Bumjin Jang

Undulatory Locomotion of Magnetic Multilink Nanoswimmers

Micro- and nanorobots operating in low Reynolds number fluid environments require specialized swimming strategies for efficient locomotion. Prior research has focused on designs mimicking the rotary corkscrew motion of bacterial flagella or the planar beating motion of eukaryotic flagella. These biologically inspired designs are typically of uniform construction along their flagellar axis. This work demonstrates for the first time planar undulations of composite multilink nanowire-based chains (diameter 200 nm) induced by a planar-oscillating magnetic field. Those chains comprise an elastic eukaryote-like polypyrrole tail and rigid magnetic nickel links connected by flexible polymer bilayer hinges. The multilink design exhibits a high swimming efficiency. Furthermore, the manufacturing process enables tuning the geometrical and material properties to specific applications.

Artificial Swimmers Propelled by Acoustically Activated Flagella

Recent studies have garnered considerable interest in the field of propulsion to maneuver micro- and nanosized objects. Acoustics provide an alternate and attractive method to generate propulsion. To date, most acoustic-based swimmers do not use structural resonances, and their motion is determined by a combination of bulk acoustic streaming and a standing-wave field. The resultant field is intrinsically dependent on the boundaries of their resonating chambers. Though acoustic based propulsion is appealing in biological contexts, existing swimmers are less efficient, especially when operating in vivo, since no predictable standing-wave can be established in a human body. Here we describe a new class of nanoswimmer propelled by the small-amplitude oscillation of a flagellum-like flexible tail in standing and, more importantly, in traveling acoustic waves. The artificial nanoswimmer, fabricated by multistep electrodeposition techniques, compromises a rigid bimetallic head and a flexible tail. During acoustic excitation of the nanoswimmer the tail structure oscillates, which leads to a large amplitude propulsion in traveling waves. FEM simulation results show that the structural resonances lead to high propulsive forces.

Fabrication of Segmented Au/Co/Au Nanowires: Insights in the Quality of Co/Au Junctions

Electrodeposition is a versatile method, which enables the fabrication of a variety of wire-like nanoarchitectures such as nanowires, nanorods, and nanotubes. By means of template-assisted electrodeposition, segmented Au/Co/Au nanowires are grown in anodic aluminum oxide templates from two different electrolytes. To tailor the properties of the cobalt segments, several electrochemical conditions are studied as a function of current density, pulse deposition, and pH. The morphology, crystal structure, and magnetic properties are accordingly investigated. Changes in the deposition conditions affect the cobalt electrocrystallization process directly. Cobalt tends to crystallize mainly in the hexagonal close-packed structure, which is the reason cobalt might not accommodate satisfactorily on the face-centered cubic Au surface or vice versa. We demonstrate that by modifying the electrolyte and the applied current densities, changes in the texture and the crystalline structure of cobalt lead to a good quality connection between dissimilar segments. In particular, lowering the bath pH, or using pulse plating at a high overpotential, produces polycrystalline fcc Co and thus well-connected Co/Au bimetallic junctions with smooth interface. These are crucial factors to be carefully considered taking into account that nanowires are potential building blocks in micro- and nanoelectromechanical systems.

A universal method for planar lipid bilayer formation by freeze and thaw

A procedure based on freezing and thawing was developed to induce the rupture of adsorbed lipid vesicles on solid surfaces into supported lipid bilayers (SLBs). The SLB assembly exploits the phase transition of both lipids and water during freezing. It enables SLB formation independent of the type of substrates and lipids as long as the vesicles spontaneously adsorb onto the surface. The created SLB is a single bilayer, and has a diffusion coefficient of (0.6–4) × 10−8 cm2 s−1 on TiO2, which is in the same range as the SLBs formed by conventional techniques. The presented approach has the advantages of both the Langmuir–Blodgett method (the versatility in the selection of lipids and substrates) and vesicle fusion (self-assembly) at the same time.

Small‐Scale Machines Driven by External Power Sources

Micro- and nanorobots have shown great potential for applications in various fields, including minimally invasive surgery, targeted therapy, cell manipulation, environmental monitoring, and water remediation. Recent progress in the design, fabrication, and operation of these miniaturized devices has greatly enhanced their versatility. In this report, the most recent progress on the manipulation of small-scale robots based on power sources, such as magnetic fields, light, acoustic waves, electric fields, thermal energy, or combinations of these, is surveyed. The design and propulsion mechanism of micro- and nanorobots are the focus of this article. Their fabrication and applications are also briefly discussed.

