the inner side of the robot body to the direction of a magnetic field

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2.4. Illumination-Induced Actuation 2.4.1. Photoelectric Materials

Fig. 5. (a) Schematic diagram of the photocurrent generation process. Reproduced by permission of American Chemical Society from [48]; (b) Photoresponse characteristics of MoS2-WS2 heterojunction. Reproduced by permission of John Wiley and Sons from [49]; (c) Model of photoinduced bending. Reproduced by permission of American Chemical Society from [50]; (d) I-V characteristics of MoS2-WS2 heterojunction Reproduced by permission of John Wiley and Sons from [49]; (e) Schematic diagram of the wheels with liquid crystalline polymer Reproduced by permission of John Wiley and Sons from [51]; (f) Linear relationship between their photocurrent and light power density. Reproduced by permission of John Wiley and Sons from [52].

15

Photoelectric Materials possess unique self-actuation photoelectric characteristics, which means their photoexcited electron-hole pairs could separate from the interface to generate selfactuation photocurrent with illumination (Fig. 5 (a)). In Fig. 5 (f), there is a linear relationship between their photocurrent and light power density. Heterostructures feature thickness, an intrinsic surface without dangling bonds and strong light-matter interactions, which are promising candidates for photoelectric materials due to their excellent photoresponse and optoelectronic properties. However, there are still some drawbacks, such as low light absorption and the lack of high-quality p-n junction, impeding their full application. Several coworkers attempted to address these problems with compounded heterostructures. Since MoS2 film has a strong ability of light absorption due to its vertically standing layered structure, considerable amounts of research on the preparation and application of MoS2 film has been extensively conducted in recent years. Huo et al. utilized a MoS2-WS2 heterostructure because they have similar lattice constants and crystalline structure, which could be strain-free with small structural defects and thus could exhibit decent rectifying behavior [52]. Wang and coworkers actualized the MoS2-Si heterojunction to improve photovoltaic activity. Photoresponse and I-V characteristics of this heterojunction are shown in Fig. 5 (b) and Fig. 5 (d). This as-fabricated structure could provide photo-generated carriers with high-speed paths of separation and transportation [49]. Yang et al. provided a new sight via fabricating a GaTe-MoS2 heterostructure for the reason that GaTe is a p-type semiconductor with a less symmetrical structure and in-plane anisotropic characteristics, which might contribute to the transport of the electrons and hole carriers [48]. Due to two different Ga-Ga bonds existing in a single layer, there are also some researches focused on Ga based heterojunction. A novel GaAs nanocone array heterostructure was designed by Luo et al.

16

[53]. Its unique nanostructures could trap incident light and make it have potential applications in future optoelectronic devices.

2.4.2. Photostrictive Materials

Unlike photoelectric materials mentioned above, which could generate a photocurrent in response to illumination, photostrictive materials might undergo such deformation when exposed to light, as shown in Fig. 5 (c). The mechanism was proved by Nath’s group [50]. They found that the structure might generate an inner residual strain with light that causes the macroscopic flexure of this material. Liquid crystalline polymer, with photo-induced bending behavior that endows it with a reversible deformation upon UV-light irradiation, has been attractive in recent years. Based on this phenomenon, Lu’s team constructed wheels with liquid crystalline polymer as well. In Fig. 5 (e), they found that when the liquid crystalline polymer set as the outer layer, the wheel could roll away from light, and it is the opposite when the polymer is set as the inner layer. Thus its rolling direction or speed could be controllable [51]. With low cost and simple fabrication process, this wheel might have practical applications. Similar to thermosalient materials, which have been discussed in the following section in detail, there are also photosalient materials. When the inner stain released suddenly, it might cause macroscopically mechanical motion as well. Naumov et al. emphasized that one photosalient material could jump up to tens of centimeters from the surface [54]. However, most photostrictive materials could only be induced by UV-light, which is not cost-effective concerning practical uses. Hence, more efforts should be spent on visual-light induced materials.

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2.5. Temperature-Induced Actuation

Though motility is habitual for beings, it might be hardly intuitive for simple objects. Recently, some reports about temperature-induced crystalline materials, which can jump or dart when heated. Among them, thermosalient crystals are the most common heat-induced materials. When heated, the material undergoes a rapid phase transition, where internal strain is accumulated, and then the strain is released by structural transformation, resulting in crystal displacement, namely the thermosalient effect. The mechanistic aspects of this phenomenon were investigated by Panda’s group [55]. Then, in Fig. S2 (a), they ascertained that thermosalient structures must be soft materials lacking in tridimensional hydrogen bonding after they compared L-pyroglutamic acid, D-pyroglutamic acid, and racemic pyroglutamic [56]. Sahoo et al. also explored 1, 2, 4, 5-tetrabromobenzene to establish a relation between its kinematic profile and mechanical property and provided a more detailed analysis of the thermosalient effect [57]. Subsequently, they continuously analyzed the thermodynamic and kinematic behaviors of four thermosalient materials comparatively [58]. More details about their study are shown in Fig. S2 (b). Apart from thermosalient materials, some other novel materials might exhibit the capability of spontaneous thermal deformation as well. Yang’s group studied a series of mesogen-jacketed liquid crystalline polymers before demonstrating that they can shrink reversibly when heated, favoring their use as self-actuation switches [59] (indicated in Fig. S3 (a) and (b)). Depa and coworkers fabricated electrical switches via temperature-responsive hydrogels because it could shrink when heated and expand to the original state when cooled down [60] (Fig. S3 (c) and (d)), whose performance indicates the feasibility of industrial production.

