Emulsion Hydrogel Soft Motor Actuated by Thermal ... - ACS Publications

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An Emulsion-Hydrogel Soft Motor Actuated by Thermal Stimulation Hui Wang, Yuling Liang, Wei Gao, Renfeng Dong, and Chaoyang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08661 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

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ACS Applied Materials & Interfaces

An Emulsion-Hydrogel Soft Motor Actuated by Thermal Stimulation Hui Wang,a Yuling Liang,a Wei Gao,b Renfeng Dong*,c and Chaoyang Wang*,a a

Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China

b

Department of Electrical Engineering & Computer Sciences, University of California, Berkeley, California 94720, United States. c

School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, China

Submitted to: ACS Applied Materials & Interfaces Contains SI

*Corresponding authors:

[email protected]; [email protected]

Keywords: emulsion-hydrogels, soft-motor, thermal-driven, NIR-driven, sterilization

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ACS Paragon Plus Environment


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Abstract An emulsion hydrogel motor (E-H-motor) constituted by low-boiling oil fuel and hydrogel matrix, is prepared through a simple yet versatile oil-in-water (O/W) emulsion template method. The E-H-motor can be efficiently propelled by the bubbles generated under thermal stimulus. As thermal-inducted explosion occurs inside the E-H-motor (diameter ~4.0 mm, 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 oC aqueous solution. Additionally, multiple water-insoluble organic solvents can serve as the fuel of self-propulsion that demonstrates the favorable universality of E-H-motor. The magnetic navigation and near-infrared (NIR) propulsion can be realized through incorporating the hydrophilic iron oxide (Fe3O4) nanoparticles and graphene oxide (GO) into 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 hydrogel based miniaturized soft-motor hold great promise for numerous practical applications.

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1. Introduction Self-propelled miniaturized motors have raised 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 are also able to use near-infrared (NIR) light as the power to achieve a remote intervention in 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 nanowires,24 and suffer from some problems: 1) some of inorganic or metal material based micromotors are not fully biocompatible, which hinder the application of micromotors in biomedical field. 2) Most inorganic or metal material based motors have restricted applications on water surface due to their high material density.8 3) The micromotors are too small for dynamic decontamination action on larger scales and for loading sufficient

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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 low-boiling-point oil fuel (pre-stored in the hydrogel skeleton) could transform from liquid to gas and liberate to propulsion under thermal stimulus. The speed of the thermoresponsive E-H-motor with 1:1 W/O volume ratio is 14.78  4.82 mm s−1 in a 60 °C aquatic environment, which is triple 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 white, and after the stored CH2Cl2 fuel is nearly released completely, the motor becomes transparent which is the same color as the hydrogel fabricated without CH2Cl2. As a result, we think the fuel availability of the proposed E-H-motor is nearly 100%.) which greatly exceeds that of previously reported chemically powered

hydrogel-motor.32

Additionally,

the

thermoresponsive

4


E-H-motor

has

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preliminarily realized the integration of multiple sophisticated functionalities. To expand the applicability of the E-H-motor actuated by sprayable oil, different organic solvents are employed as oil phase of the O/W emulsion, exemplified by dichloromethane, cyclopentane and cyclohexane. The fabrication strategy for multiple water-insoluble oils demonstrates considerable universality based on different emulsifier systems. The highly integrated E-H-motor can realize multiple custom-designed functions: the incorporation of iron oxide (Fe3O4) nanoparticles enables magnetic guidance; the implanted graphene oxide (GO) allows a remote and precise motion control in direction, speed and trajectory through NIR light; the functional oil-soluble enrofloxacin provides the potential in sterilization applications. The multi-functional E-H-motors hold great promise for practical applications in biomimetic soft robot,

33-37

with functions of

self-driving, sensing, and sterilization.

2. Experimental 2.1 Materials Acrylamide (AM), N,N’-methylenebis-acrylamide (MBA), potassium persulfate (K2S2O8, KPS), ferric chloride (FeCl3), ferrous chloride (FeCl2) were purchased from Guangzhou Chemical Reagent Co. (China). Polyvinyl alcohol (PVA), N,N,N’,N’-tetramethyldiamine (TEMED), cyclopentane (C5H10) were purchased from Aladdin Industrial Corporation (USA). Tween 80, dichloromethane (CH2Cl2), cyclohexane (C6H12) were purchased from Kemiou Chemical Reagent Co. Ltd. (China). Natural graphite powder and enrofloxacin

