Key words: Nickel nanowire; Microbead; Microorganism; Cell manipulation; Rotating magnetic field
Single-cell analysis is challenging but provides an opportunityto understand cells and microorganisms at unprecedented level ofdetails.2,3 To perform single-cell analysis, various bio-manipula-tion technologies and tools have been developed, such asmicrogrippers, magnetic tweezers, and optical tweezers.4,5 Inprinciple, two different manipulation strategies can be applied4-8:(i) contact manipulation, in which the end-effector contacts themicro-object directly for manipulation, for instance using amicrogripper to perform a pick-and-place task; and (ii) non-contactmanipulation, in which the micro-object is manipulated wirelessly,for instance by applying an electric field, a focused laser beam, oran acoustic force. Although these methods are efficient for precisemanipulation and trapping, they usually require complex exper-imental setups, and the electrical and optical field may haveadverse effects on biological samples. In the case of contact-manipulation approaches, releasing a microscale or nanoscaleobject can be challenging because van der Waals forces and othersurface forces dominate gravitational and inertial forces at this
No conflict of interest was reported by the authors of this paper.The topic of this invited contribution has been presented at the 4th IEEE
International Conference on Nano/Molecular Medicine and Engineering(IEEE-NANOMED 2010), 5-9 December, 2010, Hong Kong SAR, China.1
This research was partially supported by the European Commission underthe 7th Framework Program (FP7) from the project “Nano-Actuators andNano-Sensors for Medical Applications (NANOMA)” and the European ResearchCouncil Advanced Grant “Microrobotics and Nanomedicine (BOTMED)”.
⁎Corresponding author: Institute of Robotics and Intelligent Systems,ETH Zurich, CH-8092 Zurich, Switzerland.
Please cite this article as: L. Zhang, T. Petit, K.E. Peyer, B.J. Nelson, Targeted cxx:1-7, doi:10.1016/j.nano.2012.03.002
scale. In contrast, for noncontact-manipulation approaches,problems of that kind are negligible because no mechanicalcontact between the micro-object and the end-effector occurs.Among the various noncontact-manipulation approaches, micro-fluid flow-control techniques are promising for bio-manipulationbecause they handle living cells gently. The flow is typicallycontrolled by microfluidic devices, e.g., Lab-on-a-Chip devices.9
Recently, researchers also showed that magnetically drivenmicrorobots are capable of localized fluid flow control.7,8,10 Themain advantage of the low-field-strength magnetic actuation is thatit has a minimal impact on living cells and biological tissues, andtherefore it is promising for biomedical applications.11
Among the different magnetic materials, magnetic nanowires(NWs), such as nickel nanowires (Ni NWs), are interestingcandidates for cell manipulation. Due to shape anisotropy, amagnetic torque can be applied to actuate individual magneticNWs with a low-strength magnetic field.12-17 Researchers havereported that Ni NWs are easily internalized by cells, and nosignificant cytotoxicity was observed due to several layers ofnative nickel oxide on the surface.18-20 Moreover, it was shownthat Ni NWs could be remotely heated by using radio frequencyfields at 810 MHz to induce hyperthermia in living cells.21
Methods
Fabrication and characterization of Ni NWs
Ni NWs were synthesized by a template-assisted electro-chemical deposition method. Anodic aluminum oxide (AAO)
argo delivery using a rotating nickel nanowire. Nanomedicine: NBM 2012;
Figure 1. FESEM micrograph of Ni NW arrays embedded in an AAOtemplate. Inset: FESEM micrograph of the top view of the AAO templatebefore electrodeposition.
Figure 2. COMSOL simulation of the flow field around a rotating NW at50 Hz. The length and thickness of the NW were set to 12 μm and 200 nm,respectively.
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membranes containing ordered channels with pore sizes ofapproximately 100 nm or 200 nm were used as templates. Thelength of the NWs is controlled by the electroplating time.Figure 1 shows a field emission scanning electron microscopy(FESEM) micrograph of Ni NW arrays embedded in an AAOtemplate. The details of the synthesis procedure are reportedelsewhere.16,22 The magnetic properties of the Ni NW arraysembedded in AAO templates were characterized by a vibratingsample magnetometer (see Supplementary Materials, availableonline at http://www.nanomedjournal.com).
Magnetic actuation: 3-axis coils set-up and principle
For precise control of the motion of the Ni NW under anoptical microscope, three orthogonal Helmholtz coil pairswere constructed to generate a uniform rotating magneticfield. A maximum magnetic field strength of 15 mT can beapplied. Magnetic field strengths of 1–3 mT are, however,sufficient to perform most cell manipulation tasks. Details ofthe three-axis Helmholtz coil setup are available in theSupplementary Materials.
