Emulsion Hydrogel Soft Motor Actuated by Thermal Stimulation - ACS

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43211−43219

Emulsion Hydrogel Soft Motor Actuated by Thermal Stimulation Hui Wang,† Yuling Liang,† Wei Gao,‡ Renfeng Dong,*,§ and Chaoyang Wang*,† †

Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China Department of Electrical Engineering & Computer Sciences, University of California, Berkeley, California 94720, United States § School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China ‡

S Supporting Information *

ABSTRACT: An emulsion hydrogel motor (E-H motor), constituted by low-boilingpoint oil fuel and a hydrogel matrix, is prepared through a simple yet versatile oil-inwater (O/W) emulsion template method. The E-H motor can be efficiently propelled by the bubbles generated under a thermal stimulus. As thermally induced explosion occurs inside the E-H motor (diameter ∼4.0 mm and length ∼6.0 mm), the gas bubbles resulting from thermotropic phase transition are violently ejected from one side, leading to a fast speed of 14.78 ± 4.82 mm s−1 in a 60 °C aqueous solution. Additionally, multiple water-insoluble organic solvents can serve as the fuel for self-propulsion, which demonstrates the favorable universality of the E-H motor. The magnetic navigation and near-infrared propulsion can be realized through incorporating hydrophilic iron oxide (Fe3O4) nanoparticles and graphene oxide (GO) into the aqueous phase. Moreover, the synchronous integration of GO and enrofloxacin bactericide can enable intelligent targeted cargo transportation and delivery. The attractive self-propulsion performance, precise locomotion control, and formidable integration ability of the emulsion hydrogelbased miniaturized soft motor hold great promise for numerous practical applications. KEYWORDS: emulsion hydrogels, soft motor, thermally driven, NIR-driven, sterilization scales and for loading sufficient remediation agents.25,26 Hydrogels could serve as an ideal candidate material for fabricating self-propelled soft motors owing to their great biocompatibility and tunable mechanical properties.27,28 In addition, hydrogels naturally possess a favorable integration and encapsulation capability which can implement a stabilized encapsulation of functionalized substances through gelatinization to realize multifunctional integration. Herein, an ultrafast, biocompatible, and integrated soft motor is fabricated through a facile and scalable fabrication strategy based on an oil/water (O/W) emulsion template method. The conventional motors usually rely on the addition of external chemical fuels such as hydrogen peroxide,6,29 hydrochloric acid,30 or hydrazine.31 The proposed fuel-self-contained motor can extricate from the external fuel dependence: the lowboiling-point oil fuel (prestored in the hydrogel skeleton) could transform from liquid to gas and liberate to regulate and accelerate propulsion under a thermal stimulus. The speed of the thermoresponsive E-H motor with a 1:1 W/O volume ratio is 14.78 ± 4.82 mm s−1 in a 60 °C aquatic environment, which is triple of that of the previously reported hydrogel motor (4.33 mm s−1);32 meanwhile, the fuel availability of the thermally actuated E-H motor is nearly 100% (as shown in Figure S10a, the E-H motor with CH2Cl2 before self-propulsion is milky

1. INTRODUCTION Self-propelled miniaturized motors have gained great interest owing to their abundant applications, including directed drug or protein delivery,1−4 accelerated environmental remediation,5,6 and efficient biological detoxification.7 Chemically powered miniaturized motors with efficient locomotive properties in the presence of hydrogen peroxide fuel have been widespread reported in the past decade.8,9 However, the strong fuel dependence from external environment has greatly hindered the practical utilities of the chemically powered motors. Therefore, there have been considerable interests in designing fuel-free artificial motors driven by magnetic fields,10,11 electrical fields,12,13 ultrasound,14 self-thermophoresis,15,16 and light.18−23 In particular, heat is a versatile and powerful external stimulus to regulate and accelerate the propulsion of miniaturized motors. Additionally, the thermally propelled motors can also use near-infrared (NIR) light as the power to achieve a remote intervention in the propulsion direction and speed of the motor motion.17 Nowadays, the reported thermally driven or NIR-light-driven micromotors are typically fabricated based on inorganic microparticles or metal nanowires24 and suffer from some problems: (1) Some of the inorganic or metal material-based micromotors are not fully biocompatible, which hinders the application of micromotors in the biomedical field. (2) Most inorganic or metal materialbased motors have restricted applications on water surface because of their high material density.8 (3) The micromotors are too small for dynamic decontamination action on larger © 2017 American Chemical Society

