A Hardware Test Bed for Micro-Scale Multi-Agent Control

Tunaidi Ansari Bioengineering, UIUC

INTRODUCTION Our ultimate goal is to control the motion of multiple micro-robots and micro-organisms using visual feedback. One challenge is that, at the micro-scale, it is difficult to apply a specific control input to each agent individually. Instead, we must apply a gross input, such as a magnetic field, to all agents. What type of motion is it possible to achieve under such a constraint? What control strategy should be used? The purpose of this project is to design and build a hardware test bed that will allow us to explore these questions. The hardware test bed has two basic components: a test sample (including the micro-robots/organisms, an underlying substrate, and an actuator to apply gross inputs) and a vision system. First, we will describe how we selected the test sample, based on a review of potential applications in biology. Then, we will describe how we fabricated the test sample. Finally, we will describe how we selected and set up the vision system, and conclude with some preliminary experimental results. RESEARCH METHOD 1. Selecting the Test Sample

Our current research is inspired by recent studies regarding the galvanotaxis of

Paramecia, a genus of protozoa, in which an approach to regulating the relocation of micro-robots or microelectromechanical systems (MEMS) has been discovered [4][5]. The general premise of the proposed mechanism was to push a micro-robot horizontally across the x-axis. Calling upon the principle of galvanotaxis, an electric field was induced to a cluster of Paramecia from four different sides, each in turn, to cause the microorganisms to swarm in a circular fashion. Each cycle would then inevitably nudge the micro-robot in the same direction upon impact. With proper equipment and suitable optics, these events were tracked and visualized [2].

A taxis is an organism’s intrinsic and involuntary response engendered by certain stimuli. Hence, galvanotaxis is the innate motion response to electric current, and is frequently observed in various types of microorganisms and cells [3]. The taxis phenomenon, as well as its derivatives, will be the main sources of gross input. In apprehending a suitable test subject, its collective response to certain gross inputs, in addition to other factors such as cost and efficiency, must be considered. Galvanotaxis (gross input: electric fields)

• Physarum polycephalum (slime mold) • Amoeba (genus of protozoa) • Epithelial cells (specifically keratinocytes) • Mouse sperm

Phototaxis (gross input: light) • Euglena (genus of protozoa) • Volvox (genus of protozoa)

Chemotaxis (gross input: chemicals) • Escherichia coli (bacteria) • Listeria monocytogenes (bacteria)

Magnetotaxis (gross input: magnetic fields) • Magnetospirillum (genus of bacteria) • Metal shavings • Ferrofluid

Other potential test samples (gross input: unknown) • Plasmids (small, circular DNA molecules) • Human red blood cells (erythrocytes) • Nerve cells (neurons) and muscle cells

Ferrofluids possess unique properties and offers much foresight into future research, such as loud speaker cooling and rotary shaft sealing, as well as various medical applications [1]. Ferrofluids are essentially nano-scale ferromagnetic particles, usually magnetite or other compounds containing iron, suspended in a carrier fluid, often water or other organic solvents. The particles are prevented from aggregating to give ferrofluid its mobility. Credited to its versatility in simultaneously, and at times instantaneously, creating various shapes and patterns in the presence of magnetic fields, it is indisputable that with the correct adjustments, these phenomena can be utilized in our proposed experiment, easily able to travel at any anticipated speed in any specified direction.

Initially, the Paramecia caudatum was the species of interest since it was utilized in previous works. However, we concluded that having ferrofluid as the primary testing subject would be most efficient: no culturing is required; lifespan is not a restriction; a special environment is not necessary; cost is low; and experimentation is not as complex. In addition, ferrofluid represents a good model of Paramecia and other micro-organisms in that the amount of ferrofluid analyzed can always vary – similar to increasing or decreasing the number of micro-organisms – and that these changes in turn affect the overall motion control aspect. While Paramecia are attracted to the positive end of electric fields, ferrofluid correspondingly is attracted to the positive end of magnetic fields. Parallels can be drawn between the two in that electric and magnetic fields are related by the Biot-Savart law. As such, modifications were prepared and research continued with ferrofluid as the final testing subject. 2. Fabricating the Test Sample