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were purchased from J&K Scientific Ltd. (China). All the reagents are analytical grade and used without further purification. Deionized water (Resistivity  18.2 MΩ cm) was purified by Millipore purification apparatus (AquaPro2S) and was de-aired with argon for 30 min before use. 2.2 Preparation of E-H-Motor The thermal driven E-H-motor was prepared by the following steps. 6 mL 20 wt% AM solution was prepared in deionized water. Then, added 1.2 mg chemical cross-linker MBA, 0.06 g initiator KPS into the solution. PVA (0.06 g) which served as an emulsifier was dispersed in the mixture through sonication as follows. After that, the CH 2Cl2 (6 mL/ 12mL/ 18mL) used as the fuel of 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 L 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 oC 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) nanoparticles were prepared by the precipitation oxidation method.38 6 mL 2 mg/mL Fe3O4 homogeneous dispersion was obtained with the assistance of ultrasonic bath for 1 h. 0.06 g PVA, 1.2 g AM, 0.6 mg MBA, 0.06 g KPS, 20 L TEMED were orderly added and mixed homogeneously by sonication before each 6


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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 oC for 24 h. 2.4 Preparation of GO-E-H-Motor Graphene oxide (GO) was obtained from the natural graphite powder via the modified Hummer's method.39,40 6 mL 4 mg/mL GO dispersion was prepared in deionized water by ultra-sonication. Then, 40 L Tween 80 was dispersed in it through sonication. The chemicals for gelatinization (1.2 g AM, 1.2 mg MBA, 0.06 g KPS) was orderly dissolved in the solution and mixed well. Thereafter, 12 mL cyclopentane was added and emulsified with a high speed shearing machine (5000 rpm, 2 min). After that, 10 mL TEMED was added with stirring. The obtained emulsion was transferred into glass tubes and polymerized at 6 oC 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 the enrofloxacin (2.4 mg) were 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 the GO (4 mg/mL) was dispersed in deionized water (6 mL) simultaneously. 2.7 Characterizations 7



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Tracking. The self-propulsion process of E-H-motor was performed in a thermostat water bath cauldron. The video of 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 NIR laser (1.6 W, 808 nm, 5 mm5 mm spot) downward from the top during the NIR driven process. Optical Microscopy (OM). The Axiolab Polarizing Microscope (Carl Zeiss, Germany) equipped with a camera was utilized for imaging O/W emulsion to evaluate the emulsion particle size and distribution. Scanning Electron Microscopy (SEM). Internal morphology of 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-drying for 72 h before observation. Ultraviolet-Visible Absorption Spectra (UV-vis). The UV-vis spectra was measured on a Shimadzu UV-240 spectrophotometer to evaluate the release effect of Enro-E-H-motor and GO-Enro-E-H-motor. The halogen tungsten lamp with a filter having cutoff  600 nm was used as the NIR light source located 15 cm above the samples during the NIR-controlled release.

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

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, water-soluble monomer, chemical crosslinking agent and initiator are dissolved in aqueous phase, while low-boiling-point organic solvent serves as oil phase. The composite solution is emulsified with the assistant of high speed shearing to form the O/W emulsion. The process of free radical polymerization and gelatinization will 9



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generate a hydrogel matrix with 3D network structures, 43,44 which provides a shelter for the low-boiling-point fuel. Figure 1b shows the self-propulsion mechanism of a thermally triggered E-H-motor. When the E-H-motor is heated, the stored CH2Cl2 fuel transforms from liquid phase to gas phase. The generated gas molecules are restricted inside the hydrogel matrix. Once the gas pressure reaches to a threshold value, the CH2Cl2 vapor ejects violently from the weak spots of 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 E-H-motor, the directive bubbles released from 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, trepanning means artificially create a CH2Cl2 (g) release path, the gas tend to ejected via this prescribed path 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 section of E-H-motor.

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Figure 2. (a) Trajectories of E-H-motors with W/O volume ratios of 1:1, 1:2, 1:3. (b-d) Initial velocity (b), average speed (c) and life time (average motion time) (d) of E-H-motors with different W/O volume ratios. Time interval is 0.5 s; tracking temperature is 60 oC. Scale bar, 20 mm. Unlike the chemically-powered catalytic motors which require fuel pool for the self-propulsion,29,47 the proposed E-H-motor that can store the liquid fuel in itself and generate bubbles in aqueous media induced by heat stimulus. The propulsion of the E-H-motor originates from the thermally induced phase transition. The trajectories and instantaneous speeds of E-H-motors with different W/O volume ratios are shown in Figure 2a and Figure S1. The E-H-motors move rapidly in spiral and circular trajectories. As shown in Figure 2b, the initial velocity of a E-H-motor with 1:1 W/O volume ratio is 11