Received: June 16, 2017 Accepted: November 22, 2017 Published: November 22, 2017 43211

DOI: 10.1021/acsami.7b08661 ACS Appl. Mater. Interfaces 2017, 9, 43211−43219

Research Article

ACS Applied Materials & Interfaces

by ultrasonication. Then, 40 μL of Tween 80 was dispersed in it through sonication. The chemicals for gelatinization (1.2 g AM, 1.2 mg MBA, and 0.06 g KPS) were orderly dissolved in the solution and mixed well. Thereafter, 12 mL of cyclopentane was added and emulsified with a high-speed shearing machine (5000 rpm, 2 min). After that, 10 mL of TEMED was added with stirring. The obtained emulsion was transferred into glass tubes and polymerized at 6 °C for 24 h. 2.5. Preparation of Enro-E-H Motor. The fabrication process of Enro-E-H motor was the same as the E-H motor except that enrofloxacin (2.4 mg) was dissolved in CH2Cl2 (12 mL) through sonication in an ice bath before emulsification. 2.6. Preparation of GO-Enro-E-H Motor. The fabrication strategy of GO-Enro-E-H motor is the combination of Enro-E-H motor and GO-E-H motor: enrofloxacin (2.4 mg) was dissolved in cyclopentane (12 mL) and GO (4 mg/mL) was dispersed in deionized water (6 mL) simultaneously. 2.7. Characterizations. 2.7.1. Tracking. The self-propulsion process of the E-H motor was performed in a thermostat water bath cauldron. The movement was captured by a Canon IXUS 220 HS camera. The trajectory and speed of the E-H motor were tracked using the NIS-Elements AR 4.3 software. Additionally, the GO-E-H motor or the E-H motor was irradiated by an NIR laser (1.6 W, 808 nm, 5 mm × 5 mm spot) downward from the top during the NIR-driven process. 2.7.2. Optical Microscopy. The Axiolab polarizing microscope (Carl Zeiss, Germany) equipped with a camera was utilized for imaging the O/W emulsion to evaluate the emulsion particle size and distribution. 2.7.3. Scanning Electron Microscopy. Internal morphology of the E-H motors was observed by a scanning electron microscope (SEM, Zeiss EVO 18) operated at a 10 kV acceleration voltage. The E-H motors were freeze-dried for 72 h before observation. 2.7.4. Ultraviolet−Visible Absorption Spectra. UV−visible spectra were measured on a Shimadzu UV-240 spectrophotometer to evaluate the release effects of Enro-E-H motor and GO-Enro-E-H motor. A halogen tungsten lamp with a filter having a cutoff 18.2 MΩ cm) was purified by a Millipore purification apparatus (AquaPro2S) and was de-aired with argon for 30 min before use. 2.2. Preparation of E-H Motor. The thermally driven E-H motor was prepared by the following steps. Six milliliters of 20 wt % AM solution was prepared in deionized water. Then, added were 1.2 mg of chemical cross-linker MBA and 0.06 g of initiator KPS into the solution. PVA (0.06 g) which served as an emulsifier was dispersed in the mixture through sonication. After that, CH2Cl2 (6 mL/12 mL/18 mL) used as the fuel of the motor was added. The complex solution was stirred at 3000 rpm for 2 min to allow better homogenization of the O/W emulsion. Thereafter, 10 μL of the accelerator TEMED was added into the O/W emulsion under stirring. Subsequently, the emulsion was transferred into the mold (glass tubes of 4 mm diameter × 150 mm length) and polymerized at 6 °C for 24 h. The obtained emulsion hydrogel was cut into 6 mm length to serve as an E-H motor. The E-H motor using cyclopentane or cyclohexane as fuel was also fabricated based on the above method. 2.3. Preparation of Mag-E-H Motor. Ferroferric oxide (Fe3O4) NPs were prepared by the precipitation−oxidation method.38 Six milliliters of 2 mg/mL Fe3O4 homogeneous dispersion was obtained with the assistance of an ultrasonic bath for 1 h. PVA (0.06 g), 1.2 g of AM, 0.6 mg of MBA, 0.06 g of KPS, and 20 μL of TEMED were orderly added and mixed homogeneously by sonication before each addition. Then, an E-H motor was placed in the bottom of a glass tube (diameter = 5 mm), and the Fe3O4-mixed solution (0.5 mL) was injected into the glass tube. Subsequently, the glass tube was sealed, and the hydrogel was polymerized at 6 °C for 24 h. 2.4. Preparation of GO-E-H Motor. GO was obtained from the natural graphite powder via the modified Hummers’ method.39,40 Six milliliters of 4 mg/mL GO dispersion was prepared in deionized water