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2.6. Humidity-Induced Actuation

Fig. 6. (a) Schematic representation of humidity-induced Actuation. Reproduced by permission of John Wiley and Sons from [61]; (b) Schematic illustration and photograph of PPy motor. Reproduced by permission of John Wiley and Sons from [62]; (c) Chemical structure and mechanism of water exchange. Reproduced by permission of Springer Nature from [63].

Plants could translate environmental stimulations into various mechanical motions. For instance, pine cones can release nuts for reproduction in response to a change in humidity [64]. Analogously, humidity-sensitive materials undergo rapid and reversible mechanical deformation in response to water (Fig. 6 (a)), which can be applied to produce self-driven actuators. Wang et al. proved the progress of moisture absorption and desorption in Fig. S4 (d) [65]. Humidity sensitive materials might bend in damped conditions and recover to their original shape when the relative humidity decreases.

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Inspired by pine cones, a moisture-induced self-shaping biocomposite actuator was invented by Duigou et al. with plant fibers due to their high ability of water absorption and swelling [66]. However, the self-shaping motion was relatively slow in contrast to natural actuators. Thus, further optimizations are still needed. Obiwelouzor’s team demonstrated a poly(vinyl alcohol) film with excellent humidity-responsive properties due to hydrogen bonding [67], which undergoes fast, dramatic, and reversible deformation when exposed to moisture. The chemical structure of the film and mechanism of water exchange was illuminated by Zhang’s group [63] (Fig. 6 (c)). Similarly, Arazoe et al. fabricated a humidity-driven actuator with π-stacked nitride polymer [68], which could respond to a small amount of water. It would bend in the ambient humidity and relax so rapidly when the water droplet was removed that it could jump high vertically due to its highly anisotropic layered structure. Liu et al.'s work was proved valuable in addressing problems of humidity-responsive film’s inconsistency in the direction of bending [61]. They chose cross-linked liquid crystals polymers as preferred candidates for humiditydriven actuators, whose orientation enables a fast and reversible macroscopic deformation in a predefined direction (Fig. S4 (a)). Fig. S4 (b) describes the in-situ AFM height images of dry state, after water absorption and desorption. Applications of functional materials are driven mostly by their utility in society. Okuzaki and coworkers also found that polypyrrole (PPy) films exhibit linear elongation when relative humidity increases [62]. Based on this phenomenon, they described a ‘PPy motor’ with a closed PPy film belt, whose surface would expand when a water container was placed on the pully, and it would decrease when another organic solvent was placed onto the other side. The expansion-contraction cycle might cause a continuous clockwise rotation (Fig. 6 (b)). Chen’s group proposed a smart curtain of a model house based on bilayer

20

carbon nanotube soft actuator [69]. In Fig. S4 (c), it would keep a rolled-up state in fine weather and display a close state to shield the window when the relative humidity is high.

2.7. Applications of Self-Actuation Materials

Self-actuation materials could be applied to energy harvesters and self-healing systems. Energy harvesters could gather mechanical, thermal and solar energy based on the triboelectric effects [70] and piezoelectric effects [71]. Thus, energy harvesting devices could harvest energy from the ambient environment to realize energy conversions, such as piezoelectric generators, triboelectric generators, and some other novel generators. Moreover, many self-actuation materials can damp the vibrations of mechanical structures by converting mechanical energy into other forms of energy [72]. These materials can also harvest energy from vibration, which can be used to recharge vehicle batteries or feed monitoring sensors for structural monitoring of the health of aging bridges [73]. Self-healing materials could heal various damages, such as cuts, cracks, dents, and delamination spontaneously or with the aid of external triggers. Therefore, with an extended lifespan, these materials eliminate the necessity of regular replacement, which makes them more reliable and economically viable [74].

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3. Self-Sensing Materials

Fig. 7. (a) Mechanophore could set off a fluorescent signature with mechanical stress. Reproduced by permission of Elsevier from [75]; (b) Fiber fracture results in a sheer increase of resistance. Reproduced by permission of Elsevier from [76]; (c) Fluorescence-Respond fibers might lose fluorescence when fractured. Reproduced by permission of Royal Society of Chemistry from [3]; (d) Relationships of strain and electrical resistance. Reproduced by permission of Elsevier from [77].