Before magnetic actuation, a Ni NW suspension in DI waterwas prepared by 15-minute sonication. Afterwards, the NWsuspension was transferred into a tank with dimensions of 20 mm(length) × 15 mm (width) × 2 mm (height). A Si substrate coatedwith a SiO2 layer was placed at the bottom of the tank for thetests. The magnetic field was turned on after the NWs sank nearthe Si substrate. When a rotating magnetic field is turned on, theNi NWs rotate simultaneously in the same plane as the plane ofrotation of the magnetic field because the Ni NWs, driven by theinduced magnetic torque (τm), attempt to align their long axis tothe field. In a uniform field B, the magnetic torque for a magneticNW is expressed as:
sm = VM × B ð1Þ
where V is the volume of the NW, M is the magnetization of theNW and B is the magnetic field. The steering of a NW is realized
by changing the plane of rotation of the applied magnetic field.Our results show that the tumbling NW was steered withmicrometer positioning precision (see Supplementary Data).More details on the magnetic actuation were also publishedelsewhere.16
Results
COMSOL simulation of fluid flow surrounding the rotating NW
The fluid flow generated by a swimming microorganism caninfluence the motion of the nearby microorganisms due to thehydrodynamic interactions between the cells.23 The fluid flowgenerated by the microorganisms was also applied for thecontrol of pumping and mixing in Lab-on-a-Chip devices.24-26
Recently, it was demonstrated that the fluid flow induced bywirelessly controlled microrobots is capable of noncontactmanipulation.7,8,10 In comparison with natural microswimmers,swimming microrobots are less demanding of the environmentalconditions, such as pH value and temperature. Moreover, thelocalized flow field can bemodified by designing the microrobotsin a different shape and by actuating the microrobots with adifferent field, such as an oscillating field or a rotating field.
A Computational Fluid Dynamics (CFD) model wasdeveloped with COMSOL Multi-physics to investigate thefluid flow profile of a rotating NW. It was assumed that the NW,which has a length of 12 μm and a thickness of 200 nm, rotatessynchronously with a rotating magnetic field at 50 Hz. Tosimplify the simulation, the flow field generated by a rotatingNW was simulated in free space. No-slip boundary conditions
Figure 3. (A) Image sequence showing a 3-μm-diameter microbead pushed by a 7-μm-long tumbling Ni NW (also see video S1). The NW and microbead hadapproximately the same vx, and the distance between the NW and the microbead became constant. The NW is propelled upward in the image. The inputfrequency (f) was 21 Hz and the field strength (B) was 3 mT. (B) The curves show dependence of the translational displacement of the NW and microbead on thepushing time, and the dependence of the gap distance between the NW and microbead on the pushing time. The error bars are attributed to the uncertainty of theposition of the NW and the bead based on the resolution of the recorded images. (C) Image sequence showing a 3-μm-diameter microbead pulled by a 7-μm-long tumbling Ni NW (also see video S1). The Ni NW tumbles downward in the image. The input frequency (f) was 31 Hz and the field strength (B) was 3 mT.(D) The curves show dependence of the translational displacement of the NW and microbead on the pulling time, and the dependence of the gap distancebetween NW and microbead on the pulling time.
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were used for the surface of the NW, which is assumed to be non-deformable. All calculations were done in a rectangular cuboidvolume with flow velocities set to 0 μm/s on its faces due to anassumption of zero external flow. The flow field around therotating NW in free space is depicted in Figure 2, which showsthat the fluid is in a laminar flow condition. The flow velocity isapproximately zero close to the center of the NW and maximalnear the two extremities of the NW. The maximal flow velocityis approximately 1.9 mm/s at 50 Hz, for which the Reynoldsnumber is about 10-2 with regard to the 12 μm long NW. Thesimulation result also reveals that the velocity of the fluiddecreases about one order of magnitude in a few microns due tothe predominance of viscous forces in the Stokes flow.
Micromanipulation of individual microbeads
For the manipulation tests of individual microbeads, a singleNi NW was used as a magnetic end-effector to move 3 μm- and6-μm-diameter polystyrene (PS) microbeads. In the experiments,the 200 nm diameter Ni NWs and microbeads were immersed inDI water in a tank, and then a rotating magnetic field wasapplied. To demonstrate noncontact manipulation, three basictasks, i.e., pushing, pulling, and rotation of a microbead, wereconducted. For contact manipulation, targeted delivery of a6-μm-diameter PS microbead was performed on a complexsurface by assembling it on one end of the rotating Ni NW.