Silicon-supported aluminum oxide membranes with ultrahigh aspect ratio nanopores

AAO membranes become essential for fabricating nano-building blocks. However, the integration of these nano-building blocks in complex machinery is still challenging, mainly due to the fragility of these membranes. In this work, we overcome this drawback by developing a new integrative process which enables the support of a highly-ordered nanoporous membrane onto a mechanically robust substrate such as silicon. The fabrication of supported AAO (SAAO) membranes is achieved by transferring an AAO layer onto a Si substrate via a Au/Au compressive bonding process. Two types of AAOs were prepared for this bonding process to demonstrate the universality of our technology: mild-anodized AAO (MA-AAO) and pulse-anodized AAO (PA-AAO). We also demonstrate that the newly developed SAAO membranes are suitable for electrodeposition of nanostructures. Problems such as membrane handling or electrolyte leakage occurring in conventional AAO membranes are avoided, so that Ni nanostructures with well-controlled dimensions and uniform lengths are obtained. The high-aspect ratio Ni nanostructures have the potential to be used in various applications, such as biosensing and energy storage.

Magnetic imaging of a single ferromagnetic nanowire using diamond atomic sensors

Recent advances in nanorobotic manipulation of ferromagnetic nanowires bring new avenues for applications in the biomedical area, such as targeted drug delivery, diagnostics or localized surgery. However, probing a single nanowire and monitoring its dynamics remains a challenge since it demands high precision sensing, high-resolution imaging, and stable operations in fluidic environments. Here, we report on a novel method of imaging and sensing magnetic fields from a single ferromagnetic nanowire with an atomic-scale sensor in diamond, i.e. diamond nitrogen-vacancy (NV) defect center. The distribution of static magnetic fields around a single Co nanowire is mapped out by spatially distributed NV centers and the obtained image is further compared with numerical simulation for quantitative analysis. DC field measurements such as continuous-wave ODMR and Ramsey sequence are used in the paper and sub Gauss level of field sensing is demonstrated. By imaging magnetic fields at a single nanowire level, this work represents an important step toward tracking and probing of ferromagnetic nanowires in biomedical applications.

Magnetometry of Individual Polycrystalline Ferromagnetic Nanowires

Ferromagnetic nanowires are finding use as untethered sensors and actuators for probing micro- and nanoscale biophysical phenomena, such as for localized sensing and application of forces and torques on biological samples, for tissue heating through magnetic hyperthermia, and for microrheology. Quantifying the magnetic properties of individual isolated nanowires is crucial for such applications. Dynamic cantilever magnetometry is used to measure the magnetic properties of individual sub-500 nm diameter polycrystalline nanowires of Ni and Ni80 Co20 fabricated by template-assisted electrochemical deposition. The values are compared with bulk, ensemble measurements when the nanowires are still embedded within their growth matrix. It is found that single-particle and ensemble measurements of nanowires yield significantly different results that reflect inter-nanowire interactions and chemical modifications of the sample during the release process from the growth matrix. The results highlight the importance of performing single-particle characterization for objects that will be used as individual magnetic nanoactuators or nanosensors in biomedical applications.

Dually actuated atomic force microscope with miniaturized magnetic bead-actuators for single-molecule force measurements

We report a novel atomic force microscopy (AFM) technique with dual actuation capabilities using both piezo and magnetic bead actuation for advanced single-molecule force spectroscopy experiments. The experiments are performed by manipulating magnetic microbeads using an electromagnet against a stationary cantilever. Magnetic actuation has been demonstrated before to actuate cantilevers, but here we keep the cantilever stationary and accomplish actuation via free-manipulated microstructures. The developed method benefits from significant reduction of drift, since the experiments are performed without a substrate contact and the measured force is inherently differential. In addition, shrinking the size of the actuator can minimize hydrodynamic forces affecting the cantilever. The new method reported herein allows for the application of constant force to perform force-clamp experiments without any active feedback, profiled for a deeper understanding of biomolecular interactions.