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much higher than that of 1:2 or 1:3 W/O volume ratio in tracking temperature of 60 °C. Although the motors with higher oil content are normally expected to have a stronger propulsive force and a longer life time, 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 non-directional 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 phase transition induced from the time-delay effect of temperature, the driving force resulting from the vaporization of low-boiling 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 1:1 W/O volume ratio. The propulsion of E-H-motor depends on the continuous thrust of CH2Cl2 (g) bubbles generated by the CH2Cl2 (l) stored in 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 oC (Movie S1). As shown in Figure 2c, the average speeds of the E-H-motors don’t increase with the increment of oil content. The E-H-motor with 1:1 W/O volume ratio has the highest average speed, while the E-H-motor with 1:2 W/O volume ratio possesses the lowest speed. This phenomenon reveals that the speeds of E-H-motor don’t primarily rely on the quantity of fuel stored in hydrogel matrix. Several other factors play important roles on the motors’ motion. Firstly, no matter how much fuel the motor stockpiles, only a dominant gas release can provide an effective movement. In addition, the bubbles escaped 12


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from the surface of E-H-motor are omnidirectional. The two factors are ultimately determined by the structure of hydrogel matrix. A more pyknotic structure of hydrogel matrix is helpful to restrict the gas inside it until the gas concentrates to break the weak point of hydrogel matrix to form a directional movement. A more pyknotic 3D network structure will also cause less gas escape from the surface of motor. Accordingly, the E-H-motors with higher W/O volume ratio (lower oil content), which correspond to a more pyknotic structure, produces small surface resistance, and facilitates rapid unidirectional-propulsion. Thus, the E-H-motors with 1:1 W/O volume exhibit higher average speed than those with 1:2 or 1:3 W/O volume ratios (Movie 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 have incompact structures, more fuel means faster speed. Furthermore, the average life time mainly depends on the amount of fuel stored inside the E-H-motor. The average life time increases as W/O volume ratios 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 hydrogel matrix will burst at first under thermal stimulus, resulting in a relatively unidirectional liberation of oil bubbles. Inspired by this, we trepan the E-H-motor with 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 ratio of 1/2 and 1/3 increase by 69.9 % and 34.4 % respectively in the same 13



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tracking conditions. In order to further investigate the influence of structures on the motor motion，the internal morphology of E-H-motor is thoroughly probed. As the E-H-motor is templated from the Oil/Water (O/W) emulsion, the properties of the emulsion droplet have a major impact on the structure of hydrogel matrix. Figure 3a-c displays the size and mean diameter distributions of the oil-in-water emulsion with different O/W volume ratios. The CH2Cl2/Water emulsion can be readily stabilized by emulsifier PVA even increases the oil content from 50 % to 75 % (v/v). With the increasing of oil content, the average diameter of the CH2Cl2 droplets increases from 14.1 m to 16.0 m and the distribution density of this droplets obviously becomes larger, which changes the droplet shape from a well-defined spherical shape to a slightly deformed sphere. The existence of oil droplets is equivalent to the artificial creation of pores and reduces the crosslinking density of 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. The observation is consistent with that found in the corresponding emulsion templates, indicating the good emulsion stability during the gelatinization process.49 The SEM images of higher magnification are shown in Figure 3d-f. It is obvious that the E-H-motor with O/W volume ratio of 1 has a tough hydrogel network, relatively. While, increasing the oil content leads to a gradual increase in the number of pore throats on the void walls. Furthermore, the skeleton of the network becomes thinner as the oil content increased, and the structure of motor with O/W volume 14


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ratio of 3 is even analogous to a spongeous structure. This amply demonstrates that the structure of 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 in micro scale, which can put forward a new fabrication strategy for the soft and autonomous robots.

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Figure 3. a-c, Mean diameter and size distributions of the emulsion with different W/O volume ratios: (a) 1:1, (b) 1:2, (c) 1:3. Insets in a-c show the OM images of the emulsion. d-f, SEM images of E-H-motors with different W/O volume ratios: (d) 1:1, (e) 1:2, (f) 1:3 (Freeze-drying for 72 h).

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Figure 4. a-d, the trajectories (a, c) and speed (b, d) of E-H-motors with different tracking temperatures: (a, b) 37 oC, (c, d) 70 oC. Time intervals are 3 s at 37 oC and 0.5 s at 70 oC, respectively. e-f, average speed (e) and (f) life time 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.