The noncontact pushing tests were conducted by a 7-μm-longNi NW and a 3-μm-diameter microbead. Figure 3, A shows thatwhen the NW was propelled towards a 3-μm-diameter bead withan input frequency of 21 Hz, the bead was repelled in front of thetumbling NW without a mechanical contact due to the flowprofile around the NW, as shown by the simulation results. Twostages were found when the microbead was pushed by the fluidflow. In the first stage, the NW approaches the microbead, andthe microbead started moving when the tumbling NW wasnearby. However, the velocity (vx) of the NW was higher thanthat of the microbead. In the second stage, the microbead reachedthe same velocity as the tumbling NW and maintains the gapdistance (see Figure 3, A). The increase of the velocity of themicrobead resulted from the stronger hydrodynamic interactionbetween the microbead and the NW as their distance becamesmaller. The dependence of the translational displacement of theNW and the microbead on the time is shown in Figure 3, B inwhich the two stages are marked as the approach region and thelinear region, respectively. The plot also shows that the velocityof the NW was reduced after it pushed the microbead with aconstant velocity, whereas the velocity of the microbead wasincreased until it reached the same value as the tumbling NW.Moreover, the experimental results show that the distancebetween the NW and the microbead was reduced duringthe pushing operation as the rotational speed of the NWwas decreased.
Figure 4. (A) Image sequence showing two 6-μm-diameter PS microbeads rotated by rotating the Ni NWs simultaneously in a horizontal plane. The inputfrequency (f) was 23.5 Hz and the field strength (B) was 1.5 mT. (B) Several microbeads were manipulated by a rotating the NW to form a two-character pattern,i.e., “T” and “H,” on a surface.
Figure 5. Image sequence showing pick-and-place of a 6-μm-diameter PS microbead using a rotating Ni NW (also see video S2). After the microbead wastransported and positioned on the epidermal cell, the NW was separated from it. The dashed arrow represents the trajectory line of the tumbling NW-microbeaddoublet. The input magnetic field strength was 3 mT. The input frequency was 23 Hz for assembly and transport of the microbead.
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Noncontact pulling tests were performed by propelling the7-μm-long NW away from a 3-μm-diameter microbead. It wasdifficult, however, to drag the microbead for a long distance asshown in Figure 3, C. The microbead stopped moving inapproximate one second as the NW tumbled away from it with afrequency of 31 Hz. By reducing the rotational frequency of theNW, the interaction time between the NW and the microbeadwas increased; however, the displacement of the microbead wasnot significantly increased. Figure 3, D shows the dependence of
the average translational velocity of the microbead and the NWon the manipulation time.
To perform the rotation manipulation task of individualmicrobeads on a flat surface, two Ni NWs were propelled nearthe object and then rotated horizontally to generate a rotatingflow as shown by the simulation results (Figure 2). Figure 4, Ashows two NWs rotating clockwise with an input frequency of23.5 Hz, and the result shows that the rotational speed of the beadis related to the length of the corresponding NW. With the same
Figure 6. (A) Image sequence showing a flagellated microorganism transported using a tumbling Ni NW (also see video S3). The magnetic field was 2 mT andthe input frequency was increased from 1 Hz to 10 Hz. (B) Image sequence showing blood cell manipulation using a rotating Ni NW (also see video S4). Theblood cell was rotated on a Ni NW (left two images), and then it was transported and positioned on an epidermal cell (last four images). The magnetic fieldstrength was 3 mT. The input frequencies were tuned in a range of 0–3 Hz. Both scale bars are 30 μm.
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input frequency, i.e., 23.5 Hz, the long NW was able to rotate amicrobead for 1 turn in approximately 0.9 second, whereas theshort NW rotated the other microbead for about a half turn.Apparently, the long NW generated higher flow velocities nearits two ends in comparison with the shorter NW driven at thesame input frequency; thus the applied force on the microbeadwas larger. The microbeads, randomly distributed on a SiO2
surface, were also manipulated one by one to form a two -dimensional pattern by a rotating Ni NW. The previousmanipulation methods were used, i.e., pushing, pulling, androtation, to generate a pattern with two characters, i.e., “T”and “H,” as shown in Figure 4, B.
For cargo transport using a NW, a straightforward andefficient approach is to assemble the microobject on the NW.However, releasing the micro-object from the NW can betricky.16,27-29 Figure 5 shows a 6-μm-diameter PS microbeadthat was directly assembled onto one end of the tumbling Ni NW.The assembled NW and microbead doublet were steered to makea “U-shaped” turn and eventually the bead was placed on anepidermal cell, and the NW detached. (A video clip is alsoavailable in the Supplementary Materials.)
Micromanipulation of microorganism and individual blood cell
To investigate cell manipulation, flagellated microorganismsand human blood cells are used. Microorganisms were collectedfrom Lake Zurich. Under a light microscope we can observe that
many are flagellated microorganisms. The swimming speed ofthese microorganisms on a SiO2 surface is comparable with thatof the tumbling nanowire. In the test the tumbling NW waspropelled toward the flagella of a flagellated microorganism.When the NW and the flagella were sufficiently close, theyentangled. Figure 6, A shows a flagellated microorganismtransported by a tumbling NW near the substrate. (A video clipis available in the Supplementary Materials.)
For blood cell manipulation blood cells were attached on theNWs before the magnetic field was applied. Contact manipula-tion was performed using a cell tethered to a NW with a lowinput frequency, i.e., in a range of 0-3 Hz, as shown in Figure 6,B. By applying a rotating field in the horizontal plane, the cellwas driven by a simple rotation near a flat surface; thus itsorientation could be adjusted quickly as shown in the top row ofFigure 6, B. To demonstrate the targeted delivery of cells, ablood cell was transported and positioned on an epidermal cellwith a tumbling NW. (A video clip is available in theSupplementary Materials.)
Discussion
To perform pushing manipulation on a surface withouttouching between the end-effector and the micro-object, themicro-object must reach the same velocity as the end-effectorbefore it is overtaken. When the size of the micro-object and its
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interaction with the boundary, such as friction, becomes smaller,less force needs to be applied to push the micro-object with thesame speed as the end-effector. For the end-effector, i.e., atumbling NW, the rotational speed of the NW and thehydrodynamic interaction between the boundary and the rotatingNW are crucial for noncontact manipulation. A higher rotationalspeed of the NW generates larger fluid flow field, which ispreferred for a noncontact pushing manipulation because a largerforce is applied on the micro-object. However, the propulsionvelocity of the rotating NW also increases linearly,16 whichimplies that a larger force needs to be applied to the object forreaching the same translational velocity. Therefore, a tradeoffexists as the rotational speed of the NW is increased fornoncontact pushing. Additionally, to perform noncontactpushing manipulation continuously, the NW axis should bealigned to the object. In practice it is difficult to avoid thesidewise drifting of the object during the pushing tests due to thesurface roughness and other imperfect boundary conditions. Incontrast to noncontact pushing manipulation, the translationalvelocity of the NW is approximately the same during thenoncontact pulling test and the velocity of the microbead isreduced continuously until it is static due to the negligibleinfluence from the fluid flow (see Figure 3, D).
In the microbead-transport experiment shown in Figure 5, theNi NW was propelled toward the microbead and assembled on itwith an input frequency of 23 Hz. No significant impact wasobserved for steering after the assembly of the colloidal cargo.The disassembly of the microbead and the NW occurred whenthe NW rotated horizontally on the epidermal cell and the inputfrequency was reduced to approximately 5 Hz. An alternativemethod to disassemble the NW-microbead doublet on a flatsurface is by reversing the direction of rotation and tuning theinput frequency.16 High-positioning precision, i.e., ± 1–2 μm,was achieved for microbead manipulation using a rotating NWwith or without a physical contact.16,30
An alternative approach for micromanipulation is to utilizenatural microorganisms.31-33 In comparison with manipulationby means of an individual NW, the applied force formanipulation can be significantly increased by using a bacteriaswarm.33 Contrary to the cargo transport of a microbead, thetransport direction of a flagellated microorganism wasapproximately perpendicular to the long axis of the NW (seeFigure 6, A), which implies that the flagella were actuated bythe rotating Ni NW, and most likely a corkscrew motion wasgenerated to move the microorganism backward. The NW canbe “transported” by the microorganism if the torque generatedfrom the molecular motors of the microorganism is larger thanthe magnetic torque. For blood cell transport, unlike that of PSmicrobead delivery, it was difficult to detach the NW from thecell by tuning the input frequency. This was probably due tothe deformability of the cell membrane and the affinity ofbinding the cell membrane with the native nickel oxide layeron the NW.12,28 The surface contact area of the cell membraneon the NW was likely to be large due to membranedeformation and, hence, the van der Waals forces increased.The large contact area of a cell on a NW may offer advantagesin treating single cells, e.g., for targeted drug delivery byfunctionalized NWs.
In summary, noncontact manipulations, i.e., pushing, pulling,and rotation of individual polymer microbeads, were demon-strated by using localized fluid flow induced by a rotating NiNW. Furthermore, targeted delivery of a polymer microbead, amicroorganism, and a human blood cell was achieved in fluid.Rotating ferromagnetic NWs can be applied as wirelesslycontrolled end-effectors of manipulators for versatile manipula-tion tasks of individual micro-objects in fluid and have potentialfor single-cell analysis.
Acknowledgments
The authors thank the FIRST lab at ETH Zurich for technicalsupport. The authors also thank Prof. Jun Lou (Rice University)for providing us with Ni NWs and Dr. Bradley Kratochvil (ETHZurich) for support of the three-axis Helmholtz coils setup.
Appendix A. Supplementary data
Supplementary data to this article can be found online atdoi:10.1016/j.nano.2012.03.002.
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