Proposal of a transmitting and receiving coil configuration focusing on contactless power supply to biometric information acquisition devices for small experimental animals

In the field of drug discovery and medical technology development, it is essential to conduct animal experimentation to study the effects and side effects of drugs and technologies. To examine the effects of drugs and technologies on experimental animals, it is necessary to record changes in their biological information, such as body temperature, heart rate, blood pressure, and activity level, that occur during experiments. There are a wide variety of ways to record biometric information. To take the recording of activity as an example, methods exist such as analyzing video captured by an infrared camera, using a collar-shaped wearable device, and referring to the number of revolutions of a spinning wheel.^1–3 In recent years, however, biometric data acquisition devices implanted in the body have been attracting significant attention.^4,5 The reason for this is the usefulness of implanting the device directly into the body, which enables more accurate and continuous acquisition of biological information from experimental animals than the above methods. However, the issue here is the power supply to the device. Currently, batteries are the primary means of power supply in such devices.^4,6–8 Batteries supply power to a device by being embedded with it, enabling a stable power supply without the need for cables outside the body. However, when a battery is implanted with a device, it must be small and lightweight so that the weight of the battery does not become a burden on the experimental animal. As a result, it is difficult to increase battery capacity in conducting long-term experiments. In addition, the system consumes a lot of power when transmitting the acquired biometric information to an external terminal, such as a PC, making it impossible to monitor the biometric information in real time, creating limitations in experiments.⁹ To solve these problems simultaneously, we propose a contactless power supply system for biometric data acquisition devices. A variety of contactless power transfer methods exist for implantable devices. These include the use of electromagnetic induction between coils, ultrasonic waves,^9,10 and infrared rays.^9,11 We used the electromagnetic induction method because, by optimizing the resonance frequency of the coils, it is possible to minimize the effects of magnetic fields on living tissue.¹² In this basic system, a power-receiving coil is implanted in the experimental animal instead of a battery, and a power-transmitting coil is installed under the rearing cage to provide a constant power supply without cables. The acquired data is transferred to an external recording terminal, such as a PC, for storage. Figure 1 shows the experimental setup.

This study examines the appropriate shape of a transmitting and receiving coil, which is key in the construction of a contactless power supply system for an electromagnetic induction implantable biometric device. The following points are considered: Experimental animals move freely in their rearing cages. They also assume various postures, such as standing up and, in some cases, hanging from the ceiling by their tails for long periods of time. For this reason, biometric data acquisition devices on the market that employ a contactless power supply often have points in the breeding cage where the power supply becomes unstable. If the power supply is interrupted, the acquisition of biometric information is also interrupted. Therefore, power must be continuously transmitted regardless of the position or posture of the experimental animal in which the power-receiving coil is embedded.

To address this problem, we considered generating a rotating magnetic field of uniform intensity inside the rearing cage that excited the receiving coil from multiple directions. However, this was not possible because a single excitation coil generates a magnetic field in only one direction. To realize a rotating magnetic field, magnetic fields in at least two directions are required at the point where a rotating magnetic field is needed.^13,14 In other words, two or more excitation coils should be used, and the direction of the magnetic field generated from each coil must be parallel at the point where a rotating magnetic field is required.¹⁵ The appearance of the transmission coil designed under these conditions is shown in Fig. 2(a). The two coil axes are wound orthogonally to each other around the periphery of a rectangular ferrite, which is named a biaxially orthogonal coil for the sake of convenience. The biaxially orthogonal coils have two distinctive geometrical features.

The first is that the coils are wound so that they are symmetrical. This is to prevent magnetic coupling between the coils. Preventing magnetic coupling solves the problem of double resonance causing the power supply to become uncontrollable and the problem of the induced voltage generated when the coil is excited loading the power supply.¹⁵ 

The second feature is that the number of windings at the center of the coil must be greater than the number of windings at both ends of the coil. This is because the magnetic flux density at both ends of the coil is larger than that at the center of the coil due to an antimagnetic field effect. By taking these measures, the strength of the magnetic flux density distributed on the same plane as the power transmission coil-shaped rearing cage becomes uniform, enabling a stable power supply to the biometric data acquisition device.

The receiving coil was designed to have a shape that can efficiently receive a rotating magnetic field. The designed receiving coil is shown in Fig. 2(b). This coil is perpendicular to the longitudinal direction and has four curved rectangular coils, two each on the upper and lower surfaces of a circular magnetic core. The coils facing each other have a summing connection. The summing connection prevents the double resonance that would be caused by magnetic coupling between the coils. The coils attached to the upper part of the receiving coil and the coils attached to the lower part of the receiving coil are offset by 90°. This is because the rotating magnetic field draws a circular path in the plane, and both coils feed power if the receiving coil is oriented vertically. If the coils are oriented horizontally, the coil facing a different direction from the axis of rotation of the circular orbit of the rotating magnetic field feeds the power. Therefore, the combination of these coils enables stable power supply regardless of the animal’s posture.

The rotating magnetic field generated by the power transmission coils should be uniform. Figures 3(a) and 3(b) show the magnetic field distribution and direction of the magnetic field obtained by simulation. The simulation results show that the magnetic field density in the XY-direction of the coil plane is uniform, although the magnetic field density weakens as the height Z from the bottom of the coil increases. The rotating magnetic field was adjusted so that the magnetic flux density at the center of the coil was 100 μT. If the magnetic flux densities given to the two coils forming the biaxial orthogonal coils are B₁ and B_2, the effective value of the magnetic flux density due to the rotating magnetic field, B_rorate(rms), can be calculated as follows.

Equation (1) is the equation for the instantaneous value of the magnetic flux density of the rotating magnetic field, $Brotate⃗t$⁠. Note that $B1m⃗$ and $B2⃗$ in Eq. (1) are the amplitudes of the magnetic flux density when the two coils are driven independently of one another. Equation (2) is the equation for obtaining the effective value of the current, A_(rms), where T is the period of the current and A(t) is the instantaneous value of the current. The rms value of the magnetic flux density of the rotating magnetic field can be calculated by applying the procedure for obtaining the rms value of the current. Integrate Eq. (1) according to the definition of the rms value of current, A_(rms), in Eq. Substituting $Brotate⃗t$ for the instantaneous current value A(t) and performing the calculation, the effective value of the magnetic flux density of the rotating magnetic field in Eq. (3) is obtained. From the above equation, it is required that the values of B₁ and B₂ must be 70.7 μT to output a rotating magnetic fieldof 100 μT.

Therefore, the rms value of the rotating magnetic field is larger than the rms value of the magnetic field in one direction generated by a single coil and if the current phase difference between the two coils is 90°, the angle between the magnetic flux densities generated by each coil has no effect on the rms value of the rotating magnetic field.

We actually conducted a power-feeding experiment using the proposed transmission and receiving coil configuration. To demonstrate that stable power supply is possible regardless of the animal’s posture, the direction and angle of the receiving coil were varied, and the feed voltage was confirmed. The transmitter coil was driven at 395 kHz and the current value was fixed at 2 A. One vertical and one horizontal receiving coil were installed at a position where the magnetic field density on the transmission coil was 50 μT. Then, when the installed coil was rotated 360° in 15° increments, the change in output voltage when the load was released was recorded. The results are shown in Figs. 4(a) and 4(b) and show that in the horizontal direction, the output voltage fluctuates, but the power is supplied without interruption. In the vertical direction, the output voltage only fluctuates marginally. Therefore, a stable power supply is possible regardless of the direction and angle of the receiving coil; and omni-directionality of the system, important for contactless power supply in a biometric device embedded in an experimental animal, is ensured.

This study confirmed the usefulness of wireless power transfer in a monitoring system designed for laboratory animals. Such a system must be able to transfer power regardless of the posture and position of the animal. Therefore, the construction of a contactless power transfer system using a rotating magnetic field was described. We proposed a shape for the transmitter coil that generated a uniform rotating magnetic field, and a shape for the receiver coil that meant it efficiently received the rotating magnetic field. We showed that the power transfer via the rotating magnetic field reduced the deterioration of power feeding performance due to animal position and posture in contactless power transmission systems.

The authors have no conflicts to disclose.

Tatsuki Omori: Validation (equal). Fumihiro Sato: Conceptualization (equal); Project administration (equal). Yoshiki Furuya: Data curation (equal). Syu Sasaki: Funding acquisition (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.