Point-Based sensors, which are sensors embedded into materials, might suffer from the degradation of mechanical properties and the incompleteness of captured information. Thus, there is a risk of missing critical information in regions without a sensor and a great incentive to develop self-sensing materials that could sense some parameters everywhere. Self-sensing refers to the ability that material could sense its physiochemical conditions, such as kinetic,

22

thermodynamic, and chemical parameters, and deliver dynamic information in real time without the requirement for extra sensors [8,9]. For example, physical parameters self-sensing materials could monitor their structural health or other conditions when used. Thus, they are particularly necessary for structures vulnerable to aging, such as aircraft and highrise buildings. Such a selfsensing system may undergo constitutional reorganization in response to external effectors and then generate a signal [9]. These signals incorporate changes in resistance, voltage, fluorescence, etc. To date, various types of self-sensing sensors with different mechanisms to respond to external changes are reported. Hence, we cover the recent advances of these sensors through their responsive principles.

3.1. Resistance-Respond Self-Sensing Materials

Generally speaking, resistance-respond self-sensing materials lead to the changes in the conductivity signal upon the external force, and most of these materials are the incorporation of conductive nanoparticles or nanofibers, which could establish a conductive network. Their resistance could be sensitive to applied pressure owing to the arrangement and interfacial microstructure of the conductive network (Fig. 7 (d)). For instance, Ramirez et al. proposed selfsensing carbon nanofibers (CNF) [76], whose fracture could be considered as an open circuit and result in an abrupt increase of resistance, and thus some minor damage could be revealed (Fig. 7 (b)). CNT might be the most common carbon fibers in resistance-respond self-sensing materials with modest improvement in impact strength with interfacial bond-strength when added into other materials [78]. Bi et al. integrated SiO2 with CNTs to enhance compressive strength, and thus, it could be used as an ideal self-sensing structural material [4]. Besides, multi-walled

23

carbon nanotubes (MWCNTs) with high specific surface area and enhanced Van der Waals attraction forces among nanotubes have also received enough attention. Antonella and coworkers presented an in-depth study of strain self-sensing ability of MWCNTs with various fabrication processes and ascertained the crucial role played by sonic treatment and dispersing additive [79]. Similarly, some nanoparticles could also be mingled to strengthen both mechanical ability and self-sensing ability. Such as graphene nanoplatelets (GNPs) and carbon black particles [80], which could improve mechanical properties as well as endow better stress transfer during deformation to generate an electrical resistance response [81]. Moreover, there are also some novel piezoresistive materials such as graphene films [82], piezoresistive cement [77], which might possess a crucial promotion in the future.

3.2. Voltage-Respond Self-Sensing Materials

These kinds of materials could accumulate electric charges with mechanical stress and generate an electrical signal. Among them, most common voltage-respond self-sensing materials are the piezovoltage one, which could produce output voltage when a mechanical impact is applied and estimate impact location and problems. This material could convert the kinetic energy into electrical energy, while the electrical energy generated is minuscule. Therefore, several peers dedicated their work to amplifying the signal to improve the sensitivity. Guin et al. reported that the output voltage depends on the active area and the direction of stretching [83]. The larger the active area, the higher the output voltage. Thus, they proposed a ridge-like shape piezoelectric polyvinylidene fluoride (PVDF) with higher inner surfaces and higher conversion efficiency could be obtained. Very recently, Yan and coworkers improved mechanical properties

24

and electrical resistance of PbTiO3 ceramic via Sn and Mn doping, which could enhance the piezoelectric properties with high Curie temperature [84]. There are some magnetoelectric materials, which could sense the magnetic fields and give a voltage output as well [85].

3.3. Fluorescence-Respond Self-Sensing Materials 3.3.1. Fluorescent fibers

Some fluorescence-respond self-sensing materials with inherent fluorescent ability are added into fibers, and these fibers might lose their fluorescence when they are fractured (Fig. 7 (c)) [3]. When these fibers are arrayed into networks, even the naked eye could observe the changes in a fluorescence pattern (Fig. S5 (a)) [86]. Another advantage of fluorescent fibers could be that they could provide the approximate location of damage via the time interval between the input signal and reflected signal [87].

3.3.2. Mechanophore

Another self-reporting material is the mechanically sensitive molecule, known as mechanophore. It could produce chemical reactions upon mechanical loading [75]. It could convert the mechanophore form into the merocyanine form upon external force and set off a fluorescent signature to report the mechanical stress (Fig. 7 (a)).

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3.4. Brief Summary

Recent progress in these materials, as we mentioned above, are summarized in Table 2, in terms of their advantages, issues, and applications.