The highly efficient locomotion of the E-H-motor involves the bubble propulsion 17



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mechanism

which

utilizes

dichloromethane

bubbles

produced

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from the

phase

transformation of liquid dichloromethane inside hydrogel matrix. Owing to the temperature dependence of phase transformation, the relationship between tracking temperature and the motor motion is investigated. Figure 4a-d and Figure S5 display the trajectories and speed of E-H-motors in different tracking temperatures. Apparently, the E-H-motors don’t have a rigorous requirement for the operation temperature, which can sail well at physiological temperature (37 oC). 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 thermal-induced E-H-motor is also determined by the tracking temperature. There is an obvious motion startup stagnate (about 30 s) at the tracking temperature of 50 oC while the responsive time of the E-H-motor is negligible if the temperature exceeds 50 oC. As indicated from Figure 4e, 4f, there is a positive correlation between tracking temperature and average speed, while the average life time is inversely correlated with the tracking temperature. The speed of such thermally-actuated E-H-motor (w/o volume ratio of 1:1) can reach 17.04  4.53 mm s−1 at 75 oC, which is superior to common chemically propelled miniature motors.27,28 The longest average life time is 2758  34 s at the tracking temperature of 37 °C. There is an inverse relationship between average speed and average life time due to 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, in 18


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order to simultaneously increase the operating time and speed of E-H motor, we can use surface sealing techniques (such as using atomic layer deposition for a silica film) or enhance surface strength methods (eg: enhance surface crosslinking) for the purpose of blocking the bubbles ejected from the side wall and reducing the fuel loss and motion resistance.

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

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A novel and effective method is applied for the fabrication of E-H-motor with pre-stored fuel via O/W emulsion templates. Many different low-boiling oil phases can be incorporated inside the hydrogel matrix to serve as the propellants of E-H-motor. As shown in Figure 5, 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 S6 (a, 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 stored cyclopentane (Density:  < 1) can suspend on water in the self-propulsion 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 with a speed of 14.78  2.80 mm s−1 at a tracking temperature of 85 oC. Even, obvious movement stagnation was found at beginning of self-propelling process at this temperature. Nevertheless, a widely used solvent—cyclohexane is an appropriate choice for E-H-motor, on account of having good dissolving capacity for multiple drugs (e.g.: bactericide), thus providing diversified function for the application of environmental remediation. 20


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Figure 6. (a) The incorporation of Fe3O4 NPs into the E-H-motor for magnetic guidance. (b) The 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. Directional control is a pivotal capacity for various practical applications. For instance, Mair et al. previously reported magnetic microkayaks that can be propelled and directional controlled by special magnetic fields.51 Accordingly, the magnetic guidance of the E-H-motor can be readily realized by incorporating ferroferric oxide (Fe3O4) nanoparticles in the hydrogel matrix. Figure 6a illustrates the incorporation process of such magnetic guided E-H-motor (Mag-E-H-motor). The gelatinization of the nano-Fe3O4 mixed hydrogel precursor solution integrates magnetic function into the E-H-motor, forming a new Mag-E-H-motor. The trajectory in Figure 6b, corresponding to Movie S5, illustrates 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 results, the combination the nano-Fe3O4 encapsulation method and gelatinization offers controllable and attractive propulsion for practical applications.52 21



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Figure 7. (a) The structure of GO-E-H-motor. The inset in (a) is the fitted trajectory of GO-E-H-motor under NIR laser obtained by NIS Elements AR 4.3 software. Scale bar, 50 mm. (b) The trajectories of a GO-E-H-motor and a E-H-motor under NIR laser. (c) The speed of a GO-E-H-motor and an E-H-motor under the NIR laser. Time interval is 3 s; W/O volume ratio of these motors is 1:2. 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 implant of graphene oxide, thus implement a remote driving control through an NIR laser irradiation.53,54 The structure of the graphene oxide intermingled E-H-motor (GO-E-H-motor) is schematically illustrated 22


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in Figure 7a. The graphene oxide nanoplatelets are dispersed homogeneously in aqueous phase, the low-density cyclopentane serves as oil phase. The two-phase mixture can be readily emulsified with the high-speed shear, not affected by the addition of GO nanoplatelets. As shown in Figure S6c, the OM image of GO mixed cyclopentane/water emulsion reveals the favourable stability of the O/W emulsion. The temperature changes of GO-Enro-E-H according to NIR irradiation (1.6 W, 808 nm, 5 mm*5 mm spot) is shown in Figure S7. The trajectories of GO-E-H-motor and E-H-motor are displayed in Figure 7b and 7c under NIR laser. It is obvious that the GO-E-H-motor exerts a superior driving capability than E-H-motor under the same condition of IR irradiation. The average speed of 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 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 vibrations absorption of GO in the NIR region.56 The thermal GO platelets serve as a nano heater, 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 hydrogel matrix increased, the gases will explode from the weak spot of soft motor and then the GO-E-H-motor is actuated by explosive bubbles in a direction. The GO-E-H-motor with more thermal initiated cyclopentane gas liberation exerts greater impulse force on the self-propulsion than the E-H-motor that only actuated by the volatilized cyclopentane gas 23



