Shape-Transformable, Fusible Rodlike Swimming Liquid Metal

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Shape-Transformable, Fusible Rod-Like Swimming Liquid Metal Nanomachine Daolin wang, Changyong Gao, Wei Wang, Mengmeng Sun, Bin Guo, Hui Xie, and Qiang He ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05203 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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Shape-Transformable, Fusible Rod-Like Swimming Liquid Metal Nanomachine Daolin Wang, # Changyong Gao, # Wei Wang, Mengmeng Sun, Bin Guo*, Hui Xie and Qiang He* State Key Laboratory of Advanced Welding and Joining (HIT), Micro/Nanotechnology Research Center, Harbin Institute of Technology, 2 Yikuang Street, Harbin 150080 (China) E-mail: [email protected]; [email protected]

ABSTRACT: The T-1000 liquid metal terminator, which can transform and self-repair, represents a dream for decades that robots can fundamentally change our daily life. To date, some large-scale liquid metal machines have been developed. However, there is no report on nanoscaled liquid metal machines and their biomedical applications. We describe here a shapetransformable and fusible rod-like swimming liquid metal nanomachine, based on the biocompatible and transformable liquid metal gallium. These nanomachines were prepared by a pressure-filter-template technology, and the diameter and length could be controlled by adjusting the nanoporous templates, filter time and pressure. The as-prepared liquid gallium nanomotors display a core-shell nanorod structure composing of a liquid gallium core and solid gallium oxide shell. Upon exposure to an ultrasound field, the generated acoustic radiation force in the levitation plane can propel them to move autonomously. The liquid metal nanomachine can

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actively seek cancer cells, and transform from a rod to a droplet after drilling into cells owing to the removal of gallium oxide layers in the acidic endosomes. These transformed nanomachines could fuse together inside cells, and photothermally kill cancer cells under illumination of nearinfrared

light.

Such

acoustically-propelled

shape-transformable

rod-like liquid

nanomachines have great potential for biomedical applications.

KEYWORDS: liquid metal, gallium, nanomachine, transformation, fusion


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Swimming nanomachines with fascinating characteristics of locomotion and controllability represent an emerging and desirable type of nanomachines, and have potential application in the biomedicine field, such as cargo delivery, disease monitoring, cell manipulation and surgical tools.1-8 Inspired by natural swimming bacteria, which can convert biological energy unitsadenosine triphosphate (ATP) into mechanical motion, diverse artificial inorganic nanomachines such as rod-shaped Pt-Au nanomotor, Au-Ag-Ni nanoswimmer, Pt-coated TiO2 micromotor, and Janus mesoporous silica nanomotor have been demonstrated over the past decade.9, 10 They could convert surrounding chemical energy11, 12 or other energy sources such as electrical,13, 14 light,15, 16

magnetic,17-19 or acoustic energy20-22 into mechanical motion. However, due to the limitation

of flexibility and their poor biocompatibility and biodegradability, the reported nanomachines in the application of biomedical applications are still challenging.10 Liquid metals such as gallium and its alloys exhibit low melting points, high surface tension, high thermal and electrical conductivity, low toxicity and degradability in mildly acidic conditions, which have potential in many fields.23-28 The introduction of T-1000 in Terminator 2 presented viewers an impressive liquid metal robot which can transform from solid to silvery liquid, flow through narrow channel, and then self-repair itself into the solid shape again, which inspires the design of liquid metal robots with similar fascinating performances, particularly at the micro- and nanoscale for biomedical applications. Recently, several groups have reported the millimeter- and centimeter-scaled liquid metal motors, propelled by externally electric fields,29-32 chemical reactions,33 and ionic gradients.34 To our best knowledge, the use of liquid metals to fabricate a shape-transformable nanomachine for biomedical applications has not been reported yet.