Table 2. Summary of self-sensing materials and their advantages and issues. Self-sensing sensors

Materials

Advantages

Issues

Refs

Piezoresistive Sensors


Carbon Nanomaterials


Low Power Consumption


Complex Electronic Circuits

[76,78–81]


Piezoresistive Materials

Piezoelectric Sensors

Capacitive Sensors

[77,82]


Piezoelectric Materials


Read-Out Mechanisms


Magnetoelectric Materials


Self-power Ability


Piezocapacitive Materials


Temperature Independent


Graphene


Poor Sensitivity

[85]
Low Resolution

[88]


Environmental Interference

[89]


Ionic Liquids Fluorescent Sensors


Fluorescent Fibers

[90]
Macroscopic Changes


Mechanophores Microcantilever Sensors

[84]


Piezo Materials


Label-Free


Optical-Responsive Materials


Real-Time Monitoring


Usually in Qualitative Way

[86,87]


Complex Pretreatment

[75]


Detection Limit

[91–93] [94]


Portable
Inexpensive

3.5. Applications of Self-Sensing Materials 3.5.1. Kinetic Parameter Sensors Measurement of Strain/Stress

Some carbon materials exhibit electrical-resistance sensitivity to applied stress, which could be utilized for self-sensing strain or stress. There is also a linear relationship between stress and resistance so that the external stress could be obviously indicated. Standard resistance

26

measurement uses the two-point probe method (both probes serve for passing current and voltage measurement) or four-point probe method (two probes serve for passing current and two probes serve for voltage measurement). Whereas, though the two-point probe method is relatively convenient, it suffers from some significant drawbacks. It introduces a time-dependent reduction in conductivity, and its polarization might cause the measurement error. Therefore, the four-point probe method could be the preferred approach for conductivity measurement. Based on this method, Ramirze proposed a stress sensor based on carbon fiber [76], whose resistance increases monotonically with the increasing strain amplitude. Apart from carbon nanofibers and nanoparticles, piezoelectrical materials could also be applied in strain or stress sensors. Emamian et al. fabricated a screen printing flexible piezoelectric via Polyethylene terephthalate (PET) [95], which had a linear relationship between voltage and varying applied forces. Moreover, they used PVDF as well and pointed out that these printed sensors can be used for self-sensing both touch and force (Fig. S5 (b)) [96]. Seminara et al. manufactured a robotic skin system based on arrays of PVDF film transducers [97]. This system enables robots to interact safely and effectively with unstructured environments and humans in the case of both voluntary and reactive interaction tasks.

Measurement of Breakage/Cracks

Microcrack is the major type of structural damage and even leads to a sudden failure of the structure. Practically, it is indispensable to detect the cracks and avoid more severe failure. Early detection of small damages of materials could extend its service life as well as increase its operational safety. Breakage sensors could be some fibers embedded into materials. When the

27

breakage happens, some physical characters of the fibers such as electrical resistance will change, and the location of the breakage could be determined. In this way, some invisible cracks could be suggested in the form of resistance, which could be detected. For instance, Hirano et al. fabricated a breakage sensor via conductive fibers to monitor delamination cracks with the measurement of electrical resistance [98]. Besides, Lorcher et al. invented a micron-sized damage sensor with fluorescent fiber [3], which might lose its fluorescence when fractured in response to damage. Apart from the potential risk of breakage, Kwon’ s group conceived a multifunctional selfsensing system based on piezo materials, providing an efficient and economical way for selfsensing various jet failures such as abnormal temperature and backpressure [99]. Chen and coworkers invented an elastic (mechanical) wave sensor via a piezoelectric system and amplified the electrical signal successfully with adaptive circuits [100], which provides more opportunities for integral self-sensing systems.

Measurement of Displacement

Displacement self-sensing materials are usually estimated based on some other parameters, such as resistance or permittivity. Most of them could be piezoelectric materials or shape memory materials. The displacement gives an output in the form of stress or strain, which could be estimated by electrical resistance. Rakotondrabe et al. proposed a piezoelectric cantilevered [101], which could deflect in response to displacement and its electrical resistance changes correspondingly. Saigusa’s group utilized a self-sensing method for detecting the displacement

28

from the permittivity change [102]. Yu and coworkers improve the positioning accuracy further by increasing the load and decreasing the surface [103].

3.5.2. Thermodynamic Parameter Sensors Humidity sensing

Humidity sensors are widely used to monitor the air of environment and industrial production. Some piezo-humidity sensors could have an output voltage when exposed to a humid environment owning to lots of water molecules replace the adsorbed oxygen ions [104]. Gu et al. described a piezoelectric humidity sensor [105], which exhibited a negative correlation between output voltage and environmental humidity. The dependence could be attributed to the increased leakage current generated by proton hopping among the H3O+ under the driving force of the piezoelectric potential in the piezoelectric materials. Jang and coworkers fabricated the CNT/cement composites, whose electrical conductivity change with moisture effect and could be used as a moisture sensor as well [106].

Temperature sensing

During the phase transition, some materials show significant changes in electrical, mechanical, electromagnetic, and optical properties, which could be used for temperature selfsensing. For example, VO2 could indicate the temperature based on its resistance measurement because its thermally induced phase transition could be linked to temperature, and thus, its resistance has a relationship with temperature [107]. Tang and et al. found a new type of self-

29

sensing fiber reinforced polymer [108], which exhibits a perfect linear relationship between Brillouin frequency shift and temperature.