3. RESULTS AND DISCUSSION The E-H motor is prepared by an emulsion template synthesis method.41,42 As presented in Figure 1a, the emulsifier, watersoluble monomer, chemical cross-linking agent, and initiator are dissolved in the aqueous phase, whereas a low-boiling-point organic solvent serves as the oil phase. The composite solution is emulsified with the assistance of high-speed shearing to form the O/W emulsion. The process of free-radical polymerization and gelatinization will generate a hydrogel matrix with 3D network structures,43,44 which provides a shelter for the lowboiling-point fuel. Figure 1b shows the self-propulsion mechanism of a thermally triggered E-H motor. When the EH motor is heated, the stored CH2Cl2 fuel transforms from the liquid phase to the gas phase. The generated gas molecules are restricted inside the hydrogel matrix. Once the gas pressure reaches a threshold value, the CH2Cl2 vapor ejects violently from the weak spots of the hydrogel matrix. This explosive process forms a relatively oriented gas-releasing passage and provides a unidirectional and continuous propulsive force. Though the CH2Cl2 bubbles may also escape from the surface of the E-H motor, the directive bubbles released from the interior of the hydrogel matrix still dominate the movement of the motor. The propulsion of this hydrogel-based motor can be deeply affected by trepanning (shown in Figure 1b), microfluidics technology,45 or other methods.46 Because of trepanning, which means artificially creating a CH2Cl2 (g) release path, the gas tends to be ejected via this prescribed path 43212


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velocity of an E-H motor with a 1:1 W/O volume ratio is much higher than that of an E-H motor with a 1:2 or 1:3 W/O volume ratio at a tracking temperature of 60 °C. Although the motors with higher oil content are normally expected to have a stronger propulsive force and a longer lifetime, the higher oil content leads to a movement stagnation effect, especially in the initial self-propulsion stages. As shown in Figure S2, the movement stagnation is due to the nondirectional bubbles escaped from the motor surface. It is obvious that the motors with higher oil content possess more surface bubbles (Figure S2b,c). Owing to the phase transition induced from the timedelay effect of temperature, the driving force resulting from the vaporization of the low-boiling-point oil is not that powerful during the inception phase. The stronger resistance exerts a more obvious influence on the motors with higher oil content, causing a smaller initial velocity compared to that of the E-H motors with a 1:1 W/O volume ratio. The propulsion of the E-H motor depends on the continuous thrust of CH2Cl2 (g) bubbles generated by the CH2Cl2 (l) stored in the hydrogel matrix and gives rise to an average speed of 14.78 ± 4.82 mm s−1 with a W/O volume ratio of 1:1 at 60 °C (Movie S1). As shown in Figure 2c, the average speeds of the E-H motors do not increase with the increment of the oil content. The E-H motor with a 1:1 W/O volume ratio has the highest average speed, whereas the E-H motor with a 1:2 W/O volume ratio possesses the lowest speed. This phenomenon reveals that the speeds of the E-H motor do not primarily rely on the quantity of fuel stored in hydrogel matrix. Several other factors play important roles on the motion of the motors. First, no matter how much fuel the motor stockpiles, only a dominant gas release can provide an effective movement. In addition, the bubbles escaped from the surface of the E-H motor are omnidirectional. The two factors are ultimately determined by the structure of the hydrogel matrix. A more pyknotic structure of the hydrogel matrix is helpful to restrict the gas inside it until the gas concentrates to break the weak point of the hydrogel matrix to form a directional movement. A more pyknotic 3D network structure will also cause less gas escape from the surface of the motor. Accordingly, the E-H motors with a higher W/O volume ratio (lower oil content), which corresponds to a more pyknotic structure, produces a small surface resistance and facilitates rapid unidirectional propulsion. Thus, the E-H motors with a 1:1 W/O volume ratio exhibit a higher average speed than those with a 1:2 or 1:3 W/O volume ratio (Movies S2 and S3). Only when the structures have very small differences, the reserve of fuel levels inside the E-H motors becomes a dominant factor on the moving speed. For the motors having incompact structures, more fuel means faster speed. Furthermore, the average lifetime mainly depends on the amount of fuel stored inside the E-H motor. The average lifetime increases as the W/O volume ratio decreases from 1 to 1/3 (Figure 2d). The O/W-templated E-H motor has an inhomogeneous 3D network structure, and the hydrogel matrix has an anisotropic mechanical property.48 The weak spot of the hydrogel matrix will burst first under a thermal stimulus, resulting in a relatively unidirectional liberation of oil bubbles. Inspired by this, we trepan the E-H motor with a W/O volume ratio of 1:2 or 1:3 beforehand to improve this unidirectional propulsive behavior. As shown in Figure S3, by this means, the average speeds of the E-H motors with W/O volume ratios of 1/2 and 1/3 increase