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at room temperature. In addition to the acceleration effect presented by GO nano heater, the GO-E-H-motor can perform a remote NIR-controlled transformation in direction and location. When arbitrarily changing the irradiation site of NIR laser in GO-E-H-motor, which means altering the heating position, the ejection direction of the vaporized cyclopentane changes along with it. The fuel-contained 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 the speed of the E-H motors showed some fluctuation. The fluctuation is closely related to the bubble size in E-H motors. In the future works, we will try to preset the ejection path for the low-boiling liquid in E-H-motors matrix by microfluidics to control the bubble size.

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Figure 8. a-b, Release curves of Enro-E-H-motors (a) and GO-Enro-E-H-motors (b). Ct and C0 represent the released enrofloxacin concentration of at time t and the totally released enrofloxacin concentration, respectively. The inset figures in (a,b) are the release process 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 completely released from motors.

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By incorporating the enrofloxacin bactericidal agent, we demonstrate one of the E-H-motor’s utility in sterilization applications. The structure of the enrofloxacin imported emulsion

hydrogel

motor

(Enro-E-H-motor)

is

displayed

in

Figure

8a.

The

water-insoluble enrofloxacin bactericide is dissolved in cyclopentane solvent which acts as the oil phase in the O/W typed precursor emulsion of the hydrogel matrix. The free radical polymerization and gelatinization of the gel precursor solution accomplish the encapsulation of enrofloxacin inside E-H-motor. 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 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 oC was also investigated as shown in Figure S9. The drug-release process can be divided into three periods: firstly, 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, the Enro-E-H-motor have stayed at a defined location (1.5~15 min). As bubbles liberation breaks the hydrogel matrix, the residual enrofloxacin will quickly and increasingly escape from 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 26


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rate is about 2.0% per min. The third period is a typical hydrogel based sustained-release process that the swelling property of hydrogel and diffusibility of enrofloxacin dominate the releasing effect (the swollen Enro-E-H-motor is shown in Figure S10).61,62 The cumulative release rate ultimately reaches to 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-robot.50 In order 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 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 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 oC 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 27



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GO-Enro-E-H-motor with 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 biological 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 E-H-motor’s utilization 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 one hand, the motors are composed by hydrogel which is biocompatible 27,28, on the other hand, the mechanism for the motors’ propulsion is bubble driven which can propel the motors in kinds of real-life environments.63 As a result, such motor hold great potential in practical applications. In this work, we are 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.

4. Conclusions In summary, we have proposed a novel fuel-contained, thermally driven miniaturized 28


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soft-motor fabricated via the O/W emulsion template. The sprayable oil phase stored in E-H-motor liberates violently under thermal stimulus, thereby yielding a superfast bubble-based propulsion. In addition, multiple water-insoluble organic solvents can act as oil phase of O/W emulsion, which expands the applicability of E-H-motor. Multifunctional and intelligent E-H-motors can be prepared by incorporating various functional substancesFe3O4 nanoparticles for magnetic navigation, GO for the remote driving control and release control, enrofloxacin for the bactericidal effect. More custom-designed functions can be implemented utilizing the intrinsic integration capability of emulsion hydrogel. The controllable locomotion property of the NIR-driven soft-motor coupled with the bactericidal capability of the incorporated enrofloxacin may accomplish a targeted sanction to bacteria in water remediation. The thermally-driven, NIR-actuated, precisely controllable, and efficiently germicidal emulsion hydrogel-based soft motor could serve as a foundation for the integrated design of the autonomous bio-robot50 composed solely of soft materials toward the foremost applications ranging from environmental remediation,63,64 to intelligent sensing.65,66

ASSOCIATED CONTENT Supporting Information. It is available free of charge via the Internet at http://pubs.acs.org. The trajectories and speed of E-H-motors, characterization, release data (PDF)

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The movement of E-H-motors (AVI)

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (CW); [email protected] (RD). Notes The authors declare no competing financial interest.

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

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Emulsion Hydrogel Soft Motor Actuated by Thermal ... - ACS Publications

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