3.5.3. Chemical Parameters Sensors

Today, many available biosensors all come at a very high cost and need complex optical measurement systems. Microcantilever sensors have emerged as a new technology for detecting ultrasmall masses and biological concentration with cost-efficiency, sensitivity and convenience [109]. The deflection of microcantilever could give a signal for real-time self-sensing, and these sensors could operate in two main modes: (i) static mode, where changes of surface stress are measured, and (ii) dynamic mode, where other physical parameters indicating the surface stress are measured. Common physical parameters include capacitance, resistance, voltage, frequency, or some optical properties. The dynamic mode provides higher sensitivity than static mode with the ability of self-sensing ultrasmall mass. Among them, capacitive-respond microcantilever sensors could monitor the deflection of the microcantilever in the form of capacitance change. The main drawback of it is the low resolution [91]. Piezoresistive-Respond microcantilever sensors work based on the changes of resistance in response to the deformation. With this method, Lin’s group demonstrated high reliability of realtime blood coagulation monitoring [92]. Blood coagulation increased the force of microcantilever, which can be detected with the signal of resistance change (Fig. S6 (b)). Although it exhibited potential in miniaturization for personal diagnosis, the measurement of resistance might need complex electronic circuits.

30

Most voltage-respond microcantilever sensors use piezoelectric materials, which could generate a voltage signal in response to the deformation of the microcantilever and provide a simple sensitive read-out mechanism. Also, owing to the inverse property of piezoelectric materials, when the deformation generates, it might bring the system into vibration. Thus, the deformation of microcantilever could also be measured in the form of a frequency shift with improved accuracy. Faegh illustrated microcantilever sensors with a relationship between frequency shift and adsorbed mass [91] (Fig. S6 (a)), which increased the accuracy to 99.82 %. Additionally, they pointed out that the amount of shift in the circuit’s resonance frequency could provide qualitative and quantitative insight into the amount of target protein and could be applied in rapid, continuous, and highly sensitive self-sensing [93]. There are also some optical-propertyrespond microcantilevers. For instance, a laser vibrometer was used to measure the response of microcantilever optically by Faegh et al. [94]. Several peers devote their work to improving the property of self-sensing microcantilever sensors. Lower detection limit and higher sensitivity are urgently required. Their methods include geometry modification, employing nanoparticles, or operating MCs in lateral and torsional modes. Alodhayb and coworkers described a microcantilever array sensor system with 16 microcantilevers, which could be constructed in little time and is affordable [110]. Microcantilevers with different geometrical dimensions were designed by Kucera and coworkers to determine the optimal geometry modification [111]. Zhang et al. simplified the preparation of microcantilever sensors via a one-step immobilization of the thiol group [112]. Besides, Plata and coworkers fabricated a microcantilever sensor whose probe response could be used to determine the carbonate content in soil specimens[113].

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Fig. 8. (a) Deflection responses of immunosensors in injections of avian influenza virus (AIV), 1 the blank solution (phosphate buffer solution), 2-7 AIV solutions of different concentrations. Reproduced by permission of Springer Nature from [114]; (b) Relationship between the deflection response and the logarithm of the concentration of AIV. Reproduced by permission of Springer Nature from [114]; (c) Deflection responses of the immunosensor in sequential injections of carbofuran. Reproduced by permission of Elsevier from [115]; (d) Relationship between the deflection response and the logarithm of the concentration of carbofuran. Reproduced by permission of Elsevier from [115].

Our research group also focused on self-sensing microcantilever sensors. We reviewed the working principle, excitation method, detection mechanism, and latest applications of microcantilever biosensors [116]. Moreover, based on previous studies, our group has

32

successfully prepared a microcantilever sensor, which could be applied in determining the concentration of lysosome in a pharmaceutical formulation sample [117], avian influenza virus H9 [114], and carbofuran in soil and vegetable samples [115]. The label-free probing of avian influenza virus H9 was achieved by a microcantilever covalently modified by specific antibodies of the virus. The functionalized microcantilever exhibited satisfying sensitivity and broad linear range (Fig. 8 (a), (b)). The sample testing of carbofuran was conducted through a similar microcantilever immunosensor in the previous study. The surface was functionalized with Lcysteine (L-cys)/glutaraldehyde (GA), specific antibodies of carbofuran and gelatin (blocking agent) successively, thereby giving the microcantilever sensing ability of carbofuran in soil and vegetable samples with excellent specificity and stability. Our microcantilever sensors for carbofuran detection are with higher sensitivity, a more significant linear response and without any need of fluorescent or radioactive molecules label (Fig. 8 (c), (d)). Furthermore, their portability and inexpensiveness enable the applications in medicine, agriculture, food safety, and environmental monitoring in the future. At present, most of the readout systems of microcantilever sensors use complex and expensive optical lever systems, and the multi-array microcantilever measurement is limited [116]. Our research group also fabricated self-actuation and self-sensing microcantilever sensors. The working principle of our self-actuation and selfsensing microcantilever sensors is that when the functional molecules, polymers, antibodies, or enzymes on the surface of the microcantilever interact with the target (such as the affinity between antibodies and antigens), the microcantilever deforms and generates a certain potential change [118]. Our fabricated immunosensor was used to determine clenbuterol in pork samples with satisfactory results [119]. The developed method provides potential for the construction of immunosensors for the trace determination in the future.