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Here, we demonstrate a shape-transformable, fusible rod-like liquid metal gallium nanomachine (LGNM) propelled by an external ultrasound field. A pressure-filter-template method35 was used to prepare rod-like LGNMs whose size is dependent on the used nanoporous membranes, filter time and pressure. The LGNMs were propelled by an ultrasound field, which is considered as a safe and efficacious technology for actuating nanomachines in a bio-system.36, 37

The as-prepared LGNMs show strong fluorescence and are helpful to actively navigate to the

predefined HeLa cells. The nanorod-to-nanodroplet transformation, nanodroplet fusion and degradation processes were observed in both acidic buffer solution and the acidic endosomes of HeLa cells. The strong plasmon resonance of LGNMs in the near-infrared (NIR) region permits the potential for cancer therapy by photothermal effect. These liquid metal nanomachines represent a multifunctional platform with capacities of shape-transformation, fusion and biocompatibility for precision theranostics in future. RESULTS & DISCUSSION The LGNMs were fabricated by a pressure-filter-template method, as illustrated in Figure 1a. The setup is made of a nanoporous membrane as the template and the vacuum accessories to filter the liquid metal into the nanoporous template in a controlled manner. Gallium blocks becomes flowing liquid droplets above its melting point (29.8oC) (Figure 1b). Briefly, a droplet of gallium was put onto the surface of a commercially porous polycarbonate (PC) membrane with a diameter of 400 nm, following by filtration under a pressure of 105 Pa for 6 h at 35oC. Homogeneous LGNMs were obtained after removing the excess gallium and dissolving the PC templates. The scanning electron microscopy (SEM) image in Figure 1c shows monodispersed LGNMs with a length of about 5.5 µm. Note that the two ends of as-synthesized LGNMs differed in size and their diameters are 389±29 nm and 590±38 nm, which is essential for


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acoustically propelled movement.20 The energy-dispersive X-ray (EDX) mapping image confirms the presence of both gallium and oxygen elements in the as-synthesized nanomachine (Figure 1d). The corresponding EDX spectrum (Figure 1e) illustrates the 0.95% weight fraction of oxygen element in the LGNMs. The X-ray photoelectron spectroscopy (XPS) measurement of the LGNMs shows the existence of zero-valent gallium and trivalent gallium, confirming the formation of gallium oxide (Ga2O3) (Figure 1f). The transmission electron microscopy (TEM) image in Figure 1g displays the asymmetry and core-shell structure of the as-synthesized LGNM. The enlarged TEM image shows that the shell thickness was about 7 nm, which should be the gallium oxide layer around the inner gallium. It could be found that all the 0.95wt% oxygen comes from the oxide layer after calculation. To further identify the state of the gallium core, the force-displacement behavior of LGNMs was measured by using atomic force microscopy (AFM) at room temperature (Figure 1h). It can be observed that a noticeable peak existed at the indentation profile of -160 nm, indicating that the tip pierced the gallium oxide shell and contacted the gallium core. According to the Derjaguin-Muller-Toporov (DMT) model, the Young’s modulus of the LGNM was calculated to be 17.9 MPa, much smaller than that of the solid gallium (7.54 GPa), representing that the gallium core of LGNMs is in a liquid state. In contrast, the previously reported eutectic gallium-indium (EGaIn) alloy nanorods through a photothermal treatment are solid since the due to the gallium composition of EGaIn alloy has completely been oxidized into gallium oxide monohydroxide (GaO)OH.28 Fluorescence spectroscopy

measurement

demonstrates

that

the

LGNMs

exhibit

a

noticeable

photoluminescence at 582 nm and 640 nm at an excitation wavelength of 488 nm (Figure 1i). The subsequent laser scanning confocal microscope images (CLSM) also show that the LGNMs displayed strong green and red fluorescence under the excitation wavelengths of 488 nm and 633


