Identification of Aquatic Organisms Using a Magneto-Optical Element

Abstract

In recent advanced information society, it is important to individually identify products or living organisms automatically and quickly. However, with the current identifying technology such as RFID tag or biometrics, it is difficult to apply to amphibians such as frogs or newts because of its size, stability, weakness under a wet environment and so on. Thus, this research aims to establish a system that can trace small amphibians easily even in a wet environment and keep stable sensing for a long time. The magnetism was employed for identification because it was less influenced by water for a long time. Here, a novel magnetization-free micro-magnetic tag is proposed and fabricated with low cost for installation to a living target sensed by Magneto-Optical sensor for high throughput sensing. The sensing ability of the proposed method, which was evaluated by image analysis, indicated that it was less than half of the target value (1 mm) both in the water and air. The FEM analysis showed that it is approximately twice the actual identification ability under ideal conditions, which suggests that the actual sensing ability can be extended by further improvement of the sensing system. The developed magnetization-free micro-magnetic tag can contribute to keep up the increasing demand to identify a number of samples under a wet environment especially with the development of gene technology.

Keywords: MEMS; magnetism; magneto-optics.

Figure 1

Principle of the magneto-optical sensor, which visualizes the magnetic flux density. Rotation angle of linearly polarized light is proportional to the strength of the magnetic field. Rotated polarized light is cutoff by an analyzer according to its angle.

Figure 2

Concept of a magnetization-free magnetic tag by applying an external magnetic field to a magnetic tag and sensing by the MO sensor.

Figure 3

Detailed magnetic tag creation process. (a) Nickel plating is selectively performed by forming SU-8 insulation in the conducting part made by gold sputtering, and a pattern carrying information is produced. (b) Mixture of PDMS and iron powder is poured into the depression of SU-8 made by photolithography. (c) Tag-type photolithography is applied to gold sputtered on a substrate. Exposed gold is dissolved by immersion in the gold etchant, leaving a tag-shaped gold thin film.

Figure 4

Magnetic tag using different materials. (a) Tag created by nickel electrolytic plating. Upper left section is a section without nickel because it is not plated well. (b) Tag created using PDMS and iron powder. Iron powder is sufficiently contained in each section. (c) Tag created by gold etching.

Figure 5

Experimental setup. After polarized light from the light source is rotated by the magneto-optical element, the analyzer cuts off the light according to its angle. Half mirror has a role of passing through half of the irradiated light and reflecting the rest. External magnetic field is controlled by changing the magnet arrangement around the tag.

Figure 6

Two conditions of the magnetic field direction. (a) Magnets are placed opposite to the top and bottom of the off-plane magnetic field tag, and a magnetic field is applied perpendicular to the tag and the magneto-optical element. (b) Magnets are exposed opposite on the left and right of the in-plane magnetic field tag, and a magnetic field is applied in parallel with the tag and magneto-optical element.

Figure 7

(a–c) Off-plane magnetic field to nickel, iron and gold tags, respectively. (d–f) In-plane magnetic field to nickel, iron and gold tags, respectively. Gold tag has a small magnetization and tag identification is difficult, but the conditions of (d,e) are the most suitable for tag identification.

Figure 8

Change in the tag photographed image due to a change in sensing distance. (a) Sensing image in air. (b) Sensing image in water. Both are less than 300 μm, making it possible to identify tag sections with the naked eye.

Figure 9

Relation between MO sensing distance and brightness by image analysis software. As the sensing distance D increases, the brightness approaches a constant value, and eventually tag identification becomes difficult. Sensing ability does not significantly differ in air or water. Tag identification is possible in the range of less than 250 μm.

Figure 10

Sensing distance by FEM analysis software. Tag identification is possible even at 400 μm, which is difficult to identify with an actual MO sensor under ideal conditions.

Figure 11

Relation between sensing distance and luminance by FEM analysis. The sensing ability has been reinforced and the limit is extended to around 500 μm.