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Other self-sensing chemical parameters sensors could be indicators employed as visualization of external environmental changes. For example, colorimetric methods are gaining increasing attention in the application of self-sensing and removing toxic metal ions from wastewater due to the advantages of simplicity and cost-effectiveness [120]. The excessive presence of these ions has become a major environmental challenge worldwide that affects human beings with long term and acute toxicity [121,122]. Although modern instrumental techniques are highly sensitive and selective, they are time-consuming and they often need highcost instruments, trained operators as well as complicated sample preparation [123,124]. In contrast, colorimetric methods exhibit an obvious color change with specific ions as a signal visible to the naked eyes [125,126]. These methods are much faster, more convenient and suitable for online toxic metal ions analysis, which can detect and remove toxic metal ions from water automatically. In recent years, Md Rabiul Awual’s research group developed several types of colorimetric ordered mesoporous materials with tunable porosity. These materials are highly sensitive with excellent sorption capability and can be considered as a promising adsorbent for heavy metal ions monitoring and removal from wastewater to safeguard the public health especially in developing countries [127–130]. Moreover, phenolphthalein turns pink when the pH is higher than 8.2. Based on this phenomenon, Maia et al. fabricated a corrosion self-sensing coating via incorporating polyurea microcapsules into aluminum and magnesium alloys coating as a corrosion sensor [131]. In Fig. S6 (c), this sensor could turn pink as a result of local pH increases during the corrosion process and could be applied to identify and locate the corrosion degradation in early stages. Thus, reducing the possibility of potential structural failures and costs associated with repairing could be reduced. Similarly, Kenneth’s group added BChl-lipid into porphysomes [6], which

34

accumulate in tumor, in order to self-sense its vesicle structure qualitatively. This system would emit fluorescence when vesicles are broken and the optimum period of oncotherapy could be determined. However, these materials could only be qualitative rather than quantitative. Till now, a wide variety of quantitative self-sensing materials have been developed. Ye et al. fabricated a self-sensing microcapsule with photonic crystal [132], which could recognize and react with the targets, and there is a relationship between the amounts of the targets and the Bragg diffraction peak (Fig. S7 (a)). Faegh and coworkers designed a piezoelectric material based biological sensor for glucose detection [93]. The capacitance of its interface changes with the mass of immobilized glucose and thus, the resonance frequency could indicate the concentration of glucose (Fig. S7 (b)) Omidi’s group reported a biosensor for prostate-specific antigen (PSA) detection based on piezoresistive materials [133]. The surface stress changes with the antigen-antibody interaction, and there is a corresponding change of voltage. Fig. S7 (c) shows the linear relationship between concentration and voltage. Jia and coworkers also used fluorescence self-sensing technology [5]. Interestingly, they proposed a self-referencing signal that is difficult to be interfered with by environmental factors.

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4. Self-Actuation and Self-Sensing Materials 4.1. Piezoelectric Materials

Fig. 9. (a) Sensing ability of the CNF and the PVDF reference sensors. Reproduced by permission of American Chemical Society from [134]; (b) Comparisons about the physicochemical characterization of BNT and PZT. Reproduced by permission of Royal Society of Chemistry from [135]; (c) Comparisons about the piezoelectric ability of various piezoelectric materials. Reproduced by permission of Elsevier from [136].

Piezoelectric materials are experiencing fast growth due to the strong demand from industry and manufacturing, followed by medical instruments and telecommunications [137]. Piezoelectricity is that electric charge accumulates in certain materials in response to mechanical stress, and there is a linear electromechanical interaction between the electrical and the

36

mechanical state. Aside from the direct piezoelectric effect (internal electrical charge resulting from pressure), these materials also exhibit the reverse piezoelectric effect, namely a mechanical strain generated with an applied electrical field. Therefore, they could detect the external change powered by the generated electric potential instead of an external power supply. Their unique piezoelectric ability favoring their use as self-actuation and self-sensing materials, and thus they could be applied in actuators and sensors [111,138]. There are four main categories, piezoelectric multi crystals, single crystals, polymers, and compounds. Chen’s group concluded that the piezoelectric ability of piezoelectric single crystals is much better than that of other materials [136] (Fig. 9 (c)).

