U Kei Cheang

Fabrication and magnetic control of bacteria-inspired robotic microswimmers

A biomimetic, microscale system using the mechanics of swimming bacteria has been fabricated and controlled in a low Reynolds number fluidic environment. The microswimmer consists of a polystyrene microbead conjugated to a magnetic nanoparticle via a flagellar filament using avidin-biotin linkages. The flagellar filaments were isolated from the bacterium, Salmonella typhimurium. Propulsion energy was supplied by an external rotating magnetic field designed in an approximate Helmholtz configuration. Further, the finite element analysis software, COMSOL MULTIPHYSICS, was used to develop a simulation of the robotic devices within the magnetic controller. The robotic microswimmers exhibited flagellar propulsion in two-dimensional magnetic fields, which demonstrate controllability of the biomimetically designed devices for future biomedical applications.

Multiple-robot drug delivery strategy through coordinated teams of microswimmers

Untethered robotic microswimmers are very promising to significantly improve various types of minimally invasive surgeries by offering high accuracy at extremely small scales. A prime example is drug delivery, for which a large number of microswimmers is required to deliver sufficient dosages to target sites. For this reason, the controllability of groups of microswimmers is essential. In this paper, we demonstrate simultaneous control of multiple geometrically similar but magnetically different microswimmers using a single global rotating magnetic field. By exploiting the differences in their magnetic properties, we triggered different swimming behaviors from the microswimmers by controlling the frequency and the strength of the global field, for example, one swim and the other does not while exposed to the same control input. Our results show that the balance between the applied magnetic torque and the hydrodynamic torque can be exploited for simultaneous control of two microswimmers to swim in opposite directions, with different velocities, and with similar velocities. This work will serve to establish important concepts for future developments of control systems to manipulate multiple magnetically actuated microswimmers and a step towards using swarms of microswimmers as viable workforces for complex operations.

Harnessing bacterial power in microscale actuation

This paper presents a systematic analysis of the motion of microscale structures actuated by flagellated bacteria. We perform the study both experimentally and theoretically. We use a blotting procedure to attach flagellated bacteria to a buoyancy-neutral plate called a microbarge. The motion of the plate depends on the distribution of the cells on the plate and the stimuli from the environment. We construct a stochastic mathematical model for the system, based on the assumption that the behavior of each bacterium is random and independent of that of its neighbors. The main finding of the paper is that the motion of the barge plus bacteria system is a function of a very small set of parameters. This reduced-dimensional model can be easily estimated using experimental data. We show that the simulation results obtained from the model show an excellent match with the experimentally-observed motion of the barge.

Self-assembly of robotic micro- and nanoswimmers using magnetic nanoparticles

Micro- and nanoscale robotic swimmers are very promising to significantly enhance the performance of particulate drug delivery by providing high accuracy at extremely small scales. Here, we introduce micro- and nanoswimmers fabricated using self-assembly of nanoparticles and control via magnetic fields. Nanoparticles self-align into parallel chains under magnetization. The swimmers exhibit flexibility under a rotating magnetic field resulting in chiral structures upon deformation, thereby having the prerequisite for non-reciprocal motion to move about at low Reynolds number. The swimmers are actuated wirelessly using an external rotating magnetic field supplied by approximate Helmholtz coils. By controlling the concentration of the suspended magnetic nanoparticles, the swimmers can be modulated into different sizes. Nanoscale swimmers are largely influenced by Brownian motion, as observed from their jerky trajectories. The microswimmers, which are roughly three times larger, are less vulnerable to the effects from Brownian motion. In this paper, we demonstrate responsive directional control of micro- and nanoswimmers and compare their respective diffusivities and trajectories to characterize the implications of Brownian disturbance on the motions of small and large swimmers. We then performed a simulation using a kinematic model for the magnetic swimmers including the stochastic nature of Brownian motion.

Versatile microrobotics using simple modular subunits

The realization of reconfigurable modular microrobots could aid drug delivery and microsurgery by allowing a single system to navigate diverse environments and perform multiple tasks. So far, microrobotic systems are limited by insufficient versatility; for instance, helical shapes commonly used for magnetic swimmers cannot effectively assemble and disassemble into different size and shapes. Here by using microswimmers with simple geometries constructed of spherical particles, we show how magnetohydrodynamics can be used to assemble and disassemble modular microrobots with different physical characteristics. We develop a mechanistic physical model that we use to improve assembly strategies. Furthermore, we experimentally demonstrate the feasibility of dynamically changing the physical properties of microswimmers through assembly and disassembly in a controlled fluidic environment. Finally, we show that different configurations have different swimming properties by examining swimming speed dependence on configuration size.

Towards Model-Based Control of Achiral Microswimmers

In this paper, we introduce the three bead achiral microswimmers controlled wirelessly using magnetic fields with the ability to swim in bulk fluid. The achirality of the microswimmer introduces unknown handedness of the microswimmer. Here, we propose to use a combination of rotating and static magnetic fields to eliminate the uncertainty in swimming direction. Our experimental results demonstrated excellent capability of direction control as well as agile movements. From the experimentally collected data, we estimated a control-oriented two-wheeled robot model. Finally, we design feedback control for microswimmers based on the estimated kinematic model. In particular, we show that the feedback control law moves the microswimmer from any initial conditions to a target set of microswimmer’s position and angle.Copyright

Feedback Control of an Achiral Robotic Microswimmer

Magnetic microswimmers are useful for navigating and performing tasks at small scales. To demonstrate effective control over such microswimmers, we implemented feedback control of the three-bead achiral microswimmers in both simulation and experiment. The achiral microswimmers with the ability to swim in bulk fluid are controlled wirelessly using magnetic fields generated from electromagnetic coils. The achirality of the microswimmers introduces unknown handedness resulting in un-certainty in swimming direction. We use a combination of rotating and static magnetic fields generated from an approximate Helmholtz coil system to overcome such uncertainty. There are also movement uncertainties due to environmental factors such as unsteady flow conditions. A kinematic model based feedback controller was created based on data fitting of experimental data. However, the controller was unable to yield satisfactory performance due to uncertainties from environmental factors; i.e., the time to reach target pose under adverse flow condition is too long. Following the implementation of an integral controller to control the microswimmers’ swimming velocity, the microswimmers were able to reach the target in roughly half the time. Through simulation and experiments, we show that the feedback control law can move an achiral microswimmer from any initial conditions to a target pose.

Obstacle avoidance method for microbiorobots using electric field control

This paper presents an obstacle-avoidance based approach for the control of MicroBioRobots (MBRs) using electric field. A MBR is an integrated cell-based robotic system, each of which consists of a SU-8 microstructure blotted with swarming bacteria. The concept of the MBR is to utilize inorganic structures as platforms to harness the collective propulsive power from the biomolecular motors of bacteria. We previously demonstrated motion control of MBRs using electric field. However, in the presence of obstacles in the workspace, the electric field can be distorted. In this paper we evaluate the distortion of electric field around obstacles and develop a motion control algorithm that takes the distortion into account. Our obstacle-avoidance method enhances the controllability of the MBRs by allowing them to avoid collision with static obstacles in real time. Artificial potential field was used in our approach to generate the objective function regarding the controllability of the MBRs under electric field. Furthermore, we use COMSOL Multiphysics engineering simulation software to model an electric field applied across the testbed to characterize distortions of the field around the boundaries of static obstacles. We demonstrate the feasibility of our obstacle avoidance algorithm through experiment and simulation.

A comparison of vision-based tracking schemes for control of microbiorobots

There has been significant recent interest in micro-nano robots operating in low Reynolds number fluidic environments. Even though recent works showed the success of controlling micro-nano robots, there are some limitations because of the tracking method. In this paper, we introduce and implement a feature-based tracking method (FTM). Scale invariant feature transform (SIFT) is a well-explored technique at much larger length scales for research fields regarding robotics and vision. Here, the technique is extensively investigated and optimized for microbiorobots (MBRs) in low Reynolds number environments. Also, we compare the FTM with the conventional tracking method for cells, which is known as the region-based tracking method (RTM). We clearly show that the FTM can track more accurate positions of the objects in comparison with the RTM in cases where objects are in close contact or overlapped. Also, we demonstrate that the FTM allows tracking microscopic objects even though illumination changes over time or portions of the object are occluded or outside the field of view.

Galvanotactic Control of Self-Powered Microstructures

We are examining microactuation techniques by employing the electrokinetic and galvanotactic behavior of certain bacteria. We cultured selected strains of swarming Serratia marcescens which were attached to microstructures using a blotting technique that creates a bacterial monolayer carpet. These bacterial carpets naturally self-coordinate to propel the microstructures. The microstructures were placed in an open channel and a voltage was applied and polarity was switched. We have demonstrated directional control of the motion of the microstructures patterned with bacteria. This mobility is due to the patterning of bacteria on the microstructure surface and arises from a combination of electrokinetic effects and galvanotaxis.Copyright