Figure 1. (a) Fabrication process of an E-H motor. The scale bar in the picture of the E-H motor is 4 mm (scale bar, 100 μm). (b) Propulsion mechanism of the E-H motor. The inset shows the fitted trajectory of the E-H motor (scale bar, 20 mm).

and the motor is more inclined to do linear motion. As shown in the inset of Figure 1b, the E-H motor is rotating before trepanning, but it is nearly moving straight after a cylindrical hole was trepanned in the kernel of the section of the E-H motor. Unlike the chemically powered catalytic motors which require fuel pool for self-propulsion,29,47 the proposed E-H motor can store the liquid fuel in itself and generate bubbles in aqueous media, which is induced by a heat stimulus. The propulsion of the E-H motor originates from the thermally induced phase transition. The trajectories and instantaneous speeds of the E-H motors with different W/O volume ratios are shown in Figures 2a and S1. The E-H motors move rapidly in spiral and circular trajectories. As shown in Figure 2b, the initial

Figure 2. (a) Trajectories of E-H motors with W/O volume ratios of 1:1, 1:2, and 1:3. (b−d) Initial velocity (b), average speed (c), and lifetime (average motion time) (d) of E-H motors with different W/O volume ratios. The time interval is 0.5 s; the tracking temperature is 60 °C (scale bar, 20 mm). 43213


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network becomes thinner as the oil content increased, and the structure of the motor with an O/W volume ratio of 3 is even analogous to a spongeous structure. This amply demonstrates that the structure of the E-H motor is deeply influenced by the internal phase content under a stability of emulsion premise. The higher the internal phase content is, the more incompact structure the E-H motor has. Moreover, a reported soft robot is controlled with microfluidic logic that autonomously regulates fluid flow to achieve an automatic control.50 The emulsion template method is also a considerable measure to create a passage for gas escape on microscale, which can put forward a new fabrication strategy for the soft and autonomous robots. The highly efficient locomotion of the E-H motor involves the bubble propulsion mechanism which utilizes the dichloromethane bubbles produced from the phase transformation of liquid dichloromethane inside the hydrogel matrix. Owing to the temperature dependence of the phase transformation, the relationship between the tracking temperature and the motor motion is investigated. Figures 4a−d and S5 display the

by 69.9 and 34.4%, respectively, in the same tracking conditions. To further investigate the influence of structures on the motor motion, the internal morphology of the E-H motor is thoroughly probed. As the E-H motor is templated from the O/ W emulsion, the properties of the emulsion droplet have a major impact on the structure of the hydrogel matrix. Figure 3a−c displays the size and mean diameter distributions of the

Figure 3. (a−c) Mean diameter and size distributions of the emulsions with different W/O volume ratios: (a) 1:1, (b) 1:2, and (c) 1:3. The insets in (a−c) show the optical microscopy (OM) images of the emulsions. (d−f) SEM images of E-H motors with different W/O volume ratios: (d) 1:1, (e) 1:2, and (f) 1:3 (freeze-dried for 72 h).

O/W emulsions with different O/W volume ratios. The CH2Cl2/water emulsion can be readily stabilized by the emulsifier PVA even though it increases the oil content from 50 to 75% (v/v). With the increase of oil content, the average diameter of the CH2Cl2 droplets increases from 14.1 to 16.0 μm and the distribution density of these droplets obviously becomes larger, which changes the droplet shape from a welldefined spherical shape to a slightly deformed sphere. The existence of oil droplets is equivalent to the artificial creation of pores and reduces the cross-linking density of the hydrogel. As shown in Figure S4, at a constant cross-linking agent concentration in the water phase, adjusting the CH2Cl2 concentration from 50 to 75% (v/v) causes a steady increase in the average void size and numbers. This observation is consistent with that found in the corresponding emulsion templates, indicating the good emulsion stability during the gelatinization process.49 Scanning electron microscopy (SEM) images of higher magnification are shown in Figure 3d−f. It is obvious that the E-H motor with an O/W volume ratio of 1 relatively has a tough hydrogel network. However, increasing the oil content leads to a gradual increase in the number of pore throats on the void walls. Furthermore, the skeleton of the

Figure 4. (a−d) Trajectories (a,c) and speeds (b,d) of E-H motors with different tracking temperatures: (a,b) 37 and (c,d) 70 °C. The time intervals are 3 s at 37 °C and 0.5 s at 70 °C. (e,f) Average speed (e) and (f) lifetime of E-H motors with different tracking temperatures. The break in (f) represents an ellipsis of time. The W/O volume ratio of these E-H motors is 1:1.

trajectories and speeds of the E-H motors at different tracking temperatures. Apparently, the E-H motors do not have a rigorous requirement for the operation temperature and can sail well at physiological temperature (37 °C). In addition, the E-H motors can be accelerated by increasing the tracking temperature. The trajectories are obviously stretched, and the instantaneous speed is significantly increasing along with the increase of tracking temperature. Furthermore, the responsive time of the thermally induced E-H motor is also determined by 43214


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process; thus, the resistance of the motion will be greatly reduced. Taking advantage of this property, precise positioning and remote control of the E-H motor could be implemented in the near future. The cyclohexane/water emulsion-based E-H motor can sail on water at a speed of 14.78 ± 2.80 mm s−1 at a tracking temperature of 85 °C even though obvious movement stagnation was found at the beginning of the self-propelling process at this temperature. Nevertheless, a widely used solvent, cyclohexane, is an appropriate choice for the E-H motor on account of its good dissolving capacity for multiple drugs (e.g., bactericide), thus providing diversified function for the application of environmental remediation. Directional control is a pivotal capacity for various practical applications. For instance, Mair et al. previously reported magnetic microkayaks that can be propelled and directionalcontrolled by special magnetic fields.51 Accordingly, the magnetic guidance of the E-H motor can be readily realized by incorporating ferroferric oxide (Fe3O4) NPs in the hydrogel matrix. Figure 6a illustrates the incorporation process of such a

the tracking temperature. There is an obvious motion startup stagnation (about 30 s) at the tracking temperature of 50 °C, whereas the responsive time of the E-H motor is negligible if the temperature exceeds 50 °C. As indicated in Figure 4e,f, there is a positive correlation between the tracking temperature and the average speed, whereas the average lifetime is inversely correlated with the tracking temperature. The speed of such a thermally actuated E-H motor (a W/O volume ratio of 1:1) can reach 17.04 ± 4.53 mm s−1 at 75 °C, which is superior to those of common chemically propelled miniature motors.27,28 The longest average lifetime is 2758 ± 34 s at a tracking temperature of 37 °C. There is an inverse relationship between the average speed and the average lifetime because of the limited quantity of fuel. Hence, we can readily regulate and control the two variables to obtain a suitable E-H motor to meet the requirements of the practical applications. Furthermore, to simultaneously increase the operating time and speed of the E-H motor, we can use surface-sealing techniques (such as using atomic layer deposition for a silica film) or enhance surface strength methods (e.g., enhance surface cross-linking) for the purpose of blocking the bubbles ejected from the sidewall and reducing the fuel loss and motion resistance. A novel and effective method is applied for the fabrication of the E-H motor with prestored fuel via O/W emulsion templates. Many different low-boiling-point oil phases can be incorporated inside the hydrogel matrix to serve as the propellants of the E-H motor. As shown in Figure 5,

Figure 6. (a) Incorporation of Fe3O4 NPs into the E-H motor for magnetic guidance. (b) Trajectory of Mag-E-H motors under the magnetic field. The inset in (b) is the fitted trajectory obtained by NIS-Elements AR 4.3 software (scale bar, 20 mm).

magnetically guided E-H motor (Mag-E-H motor). The gelatinization of the nanoFe3O4-mixed hydrogel precursor solution integrates a magnetic function into the E-H motor, forming a new Mag-E-H motor. The trajectory in Figure 6b, corresponding to Movie S5, illustrates the remote guidance of the Mag-E-H motor achieved by a horizontal nearby magnet. The Mag-E-H motor can be precisely navigated for linear motion by using external magnetic field. As a result, the combination of the nanoFe3O4 encapsulation method and gelatinization offers controllable and attractive propulsion for practical applications.52 The propulsion of cyclopentane-based soft E-H motor on the water surface suffers from hydraulic resistance.9 Owing to the flotage effect of the cyclopentane-based E-H motor, we can readily reform the existing E-H motor with the implantation of GO, thus implementing a remote-driving control through NIR laser irradiation.53,54 The structure of the GO-intermingled E-H motor (GO-E-H motor) is schematically illustrated in Figure 7a. The GO nanoplatelets are dispersed homogeneously in the aqueous phase; the low-density cyclopentane serves as the oil phase. The two-phase mixture can be readily emulsified with a high-speed shear and is not affected by the addition of GO nanoplatelets. As shown in Figure S6c, the OM image of the GO-mixed cyclopentane/water emulsion reveals the favorable stability of the O/W emulsion. The temperature changes of the GO-Enro-E-H motor upon NIR irradiation (1.6 W, 808 nm, 5 mm × 5 mm spot) are shown in Figure S7. The trajectories of the GO-E-H motor and the E-H motor under an NIR laser are displayed in Figure 7b,c. It is obvious that the GO-E-H motor exhibits a superior driving capability than the E-H motor under

Figure 5. (a−d) Trajectories (a,c) and speeds (b,d) of E-H motors using cyclopentane (a,b) and cyclohexane (c,d) as fuel. The tracking temperatures are 65 °C for cyclopentane and 85 °C for cyclohexane. The selection of tracking temperature is based on the boiling points of the fuels. The W/O volume ratio of these E-H motors is 1:2.

cyclopentane and cyclohexane are chosen as the fuel and exert a significant propulsion effect, indicating the wide range of fuel selection of the E-H motor. The optical microscopic images of the cyclopentane/water emulsion and cyclohexane/ water emulsion in Figure S6a,b display a good stability at 66.7% (v/v) oil content. Accordingly, the average speed of the cyclopentane/water emulsion-based E-H motor reaches 15.18 ± 1.43 mm s−1 at 65 °C (Movie S4), and the initial movement stagnation effect is not apparent. Owing to the low density of cyclopentane, the E-H motor that stored cyclopentane (density: ρ < 1) can suspend on water in the self-propulsion 43215


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Figure 8a. The water-insoluble enrofloxacin bactericide is dissolved in the cyclopentane solvent which acts as the oil

Figure 7. (a) Structure of the GO-E-H motor. The inset in (a) is the fitted trajectory of the GO-E-H motor under an NIR laser obtained by NIS-Elements AR 4.3 software (scale bar, 50 mm). (b) Trajectories of a GO-E-H motor and an E-H motor under an NIR laser. (c) Speeds of a GO-E-H motor and an E-H motor under an NIR laser. The time interval is 3 s; the W/O volume ratio of these motors is 1:2.

the same condition of IR irradiation. The average speed of the GO-E-H motor reaches 1.34 ± 0.17 mm s−1 (Movie S6), nearly twice as fast as the E-H motor (0.78 ± 0.18 mm s−1) in the same circumstances (Figure 7b). The remarkable motion capability of the GO-E-H motor can be attributed to the photothermal effect,55 where the implanted GO converts the adsorbed optical energy to thermal power by virtue of atomic and molecular vibration absorption of GO in the NIR region.56 The thermal GO platelets serve as a nanoheater, which uniformly and rapidly increases the temperature of the motor. The superheated liquid cyclopentane fuel in the vicinity of the GO nanoplatelets is transformed into gases. As the internal pressure of the hydrogel matrix increased, the gases will explode from the weak spot of the soft motor, and then the GO-E-H motor is actuated by the explosive bubbles in a direction. The GO-E-H motor with more thermally initiated cyclopentane gas liberation exerts greater impulse force on the self-propulsion than the E-H motor which is only actuated by the volatilized cyclopentane gas at room temperature. In addition to the acceleration effect presented by the GO nanoheater, the GO-EH motor can perform a remote NIR-controlled transformation in direction and location. When arbitrarily changing the irradiation site of the NIR laser in the GO-E-H motor, which means altering the heating position, the ejection direction of the vaporized cyclopentane changes along with it. The fuelcontained GO-E-H motor with arbitrarily adjustable capacity in motion control can be broadly applied to designing bionic soft robots with complex custom-designed functions in the near future.57,58 Additionally, the speed of the E-H motors showed some fluctuation. The fluctuation is closely related to the bubble size in the E-H motors. In the future works, we will try to preset the ejection path for the low-boiling-point liquid in the E-H motor matrix by microfluidics to control the bubble size. By incorporating the enrofloxacin bactericidal agent, we demonstrate one of the E-H motor utilities in sterilization applications. The structure of the enrofloxacin-imported emulsion hydrogel motor (Enro-E-H motor) is displayed in

Figure 8. (a,b) Release curves of Enro-E-H motors (a) and GO-EnroE-H motors (b). Ct and C0 represent the released enrofloxacin concentration at time t and the totally released enrofloxacin concentration, respectively. The inset figures in (a,b) are the release processes of the Enro-E-H motors and the GO-Enro-E-H motors, respectively. The concentration of enrofloxacin in the experimental environment would reach 8 μg mL−1 when it is completely released from the motors.

phase in the O/W-type precursor emulsion of the hydrogel matrix. The free-radical polymerization and gelatinization of the gel precursor solution accomplish the encapsulation of enrofloxacin inside the E-H motor.59 The bactericidal activity of enrofloxacin is concentration-dependent.60 To evaluate the sterilization efficacy of the Enro-E-H motor, we investigate the release curves of the Enro-E-H motor in 50 °C water environment (Figure 8a; the absorbance standard curve equation of enrofloxacin is shown in Figure S8). The release of the hydrogel at body temperature 37 °C was also investigated, as shown in Figure S9. The drug-release process can be divided into three periods: first, the Enro-E-H motor is propelled by the liberated cyclopentane bubbles; meanwhile, the dissolved enrofloxacin rapidly releases along with cyclopentane (0−1.5 min). The cumulative release rate in this period is about 15.6%, and the releasing rate is up to 10.4% per min. Then, the cyclopentane fuel runs out, and the Enro-E-H motor has stayed at a defined location (1.5−15 min). As bubble liberation breaks the hydrogel matrix, the residual enrofloxacin will quickly and increasingly escape from the Enro-E-H motor, which may lead to the death of some susceptible bacteria by virtue of the high concentration of enrofloxacin exposure. The cumulative release rate in the second period is up to 43%; meanwhile, the releasing rate is about 2.0% per min. The third period is a typical 43216


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the E-H motor. Multifunctional and intelligent E-H motors can be prepared by incorporating various functional substances such as Fe3O4 NPs for magnetic navigation, GO for remote driving control and release control, enrofloxacin for bactericidal effect. More custom-designed functions can be implemented utilizing the intrinsic integration capability of the emulsion hydrogel. The controllable locomotion property of the NIRdriven soft motor coupled with the bactericidal capability of the incorporated enrofloxacin may accomplish a targeted sanction to bacteria in water remediation. The thermally driven, NIRactuated, precisely controllable, and efficiently germicidal emulsion hydrogel-based soft motor could serve as a foundation for the integrated design of autonomous biorobots50 composed solely of soft materials toward the foremost applications ranging from environmental remediation63,64 to intelligent sensing.65,66

hydrogel-based sustained-release process in which the swelling property of the hydrogel and the diffusibility of enrofloxacin dominate the releasing effect (the swollen Enro-E-H motor is shown in Figure S10).60,61 The cumulative release rate ultimately reaches 81.1% in 150 min. In conclusion, the Enro-E-H motor with multiple attributes of medicine encapsulation and transportation, regional drug delivery, and chronic drug release has displayed a high automation, intelligence, and good adaptability, which may serve as a foundation for autonomous soft robots.50 To enhance the adjustable capacity, we incorporate GO in the Enro-E-H motor, thus achieving a remote motion control and release control synchronously. The fabrication strategy of the GO-embedded Enro-E-H motor (GO-Enro-E-H motor) is similar to that of the Enro-E-H motor; the only dissimilitude lies in the introduction of homogeneously dispersed GO in the aqueous phase. To further evaluate the controlled release efficiency, the GO-Enro-E-H motor is exposed to an NIR irradiation with an energy density of 0.8 W·cm−2 in 25 °C water environment. Compared to the Enro-E-H motor, the movement of the NIR light-driven GO-Enro-E-H motor lasts for 30 min. Simultaneously, the cumulative release rate in this period is about 35.3%, and the releasing rate is up to 61.8% per hour. In 0.5−2 h, the GO-Enro-E-H motor reaches a predesigned position and large quantity of enrofloxacin bactericide escapes from the broken motor. The cumulative release rate is 55.3% with a releasing speed of 31.3% per hour in the burst release stage. Eventually, the cumulative release rate reaches 81.7% in 24 h (Figure 8b). The GO-Enro-E-H motor with a controllable drug delivery system displays three advantages: (1) The hydrogel matrix (the drug carrier and medium for sustained release) greatly reduces the drug depletion caused by direct exposure. (2) The biologically harmless NIR irradiation can remotely drive the soft motor in a precise pattern, involving controllable activation, acceleration, and directional control. (3) The combination of controlled release and controlled propulsion results in a site-specific transportation and sentinel release of enrofloxacin, thereby yielding to a targeted bactericidal effect. The incorporation of enrofloxacin just serves as a proof-of-concept demonstration of the utilization of the EH motors in a sterilization context. This facile integration can be broadly extended to incorporating other functionalized substances into the intelligent soft-motor system,50 for the applications including but not limited to sterilization, detoxification,28 and water purification,25 in environmental conservation. Additionally, in our work, on the one hand, the motors are composed by hydrogel which is biocompatible,27,28 and on the other hand, the mechanism for the propulsion of the motors is bubble-driven, which can propel the motors in different real-life environments.62 As a result, such motors hold great potential in practical applications. In this work, we have focused on demonstrating a brand-new emulsion hydrogel soft motor actuated by thermal stimulation; the following work will pay more attention on the applications systematically.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b08661. Trajectories and speeds of the E-H motors; characterization; release data; images of self-propulsion process of the E-H motors; SEM images of the E-H motors; OM images of the cyclopentane/water emulsion, cyclohexane/water emulsion, and cyclopentane/water emulsion mixed with GO; temperature of the GO-Enro-E-H motor upon NIR irradiation; absorbance standard curve equation of enrofloxacin; and photographs of the E-H motor before and after self-propulsion and Enro-E-H motor before and after enrofloxacin release (PDF) Movement of the E-H motor at 60° C (W/O volume =1, fuel: CH2Cl2) (AVI) Movement of the E-H motor at 60° C (W/O volume =2, fuel: CH2Cl2) (AVI) Movement of the E-H motor at 60° C (W/O volume =3, fuel: CH2Cl2) (AVI) Movement of the E-H motors at 65° C (W/O volume =2, fuel: cyclopentane) (AVI) Magnetically guided movement of the Mag-E-H motor at 50° C (W/O volume = 1, fuel: CH2Cl2) (AVI) NIR-driven movement of the GO-E-H motor at 50° C (W/O volume = 2, fuel: cyclopentane) (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.D.). *E-mail: [email protected] (C.W.). ORCID

Wei Gao: 0000-0002-8503-4562 Chaoyang Wang: 0000-0002-7270-5451 Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In summary, we have proposed a novel fuel-contained, thermally driven miniaturized soft motor fabricated via the O/W emulsion template. The sprayable oil phase stored in the E-H motor liberates violently under a thermal stimulus, thereby yielding a superfast bubble-based propulsion. In addition, multiple water-insoluble organic solvents can act as the oil phase of the O/W emulsion, which expands the applicability of

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21474032). REFERENCES

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Emulsion Hydrogel Soft Motor Actuated by Thermal Stimulation - ACS

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