4.1.1. Piezoelectric Multi Crystals

PZT is the most common piezoelectric ceramic in use today owing to its high piezoelectric responsivity and large-scale production capability. Recent advances focus on the modification of PZT. Patel et al. added manufactured metallic Inconel substrates [139], whose interface might form an oxide layer during the high temperature crystallization, and thus this material could be used in high temperature with promising electrical properties. However, its toxicity might be a potential risk for the environment and human beings in the manufacturing process and waste disposal. European Union has published a health normative avoiding the use of lead recently and then comes the more environmental-friendly lead-free substitutions. Researches have been conducted on potassium sodium niobate based materials with enhanced piezoelectric properties, a kind of advanced materials that might replace PZT in the future. As mentioned before, various lead-free BNKT based ceramics, which is a perovskite structure piezoelectric material,

37

exhibiting highly effective direct and converse piezoelectric responses, and more importantly, they are non-toxic, and thus could be the substitution of PZT [140]. Esquivel-Gaon’s group presented comparisons about the physicochemical characterization of sodium bismuth titanate (BNT) and PZT [135] (Fig. 9 (b)). However, piezoelectric multi crystals’ piezoelectric ability is relatively unsatisfactory when compared with piezoelectric single crystals.

4.1.2. Piezoelectric Single Crystals

Quartz crystal is a widely used piezoelectric crystal. It is a mineral composed of silicon and oxygen atoms, which commonly used as crystal oscillators and piezoelectric quartz crystal sensors [141]. For quartz crystals, changes in the electrical parameters could be detected efficiently so that they could be used as low-cost and easy-operate self-actuation and self-sensing materials. Recently, piezoelectric single crystals could be reduced to micro/nano level, and thus the piezoelectric ability and machinability could be improved significantly. Semiconductors, such as ZnO and AlN, possess higher values of piezoelectric coupling coefficient [142], which are the most common piezoelectric single crystals. Kucera presented a self-actuated and self-sensing cantilevers based on AlN owing to its unique piezoelectric ability [111]. Cai’s group proposed the [Zn(L)2(OH)2]n·Guest [143]. Moreover, its giant piezo-mechanical response is tunable with various guests. Transition metal dichacol genides such as MoS2 also process a high piezoelectric coupling coefficient that is comparable to and even higher than AlN and ZnO [144].

38

4.1.3. Piezoelectric Polymers

Piezoelectric polymers with low density could be more flexible and biocompatible, which could be applied in sensors and actuators of tissue engineering [145], whereas their piezoelectric ability depends on hydrogen bonding, so the piezoelectric response is low. PVDF, one of the most common piezoelectric polymers, is much more flexible than piezoelectric crystals and could withstand much larger strains. Piezoelectric PVDF is promising materials for sensors due to its wide frequency coverage and high conversion efficiency [83]. This kind of flexible, light-weight, and inexpensive polymer is suitable for wearable, portable, or disposable sensing devices. Lei and coworkers designed a sensor patch for respiration detection based on PVDF [146]. The sensor could deform periodical on a human chest during respiratory and turn the mechanical stretch into electrical signals and is capable of preventing dynamic noises. Apart from sensing, piezoelectric PVDF films are emerging as energy harvesting as well due to the generated energy from the external strain. In this way, Li’s group developed an energy harvester of bi-resonant structure [147], which could generate electric energy from the strain caused by random vibration sources and could be applied in other vibrational energy harvesters based on electromagnetic or electrostatic conversion.

4.1.4. Piezoelectric Compounds

Piezoelectric compounds are usually obtained by integrating bulky or powdery piezoelectric inorganic substances into piezoelectric organic polymers. Therefore, they could combine the advantages of both inorganic and organic materials, such as high flexibility and excellent

39

piezoelectric ability. Some nanoparticles could be doped in PVDF, acting as a catalyst for crystallographic phase transformation to improve piezoelectric properties [148]. Babu et al. combined PZT with poly dimethyl siloxane [149]. The polymer could offer the advantage of flexibility and formability with reduced sizes. These piezoelectric materials could be applied in microsensors, mechanical energy harvesters, and microactuators and might be the direction of future development.

4.2. Shape Memory Materials

Fig. 10. (a) Schematic of magnetic field-induced strain change in magnetically sensitive shape memory materials. Reproduced by permission of Elsevier from [150]; (b) The corresponding change of electrical resistance with strain. Reproduced by permission of Elsevier from [150]; (c) Schematic of the relationship between pH and contact angle. Reproduced by permission of Royal Society of Chemistry from [151]; (d) Organic liquid-induced shape memory material which 40

could recover when exposed to organic liquids. Reproduced by permission of American Chemical Society from [152].