To fully understand the characteristics and mysteries of ferrofluid, experiments were run on a larger scale before emulating on a micro scale. Larger Scale: In conducting research with the naked eye, a 10 cm-diameter petri dish was chosen as a container for the fluid components. Approximately 10 ml of isopropyl alcohol was added into the petri dish to ensure easy flow of the ferrofluid when motion is activated, followed by around 100

µl of ferrofluid. Magnets, which are the preferred gross inputs, are utilized in various ways in order to study their affect on the mobilization and structure of ferrofluid. Micro Scale: In handling the liquid-like nature of ferrofluid, a microscope slide was selected as its substratum. Isopropyl alcohol was spread over the slide’s surface and in order to prevent ferrofluid staining the glass. Then, miniscule amounts of ferrofluid were added onto the alcohol-laid microscope slide. The cover slip was placed on top of the sample, ensuring no bubbles were formed, in order to restrict the ferrofluid’s movement. To prevent leaking and to preserve sample integrity, the cover slip was glued down onto the glass slide for permanent containment. The mounted microscope slide is then placed upon the test platform and securely held by the two clamps. A powerful but miniature magnet is manually positioned underneath the slide and in between the two aluminum blocks of the platform. Data and results are gathered by the effects of the location and movement of the gross input. 3. Selecting and Setting Up the Vision System The Optics:

In order to perform the expected experiments, a camera with certain basic features is required. These include: a high frame rate for capturing sequential images; a connection via a standard IEEE 1394 firewire port to circumvent a frame grabber and to use modifiable open source drivers; and a high resolution to view small details. After surveying several reliable manufacturers, we found a suitable camera and tracking system. The Basler A602FC is a color video camera that possesses the necessary features prescribed; the

Edmund Optics VZM 300i video lens is attached the A602FC to provide a 4x zoom; and a 115-volt illuminator is connected to the video lens in order to supply sufficient lighting (see Figure 1).

Suitable software for Linux was required for realizing images and motion pictures on the computer. Coriander, a functional GUI for firewire cameras, was acquired (see Figure 2). Testing and results were thus achieved via this program.

The Stage: The constructed optics is held in place with an adjustable arm onto a stable workstation. In addition, a testing platform was machined to fulfill several roles (see Figure 3): to hold the slide in place, to allow placement of gross input, and the raise the slide to the cameras optimal focal range. A 6” x 6” aluminum board was utilized as the base of the platform. Two additional aluminum bricks were attached and centered onto the board 3” apart in parallel. At the top of each brick are indented grooves, or

Figure 1. Setup of visual tracking system..

Figure 2. Coriander 1.01, the software for image capturing.

Figure 3. Machined test platform.

slots, with the same width as that of a microscope slide and horizontally machined across its narrow end. These grooves have the purpose of stabilizing a standard 3” microscope slide, which would be placed on top and across the two aluminum bricks. In addition, the slots provide a constancy in experiments since it provides only one position for the glass slide to rest upon. PRELIMINARY RESULTS 1. Experiments Performed on Both a Larger and Micro Scale:

Whenever the magnet is just placed in a standstill position underneath the ferrofluid, a three dimensional pattern of projecting spikes is formed in-bounded by the shape of the magnet (see Figure 4). The closer the magnet is to the ferrofluid, the more spikes that are generated. The farther away the magnet is to the ferrofluid, the fewer spikes that are generated until no spikes are visible.

When moving to same position as a final state from various starting points, the resulting pattern is always the same with the exception of a thin lagging streak of ferrofluid coming from the direction of the starting point.

When sliding the magnet under the glass slide, the ferrofluid follows in a line. When pulled slower, the line is thicker with more ferrofluid attracted to the magnet. On the contrary, when pulled faster, the line appears much thinner since there is less time for the magnet to attract the ferrofluid. Dragging a magnet throughout the ferrofluid yields continuous lines, and drawings and patterns can be formed as desired. The thickness of the lagging ferrofluid does not depend on the starting direction, but only the speed of the passing magnet when tugged by the string.

When the magnet is lowered from above, a mass of ferrofluid is collected and gathered at the landing point of the magnet and appears in the outlined shape of the magnet. The same occurs when the magnet is brought from under the slide and makes contact from below.

When using two magnets placed on opposite ends under the microscope slide and having the majority of the ferrofluid in the center, both magnets drag the ferrofluid away from the middle towards themselves. Once sufficient ferrofluid is attracted to both magnets, a three dimensional pattern is created in the shape of the magnets. For example, a ring shaped magnet forms a ring of ferrofluid with projections in the shape of a ring; a disk shaped magnet forms a circular structure of ferrofluid spikes.

2. Experiments Performed Only on a Larger Scale:

A disk shaped magnet was placed outside the petri dish

on the side and attracted ferrofluid to the edge. The magnet was given a push. Since the magnet was attracted to the encased ferrofluid, the magnet followed a path encircling the outer circumference of petri dish, dragging ferrofluid around the inside edge of the petri dish (see Figure 5).

Figure 4. Three dimensional projections (A602FC under 4x zoom).

Figure 5. Ferrofluid attracted to the edge of petri dish..

When two magnets were placed on opposite ends under the petri dish with the majority of ferrofluid in the center of the petri dish, the ferrofluid migrated towards both magnets. The petri dish was manually and gradually turned in a circular fashion. The resulting pattern was a ferrofluid ring shape with a straight line cutting it in halves. In the very center of this circle was a lobe of a portion of the original ferrofluid. CONCLUSION AND FUTURE WORK Control of groups on the micro-scale is probable and has yet to be made feasible. Using the unique properties inherent within specific agents, organized motion control can be realized with corresponding gross inputs. In the case of ferrofluid, its magnetic capabilities are essential in directing movement via the magnetic fields shed by miniature magnets. This research is not nearly complete, but is just a start in revealing the basic knowledge behind specific agents and acquiring intuition regarding the functionality of ferrofluid motility. Interesting studies that may be further conducted include accuracy and precision of the ferrofluid control, use of ferrofluid traversals as a means of pushing groups of micro-robots in a specified direction, varying the amounts of ferrofluid utilized in each experiment, creating a more durable and permanent slide, and automating the gross input source and positioning. In addition to improving these features, future work may be focused on obtaining other usable agents and gross inputs as well as perfecting each aspect of control. ACKNOWLEDGMENT The author gratefully acknowledges Professor Timothy Bretl (Aerospace Engineering, UIUC), for generously providing lab space and financial funds for equipment. Main funding sources were provided by the Boeing Co., the Illinois Space Grant Consortium, and NASA. Appreciation is given to Miles Johnson (Aerospace Engineering, UIUC) and Sharath Avadhanam (Industrial Engineering, UIUC) for assisting in the acquiring and operating of the Coriander software, and to Kent Elam for machining necessary parts. REFERENCES [1] S. Odenbach. Preface: Ferrouids. Journal of Physics: Condensed Matter, 18, 2006. [2] N. Ogawa, H. Oku, K. Hashimoto, and M. Ishikawa. Microrobotic visual control of motile

cells using high-speed tracking system. IEEE Transactions on Robotics, 21:704-712, 2005. [3] N. Ogawa, H. Oku, K. Hashimoto, and M. Ishikawa. A physical model for galvanotaxis of

paramecium cell. Journal of Theoretical Biology, 242:314-328, 2006. [4] N. Ogawa, H. Oku, K. Hashimoto, and M. Ishikawa. Single-cell level continuous observation

of microorganism galvanotaxis. In IEEE International Symposium on Biomedical Imaging, pages 1331-1334, Arlington, VA, 2004.

[5] K. Takahashi, N. Ogawa, H. Oku, and K. Hashimoto. Organized motion control of a lot of microorganisms using visual feedback. In IEEE International Conference on Robotics and Automation, pages 1408-1413, Orlando, FL, May 2006.