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nm, further indicating that these LGNMs can be used as in vivo image-guided disease theranostics without addition of any fluorescent probes (Figure 1j). Also, the fluorescence intensity of LGNMs with a diameter of 5 µm, 3 µm, 1µm and 200 nm at an excitation wavelength of 488 nm is almost identical, indicating that both the length and the diameter of the as-prepared LGNMs did not influence the fluorescence intensity (Figure S1). The UV-vis-NIR absorption spectrum of the LGNMs in water shows the distinct absorption in the NIR wavelength region, which is important for photothermal therapy (Figure 1k). Moreover, the length of the obtained LGNMs could be modulated by adjusting the filtering time and pressure. The fitted curve in Figure S2 shows that there was a linear relationship between the length of LGNMs and the filter time or pressure. The length of as-synthesized LGNMs increased with the increasing time and pressure. The longest liquid metal nanorod was about 15 µm, in consistent with the thickness of the used PC template. More interestingly, Figure S3 shows that the as-prepared LGNMs maintains the rod-shaped structure after incubating 72 h at 37oC (above gallium melting point). It is supposed that these LGNMs should transform from nanorods to nanodroplets due to the high surface tension of liquid gallium. We speculate that the outside Ga2O3 layer stabilizes the rod-shaped structure. Next, we examined the role of the outer Ga2O3 layer on the stability of the rod-shaped LGNMs. To this end, the gallium rods with diameters of 5 µm, 1 µm, 800 nm and 400 nm, respectively, were immersed in a phosphate buffer solution (PBS) at pH 5 since the gallium oxide could be decomposed in an acidic condition. Microscopic images in Figure 2a show that these gallium rods completely transformed from rods to droplets after 2 h at 37oC. The gallium rods with the length of 14.22 µm, 12.90 µm, 11.31 µm and 7.96 µm, respectively, transformed to droplets with the diameters of 6.99 µm, 2.30 µm, 1.81 µm and 0.94 µm, respectively. This rod-


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to-droplet transformation obviously arises from the presence of the high surface tension of rodlike liquid gallium cores once the removal of Ga2O3 layers. This clearly demonstrates our speculation that the outside gallium oxide shells stabilize the rod-shaped structure of LGNMs. Note that the volumes of the resulting gallium droplets decrease about 36%, 42%, 46%, 57%, compared to the gallium rods with the diameter of 5 µm, 1 µm, 800 nm and 400 nm, respectively. It suggests that the exposed liquid gallium also is partially dissolved by acid after removal of the Ga2O3 shells. More particularly, similar rod-to-droplet transformation of the LGNMs were also observed at pH 5 at room temperature (below gallium melting point) (Figure S4). It indicates that the inner rod-like gallium cores of LGNMs remained in the liquid phase at room temperature, which is probably because of the pre-melting effect of gallium nanoparticles at room temperature.38-41 We further studied the structural transformation process of LGNMs with a diameter of 400 nm in a higher gallium concentration in PBS of pH 5 at 37oC. Figure 2b shows that the lengths of the LGNMs gradually became shorter in the first half hour, indicating that the two ends of LGNMs preferred to react with protons. This is because that there are more defects of the outside oxide layers in the two ends of LGNMs. With the prolonged reaction time, the rod-like LGNMs gradually transformed into droplets. Interestingly, LGNMs near one another could coalesce into larger droplets. Also, these droplets slowly degraded into smaller droplets due to the chemical reaction between gallium and acid, and some of them disappeared completely within 72 h. Taken together, these results suggest that our synthesized LGNMs exhibit good pH-responsive transformation, fusion and degradation capabilities. To acoustically drive the LGNMs, LGNMs were dispersed in water and dropped into a cell with a diameter of 5 mm and a height of 1.5 mm. Firstly, we tested the effect of the frequency


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on the velocity. Figure 3a illustrates the dependence of velocity of the LGNMs on the frequency of the ultrasound field ranging from 330 kHz to 550 kHz, while maintaining the applied voltage at 10 V. It can be found that the velocity of LGNMs initially increase and then decrease with the increasing excitation frequency. The maximum velocity could reach to about 23 µm s-1 at a frequency of 420 kHz, and thus the frequency of 420 kHz was employed to propel the LGNMs in the following experiments. Figure 3b further shows that a quadratic relationship existed between the ultrasound voltage and the velocity of LGNMs, indicating that the voltage could be used to modulate the velocity of motion. The time-lapse image in Figure 3c (taken from Video S1) shows that the LGNMs with a length of 2.0 µm, 3.5 µm, 4.6 µm, 5.4 µm and 5.8 µm traveled 24.2 µm, 33.6 µm, 47.4 µm, 29.1 µm, and 17.5 µm over a period of 2 s, respectively, under an ultrasound frequency of 420 kHz at an applied voltage of 10 V. The mean square displacement (MSD) versus time interval curves of the LGNMs show that the slopes firstly raised and then decreased with the increasing length of LGNMs (Figure 3d). More particularly, the instantaneous velocity of LGNMs exhibited a similar trend with the value of (ln(L/R)-0.72)/2πηL, where the L and R are the length and diameter of the LGNMs, and η is the dynamic viscosity of water (Figure 3e). Taken together, these results demonstrate that the velocity of motion of LGNMs did not decrease with the increase of the length of LGNMs, suggesting that the driving force is not the acoustic streaming force.42-44 To better investigate the acoustic propulsion, liquid metal gallium particles with a diameter of 1 µm were added as traced particles. Microscopic image in Figure 3f, captured from Video S2, shows that two LGNMs with a diameter of 400 nm and a length of 5.5 µm moved a distance of 83 µm and 70 µm in 4 s at an ultrasound frequency of 420 kHz and applied voltage of 10 V.


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Also, the movie in Video S2 clearly demonstrates that two LGNMs swim in a rotational style. Note that the direction of motion of LGNMs (red lines) obviously differed from that of the traced particles (blue arrow lines). This difference arises from different driving forces of two kinds of particles. In other words, the traced particles are propelled by the acoustic streaming flow, however, the motion of two LGNMs originates from the primary acoustic radiation force20, 45 on the asymmetric shape of the LGNMs in the levitation plane by acoustic energy that is generated at the base of the cell (Figure 3g). To confirm this mechanism, a finite element method (FEM) model was established using the acoustic pressure module in the COMSOL Multiphysics (Figure S5). The simulation result suggests that an acoustic pressure gradient field presents in the levitation plane (Figure 3h). According to the simulated gradient field in Figure 3h, the acoustic pressure difference (∆P, blue line) along their long axis gradually increases, which is in parallel with the measured velocity of motion (black line) along the trajectory of LGNMs (Figure 3h). To assess the cytotoxicity of LGNMs, human L-02 hepatocytes (L-02) and HeLa cancer cells were co-cultured with the LGNMs at concentrations from 10 to 300 mg L-1 for 24 h. Then, the cell viability was evaluated by using the 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) assay. As shown in Figure S6, even at a concentration of 300 mg L-1, approximately 90% of them remained alive, indicating the negligible cytotoxicity of the asprepared LGNMs. Next, we evaluated the capacities of nanomachines for actively seeking and drilling into the predefined HeLa cells (Figure 4a). Figure 4b and the corresponding Video S3 show an acoustically-propelled LGNM to actively target a HeLa cell. Once the LGNM reached to the predefined HeLa cell, it continued its rotational motion and thus drilled the cell membrane by sustained ultrasonic propulsion. After 24 h, CLSM images in Figure 4c and corresponding 3D reconstruction image in Figure 4d further confirmed the intracellular localization of the LGNM.


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Here, the red fluorescence represents the DID-dyed HeLa cancer cell membrane, whereas the rod-like green fluorescence comes from the LGNM. We further explored whether the LGNMs could also transform, fuse and degrade inside cells like those in acidic conditions as schematically illustrated in Figure 5a. As shown in Figure 5b, the shape of two internalized LGNMs with diameters of 400 nm and lengths of 3.5 µm and 5.5 µm did not obviously change in the beginning. However, the shape-transformation was gradually observed with the increasing incubation time. After 3 h, two rod-like LGNMs almost transformed into two spherical droplets with volume of 0.45 µm3 and 0.70 µm3, respectively. More interestingly, two transformed droplets started to fuse together. The CLSM images in Figure 5c show that two droplets inside the cell completely fused into one big droplet with a volume of about 1.10 µm3, which is approximately equal to the total volume of two nanomachines. The 3D reconstruction CLSM image in Figure 5d reveals that the fused liquid metal droplet locates inside the HeLa cell. Moreover, the TEM image of the cell slice in Figure 5e confirmed that the fused liquid metal droplets are encapsulated in the endosomes of HeLa cells. This means that the rod-to-droplet transformation and fusion of the LGNMs happened in acidic endosomes of HeLa cells, in agreement with the results discussed above (Figure 2b). Finally, we studied the fused liquid metal droplets as a photothermal agent for cancer therapy since the LGNMs have a strong absorption in the NIR region as mentioned above. Optical image in Figure 5f shows that the fused liquid metal droplets were internalized in the HeLa cells and all of cells reserved their membrane integrity. Under exposure of 808 nm focused NIR laser with a spot diameter of 7 µm at a power of 20 mW·µm-2 for 5 s, we clearly observed a broken region in the cell membrane as shown in Figure 5g. Then, two fluorophores, propidium iodide (PI) and Calcein acetoxymethyl (Calcein-AM) were added to indicate the cell viability.


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Fluorescence image in Figure 5h shows that a red fluorescence appeared from the irradiated HeLa cells, while the untreated cells displayed green fluorescence, indicating the necrosis of the treated HeLa cell. In the bare control group, the irradiated HeLa cell maintained its activity under the same irradiation conditions (Figure S7). These results suggest that the photothermal effect of the liquid metal droplet leads to a significant increase of temperature in the cell and subsequently killed the cell. CONCLUSION We have demonstrated a shape-transformable, fusible and biodegradable rod-like liquid metal gallium nanomachine that could be propelled by ultrasound field. This liquid metal nanomachine was prepared by using a pressure-filter-template method, and their diameter is dependent on the inner porous diameter of the templates and the length is determined by the thickness of the templates, the used filtering time and vacuum pressure. The as-prepared rod-like nanomachines show an asymmetric and core-shell structure. The shell is composed of gallium oxide, which stabilize the rod-like core of liquid gallium. Both the asymmetric structure and high density of nanomachines allow for ultrasound-propelled motion and precise speed control. The computer simulation result reveals that the propelling force mainly comes from the primary acoustic radiation force on the asymmetric shape of the rod-like nanomachines in the levitation plane. The resulting propulsion force by acoustic energy is able to actively piercing and internalization of liquid metal nanomachines into HeLa cells. More importantly, these internalized liquid metal nanomachines exhibit a rod-to-droplet transformation, and subsequent droplet fusion in the mild acidic endosomes of HeLa cells - like some features of the T-1000 Terminator. Upon irradiation of NIR light, the generated photothermal effect of liquid metal nanomachines could rapidly kill cancer cell and subsequently be degraded. In comparison with


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the existing gold-based ultrasound-driven nanomachines, LGNMs have unique capabilities of shape-transformation, fusion and photoluminescence. Such liquid metal nanomachine could be used as a multifunctional platform integrating drug delivery, active targeting, imaging and photothermal cancer therapy and represents a state-of-the-art inorganic nanomachine capable of advancing the implementation of next-generation precision theranostics. EXPERIMENTAL SECTION Preparation of LGNMs: Firstly, a bulk liquid metal gallium was poured into the polycarbonate (PC) membranes coated filters, followed by filtration under a pressure of 105 Pa at 35oC. After filtration for 6 h, the excess gallium was removed by wiping the surfaces of the template. Then, the liquid metal nanomachines were obtained by dissolving the templates with CH2Cl2 and redisposing into water. Acoustic experiments: Polyimide tape was used to prepare the cell on the surface of glass slide, the cell has a cylindrical structure with a diameter of 5 mm, a height of about 1.5 mm. The other side of the glass sheet was attached by a ceramic transducer to generate acoustic waves. By connecting with a function generator and a signal amplifier, a sine wave transferred to the ceramic transducer. In a typical experiment, liquid metal nanomachine solution was added in the cell, followed by covering the cell with a square glass coverslip which acted as the sound reflector. An Olympus optical microscope was used for observing and recording the acousticallypropelled motion of the liquid metal nanomachines. Numerical Simulation: The simulation was conducted using the commercial software COMSOL Multiphysics. Four different physical modules of COMSOL Multiphysics were used to study the acoustic field of motion plane. The simulation space was a cylinder of fluid with 160 µm in


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length and 120 µm in diameter. A predefined finer mesh for fluid hydrodynamics was used for the fluid domain with an accuracy of 3 µm. Meanwhile, a free triangular mesh was used to generate 86341 elements. All the domains were fully coupled and a frequency simulation was carried out to calculate the system. In vitro cytotoxicity: HeLa and L-02 cells were seeded in the 96-well plates (104 cells per well). After culture for 24 h, the cells were co-cultured with the liquid metal nanomachines at different concentrations for 24 h, followed by adding 20 ml of the 3-(4, 5-dimethylthiazol-2-yl)-2, 5diphenyltetrazolium bromidesolution (5 mg mL-1). After 4 h of incubation, the medium was removed, and the cells were mixed with 150 mL of dimethyl sulphoxide. The cell viability was tested by measuring the absorbance at a wavelength of 570 nm by a microplate reader. Intracellular position measurement of LGNMs: The cell membranes of liquid metal nanomachines penetrated HeLa cells was labelled by DID dye for 15 min and then washed with PBS. Then, the intracellular position of liquid metal nanomachines was measured by using a Leica TCS SP5 II confocal laser scanning microscope (CLSM) (Leica, Heidelberg, Germany). The excitation wavelength of liquid metal nanomachines was 488 nm and the excitation wavelength of cell membrane dye DID was 633 nm. Photothermal cancer therapy: A modified inverted fluorescence microscope (Olympus IX 71, Tokyo, Japan) was used to observe the photothermal cancer therapy process of liquid metal nanomachines. The HeLa cells with fused liquid metal nanomachines were irradiated by an 808 nm laser at the power of 20 mW·µm-2 for 5 s. Subsequently, two fluorophores, PI and CalceinAM were added to indicate the cell viability. Finally, the dyes were excited by a 488 nm light and the resulting fluorescence image was captured simultaneously.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional information (PDF) The acoustic propelled motion of liquid metal nanomachines with different lengths (AVI) The motion of liquid metal nanomachines under an ultrasound field (AVI) Liquid metal nanomachine moves to the predefined HeLa cell under the propulsion of ultrasound field (AVI) AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected] Author Contributions D. Wang and C. Gao contributed to this wok equally. ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (No. 21573053) and National Postdoctoral Program for Innovative Talents (BX201700065). REFERENCES (1) Guix, M.; Mayorga-Martinez, C. C.; Merkoçi, A. Nano/Micromotors in (Bio)chemical Science Applications. Chem. Rev. 2014, 114, 6285-6322.


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(2) Li, J.; Esteban-Fernández de Ávila, B.; Gao, W.; Zhang, L.; Wang, J. Micro/Nanorobots for Biomedicine: Delivery, Surgery, Sensing, and Detoxification. Sci. Robot. 2017, 2, eaam6431. (3) Peng, F.; Tu, Y.; van Hest, J. C. M.; Wilson, D. A. Self-Guided Supramolecular CargoLoaded Nanomotors with Chemotactic Behavior towards Cells. Angew. Chem. Int. Ed. 2015, 127, 11828-11831. (4) Palacci, J.; Sacanna, S.; Steinberg, A. P.; Pine, D. J.; Chaikin, P. M. Living Crystals of Light-Activated Colloidal Surfers. Science 2013, 339, 936-940. (5) Walker, D.; Käsdorf, B. T.; Jeong, H.-H.; Lieleg, O.; Fischer, P. Enzymatically Active Biomimetic Micropropellers for the Penetration of Mucin Gels. Sci. Adv. 2015, 1, e1500501. (6) Cheng, M.; Liu, Q.; Ju, G.; Zhang, Y.; Jiang, L.; Shi, F. Bell-Shaped SuperhydrophilicSuperhydrophobic-Superhydrophilic Double Transformation on a pH-Responsive Smart Surface. Adv. Mater. 2014, 26, 306-310. (7) Simmchen, J.; Baeza, A.; Ruiz, D.; Esplandiu, M. J.; Vallet-Regí, M. Asymmetric Hybrid Silica Nanomotors for Capture and Cargo Transport: Towards a Novel Motion-Based DNA Sensor. Small 2012, 8, 2053-2059. (8) García-López, V.; Chen, F.; Nilewski, L. G.; Duret, G.; Aliyan, A.; Kolomeisky, A. B.; Robinson, J. T.; Wang, G.; Pal, R.; Tour, J. M. Molecular Machines Open Cell Membranes. Nature 2017, 548, 567-572. (9) Kim, K.; Guo, J.; Xu, X.; Fan, D. L. Recent Progress on Man-Made Inorganic Nanomachines. Small 2015, 11, 4037-4057. (10) Wang, H.; Pumera, M. Fabrication of Micro/Nanoscale Motors. Chem. Rev. 2015, 115, 8704-8735. (11) Ma, X.; Hahn, K.; Sanchez, S. Catalytic Mesoporous Janus Nanomotors for Active Cargo Delivery. J. Am. Chem. Soc. 2015, 137, 4976-4979. (12) Zheng, J.; Dai, B.; Wang, J.; Xiong, Z.; Yang, Y.; Liu, J.; Zhan, X.; Wan, Z.; Tang, J. Orthogonal Navigation of Multiple Visible-Light-Driven Artificial Microswimmers. Nat. Commun. 2017, 8, 1438. (13) Loget, G.; Kuhn, A. Electric Field-Induced Chemical Locomotion of Conducting Objects. Nat. Commun. 2011, 2, 535. (14) Yan, J.; Han, M.; Zhang, J.; Xu, C.; Luijten, E.; Granick, S. Reconfiguring Active Particles by Electrostatic Imbalance. Nat. Mater. 2016, 15, 1095.


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(41) Parravicini, G. B.; Stella, A.; Tognini, P.; Merli, P. G.; Migliori, A.; Cheyssac, P.; Kofman, R. Insight into the Premelting and Melting Processes of Metal Nanoparticles through Capacitance Measurements. Appl. Phys. Lett. 2003, 82, 1461-1463. (42) Ahmed, S.; Wang, W.; Bai, L.; Gentekos, D. T.; Hoyos, M.; Mallouk, T. E. Density and Shape Effects in the Acoustic Propulsion of Bimetallic Nanorod Motors. ACS Nano 2016, 10, 4763-4769. (43) Barnkob, R.; Augustsson, P.; Laurell, T.; Bruus, H. Acoustic Radiation- and StreamingInduced Microparticle Velocities Determined by Microparticle Image Velocimetry in an Ultrasound Symmetry Plane. Phys. Rev. E 2012, 86, 056307. (44) Nadal, F.; Lauga, E. Asymmetric Steady Streaming as a Mechanism for Acoustic Propulsion of Rigid Bodies. Phys. Fluids 2014, 26, 082001. (45) Wang, W.; Duan, W.; Ahmed, S.; Mallouk, T. E.; Sen, A. Small Power: Autonomous Nanoand Micromotors Propelled by Self-Generated Gradients. Nano Today 2013, 8, 531-554.


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Figure 1. Preparation and characterization of liquid metal gallium nanomachines (LGNMs). (a) Scheme for the preparation of LGNMs through the pressure-filter-template method. (b) Optical image of flowing liquid metal gallium droplets. Scale bar, 1 cm. (c) SEM image of the asfabricated LGNMs by using the polycarbonate membrane with a pore size of 400 nm. Scale bar, 5 µm. (d) EDX mapping images of the LGNM show the existence of gallium and oxygen element. Scale bar, 1 µm. (e) EDX spectrum of the LGNMs detects the content of gallium and oxygen. (f) The Ga 3d XPS spectrum of the LGNMs. (g) TEM image confirms the core-shell structure of as-synthesized LGNM. Scale bar, 1 µm. The enlarged TEM image indicates that the thickness of oxide layer is about 7 nm. Scale bar, 50 nm. (h) The force-displacement curves of the LGNM and solid gallium (inset). (i) The fluorescence emission spectrum of LGNMs at an excitation wavelength of 488 nm. (j) CLSM images of LGNMs, scale bars, 5 µm. (k) UV-VisNIR spectrum of the LGNMs in water.


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Figure 2. The transformation, fusion and degradation of LGNMs in a PBS of pH 5 at 37oC. (a) Microscopic images showing the pH-responsive rod-to-droplet transformation of LGNMs with diameters of 5 µm, 1 µm, 800 nm and 400 nm, respectively, after incubating 2 h. (b) The timelapse images illustrating the transformation, fusion and degradation of the LGNMs. All scale bars, 10 µm.


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Figure 3. Acoustically-propelled LGNMs with a diameter of 400 nm. (a) The effect of the ultrasound frequency on the velocity of LGNMs at an applied voltage of 10 V. (b) The average velocity of LGNMs under different voltages at a frequency of 420 kHz. (c) Time-lapse images of five LGNMs with the diameter of 400 nm and different lengths under an ultrasound frequency of 420 kHz at an applied voltage of 10 V in a period of 2 s. The lengths of the LGNMs are 2.0 µm (1), 3.5 µm (2), 4.6 µm (3), 5.4 µm (4) and 5.8 µm (5), respectively. Scale bar, 10 µm. (d) The MSD verus time interval of the LGNMs. (e) The instantaneous velocities of five LGNMs in (c). (f) Time-lapse images showing the trajectories of two LGNMs (red lines) in 4 s at an ultrasound frequency of 420 kHz and applied voltage of 10 V. The blue arrow lines represent the direction of motion of the traced liquid gallium particles with a diameter of 1 µm. Scale bar, 20 µm. (g) Schematic illustration of the autonomous motion of the asymmetric LGNMs in the levitation plane by acoustic energy that is generated at the base of the cell. (h) COMSOL simulation of the generated pressure field. (i) The relationship between the experimental velocity (black) and the simulated pressure difference (∆P) (blue) of the LGNMs in the levitation plane.


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Figure 4. Schematic illustration (a) and time-lapse images (b) of acoustically-propelled LGNMs for actively seeking and target the cancer cells. (c) CLSM images illustrating the internalization of the LGNM into a HeLa cell after 24 h. (d) The 3D reconstruction CLSM image of the cell in (c). All scale bars, 10 µm.


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Figure 5. (a) Scheme of the intracellular transformation, fusion and photothermal cancer cell therapy of LGNMs. (b) Time-lapse microscopic images demonstrating the intracellular transformation and fusion process of two LGNMs. Scale bars, 10 µm. (c) CLSM images of the fused liquid metal droplets inside the HeLa cell. Scale bars, 10 µm. (d) The 3D reconstructed CLSM image of the HeLa cell in (c). Scale bars, 10 µm. (e) Representative TEM image of the HeLa cell containing the fused liquid metal droplets. Scale bar, 2 µm. Microscopic images of HeLa cells with the fused liquid metal droplets (f) before and (g) after NIR laser irradiation. Scale bars, 20 µm. (h) Fluorescence image of HeLa cells stained with Calcein-AM and PI showing the dead cell (red) and normal cells (green) after NIR irradiation. Scale bars, 20 µm.


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TABLE OF CONTENTS


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Shape-Transformable, Fusible Rodlike Swimming Liquid Metal

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