Shape memory materials are able to undergo reversible deformation with external stress or temperature and could recover completely upon certain stimuli [153]. During the deforming process, its electrical resistivity changes significantly along with the mechanical properties, thus could realize the function as self-sensing. Besides, during the recovering process, it could generate a sizeable actuating force thus could realize the function as self-actuation [154]. For instance, Yin et al. fabricated a magnetic shape memory microactuator, which could recover to its original state in a magnetic field (Fig. 10 (a)) and could be used as a strain sensor owing to its electrical resistance changing with strain [150]. Fig. 10 (b) shows the linear correlation between magneto strain and electrical resistance. Wu’s group highlighted an organic liquid-induced shape memory material that could recover when exposed to organic liquids [152] (Fig. 10 (d)). Then they invented an organic liquid sensor, the dramatic increase of which electrical resistivity might lead an LED lampion goes off immediately when dripped 16 mg organic liquid. Aside from chemo-responsive shape memory materials and magnetically sensitive shape memory materials, there are microwave heating triggered shape memory materials as well. Among them, the light-driven and chemical liquid-induced method with relatively clean energy sources could be controlled precisely and instantly, which could be utilized in future shape memory materials. Several peers also gave a further enhancement of shape memory materials’ performance. Zheng et al. utilized a layer multiplying co-extrusion method to make shape memory materials with higher shape fixity and recovery ability [155]. Ma’s group indicated the dynamics study of

41

external mechanical constraints [156]. Ruth and coworkers invented a closed loop to improve the linearity and response rate of the self-sensors [157]. With improved properties, the as-fabricated sensors could have a more extensive application.

4.3. Novel Self-Actuation and Self-Sensing Materials

Other materials could also change their properties in response to external stimuli. For example, some materials could switch between hydrophilicity and hydrophobicity reversibly. In Fig. 10 (c), Jiao et al. described a “smart valve” that could be too hydrophobic to let solution passing through at pH 7.0 due to the protonation of pH groups [151], and pH could be measured with the contact angles.

4.4. Applications of Self-Actuation and Self-Sensing Materials

self-actuation and self-sensing materials have a wide application in self-powered systems, which could harvest environmental energy to put out signals without external power sources and work sustainably and continuously. Therefore, mechanical energy harvesting from the human body in daily life could be converted into electrical energy for the continuous power of sensing systems, which could be applied to wearable devices [158,159] and artificial muscles or skins [88,160]. Wearable devices could be integrated into textile fiber or clothes to monitor and record information of one’s physiological condition, such as sweat rate and body temperature or motion activities [161]. Artificial muscles and skins could transform chemical energy into mechanical energy and respond to different tactile and biological stimuli like natural ones [162].

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5. Conclusions and Outlook

Self-Actuation and self-sensing materials have been arousing enormous attention in the fields of medicine, environment, and manufacture. There are significant interest and research activities in these materials, which would likely take on more significant roles in the next generation of smart materials. Each kind of new material is developed to meet certain demands. Initially, self-actuation materials could be classified into non-autonomous and autonomous systems. Entirely based on spontaneous reaction, without the requirement of external power, self-actuation materials have a relatively narrow scope of application compared with nonautonomous systems. Self-sensing materials deserve adequate attention as well because they can reflect some conditions of their own without additional sensors. Self-Sensors of physical parameters prepared from this kind of material can be used as health monitoring materials. Meanwhile, self-sensors of chemical parameters, such as microcantilever sensors, are even able to conduct real-time monitoring of the concentration of certain materials. Self-Actuation and self-sensing materials combine both functions. However, challenges remain for further application of these materials, such as slow responsiveness, reduced sensitivity and reproducibility, and single stimuli-responsive mode, where future efforts should be focused. We anticipated reasonable improvements in selfactuation and self-sensing materials. There is a scope to develop materials that could be responsive to multiple stimuli with adaptable behavior toward different types of targets or measure multiple signals simultaneously. Self-Actuation and self-sensing materials could be used extensively in wearable devices and even artificial muscles and skins. In the future, this material might be flexible enough to be

43

weaved into clothes powered by body motion energy in order to report health status or automatically respond to the surrounding environment. Self-sensing materials can be used for various biomedical, biomimetic, micro or mobile robotic applications where external sensing systems are usually unavailable. Further, these materials might have more practical applications in our daily life, such as smart windows/curtains that would open in the sunlight and close in the dark at night or some wearable device for health monitoring. These features could be incorporated into the Internet of Things (IoT) and we can use Artificial Intelligence (AI) to help make sense of the collected data. More importantly, future research should give priority to comprehensive benefits, such as safety, cost, as well as user-and-environment-friendliness rather than only focusing on remarkable properties or functions. In summary, we have presented a comprehensive review of recent progress in self-actuation and self-sensing materials, including the fabrication strategies, stimuli-responsive performance, and emerging applications. There is still much room for further development in this field, and research towards this trend should be encouraged and proceeded. Overall knowledge of these materials is of utmost importance, and thus, we hope this systematic review could provide some useful clues and be an inspiration for scientists in an academic circle and industry to motivate new research effort and generate further interest in this area.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 21375116), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Jiangsu Province research program on analytical methods and

44

techniques on the shared platform of large-scale instruments and equipment (BZ 201409). Student’s Platform for Innovation and Entrepreneurship Training Program of Yangzhou University (X20190268).

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Highlights •

This review concerns recent progress and challenges in the development of self-actuation and self-sensing materials.

•

Explanation the mechanisms of stimuli responsiveness together with evaluation of the properties of these materials.

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Perspectives of the applications of self-sensing materials in analytical techniques and portable sensing systems.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: